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2016 March–April 97www.americanscientist.org96 American Scientist, Volume 104
In Teller’s experiment, an infant
was held facing a large, uniform gray
screen on which was presented a
black-and-white bar grating pattern.
The holder was shielded from view-
ing the grating. An adult observer, also
blind to the grating location, was hid-
den behind the screen, and observed
the infant through a central peephole.
(See a linked video of the procedure at
www.americanscientist.org.)
On each experimental trial, the
grating was presented randomly
on the left or right side of the screen.
The observer’s task was to use any of
the infant’s behaviors, such as eye or
body-orienting movements, to make a
forced-choice guess as to whether the
grating was located on the left or the
right. Thus, Teller dubbed her method
forced-choice preferential looking.
The infant had a view of a large, gray
screen with a grating on one side of the
center and a patternless gray patch on
the other side. The grating and plain
patches were designed to have a lu-
minance equal to that of the screen, so
that if the infant’s visual systems could
not resolve the grating, there would be
nothing to bias their looking and orient-
ing behavior toward either side. But,
because infants have strong (apparently
innate) preference for patterned versus
unpatterned stimuli, if they can resolve
the grating, their behavior should re-
veal the location of the grating to the
hidden observer behind the screen.
Teller’s key innovation was to make
sure each guess of the observer had
an objective outcome—correct or
incorrect—with a 0.5 probability of
being correct by chance. If the adult
correctly identified the randomized
location of the grating for five trials in
a row, for example, the probability of
that happening by chance was 1/25,
or 0.031. Early studies generally pre-
sented each test stimulus 20 times (or
more) to increase the statistical reli-
ability of the results.
The concept underlying the ap-
proach was one of information trans-
fer. As Teller characterized it, the
information about the location of
the grating would be “transmitted”
through the infant’s visual system
to the perceptual and motor centers,
which would “pass” the information
to the observer via the infant’s behav-
ior. Teller described the idea in a 1979
paper: “If the observer’s performance
is above chance, it follows that the in-
fant can discriminate that particular
stimulus from its surrounding visual
field. If the observer’s performance
is at chance, it follows that the infor-
mation concerning the location of the
stimulus was lost somewhere between
the display and the observer, hope-
William James, the great
19th century philoso-
pher and psychologist,
once described the sen-
sory-perceptual world of infants as a
“blooming, buzzing confusion.” The
image James painted raises profound
questions that are pragmatic, scien-
tific, and philosophical: How can we
know what infants see? Or how can
we know what any beings see if they
cannot tell us, via language or other
unambiguous communicative gesture,
of their internal experience?
Of course, James could not know
what an infant’s perceptual world
was like. And for many decades after
James, a series of myths and errone-
ous ideas about the sensory world of
infants were propagated. Indeed, re-
searchers as recently as the mid-1950s
believed that newborn infants were
unable to see patterns because of im-
maturities in the optics of the eye, the
retina, and the visual cortex. More-
over, as recently as the 1970s, some
physicians, including ophthalmolo-
gists, told new mothers that their new-
born could see almost nothing, and
was essentially blind.
Can we picture what the infant’s
visual world is like? If we were to
adopt James’s view, we might reason-
ably envision a newborn’s perception
of the world to be a sort of jumble,
like a dynamic Jackson Pollock paint-
ing or a Picassoesque montage of
deconstructed elements of objects—
impoverished in color, spatial detail,
and contrast, and without organized
perceptual meaning. However, many
neuroscientific findings and the results
of carefully designed perceptual tests
now give us good reason to believe
that such a characterization is very far
from the truth. The newborn infant’s
visual world is almost certainly neither
a Jamesian confusion, nor a patternless
haze, nor the equivalent of blindness.
We have a high level of confidence that
it is a highly organized (albeit imma-
ture), rapidly developing version of
adult vision, rich in pattern, contrast,
and color, and that it possesses some
remarkable abilities for discrimination
and complex pattern recognition. In
addition, the methods used to study
visual development have wide appli-
cability across sensory modalities in
humans as well as in other species.
A Conceptual Breakthrough
The ultimate demonstration of vision
is behavior that can be correlated in
a systematic and reliable way with
a visual stimulus. In testing adults,
we have the luxury of being able to
devise a reliable objective test requir-
ing only that they correctly identify
the letters on an eye chart. Objective
quantification of a preverbal human’s
visual capacity, however, poses formi-
dable challenges.
In the late 1950s and early 1960s,
developmental psychologist Robert L.
