Synesthesia for color is linked to improved color perception but reduced motion perception

Perceptual processing links autism and synesthesia: A co-twin control study

Synesthesia occurs more commonly in individuals fulfilling criteria for an autism spectrum diagnosis than in the general population. It is associated with autistic traits and autism-related perceptual processing characteristics, including a more detail-focused attentional style and altered sensory sensitivity. In addition, these characteristics correlate with the degree of grapheme-color synesthesia (consistency of grapheme-color associations) in non-synesthetes. We investigated a predominantly non-synesthetic twin sample, including individuals fulfilling criteria for an autism spectrum diagnosis or other neurodevelopmental disorders (n = 65, 14-34 years, 60% female). We modelled linear relationships between the degree of grapheme-color synesthesia and autistic traits, sensory sensitivity, and visual perception, both within-twin pairs (22 pairs) where all factors shared by twins are implicitly controlled (including 50-100% genetics), and across the entire cohort. We found that the degree of grapheme-color synesthesia was associated with autistic traits within the domain of Attention to Details and with sensory hyper-, but not hypo-sensitivity. These associations were stronger within-twin pairs than across the sample. Further, twins with a higher degree of grapheme-color synesthesia were better than their co-twins at identifying fragmented images (Fragmented Pictures Test). This is the first twin study on the association between synesthesia and autism-related perceptual features and traits. The results suggest that investigating these associations within-twin pairs, implicitly adjusting for potential confounding factors shared by twins, is more sensitive than doing so in non-related individuals. Consistent with previous findings, the results suggest an association between the degree of grapheme-color synesthesia and autism-related perceptual features, while utilizing a different measure for sensory sensitivity. The novel finding of enhanced fragmented picture integration in twins with a higher degree of grapheme-color synesthesia challenges the view of a generally more detail-focused attentional style in synesthesia and might be related to enhanced memory or mental imagery in more synesthetic individuals.

Synaesthesia as a model system for understanding variation in the human mind and brain

The aim of this article is to reposition synaesthesia as model system for understanding variation in the construction of the human mind and brain. People with synaesthesia inhabit a remarkable mental world in which numbers can be coloured, words can have tastes, and music is a visual spectacle. Key questions remain unanswered about why it exists, and how the study of synaesthesia might inform theories of the human mind. This article argues we need to rethink synaesthesia as not just representing exceptional experiences, but as a product of an unusual neurodevelopmental cascade from genes to brain to cognition of which synaesthesia is only one outcome. Specifically, differences in the brains of synaesthetes support a distinctive way of thinking (enhanced memory, imagery etc.) and may also predispose towards particular clinical vulnerabilities. In effect, synaesthesia can act as a paradigmatic example of a neuropsychological approach to individual differences.

Enhanced perception and memory: Insights from synesthesia and expertise

Memory research has identified many strategies to enhance memory. However, natural foundations of enhanced memory are vastly underexplored. Interestingly, numerous studies show that synesthesia is associated with enhanced memory performance. Although it has been hypothesized for years that wider changes in visual perception are closely linked with enhanced memory functions in synesthesia, the hypothesis has never been directly put to the test. Here, we investigated whether visual perceptual abilities in synesthesia are linked with higher memory performance by comparing synesthetes who experience colors for letters with non-synesthetic color experts and non-synesthetic individuals from the more general population. Our results showed that synesthesia and expertise share a common profile of enhanced visual perceptual ability and memory in contrast to non-synesthetic individuals from the more general population. Overall, our findings suggest that visual perception and visual memory are more closely connected than previously thought.

The time course of synaesthetic colour perception

Grapheme-colour synaesthesia is a neurodevelopmental condition wherein perception of numbers and letters consistently and involuntarily elicits concurrent experiences of colour photisms. Accumulating evidence suggests that heterogeneity in the visuospatial phenomenology of synaesthesia is attributable to the operation of top-down processes underlying photisms experienced as representations in associator synaesthetes and bottom-up processes subserving photisms experienced as spatially localized in projector synaesthetes. An untested corollary of this hypothesis is that bottom-up mechanisms will actuate earlier photism perception in projector than associator synaesthetes. We tested this prediction in a pre-registered study wherein associators and projectors completed adaptive temporal order judgement tasks for graphemes, colours, and photisms. In corroboration of the hypothesis of differential photism access across subtypes, projectors displayed lower photism colour thresholds than associators whereas the two subtypes did not significantly differ in veridical colour thresholds. Synaesthetes did not differ in grapheme or colour thresholds relative to non-synaesthete controls. These results are consistent with the proposal of differential neural mechanisms underlying photism perception in subtypes of grapheme-colour synaesthesia and warrant renewed attention to heterogeneity in the mechanisms and phenomenology of this condition.

