Binocular Coordination During Reading and Non-Reading Tasks

Article (PDF Available)inPsychological Bulletin 134(5):742-63 · October 2008with234 Reads
DOI: 10.1037/a0012979 · Source: PubMed
Abstract
The goal of this review is to evaluate the literature on binocular coordination during reading and non-reading tasks in adult, child, and dyslexic populations. The review begins with a description of the basic characteristics of eye movements during reading. Then, reading and non-reading studies investigating binocular coordination are evaluated. Areas of future research in the field are identified and discussed. Finally, some general conclusions are made regarding binocular coordination. The review demonstrates that findings from traditionally independent areas of research are largely consistent and complementary. Throughout the review, theoretical and methodological commonalities are identified and clarified in order to advance current understanding of this fundamental aspect of human visual processing.
Binocular Coordination During Reading and Non-Reading Tasks
Julie A. Kirkby
University of Southampton
Lisa A. D. Webster
Durham University
Hazel I. Blythe and Simon P. Liversedge
University of Southampton
The goal of this review is to evaluate the literature on binocular coordination during reading and
non-reading tasks in adult, child, and dyslexic populations. The review begins with a description of the
basic characteristics of eye movements during reading. Then, reading and non-reading studies investi-
gating binocular coordination are evaluated. Areas of future research in the field are identified and
discussed. Finally, some general conclusions are made regarding binocular coordination. The review
demonstrates that findings from traditionally independent areas of research are largely consistent and
complementary. Throughout the review, theoretical and methodological commonalities are identified and
clarified in order to advance current understanding of this fundamental aspect of human visual process-
ing.
Keywords: binocular coordination, fixation disparity, oculomotor control, dyslexia
Our goal in this article is to review the literature on binocular
coordination of eye movements in adult, child, and dyslexic pop-
ulations. We focus primarily on binocular coordination during
reading, but we also include some other visual tasks. In order to
evaluate research in this area fully, we begin by briefly describing
basic characteristics of eye movements in reading that have been
well established from monocular eye tracking studies. We then
discuss adults’ and children’s binocular coordination during read-
ing and during some non-reading tasks. We subsequently consider
the influence of binocular coordination in reading disability/
dyslexia. The data reported in this review all involve English
readers unless otherwise specified. Furthermore, we report fixation
disparity magnitudes for all studies in terms of characters (wher-
ever possible).
1
Over the last 40 or so years, it has become increasingly apparent
that eye movement methodology is an extremely valuable tool for
scientists who want to investigate aspects of human cognitive
processing. Eye movement measurement has traditionally been
associated with research investigating visual processing and writ-
ten language comprehension (see Liversedge & Findlay, 2000;
Rayner, 1978b, 1998). Within the last decade or so, however,
something of an explosion has occurred in the use of eye move-
ment measurement more generally within the field of experimental
psychology.
A substantial body of research currently exists that explores the
basic characteristics of eye movement behavior during reading
(see Rayner, 1978b, 1998). It is somewhat surprising (if not
alarming), given the depth of this research and its implications,
that comparatively little research has investigated how the two
eyes move in relation to each other, that is, binocular coordination.
The majority of eye movement researchers record from only one of
the eyes. Ocular alignment is of little relevance to many research-
ers; thus, binocular recording is often not necessary. The principal
reason for the lack of binocular research within the field of reading
is probably the prevalent assumption that each eye fixates the same
character within a word. In fact, as we will see from this review,
an increasing body of research indicates that this is not always the
case.
An important point to note is that binocular coordination has not
been recognized as a coherent research topic. Two factors have
contributed to this lack of recognition: (a) different disciplines
have focused on different theoretical issues, and (b) different
methodological approaches have been adopted to study different,
but clearly related, issues. For example, vision scientists have
produced a body of research examining low-level characteristics of
binocular coordination during saccades, typically with participants
moving their eyes between stimuli that are composed of light-
emitting diodes (LEDs). A second body of research has examined
binocular coordination during fixations in reading; this work has
been conducted by researchers who are often focused on aspects of
linguistic as well as visual processing. Obviously, words, sen-
tences, and passages have been used as stimuli in their experi-
1
The authors are very grateful to Keith Rayner and three anonymous
reviewers for their helpful comments on a draft of this article.
Julie A. Kirkby, Hazel I. Blythe, and Simon P. Liversedge, School of
Psychology, University of Southampton, Southampton, United Kingdom;
Lisa A. D. Webster, Department of Psychology, Durham University,
Durham, United Kingdom.
Julie A. Kirby was supported by Leverhulme Grant F/00 128/AG. This
work was also supported by the Biotechnology and Biological Sciences
Research Council Grant 519168. The research is based on a doctoral thesis
by Julie A. Kirkby at the University of Southampton.
Correspondence concerning this article should be addressed to Simon P.
Liversedge, Centre for Visual Cognition, School of Psychology, Shackle-
ton Building, University of Southampton, Southampton, SO17 1BJ United
Kingdom. E-mail: s.p.liversedge@soton.ac.uk
Psychological Bulletin Copyright 2008 by the American Psychological Association
2008, Vol. 134, No. 5, 742–763 0033-2909/08/$12.00 DOI: 10.1037/a0012979
742
ments. A third body of research has assessed binocular coordina-
tion in children with dyslexia to examine the relation between
binocular coordination and reading difficulty. The studies in this
area have frequently used somewhat subjective methods of assess-
ment; indeed, some studies have not assessed eye movements
directly. Nonetheless, there are interesting and important common-
alities across studies in different areas. These commonalities lead
us to similar conclusions concerning binocular coordination de-
spite different approaches. We start this review by considering the
basic characteristics of eye movements during reading based on
monocular experimentation before describing work investigating
binocularity.
Basic Characteristics of Eye Movements During Reading
The two principal, defining features of eye movement behavior
during reading are saccades and fixations. Saccades are rapid,
ballistic eye movements of up to 500° per second. They vary in
their magnitude and duration, with small saccades having shorter
durations than do long ones (e.g., a saccade will be of approx-
imately 30-ms duration, and a saccade will be of approximately
40 –50-ms duration; Abrams, Meyer, & Kornblum, 1989; Rayner,
1978a). On average, saccades are larger during scene perception
than during reading. Fixations are made between saccades, during
which the eyes are relatively still for periods within the range of
150 –500 ms (though the majority are in the order of 200 –250 ms).
The purpose of saccadic eye movements is to bring new informa-
tion onto the fovea. The human retina is not uniform with respect
to visual acuity; instead, it is composed of three regions. These
regions are not categorically distinct, separate portions of the retina
but rather continuous regions across which visual acuity varies.
The three regions are the fovea, the parafovea, and the periphery
(Balota & Rayner, 1983; Rayner, 1978b). The fovea is the central
of our vision within which visual acuity is highest. The parafo-
vea is the region that extends outward from the fovea to either
side of fixation; the periphery, as the name suggests, is the region
extending beyond this. The further away from the foveal region of
the retina that light from a visual stimulus falls, then the poorer the
acuity with which that stimulus will be perceived. Thus, to see
something clearly, it is necessary to orient the eyes such that light
from a target stimulus falls on the high acuity region of the fovea.
Saccadic eye movements serve this purpose.
Most eye fixations are between 200 and 250 ms during silent
reading, and the mean saccade amplitude is 7–9 character spaces.
Saccade lengths are measured in character spaces rather than
absolute distances because saccade lengths are the same (in terms
of the number of characters) regardless of whether the text is
presented close or distant from the reader, or at different visual
angles (Morrison, 1983; Morrison & Rayner, 1981; O’Regan,
1983; O’Regan, Levy-Schoen, & Jacobs, 1983).
It is virtually impossible to read if the text is visible only in the
parafovea or periphery due to acuity limitations (Rayner &
Bertera, 1979; Rayner, Inhoff, Morrison, Slowiaczek, & Bertera,
1981); nonetheless, not all words are fixated. Readers fixate con-
tent words (words that have a definable lexical meaning) on about
85% of occasions and function words (words that serve to express
grammatical relations with other words in a sentence) on about
35% of occasions (Carpenter & Just, 1983; Rayner & Duffy,
1986). The probability of a word being fixated is further dependent
on word length; the longer the word, the more likely it is to be
fixated (Rayner & McConkie, 1976). Words consisting of two to
three characters are fixated about 25% of the time; words consist-
ing of eight characters or more are nearly always fixated and may
often require more than one fixation.
Most saccades are made from left to right when reading English.
However, approximately 10%–15% of saccades are made from
right to left. Two types of right to left movements occur during
English reading: return sweeps and regressions. Return sweeps are
saccadic eye movements that are made after a reader has fixated
the right end of a line of text. These saccades are made in order to
read the next line of text. In contrast, regressive eye movements
occur at any point during reading and are usually made in order to
refixate material that has already been fixated at least once. Small
regressions of a few character spaces in length are likely to be
within-word regressions and usually occur when the reader makes
a saccade that positions the fixation non-optimally on a word. In
such situations, a regression is often made in order to compensate.
Other regressions, usually between words, often occur with dis-
ruptions in lexical, syntactic, or semantic processing. It follows
then that text difficulty and regressions are related. When a text
becomes conceptually difficult, the frequency of regressions in-
creases as does the duration of fixation, whereas saccade length
consequently decreases (Jacobson & Dodwell, 1979; Rayner &
Pollatsek, 1989).
Readers are unable to acquire new visual information during a
saccade since the velocity of the eye movement results in smearing
of the visual information (Uttal & Smith, 1968). Blur is not
perceived during a saccade because the visual information that is
sampled at the beginning and end of a saccade masks the visual
input that occurs during a saccade (Brooks, Impelman, & Lum,
1981; Campbell & Wurtz, 1978; Chekaluk & Llewellyn, 1990).
Sensitivity to visual stimuli is reduced during a saccade since
readers are unable to acquire new information. This phenomenon
is referred to as saccadic suppression (Matin, 1974).
A saccade is a motor movement; thus, a certain amount of time
is needed to plan it. Whenever a saccade is generated, two impor-
tant metrics can be computed: where the saccade is targeted and
when the saccade will be executed. Some researchers have argued
that “when” to move the eyes (reflected in fixation durations) and
“where” to move the eyes (reflected in fixation locations) are
determined via independent processes (see Aslin & Shea, 1987;
Becker & Ju¨rgens, 1979; Findlay & Walker, 1999; Rayner and
McConkie, 1976; Rayner & Pollatsek, 1981, 1987, 1989). A large
number of visual and linguistic factors influence both where and
when readers make fixations during reading, though for brevity’s
sake, these will not be discussed in detail here (see Rayner, 1998,
for a thorough review). Having provided a brief synopsis of some
basic characteristics of eye movements based on studies employ-
ing monocular recordings, we turn our attention to the main focus
of this review, namely, binocular coordination of the eyes.
Adult Binocular Coordination in Non-Reading Tasks
As one would expect when discussing binocular vision, both the
direction and the distance of a visual object are important factors
in oculomotor control. Information regarding the distance and
direction of an object relative to the observer is utilized such that
each eye can accurately fixate the object. Accurate fixation is
743
BINOCULAR COORDINATION
achieved by utilizing disjunctive saccades (saccades in which the
point of fixation changes in both depth and direction), conjugate
saccades (saccades that involve movement exclusively in the same
direction), and pure vergence movements (a depth only move-
ment). Note that some researchers refer to conjugate saccades as
pure version movements, those that occur when the eyes are
seemingly yoked and move in the same direction, maintaining a
constant angle of sight between them. Vergence eye movements
are the opposite, in that the eyes move in different directions so
that the angle of sight between them changes. Disjunctive eye
movements are combined movements in which the eyes move in
the same direction but by different amounts.
Binocular vision allows us to perceive and move around our
environment with more accuracy than monocular vision would
permit (Jones & Lee, 1981). Although humans have binocular
vision, they perceive the world as a single, unified, cyclopean
percept. Thus, the visual system must coordinate the input of the
two eyes very precisely and systematically. The issue of binocular
coordination is central to this process.
As mentioned earlier, many researchers assumed until recently
that the eyes fixated on the same letter within a word during
reading. Similarly, it was often assumed that human saccadic eye
movements were entirely conjugate during scanning of non-
linguistic material. Specifically, when making eye movements to
change either the horizontal or vertical fixation location but not the
depth location, it is generally assumed that a binocular saccade is
perfectly conjugate. However, it has become apparent with ad-
vances in binocular experimentation that this is not necessarily the
case and that saccade metrics for each eye can have different
characteristics.
Collewijn, Erkelens, and Steinman (1988) developed a tech-
nique to investigate how target direction and depth within the
oculomotor range influence saccadic parameters such as peak
velocity, amplitude, and duration of saccades. They used a revolv-
ing magnetic field–sensor coil technique (Collewijn, Martins, &
Steinman, 1981) to record absolute horizontal and vertical eye
positions of both eyes in space. Participants were required to make
saccadic eye movements between two LEDs. The saccades were
temporally dictated by the pace of a metronome. Pairs of LED
targets appeared in two conditions. In one, the LEDs were posi-
tioned symmetrically around an iso-vergence circle (in which eye
movements between any two points required only a horizontal
change in position without a vergence component). In the second
condition, the central LED was illuminated in combination with an
LED in the periphery, potentially requiring movements in both
horizontal and depth planes. The target direction and the plane on
which the target was placed were determined in a pseudorandom
sequence.
Collewijn et al. (1988) found that saccades were unequal in the
two eyes when the eyes made horizontal saccades between sta-
tionary targets located on the iso-vergence circle. Saccades of the
abducting eye (the eye moving temporally) relative to the adduct-
ing eye (the eye moving nasally) were significantly larger in size,
had higher maximum velocities, and had shorter durations. More-
over, saccades of the abducting eyes were more skewed than those
of the adducting eye. (Skewness refers to the time between saccade
onset and peak velocity [acceleration period] as a fraction of the
total saccade duration.) These differences resulted in the eyes
becoming transiently diverged during a saccade.