Fantz of Case Western Reserve Univer-
sity in Ohio began a systematic study
of vision in infants using a method he
called preferential looking. He observed
infants viewing a pair of visual stimuli
and recorded which stimulus the in-
fants looked at, how many times they
looked at each stimulus, and how long
each look lasted. By this means, he
quantified which patterns, and what
features of the patterns, infants could
perceive or perhaps “preferred.” His
observations clearly showed that in-
fants “preferred” patterned versus un-
patterned, homogeneous stimuli. This
observation set the stage for powerful
additional methods to be introduced.
In 1974 psychologist Davida Y.
Teller of the University of Washing-
ton introduced a modification to the
preferential looking technique that,
although subtle, was conceptually
profound. She changed the role of the
adult observer from having an essen-
tially subjective task to one with an
objective outcome.
Russell D. Hamer received his doctorate in
sensory science in 1979 from Syracuse Uni-
versity. He is currently an affiliate research
professor in psychology at Florida Atlantic
University and an affiliate scientist at the
Smith-Kettlewell Eye Research Institute. From
2007 to 2013, he continued his research on vi-
sual function in infants and adults as a visit-
ing professor at the Institute of Psychology at
the University of São Paulo in Brazil. Email:
russhamer2@gmail.com
The Visual World of Infants
Discovering what babies can see has been a formidable challenge, but research
methods now provide an objective picture of their surprising visual abilities.
Russell D. Hamer
A standard eye examination can tell researchers a lot about the normal anatomy of an infant’s
retina and optical system, but not what the infant actually sees. As infants are unable to ver-
bally tell us what they see, researchers over the past 40 or so years have had to develop other
methods and tools that key into an infant’s natural looking behavior and body movements to
objectively measure how much detail an infant can actually see.
Amelie-Benoist/BSIP SA/Alamy Stock Photo
How can we know what infants see? Or
how can we know what any beings see if
they cannot tell us, via language or other
unambiguous communicative gesture, of
their internal experience?
98 American Scientist, Volume 104 2016 March–April 99www.americanscientist.org
fully (but not necessarily) within the
infant’s sensory visual system.”
To determine the visual acuity of an
individual infant, the percentage of trials
for which the observer correctly identi-
fied the location of the grating had to
be measured for each of set of gratings
with different bar widths (some too fine
to see, others extremely visible) that en-
compassed the infant’s visual threshold.
The resulting data were encouraging-
ly systematic—for gratings with large
bars, the adult observers’ performance
was always significantly above chance,
and often nearly 100 percent of grating
locations were correctly identified. For
the finer gratings, the observer’s perfor-
mance dropped to chance performance
(50 percent correct), and intermediate
performance resulted when in-between
bar widths were displayed.
Such a performance curve is called a
psychometric function (see figure on page
100). The threshold bar width is esti-
mated as the width where performance
is significantly above chance, in this
case being the bar width corresponding
to 75 percent correct. When this proce-
dure was used to test a large sample of
infants of different ages, the thresholds
shifted systematically from coarse grat-
ings (low acuity) to finer gratings (high-
er acuity) with increasing infant age,
mapping out a regular developmental
sequence for grating acuity.
The forced-choice preferential look-
ing procedure has valuable internal
controls based on performance at ei-
ther end of the psychometric function.
Chance performance reassures us that
the stimulus display did not have any
detectable artifacts that would draw
the infants’ visual attention even if
they could not see the grating. Perfor-
mance at or near 100 percent correct
shows that when the grating is above-
threshold for the infant, the infant has
the sensory and motor mechanisms
adequate to do the task and orient to a
visual stimulus.
With the introduction of this tech-
nique, the characterization of infants’
visual development joined the domain
of the modern, objective psychophys-
ics traditionally used to study sensory
capacity in cooperative, linguistically
competent adult observers.
Mapping Out Acuity
Between the mid-1970s and mid-1990s,
research coming out of labs worldwide,
including Teller’s, used the forced-
choice preferential looking method to
establish reliable, objective norms for the
development of visual acuity in humans
and nonhuman primates.
Grating acuity is measured in terms
of spatial frequency, in units of cycles
per degree. A cycle is one repeat of a pair
of the light and dark grating bars, and a
degree is a measure of bar size in terms
of visual angle. One degree of visual
angle encompasses 60 arcminutes (one
arcminute equal to 1/60th of a degree).
The individual line-strokes of the 20/20
letters on the widely used Snellen eye
chart encompass 1 arcminute, which is
the width of each bar in a grating with a
spatial frequency of 30 cycles per degree.
