Crossmodal interactions: lessons from synesthesia

Induced awareness of synesthetic sensations in synesthetically predisposed "Borderline Non-synesthetes"

A long-standing issue concerning synesthesia is whether the trait is continuous or discontinuous with ordinary perception. Here, we found that a substantial proportion of non-synesthetes (>10 % out of >200 unselected participants) spontaneously became aware of their synesthesia by participating in an online survey that forced them to select colors for stimuli that evoke color sensations in synesthetes. Notably, the test-retest consistencies of color sensation in these non-synesthetes were comparable to those in self-claimed synesthetes, revealing their strong though latent synesthetic dispositions. The effect was absent or weak in a matched control survey that did not include the color-picking test. Therefore, the color-picking task likely provided the predisposed "borderline non-synesthetes" with an opportunity to dwell on their tendency toward synesthesia and allowed their subconscious sensations to become conscious ones. The finding suggests that the general population has a continuum of synesthetic disposition that encompasses both synesthetes and non-synesthetes.

The timing of cortical activation in associator grapheme-colour synaesthetes using MEG

Grapheme-colour synaesthetes experience an anomalous form of perception in which graphemes systematically induce specific colour concurrents in their mind's eye ("associator" type). Although grapheme-colour synaesthesia has been well characterised behaviourally, its neural mechanisms remain largely unresolved. There are currently several competing models, which can primarily be distinguished according to the anatomical and temporal predictions of synaesthesia-inducing neural activity. The first main model (Cross-Activation/Cascaded Cross-Tuning and its variants) posits early recruitment of occipital colour areas in the initial feed-forward sweep of brain activity. The second (Disinhibited Feedback) posits: (i) later involvement of a multisensory convergence zone (for example, in parietal cortices) after graphemes have been processed in their entirety; and (ii) subsequent feedback to early visual areas (i.e., occipital colour areas). In this study, we examine both the timing and anatomical correlates of associator grapheme-colour synaesthetes (n = 6) using MEG. Using innovative and unbiased analysis methods with little a priori assumptions, we applied Independent Component Analysis (ICA) on a single-subject level to identify the dominant patterns of activity corresponding to the induced, synaesthetic percept. We observed evoked activity that significantly dissociates between synaesthesia-inducing and non-inducing graphemes at approximately 190 ms following grapheme presentation. This effect is present in grapheme-colour synaesthetes, but not in matched controls, and exhibits an occipito-parietal topology localised consistently within individuals to extrastriate visual cortices and superior parietal lobes. Due to the observed timing of this evoked activity and its localization, our results support a model predicting relatively late synaesthesia-inducing activity, more akin to the Disinhibited Feedback model.

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.

Computational Paradigm to Elucidate the Effects of Arts-Based Approaches: Art and Music Studies and Implications for Research and Therapy

Art therapy and music therapy, as well as other arts-based approaches and interventions, help to mitigate symptoms in serious and chronic diseases and to improve the well-being and quality of life for both healthy individuals and patients. Artistic creation is also researched and practiced intending to empower and understand individuals, groups, and communities. However, much research is required in order to learn how arts-based approaches operate and to enhance their effectivity. The complex and simultaneous occurrences involving the dynamics of the creation work, the client, and the therapist in a typical arts setting are difficult to grasp, consequently affecting their objective analyses. Here we employ our Computational Paradigm which enables the quantitative and rigorous tracking, analyzing, and documenting of the underlying dynamic processes, and describe its application in recent past and current real-world art and music studies with human participants. We aim to study emergent artistic behaviors of individuals and collectives in response to art and music making. Significant insights obtained include demographic variation factors such as gender and age, empirical behavioral patterns, and quantitative expressiveness and its change. We discuss the implications of the findings for therapy and research, such as causality for behavioral diversification and audio-visual cross-modality, and also offer directions for future applications and technology enhancements.

Cross-Modal Perceptual Organization in Works of Art

This study investigates the existence of cross-modal correspondences between a series of paintings by Kandinsky and a series of selections from Schönberg music. The experiment was conducted in two phases. In the first phase, by means of the Osgood semantic differential, the participants evaluated the perceptual characteristics first of visual stimuli (some pictures of Kandinsky's paintings, with varying perceptual characteristics and contents) and then of auditory stimuli (musical excerpts taken from the repertoire of Schönberg's piano works) relative to 11 pairs of adjectives tested on a continuous bipolar scale. In the second phase, participants were required to associate pictures and musical excerpts. The results of the semantic differential test show that certain paintings and musical excerpts were evaluated as semantically more similar, while others were evaluated as semantically more different. The results of the direct association between musical excerpts and paintings showed both attractions and repulsions among the stimuli. The overall results provide significant insights into the relationship between concrete and abstract concepts and into the process of perceptual grouping in cross-modal phenomena.

