Synesthesia: When colors count

A possible key role of vision in the development of schizophrenia

Based on a brief overview of the various aspects of schizophrenia reported by numerous studies, here we hypothesize that schizophrenia may originate (and in part be performed) from visual areas. In other words, it seems that a normal visual system or at least an evanescent visual perception may be an essential prerequisite for the development of schizophrenia as well as of various types of hallucinations. Our study focuses on auditory and visual hallucinations, as they are the most prominent features of schizophrenic hallucinations (and also the most studied types of hallucinations). Here, we evaluate the possible key role of the visual system in the development of schizophrenia.

Colours + Numbers differs from colours of numbers: cognitive and visual illusions in grapheme-colour synaesthesia

This study investigates the bi-directionality of synaestesic experience by means of a flanked bisection paradigm in TT, a number-colour synaesthete. Previous studies have shown that bisection is shifted towards the larger digit flanker (e.g., Ranzini & Girelli, 2012). TT and controls performed line bisections with lines flanked by black digits (experiment 1), by TT's photism colours (experiment 2), and by congruently (experiment 3), or incongruently coloured digits (experiment 4). While the results of the control group mainly replicated previous findings, only the colour-digit congruence elicited in TT the larger-digit bias. TT's absence of effects in the other conditions was not due to reduced sensitivity to luminance effects (experiment 5), or to mathematical expertise (experiment 6). We suggest that grapheme-colour synaesthesia might be characterised by a rigid access to semantic representation when the inducer is task-irrelevant.

Orange Is Less Than Green: An Examination of Bidirectionality in Grapheme-Color Synesthesia

Grapheme-color synesthetes experience a sense of color when viewing graphemes (e.g., digits and letters). Traditionally, these synesthetic perceptions are considered to be unidirectional, where viewing a grapheme elicits a nonveridical sensation of color, but viewing a color does not induce a reciprocal sense of a grapheme. A growing body of research has emerged that suggests the potential for bidirectional percepts, wherein color facilitates additional grapheme perception. We present here a novel paradigm in which we presented two sets of pure color patches, based on synesthete's reported colors, side-by-side and asked participants to indicate the color patch with the greater affiliated magnitude. Results indicated that the odds of answering correctly on trials were significantly greater for synesthetes (80.2% accuracy) than nonsynesthetes (52.1% accuracy). These results are aligned with other reports that support the notion of inducing a sense of magnitude from color in synesthetes. These findings challenge the traditional model of synesthesia as a unidirectional phenomenon and have implications of the neuronal communications that underlie perception in general.

Searching for Shereshevskii: What is superior about the memory of synaesthetes?

Some individuals with superior memory, such as the mnemonist Shereshevskii (Luria, 1968), are known to have synaesthesia. However, the extent to which superior memory is a general characteristic of synaesthesia is unknown, as is the precise cognitive mechanism by which synaesthesia affects memory. This study demonstrates that synaesthetes tend to report subjectively better than average memory and that these reports are borne out with objective testing. Synaesthetes experiencing colours for words show better memory than matched controls for stimuli that induce synaesthesia (word lists) relative to stimuli that do not (an abstract figure). However, memory advantages are not limited to material that elicits synaesthesia because synaesthetes demonstrate enhanced memory for colour per se (which does not induce a synaesthetic response). Our results suggest that the memory enhancement found in synaesthetes is related to an enhanced retention of colour in both synaesthetic and nonsynaesthetic situations. Furthermore, this may account for the fact that synaesthetic associations, once formed, remain highly consistent.

Cross-modal, bidirectional priming in grapheme-color synesthesia

Grapheme-color synesthetes perceive achromatic graphemes to be inherently colored. In this study grapheme-color synesthetes and non-synesthetes discriminated (1) the color of visual targets presented along with aurally presented digit primes, and (2) the identity of aurally presented digit targets presented with visual color primes. Reaction times to visual color targets were longer when the color of the target was incongruent with the synesthetic percept reported for the prime. Likewise, discriminating aurally presented digit targets took longer when the color of the prime was incongruent with the synesthetic percept for the target. These priming effects were absent in non-synesthetes. We conclude that binding between digits and colors in grapheme-color synesthetes can occur bidirectionally across senses. The results are in line with the idea that synesthesia is the result of linking inducing stimuli (e.g. digits) to synesthetic percepts (colors) at an abstract - supra-modal - conceptual level of processing.