The line-strokes of the big “E” on an eye
chart (for 20/200 vision) are 10 times
larger, or 10 arcminutes, corresponding
to a grating with spatial frequency of 3
cycles per degree. As acuity improves
with age, infants can see gratings with
smaller and smaller bar widths (or high-
er and higher spatial frequency).
The fractions in a Snellen eye chart,
originally developed by Dutch oph-
thalmologist Herman Snellen in 1862,
quantify someone’s acuity compared
to the average normal adult acuity of
20/20. For example, consider a person
with 20/200 acuity. The fraction means
that smallest letters he or she can read
from 20 feet (the numerator) could be
read by someone with normal acuity
from 200 feet (the denominator).
Data shows that newborn healthy
infants have pattern vision starting
with an acuity of 1 to 2 cycles per de-
gree. Acuity increases steadily and
rapidly over the first 1 to 2 years of
life, achieving 12 cycles per degree by
the age of 1 year. So a 3-month old, on
average, has an acuity of 3 cycles per
degree, a 6-month old’s acuity is 6 cy-
cles per degree, and so on until about
18 months to 2 years of age. Behavioral
grating acuity reaches adult levels by
the age of 3 to 5 years.
Acuity in Perspective
Although a 3-month-old’s acuity of 3
cycles per degree of visual angle (the
equivalent of 20/200 vision) seems
quite poor compared with that of
an adult, consider what an acuity of
20/200 permits one to see. If an infant
could read, the 20/200 “E” on an eye
chart would be readable at a distance
of about 6 meters. If you hold up your
thumb at arm’s length, it is about 2
degrees of visual angle, or about 12
times wider than the line-strokes of
that big “E.” In other words, an infant
in your arms can easily see the impor-
tant features of your face: your eyes,
nose, lips, and smile. The baby can
also see his or her own hands, fingers,
feet, and toes. The irises of your eyes
(which are about 1.3 centimeters in
diameter) would be visible from 4.5
meters; your whole eye would be vis-
ible from about 12 meters; your mouth
would be visible from about 22 me-
ters. The baby could see something the
apparent size of the Moon, which is
visually about three times larger than
the line strokes of the big “E” on the
eye chart. It’s also the about the same
size as a 70-meter long Boeing 747 air-
craft when it is 24 kilometers away.
But can infants focus on objects
that distant? From the time of Fantz’s
early studies in the 1950s and 1960s,
it was widely believed that young
infants could only focus up to 18 to
25 centimeters in front of their fac-
es—and this myth is still propagated
today in popular books and on web-
sites on parenting and development.
However, the viewing distance of the
stimuli in most of the forced-choice
preferential looking studies has been
50 to 100 centimeters, or more in some
cases, and these thresholds are con-
sidered to be lower-bound estimates.
If an infant could focus optically no
further than 18 centimeters, for exam-
ple, and the gratings were presented at
50 centimeters, the image of the grat-
ing reaching the retina would be out
of focus by about 3.6 diopters, which
would reduce the acuity of an adult
by 4 to 5 lines on the eye chart. This
Although a 3-month-old’s acuity seems
poor compared with that of an adult, the
baby can see something the apparent
size of the Moon, and mom’s whole eye
would be visible from about 12 meters.
24
12
6
3
1. 5
0.8
30
0.4
acuity (cycles/degree)
20/25
20/50
20/100
20/200
20/400
20/800
20/20
20/1500
acuity (snellen equivalent)
newborn 241812964321 36
infant age (months)
Parents are often told that their newborns require bold contrasts such as black and white
patterns (top) and bright colors (bottom) to promote normal visual development. Although
newborns are attracted to these strong stimuli, research has shown that their visual systems are
capable of resolving much more subtle patterns and colors. Although infants aren’t capable of
verbalizing what they can see, we can nevertheless obtain objective estimates of their abilities
using what’s called the forced-choice preferential looking method, which takes advantage of in-
fants’ natural preferences to look at, and orient towards, patterned stimuli. In addition to acuity,
it has been used to track development of sensitivity to visual contrast, color, motion, and other
visual features, as well as development of other sensory modalities, such as hearing.
Visual acuity of infants is often measured in terms of the number of regularly spaced black-
and-white grating bars an infant can reliably respond to. The finer the bars in the grating, the
more cycles (pairs of black and white bars) per degree of visual angle there are (left axis; all
numbers in log scale). Adult grating acuity is about 30 cycles per degree. Babies are born with
an acuity of 1 to 2 cycles per degree, but rapidly mature to about half of an adult’s level by the
age of 2 years, and children reach adult acuity by the age of 3 to 5 years. (Adapted from D. Y.