Synaesthesia: a distinct entity that is an emergent feature of adaptive neurocognitive differences

In this article, I argue that synaesthesia is not on a continuum with neurotypical cognition. Synaesthesia is special: its phenomenology is different; it has distinct causal mechanisms; and is likely to be associated with a distinct neurocognitive profile. However, not all synaesthetes are the same, and there are quantifiable differences between them. In particular, the number of types of synaesthesia that a person possesses is a hitherto underappreciated variable that predicts cognitive differences along a number of dimensions (mental imagery, sensory sensitivity, attention to detail). Together with enhanced memory, this may constitute a common core of abilities that may go some way to explaining why synaesthesia might have evolved. I argue that the direct benefits of synaesthesia are generally limited (i.e. the synaesthetic associations do not convey novel information about the world) but, nevertheless, synaesthesia may develop due to other adaptive functions (e.g. perceptual ability, memory) that necessitate changes to design features of the brain. The article concludes by suggesting that synaesthesia forces us to reconsider what we mean by a 'normal' mind/brain. There may be multiple 'normal' neurodevelopmental trajectories that can sculpt very different ways of experiencing the world, of which synaesthesia is but one. This article is part of a discussion meeting issue 'Bridging senses: novel insights from synaesthesia'.

Inducing synesthesia in non-synesthetes: Short-term visual deprivation facilitates auditory-evoked visual percepts

Sounds can modulate activity in visual cortex, facilitating the detection of visual targets. However, these sound-driven modulations are not thought to evoke conscious visual percepts in the general population. In individuals with synesthesia, however, multisensory interactions do lead to qualitatively different experiences such as sounds evoking flashes of light. Why, if multisensory interactions are present in all individuals, do only synesthetes experience abnormal qualia? Competing models differ in the time required for synesthetic experiences to emerge. The cross-activation model suggests synesthesia arises over months or years from the development of abnormal neural connections. Here we demonstrate that after ∼5 min of visual deprivation, sounds can evoke synesthesia-like percepts (vivid colors and Klüver form-constants) in ∼50% of non-synesthetes. These results challenge aspects of the cross-activation model and suggest that synesthesia exists as a latent feature in all individuals, manifesting when the balance of activity across the senses has been altered.

The Effects of Visual Cues, Blindfolding, Synesthetic Experience, and Musical Training on Pure-Tone Frequency Discrimination

How perceptual limits can be reduced has long been examined by psychologists. This study investigated whether visual cues, blindfolding, visual-auditory synesthetic experience, and musical training could facilitate a smaller frequency difference limen (FDL) in a gliding frequency discrimination test. Ninety university students, with no visual or auditory impairment, were recruited for this one-between (blindfolded/visual cues) and one-within (control/experimental session) designed study. Their FDLs were tested by an alternative forced-choice task (gliding upwards/gliding downwards/no change) and two questionnaires (Vividness of Mental Imagery Questionnaire and Projector⁻Associator Test) were used to assess their tendency to synesthesia. The participants provided with visual cues and with musical training showed a significantly smaller FDL; on the other hand, being blindfolded or having a synesthetic experience before could not significantly reduce the FDL. However, no pattern was found between the perception of the gliding upwards and gliding downwards frequencies. Overall, the current study suggests that the inter-sensory perception can be enhanced through the training and facilitation of visual⁻auditory interaction under the multiple resource model. Future studies are recommended in order to verify the effects of music practice on auditory percepts, and the different mechanisms between perceiving gliding upwards and downwards frequencies.

Beyond the subjective experience of colour: An experimental case study of grapheme-texture synesthesia

This study was a case investigation of grapheme-texture synestheste TH, a female who subjectively reported experiencing a visual association between grapheme and colour/texture. First, we validated the existence of a synesthetic association in an objective manner. Involuntarily elicited experience is a major hallmark that is common to different types of synesthetes. Our results indicated interference between physical and synesthetic texture, suggesting the involuntary occurrence of synesthetic textural experience. We analysed the behavioural measures using the EZ diffusion model. The result suggested that TH's synesthetic experience was dissociable from that of briefly trained associative processing of non-synesthetes. Second, we investigated how the synesthetic experience of colour and texture dimensions was bound in the visual representation. We found that the interference effects of colour and texture were not independent. This suggested that in the elicited experience, the colour and texture features construct an integrated representation.