Implicit interactions between number and space in digit-color synesthesia

In digit-color synesthesia, a variant of grapheme-color synesthesia, digits trigger an additional color percept. Recent work on number processing in synesthesia suggests that colors can implicitly elicit numerical representations in digit-color synesthetes implying that synesthesia is bidirectional. Furthermore, morphometric investigations revealed structural differences in the parietal cortex of grapheme-color synesthetes, i.e., in the brain region where interactions between number and space occur in non-synesthetic subjects. Based upon these previous findings, we here examined whether implicitly evoked numerical representations interact with spatial representations in synesthesia in such a way that even a non-numerical, visuo-spatial task (here: line bisection) is modulated, i.e., whether synesthetes exhibit a systematic bisection bias for colored lines. Thirteen digit-color synesthetes were asked to bisect two sets of lines which were colored in their individual synesthetic colors associated with a small or a large digit, respectively. For all colored line stimuli combined, digit-color synesthetes showed--like control subjects (n = 13, matched for age, gender, IQ and handedness)--a pseudo-neglect when bisecting colored lines. Measuring the color-induced change of the bisection bias (i.e., comparing the biases when bisecting lines colored according to a small number vs those lines corresponding to a large number) revealed that only digit-color synesthetes were significantly influenced by line color. The results provide further evidence for the bidirectional nature of synesthesia and support the concept of a mental number line. In addition, they extend previous reports on bidirectionality in synesthesia by showing that even non-numerical, visuo-spatial performance can be modulated by implicit bidirectional processes.

Why we are not all synesthetes (not even weakly so)

A little over a decade ago, Martino and Marks (Current Directions in Psychological Science 10:61-65, 2001) put forward the influential claim that cases of intuitive matchings between stimuli in different sensory modalities should be considered as a weak form of synesthesia. Over the intervening years, many other researchers have agreed-at the very least, implicitly-with this position (e.g., Bien, ten Oever, Goebel, & Sack NeuroImage 59:663-672, 2012; Eagleman Cortex 45:1266-1277, 2009; Esterman, Verstynen, Ivry, & Robertson Journal of Cognitive Neuroscience 18:1570-1576, 2006; Ludwig, Adachi, & Matzuzawa Proceedings of the National Academy of Sciences of the United States of America 108:20661-20665, 2011; Mulvenna & Walsh Trends in Cognitive Sciences 10:350-352, 2006; Sagiv & Ward 2006; Zellner, McGarry, Mattern-McClory, & Abreu Chemical Senses 33:211-222:2008). Here, though, we defend the separatist view, arguing that these cases are likely to form distinct kinds of phenomena despite their superficial similarities. We believe that crossmodal correspondences should be studied in their own right and not assimilated, either in terms of the name used or in terms of the explanation given, to synesthesia. To conflate these two phenomena is both inappropriate and potentially misleading. Below, we critically evaluate the evidence concerning the descriptive and constitutive features of crossmodal correspondences and synesthesia and highlight how they differ. Ultimately, we wish to provide a general definition of crossmodal correspondences as acquired, malleable, relative, and transitive pairings between sensory dimensions and to provide a framework in which to integrate the nonsystematic cataloguing of new cases of crossmodal correspondences, a tendency that has increased in recent years.

Training, hypnosis, and drugs: artificial synaesthesia, or artificial paradises?

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 the majority of cases of developmental synaesthesia in terms of learning patterns: First, their resistance to change through exposure once the synaesthetic repertoire has been fixed; 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 development of synaesthesia, probably only in those with a genetic predisposition to develop the condition.