Bains, Crawford, Cadera, and Vilis (1992) investigated when
the oculomotor system becomes non-conjugate during a saccade.
In particular, they were interested in whether a single saccade
generator guides oculomotor control of the two eyes or whether
separate saccade generators exist for each eye. Binocular measure-
ments of 5 adult participants were taken by using a three-
dimensional implementation search coil technique (Tweed,
Cadera, & Vilis, 1990). Participants were seated at a distance of 2
meters from a target board. This was a distance that would neces-
sitate no significant changes in the angle between the two eyes
when fixating different positions on the board. The vergence angle
between the eyes at the central target was 1.718°, whereas the
angle at the peripheral target was 1.703°. Target positions were
indicated by a 0.23° red dot on a yellow background and were
continuously visible. Participants were seated in a position such
that the central target was displayed straight ahead and at eye level.
Twelve targets (hour labels) were placed 30° eccentrically around
the center target, spaced as on a clock. Consecutive saccades were
made from the center target to each of the eccentric targets.
Approximately 15 saccades were made to each target as binocular
recordings were taken.
The results indicated that successive saccades of the same
direction and amplitude showed variations in velocity, duration,
and curvature. Peak saccadic velocity varied systematically with
saccade direction. The mean peak velocities of the two eyes were
extremely similar, although small differences between the two
eyes were noted. The abducting eye had a higher peak velocity
during horizontal saccades and also started to saccade earlier than
did the adducting eye. These findings are consistent with Collewijn
et al.’s (1988) findings reported above, demonstrating transient
divergence of the two eyes during the saccade. Bains et al. (1992)
suggested that these differences might be attributed to differences
in synaptic delays and/or mechanical dynamics of the muscles
controlling the two eyes. They, therefore, inferred that transient
divergence did not necessarily result from lack of yoking between
the two eyes.
Disjunctive saccades (combinations of vergence and version,
driven by a stimulus which requires movements in both the hori-
zontal/vertical planes and in depth) differ in their characteristics to
conjugate saccades (pure version) in that they have a lower veloc-
ity and longer duration. Some researchers have argued that the two
are generated in different ways. Zee, Fitzgibbon, and Optican
(1992) recorded binocular eye movements in 4 adult participants
who were asked to saccade between targets. The task involved
various combinations of version and vergence movements. Their
results also supported findings from Collewijn et al. (1988), show-
ing transient changes in horizontal alignment during both horizon-
tal and vertical version movements in the absence of a required
depth movement. The eyes generally became divergent during the
initial stage of a saccade and subsequently became convergent.
Alignment changes tended to be divergent during upwards-vertical
version movements and convergent during downwards-vertical
version movements. More generally, their results showed that the
velocity of vergence movements was greater when they were part
of a combined vergence–version movement than when they were
pure vergence movements.
Collewijn, Erkelens, and Steinman (1995) examined the inter-
action between vergence and version. They studied the dynamics
of voluntary, horizontal, binocular gaze shifts between pairs of
744
KIRKBY, WEBSTER, BLYTHE, AND LIVERSEDGE
continuously visible three-dimensional targets. Target angle dif-
fered in depth only (pure vergence), direction only (pure version;
conjugate saccades), or in both depth and direction (combined
vergence/version movements; disjunctive saccades). Their exper-
iments focused on gaze shifts between targets that were located in
what they referred to as the manual working space, that is, the area
in which the majority of human visuo-motor activity occurs. Nat-
ural gaze shifts that occur in this area require alterations in version
and vergence. Their manipulations required interactions of ver-
gence and version during disjunctive gaze shifts.
Version and vergence were well integrated for the purpose of
achieving three-dimensional binocular gaze shifts with speed and
efficiency in a large part of manual working space. This is an
important aspect of binocular vision as it enables stereopsis (three-
dimensional vision) and precise coordination of prehension (Epel-
boim et al., 1995; Loftus, Servos, Goodale, Mendarozqueta, &
Mon-Williams, 2004). Collewijn and colleagues found that binoc-
ular gaze shifts between locations within manual working space
were mostly disjunctive. Furthermore, stimuli that were designed
to elicit pure vergence still induced disjunctive movements be-
cause participants made small version movements even though
these were not required. Gaze behavior at further distances neces-
sitated little vergence, and gaze shifts were usually conjugate.
These results were again in agreement with those of Collewijn et
al. (1988) and Bains et al. (1992) mentioned previously, all of
which indicate a degree of disconjugacy with respect to binocular
control during saccades in non-reading tasks.
Erkelens and Sloot (1995) found results similar to those de-
scribed above. The objective of their study was to quantify the
spatial variability in trajectories of binocular saccades. Self-paced
saccades were made between a number of stationary visual targets
located in the frontal plane. In excess of 75 saccades were made to
each target. Binocular measurements of horizontal and vertical eye
movements were recorded by using the scleral coil technique. The
effective direction was defined as that from the starting position to
the end position of each primary saccade. The initial direction of
the saccade was defined as the direction from the starting position
to the eye position when the saccade had covered a distance of
2.5°. Erkelens and Sloot found that variability was between two to
seven times larger in initial directions than in effective directions.
The curvedness of the saccades appeared to result from a purpose-
ful control strategy, whereby, initially, the eyes accelerated in
roughly the direction of the target after which they were guided
specifically to the target. However, these irregular patterns of
saccadic direction were highly correlated between the eyes, indi-
cating that the variability for the two eyes was very similar.
Erkelens and Sloot suggested that saccades, for both eyes, were
generated from a common source or spatial map.
Collewijn et al. (1995) provided important information about
eye movements during non-reading tasks; however, their data were
confined exclusively to descriptions of gaze changes as a function
of time. In a later study, Collewijn, Erkelens, and Steinman (1997)
focused on the spatial trajectories of the binocular fixation point
(i.e., the intersection point of the two lines of sight) associated with
various conjugate and non-conjugate gaze shifts within a horizon-
tal plane of regard. The trajectories of conjugate and convergent
gaze shifts were highly curved, whereas divergent gaze shifts
produced relatively straight trajectories (see also Doyle & Walker,
2001, for evidence of curved saccades in both eyes). Collewijn et
al. (1997) also noted that Collewijn et al. (1995) found unique
dynamic characteristics that were associated with version, ver-
gence, and disjunctive saccades. Their results (Collewijn et al.,
1997) suggested that control of the vergence and version compo-
nents of the gaze shift could be, to a degree, dissociated for targets
that differed in both depth and direction. These results are incon-
sistent with models of binocular oculomotor control claiming that
each eye responds to its own target. Rather, target depth and target
direction can be processed and responded to separately by ocular
vergence and version, and this does not have to occur at exactly the
same time. If it were such that only one system, vergence or
version, was active at a given moment, then respective, discern-
ible, velocity characteristics would be displayed. At all other times,
when both systems operate simultaneously (i.e., when the head is
free and targets are within a range that relies heavily on vergence),
a strong interaction would occur with an acceleration in vergence,
and the movement would become disconjugate.
Another relevant study was conducted by Kloke and Jaschinski
(2006). They described the extent of individual variability in
binocular transient asymmetry. Their stimuli consisted of red
points of light, 2 mm in diameter, presented by laser diodes.
Participants were asked to make eye movements of to the left or
the right of a centrally presented fixation point. Eye movements
were recorded by an infrared reflection technique (Reulen et al.,
1988). Transient divergence resulting from binocular latencies
and/or velocity asymmetry resulted in a maximum divergence of
1°. The effect of saccadic direction was negligible in relation to
latency and divergence differences, accounting for less than 0.3%
of variance. Kloke and Jaschinski demonstrated that the degree of
divergence occurring during a saccade was accounted for by the
asymmetry of saccadic velocity rather than by latency differences
or fixation disparities prior to saccade onset. Individual divergence
variability correlated strongly with asymmetric binocular velocity.
Although they used non-linguistic stimuli in their study, Kloke and
Jaschinski suggested that individual differences in transient asym-
metries of the two eyes may be related to normal and abnormal
reading abilities. We will discuss this possibility later in the
review.
On the basis of the seven non-reading binocular studies that we
have reviewed, it is apparent that saccades are not temporally and
spatially conjugate as has often been assumed. Note that these
studies have used two different eye tracking mechanisms (scleral
coils and infrared reflections) to measure binocular saccades
across a range of similar tasks, and they have all found transient
divergence between the two eyes during saccades. Saccades be-
tween targets differing in direction, but not in depth, are actually
non-conjugate. Subtle differences in the timing of binocular sac-
cades may modulate the coordination of the eyes, with the abduct-
ing eye (the eye moving temporally) initiating a saccade slightly in
advance of the adducting eye (the eye moving nasally). Differ-
ences in the peak velocity, duration, and skewness of abducting
and adducting saccades have also been demonstrated. Further-
more, inequalities in spatial dimensions have been observed, with
saccades of the abducting eye having larger amplitudes than those
of the adducting eye. However, note that the divergence that
occurs within the saccade may not necessarily result from a lack of
yoking between the eyes. Instead, such divergences may be attrib-
uted to synaptic transmission differences or differing muscular
control between the eyes.
745
BINOCULAR COORDINATION
To summarize, binocular coordination studies have demon-
strated two dissociated systems responsible for version and ver-
gence components of eye movements, which operate within dis-
tinct temporal frameworks. These systems are highly interactive.
The vergence component of the oculomotor system becomes tem-
porally similar to the version component during combined eye
movements. One final point is that people have no perceptual
experience of the transient divergence that occurs during saccades.
There are at least two reasons for this. First, saccades are very fast
and are ordinarily short in duration; hence, there is little opportu-
nity for detection. Second, as described in the previous section,
saccadic suppression occurs during a saccade. Consequently, tran-
sient disparity of retinal inputs would not be detected.
The studies described above have focused on saccade metrics;
few studies, however, have investigated the binocular disparity
that occurs during a fixation. Perhaps this is not too surprising
since it is primarily during fixations that visual information is
extracted and processed. Therefore, it is perhaps reasonable to
assume that it would be more likely to observe alignment of the
points of fixation during fixations than during saccades. This need
not be the case, however. In the following section, we review
studies that have looked specifically at disparity during fixations in
reading. Again, these studies have used a variety of methods to
record eye movements but have used stimuli that were always
linguistic in nature (words, sentences, texts). As will be seen, the
results are highly consistent with those that we have already
reviewed despite differences in theoretical and methodological
approaches.
Adult Binocular Coordination During Reading
Few studies have specifically examined binocular coordination
during reading in adults. Early results suggested good binocular
coordination and exact synchrony (e.g., Tinker, 1958; Yarbus,
1967). Smith, Schremser, and Putz (1971) used real-time computer
methods to investigate the extent to which the eyes were coordi-
nated in directional motion. They measured the difference in
timing of saccade initiation between the two eyes during reading of
texts that differed in difficulty and orientation (the text was or was
not rotated clockwise or anticlockwise from the horizontal). Smith
et al.’s main finding was a time difference between saccade onsets
for each eye; these seemed to cluster around three values. In each
case, the left eye led the right eye (by either 1 ms, 7–9 ms, or by
14 ms). Saccade asymmetry was also affected when the reading
display was rotated by 15° from the horizontal direction, but it was
not influenced by the difficulty of the text. The Smith et al. results
conflict with the view that the eyes are exactly conjugate in
saccadic motion. Instead, they indicate that very small differences
in the timing of binocular saccadic initiation administrate the
coordination of the eyes. Smith et al. suggested that the neurons of
the cortex and midbrain (which govern eye motion) may be time
and direction specific when determining the guidance and coordi-
nation of the eyes. Note, however, that Smith et al. reported precise
data for the timing of differences in peak saccadic velocities. From
the peak saccadic velocity data they inferred differences in saccade
onset latencies for the two eyes. Also, the saccade onset asymme-
tries reported by Smith et al. were of considerable magnitude and
the direction of the asymmetry was opposite to that reported in
subsequent research (e.g., see the discussion of Collewijn et al.,
1988, in Adult Binocular Coordination in Non-Reading Tasks).
Another aspect of Smith et al.’s data that deserves mention is their
failure to find a text difficulty effect on the magnitude of binocular
coordination asymmetries, a finding that is consistent with Juhasz,
Liversedge, White, and Rayner (2006, see below).
Williams and Fender (1977) questioned Smith et al.’s (1971)
findings, noting that this study was one of the few to find a
binocular saccade onset time difference. Williams and Fender
suggested that the onset latency discrepancy in the Smith et al.
study may have resulted from the way that they had inferred
saccade onset times. Williams and Fender conducted an experi-
ment to determine whether the discrepancy reflected differences in
each eye’s acceleration slightly after saccadic onset. This was not
a reading experiment; however, it is relevant to our discussion of
binocular saccade onsets. Thus, we briefly describe it here.
Williams and Fender (1977) used stimuli that were 2-ft diameter
bright spots on a dark surround presented on Maxwellian viewing
apparatus. The spot randomly jumped between a central and left
position, a central and right position, or a left and right position.
All jumps were or of visual angle. There were three viewing
conditions: monocular with the left eye stimulated, monocular with
the right eye stimulated, or binocular viewing. Williams and
Fender found that the degree of synchronization between the two
eyes was independent of viewing condition (monocular vs. binoc-
ular) and saccade direction (left vs. right). Differences were found
for the abducting and adducting saccade velocity. Most important
for our purposes, no differences in saccade onset were found in any
condition.
Recent studies have investigated the degree to which disparity
exists during fixations in reading. Hendriks (1996) examined post-
saccadic vergence velocity during fixations in adults. She moni-
tored the binocular eye movements of 12 participants while they
read words in context (prose passages) or in a list of unrelated
words. Participants were asked to read normally while attending to
the meaning of the word or to read the words in order to sound
them out subvocally. She found that passage reading produced a
higher vergence velocity during fixations than did reading unre-
lated words. Also, vergence velocities were higher when reading
for meaning than when making subvocal pronunciations. Hendriks
attributed this latter finding to the fact that readers must rely solely
on visual information when words appear in unrelated lists because
a helpful context is unavailable. She argued that, under such
circumstances, saccades are smaller in amplitude and fixations
more stable than they are when words are read in passages.
