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Metacontrast masking is processed before grapheme–color
synesthesia
Michael Patrick Bacon &Bruce Bridgeman &
Vilayanur S. Ramachandran
Published online: 28 November 2012
#Psychonomic Society, Inc. 2012
Abstract We investigated the physiological mechanism of
grapheme–color synesthesia using metacontrast masking. A
metacontrast target is rendered invisible by a mask that is
delayed by about 60 ms; the target and mask do not overlap
in space or time. Little masking occurs, however, if the
target and mask are simultaneous. This effect must be cor-
tical, because it can be obtained dichoptically. To compare
the data for synesthetes and controls, we developed a meta-
contrast design in which nonsynesthete controls showed
weaker dichromatic masking (i.e., the target and mask were
in different colors) than monochromatic masking. We ac-
complished this with an equiluminant target, mask, and
background for each observer. If synesthetic color affected
metacontrast, synesthetes should show monochromatic
masking more similar to the weak dichromatic masking
among controls, because synesthetes could add their synes-
thetic color to the monochromatic condition. The target–
mask pairs used for each synesthete were graphemes that
elicited strong synesthetic colors. We found stronger mono-
chromatic than dichromatic U-shaped metacontrast for both
synesthetes and controls, with optimal masking at an asyn-
chrony of 66 ms. The difference in performance between the
monochromatic and dichromatic conditions in the synes-
thetes indicates that synesthesia occurs at a later processing
stage than does metacontrast masking.
Keywords Visual perception .Visual awareness .Neural
mechanisms .Metacontrast .Synesthesia
After more than a century of being on the fringes of perceptual
science (Galton, 1880), synesthesia has seen renewed interest in
the past few years with the application of modern psychophys-
ical methods (Cytowic, 2002; Ramachandran & Hubbard,
2001a,2001b). Synesthesia is both involuntary and stable
(Brang & Ramachandran, 2011; Cytowic & Eagleman,
2009); as a perception in one modality that occurs as a result
of stimulation in another, it represents a failure of accurate
perception of the properties of the world. In this way, synesthe-
sia is a tool for uncovering perceptual mechanisms, which are
often investigated by exploring the limits of perceptual capa-
bility. Investigating when and how perception breaks down
(e.g., measuring thresholds) often informs researchers about
the mechanisms of perception. The natural breakdown of cor-
respondence between physical stimulation and perception can
thus be informative about perceptual mechanisms.
We can begin to locate the level of grapheme–color
synesthesia in the brain by examining the point of percep-
tual breakdown using direct psychophysical methods. The
best-documented type of synesthesia is grapheme–color
synesthesia, in which letters and numbers evoke an idiosyn-
cratic experience of color for each grapheme. In this study,
we employed a metacontrast-masking paradigm to compare
the performance of grapheme–color synesthetes and non-
synesthete control participants using both dichromatic and
monochromatic stimuli. If both synesthetes and controls
experienced stronger masking under monochromatic than
under dichromatic conditions, this would indicate that meta-
contrast masking is processed before synesthesia; the syn-
esthetic advantage of experiencing color would not affect
metacontrast perception. This would suggest a later process-
ing site for synesthesia within the visual stream. If synes-
thete performance were dissimilar from nonsynesthete
performance with monochromatic stimulus presentation,
such that metacontrast masking was weakened or eliminat-
ed, this would indicate that metacontrast masking is pro-
cessed either after synesthesia or at the same level. The
M. P. Bacon :B. Bridgeman (*)
Department of Psychology, Social Sciences 2 UCSC,
Santa Cruz, CA 95064, USA
e-mail: bruceb@ucsc.edu
V. S. Ramachandran
Department of Psychology, University of California,
San Diego, La Jolla,
San Diego, CA, USA
Atten Percept Psychophys (2013) 75:5–9
DOI 10.3758/s13414-012-0401-1
idiosyncratic experience of color in this case would interfere
with metacontrast masking, which would suggest an early,
or perhaps a multilevel, processing site for synesthesia.