Synesthesia does not help to recover perceptual dominance following flash suppression

Grapheme-colour synesthesia occurs when letters or numbers elicit an abnormal colour sensation (e.g., printed black letters are perceived as coloured). This phenomenon is typically reported following explicit presentation of graphemes. Very few studies have investigated colour sensations in synesthesia in the absence of visual awareness. We took advantage of the dichoptic flash suppression paradigm to temporarily render a stimulus presented to one eye invisible. Synesthetic alphanumeric and non-synesthetic stimuli were presented to 21 participants (11 synesthetes) in achromatic and chromatic experimental conditions. The test stimulus was first displayed to one eye and then masked by a sudden presentation of visual noise in the other eye (flash suppression). The time for an image to be re-perceived following the onset of the suppressive noise was calculated. Trials where there was no flash suppression performed but instead mimicked the perceptual suppression of the flash were also tested. Results showed that target detection by synesthetes was significantly better than by controls in the absence of flash suppression. No difference was found between the groups in the flash suppression condition. Our findings suggest that synesthesia is associated with enhanced perception for overt recognition, but does not provide an advantage in recovering from a perceptual suppression. Further studies are needed to investigate synesthesia in relation to visual awareness.

Synaesthesia and autism: Different developmental outcomes from overlapping mechanisms?

Synaesthesia, a mixing of the senses, is more common in individuals with autism. Here, we review the evidence for the association between synaesthesia and autism with regard to their genetic background, brain connectivity, perception, cognitive mechanisms and their contribution to exceptional talents. Currently, the overlap between synaesthesia and autism is established most convincingly at the level of alterations in sensory sensitivity and perception, with synaesthetes showing autism-like profiles of sensory sensitivity and a bias towards details in perception. Shared features may include a predominance of local over global connectivity in the brain. When autism and synaesthesia co-occur in the same individual, the chance of developing heightened cognitive and memory abilities is increased. We discuss how the same theoretical models could potentially explain both conditions. Given the evidence, we believe the phenotypical overlap between autism and synaesthesia has been established clearly enough to invite future research to confirm overlapping mechanisms.

A case of co-occuring synesthesia, autism, prodigious talent and strong structural brain connectivity

BACKGROUND

Synesthesia is a sensory phenomenon where certain domain-specific stimuli trigger additional sensations of e.g. color or texture. The condition occurs in about 4% of the general population, but is overrepresented in individuals with Autism Spectrum Disorder (ASD), where it might also be associated with the presence of prodigious talents.

CASE PRESENTATION

Here we describe the case of a young transsexual man with Asperger Syndrome, synesthesia and a prodigious talent for foreign language acquisition. In our case, not only letters, numbers, spoken words, music, noises, weekdays and months lead to highly consistent, vivid color sensations but also his own and others' emotions, geometric shapes, any mathematical symbol, and letters from an unfamiliar alphabet (Hebrew). These color associations seem to aid categorization, differentiation and storage of information and might thereby contribute to the young man's language acquisition ability. We investigated the young man's structural brain connectivity in comparison to adults with or without ASD, applying global fiber tracking to diffusion-weighted Magnetic Resonance Imaging (MRI) data. The case presented with increased connectivity, especially between regions involved in visual and emotion processing, memory, and higher order associative binding regions. An electroencephalography experiment investigating synesthetic color and shape sensations while listening to music showed a negligible occipital alpha suppression, indicating that these internally generated synesthetic sensations derive from a different brain mechanism than when processing external visual information.

CONCLUSIONS

Taken together, this case study endorses the notion of a link between synesthesia, prodigious talent and autism, adding to the currently still sparse literature in this field. It provides new insights into the possible manifestations of synesthesia in individuals with ASD and its potential contribution to prodigious talents in people with an otherwise unexceptional cognitive profile. Additionally, this case impressively illustrates how synesthesia can be a key element not only of sensory perception but also social and emotional processing and contributes to existing evidence of increased brain connectivity in association with synesthesia.