Vergence velocity during fixations was faster after long preceding
saccades than after short preceding ones (Collewijn et al., 1988;
Zee et al., 1992). Therefore, readers tended to make large saccades
when reading for meaning, and this, in turn, led to faster vergence
velocities during fixations. Note that this finding is consistent with
studies using LEDs rather than text as stimuli.
Heller and Radach (1999) reported three experiments to exam-
ine different aspects of binocular coordination in reading. They
investigated how binocular fixation disparity was influenced by
fixation position on a page of text, specifically whether or not the
residual disparity remaining at the end of a fixation accumulates as
the reader progresses through multiple lines of text. Heller and
Radach also examined differences in fixation disparity magnitude
between binocular and monocular reading conditions and the in-
fluence of text difficulty on binocular parameters. Their results
746
KIRKBY, WEBSTER, BLYTHE, AND LIVERSEDGE
regarding fixation disparity were consistent with findings from
Collewijn et al. (1988). The saccade amplitude asymmetries that
were observed in the simple scanning paradigm were also present
in reading. In contrast to Collewijn et al.’s (1988) scanning data,
however, vergence movements in reading were notably slower and
a residual disparity at the end of each reading fixation was com-
mon (see also Hendriks, 1996). In order to determine whether this
residual disparity would lead to an accumulation of fixation error
over several lines of text, 7 participants were asked to read 20 short
passages, each consisting of six lines. Participants were asked to
read fluently without resting at the beginning of each new line. The
results showed different disparity magnitudes for the first line of
each passage than for the remaining lines. Fixation error accumu-
lated over the first line, with an average value of approximately
two character spaces. This trend, however, then slowed and re-
versed over the remaining lines, leading to a reduced mean fixation
error of approximately 1.5 character spaces. Heller and Radach
suggested that the visual system does not tolerate the accumulation
of disparity beyond a certain point. Note, however, that these data
are descriptive and should be treated with some caution.
Heller and Radach (1999) designed a second experiment to
examine differences between monocular and binocular reading.
Eight participants were asked to read 200 lines of text under
normal binocular conditions and then read 200 lines with one eye
occluded and 200 lines with the other eye occluded. When partic-
ipants read monocularly, fixation durations, the proportion of
regressive saccades, and the number of fixations per line increased,
suggesting that reading monocularly was more difficult than read-
ing binocularly (see also Jones & Lee, 1981). A substantial and
unexpected increase in the amplitude of progressive saccades
under monocular viewing was also found. Heller and Radach
speculated that the field of view may be somewhat reduced under
monocular conditions and that the saccadic system might be un-
able to adapt easily to this change. Importantly, however, as with
Collewijn et al. (1988), no differences were found between sac-
cade amplitudes for monocular left and right eye saccades. Despite
the occlusion of one eye, the movement amplitude of the non-
occluded eye during the fixation remained comparable with that
observed under binocular viewing conditions. Further, no differ-
ences between the monocular and binocular viewing conditions
were observed in the slow vergence movements that occurred
during fixations (see also Inhoff, Solomon, Seymour, & Radach,
2008). This suggests that the slow vergence movements during
fixations are reflex-like and preprogrammed, rather than a specific
response to the current visual stimuli.
The final question that Heller and Radach (1999) asked was
whether task demands modulate binocular saccade metrics during
reading. They predicted that differences in saccade amplitudes
between the two eyes would be reduced under difficult reading
conditions, consistent with the view that reduced fixation disparity
might occur when reading is difficult. To test this hypothesis, they
compared binocular coordination during reading when text was
presented in normal case with that when text was presented in
MiXeD cAsE (since mixed case text is known to cause increased
processing difficulty during reading). Basic oculomotor measures
(e.g., fixation durations, number of fixations, etc.) showed that
reading mixed case text was more difficult than reading text
presented normally. Critically, they also reported that differences
in binocular saccade amplitudes were reduced for mixed case text
relative to normal text. Subsequently, vergence velocity during
fixations was slower for mixed case text than for normal text.
These effects held for the entire range of saccades. Heller and
Radach concluded that the visual system is able to tolerate larger
binocular fixation errors in normal reading compared with those in
difficult reading. Heller and Radach interpreted these results, along
with the finding that vergence movements remain similar under
monocular and binocular viewing conditions, as indicating that
fixational vergence is reflexive and does not require binocular
visual input in order to manifest. It appears that task demands
primarily influence saccade coordination, the vergence movements
being largely a consequence of the binocular saccade metrics.
Kliegl, Nuthmann, and Engbert (2006) further demonstrated a
systematic disparity between the points of fixation of the two eyes
during reading. They found that the eyes fixated different letters
within a word on 41% of fixations. The data set in their study was
large, in that 222 participants read 144 sentences. Although bin-
ocular coordination during reading was not the primary focus of
their study, they did investigate whether binocular disparity af-
fected the duration of first-pass, single fixations and saccade
amplitudes. Their data showed no influence of disparity on fixation
durations or on the amplitude of the incoming or outgoing sac-
cades.
Kliegl et al.’s (2006) finding that left and right eye saccade
amplitudes did not differ may initially appear somewhat puzzling,
given that the majority of studies investigating binocular saccade
metrics have shown differences in saccade amplitudes for the
adducting and abducting eyes. We suspect that this null effect may
be due to correlations between saccade amplitude and disparity
that arose from how the measures were computed. Disparity was
assessed as mean disparity during a fixation rather than by com-
puting saccade amplitudes on the basis of disparity that exists at
the beginning and end of a saccade. Perhaps a more important
point to note from Kliegl et al.’s data concerns the null effect of
disparity on fixation durations. This strongly suggests that the ease
with which linguistic processing could proceed was uninfluenced
by fixation disparity. We will return to this point later in the
section.
Another noteworthy aspect of Kliegl et al.’s (2006) data con-
cerns the prevalence of fixations on which the lines of sight were
crossed (with the left point of fixation to the right of the right point
of fixation by more than one character). Kliegl et al. found that the
eyes were more likely to be crossed than uncrossed on disparate
fixations. This result is notable in that several other studies have
found the opposite pattern such that the prevalent disparity was
uncrossed rather than crossed (see discussion immediately below).
As yet, it is unclear why different experiments have yielded
different patterns of disparity; clearly, further research is required
in order to elucidate this issue.
Finally, Kliegl et al. (2006) reported the frequency with which
both eyes fixated the same word during reading. To our knowl-
edge, no other study has reported data pertaining to this issue.
Kliegl et al. found that both eyes fixated the same word on 77% of
first-pass single fixations; thus, by implication, each eye fixated a
different word on 23% of fixations. Given the paucity of data in
respect to this issue, we decided to re-examine some of our own
data collected in experiments reported previously. We consider
adult eye movement data from an experiment reported by Blythe et
al. (2006) in which participants were asked to read single sentences
747
BINOCULAR COORDINATION
containing a target word that was short (4 characters), medium (6
characters), or long (10 characters). These words were matched for
frequency. Blythe et al.’s (2006) data provide an opportunity to
examine how word length affects the frequency with which both
eyes fixate the same word.
The data from Blythe et al. (2006) are comparable with data
reported by Kliegl et al. (2006), with participants fixating the same
word with each eye on 85% of fixations. Also, the pattern changed
in an entirely systematic and expected way when words of differ-
ent lengths were considered. The frequency with which the same
word was fixated was lower for short words (79%) than for
medium words (81%) and long words (95%). This result presum-
ably indicates that disparity occurs during reading and that it is
probabilistically more likely that each eye will fixate a different
word for short words than for long words.
Another interesting aspect of the Blythe et al. (2006) data
concerns which eye fixated the left and right word of an adjacent
pair in the sentence. The proportion of fixations on which the right
eye fixated a word to the left of the one fixated by the left eye was
small compared with the proportion of fixations on which the right
eye fixated a word to the right of the one fixated by the left eye.
This result is consistent with the finding that readers make fewer
crossed fixations than uncrossed fixations (cf. Kliegl et al., 2006).
Furthermore, the increased likelihood of both eyes fixating the
same word when the word was long than when it was short was
due entirely to a reduction in the proportion of fixations in which
the eyes showed an uncrossed disparity and in which the right eye
fixated a word to the right of the one fixated by the left eye. This
effect probably occurs because readers are more likely to fixate the
beginning than the end of a long word.
Another study that examined binocular coordination during
reading was conducted by Liversedge, White, Findlay, and Rayner
(2006). They investigated the magnitude and nature of fixation
disparity, attempting to replicate and extend the findings of Heller
and Radach (1999) by determining how frequently fixation dis-
parity occurred and its direction. Additionally, they compared
precise fixation positions at fixation onset with those at fixation
offset to examine the vergence movements made throughout fix-
ations. They analyzed the movements of the eyes during fixations
per se and the movements in relation to the nature and magnitude
of fixation disparity at fixation onset. They found that the points of
fixation were disparate by one character space or more at the end
of a fixation on 47% of fixations made across the entire sentence.
These results were in agreement with those reported by Heller and
Radach (1999). On the basis of these findings, Liversedge et al.
disputed the widely held assumption that both eyes always fixate
the same character during reading. Liversedge et al. also reported
that the proportions of aligned (53%), crossed (8%), and uncrossed
(39%) fixations were approximately constant for all 15 partici-
pants. Moreover, they showed that the magnitude of fixation
disparity was not modulated by eye dominance, nor were the
proportions of aligned, crossed, and uncrossed fixations.
In answer to the question of whether disparity alignment and
magnitude change during a fixation, Liversedge, White, et al.
(2006) compared the proportions of fixations that were aligned at
the beginning with those at the end of a fixation. At the end of a
fixation (53%), the proportion of aligned fixations was greater than
at the beginning of a fixation (48%). Fixation disparities had
greater magnitudes at the beginning of a fixation than at the end of
one. These findings support those of Hendriks (1996). Vergence
movements do occur during fixations in reading, and on average,
vergence movements aid in reducing disparity during a fixation.
Liversedge, White, et al. (2006) also clarified the nature of the
vergence movements that were recorded. This was achieved by
grouping qualitatively different movements into categories. Con-
vergence movements occurred with the most frequency (52% of
fixations), whereas divergence movements occurred only half as
often (25% of fixations). Drift movements, in which the two eyes
moved in the same direction by an equal amount (13% of fixa-
tions) and stable fixations (10%) occurred in roughly in the same
proportions. Note that drift movements are slow movements of
both eyes by an equal amount in the same direction. Importantly,
however, drift movements are not the same as version movements
since drift movements occur during a fixation, whereas version
movements occur during saccades. Note also that the two types of
movement have very different temporal characteristics. The ver-
gence data reported by Liversedge, White, et al. (2006) illustrate
that convergence movements usually predominate during fixa-
tions, but other types of eye movements and stable fixations also
occur. Further, the occurrence of movement during fixations was
partially dependent on the alignment of the fixation. Vergence was
more likely to occur during unaligned fixations than during aligned
fixations, and its direction tended to be corrective in relation to the
direction of the disparity.
It may be helpful at this point to make explicit the close relation
between studies that have focused on disparity during fixations and
studies that have focused on convergence or divergence of the eyes
during saccades. Clearly, the terminology and the manner in which
the analyses were conducted have been quite different in these two
bodies of work. It is important to realize, however, that data from
both sets of studies are complementary.
Recall that studies of adult saccadic binocular coordination have
demonstrated a transient divergence of the eyes during a saccade.
That is to say, the two eyes move apart from each other such that
the eye moving outward becomes further ahead than the eye
moving in a nasal direction. This divergence of the eyes is com-
parable with what reading researchers have defined as uncrossed
(although this terminology has always been applied to the state of
the eyes during fixations). Note, though, that this divergent state is
temporary in that prior to saccade completion, the eye moving
nasally starts to catch up with the outward moving eye. Recall,
however, that several studies in which saccadic divergence was
analyzed reported that residual divergence between the two eyes
persisted even at the end of a saccade. Collewijn et al. (1988)
reported that the residual divergence was, on average, 0.3° in
magnitude. The clear implication of this finding is that some
uncrossed disparity between the positions of the eyes should occur
at the beginning of fixations; indeed, this is what Liversedge,
White, et al. (2006) found. The mean disparity between the eyes at
the beginning of fixations was 1.3 character spaces, which equated
to 0.38°. Thus, findings showing saccadic divergence in non-
reading tasks and those showing prevalence for uncrossed binoc-
ular disparity during fixations in reading are consistent with each
other. Note, also, that this holds despite the use of different types
of eye tracking systems, different theoretical motivations, and
different approaches to the analyses.
We now return to our discussion of the experimental work
investigating reading, specifically focusing on linguistic and visual
748
KIRKBY, WEBSTER, BLYTHE, AND LIVERSEDGE
processing of text in relation to binocular coordination. As re-
ported earlier, Heller and Radach (1999) proposed that the visual
system tolerates less binocular disparity when presented with
MiXeD cAsE text than with normal text. Juhasz et al. (2006)
investigated whether properties of the text affect binocular coor-
dination during reading. Readers’ eye movements were tracked
binocularly as they read sentences containing high and low fre-
quency words or trials containing horizontal rows of equally
spaced Xs. Half of the sentences were presented in normal case
and half in mixed case. The inclusion of high and low frequency
words allowed Juhasz et al. to determine whether word-processing
difficulty influenced disparity since high frequency words are
easier to process than are low frequency ones (Inhoff & Rayner,
1986; Rayner & Duffy, 1986). Additionally, the horizontal rows of
Xs provided an opportunity to examine disparity during scanning
under conditions in which language processing did not occur.
Finally, mixed case text is more difficult to visually process than
is normal case text; thus, Juhasz et al. could determine whether
visual-processing difficulty reduced disparity.