Two plus blue equals green: grapheme-color synesthesia allows cognitive access to numerical information via color

In grapheme-color synesthesia, graphemes (e.g., numbers or letters) evoke color experiences. It is generally reported that the opposite is not true: colors will not generate experiences of graphemes or their associated information. However, recent research has provided evidence that colors can implicitly elicit symbolic representations of associated graphemes. Here, we examine if these representations can be cognitively accessed. Using a mathematical verification task replacing graphemes with color patches, we find that synesthetes can verify such problems with colors as accurately as with graphemes. Doing so, however, takes time: ~250 ms per color. Moreover, we find minimal reaction time switch-costs for switching between computing with graphemes and colors. This demonstrates that given specific task demands, synesthetes can cognitively access numerical information elicited by physical colors, and they do so as accurately as with graphemes. We discuss these results in the context of possible cognitive strategies used to access the information.

Enhanced dimension-specific visual working memory in grapheme-color synesthesia

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Grapheme–color synesthetes display superior color working memory than controls.

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This effect is independent of color familiarity and color discrimination abilities.

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Controls and synesthetes do not differ in grapheme working memory.

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These results support enhanced color processing in synesthesia.

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They also support research linking sensory processing and working memory.

There is emerging evidence that the encoding of visual information and the maintenance of this information in a temporarily accessible state in working memory rely on the same neural mechanisms. A consequence of this overlap is that atypical forms of perception should influence working memory. We examined this by investigating whether having grapheme–color synesthesia, a condition characterized by the involuntary experience of color photisms when reading or representing graphemes, would confer benefits on working memory. Two competing hypotheses propose that superior memory in synesthesia results from information being coded in two information channels (dual-coding) or from superior dimension-specific visual processing (enhanced processing). We discriminated between these hypotheses in three n-back experiments in which controls and synesthetes viewed inducer and non-inducer graphemes and maintained color or grapheme information in working memory. Synesthetes displayed superior color working memory than controls for both grapheme types, whereas the two groups did not differ in grapheme working memory. Further analyses excluded the possibilities of enhanced working memory among synesthetes being due to greater color discrimination, stimulus color familiarity, or bidirectionality. These results reveal enhanced dimension-specific visual working memory in this population and supply further evidence for a close relationship between sensory processing and the maintenance of sensory information in working memory.

Survival of the synesthesia gene: why do people hear colors and taste words?

This Unsolved Mystery reviews the biological evidence for why synesthesia, a condition in which stimuli presented through one modality spontaneously evoke sensations in an unrelated modality, may have been conserved in the population.

Synesthesia is a perceptual experience in which stimuli presented through one modality will spontaneously evoke sensations in an unrelated modality. The condition occurs from increased communication between sensory regions and is involuntary, automatic, and stable over time. While synesthesia can occur in response to drugs, sensory deprivation, or brain damage, research has largely focused on heritable variants comprising roughly 4% of the general population. Genetic research on synesthesia suggests the phenomenon is heterogeneous and polygenetic, yet it remains unclear whether synesthesia ever provided a selective advantage or is merely a byproduct of some other useful selected trait. Progress in uncovering the genetic basis of synesthesia will help us understand why synesthesia has been conserved in the population.

Categorization of Synaesthesia

This article is an attempt to synthesize the current knowledge about synaesthesia from many fields such as literature, arts, multimedia, medicine, or psychology. The main goal of this paper is to classify various types and forms of synaesthesia. Besides developmental synaesthesia being likely to play a crucial role in developing cognitive functions (constitutional or neonatal synaesthesia) there are types of synaesthesia acquired during adulthood (e.g., phantom or artificial synaesthesia), momentary synaesthesia triggered temporarily in people who do not show signs of synaesthesia every day (e.g., virtual, narcotic, or posthypnotic synaesthesia), and associational synaesthesia which, semantically speaking, refers to some universal sense relations (e.g., literary, artistic, and multimedia synaesthesia). There is a hypothesis that every kind of synaesthesia holds a different function—compensatory or integrative. It was suggested that synaesthesia can be described in one dimension, showing the intensity of this phenomenon. The stronger types of synaesthesia are: semantic, conceptual, intermodal, synthetic, comprehensive, external and bidirectional. The weaker types of synaesthesia are: sensory, perceptual, intramodal, analytic, partial, internal and unidirectional. There are huge individual differences in the manner that synaesthesia presents itself. By including a classification of kinds, types, and forms of synaesthesia into future experimental research will ensure a better understanding of the nature of this phenomenon, its mechanisms and the role that it plays in developing cognitive processes.