Association between synesthetic colors and sensitivity to physical colors changed by type of synesthetic experience in grapheme-color synesthesia

Grapheme-color synesthesia is a condition in which visual perception of letters induces simultaneous perception of a specific color. Previous studies indicate that grapheme-color synesthetes are more sensitive to physical colors than non-synesthetes. Synesthetic colors are found to be concentrated in multiple regions of the color space, forming "synesthetic color clusters". The present study investigated whether color sensitivity corresponding to synesthetic color clusters (clustered colors) is higher than color sensitivity that does not correspond to synesthetic color clusters (non-clustered colors). However, we found no difference in the color sensitivity for clustered and non-clustered colors. We also investigated whether the color sensitivity is dependent on the synesthetic experience (associators and projectors). We found that the greater the tendency toward associator characteristics, the greater the sensitivity for clustered colors compared to that for non-clustered colors. Our findings suggest an association between synesthetic colors and physical color sensitivity that is modulated by synesthetic experience.

The Relation Between Autistic Traits, the Degree of Synaesthesia, and Local/Global Visual Perception

Synaesthesia is highly prevalent in autism spectrum disorder. We assessed the relation between the degree of autistic traits (Autism Spectrum Quotient, AQ) and the degree of synaesthesia in a neurotypical population, and hypothesized both are related to a local bias in visual perception. A positive correlation between total AQ scores and the degree of synaesthesia was found, extending previous studies in clinical populations. Consistent with our hypothesis, AQ-attention to detail scores were related to increased performance on an Embedded Figures Task and reduced susceptibility to visual illusions. We found no relation between autistic traits and performance on a motion coherence task, and no relation between synaesthesia and local visual perception. Possibly, this relation is reserved for supra-threshold synaesthetes.

Substantiating synesthesia: a novel aid in a case of grapheme-colour synesthesia and concomitant dyscalculia

Here we present the case of SP, a 21-year-old female with life-long dyscalculia. SP was subsequently diagnosed with grapheme-color synesthesia, a diagnosis that serendipitously catalyzed our development of a novel aid:The digit-color calculator (DCC). The DCC substantiates SP's color concurrents, dramatically ameliorating her difficulties with basic calculations. We envisage the DCC and its analogues may assist others in educational settings, particularly if they experience difficulties with the acquisition of literacy and numeracy. Further devices that leverage synesthesia may also have the potential to improve the quality of life for others with trait synesthesia regardless of concomitant disorder.

A single system account of enhanced recognition memory in synaesthesia

Researchers often adjudicate between models of memory according to the models' ability to explain impaired patterns of performance (e.g., in amnesia). In contrast, evidence from special groups with enhanced memory is very rarely considered. Here, we explored how people with unusual perceptual experiences (synaesthesia) perform on various measures of memory and test how computational models of memory may account for their enhanced performance. We contrasted direct and indirect measures of memory (i.e., recognition memory, repetition priming, and fluency) in grapheme-colour synaesthetes and controls using a continuous identification with recognition (CID-R) paradigm. Synaesthetes outperformed controls on recognition memory and showed a different reaction-time pattern for identification. The data were most parsimoniously accounted for by a single-system computational model of the relationship between recognition and identification. Overall, the findings speak in favour of enhanced processing as an explanation for the memory advantage in synaesthesia. In general, our results show how synaesthesia can be used as an effective tool to study how individual differences in perception affect cognitive functions.

Autistic traits in synaesthesia: atypical sensory sensitivity and enhanced perception of details

In synaesthetes, specific sensory stimuli (e.g. black letters) elicit additional experiences (e.g. colour). Synaesthesia is highly prevalent among individuals with autism spectrum disorder (ASD), but the mechanisms of this co-occurrence are not clear. We hypothesized autism and synaesthesia share atypical sensory sensitivity and perception. We assessed autistic traits, sensory sensitivity and visual perception in two synaesthete populations. In Study 1, synaesthetes (N = 79, of different types) scored higher than non-synaesthetes (N = 76) on the Attention-to-detail and Social skills subscales of the autism spectrum quotient indexing autistic traits, and on the Glasgow Sensory Questionnaire indexing sensory hypersensitivity and hyposensitivity which frequently occur in autism. Synaesthetes performed two local/global visual tasks because individuals with autism typically show a bias towards detail processing. In synaesthetes, elevated motion coherence thresholds (MCTs) suggested reduced global motion perception, and higher accuracy on an embedded figures task suggested enhanced local perception. In Study 2, sequence-space synaesthetes (N = 18) completed the same tasks. Questionnaire and embedded figures results qualitatively resembled Study 1 results, but no significant group differences with non-synaesthetes (N = 20) were obtained. Unexpectedly, sequence-space synaesthetes had reduced MCTs. Altogether, our studies suggest atypical sensory sensitivity and a bias towards detail processing are shared features of synaesthesia and ASD. This article is part of the discussion meeting issue 'Bridging senses: novel insights from synaesthesia'.