The results were very similar to Liversedge, White, et al.’s
(2006) findings. Three types of fixation patterns were found:
aligned, uncrossed, and crossed, with 55% of fixations belonging
to the aligned category. Average binocular disparity magnitude
was not different for uncrossed fixations than crossed fixations,
and fixation durations were not affected by the nature of the
binocular fixation pattern. More importantly, however, binocular
disparity was not affected by the nature of the text, sentences, or
Xs, nor was it affected by the case or frequency manipulations.
There was, however, a standard frequency effect (Rayner, 1998) in
which fixations were longer on low frequency words than on high
frequency words. The main findings of Juhasz et al. were incon-
sistent with those obtained by Heller and Radach (1999).
The final study that we consider in this section is one by
Liversedge, Rayner, White, Findlay, and McSorley (2006). They
investigated how readers program saccadic eye movements and
were particularly interested in whether saccades for each eye are
programmed independently (e.g., Helmholz, 1910, as cited in
Howard, 1999), or if they are driven by a single neural signal (e.g.,
Hering, 1868, as cited in Howard, 1999). Experimental sentences
were constructed containing a target compound noun (e.g., cow-
boy) that was 6, 8, or 10 characters long (each morphological
constituent was the same length). Shutter goggles were used to
block visual input to each eye alternately every 8 ms. These
alternations were synchronized with changes in the display screen
such that all words in the sentence other than the target word were
presented in their entirety to both eyes, whereas a different portion
of the target word was presented alternately and separately to each
eye. Additionally, the movements of each eye were precisely
measured. Three target word presentation conditions were em-
ployed: congruent (cowb to the left eye and wboy to the right eye);
incongruent (wboy to the left eye and cowb to the right eye) and a
binocular control condition (cowboy to both eyes). Importantly,
participants perceived the whole target word regardless of the
particular presentation condition. Also, the two character overlap
served to anchor the word portions together in the vertical plane.
Liversedge, Rayner, et al. (2006) made three predictions. First,
if saccade metrics were computed for each eye independently on
the basis of each eye’s unique retinal stimulation, then different
saccade sizes and different landing positions on the word would be
expected for each eye (since each eye would target a different
portion of the word). Second, if the input from one of the two eyes
was suppressed, and saccade metrics were computed on the basis
of one or other visual input, then saccades would be of different
lengths (depending on whether the right or the left part of the word
was being suppressed). Finally, if saccade metrics were computed
on the basis of a representation that is unified from the two
different retinal signals, then the mean saccade lengths and fixation
positions on the target word should be uninfluenced by the differ-
ent dichoptic presentation conditions.
Fixation durations on the target word were significantly in-
creased under the dichoptic presentation conditions compared with
those of the control condition; nonetheless, landing positions on
the target word were uninfluenced by dichoptic presentation. Thus,
Liversedge, Rayner, et al. (2006) concluded that saccade metrics
for a non-foveal target word were computed on the basis of a
unified perceptual representation obtained from distinct retinal
signals (note that Collewijn et al., 1997, formed a similar conclu-
sion on the basis of independent evidence). Liversedge et al. also
proposed that this unified visual percept is achieved through a
process of fusion rather than suppression.
In summary, a number of important conclusions follow from
studies investigating binocular coordination during reading. First,
a crucial function of the oculomotor system is to coordinate
binocular eye movements in order to position the points of fixation
sufficiently close to one another that a single, unified visual
percept of the text may be achieved. Nonetheless, several studies
indicate that fixations are disparate by more than one character
space (but seldom by more than two characters) in just under half
of the fixations made during reading. Additionally, it appears that
binocular disparity is not affected by manipulations of processing
difficulty, such as case or frequency. Thus, based on both reading
and non-reading experiments, it appears that a significant amount
of disparity between the two retinal inputs can be, and is, tolerated
on a fixation by fixation basis. The visual system can tolerate more
disparity when text is read for meaning than when the reader is
asked to sound out each word subvocally (although disparity does
not accumulate beyond one and a half character spaces). Small
vergence movements occur during fixations, and these are often
corrective in that they typically reduce disparity, though residual
disparity does remain immediately prior to the following saccade.
Vergence movements are similar under monocular and binocular
viewing conditions; thus, it seems plausible that vergence move-
ments are preprogrammed and reflexive rather than determined by
specific visual input. Finally, researchers are beginning to piece
together an understanding of how the visual system accomplishes
a unified cyclopean percept based on differing retinal inputs using
dichoptic presentation techniques (e.g., Blythe, Joseph, Findlay, &
Liversedge, 2008). A unified representation appears to be achieved
through a process of fusion rather than suppression of one retinal
image. Fusion seems to occur at an early stage in visual process-
ing, and binocular saccade matrices are computed on the basis of
this unified percept.
The findings described above have implications for visual and
linguistic processing during written language comprehension.
First, we have focused on the disparity that occurs during fixations
in reading; however, a large proportion of fixations (just over half
in those studies that provide these data) are aligned (i.e., fixations
are less than a character apart). Very clearly, disparity does not
749
BINOCULAR COORDINATION
occur all of the time. What this also means is that the visual system
that processes disparate retinal inputs cannot operate on a fixed
basis but must be able to process variable degrees of disparity that
change on a fixation by fixation basis. Clearly, a greater degree of
flexibility is required than would be the case if disparity magnitude
was constant across fixations. Another general point is that the size
of fixation disparities during reading is not large. To be clear, we
are not suggesting that a reader fixates a word at the beginning of
a line of text with one eye and a word at the end of a line with the
other, in a chameleon-like manner. Instead, there is a small offset
between the points of fixation. Thus, a reasonable conclusion is
that the eyes are aligned on the majority of fixations, and when
they are not, they are unaligned by only a small amount. Contin-
gent on one’s perspective, this is a case of the glass half empty or
half full.
The findings described above also have methodological impli-
cations for researchers investigating aspects of visual and linguis-
tic processing during reading. Eye movement research in reading
has traditionally involved recording movements from only one
eye. Consequently, the single point of fixation has been taken as a
precise index of the point at which overt visual attention is allo-
cated within the text. Furthermore, the perceptual span, the area of
text from which the reader can extract useful information (Rayner,
1986), is assumed to be oriented about this point. In adults, the
perceptual span extends from the beginning of a fixated word, no
more than 4 character spaces to the left of fixation, to around 15
character spaces to the right of fixation (see Rayner, 1998, for a
summary). Thus, the single point of fixation of one eye (usually
the right one) has been instrumental in determining researchers’
assessments of the exact portion of the text from which informa-
tion may be extracted. However, from the studies reviewed above,
it is clear that the eyes are approximately one or two characters
apart on about half of fixations. Also, it appears that a unified
percept is achieved through fusion. Thus, the true, precise point at
which overt attention is allocated is potentially one or the other
point of fixation, or perhaps, some point between the two (pre-
sumably, the perceptual span will be anchored at this point and will
be a character or two larger than has been estimated). This might
sound slightly worrisome for investigators who have implicitly
assumed that the eyes are fixated on a single point. However, our
view is that researchers should not be concerned, as the vast
majority of experiments investigating reading do not require this
degree of precision regarding the location of attention. Further-
more, the studies described above consistently show no relation
between linguistic processing and fixation disparity. For those
researchers who may still be concerned, a relatively simple meth-
odological solution would be to patch one eye and record from the
other during reading. In summary, fixation disparity during read-
ing seems to be largely visually based, reflecting fundamental,
low-level aspects of oculomotor behavior.
Although we believe that there is no relation between linguistic
processing and fixation disparity, fixation disparity in reading may
have important implications for a small number of theoretical
issues. For example, a phenomenon that has received considerable
attention in the literature in recent years is the parafoveal-on-
foveal effect (see Kennedy, 1998). Parafoveal-on-foveal effects
occur when some lexical characteristic of a word (word n 1) in
the parafovea, that has not yet been directly fixated, affects pro-
cessing on the fixated word (word n). These effects are often quite
small and are usually more pronounced for a subset of fixations
that are very close to word n 1 (e.g., Rayner, Warren, Juhasz, &
Liversedge, 2004). These effects have come under considerable
scrutiny since the phenomenon is central to a fundamental distinc-
tion between two categories of eye movement control models:
serial models (e.g., Reichle, Rayner, & Pollatsek, 2003) and par-
allel ones (e.g., Engbert, Nuthmann, Richter, & Kliegl, 2005). A
central assumption of serial models is that words are identified
serially and sequentially, whereas parallel models stipulate that
words are identified in parallel. Thus, the existence of parafoveal-
on-foveal effects, whereby lexical characteristics of word n 1
affect processing of word n constitute strong evidence against
serial models and in favor of parallel ones. Advocates of serial
models have argued that parafoveal-on-foveal effects arise due to
misdirected fixations (i.e., those saccades that are targeted at, but
land just short of, word n 1). Hence, the processing that occurs
during these fixations is driven by the word to which the saccade
was targeted rather than by the word on which the fixation was
made. However, an alternative explanation of these effects in
terms of binocular disparity is possible. The fixations involving
parafoveal-on-foveal effects may be those on which disparity
occurs such that the eye that is not recorded actually fixates on
word n 1. This alternative account may also be relevant when
considering word skipping rates. Thus, further experimental inves-
tigation is warranted.
Binocular disparity also has implications for the split fovea
model of word identification (see Shillcock, Ellison, & Monaghan,
2000). According to this model, lexical identification is directly
influenced by the physiological makeup of the retina. The retina is
vertically split about the center of the fovea with the left hemifield
of the retina projecting to the right side of the brain, and the right
hemifield projecting to the left side. An implicit assumption of this
account is that both eyes fixate the same point within a word;
therefore, the word is precisely split about this single point of
fixation. However, the body of research described above indicates
that this is not the case. Differing degrees of binocular disparity
occur on a fixation by fixation basis during reading; thus, the
precise location of the “split” within a word may vary for each eye.
Thus, the assumption of a single point of fixation for both eyes on
a fixated word is not warranted for a substantial proportion of
fixations; thus, the split fovea model does not appear to offer a
realistic account of processing. We should note, however, that it
may be possible to modify the basic model such that it can handle
the fact that disparity does occur to differing degrees during
reading, but in its present form there is no mechanistic account of
how this occurs. In the next section, we review developmental
aspects of binocularity during reading.
Basic Developmental Characteristics of Eye Movements
During Reading
Several studies have demonstrated developmental trends in eye
movements during reading (Buswell, 1922; Rayner, 1986; McCo-
nkie et al., 1991). As age and reading skill increase, fixation
durations decrease, saccade lengths increase, the number of fixa-
tions decreases, and the frequency of regressions decreases
(Rayner, 1998). The developmental changes in oculomotor mea-
sures largely reflect improvements in reading ability.
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KIRKBY, WEBSTER, BLYTHE, AND LIVERSEDGE
The most recent of these studies (McConkie et al., 1991) pre-
sented new data alongside previously reported data by Buswell
(1922) and Taylor, Frackenpohl, and Pettee (1960). The develop-
mental trends in these studies were highly similar. Fixation dura-
tions decreased, with 200-ms fixations in adults, and 304-ms
fixations in 6- and 7-year-olds. Adults made proportionally fewer
fixations than did children (decreasing from 168 fixations per 100
words for 6- and 7-year-olds to 118 fixations per 100 words for
adults). The frequency of regressions decreased from approxi-
mately 34% of eye movements in 6- and 7-year-olds to 21% of eye
movements in adults. Differences between adults and children
were also observed in the frequency with which words were
refixated. Adults refixated 5 character words 15% of the time,
whereas children refixated them 57% of the time. Another oculo-
motor parameter in which developmental change was observed
was saccade targeting characteristics. Children between the ages of
6 and 7 made more short saccades than did children between the
ages 10 and 11. In young children, 90% of saccades were less than
two character spaces. By the age of 11, such short saccades were
made only 4% of the time and by adulthood hardly any short
saccades were produced at all.
Saccades in reading typically serve the purpose of bringing new
portions of the text—that is, words— onto the fovea. The landing
position on a word, regardless of word length, is usually near the
center (Rayner, 1979), indicating that words serve as units that are
targeted during the control of eye movements. After the 1st year of
reading tuition, children typically display the same landing posi-
tion characteristics as adults (McConkie et al., 1991). Although
initial within-word landing positions are highly similar in adults
and children, small refixations are more common in children.
These refixations seem to be driven by cognitive-processing fac-
tors, as the lack of a developmental change in landing positions
suggests that refixations are not a consequence of poor saccade
targeting accuracy.
In general, children display substantial variability in their eye
movement behavior; this systematically decreases with increased
reading ability. McConkie et al. (1991) have speculated that the
variability in eye movements results from differences in the strat-
egies that children adopt when learning to read. As they become
more skilled, they converge on a common set of strategies.
Another key difference between adults and children is in terms
of the characteristics of their perceptual span, the area of text from
which the reader can extract useful information (Rayner, 1986).
Rayner (1986) found that the perceptual span of beginning readers,
7–11 years old, was smaller than that of more mature readers,
indicating that young readers process smaller areas of text during
fixations. Further, adults and children extracted differing levels of
information from the area to the right of fixation, such that adults
were more disrupted by decreases in available text than were
children. Despite these differences, however, children’s perceptual
span, like adult span, is smaller to the left than to the right of
fixation beginning about age 6. This suggests that as little as 1 year
of reading experience is necessary to develop the asymmetric
pattern that is seen in adult readers and that the development of
perceptual span with age is largely quantitative.
In summary, several eye movement studies have led researchers
to conclude that eye movement behavior during reading gradually
develops with age. These changes are largely associated with
increases in the ease of text processing. As reading ability in-
creases, fixation duration and quantity decrease, saccade lengths
increase, and the number of regressive movements decrease. Chil-
dren also make considerably more short saccades than do adults;
this is linked to their increased within-word refixation rates. The
occurrence of short saccades, however, decreases to adult levels by
age 11. Research into landing site distributions has also demon-
strated that the ability to target the eyes to an optimum position
within a word is equivalent for children and adults following 1
year of reading instruction. Also, age-related changes in eye move-
ments indicate that perceptual span develops progressively, with
beginning readers having smaller spans than those of skilled read-
ers. Nonetheless, children display span asymmetries similar to
adults, with more information available to the right than to the left
of fixation. An important question is whether maturation occurs in
binocular eye movement behavior alongside improvement in read-
ing skill. We discuss this question below.