Teller and J. A. Movshon, Vision Research 26:1483.)
Ron Sutherland/Science Source
David Parker/Science Source
100 American Scientist, Volume 104 2016 March–April 101www.americanscientist.org
amount of defocus would mean that
the “true” behavioral acuity (if opti-
cal blur were corrected) might be con-
siderably higher than the measured
acuity. Although researchers did not
in fact know the plane of focus of the
infants during testing, evidence from
independent studies of this issue, in-
cluding those by optometrists and neu-
roscientists Grazyna M. Tondel and T.
Rowan Candy of Indiana University,
suggest that by at least eight weeks
of age, infants in these studies were
capable of adjusting their focus to be
appropriate to the test distances. Over-
all, the research indicates that infants
have the capacity to focus on objects at
virtually any distance, from infinity to
very close to their own face, with rela-
tively accurate control of focus start-
ing at the age of about eight weeks,
and improving thereafter. There is no
compelling evidence, therefore, that
the acuity values measured were se-
verely underestimated as the result of
impediments to optical focus.
If the infants’ optics are not limit-
ing their acuity, what is? Anatomical
evidence obtained from the retinas and
brains of infant cadavers, along with
physiological data from nonhuman
species, and mathematical modeling
of visual processing, all point to imma-
turities in the retina and the visual cen-
ters of the brain as the limiting factors
in infants’ visual sensitivity and acuity.
The improvement in acuity tracks the
maturation of the spatial processing
abilities of the retina and brain. One
example is the maturation of the fo-
vea, the central part of the retina that
is used to see patterns with very fine
spatial detail. The infant fovea (which
comprises about 4 degrees of visual
angle) is functional, but has a lower
density of photoreceptors packed into
it. Over the first two years, the foveal
cone density increases and the cells
themselves assume adult form, with
increased light-capturing ability and
sensitivity. The visual acuity limit of
adults closely matches the spacing of
their foveal cone photoreceptors (ap-
proximately 1 arcminute), equivalent
to the dimensions of the line strokes
of the 20/20 eye chart letters and the
separation between the strokes.
Looking Forward
The forced-choice preferential looking
paradigm has had enormous influence
not only on the study of visual devel-
opment, but also on research into other
sensory modalities, and the investiga-
tion of perceptual and cognitive de-
velopment in humans and in animals.
Within the domain of visual develop-
ment, the paradigm was used to show
that infants can resolve dark and light
features of relatively low contrast, not
just completely black and white. It was
used in the first demonstrations that
young infants have true color vision,
and can detect more than just bright-
ness differences. Both of these topics
also remain the subject of persistent
myths perpetuated to parents by pop-
ular books and articles, suggesting that
infants can only see high-contrast pat-
terns in black and white.
The first year of life is an intense pe-
riod of development, involving many
complex neural and physical changes
necessary to create the dynamic expe-
rience of vision. But behavioral stud-
ies, augmented with complementary
research using brain wave measures
to follow the maturation of the visual
cortex, have shown that, although in-
fant vision is not as good as that of an
adult, the visual experience of babies
is quite rich and well-organized. It is
certainly not a “blooming, buzzing,”
patternless confusion.
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issue of American Scientist Online:
http://www.americanscientist.org/
issues/id.119/past.aspx
The first year of life is an intense period
of development, involving many
complex neural and physical changes
necessary to create the dynamic
experience of vision.
observer’s percent correct
stripe width
2.5
(20/50)
5
(20/100)
10
(20/200)
20
(20/400)
40
(20/800)
80
(20/1600)
chance (50 percent correct)
75 percent correct
subject age: 8 weeks
arcminutes
(Snellen)
100
80
60
40
threshold acuity
20/300
An infant’s visual acuity is estimated from a curve called a psychometric function, depicted
here for an 8-week-old infant. The graph shows the observer’s percent correct (y-axis) for each
of five different gratings with bar widths of 5, 10, 20, 40, and 80 arcminutes, corresponding to
Snellen eye chart values of 20/100, 20/200,20/400, 20/800, and 20/1600 (x-axis). For the coarse
gratings, the observer’s percent correct was highly significantly above chance performance (as
high as 100 percent correct). For finer gratings, performance dropped to near chance (50 percent
correct). The threshold bar width, or acuity, was defined as the bar width that would have
yielded 75 percent correct. The acuity estimate for this 8-week old infant was 20/300, about 15
times worse than normal adult acuity of 20/20. (Adapted from D. Y. Teller, 1979.)