Do synaesthesia and mental imagery tap into similar cross-modal processes?

Synaesthesia has previously been linked with imagery abilities, although an understanding of a causal role for mental imagery in broader synaesthetic experiences remains elusive. This can be partly attributed to our relatively poor understanding of imagery in sensory domains beyond vision. Investigations into the neural and behavioural underpinnings of mental imagery have nevertheless identified an important role for imagery in perception, particularly in mediating cross-modal interactions. However, the phenomenology of synaesthesia gives rise to the assumption that associated cross-modal interactions may be encapsulated and specific to synaesthesia. As such, evidence for a link between imagery and perception may not generalize to synaesthesia. Here, we present results that challenge this idea: first, we found enhanced somatosensory imagery evoked by visual stimuli of body parts in mirror-touch synaesthetes, relative to other synaesthetes or controls. Moreover, this enhanced imagery generalized to tactile object properties not directly linked to their synaesthetic associations. Second, we report evidence that concurrent experience evoked in grapheme-colour synaesthesia was sufficient to trigger visual-to-tactile correspondences that are common to all. Together, these findings show that enhanced mental imagery is a consistent hallmark of synaesthesia, and suggest the intriguing possibility that imagery may facilitate the cross-modal interactions that underpin synaesthesic experiences. This article is part of a discussion meeting issue 'Bridging senses: novel insights from synaesthesia'.

Stochastic resonance model of synaesthesia

In synaesthesia, stimulation of one sensory modality evokes additional experiences in another modality (e.g. sounds evoking colours). Along with these cross-sensory experiences, there are several cognitive and perceptual differences between synaesthetes and non-synaesthetes. For example, synaesthetes demonstrate enhanced imagery, increased cortical excitability and greater perceptual sensitivity in the concurrent modality. Previous models suggest that synaesthesia results from increased connectivity between corresponding sensory regions or disinhibited feedback from higher cortical areas. While these models explain how one sense can evoke qualitative experiences in another, they fail to predict the broader phenotype of differences observed in synaesthetes. Here, we propose a novel model of synaesthesia based on the principles of stochastic resonance. Specifically, we hypothesize that synaesthetes have greater neural noise in sensory regions, which allows pre-existing multisensory pathways to elicit supra-threshold activation (i.e. synaesthetic experiences). The strengths of this model are (a) it predicts the broader cognitive and perceptual differences in synaesthetes, (b) it provides a unified framework linking developmental and induced synaesthesias, and (c) it explains why synaesthetic associations are inconsistent at onset but stabilize over time. We review research consistent with this model and propose future studies to test its limits. This article is part of a discussion meeting issue 'Bridging senses: novel insights from synaesthesia'.

Synaesthetic colour associations for Japanese Kanji characters: from the perspective of grapheme learning

One of the fundamental questions about grapheme-colour synaesthesia is how specific associations between the graphemes and colours are formed. We addressed this question by focusing on the determinants of synaesthetic colours for Japanese Kanji characters (logographic characters) using a psycholinguistic approach. Study 1 explored the influence meaning has on synaesthetic colours for Kanji characters representing abstract meanings by examining synaesthetic colours for antonym pairs (i.e. characters with meanings opposed to each other) in Japanese synaesthetes. Results showed that semantic relations influenced the grapheme-colour associations for characters representing abstract meanings in the early stages of learning abstract Kanji, while the influence was reduced in the grapheme-colour associations for those learned later. Study 2 examined the effect that learning new sounds or meanings of graphemes has on synaesthetic colours for those graphemes. Japanese synaesthetes were taught new sounds or new meanings for familiar Kanji characters. Results indicated that acquiring new information for graphemes slightly but significantly reduced the test-retest grapheme-colour association consistency, suggesting that synaesthetic colours can be modulated to reflect the synaesthete's latest knowledge about graphemes. Implications of these findings are discussed from the perspective of the relationship between synaesthesia and grapheme learning. This article is part of a discussion meeting issue 'Bridging senses: novel insights from synaesthesia'.

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.