Metacontrast masking
Metacontrast, a type of backward visual masking, occurs when
a briefly presented stimulus, the target, becomes less visible or
is visually eliminated if it is immediately followed by another
briefly presented stimulus, the mask. Metacontrast is itself an
interesting phenomenon, because the mask obscures visual
perception of the target backward in time. Metacontrast follows
a U-shaped masking function (Alpern, 1953), alternately re-
ferred to as a Type B masking function (Kolers, 1962;
Kahneman, 1968). The point for optimal visual suppression is
within a 50- to 60-ms stimulus onset asynchrony (SOA;
Alpern, 1953;Stigler,1910) or a 50- to 60-ms stimulus termi-
nation asynchrony (STA). By manipulating the relative dura-
tions of the target and mask, Macknik and Livingstone (1998)
found that masking functions follow the STA more closely than
the SOA; previously the two measures had been confounded,
because the target and mask generally had equal durations.
Metacontrast lends itself nicely to this project because of
what is already known about it. It can be obtained dichopti-
cally (Kolers & Rosner, 1960; Schiller & Smith, 1968;
Werner, 1940), which eliminates lateral geniculate nucleus
or retinal explanations (Bridgeman, 1971). This indicates that
the earliest possible site of metacontrast must be V1, the
earliest site of binocular convergence. Evidence of metacon-
trast has been found in single cells of V1 in both the cat
(Bridgeman, 1975) and the monkey (Bridgeman, 1980).
However, metacontrast can also be obtained using illusory
or subjective contours (Gilden,MacDonald,&Lasaga,
1988), and area V2 is the first processing site for subjective
contours (Petry, 1987). If metacontrast were to be unaffected
by synesthesia, we could therefore further narrow the seat of
grapheme–color synesthesia as being beyond the level of V2.
Method
Measure of equiluminance
Metacontrast is generally considered to be stronger if the
target and mask are of the same color, but the relationship is
complex (Breitmeyer, 1984). By displaying target–mask
pairs that are equiluminant with the background for each
participant, we were able to record two distinct masking
functions for dichromatic and monochromatic stimulus pre-
sentations for nonsynesthete control participants.
Finding an observer’s equiluminant point is often a dif-
ficult and time-consuming process, as well as being prone to
error because of chromatic adaptation during a series of
trials. To locate each observer’s subjective equiluminant
point quickly and efficiently, we used a graphic interface
1
in which two flickering fields alternated two opposed color
gradients. The top of the first field was bright red and faded
gradually to black at the bottom. This field alternated with a
second field that was bright green at the bottom and faded
gradually to black at the top. These two fields were alter-
nated at a flicker rate above the chromatic flicker fusion rate
but below the luminance flicker fusion rate.
At some intermediate height in the field, the decreasing red
and the increasing green will have the same luminance for the
observer. That individual’s equiluminant values will then pop
out by flicker fusion, the location at which the observer sees
the least amount of flicker, or even a stationary line; in other
words, a location where the luminances of the red and green
gradients match. However, this is a false perception, and thus a
garden path, because the entire array is flickering at the same
rate. Each participant is asked to adjust the relative brightness
levels of the two fields until the equiluminant point is exactly
in the middle of the pattern. For the present study, this process
was repeated for each participant in order to locate the correct
RGB triplet values for equiluminant red and blue and for
equiluminant red and yellow, for the example of a red back-
ground, blue target, and yellow mask.
Procedure
Four female grapheme–color synesthetes, as well as seven
female and two male nonsynesthete controls, were recruited
from the undergraduate student body at the University of
California, Santa Cruz. All of the synesthetes volunteered
their time, while the nonsynesthete control observers vol-
unteered for credit for a class requirement.
For each of our synesthetes, we validated with a test–
retest method their synesthetic associations of letters, numb-
ers, and ordered time units (days of the week and months of
the year). All four synesthetes reported the same synesthetic
associations two days apart with 100 % consistency between
the two time periods. If the synesthetic observers did not
associate a particular grapheme with a specific color, they
were asked to leave it blank.
All synesthetes and nonsynesthete controls were run in-
dividually in the same darkened experimental room using
the same computer and screen for both determination of
equiluminance and metacontrast masking. Each participant
sat with eyes 60 cm from the center of the screen. We
located each participant’s equiluminant point using
Bridgeman’s garden path procedure. The RGB triplet
1
We thank Kevin Samii for programming the “garden path”graphics
and interface.
