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Metacontrast masking is processed before grapheme-color synesthesia



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 cortical, because it can be obtained dichoptically. To compare the data for synesthetes and controls, we developed a metacontrast design in which nonsynesthete controls showed weaker dichromatic masking (i.e., the target and mask were in different colors) than monochromatic masking. We accomplished 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 synesthetic color to the monochromatic condition. The target-mask pairs used for each synesthete were graphemes that elicited strong synesthetic colors. We found stronger monochromatic than dichromatic U-shaped metacontrast for both synesthetes and controls, with optimal masking at an asynchrony of 66 ms. The difference in performance between the monochromatic and dichromatic conditions in the synesthetes indicates that synesthesia occurs at a later processing stage than does metacontrast masking.
Metacontrast masking is processed before graphemecolor
Michael Patrick Bacon &Bruce Bridgeman &
Vilayanur S. Ramachandran
Published online: 28 November 2012
#Psychonomic Society, Inc. 2012
Abstract We investigated the physiological mechanism of
graphemecolor 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 graphemecolor
synesthesia in the brain by examining the point of percep-
tual breakdown using direct psychophysical methods. The
best-documented type of synesthesia is graphemecolor
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 graphemecolor 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
V. S. Ramachandran
Department of Psychology, University of California,
San Diego, La Jolla,
San Diego, CA, USA
Atten Percept Psychophys (2013) 75:59
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
graphemecolor synesthesia as being beyond the level of V2.
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 targetmask
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 observers 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 observers subjective equiluminant
point quickly and efficiently, we used a graphic interface
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 individuals 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.
Four female graphemecolor 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 participants equiluminant point using
Bridgemans garden path procedure. The RGB triplet
We thank Kevin Samii for programming the garden pathgraphics
and interface.
6 Atten Percept Psychophys (2013) 75:59
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 targetmask pairs. Each par-
ticipant completed 3050 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 targetmask 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 targetmask pair
contained the shorter target bar. The two targetmask 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 dichromaticcon-
dition, the target and mask were presented in different,
equiluminant colors. For the control, monochromaticcon-
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 colorfor 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,
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.
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.
Fig. 1 Stimulus array, scaled as in the experiment. In this example, the
shorter target is in the lower targetmask 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) 75:59 7
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,
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.
STA (msec)
-33 0 66 99 132 167 199
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
-33 0 66 99 132 167 199
Percent correct
STA (msec)
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) 75:59
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 perceptionfor 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 graphemecolor 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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Atten Percept Psychophys (2013) 75:59 9
... It also differs from all other conditions by the number of sequentially presented visual transients: With an SOA of zero, there are two visual transients less, as the disks (target and distractors) and rings (masks) start and end at the same time. We expected a u-shaped distribution of the accuracy rates as a function of the different SOA steps (e.g., Alpern, 1953;Enns and Di Lollo, 2000;Tata, 2002;Boyer and Ro, 2007;Bacon et al., 2013;Agaoglu et al., 2018). We presented 24 stimuli, whereof only four (in the inner corners of the virtual square) were potential targets (highlighted by lines pointing toward these positions; Figure 1). ...