Grapheme-Color Synesthesia is Associated with PTSD Among Deployed Veterans: Confirmation of Previous Findings and Need for Additional Research

Post-traumatic stress disorder (PTSD) is related to alteration in neuropsychological functioning, including visual and other cognitive processes. Grapheme-color synesthesia is a phenomenon in which a letter or number elicits response of a concurrent image or color perception. Since we earlier reported an association between grapheme-color synesthesia and PTSD, our objective in the current study was to validate this association among a new study group and assess risk factors. For this, we surveyed 1,730 military veterans who have been outpatients in the Geisinger Clinic, a multi-hospital system in Pennsylvania, USA. All the study veterans served in a warzone deployment. The association between PTSD and Grapheme-color synesthesia was evaluated. The average age of veterans was 59.6 years among whom 95.1% were male. Current PTSD prevalence rate was observed to be 7.6% (95% C.I. = 6.5-9.0) and in 3.4% of veterans (95% C.I. = 2.7-4.4) grapheme-color synesthesia was found to be positive. Initial bivariate analyses suggested that synesthesia was associated with current PTSD [odds ratio (OR) = 3.3, p<0.001]. Multivariable stepwise logistic regression evaluating the age, sex, education, trauma exposure, current psychological stress, psychotropic medication use, combat exposure, history of concussion, and current depression, confirmed this association (OR = 2.33, p = 0.019). The present study corroborated that Grapheme-color synesthesia was linked to PTSD among a second cohort of deployed military veterans. Further research is recommended in order to validate this observation and to determine whether synesthesia is a risk factor for PTSD.

Exploring the functional nature of synaesthetic colour: Dissociations from colour perception and imagery

Individuals with grapheme-colour synaesthesia experience anomalous colours when reading achromatic text. These unusual experiences have been said to resemble 'normal' colour perception or colour imagery, but studying the nature of synaesthesia remains difficult. In the present study, we report novel evidence that synaesthetic colour impacts conscious vision in a way that is different from both colour perception and imagery. Presenting 'normal' colour prior to binocular rivalry induces a location-dependent suppressive bias reflecting local habituation. By contrast, a grapheme that evokes synaesthetic colour induces a facilitatory bias reflecting priming that is not constrained to the inducing grapheme's location. This priming does not occur in non-synaesthetes and does not result from response bias. It is sensitive to diversion of visual attention away from the grapheme, but resistant to sensory perturbation, reflecting a reliance on cognitive rather than sensory mechanisms. Whereas colour imagery in non-synaesthetes causes local priming that relies on the locus of imagined colour, imagery in synaesthetes caused global priming not dependent on the locus of imagery. These data suggest a unique psychophysical profile of high-level colour processing in synaesthetes. Our novel findings and method will be critical to testing theories of synaesthesia and visual awareness.

Gray Bananas and a Red Letter A - From Synesthetic Sensation to Memory Colors

Grapheme-color synesthesia is a condition in which objectively achromatic graphemes induce concurrent color experiences. While it was long thought that the colors emerge during perception, there is growing support for the view that colors are integral to synesthetes' cognitive representations of graphemes. In this work, we review evidence for two opposing theories positing either a perceptual or cognitive origin of concurrent colors: the cross-activation theory and the conceptual-mediation model. The review covers results on inducer and concurrent color processing as well as findings concerning the brain structure and grapheme-color mappings in synesthetes and trained mappings in nonsynesthetes. The results support different aspects of both theories. Finally, we discuss how research on memory colors could provide a new perspective in the debate about the level of processing at which the synesthetic colors occur.

Oscillatory Mechanisms of Response Conflict Elicited by Color and Motion Direction: An Individual Differences Approach

Goal-directed behavior requires control over automatic behavior, for example, when goal-irrelevant information from the environment captures an inappropriate response and conflicts with the correct, goal-relevant action. Neural oscillations in the theta band (∼6 Hz) measured at midfrontal electrodes are thought to form an important substrate of the detection and subsequent resolution of response conflict. Here, we examined the extent to which response conflict and associated theta-band activity depend on the visual stimulus feature dimension that triggers the conflict. We used a feature-based Simon task to manipulate conflict by motion direction and stimulus color. Analyses were focused on individual differences in behavioral response conflict elicited across different stimulus dimensions and their relationship to conflict-related midfrontal theta. We first confirmed the presence of response conflict elicited by task-irrelevant motion and stimulus color, demonstrating the usefulness of our modified version of the Simon task to assess different sensory origins of response conflict. Despite titrating overall task performance, we observed large individual differences in the behavioral manifestations of response conflict elicited by the different visual dimensions. These behavioral conflict effects were mirrored in a dimension-specific relationship with conflict-related midfrontal theta power, such that, for each dimension, individual midfrontal theta power was generally higher when experienced response conflict was high. Finally, exploratory analyses of interregional functional connectivity suggested a role for phase synchronization between frontal and parietal scalp sites in modulating experienced conflict when color was the task-relevant visual dimension. Highlighting the importance of an individual differences approach in cognitive neuroscience, these results reveal large individual differences in experienced response conflict depending on the source of visual interference, which are predicted by conflict-related midfrontal theta power.