Children’s Binocular Coordination During Non-Reading
Tasks
To further our understanding of developmental trends in oculo-
motor control, we discuss studies investigating binocular eye
movements during non-reading tasks. Although few developmen-
tal studies have focused on binocular control during saccades,
Yang, Bucci, and Kapoula (2002) examined the latency of eye
movements in three-dimensional space (version, vergence, and
combined version–vergence). They found that the mean latencies
for version, vergence, and combined eye movements were all
longer for children than for adults as well as that the variability of
the latency values was larger in children than in adults. Both pure
and combined movement latencies gradually decreased with age,
reaching adult levels roughly between the ages of 10 and 12.
Yang et al. (2002) also found that children tended to trigger the
vergence component first when making combined version–
vergence movements, whereas adults did not show a dominant
pattern (the two systems were often initiated simultaneously).
However, some similarities did exist between children and adults;
for example, saccadic latencies were shorter when viewing was
close (20 cm) than when viewing was distant (150 cm). The
majority of children in the study also displayed the same pattern of
vergence latencies as adults; convergence latencies were longer
than divergence latencies.
These results support the prior suggestion that version and
vergence triggering mechanisms are distinct. Further, the results
indicate that these distinct mechanisms mature progressively with
age, perhaps in association with prefrontal changes (Yang et al.,
2002). Perhaps the most important finding in this study was that
the capacity for synchronization of the two components of com-
bined eye movements (parallel processing of sensory information
of direction and depth) develops at a slower rate than does matu-
ration of the two components (vergence and version) separately;
this synchrony was below adult levels even at age 12.
As noted earlier, it is well established that adults have discon-
jugate saccade metrics for the adducting and abducting eyes (Col-
lewijn et al., 1988). Fioravanti, Inchingolo, Pensiero, and Spanios
(1995) investigated developmental trends associated with discon-
jugacy of binocular saccades. Asymmetric movements of the ad-
ducting and abducting eyes were found for younger (5–9 years old)
and older (11–13 years old) children as well as for adults. How-
751
BINOCULAR COORDINATION
ever, the pattern of asymmetry was reversed in younger children
relative to older children and adults. Younger children compared
with older children displayed smaller amplitudes, smaller peak
velocities, longer durations, and larger acceleration times for sac-
cades of the adducting eye in relation to the abducting eye. As a
consequence, young children’s eyes tended to become converged
during saccades. This was in contrast to the reversed pattern that
occurred in adults (Collewijn et al., 1988) and that Fioravanti et al.
found in older children, that is, the abducting eye made a larger,
faster saccade than that of the adducting eye. Subsequently, older
children’s and adults’ eyes tended to become diverged during
saccades.
Fioravanti et al. (1995) also assessed children’s and adults’
saccadic accuracy. Adults consistently undershot their target and
made very small post-saccadic drift movements. Children in both
age groups initially underestimated amplitudes of large target
jumps and overestimated amplitudes of small target jumps. How-
ever, landing position error was consistently reduced during post-
saccadic drift, similar to but very often larger than the corrective
movements carried out by adults. This indicates that saccade
metric mechanisms were working as well in children as in adults.
The initial overshoot of the saccade, in response to small target
jumps, was gradually reduced with age.
In Fioravanti et al.’s (1995) study, saccades were made at a
viewing distance of 100 cm, yet a comfortable reading distance is
approximately 30 40 cm (Yang & Kapoula, 2003). With this in
mind, Yang and Kapoula (2003) examined the influence of view-
ing distance on the quality of binocular coordination of saccades
(to LEDs) in children and adults. They used two distances: 20 cm
and 150 cm. Children’s binocular coordination of saccades was
poor both during and after saccades, particularly in children ages
4 6. Poor binocular coordination of saccades in children was
distance dependent, such that saccade disconjugacy at the close
distance was accentuated, amounting to 10% of the size of the
saccade. This did not occur in adults. Yang and Kapoula attributed
the distance-dependent disconjugacy to immaturity of cortical or
subcortical saccade control rather than to some muscular differ-
ence in children and adults. The disconjugancy of saccades was
dramatically reduced with age and eventually reached adult levels
around the age of 10 to 12. The stability of the eyes after the
saccade and the quality of binocular alignment during fixation,
however, was poor in younger children regardless of the viewing
distance.
Notably, as was the case with adult investigations of binocular
vision involving non-reading tasks, the two studies described
above examined movements of the eyes rather than fixations.
Nonetheless, the results of the two are consistent. Thus, these
experiments serve to establish some of the systematic changes in
binocular coordination that occur with age.
Development of Binocular Coordination During Reading
Developmental studies of binocular coordination during reading
are relatively sparse. Bucci and Kapoula (2006) used binocular
recordings (by way of electro-oculography) to evaluate the quality
of binocular saccade coordination to single words and LEDs in
normally reading children at age 7 compared with that of adults.
(Note that the recordings were of low resolution due to the manner
in which eye movements were recorded.) The single word reading
task involved fixating a cross on the left side of the screen then
subvocally pronouncing a word containing either five, seven, or
nine characters, centrally presented, then fixating a cross on the
right side of the screen. The LED presentation involved a standard
paradigm; a target LED jumped horizontally from to 10° or 20°,
leftwards or rightwards. Participants’ performance was compared
across the two tasks for rightwards saccades only.
Bucci and Kapoula (2006) found that the latency and coordina-
tion of binocular saccades were not influenced by type of task,
consistent with findings from Juhasz et al. (2006). Binocular
control during isolated word reading was poor in children relative
to adults, confirming and extending the findings of Yang and
Kapoula (2003). They suggested that Hering’s law (1868; in
Howard, 1999), stating that the two eyes are well yoked because
they receive equal innervations, is not always applicable during
reading. Furthermore, they suggested that poor binocular control in
children could interfere with learning to read and could contribute
to the long fixation durations observed in beginning readers.
Blythe et al. (2006) noted that previous studies examining the
coordination of the two eyes have rarely employed natural reading
tasks. Thus, it is not clear that conclusions from these studies
generalize to normal reading performance. To address this gap in
the literature, Blythe et al. (2006) measured the binocular coordi-
nation of children and adults during sentence reading. The eye
movements of 12 adult and 12 child participants were recorded
binocularly. The groups showed binocular disparity during fixa-
tions, both at the start and at the end of a fixation. Children,
however, displayed greater disparity magnitudes than did adults as
well as a greater proportion of crossed fixations. Blythe et al.
(2006) argued that the larger proportion of crossed fixations in
children than in adults was due to a pattern of asymmetric saccadic
movements for the adducting and abducting eyes that was opposite
to the one observed in adults. These data demonstrate that binoc-
ular disparity during processing of complex visual stimuli is sim-
ilar to that observed in studies examining disparity during the
processing of simple visual stimuli. Blythe et al. (2006) argued that
the visual system is able to tolerate considerable disparity between
the two retinal inputs and still achieve a unified percept during
reading. This is particularly striking since the cognitive demands
associated with reading are greater for beginning than for skilled
readers; nonetheless, beginning readers tolerated a greater visual
disparity than did skilled readers. Thus, this finding is consistent
with others in which no strong relation has been found between
text-processing difficulty and the magnitude of the disparity ob-
served during reading.
In Blythe et al.’s (2006) study, children exhibited a greater
proportion of crossed alignments during fixations than did adults,
although still proportionally fewer crossed than uncrossed fixa-
tions. However, 42.7% of younger children’s (7–9 years) un-
aligned fixations were crossed, compared with 35.9% of older
children’s (10–11 years) fixations. This demonstrates a develop-
mental difference in the alignment of fixation disparity. These
results are consistent with evidence for developmental trends in
binocular coordination that are observed in non-reading tasks. As
mentioned above, these studies have demonstrated that the ampli-
tudes of the saccadic metric for the adducting and abducting eyes
are reversed for younger children compared with those of adults
(Fioravanti et al., 1995). This reversal in asymmetrical amplitude
may contribute to the greater proportion of crossed fixations found
752
KIRKBY, WEBSTER, BLYTHE, AND LIVERSEDGE
in younger children than those in older children. Combining the
results of Blythe et al. (2006) and Fioravanti et al. (1995), it
appears that developmental changes in binocular coordination can-
not be attributed solely to high-level cognitive processes that occur
during reading, as the level of complexity of the visual stimuli
differed in the two studies considerably. Rather, the differences
appear to be due to low-level visual/ocular development. Thus, it
appears that trends of maturation do exist in eye movement be-
havior alongside cognitive development in terms of improvement
in reading skill.
Again, it is worth noting that reading researchers have focused
on binocular coordination during fixations, rather than during
saccades. Despite this, however, differences between adults and
children appear to be highly consistent regardless of whether
coordination is assessed by means of fixations or saccades.
Younger children’s eyes tend to become transiently converged
during saccades and we observe crossed disparity during fixations,
whereas older children’s and adults’ eyes become transiently di-
verged during saccades and we observe uncrossed disparity during
fixations. Furthermore, developmental studies provide converging
evidence that binocular coordination systematically changes with
age and that both temporal and spatial coordination is poorer in
children than in adults. Limited data also suggest that viewing
distance modulates binocular coordination in children. Differences
in binocular coordination (either conjugate or disjunctive move-
ments) achieve adult levels at about 12 years of age. In real world
viewing situations, most eye movements involve combined move-
ments, responding to both depth and direction. The capacity to
perform these movements in synchrony matures more slowly than
does the capacity to perform the individual pure movements. These
developmental trends were observed in experiments using visual
stimuli of differing levels of complexity and in tasks that involved
the engagement of cognitive processes to differing degrees. Thus,
it appears that age-related differences in binocular coordination are
not driven by cognitive development.
All of the studies discussed thus far have reported data from
experiments in which binocular eye movement recordings were
taken. The findings provide a coherent and consistent account of
binocular coordination in adults and children across different tasks.
Studies that have examined binocular coordination in individuals
with dyslexia, however, have often used quite different methods;
in many cases, no eye movement data have been recorded. In the
next section, we review these studies to evaluate the importance of
binocular coordination in reading. First, however, we briefly sum-
marize the basic characteristics of oculomotor control in dyslexia.
Dyslexia and Oculomotor Control
Despite adequate instruction and intellectual capacity, some
children find learning to read to an age-appropriate standard ex-
tremely difficult. This deficit is often referred to as dyslexia and
affects around 5%–10% of the population (Ramus, 2001). The
literature on dyslexia is vast and often highly controversial; to date,
there is no single unified theoretical account of its cause. Current
theories are numerous, and a description of them all is beyond the
scope of this review. It is useful, however, to briefly discuss two
of the most prominent theories. The phonological theory (Liber-
man, 1973; Snowling, 2000; Stanovich, 1988) claims that dyslexia
is a direct consequence of cognitive deficits specific to the repre-
sentation and processing of speech sounds (phonology; Ramus,
2003). This deficit arises from a congenital dysfunction of the
cortical areas responsible for phonological processing (Galaburda,
Sherman, & Rosen, 1985; Paulesu et al., 2001; Temple et al.,
2001). Difficulties in learning to read are predominantly preceded
by difficulties in learning to speak.
The notion that phonological deficits are prevalent in develop-
mental dyslexia is generally accepted; however, some researchers
believe that poor phonological performance is secondary to more
general deficits in sensorimotor and learning behavior. Proposed
areas of sensorimotor deficits include auditory (Tallal, 1980),
visual (Lovegrove, Bowling, Badcock, & Blackwood, 1980), and
cerebellar/motor regions (Fawcett & Nicolson, 1995).
An alternative, and more controversial, theory of dyslexia is the
magnocellular theory (Stein, 2001). The magnocellular account of
dyslexia states that the observed phonological and visual deficits in
dyslexia are caused by a neurological impairment, in which mag-
nocells in sensory pathways do not function as they should. Ac-
cording to this account, magnocells play a central role in binocular
coordination in the visual modality, and abnormalities in the mag-
nocellular pathway cause poor oculomotor performance; this, in
turn, results in poor reading performance. Note, however, that the
magnocellular account is not specific to deficits in the visual
modality (see Stein, 2001, for further details). A number of studies
have demonstrated that dyslexic readers require larger differences
between stimuli to discriminate them than do normal readers
(Sperling, Lu, Manis, & Seidenberg, 2003; Demb, Boynton, &
Heeger, 1997). With respect to eye movement research, experi-
mental work has typically examined ocular dominance and binoc-
ular control in relation to reading skills. These studies are dis-
cussed in the next section (Stein & Fowler, 1993; Stein, Talcott, &
Walsh, 2000).
Basic Characteristics of Eye Movements in Dyslexic
Readers
Over the past three decades, a body of research has demon-
strated visual-processing abnormalities in dyslexia (Lovegrove et
al., 1980). It remains unclear, however, to what extent eye move-
ments, or a more general visual deficit, contribute to reading
problems (Biscaldi, Gezeck, & Stuhr, 1998; Fischer, Biscaldi, &
Otto, 1993; Fischer & Weber, 1990; Olson, Kliegl, & Davidson,
1983; Pavlidis, 1981; Rayner, 1998).
Some studies have shown that dyslexic readers display similar
patterns of eye movements to normal readers when matched for
reading skill (see Rayner, 1985a, 1985b, for a review). Other
studies, however, have shown different patterns of eye movements
in dyslexic readers when compared with those of control readers
either matched for reading ability or for age. The eye movement
patterns differ in several ways from those displayed by typical
readers: longer fixation durations, larger numbers of fixations,
shorter saccade lengths, and more regressive movements (Biscaldi
et al., 1998; Elterman, Daroff, & Bornstein, 1980; Griffin, Walton,
& Ives, 1974; Hutzler & Wimmer, 2004; Rayner, 1978b; Rubino
& Minden, 1973).