Full-text available
To investigate the relation between attention and awareness, we manipulated visibility/awareness and stimulus-driven attention capture among metacontrast-masked visual stimuli. By varying the time interval between target and mask, we manipulated target visibility measured as target discrimination accuracies (ACCs; Experiments 1 and 2) and as subjective awareness ratings (Experiment 3). To modulate stimulus-driven attention capture, we presented the masked target either as a color-singleton (the target stands out by its unique color among homogeneously colored non-singletons), as a non-singleton together with a distractor singleton elsewhere (an irrelevant distractor has a unique color, whereas the target is colored like the other stimuli) or without a singleton (no stimulus stands out; only in Experiment 1). As color singletons capture attention in a stimulus-driven way, we expected target visibility/discrimination performance to be best for target singletons and worst with distractor singletons. In Experiments 1 and 2, we confirmed that the masking interval and the singleton manipulation influenced ACCs in an independent way and that attention capture by the singletons, with facilitated performance in target-singleton compared to distractor-singleton conditions, was found regardless of the interval-induced (in-)visibility of the targets. In Experiment 1, we also confirmed that attention capture was the same among participants with worse and better visibility/discrimination performance. In Experiment 2, we confirmed attention capture by color singletons with better discrimination performance for probes presented at singleton position, compared to other positions. Finally, in Experiment 3, we found that attention capture by target singletons also increased target awareness and that this capture effect on subjective awareness was independent of the effect of the masking interval, too. Together, results provide new evidence that stimulus-driven attention and awareness operate independently from one another and that stimulus-driven attention capture can precede awareness.
... However, as many studies have also repeatedly provided arguments supporting the idea synaesthetic experiences are modulated by attention and have therefore claimed that synaesthetic processing resembles that of a late, post-attentive mechanism (e.g. Bacon, Bridgeman, & Ramachandran, 2013;Bargary et al., 2009;Blair & Caplovitz, 2012;Carriere et al., 2009;Edquist et al., 2006;Gheri, Chopping, & Morgoan, 2009;Hubbard et al., 2009;Mattingley, Payne, & Rich, 2006;Mattingley, 2009;Nijboer & Van der Stigchel, 2009;Price & Mattingley, 2013;Rich, Mattingley, & Payne, 2001;Rich & Mattingley, 2003;Rich & Karstoft, 2013;Sagiv, Heer, & Robertson, 2006;Specht, 2012;Teuscher, Brang, & Ramachandran, 2010;Ward et al., 2010). ...
Traditionally, synaesthesia has been understood as a neurological condition characterised by an enhanced hyperbinding or multisensory integration mechanism between the different brain areas elicited by the synaesthetic associations. Recent studies have challenged this assumed theory. However, these studies have only explored synaesthetes’ MSI through illusion perception paradigms (mostly, the Double-Flash Illusion), and thus the question whether abnormal MSI capacities in synaesthetes can be generalised to other domains remains unanswered as of yet. Furthermore, several studies have shown that the Double-Flash Illusion is modulated by attention. Critically, other former studies have also posited contradictory evidence regarding the normality of synaesthetes´ attentional and general executive control abilities. Thus, the aim of the present study was to test the hypothesis whether synaesthetes’ reduced MSI illusion perception observed in previous studies, could be better explained by enhanced attention capacities rather than reduced MSI abilities per se. In order to test this hypothesis, three different tasks were conducted: a MSI illusion task (Double-Flash Illusion), a MSI congruency task (Cross-modal Congruency Task), and a selective attention task (Flanker task). Performance in these tasks was compared between three sample groups: a group of synaesthetes, a group of people who only experienced synaesthetic traits to same degree, and a matched control group. The statistical analyses failed at showing direct abnormal capacities in multisensory integration and attention for synaesthetes: no group differences were found for the Double-Flash Illusion or the Flanker Task measures. However, both synaesthetes and synaesthetes-traits, relative to controls, did exhibit a significant smaller cross-modal congruency effects (CCE) in the Cross-modal Congruency Task. The seemingly contradictory results between the three tasks, has lead us to formulate a new, alternative hypothesis that proposes that the CCE differences found for synaesthetes could be due to specific enhanced capacities to filter out irrelevant stimuli presented in multimodal settings. The significance and implications of these findings, as well as the fact that synaesthetes and synaesthetes-traits showed same behavioural patterns are discussed.
... In the Bridgeman's lab an elegant study was conducted, examining relations between metacontrast and synesthesia mechanisms (Bacon, Bridgeman, & Ramachandran, 2013). In the synesthesia effects, a stimulus in one modality causes an illusory sensory experience in a different modality. ...