Personality predicts the vibrancy of colour imagery: The case of synaesthesia

In this study we show that personality traits predict the physical qualities of mentally generated colours, using the case of synaesthesia. Developmental grapheme-colour synaesthetes have the automatic lifelong association of colours paired to letters or digits. Although these colours are internal mental constructs, they can be measured along physical dimensions such as saturation and luminance. The personality of synaesthetes can also be quantified using self-report questionnaires relating, for example, to the five major traits of Conscientiousness, Extraversion, Agreeableness, Neuroticism, and Openness to Experience. In this paper, we bring together both types of quality by examining whether the personality of individual synaesthetes predicts their synaesthetic colours. Twenty grapheme-colour synaesthetes were tested with the Big Five Inventory (BFI) personality questionnaire. Their synaesthesia was also tested in terms of consistency and average colour saturation and luminance. Two major results were found: although personality did not influence the overall robustness (i.e., consistency) of synaesthesia, it predicted the nature of synaesthetes' colours: the trait of Openness was positively correlated with the saturation of synaesthetic colours. Our study provides evidence that personality and internal perception are intertwined, and suggests future avenues of research for investigating the associations between the two.

When synesthesia and savant abilities are mistaken for hallucinations and delusions: contribution of a cognitive approach for their differential diagnosis

OBJECTIVE

Schizophrenia is characterized by hallucinations, delusions, disorganized speech and behavior, and other symptoms that cause social or occupational dysfunction. However, some of these symptoms, such as hallucinations and delusions, can be indicative of other phenomena such as synesthesia and savant abilities. The aim of this paper is to highlight how neurological and psychiatric conditions can be confused and how formal neuropsychological evaluations can be necessary to distinguish them.

METHOD

We report the case of a young woman, VA, who perceived sounds as colors and claimed to have elaborated complex astrophysical reasoning, despite having experienced difficulties at school, especially in mathematics. VA also had difficulties to orient herself, to develop social relationships, and often became confused by daily life situations. These elements were considered as symptoms of schizophrenia.

RESULTS

Evaluations revealed that VA exhibited savant abilities in astrophysics and colored-hearing synesthesia. We also found evidence of higher-than-average cognitive functioning.

CONCLUSIONS

In complex cases, neuropsychological and formal evaluations are necessary to establish a differential diagnosis. Moreover, the case highlights the link between synesthesia and savant abilities.

Social perception in synaesthesia for colour

Synaesthesia is a rare phenomenon in which stimulation in one modality (e.g., audition) evokes a secondary percept not associated with the first (e.g., colour). Prior work has suggested links between synaesthesia and other neurodevelopmental conditions that are linked to altered social perception abilities. With this in mind, here we sought to examine social perception abilities in grapheme-colour synaesthesia (where achromatic graphemes evoke colour experiences) by examining facial identity and facial emotion perception in synaesthetes and controls. Our results indicate that individuals who experience grapheme-colour synaesthesia outperformed controls on tasks involving fine visual discrimination of facial identity and emotion, but not on tasks involving holistic face processing. These findings are discussed in the context of broader perceptual and cognitive traits previously associated with synaesthesia for colour, with the suggestion that performance benefits shown by grapheme-colour synaesthetes may be related to domain-general visual discrimination biases observed in this group.

Color Processing in Synesthesia: What Synesthesia Can and Cannot Tell Us About Mechanisms of Color Processing

Synesthetic experiences of color have been traditionally conceptualized as a perceptual phenomenon. However, recent evidence suggests a role of higher order cognition in the formation of synesthetic experiences. Here, we discuss how synesthetic experiences of color differ from and influence veridical color processing, and how non-perceptual processes such as imagery and color memory might play a role in eliciting synesthetic color experience.