Contextual Priming in Grapheme–Color Synesthetes and Yoked Controls: 400 msec in the Life of a Synesthete

Grapheme–color synesthesia is a heritable trait where graphemes (“2”) elicit the concurrent perception of specific colors (red). Researchers have questioned whether synesthetic experiences are meaningful or simply arbitrary associations and whether these associations are perceptual or conceptual. To address these fundamental questions, ERPs were recorded as 12 synesthetes read statements such as “The Coca-Cola logo is white and 2,” in which the final grapheme induced a color that was either contextually congruous (red) or incongruous (“…white and 7,” for a synesthetes who experienced 7 as green). Grapheme congruity was found to modulate the amplitude of the N1, P2, N300, and N400 components in synesthetes, suggesting that synesthesia impacts perceptual as well as conceptual aspects of processing. To evaluate whether observed ERP effects required the experience of colored graphemes versus knowledge of grapheme–color pairings, we ran three separate groups of controls on a similar task. Controls trained to a synesthete's associations elicited N400 modulation, indicating that knowledge of grapheme–color mappings was sufficient to modulate this component. Controls trained to synesthetic associations and given explicit visualization instructions elicited both N300 and N400 modulations. Lastly, untrained controls who viewed physically colored graphemes (“2” printed in red) elicited N1 and N400 modulations. The N1 grapheme congruity effect began earlier in synesthetes than colored grapheme controls but had similar scalp topography. Data suggest that, in synesthetes, achromatic graphemes engage similar visual processing networks as colored graphemes in nonsynesthetes and are in keeping with models of synesthesia that posit early feed-forward connections between form and color processing areas in extrastriate cortex. The P2 modulation was unique to the synesthetes and may reflect neural activity that underlies the conscious experience of the synesthetic induction.

The eye amputated - consequences of eye amputation with emphasis on clinical aspects, phantom eye syndrome and quality of life

In this thesis the term eye amputation (EA) covers the removing of an eye by: evisceration, enucleation and exenteration. Amputation of an eye is most frequently the end-stage in a complicated disease, or the primary treatment in trauma and neoplasm. In 2010 the literature is extensive due to knowledge about types of surgery, implants and surgical technique. However, not much is known about the time past surgery.

THE PURPOSE OF THE PHD THESIS WAS

To identify the number of EA, the causative diagnosis and the indication for surgical removal of the eye, the chosen surgical technique and to evaluate a possible change in surgical technique in Denmark from 1996 until 2003 (paper I); To describe the phantom eye syndrome and its prevalence of visual hallucinations, phantom pain and phantom sensations (paper II); To characterise the quality of phantom eye pain, including its intensity and frequency among EA patients. We attempted to identify patients with increased risk of developing pain after EA and investigated if preoperative pain is a risk factor for a later development of phantom pain (paper III); In addition we wanted to investigate the health related quality of life, perceived stress, self rated health, job separation due to illness or disability and socio-economic position of the EA in comparison with the general Danish population (paper IV).

THE STUDIES WERE BASED ON

Records on 431 EA patients, clinical ophthalmological examination and an interview study of 173 EA patients and a questionnaire answered by 120 EA patients.

CONCLUSIONS

The most frequent indications for EA in Denmark were painful blind eye (37%) and neoplasm (34%). During the study period 1996-2003, the annual number of eye amputations was stable, but an increase in bulbar eviscerations was noticed. Orbital implants were used with an increasing tendency until 2003. The Phantom eye syndrome is frequent among EA patients. Visual hallucinations were described by 42% of the patients. The content were mainly elementary visual hallucinations, with white or colored light as a continuous sharp light or as moving dots. The most frequent triggers were darkness, closing of the eyes, fatigue and psychological stress. Fifty-four percent of the patients had visual hallucinations more than once a week. Ten patients were so visually disturbed that it interfered with their daily life. Approximately 23% of all EA experience phantom pain for several years after the surgery. Phantom pain was reported to be of three different qualities: (i) cutting, penetrating, gnawing or oppressive (n=19); (ii) radiating, zapping or shooting (n=8); (iii) superficial burning or stinging (n=5); or a mixture of these different pain qualities (n=7). The median intensity on a visual analogue scale, ranging from 0 to 100, was 36 [range: 1-89]. One-third of the patients experienced phantom pain every day. Chilliness, windy weather and psychological stress/fatigue were the most commonly reported triggers for pain. Factors associated with phantom pain were: ophthalmic pain before EA, the presence of implant and a patient reported high degree of conjunctival secretion. A common reason for EA is the presence of a painful blind eye. However, one third of these patients continue to have pain after the EA. Phantom sensations were present in 2% of the patients. The impact of an eye amputation is considerable. EA patients have poorer health related quality of life, poorer self-rated health and more perceived stress than does the general population. The largest differences in health related quality of life between the EA patients and the general population were related to role limitations due to emotional problems and mental health. Patients with the indication painful blind eye are having lower scores in all aspects of health related quality of life and perceived stress than patients with the indication neoplasm and trauma. The percentage of eye amputated which is divorced or separated was twice as high as in the general population. Furthermore, 25% retired or changed to part-time jobs due to eye disease and 39.5% stopped participating in leisure activities due to their EAs.