When about half a century ago masking research emerged as one of the hot topics in psychophysics, cognitive psychology and psychophysiology, Bruce Bridgeman was among the leaders in this domain. His studies and papers on masking must not be overlooked also today. This article brings to the readers a brief review of Bridgeman's contributions to the field and directly related research from other laboratories, with an eye on the implications for consciousness studies.
... That is, the concurrent experience of color appears to influence attention and speed visual search for a projector synesthete ( Smilek, Dixon, & Merikle, 2003), but not in samples where the associator/projector distinction was not analyzed ( Edquist et al., 2006). Moreover, in samples where the projector/associator distinction was not drawn, awareness of the inducing stimulus appears necessary for the synesthetic experience to be elicited, such that the concurrent experience does not survive masking of the inducer ( Bacon, Bridgeman, & Ramachandran, 2013;Mattingley et al., 2001). In contrast, it has been reported that for one projector synesthete, their synesthetic experience of color protected against object-substitution masking ( Wagar, Dixon, Smilek, & Cudahy, 2002), a form of visual masking in which target awareness is impaired due to object-updating processes (for a review see Goodhew, Pratt, Dux, & Ferber, 2013). ...
Synesthesia is the phenomenon in which individuals experience unusual involuntary cross-modal pairings. The evidence to date suggests that synesthetes have access to advantageous item-specific memory cues linked to their synesthetic experience, but whether this emphasis on item-specific memory cues comes at the expense of semantic-level processing has not been unambiguously demonstrated. Here we found that synesthetes produce substantially greater semantic priming magnitudes, unrelated to their specific synesthetic experience. This effect, however, was moderated by whether the synesthetes were projectors (their synesthetic experience occurs in their representation of external space), or associators (their synesthetic experience occurs in their 'mind's eye'). That is, the greater a synesthetes's tendency to project their experience, the weaker their semantic priming when the task did not require them to semantically categorize the stimuli, whereas this trade-off was absent when the task did have that requirement. Crown Copyright © 2015. Published by Elsevier Inc. All rights reserved.
A review of recent research in visual masking and TMS-masking to be published in 2014. The preliminary eBook version of this volume can be obtained from
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The last few years have seen the publication of a number of studies by researchers claiming to have induced “synaesthesia”, “pseudo-synaesthesia”, or “synaesthesia-like” phenomena in non-synaesthetic participants. Although the intention of these studies has been to try and shed light on the way in which synaesthesia might have been acquired in developmental synaesthestes, we argue that they may only have documented a phenomenon that has elsewhere been accounted for in terms of the acquisition of sensory associations and is not evidently linked to synaesthesia. As synaesthesia remains largely defined in terms of the involuntary elicitation of conscious concurrents, we suggest that the theoretical rapprochement with synaesthesia (in any of its guises) is unnecessary, and potentially distracting. It might therefore be less confusing if researchers were to avoid referring to synaesthesia when characterizing cases that lack robust evidence of a conscious manifestation. Even in the case of those other conditions for which conscious experiences are better evidenced, when training has been occurred during hypnotic suggestion, or when it has been combined with drugs, we argue that not every conscious manifestation should necessarily be counted as synaesthetic. Finally, we stress that cases of associative learning are unlikely to shed light on two highly specific characteristic of developmental synaesthesia in terms of learning patterns: First, their resistance to change through exposure ; Second, the transfer of conditioned responses between concurrents and inducers after training. We conclude by questioning whether, in adulthood, it is ever possible to acquire the kind of synaesthesia that is typically documented in the developmental form of the condition. The available evidence instead seems to point to there being a critical period for the acquisition of synaesthesia, probably only in those with a genetic disposition to develop the condition.