The Emergence of Synaesthesia in a Neuronal Network Model via Changes in Perceptual Sensitivity and Plasticity

Synaesthesia is an unusual perceptual experience in which an inducer stimulus triggers a percept in a different domain in addition to its own. To explore the conditions under which synaesthesia evolves, we studied a neuronal network model that represents two recurrently connected neural systems. The interactions in the network evolve according to learning rules that optimize sensory sensitivity. We demonstrate several scenarios, such as sensory deprivation or heightened plasticity, under which synaesthesia can evolve even though the inputs to the two systems are statistically independent and the initial cross-talk interactions are zero. Sensory deprivation is the known causal mechanism for acquired synaesthesia and increased plasticity is implicated in developmental synaesthesia. The model unifies different causes of synaesthesia within a single theoretical framework and repositions synaesthesia not as some quirk of aberrant connectivity, but rather as a functional brain state that can emerge as a consequence of optimising sensory information processing.

Synaesthesia is a remarkable form of altered perception, where one attribute of a stimulus (e.g. sound) leads to the conscious experience of an additional attribute (often colour). Despite being known about for 200 years, there is no commonly agreed upon model for how and why synaesthesia emerges. This study presents a new model of synaesthesia based on computational principles that accounts for the emergence of different types of synaesthesia (acquired and developmental) as well as many of its key characteristics. The model describes how two independent neuronal systems can evolve to interact with one another even though their inputs are statistically uncorrelated. Specifically, synaesthesia arises as a result of instability in the learning process that shapes the network, which can be caused by heightened plasticity or due to sensory deprivation of one of the systems. The model unifies different aspects of synaesthesia and generates novel insights and predictions.

Examining the Relationship Between Schizotypy and Self-Reported Visual Imagery Vividness in Grapheme-Color Synaesthesia

Synaesthesia is a condition in which one property of a stimulus triggers a secondary experience not typically associated with the first (e.g., seeing achromatic graphemes can evoke the perception of color). Recent work has explored a variety of cognitive and perceptual traits associated with synaesthesia. One example is in the domain of personality, where higher rates of positive schizotypy and openness to experience and lower agreeableness have been reported in synaesthetes who experience color as their evoked sensation relative to typical adult controls. Additionally, grapheme-color synaesthetes have previously been reported to show elevated mental imagery compared to typical adults. Here, we aimed to further elucidate the relationship between personality, synaesthesia, and other cognitive traits. In Study 1, we examined self-reported schizotypy and self-reported visual imagery vividness in grapheme-color synaesthetes and typical adults. Our results partially replicated previous findings by showing that synaesthesia was associated with greater positive schizotypy and enhanced self-reported imagery vividness. The results also extend previous reports by demonstrating that differences in positive schizotypy and mental imagery vividness are not related in grapheme-color synaesthesia. In Study 2, we sought to build on prior work showing lower agreeableness and increased openness to experience in synaesthetes by examining whether grapheme-color synaesthesia is associated with other conceptually related traits; namely lower self-monitoring and increased sensation seeking. We did not find any differences between synaesthetes and controls on either of these traits. These findings are discussed in relation to potential factors that may contribute to the observed personality profile in grapheme-color synaesthesia.

Migraine in Synesthetes and Nonsynesthetes: A Prevalence Study

Synesthesia is a neurological condition in which an inducer stimulus in one sense leads to a concurrent percept in a second sense. The immune hypothesis of synesthesia links synesthesia to immune-related conditions such as migraine. More specifically, migraine with aura may be linked to grapheme-color synesthesia as both involve cortical hyperexcitability. In this study, 161 female synesthetes, and 92 female nonsynesthetes, completed an online questionnaire about synesthesia and migraine. We found no general link between migraine and synesthesia nor between migraine with aura and grapheme-color synesthesia. Exploratory analyses, however, showed that certain types of synesthetic inducer (non-linguistic visual experiences, scent, taste, emotion and personality) were associated with visual disturbances in headache among female participants, and touch as a concurrent was associated with migraine with aura. On the basis of our exploratory analyses, we hypothesize that specific subtypes of synesthesia are related to migraine. The relationship between these two conditions is likely to become clearer as research on the underlying causes of synesthesia and migraine progresses.