Bidirectionality in synesthesia: evidence from a multiplication verification task

Color-grapheme synesthetes automatically perceive achromatic numbers as colored (e.g., 7 is turquoise). Up until recently, synesthesia was believed to be unidirectional. For instance, the number 7 gives rise to the percept of turquoise but the perception of turquoise does not trigger the number 7. However, some recent studies argue for bidirectional connections Cohen Kadosh et al., 2005; Johnson et al., 2007; Knoch et al., 2005). In this study, a multiplication verification task (e.g., 7 x 2 = 14, true/false?) was used to test bidirectionality. In agreement with previous studies we observed that the presentation of colors evokes numerical magnitudes. The current findings add two important notions to previous studies: (a) The influence of color on the processing of numerical information can be extended to multiplication verification tasks and (b) The perception of color can both facilitate and interfere with the processing of digit-related information.

Phantom eye syndrome: types of visual hallucinations and related phenomena

PURPOSE

To describe the prevalence of phantom eye syndrome in eye-amputated patients, to give a description of visual hallucinations, and to identify triggers, stoppers, and emotions related to visual hallucinations.

METHODS

The hospital database was screened, using surgery codes for patients who had received ocular evisceration, enucleation, or secondary implantation of an orbital implant in the period 1993-2003. A total of 267 patients was found and invited to participate, 173 accepted. Patients who accepted participation had their records reviewed, and a structured interview about visual hallucinations and pain was performed by one trained questioner (M.L.R.R.).

RESULTS

The prevalence of phantom eye syndrome was 51%. Elementary visual hallucinations were present in 36%, complex visual hallucinations in only 1%, and other visual hallucinations in 14%. The elementary visual hallucinations were most often white or colored light, as a continuous sharp light or as moving dots. The most frequent triggers were darkness, closing of the eyes, fatigue, and psychological stress; 54% of patients had the experience more than once a week. Ten patients were so visually disturbed that it interfered with their daily life.

CONCLUSIONS

Phantom eye syndrome is common, and the authors recommend that surgeons inform their patients about the phenomenon.

Priming Letters by Colors: Evidence for the Bidirectionality of Grapheme–Color Synesthesia

In synesthesia, stimulation of one sensory modality leads to a percept in another nonstimulated modality, for example, graphemes trigger an additional color percept in grapheme–color synesthesia, which encompasses the variants letter–color and digit–color synesthesia. Until recently, it was assumed that synesthesia occurs strictly unidirectional: Although the perception of a letter induces a color percept in letter–color synesthetes, they typically do not report that colors trigger the percept of a letter. Recent data on number processing in synesthesia suggest, however, that colors can implicitly elicit numerical representations in digit–color synesthetes, thereby questioning unidirectional models of synesthesia. Using a word fragment completion paradigm in 10 letter–color synesthetes, we show here for the first time that colors can implicitly influence lexical search. Our data provide strong support for a bidirectional nature of grapheme–color synesthesia and, in general, may allude to the mechanisms of cross-modality interactions in the human brain.