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In synesthesia, experiences in one domain evoke additional experiences in another, as when musical notes or letters of the alphabet evoke colors. Both the domains and their pairings are diverse. Indeed, Day's (2013) recently tabulated 60 types of synesthesia, each referring to a different combination of inducing and induced domains. The domains conjoined through synesthesia may belong to different sense modalities, as in music-color synesthesia, but may also belong to the same modality: In grapheme-color synesthesia, seeing printed letters or numbers evokes color experiences. In music-color and grapheme-color synesthesia, the inducing stimuli are perceptual, reflecting culture-specific categories (notes of the Western musical scale, letters of the alphabet) learned by synesthetes and non-synesthetes alike. Synesthesia may be triggered not only by sounds, tastes, smells, and pains, but also by more complex signals: words(e.g., Simner, 2007), emotional states (e.g., Ward, 2004), and even personalities (e.g., Novich et al., 2011). Analogously, the domains of synesthetic responses too can range widely. The composer Rimsky-Korsakoff “saw” the key of D-major as golden (Myers, 1914), while a grapheme-personification synesthete reported, “Ts are generally crabbed, ungenerous creatures” (Calkins, 1893; p. 454). Other phenomena, however, such as cross-modally evoked images or memories, are not typically considered examples of synesthesia. In this article, we briefly describe half a dozen illustrative cases that border on traditional forms of synesthesia: cross-modal correspondence, cross-modal imagery, sensory (cross-modal) autobiographical memory, empathic perception, hallucination, and the Doppler illusion. Do any or all of the six constitute forms of synesthesia? The answer depends, we suggest, on the framework for characterizing synesthesia. Consequently, after describing the six phenomena, we sketch three frameworks that differ in how they characterize these phenomena relative to prototypical forms of synesthesia. Several investigators, taking different perspectives and coming to different conclusions, have already considered possible relations to synesthesia in three of the six: cross-modal correspondence (Martino and Marks, 2001; Deroy and Spence, 2013); cross-modal imagery (Craver-Lemley and Reeves, 2013; Spence and Deroy, 2013); and empathic perception (Fitzgibbon et al., 2010; Rothen and Meier, 2013).
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A vicarious experience is an empathetic state in response to the observation of others' sensations, emotions, and actions (Keysers and Gazzola, 2009). Vicarious experiences in response to social stimuli are quite common in the general healthy population and they may even constitute an important basis for social behavior. Interestingly, vicarious experiences recruit similar neural processes as the primary experience of a certain sensation, emotion, or action, and it is assumed that the mirror neuron system is involved in these vicari-ous neural processes (e.g., Morrison et al., 2004; Singer et al., 2004; Jackson et al., 2005). Synesthesia denotes a condition that leads to specific experiences in response to normal sensory input that is not expe-rienced by non-synesthetes. Synesthetic experiences are characterized as idiosyn-cratic, involuntarily elicited, and con-sistent over time (Grossenbacher and Lovelace, 2001; Ward, 2013; but see, Simner, 2011). Synesthesia tends to run in families suggesting a genetic compo-nent and has a neural basis (Asher et al., 2006, 2009; Barnett et al., 2008). The best studied and most accepted form of synesthesia is grapheme-color synesthe-sia. People affected by this type of synes-thesia experience colors for numbers and letters printed in black on a white back-ground (e.g., Rothen et al., 2012). On a neural basis, it has been suggested that brain regions concerned with bind-ing processes, the modality of the induc-ing stimulus, and the modality of the respective sensory experience are involved (e.g., Hubbard et al., 2011; Rouw et al., 2011). Recently, it has been suggested that also vicarious experiences represent an instance of synesthesia. In particular, the term mirror-sensory synesthesia has been introduced in the scientific literature to describe instances of overt phenomenological experiences reflecting the actual state of an observed sensation and/or emotion (i.e., a phenomeno-logically overt vicarious experience, Fitzgibbon et al., 2012b). It has been sug-gested that people who report to have explicit and consciously accessible expe-riences of touch and/or pain upon the observation of other people being touched or in pain may be called mirror-touch and mirror-pain synesthetes, respectively (Fitzgibbon et al., 2010, 2012b). However, other mirror-sensory forms, such as for example mirror-disgust experiences, seem possible. Here, we argue that the label synes-thesia should be reserved for canonical cases of synesthesia (such as grapheme-color or lexical-gustatory) and we outline similarities and differences between synesthesia and vicarious experiences (Table 1) (for the use of the term synes-thesia see also, Deroy and Spence, 2013). By using the term mirrored sensory experiences, we focus on phenomeno-logically open instances of vicarious experiences because as by the defini-tion of "mirror-sensory synesthesia" phenomenologically less overt forms are not to be regarded as instances of synesthesia. Table 1 | Commonalities and differences between synesthesia and mirrored sensory experiences.