Phosphene Perception Relates to Visual Cortex Glutamate Levels and Covaries with Atypical Visuospatial Awareness

Phosphenes are illusory visual percepts produced by the application of transcranial magnetic stimulation to occipital cortex. Phosphene thresholds, the minimum stimulation intensity required to reliably produce phosphenes, are widely used as an index of cortical excitability. However, the neural basis of phosphene thresholds and their relationship to individual differences in visual cognition are poorly understood. Here, we investigated the neurochemical basis of phosphene perception by measuring basal GABA and glutamate levels in primary visual cortex using magnetic resonance spectroscopy. We further examined whether phosphene thresholds would relate to the visuospatial phenomenology of grapheme-color synesthesia, a condition characterized by atypical binding and involuntary color photisms. Phosphene thresholds negatively correlated with glutamate concentrations in visual cortex, with lower thresholds associated with elevated glutamate. This relationship was robust, present in both controls and synesthetes, and exhibited neurochemical, topographic, and threshold specificity. Projector synesthetes, who experience color photisms as spatially colocalized with inducing graphemes, displayed lower phosphene thresholds than associator synesthetes, who experience photisms as internal images, with both exhibiting lower thresholds than controls. These results suggest that phosphene perception is driven by interindividual variation in glutamatergic activity in primary visual cortex and relates to cortical processes underlying individual differences in visuospatial awareness.

Color synesthesia. Insight into perception, emotion, and consciousness

Purpose of review

Synesthesia is an extraordinary perceptual phenomenon, in which individuals experience unusual percepts elicited by the activation of an unrelated sensory modality or by a cognitive process. Emotional reactions are commonly associated. The condition prompted philosophical debates on the nature of perception and impacted the course of art history. It recently generated a considerable interest among neuroscientists, but its clinical significance apparently remains underevaluated. This review focuses on the recent studies regarding variants of color synesthesia, the commonest form of the condition.

Recent findings

Synesthesia is commonly classified as developmental and acquired. Developmental forms predispose to changes in primary sensory processing and cognitive functions, usually with better performances in certain aspects and worse in others, and to heightened creativity. Acquired forms of synesthesia commonly arise from drug ingestion or neurological disorders, including thalamic lesions and sensory deprivation (e.g., blindness). Cerebral exploration using structural and functional imaging has demonstrated distinct patterns in cortical activation and brain connectivity for controls and synesthetes. Artworks of affected painters are most illustrative of the nature of synesthetic experiences.

Summary

Results of the recent investigations on synesthesia offered a remarkable insight into the mechanisms of perception, emotion and consciousness, and deserve attention both from neuroscientists and from clinicians.

Color synesthesia improves color but impairs motion perception

A recent study showed that color synesthetes have increased color sensitivity but impaired motion perception. This is exciting because little research has examined how synesthesia affects basic perceptual processes outside the context of synesthetic experiences. The results suggest that synesthesia broadly impacts perception with greater neural implications than previously considered.

Probing the neurochemical basis of synaesthesia using psychophysics

The neurochemical mechanisms that contribute to synaesthesia are poorly understood, but multiple models implicate serotonin and GABA in the development of this condition. Here we used psychophysical tasks to test the predictions that synaesthetes would display behavioral performance consistent with reduced GABA and elevated serotonin in primary visual cortex. Controls and synaesthetes completed the orientation-specific surround suppression (OSSS) and tilt-after effect (TAE) tasks, previously shown to relate to GABA and serotonin levels, respectively. Controls and synaesthetes did not differ in the performance parameter previously associated with GABA or in the magnitude of the TAE. However, synaesthetes did display lower contrast difference thresholds in the OSSS task than controls when no surround (NS) was present. These results are inconsistent with the hypothesized roles of GABA and serotonin in this condition, but provide preliminary evidence that synaesthetes exhibit enhanced contrast discrimination.

Enhanced recognition memory in grapheme-color synaesthesia for different categories of visual stimuli

Memory has been shown to be enhanced in grapheme-color synaesthesia, and this enhancement extends to certain visual stimuli (that don't induce synaesthesia) as well as stimuli comprised of graphemes (which do). Previous studies have used a variety of testing procedures to assess memory in synaesthesia (e.g., free recall, recognition, associative learning) making it hard to know the extent to which memory benefits are attributable to the stimulus properties themselves, the testing method, participant strategies, or some combination of these factors. In the first experiment, we use the same testing procedure (recognition memory) for a variety of stimuli (written words, non-words, scenes, and fractals) and also check which memorization strategies were used. We demonstrate that grapheme-color synaesthetes show enhanced memory across all these stimuli, but this is not found for a non-visual type of synaesthesia (lexical-gustatory). In the second experiment, the memory advantage for scenes is explored further by manipulating the properties of the old and new images (changing color, orientation, or object presence). Again, grapheme-color synaesthetes show a memory advantage for scenes across all manipulations. Although recognition memory is generally enhanced in this study, the largest effects were found for abstract visual images (fractals) and scenes for which color can be used to discriminate old/new status.