6 Atten Percept Psychophys (2013) 75:5–9
values obtained were then used for the remainder of the
study for the corresponding observer. Prior to experi-
mental participation, each synesthete was first activated
until subjective colors were experienced by displaying a
stationary target and mask pair together until the synes-
thetic color was experienced, although this was achieved
almost immediately. At least a 1-s interval was inter-
posed between the activation and masking stimuli, to
prevent the activation from distorting the masking. The
graphemes that each synesthete reported as evoking the
strongest color experience among the colors that we
used were assigned as the target–mask pairs. Each par-
ticipant completed 30–50 practice trials in both chroma
conditions prior to experimental participation.
The target stimuli consisted of a pair of isolated horizon-
tal bars composed of repeated letters or numbers 0.28º high,
one above and one below the fixation point (Fig. 1). Each
target was bordered by a mask consisting of a pair of
nonoverlapping bars, each 0.28º high, composed of a differ-
ent repeated letter or number. The target–mask separation
was 0.09º. One of the targets consisted of eight repeated
symbols, while the other consisted of seven. All of the
masks were eight letters wide.
The masking paradigm was based on a two-alternative
forced choice (2AFC) task in which participants reported by
keypress whether the upper or the lower target–mask pair
contained the shorter target bar. The two target–mask com-
binations were displayed simultaneously on a CRT screen
refreshed at 60 Hz. The target duration was one frame, and
mask duration was two frames. For the “dichromatic”con-
dition, the target and mask were presented in different,
equiluminant colors. For the control, “monochromatic”con-
dition, the same letters or numbers as in the dichromatic
condition were used in both target and mask, but both were
presented in the same color—for instance, both blue or both
green. Thus, any distortion of masking due to the use of
different letters in the target and mask would be equilibrated
across conditions.
Seven different timing conditions were based on the STA,
whichhasbeenfoundtomorereliablypredictmasking
performance than does the SOA (Macknik & Livingstone,
1998). The seven timing conditions were −33 (forward para-
contrast masking), 0, 66, 99, 132, 165, and 199 ms. These
timing conditions were presented in a randomized order for
each participant, and each participant completed the same
number of trials for each of the seven STAs. A block con-
sisted of 154 trials (22 trials at each STA) of either dichro-
matic presentation or monochromatic presentation. The
block order alternated monochromatic and dichromatic stim-
ulus presentation, and the first block alternated between
monochromatic and dichromatic stimuli for each participant.
Each participant completed one monochromatic and one
dichromatic block.
Analysis
Our analysis included one between-subjects variable, syn-
esthesia, and two within-subjects variables, chroma and
STA. The runs for each participant were averaged in order
to obtain a single masking function for each participant. We
performed a 2 (synesthesia) × 2 (chroma) × 7 (STA) mixed
design analysis of variance.
In two phases, we tested the null hypothesis that the
monochromatic and dichromatic conditions would yield
indistinguishable masking functions for the synesthetes.
This would mean that synesthetic color reduced masking
in the same way as real (physical) color. The first,
preliminary phase engaged the control observers, to
assure that our dichromatic stimulus conditions would
yield less metacontrast masking than would our mono-
chromatic stimulus conditions among the nonsynesthetic
controls. The second phase tested our synesthetes in the
monochromatic and dichromatic conditions.
Targets
xxxxxxxx
xxxxxxx
Masks
cccccccc
cccccccc
cccccccc
cccccccc
Fig. 1 Stimulus array, scaled as in the experiment. In this example, the
shorter target is in the lower target–mask pair. In the experiment, upper
and lower short targets were assigned randomly for each trial. For
backward masking, the upper panel would be presented before the
lower panel. Following the targets and masks, a decision window
remained on the screen until response
Atten Percept Psychophys (2013) 75:5–9 7
Results
In the control observers, the dichromatic condition resulted
in weaker metacontrast than did the monochromatic condi-
tion (Fig. 2), establishing a baseline difference between the
stimulus conditions against which the synesthete perfor-
mance could be compared.