Multiple dimensions in bi-directional synesthesia

Grapheme-color synesthetes report seeing a specific color when a number is perceived. The reverse, the synesthetic experience of a specific grapheme after the percept of a color is extremely rare. However, recent studies have revealed these interactions at both behavioral and neurophysiological levels. We investigated whether similar neuronal processes (i.e. perceptual and/or attentional) may underlie this bi-directional interaction by measuring event-related potentials (ERPs) during both a number-color and color-number priming task. In addition, we investigated the unitarity of synesthesia by comparing two distinct subtypes of synesthetes, projectors and associators, and assessed whether consistencies between measures (i.e. behavioral and electrophysiological) were present across synesthetes. Our results show longer reaction times for incongruent compared with congruent trials in both tasks. This priming effect is also present in the P3b latency (parietal electrode site) and P3a amplitude (frontal electrode site) of the ERP data. Interestingly, projector and associator synesthetes did not reveal distinct behavioral or electrophysiological patterns. Instead, a dissociation was found when synesthetes were divided in two groups on the basis of their behavioral data. Synesthetes with a large behavioral priming effect revealed ERP modulation at the frontal and parietal electrode sites, whereas synesthetes with a small priming effect revealed a frontal effect only. Together, these results show, for the first time, that similar neural mechanisms underlie bi-directional synesthesia in synesthetes that do not report a synesthetic experience of a grapheme when a color is perceived. In addition, they add support for the notion of the existence of both 'lower' and 'higher' synesthetes.

Is undiagnosed synaesthesia a confounding factor in the interpretation of MRI images?

Synaesthesia is a condition in which stimulation of a sensory modality evokes another sensation in the same or a different sensory modality. Currently, synaesthesia is considered a neurological condition that involves crosstalk between brain regions. Given the numerous anatomical and functional connections within the brain, it is possible that undiagnosed synaesthesia may influence the results of functional magnetic resonance imaging (fMRI) studies or even structural MRI. In this paper, we investigated the currently available literature to determine if and how the sensations invoked by synaesthesia could impact fMRI and structural MRI. Our investigation found that synaesthesia can have a profound impact on fMRI studies of sensory and cognitive functions, and there is evidence to suggest structural connections in the brain are also altered. Given the low prevalence of synaesthesia, the likelihood of synaesthesia being a confounding factor in fMRI studies of patient groups is small; however, determining the presence of synaesthesia is important for investigating individual patients.

Of Colored Numbers and Numbered Colors

Grapheme-color synesthetes experience a specific color when they see a grapheme but they do not report to perceive a grapheme when a color is presented. In this study, we investigate whether color can still evoke number-processes even when a vivid number experience is absent. We used color-number and number-color priming, both revealing faster responses in congruent compared to incongruent conditions. Interestingly, the congruency effect was of similar magnitude for both conditions, and a numerical distance effect was present only in the color-number priming task. In addition, a priming task in which synesthetes had to judge the parity of a colored number revealed faster responses in parity congruent than in parity incongruent trials. These combined results demonstrate that synesthesia is indeed bi-directional and of similar strength in both directions. Furthermore, they illustrate the precise nature of these interactions and show that the direction of these interactions is determined by task demands, not by the more vividly experienced aspect of the stimulus.

The neuronal correlate of bidirectional synesthesia: a combined event-related potential and functional magnetic resonance imaging study

The neuronal correlate of a rare explicit bidirectional synesthesia was investigated with numerical and physical size comparison tasks using both functional magnetic resonance imaging and event-related potentials. Interestingly, although participant I.S. exhibited similar congruity effects for both tasks at the behavioral level, subsequent analyses of the imaging data revealed that different brain areas were recruited for each task, and in different time windows. The results support: (1) the genuineness of bidirectional synesthesia at the neuronal level, (2) the possibility that discrepancy in the neuronal correlates of synesthesia between previous studies might be task-related, and (3) the possibility that synesthesia might not be a unitary phenomenon.