Electrophysiological correlates and psychoacoustic characteristics of hearing-motion synaesthesia

People with hearing-motion synaesthesia experience sounds from moving or changing (e.g. flickering) visual stimuli. This phenomenon may be one of the most common forms of synaesthesia but it has rarely been studied and there are no studies of its neural basis. We screened for this in a sample of 200+ individuals, and estimated a prevalence of 4.2%. We also document its characteristics: it tends to be induced by physically moving stimuli (more so than static stimuli which imply motion or trigger illusory motion); and the psychoacoustic features are simple (e.g. "whooshing") with some systematic correspondences to vision (e.g. faster movement is higher pitch). We demonstrate using event-related potentials that it emerges from early perceptual processing of vision. The synaesthetes have a higher amplitude motion-evoked N2 (165-185ms), with some evidence of group differences as early as 55-75ms. We discuss similarities between hearing-motion synaesthesia and previous observations that visual motion triggers auditory activity in the congenitally deaf. It is possible that both conditions reflect the maintenance of multisensory pathways found in early development that most people lose but can be retained in certain people in response to sensory deprivation (in the deaf) or, in people with normal hearing, as a result of other differences (e.g. genes predisposing to synaesthesia).

Ignoring Irrelevant Information: Enhanced Intermodal Attention in Synaesthetes

Despite the fact that synaesthetes experience additional percepts during their inducer-concurrent associations that are often unrelated or irrelevant to their daily activities, they appear to be relatively unaffected by this potentially distracting information. This might suggest that synaesthetes are particularly good at ignoring irrelevant perceptual information coming from different sensory modalities. To investigate this hypothesis, the performance of a group of synaesthetes was compared to that of a matched non-synaesthete group in two different conflict tasks aimed at assessing participants' abilities to ignore irrelevant information. In order to match the sensory modality of the task-irrelevant distractors (vision) with participants' synaesthetic attentional filtering experience, we tested only synaesthetes experiencing at least one synaesthesia subtype triggering visual concurrents (e.g., grapheme-colour synaesthesia or sequence-space synaesthesia). Synaesthetes and controls performed a classic flanker task (FT) and a visuo-tactile cross-modal congruency task (CCT) in which they had to attend to tactile targets while ignoring visual distractors. While no differences were observed between synaesthetes and controls in the FT, synaesthetes showed reduced interference by the irrelevant distractors of the CCT. These findings provide the first direct evidence that synaesthetes might be more efficient than non-synaesthetes at dissociating conflicting information from different sensory modalities when the irrelevant modality correlates with their synaesthetic concurrent modality (here vision).

A Is for Apple: the Role of Letter-Word Associations in the Development of Grapheme-Colour Synaesthesia

This study investigates the origins of specific letter-colour associations experienced by people with grapheme-colour synaesthesia. We present novel evidence that frequently observed trends in synaesthesia (e.g., A is typically red) can be tied to orthographic associations between letters and words (e.g., 'A is for apple'), which are typically formed during literacy acquisition. In our experiments, we first tested members of the general population to show that certain words are consistently associated with letters of the alphabet (e.g., A is for apple), which we named index words. Sampling from the same population, we then elicited the typical colour associations of these index words (e.g., apples are red) and used the letter → index word → colour connections to predict which colours and letters would be paired together based on these orthographic-semantic influences. We then looked at direct letter-colour associations (e.g., A→ red, B→ blue…) from both synaesthetes and non-synaesthetes. In both populations, we show statistically that the colour predicted by index words matches significantly with the letter-colour mappings: that is, A→ red because A is for apple and apples are prototypically red. We therefore conclude that letter-colour associations in both synaesthetes and non-synaesthetes are tied to early-learned letter-word associations.

Cortical Color and the Cognitive Sciences

Back when researchers thought about the various forms that color vision could take, the focus was primarily on the retinal mechanisms. Since that time, research on human color vision has shifted from an interest in retinal mechanisms to cortical color processing. This has allowed color research to provide insight into questions that are not limited to early vision but extend to cognition. Direct cortical connections from higher-level areas to lower-level areas have been found throughout the brain. One of the classic questions in cognitive science is whether perception is influenced, and if so to what extent, by cognition and whether a clear distinction can be drawn between perception and cognition. Since perception is seen as providing justification for our beliefs about properties in the external world, these questions also have metaphysical and epistemological significance. The aim of this paper is to highlight some of the areas where research on color perception can shed new light on questions in the cognitive sciences. A further aim of the paper is to raise some questions about color research that are in dire need of further reflection and investigation.