The synesthetic observers also showed weaker masking
in the dichromatic condition (Fig. 3), a significant main
effect, F(1, 11) 089.3, p< .001. The difference between
the monochromatic and dichromatic conditions was as
strong in the synesthetic observers as in the controls, F(1,
11) < 1, n.s., indicating that the difference between dichro-
matic and monochromatic masking occurred for both syn-
esthetes and controls. Thus, our null hypothesis of no
significant difference in masking between the conditions
for the synesthetes was rejected. The synesthetes were un-
able to use their synesthetic colors to differentiate the target
from the mask, and therefore showed strong metacontrast in
the monochromatic condition, even though they were able
to use physical color to defeat masking in the dichromatic
condition. There was also a significant main effect of STA
condition, F(6, 54) 034.40, p< .001.
The average performance of the synesthetes collapsed
across STAs was no better in the dichromatic condition
than was the performance of nonsynesthetes (M0.939,
SE 0.10, and M0.932, SE 0.10, respectively), as
tested by a post-hoc ttest, t(27) 00.081, p0.94. The
synesthetes reported seeing their synesthetic colors in the
masking trials, however. Furthermore, the nonsynesthetes
did not perform significantly better than the synesthetes
in the monochromatic condition (M0.827, SE 0.015,
and M0.862, SE 0.015, respectively), t(27) 00.395,
p0.67.
Performance in both the dichromatic and monochromatic
conditions followed a U-shaped function for the backward
side of the metacontrast function. However, performance
was categorically better in the dichromatic than in the
monochromatic condition for both synesthetes and controls.
While the point of optimal masking was the same for both
conditions, the degradation in performance was not as dra-
matic in the dichromatic condition.
Dichromatic
Monochromatic
100
90
80
70
60
STA (msec)
-33 0 66 99 132 167 199
Percent
correct
Control Data
Fig. 2 Nonsynesthesia data,
averaged over nine participants.
STA stands for stimulus
termination asynchrony; that is,
at STA 00 ms, the target and
mask terminate simultaneously.
Error bars indicate between-
subjects standard errors
100
90
80
70
60
-33 0 66 99 132 167 199
Percent correct
STA (msec)
Dichromatic
Monochromatic
Synesthetic Data
Fig. 3 Synesthesia data,
averaged over four participants.
STA stands for stimulus
termination asynchrony. Error
bars indicate between-subjects
standard errors
8 Atten Percept Psychophys (2013) 75:5–9
Discussion
The results of the present study suggest that metacontrast
masking and synesthesia are mutually exclusive and that syn-
esthesia occurs at a later processing stage than does metacon-
trast in the visual stream. This distinction is reflected
qualitatively in the differences between synesthetic and real
color perception—for instance, in the lack of a complementary-
color afterimage upon the disappearance of a letter seen in
synesthetic color (Bridgeman, Winter, & Tseng, 2010). In
Bridgeman et al.’s study, synesthetes perceived both a synes-
thetic and a real color together; when a black high-contrast
letter abruptly disappeared, one synesthete, for example, saw
the afterimage color as “white, but still red [the synesthetic
color for that figure in that person].”The present results also
suggest that synesthetic color experience does not influence
color processing in metacontrast masking. A U-shaped meta-
contrast masking function appeared in the monochromatic
stimulus condition, with an optimal masking point at 66 ms,
whether or not the observer experienced synesthetic colors.
That is, the synesthetes were incapable of using their synes-
thetic colors to identify the masked target.
A related interpretation of our results can also be made: It
is possible that, rather than coming after metacontrast, syn-
esthesia is based on a different system that is not involved in
metacontrast masking. (We thank Vince DiLollo for sug-
gesting this possibility.)
By using a metacontrast-masking procedure, we were able
to build on what we know of metacontrast and its location in
the brain to begin to locate the level of grapheme–color synes-
thesia, which must occur beyond the level of V2 if it does
involve the same system as metacontrast. Measuring thresholds
and recording natural breaks in perception with a psychophys-
ical experiment affords a more direct measure of experience
than do neuroimaging methods such as fMRI (Hubbard,
Arman, Ramachandran, & Boynton, 2005;Nunnetal.,2002;
Sperling, Prvulovic, Linden, Singer, & Stirn, 2006), though
fMRI studies are consistent with ours in identifying color–
grapheme synesthesia with activity beyond V2. Specifically,
they have identified activity in V4 with synesthetic color (see,
however, Hupé, Bordier, & Dojat, 2012). When research
locates the exact level of synesthesia in the brain and accurately
maps its neural structure, we will better understand how the
human brain creates and binds its own perceptions.
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