Lefton, Nagle, Johnson, and Fisher (1979) recorded horizontal
movements of both eyes as paragraphs of text were read. Although
binocular recordings were taken, Lefton et al. did not discuss how
both eyes were coordinated in any detail. Despite this, highly
753
BINOCULAR COORDINATION
significant differences in eye movement behavior were found
between 5th grade children who were good readers and poor
readers. The groups differed in the number and duration of fixa-
tions made after both progressive and regressive saccades. Lefton
et al. concluded that eye movements in reading-impaired individ-
uals were “nonsystematic” (p. 327). A critical question concerning
these (and other) findings is whether irregular patterns of eye
movements cause or are caused by individuals’ reading difficulties
(Hyo¨na¨ & Olson 1995; Olson, Conners, & Rack, 1991; Pavlidis,
1981; Rayner, 1985a, 1998)
Pavlidis (1981) has argued that dyslexic children have qualita-
tively and quantitatively different eye movement patterns than do
typically developing readers. He recorded monocular eye move-
ments as participants fixated five sequentially presented, illumi-
nated LEDs along a horizontal array. Dyslexic participants fixated
the LEDs less accurately than did control participants. Also, Pav-
lidis reported more regressive eye movements and shorter fixation
durations for dyslexic readers than for typical readers. Pavlidis
argued that differences in eye movement behavior during reading
tasks could be due to dysfunction in oculomotor control or cog-
nitive dysfunction associated with dyslexia. Pavlidis’s experiment
did not involve reading; therefore, he concluded that performance
difficulties in a simple, non-linguistic sequential fixation task
indicated a constitutional oculomotor disability inherent to dys-
lexia. Pavlidis argued that erratic eye movements might arise from
malfunctions in oculomotor control and/or disruptions to sequen-
tial processing of visual information over successive fixations.
Thus, Pavlidis favored an account of dyslexia in which poor
oculomotor control plays a central role. Only monocular eye
movement data were recorded, and it is therefore impossible to
know the extent to which these claims might also apply to binoc-
ular coordination during reading; however, we see no reason why
they should not generalize.
Pavlidis’s (1981) study received considerable scrutiny in sub-
sequent years, and the results have been questioned on numerous
occasions. Several attempts at replicating his findings have pro-
duced data that differed from the original results (Black, Collins,
DeRoach, & Zubrick, 1984a; Brown, Haegerstrom-Portnoy, Ad-
ams, et al., 1983; Olson, Conners, & Rack, 1991; Olson et al.,
1983; Stanley, 1994; Stanley, Smith, & Howell, 1983). Also,
several studies have failed to show significant differences between
normal and dyslexic readers’ oculomotor behavior during other
non-reading tasks (Adler-Grinberg & Stark, 1978; Black, Collins,
DeRoach, & Zubrick, 1984b, 1984c; Brown, Haegerstrom-
Portnoy, Adams, et al., 1983; Brown, Haegerstrom-Portnoy, Yin-
gling, et al., 1983; Eskenazi & Diamond, 1983; Fields, Wright, &
Newman, 1993; Olson, Conners, & Rack, 1991; Stanley, Smith, &
Howell, 1983).
Other researchers have argued that poor eye movement control
is a causal factor in dyslexia. Poor control manifests as differences
in the stability of fixation (Eden, Stein, Wood, & Wood, 1994;
Raymond, Ogden, Fagan, & Kaplan, 1988), increased numbers of
express saccades (Biscaldi & Fischer, 1993; Biscaldi, Fischer, &
Aiple, 1994; Fischer, Biscaldi, & Otto, 1993; Fischer & Weber,
1990), poor visual attention span (Bosse, Tainturier, & Valdois,
2007; Prado, Dubois, & Valdois, 2007; Valdios et al., 2003), and
limited perceptual span, whereby dyslexic readers are unable to
process as much information from the parafovea as are normal
readers (Farmer & Klien, 1995; MacKeben et al., 2004; Rayner,
Murphy, Henderson, & Pollatsek, 1989).
Rayner (1985a, 1998) has argued that much of the variability
between readers’ eye movements is associated with cognitive
processes involved in comprehension (see also Liversedge & Find-
lay, 2000). Thus, some researchers have employed tasks that
require precise saccadic orienting but do not involve linguistic
processing to investigate disruptions in visual processing indepen-
dent from linguistic processing (Hutzler, Kronbichler, Jacobs, &
Wimmer, 2006). Hutzler et al. (2006) designed a string-processing
task in which participants’ eye movements were monitored as they
searched through paragraphs of three and four or five and six
consonant character strings to identify those that contained pairs of
identical adjacent letters. In another experiment, they measured
eye movements as participants read pseudowords. The string-
processing task required saccadic eye movements and fixations
that are arguably similar to those made during reading but that did
not involve linguistic processing. In contrast, Hutzler et al. (2006)
argued that at least some linguistic processing was involved in
pseudoword reading. They found differences in oculomotor con-
trol between normal and dyslexic readers solely in the pseudoword
reading task, even though the demands on the visual and oculo-
motor systems were similar across the two tasks. Hutzler et al.
(2006) concluded that eye movement differences between dyslexic
and typical readers reflect difficulties associated with linguistic
processing in dyslexia because the groups did not differ in the
non-linguistic visual-processing task. Thus, it appears that poor
oculomotor control is not causally related to dyslexia. Note, how-
ever, that Hutzler et al. (2006) only recorded monocular eye
movements; thus, they were unable to unequivocally rule out a
causal relation between poor binocular coordination and dyslexia.
Individuals with reading difficulties are less likely to suffer from
obvious phonological deficits in relatively shallow orthographies,
such as German and Italian, in which letter–sound correspon-
dences are consistent, than in deeper orthographies, such as En-
glish, in which these correspondences are more variable (De Luca,
Di Pace, Judica, Spinelli, & Zoccolotti, 1999; Wimmer, 1993).
Slow reading speeds rather than phonological deficits are a char-
acteristic symptom of dyslexia in these shallow orthographies. The
slow reading speed may be due to overreliance on sequential,
sublexical processing, in which the reader processes the individual
letters of a word serially, rather than in parallel, as is normally the
case. Readers using a sublexical-processing strategy tend to make
more small rightwards saccades as well as longer fixations than
those of other readers. Hawelka and Wimmer (2005) concluded
that reading difficulties, for German dyslexic readers, occur when
readers adopt a sequential letter-processing strategy to compensate
for limitations in the number of letters that they can process in
parallel.
This claim is consistent with the dual route model of reading
proposed by Castles and Coltheart (1993). In this model, readers
utilize a global mode (parallel processing of all letters within a
word) and an analytic mode (sequential processing of smaller
segments of words) of word recognition. Hawelka and Wimmer
(2005) argued that German dyslexic readers demonstrated a deficit
in global word identification. In contrast, they claimed that a
phonological deficit (typically evident in a deeper orthography,
such as English) affects the analytic mode of (sequential) process-
ing.
754
KIRKBY, WEBSTER, BLYTHE, AND LIVERSEDGE
MacKeben et al. (2004) supported the claim that German dys-
lexic readers have limitations in parallel processing of letters.
Dyslexic readers made more saccades and longer fixations on
words of more than five letters than did non-dyslexic readers. The
two groups showed no differences, however, for words of five
letters or less. This suggests that dyslexic readers have more
difficulty in recognizing longer words than do typical readers,
perhaps leading them to adopt a segmentation strategy. MacKeben
et al. suggested that their results could be explained by limited
perceptual span in dyslexic readers as documented by Rayner
(1996) or by the magnocellular theory of dyslexia, which proposes
deficient processing of information by magnocells (Stein, 2001).
Several studies have examined non-phonological deficits asso-
ciated with dyslexia in Italian, another transparent orthography. De
Luca et al. (1999) found that dyslexic readers made an additional
saccade when the length of a target word increased by two letters;
however, an increase in word length of five letters was necessary
to elicit an additional saccade in typical readers. This effect was
attributed to dyslexic readers’ segmentation of words that typical
readers could process in parallel. The other main finding of the
study was that dyslexic readers skipped fewer short words than did
those in the control group. No group differences were found,
however, in non-reading tasks. The groups were similar in fixation
stability, accuracy, saccade amplitude, and the number of correc-
tive saccades. This contradictory pattern of results between reading
and non-reading tasks suggests that the distinct eye movement
behavior of dyslexic participants during reading is not caused by
slow reading speed.
A second study by De Luca, Borrelli, Judica, Spinelli, and
Zoccolotti (2002) found that dyslexic readers showed similar eye
movement patterns for word and pseudoword reading, indicating
that both were processed in a serial manner. In contrast, non-
dyslexic readers’ eye movements indicated that only pseudowords
were processed sequentially. De Luca, Borrelli, et al. (2002) ar-
gued that a serial reading mode is typical for Italian dyslexic
readers.
In summary, considerable work has indicated that both children
and adults with dyslexia exhibit eye movement behavior during
reading that is substantially disrupted relative to that of normal
readers. This seems to be, largely, a reflection of linguistic-
processing difficulties. In non-reading tasks, evidence for differ-
ences in oculomotor control between dyslexic and normal readers
is much less clear.
Binocular Coordination in Dyslexic Populations During
Non-Reading Tasks
Some researchers have suggested that atypical development of
eye dominance is related to the ability to maintain a steady visual
percept as well as influencing reading ability (Bigelow & Mc-
Kenzie, 1985; Cornelissen, Bradley, Fowler, & Stein, 1992; Cor-
nelissen, Munroe, Fowler, & Stein, 1993; Stein & Fowler, 1981,
1993; Stein, Riddell, & Fowler, 1986, 1988). Very few studies,
however, have directly recorded binocular eye movements during
both reading and non-reading tasks. The majority of research on
binocular control and dyslexia has used a reference eye test called
the Dunlop Test (DT; Dunlop, Dunlop, & Fenelon, 1973) or a
similar methodological paradigm, the Tranaglyph (see Bigelow &
McKenzie, 1985).
In the DT, a participant views two slides (Clement Clark fusion
slides F69 and F70) simultaneously, one with each eye, through a
synoptophore. Both slides depict a house; one slide displays a
small tree on one side of the door, and the other slide displays a
large tree on the opposite side of the door. Other than this differ-
ence, the slides are identical, and when they are viewed dichopti-
cally, the door is in the center of the visual field, with the large tree
on one side and the small tree on the other. The experimenter then
adjusts the synoptophore such that the two slides are slowly drawn
apart while the participant fixates on the door. At some point,
fusion is broken; immediately before this occurs, most individuals
report that one of the two trees moves. The tree that does not move
is the stable image, and the consistency with which the stable
image is associated with one of the two eyes determines the
stability of ocular dominance.
The nature of the proposed relation between unstable ocular
dominance and binocular coordination is not entirely clear. Some
researchers have argued that children who lack a stable dominant
eye are unable to make adequate vergence movements to maintain
a fused percept. The DT is used to assess the stability of ocular
dominance, which, in turn, indexes vergence capabilities required
for fusion. A serious concern regarding this methodological ap-
proach is the extent to which subjective reports on the DT ade-
quately reflect natural and spontaneous vergence movements that
occur during fixations in reading. Furthermore, it is important to
consider studies examining relations among stability of ocular
dominance, poor binocular coordination, and dyslexia, within the
context of recent binocular research (discussed earlier) that has
directly measured binocular eye movements during reading. Recall
that typically developing children (as well as adults) exhibit con-
siderable fixation disparity. Thus, binocular alignment is far from
perfect during reading in non-dyslexic populations; the two eyes
often fixate different letters within a word. This research, there-
fore, has clear implications for claims that poor binocular coordi-
nation has a causal role in reading difficulties. This is particularly
true since much of this work has not actually monitored binocular
eye movements.
In this final section of the review, we consider studies evaluating
the role of binocular coordination during reading in dyslexic
populations. Bishop, Jancey, and Steel (1979) reported a longitu-
dinal study in which the DT was administered to assess eye-
dominance development. They tested 147 children around 8 years
of age. Off-line measures of reading ability and intelligence were
administered. Bishop et al. found no significant difference in
reading ability between those children with a stable dominant eye
and those without when IQ was controlled. Further analyses
showed no eye-dominance differences between a group of 17 poor
readers (those reading 15 months below the expected level for their
ages) and an IQ-matched control group with age-appropriate read-
ing ability.
Other studies, however, have found results consistent with a
relation between eye dominance and reading performance (Stein et
al., 1986, 1988; see also Cornelissen et al., 1993). Stein et al.
(1986) reported a strong relation between age and performance on
the DT. The proportion of children with stable ocular dominance
increased steadily from 52% in 5-year-olds to almost 90% in
10-year-olds. Further, they found that participants who achieved
stable ocular dominance were reading, on average, 6.3 months
ahead of those who had not. Stein et al. (1986) argued that these
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BINOCULAR COORDINATION
differences in eye dominance reflected unstable binocular fixation,
which appeared to be related to successful reading development.
Furthermore, Stein et al. (1986) argued that these effects were a
consequence of abnormal magnocellular function. They suggested
that appropriate magnocellular function is crucial to the mainte-
nance of stable binocular fixation on small targets and is necessary
to prevent unintended eye movements that lead to visual confu-
sion, blurring, broken letters, and misordering of letters (Stein,
2001). They suggested that visual confusion arises from an inabil-
ity to fuse disparate patterns of retinal stimulation (Cornelissen et
al., 1993) or, alternatively, from fluctuating eye dominance leading
to alternating perceptions of a word (Stein, 2001). Furthermore,
they suggested that the cells involved in maintaining stable bin-
ocular visual fixation are located in the superior colliculus (Munoz
& Wurtz, 1992), which is dominated by magnocells.