Is the Sky 2? Contextual Priming in Grapheme-Color Synaesthesia

Grapheme-color synaesthesia is a neurological phenomenon in which particular graphemes, such as the numeral 9, automatically induce the simultaneous perception of a particular color, such as the color red. To test whether the concurrent color sensations in grapheme-color synaesthesia are treated as meaningful stimuli, we recorded event-related brain potentials as 8 synaesthetes and 8 matched control subjects read sentences such as “Looking very clear, the lake was the most beautiful hue of 7.” In synaesthetes, but not control subjects, congruous graphemes, compared with incongruous graphemes, elicited a more negative N1 component, a less positive P2 component, and a less negative N400 component. Thus, contextual congruity of synaesthetically induced colors altered the brain response to achromatic graphemes beginning 100 ms postonset, affecting pattern-recognition, perceptual, and meaning-integration processes. The results suggest that grapheme-color synaesthesia is automatic and perceptual in nature and also suggest that the connections between colors and numbers are bidirectional.

A synesthetic walk on the mental number line: The size effect

Are small and large numbers represented similarly or differently on the mental number line? The size effect was used to argue that numbers are represented differently. However, recently it has been argued that the size effect is due to the comparison task and is not derived from the mental number line per se. Namely, it is due to the way that the mental number line is mapped onto the task-relevant output component. Here synesthesia was used to disentangle these two alternatives. In two naming experiments a digit-color synesthete showed that the congruity effect was modulated by number size. These results support the existence of a mental number line with a vaguer numerical representation as numbers increase in size. In addition, the results show that in digit-color synesthesia, colors can evoke numerical representation automatically.

Synesthesia and number cognition in children

Grapheme-color synesthesia, when achromatic digits evoke an experience of a specific color (photisms), has been shown to be consistent, involuntary, and linked with number concept in adults, yet there have been no comparable investigations with children. We present a systematic study of grapheme-color synesthesia in children aged between 7 and 15 years. Here we show that such children (but not children with phoneme-color synesthesia) experience involuntary difficulties in numerical tasks when digits are presented in colors incongruent with their photisms. Synesthesia in children may thus have important consequences for certain aspects of numerical cognition.

Colours sometimes count: awareness and bidirectionality in grapheme-colour synaesthesia

Three experiments were conducted with 10 grapheme-colour synaesthetes and 10 matched controls to investigate (a) whether awareness of the inducer grapheme is necessary for synaesthetic colour induction and (b) whether grapheme-colour synaesthesia may be bidirectional in the sense that not only do graphemes induce colours, but that colours influence the processing of graphemes. Using attentional blink and Stroop paradigms with digit targets, we found that some synaesthetes did report "seeing" synaesthetic colours even when they were not able to report the inducing digit. Moreover, congruency effects (effects of matching the colour of digit presentation with the synaesthetic colour associated with that digit) suggested that grapheme-colour synaesthesia can be bidirectional, at least for some synaesthetes.

When conditioned responses "fire back": bidirectional cross-activation creates learning opportunities in synesthesia

In grapheme-color synesthesia, the letter "c" printed in black may be experienced as red, but typically the color red does not trigger the experience of the letter "c." Therefore, at the level of subjective experience, cross-activation is usually unidirectional. However, recent evidence from digit-color synesthesia suggests that at an implicit level bidirectional cross-activation can occur. Here we demonstrate that this finding is not restricted to this specific type of synesthesia. We introduce a new method that enables the investigation of bidirectionality in other types of synesthesia. We found that a group of grapheme-color synesthetes, but not a control group, showed a startle in response to a color-inducing grapheme after a startle response was conditioned to the specific corresponding color. These results implicate that when the startle response was associated with the real color an association between shock and the grapheme was also established. By this mechanism (i.e. implicit cross-activation) the conditioned response to the real color generalized to the synesthetic color. We suggest that parietal brain areas are responsible for this neural backfiring.

Can synaesthesia research inform cognitive science?

The renaissance of synaesthesia research has produced many insights regarding the aetiology and mechanisms that might underlie this intriguing phenomenon, which abnormally binds features between and within modalities. Synaesthesia is interesting in its own right, but whether it contributes to our knowledge of neurocognitive systems that underlie non-synaesthete experience is an open question. In this review, we show that results from the field of synaesthesia can constrain cognitive theories in numerical cognition, automaticity, crossmodal interaction and awareness. Therefore, research of synaesthesia provides a unique window into other domains of cognitive neuroscience. We conclude that the study of synaesthesia could advance our understanding of the normal and abnormal human brain and cognition.