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We investigated grapheme--colour synaesthesia and found that: (1) The induced colours led to perceptual grouping and pop-out, (2) a grapheme rendered invisible through `crowding' or lateral masking induced synaesthetic colours --- a form of blindsight --- and (3) peripherally presented graphemes did not induce colours even when they were clearly visible. Taken collectively, these and other experiments prove conclusively that synaesthesia is a genuine perceptual phenomenon, not an effect based on memory associations from childhood or on vague metaphorical speech. We identify different subtypes of number--colour synaesthesia and propose that they are caused by hyperconnectivity between colour and number areas at different stages in processing; lower synaesthetes may have cross-wiring (or cross-activation) within the fusiform gyrus, whereas higher synaesthetes may have cross-activation in the angular gyrus. This hyperconnectivity might be caused by a genetic mutation that causes defective pruning of connections between brain maps. The mutation may further be expressed selectively (due to transcription factors) in the fusiform or angular gyri, and this may explain the existence of different forms of synaesthesia. If expressed very diffusely, there may be extensive cross-wiring between brain regions that represent abstract concepts, which would explain the link between creativity, metaphor and synaesthesia (and the higher incidence of synaesthesia among artists and poets). Also, hyperconnectivity between the sensory cortex and amygdala would explain the heightened aversion synaesthetes experience when seeing numbers printed in the `wrong' colour. Lastly, kindling (induced hyperconnectivity in the temporal lobes of temporal lobe epilepsy [TLE] patients) may explain the purp...
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Metacontrast suppression was compared under conditions of monoptic and dichoptic presentation. It was found that interference was greater with dichoptic than with monoptic presentation, and that the point of maximum interference differed in the two conditions. The interference curve generated as a function of inter-stimulus interval (ISI) was also different; monoptically a “U”-shaped curve was found, whereas dichoptically the curve appeared to be more “J”-shaped. Theoretical implications are discussed, and it is suggested that binocular rivalry may play a role in dichoptic interference.
"The Perception of Illusory Contours" is a complete and comprehensive volume on one of the most important phenomena in modern perception research. An illusory contour is a demonstration in which people perceive edges, surfaces, objects, and colors that have no physical reality. The international group of distinguished researchers that comprise the contributors to the volume present new theoretical interpretations and data in addition to reviewing the extensive literature on this topic. . . . "The Perception of Illusory Contours" contains the most comprehensive set of illusory contour figures ever assembled. This volume is a highly significant reference work in an area of research at the critical intersection of perception, cognitive science, visual neurophysiology, and artificial intelligence. (PsycINFO Database Record (c) 2012 APA, all rights reserved)
Paracontrast and metacontrast magnitudes were measured in a target identification task. A particular class of illusory contours is described that did not mask in the paracontrast condition but did show a large metacontrast magnitude. The discontinuity in the masking function is interpreted in terms of the Fourier decomposition of the visual scene that is performed by cells selectively responsive to discrete bands of spatial frequencies. The class of contours that we describe can only mask through inhibition generated by their low spatial frequencies. These results are consistent with recent models of masking based on two independent modes of inhibition—within sustained visual channels, and between sustained and transient visual channels.