Stein et al. (1988) recorded binocular eye movements as an
objective measure of fixation stability during two synoptophore
vergence tests (one in which visual slides subtended 2.5°, which is
standard for DT conditions, and the other in which the slides
subtended 7°). Over a period of 3 months, children between the
ages of 8 and 11 years who had a reading level two standard
deviations below that expected for their ages and IQs were re-
cruited. Participants were asked to fuse the picture and maintain
this fused state for as long as possible as the synoptophore tubes
were abducted or adducted (at a rate of approximately 0.5° per
second). Participants pressed a key as soon as they experienced
diplopia. Stein et al. (1988) recorded eye movement data during
trials of both the 2.5° and synoptophore tests. The control group
made convergent and divergent eye movements as appropriate
under test conditions. By contrast, the group of dyslexic readers
with unstable ocular dominance made more inappropriate conju-
gate eye movements than did the group of normal readers. This
difference was particularly pronounced when participants tracked
small targets compared with large targets (though no formal sta-
tistical analyses were reported in the article).
Visual instability in dyslexia, caused by deficient oculomotor
control, was investigated by Fischer and Hartnegg (2000). They
reanalyzed data from a previous study to assess stability of fixa-
tions, as measured by the number of intrusive saccades (unneces-
sary rapid shifts in eye position) during periods when participants
fixated on a stationary target. Dyslexic and non-dyslexic children,
between 7 and 17 years, had simultaneous horizontal eye move-
ments of both eyes recorded by using an infrared-light reflection
method. With the exception of the group ages 7–8, all of the
dyslexic children generated more intrusive saccades than did the
non-dyslexic children. Furthermore, the difference between the
groups increased with age; dyslexic participants showed a system-
atic lag in the development of fixation stability. Note, however,
that no aspects of binocular coordination were analyzed even
though binocular eye movements were recorded.
Dyslexia and fixation disparity were also the focus of an inves-
tigation conducted by Jaschinski, Ko¨nig, Schmidt, and Methling
(2004). The participants were dyslexic and typically developing
children ages 7–16 years. The sample of 50 children was com-
posed of 30 dyslexic participants, who had both reading and
writing disabilities, and 20 typically developing children. A psy-
chophysical measure of fixation disparity was determined by the
use of dichoptically presented nonius lines. In the nonius line test
of disparity, the observer views two dichoptically presented ver-
tical lines, one above the other. Participants adjust the lines in the
horizontal plane until they appear collinear (vertically aligned);
any stimulus offset that remains indexes fixation disparity. An
alternative method that is sometimes employed involves the brief
presentation of pairs of nonius lines. Participants make a forced
choice (collinear or not); again, the offset at which participants
report collinearity indexes disparity.
Jaschinski et al. (2004) employed central fusion stimuli, con-
sisting of 5 squares, that were presented dichoptically by means of
shutter goggles with 0.5° disparity. After 400 ms, two dichoptic
nonius lines (each visible to only one eye) were presented for 80
ms, one above and one below the central stimulus. The amount of
horizontal offset between the lines was varied in a step procedure
designed to initiate corrective vergence eye movements. Partici-
pants indicated whether the lines appeared collinear. The experi-
mental procedure included 30 trials in which the squares required
a convergent movement to attain fusion and 30 in which a diver-
gent movement was required. Nonius line collinearity judgments
indexed the magnitude of vergence movement. Jaschinski et al.
used this method to estimate vergence velocity and found more
children with large amounts of variability in the dyslexic group
than in the control group. This, in turn, led to a mean fixation
disparity for the dyslexic group that was 0.3 min of arc larger than
for the typically developing group. Jaschinski et al. concluded that
dyslexic readers had significantly worse binocular coordination
than did typically developing children, though they also suggested
that further research is needed to investigate the temporal and
spatial characteristics of vergence movements during fixations
with respect to dyslexia.
Bigelow and McKenzie (1985) used a Tranaglyph, rather than a
synoptophore, to assess the association between unstable ocular
dominance and reading ability. Note, however, that similar prin-
ciples underlie the two methods. In the Tranaglyph test, two slides,
one green and one red, are viewed through glasses with red and
green lenses. The two slides form a picture, or in this case, letters
and blobs subtending 2.5° of visual angle, when they are overlaid
and viewed binocularly. The green section of the image stimulates
the right eye (wearing the red lens) and the red section stimulates
the left eye (wearing the green lens). The two slides are gradually
moved apart and vergence eye movements are required to maintain
a fused percept. As with the synotophore test, one of the colored
sections seems to move prior to a break in fusion. The colored
section that remains constant indicates the dominant eye. Bigelow
and McKenzie tested two groups of children, consisting of good
and poor readers. A discrepancy criterion was used to define poor
readers (i.e., reading at least 17 months behind the expected level
for their IQ); thus, poor readers were, on average, 2.1 years older
than good readers. Stability was assumed when the same eye was
dominant on 80% or more of trials, and unstable dominance was
assumed when the same eye was dominant on less than 80% of
trials. In line with findings from Stein et al. (1988), Bigelow and
McKenzie observed increased unstable eye dominance in poor
readers compared with typically developing readers. Bigelow and
McKenzie did not argue that this relation was causal; instead, they
argued that the processes by which ocular stability is operative
require further explication before a causal relation may be as-
sumed.
A recent study by Kapoula et al. (2007), investigated binocular
coordination in groups of dyslexic and non-dyslexic children. The
756
KIRKBY, WEBSTER, BLYTHE, AND LIVERSEDGE
children, ages 9 to 13 years, were assessed on several different
aspects of their vergence capacity by way of orthoptic tests. The
near point of convergence was determined by presenting a small
pen light in the midplane of the participant’s binocular visual field.
This was slowly moved nearer to the participant’s eyes until one
eye lost fixation. The authors found that dyslexic children were
less able to converge their eyes onto points very near their faces
than were non-dyslexic children. Non-dyslexic children converged
their eyes on the pen light when it was held as close as 6 cm or less.
Dyslexic children, however, lost fixation when the pen light was
around 7–10 cm in front of their eyes. A second test assessed
vergence capacity by using prism bars. Participants were shown a
small letter, presented at a far distance (400 cm) or a near one (30
cm). The prism was placed in front of one eye and the convergent/
divergent power of the prism was increased until the participant
reported blurring or diplopia (image doubling). The maximum
prism power at which clear single vision was recovered repre-
sented the maximum relative convergence or divergence capacity.
Kapoula et al. found no significant convergence capacity differ-
ence between the two groups, for near or far targets. They did,
however, demonstrate a divergence limitation for the dyslexic
children, for both near and far targets. The dyslexic children had a
6 prism diopter reduction in their divergence capacity at both
distances compared with those of the non-dyslexic children.
It is important to consider these results in the context of studies that
have examined vergence during fixations in reading. As discussed in
earlier sections of this review, vergence movements during fixations
tend to be corrective for the residual disparity from the preceding
saccade (Blythe et al., 2006). Specifically, children tend to make
divergent movements during fixations, whereas adults make conver-
gent movements. If children with dyslexia are poor at making diver-
gent movements, as found by Kapoula et al. (2007), then they might
be less effective than typically developing children at reducing dis-
parity during fixations in reading.
Many researchers have questioned the causal link between un-
stable eye dominance and dyslexia. For example, Bishop (1989)
reviewed Stein and colleagues’ work and suggested that experi-
ence with reading text affects performance on the DT. It should be
noted that unstable ocular dominance has been demonstrated in
many participants with excellent reading and spelling abilities.
This poses a problem for those who argue that lack of fixed
reference on the DT is causally related to reading difficulties
(Lennerstrand, Ygge, & Rydberg, 1994; Newman et al., 1985).
A longitudinal study was conducted by Lennerstrand, Ygge, and
Jacobson (1993) to assess whether poor binocular coordination
accurately predicts later reading ability. They compared good and
poor readers before poor readers were diagnosed as dyslexic
(Lennerstrand et al., 1993) and reanalyzed the data from 40 of the
original participants once they received a diagnosis (Lennerstrand
et al., 1994). Ophthalmological evaluations were conducted when
the children were between 8 and 9 years of age. The sensory tests
included an ocular dominance test with a synoptophore. Here (in
line with Stein & Fowler’s, 1982, methodology), the images were
gradually drawn apart until a single image was no longer per-
ceived. The children indicated when they were no longer able to
fuse the disparate images and diplopia occurred.
In addition to participants’ subjective reports, binocular eye move-
ments were recorded from children ages 8 to 9 and 11 to 12. Eye
movement recordings allowed for the assessment of vergence move-
ments that occurred during the presentation of the synoptophore test.
With regard to binocular coordination, Lennerstrand et al. (1993)
reported no significant group differences (dyslexic children compared
with typically developing children) for stable or unstable ocular dom-
inance. Furthermore, no significant group difference was found in
vergence fusion capacity for images extending 2.5° of visual angle in
either age group. For images subtending 7.5° of visual angle, no group
difference was found for children in the 8 –9 years age range; how-
ever, 11–12-year-old dyslexic children had a higher capacity for
divergence than did typically developing children of the same age.
Note that this finding is opposite the pattern reported by Kapoula et al.
(2007). Kapoula et al. found that dyslexic participants of similar age
were limited in their capacity for divergent eye movements. Lenner-
strand et al. (1993) proposed that the discrepancy between their
findings and those of Stein et al. (1986) might reflect differences in
participant selection procedures. Lennerstrand et al.’s (1993) sample
differed only with respect to reading capacity, not with respect to
socioeconomic status, visual problems, or intellectual ability.
With these studies in mind, it is at least possible that unstable
ocular dominance may prove to be a correlate of dyslexia rather
than a causal factor in reading difficulties. It may also provide a
behavioral marker for the diagnosis of dyslexia. Clearly, however,
a considerable amount of further research providing accurate bin-
ocular eye movement data regarding small vergence movements
during fixations is needed before the suggestion that visual abnor-
malities cause dyslexia, or that they are co-existent with dyslexia,
can be instantiated or rejected.
From the studies reviewed here, it should be clear that results in this
area are highly contradictory. Some studies have found a link between
poor binocular coordination and reading difficulties in dyslexia (Big-
elow & McKenzie, 1985; Kapoula et al., 2007; Stein et al., 1986;
1988; Jaschinski et al., 2004), whereas others have not (Bishop et al.,
1979; Lennerstrand et al., 1993, 1994). It is, therefore, very difficult to
draw strong conclusions in favor of, or against, the claim. What we
can do, however, is to consider why the results in this area have been
so inconsistent. An important point in relation to this question is that
only three of these studies actually recorded eye movements; all of the
other experiments used tasks in which aspects of binocular coordina-
tion were inferred without actually measuring the positions of the
eyes. Three studies used the DT, one used nonius lines, and the other
used a prism test. The validity of the DT for assessing binocular
coordination is questionable. The nonius line task provides accurate
information about binocular disparity (Jainta, Hoormann, & Jaschin-
ski, 2007), but the task may be different enough from normal reading
that findings do not generalize to binocular coordination during read-
ing. A similar argument may be leveled at the method of prism
assessment. It seems prudent in light of these inconsistent results to
maintain a position of some caution in relation to conclusions about
the role of binocular coordination in reading ability. Overall, the data
are suggestive of some correlation between dyslexia and vergence
control in some participant populations. However, in our view, a
causal link between reading difficulties and poor binocular coordina-
tion has not been compellingly demonstrated.
Binocular Coordination in Dyslexic Populations During
Reading Tasks
As mentioned previously, the two binocular visual inputs must
be successfully fused for a single visual percept to be formed
757
BINOCULAR COORDINATION
(Liversedge, Rayner, White, Findlay, & McSorley, 2006). Thus,
Cornelissen et al. (1992) asked children to read single words with
both eyes open and then with one eye occluded to investigate the
effect of unstable binocular control on reading. They reasoned that
monocular viewing would remove any visual confusion stemming
from poor integration of the two retinal inputs. Children were
asked to read two lists of single words (presented in the form of a
paragraph) that were matched for length and difficulty. Cornelis-
sen et al. (1993) predicted that children who failed the DT would
make fewer reading errors (i.e., accurate recognition of words)
when they read with one eye occluded than when they read with
both eyes. Children with unstable binocular control made fewer
non-word reading errors for monocular viewing than for binocular
viewing of the text. Cornelissen et al. (1992) concluded that failure
to successfully integrate the two retinal inputs caused visual con-
fusion and led to the non-word reading errors made by dyslexic
readers. They suggested two possible ways in which binocular
coordination may not compensate adequately for visual disparity
between the two inputs in participants who fail the DT. First, the
fusion system may not establish correspondences between the two
disparate retinal inputs; hence, fusion may not be achieved. Alter-
natively, confusion may occur regarding the direction of vergence
movements in order to correct for binocular disparity.
In a follow up study, Cornelissen et al. (1993) addressed these
possibilities by monitoring participants’ binocular eye movements
as they read lists of single words presented as paragraphs. They
found that adults had significantly smaller disparity magnitudes (or
“vergence errors,” to use their terminology) than did children ages
9 –11. Typically developing children had, on average, fixation
disparity of 0.12° (SD 0.12°); non-dyslexic adults had, on
average, fixation disparity of 0.08° (SD 0.06°). However, no
difference in the disparity magnitude of fixations was found be-
tween groups of children who passed or failed the DT. Cornelissen
et al. (1993) concluded, therefore, that poor vergence control
during reading fixations was not the primary cause of the non-
word reading errors that they had demonstrated in children who
failed the DT.
Cornelissen et al. (1992) suggested that a brief period of mon-
ocular occlusion may prove beneficial to children with reading
deficits. This was confirmed in a study by Stein, Richardson, and
Fowler (2000). Children who failed the DT later acquired a stable
dominant eye (as assessed by the DT) after wearing glasses to
occlude one eye during reading for a period of 9 months. Those
children who gained a stable dominant eye as a consequence of the
monocular occlusion intervention (64%) showed significant im-
provement in reading ability compared with children who did not
receive the intervention. Stein et al. (2000) argued that improve-
ments in reading ability were a consequence of gaining a stable
dominant eye, which led to greater binocular control during fixa-
tion. However, the results also showed that 54% of the children
who did not receive the occlusion intervention also obtained a
stable dominant eye within the time frame of the study.