Synaesthesia

Synaesthesia is the intriguing, involuntary experience of feeling one sensation in response to a different sensory stimulus. Recognised since described in 1890 by John Locke and clarified by Galton in the 1880s, it has been analysed in the last 50 years. Grapheme-colour synaesthesia is the commonest form, but many other sensory linkages are reported. Experiments show that it is a genuine immediate perception, not merely a memory or learned association. Many of the mechanisms posited are based on indirect methods, and we know little of the neurophysiological mechanisms.

Variants of synesthesia interact in cognitive tasks: Evidence for implicit associations and late connectivity in cross-talk theories

This study examines the interaction between two types of synesthesia: ordinal linguistic personification (OLP; the involuntary association of animate qualities such as gender/personality to linguistic units such as letters/numbers/days) and grapheme-color synesthesia (the involuntary association of colors to letters and/or numbers). By examining both variants in the same individual we aim to: (a) show that features of different synesthetic variants interact in cognitive tasks, (b) provide a cognitive model of this interaction, and (c) constrain models of the underlying neurological roots of this connectivity. Studies have shown inhibition in Stroop-type tasks for naming font colors that clash with synesthetic colors (e.g. slower naming of green font for synesthetically red letters). We show that Stroop-type slow-down occurs only when incongruent colors come from other letters with matching (but not mis-matching) gender (experiment 2). We also measure the speed of OLP gender judgments (e.g. a=female; experiment 1) and show that response times are slowed by incongruent colors from other letters with mis-matching (but not matching) genders. Our studies suggest that synesthetic variants interact and that their concurrents can become implicitly connected without mediation from inducing stimuli. We interpret these findings in light of recent developmental data showing protracted heterochronous neuronal development in humans, which continues through adolescence in parietal, frontal and perisylvian areas.

Neurocognitive mechanisms of synesthesia

Synesthesia is a condition in which stimulation of one sensory modality causes unusual experiences in a second, unstimulated modality. Although long treated as a curiosity, recent research with a combination of phenomenological, behavioral, and neuroimaging methods has begun to identify the cognitive and neural basis of synesthesia. Here, we review this literature with an emphasis on grapheme-color synesthesia, in which viewing letters and numbers induces the perception of colors. We discuss both the substantial progress that has been made in the past fifteen years and some open questions. In particular, we focus on debates in the field relating to the neural basis of synesthesia, including the relationship between synesthesia and attention and the role of meaning in synesthetic colors. We propose that some, but probably not all, of these differences can be accounted for by differences in the synesthetes studied and discuss some methodological implications of these individual differences.

When a line is a number: Color yields magnitude information in a digit-color synesthete

The phenomenon of synesthesia has received a great deal of interest recently in the scientific literature. Many previous studies stressed the unidirectional nature of this phenomenon. For example, color-grapheme synesthetes automatically perceive achromatic numbers as colored (e.g. 7 is turquoise). Conversely, colors do not automatically give rise to any sort of number experience (e.g. turquoise is 7). In contrast to the common view, we report on a digit-color synesthete in whom colors can evoke numerical representations in the absence of any digit presentation. It is concluded that in synesthesia there is a reciprocal rather than unidirectional flow of information between dimensions.

Crossmodal interactions: lessons from synesthesia

Synesthesia is a condition in which stimulation in one modality also gives rise to a perceptual experience in a second modality. In two recent studies we found that the condition is more common than previously reported; up to 5% of the population may experience at least one type of synesthesia. Although the condition has been traditionally viewed as an anomaly (e.g., breakdown in modularity), it seems that at least some of the mechanisms underlying synesthesia do reflect universal crossmodal mechanisms. We review here a number of examples of crossmodal correspondences found in both synesthetes and nonsynesthetes including pitch-lightness and vision-touch interaction, as well as cross-domain spatial-numeric interactions. Additionally, we discuss the common role of spatial attention in binding shape and color surface features (whether ordinary or synesthetic color). Consistently with behavioral and neuroimaging data showing that chromatic-graphemic (colored-letter) synesthesia is a genuine perceptual phenomenon implicating extrastriate cortex, we also present electrophysiological data showing modulation of visual evoked potentials by synesthetic color congruency.