Again, in our view, the data from these studies are far from
compelling with respect to a causal relation between binocular
coordination and dyslexia. Some researchers have suggested that
binocular instability (or the lack of a stable dominant eye) brings
about visual confusion, whereas others have argued that this is not
the case. The theory is supported with evidence from intervention
studies. However, the number of dyslexic children with unstable
ocular dominance is unclear as is the nature of the relation between
visual deficits and dyslexia. In studies that have controlled for IQ
differences between dyslexic and typically developing children,
reading ability has not been associated with fixed or unfixed ocular
dominance. Furthermore, a considerable proportion of the popu-
lation with excellent reading and spelling abilities has unfixed
ocular dominance.
In conclusion, the relation between binocular eye movements
and dyslexia is not yet clear. This is true for a number of reasons.
The most important, perhaps, is the lack of studies specifically
examining binocular coordination in dyslexic readers during both
non-reading and natural reading tasks. A systematic and thorough
investigation of the binocular coordination of dyslexic readers will
be necessary before any causal link can be substantiated.
Future Research
Research examining binocular coordination has evolved differ-
entially for the three populations that we have discussed in this
review: adults, children, and individuals with dyslexia. Therefore,
it seems likely that future research will continue to be somewhat
independent in these populations, to at least some degree. We
hope, however, that consideration of binocular coordination re-
search as a single coherent research topic will accelerate scientific
progress. Our review is an attempt to bring together a number of
disparate lines of research in order to identify and clarify their
theoretical and methodological commonalities. We believe a more
unified field will occur gradually over time. In the meantime, we
believe that a number of specific issues deserve attention over the
next few years.
Much of the work that we have discussed concerns reading. It is
important not to lose sight of the fact that reading is a highly
skilled task, during which eye movements are made in a very
stylized, systematic, and regular manner as a text is scanned
horizontally. In less well defined tasks, such as natural scene
viewing, scanning is far less constrained. Thus, a very important
area of future research is binocular coordination during more
naturalistic tasks. Such work will contribute to our understanding
of how visual and cognitive factors influence the precise control of
the two eyes more generally (though see Jones & Lee, 1981).
The characteristics of binocular coordination in adults are now
relatively well documented across a range of laboratory-based
viewing situations and tasks. In reading research, however, con-
siderable confusion has arisen about the extent to which crossed or
uncrossed disparity prevails. Some researchers have found a ma-
jority of crossed disparities during reading, whereas others have
observed a majority of uncrossed disparities. It is fair to say that,
to date, most published data show a higher proportion of uncrossed
fixations than crossed fixations. However, further work is required
to better understand the factors that cause crossed and uncrossed
disparity during reading. As a starting point, it may be worth
considering whether different patterns of disparity are due to
particular eye tracking systems, the illumination of the room
during data collection, the viewing distance of the text, or indi-
vidual differences in readers.
Other interesting aspects of binocular coordination that require
further investigation are the psychological mechanisms and pro-
cesses by which disparate retinal inputs are fused. This work is
particularly necessary given that variable degrees of disparity have
758
KIRKBY, WEBSTER, BLYTHE, AND LIVERSEDGE
been observed on a fixation by fixation basis during reading, even
though the stimulus is at a fixed viewing distance and little if any
variability in this respect is necessary. Clearly, the binocular
coordination system has considerable flexibility. A critical ques-
tion is how closely patterns of retinal stimulation must correspond
in order for fusion to still occur. How we attain the percept of a
single, unified, visual representation is central to research investi-
gating all aspects of binocular coordination. Further, research is
needed to extend our current understanding of how oculomotor
control affects binocular vision beyond the descriptive accounts
that are currently available.
Recent developments in computer science are also important in
understanding processes of binocular coordination. In particular,
the role of binocular coordination in the perception of depth is
critical. Computer scientists have now developed advanced three-
dimensional displays on which objects can be presented such that
they are perceived in depth without the use of special viewing
devices, such as colored glasses. These displays usually present the
stimulus such that the plane of focus is the screen itself (and
therefore accommodation must be fixed at this depth), whereas the
depth at which the object appears is variable (therefore, presum-
ably, vergence will vary). Ordinarily, the accommodation and
vergence systems work in unison during normal scene viewing,
co-varying in relation to each other. Thus, future research will be
required to evaluate how three-dimensional displays influence
oculomotor performance and, specifically, binocular coordination
during scene inspection and reading.
With respect to developmental aspects of binocular coordina-
tion, it appears that the primary changes during development
involve reduced magnitude of disparity, along with a change in the
direction of disparity. Explanations for these developments, in
particular the switch from crossed to uncrossed disparities, have
been, at best, speculative. A very important issue for future re-
search with children is to address the cause of these developmental
changes. Furthermore, one important study has identified an influ-
ence of viewing distance on children’s binocular coordination, but
not on adults’ coordination (Yang & Kapoula, 2003). In our view,
it is vital to replicate this effect and then thoroughly describe
developmental changes in relation to it. Such research must estab-
lish whether the effect is attributable to the size of the stimulus as
it falls on the retina or the vergence angle that the viewer must
maintain. In addition, it is necessary to determine why such effects
occur in children but not in adults. To us, this seems a particularly
important line of future inquiry that deserves prompt attention.
The final area of future research that we consider concerns the
relation between binocular coordination during reading and dys-
lexia. What should be clear from our review is that the causal role
of magnocellular deficits in dyslexia is an extremely contentious
issue. We reviewed a number of studies that have examined
binocular coordination in dyslexic populations; we believe that
most of them have methodological limitations. Thus, it seems
prudent to form conclusions on their basis with caution. A sys-
tematic and thorough investigation of binocular coordination in
dyslexic children needs to be conducted before any solid conclu-
sions can be drawn as to a causal relation between the two.
The only way to determine the causal relation between binocular
coordination and dyslexia is to adopt and adhere to some strict
experimental practices. For example, stringent recruitment criteria
must be adopted in selecting samples of dyslexic participants and
appropriate control participants. High quality binocular eye move-
ment data need to be recorded during both natural reading and
non-linguistic tasks to assess binocular stability and the extent to
which instability is uniquely associated with linguistic processing.
Finally, at a more general level, it would be valuable to conduct a
large scale longitudinal study in which literacy profiles for dys-
lexic and non-dyslexic children are obtained (using traditional
off-line measures) and then related to the development of binoc-
ular eye movement control. Such a study would provide a com-
prehensive view of the association between binocular coordination
and dyslexia. To date, such rigorous experimental practices have
not been the standard for researchers examining binocular coordi-
nation in dyslexia, but we believe them to be absolutely vital for
future research.
Overall Conclusions
Our goal in this review is to provide an analytical evaluation of
the literature on binocular coordination in reading and non-reading
tasks. Perhaps the most striking conclusion from our evaluation is
that the points of fixation associated with the two eyes are often
disparate during reading and non-reading tasks. A widely held, and
often implicit, assumption among researchers is that both eyes
fixate the same letter in a word, or the same point within a scene.
Clearly, this is not the case on a significant proportion of fixations.
This said, however, it is also important to note that the magnitude
of the observed disparity is not enormous.
An important implication of this conclusion is that the tradi-
tional description of the human binocular system in which the two
lines of sight adhere to a tight, rigid trigonometric, angular ar-
rangement in relation to the fixated stimulus is untrue. This de-
scription is frequently depicted in undergraduate text books, and it
may be time to revise it. The two eyes are coordinated such that
each eye fixates within a degree of proximity to the other in order
to allow fusion to occur. Thus, the oculomotor control system
subserves a visual system that is efficient in constructing a clear
and unified perceptual representation from retinal inputs that can
differ to a substantial degree. Arguably, this may be the most
important implication to emerge from our review.
Another point that is apparent from our evaluation is that most
studies investigating binocular coordination in non-reading tasks
have examined disparity that occurs during saccades, whereas
most studies investigating reading have examined the disparity that
exists during fixations. During reading, the disparity is approxi-
mately one character space and is unaffected by the processing
demands of the task. Increasing evidence has suggested that a
unified percept is achieved through a process of fusion, and bin-
ocular saccades are programmed on the basis of the fused repre-
sentation.
In adults, the abducting eye initiates a saccade, moving faster
and further than the adducting eye, such that the two lines of sight
generally become divergent during the saccade. Two dissociated
subsystems are responsible for vergence and version, and interac-
tions between these underlie the characteristic patterns of binocu-
lar asymmetry observed in both reading and non-reading tasks.
The pattern of asymmetry during saccades is reversed for young
children; the lines of sight become converged during saccades.
Systematic changes in the alignment of the eyes during fixation
occur during development; younger children demonstrate residual
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BINOCULAR COORDINATION
disparity in the convergent direction, whereas adults demonstrate
divergence. Binocular coordination is comparatively poor for
younger children (in relation to adult performance). With maturity
comes a reduction in saccade disconjugacy, and, at around 12
years of age, patterns of eye movement behavior in children are
similar to those in adults.
Some researchers have claimed that poor binocular coordination
in children is associated with reading difficulties. However, the
evidence is mixed and a causal relation between poor binocular
coordination and reading difficulties has not been unequivocally
demonstrated. What is clear, however, is that age-related differ-
ences in binocular coordination are not driven by cognitive devel-
opment.
To summarize, we have reviewed a wide range of research
investigating aspects of binocular coordination. This research has
been conducted in a number of traditionally independent areas;
however, findings are largely consistent and complementary.
These findings cast light on a number of important and theoreti-
cally interesting questions. An increasingly large number of re-
searchers are turning their attention to experimentation in this
field, and it is likely that future research will significantly improve
our understanding of this fundamental aspect of human visual
processing.
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Accepted April 7, 2008
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    • "Similar results were reported for adult German-speaking dyslexics [22]. These findings are in contrast with those on English-speaking dyslexics, whose eye movement patterns typically showed prolonged fixation durations and substantially higher numbers of regressions (for reviews, see [1,30]). As mentioned above, a large number of studies carried out in regular orthographies compared eye movements of dyslexic and typical readers. "
    [Show abstract] [Hide abstract] ABSTRACT: Over the past decades, the relation between reading skills and eye movement behavior has been well documented in English-speaking cohorts. As English and German differ substantially with regard to orthographic complexity (i.e. grapheme-phoneme correspondence), we aimed to delineate specific characteristics of how reading speed and reading comprehension interact with eye movements in typically developing German-speaking (Austrian) adolescents. Eye movements of 22 participants (14 females; mean age = 13;6 years;months) were tracked while they were performing three tasks, namely silently reading words, texts, and pseudowords. Their reading skills were determined by means of a standardized German reading speed and reading comprehension assessment (Lesegeschwindigkeits- und -verständnistest für Klassen 6-12). We found that (a) reading skills were associated with various eye movement parameters in each of the three reading tasks; (b) better reading skills were associated with an increased efficiency of eye movements, but were primarily linked to spatial reading parameters, such as the number of fixations per word, the total number of saccades and saccadic amplitudes; (c) reading speed was a more reliable predictor for eye movement parameters than reading comprehension; (d) eye movements were highly correlated across reading tasks, which indicates consistent reading performances. Contrary to findings in English-speaking cohorts, the reading skills neither consistently correlated with temporal eye movement parameters nor with the number or percentage of regressions made while performing any of the three reading tasks. These results indicate that, although reading skills are associated with eye movement patterns irrespective of language, the temporal and spatial characteristics of this association may vary with orthographic consistency.
    Full-text · Article · Jan 2016
    • "We began each experimental trial with the calibration procedure. After the calibration procedure, a nonverbal reading procedure—dot scanning—was performed (Kirkby et al., 2008Kirkby et al., , 2010Kirkby et al., , 2013). The dot scanning task consisted of 11 black points (presented for 1 s, one point at a time, from left to right; size 0.5 ) presented on a white screen in an area of 16 (with a distance of 1.45 between points); the sixth point was presented in the middle of the screen (640 px; 512 px). "
    [Show abstract] [Hide abstract] ABSTRACT: When measuring fixation disparity (an oculomotor vergence error), the question arises as to whether a monocular or binocular calibration is more precise and physiologically more appropriate. In monocular calibrations, a single eye fixates on a calibration target that is taken as having been projected onto the center of the fovea; the corresponding vergence state represents the heterophoria (the resting vergence position), which has no effect on the calibration procedure. In binocular calibrations, a vergence error may be present and may affect the subsequent measurement of the fixation disparity during binocular recordings. This study includes a test of the precision of both monocular and binocular calibrations and an evaluation of the impact of the calibration procedure on the measurement of fixation disparity during a dot scanning task. Our results show that 11 participants (out of 19) each exhibited a significant difference in fixation disparity with the two types of calibration procedures. In addition, the fixation disparity was more strongly affected by heterophoria undergoing monocular calibration, as opposed to binocular calibration. This serves as additional evidence showing that the monocular calibration produces a physiologically more plausible fixation disparity and seems to be more appropriate for studying the full extent of fixation disparity.
    Full-text · Article · Aug 2015
    • "Similar results have been observed in older adults (Rayner et al., 2011); that is to say, they did not need more time for visual processing than young adults. Children also show striking similarities to adults in binocular coordination, although there are also some differences (Blythe et al., 2006; see Kirkby, Webster, Blythe, & Liversedge, 2008, for a review). Taken together, children and adults behave very similarly with respect to where to move the eyes in text and how the encoding of visual information takes place. "
    [Show abstract] [Hide abstract] ABSTRACT: Extending our understanding of the interplay between visual and cognitive processes during reading is essential to understand how reading develops and changes across the lifespan. Monitoring readers' eye movements provides a fine-grained online protocol of the reading process as it evolves over time, but until recently eye movements have rarely been collected for young developing and ageing people. Developmental eye-tracking constitutes an emerging and innovative field that addresses various theoretical questions related to changes in the process of reading across the lifespan and the mechanisms that drive intra-individual trajectories and create inter-individual differences among readers. The aim of this editorial is to briefly summarise the current state of the field and to outline which questions are currently being investigated and presented in this Special Issue.
    Article · Jul 2015
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