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.

Mirror-Touch Synaesthesia Is Not Associated with Heightened Empathy, and Can Occur with Autism

Research has linked Mirror-Touch (MT) synaesthesia with enhanced empathy. We test the largest sample of MT synaesthetes to date to examine two claims that have been previously made: that MT synaesthetes (1) have superior empathy; and (2) only ever experience their MT synaesthesia in response to viewing a person being touched. Given that autism has been suggested to involve deficits in cognitive empathy, we also test two predictions: that MT synaesthetes should (3) be less likely than general population individuals without MT synaesthesia to have an autism spectrum condition (ASC), if MT is characterized by superior empathy; and (4) have fewer autistic traits. We selected three groups: a pure MT synaesthesia group (N = 46), a pure grapheme-colour (GC) synaesthesia group (N = 36), and a typical control group without synaesthesia (N = 46). Participants took three measures of empathy and one measure of autistic traits. MT synaesthetes did not show enhanced empathy. In addition, 30% of all MT synaesthetes recruited into this study (N = 135) reported also having ASC, and MT synaesthetes showed higher autistic trait scores than controls. Finally, some MT experiences were reported in response to viewing objects being touched. Our findings dispute the views that MT synaesthesia is linked with enhanced empathy, is less likely to occur with ASC or elevated autistic traits, and is specific to seeing a person being touched.

Lessons of synaesthesia for consciousness: Learning from the exception, rather than the general

The question of whether the mechanisms underlying synaesthesia are similar to those ones underlying widespread cases of multisensory interactions tends to overshadow the question of whether their manifestations can be thought of as being continuous with one another. Here we focus on the conscious manifestations of synaesthesia and crossmodal correspondences and consider the dimensions along which these could be organised on a continuum. While synaesthesia and crossmodal correspondences may differ regarding the degree of vividness, frequency, specificity of the conscious manifestation, and control over its content, we demonstrate that these dimensions fail to lead to the expected continuous distribution. We suggest that the traditional distinction between conscious and non-conscious conditions might not be sufficient to separate synaesthesia from crossmodal correspondences, but highlight that the conscious manifestations that characterize synaesthesia remain exceptional and cannot be reconciled with other occasional occurrences of crossmodal mental imagery documented in the case of crossmodal correspondences.

The Tactile Dimensions of Abstract Paintings: A Cross-Modal Study

In our research, we tested for the existence of cross-modal visual and tactile associations in the experience of abstract art. Specifically, we measured the association of 60 abstract paintings with four couples of antonyms related to texture, such as warm or cold, smooth or rough, lightweight or heavy, soft or hard, investigating if the different modality of presentation on a computer screen (color versions: natural colors, inverted colors, black and white) gave rise to different associations relative to the four couples of opponent qualities. Second, we tested whether there might be differences between the ratings of the paintings when they were presented as images on a computer screen versus in real life at the museum. The results confirmed that associations between visual and tactile experience with such complex stimuli exist. In the case of the couple warm or cold, a significant inversion of associated qualities occurs when the images are presented in inverted colors as opposed to natural colors; furthermore, when presented in black and white, warm evaluations are "cooled down," but cold evaluations remain the same. The degree of smoothness could be considered not associated with the color versions. When seen in black and white, both the mean softness and the mean lightweight-ness of the paintings were reduced; however, in the last case, this effect was more evident for the most lightweight pictures. There is only a slight difference between the two presentations of the paintings as images presented on a computer screen and seen in real life, relative to the warm or cold and soft or hard dimensions. Of the four opponent qualities, the three pairs warm or cold, lightweight or heavy, and soft or hard showed the most interesting results in relation to the cross-modal associations.

Moral-up first, immoral-down last: the time course of moral metaphors on a vertical dimension

Many abstract bipolar concepts are usually represented by metaphors on vertical dimensions (e.g. positive-up, negative-down). However, several studies have found an asymmetry in the way in which individuals process bipolar dimensions, with +polarities being stronger than -polarities. The current research focused on moral metaphors on a vertical dimension (e.g. moral-up and immoral-down) and examined the asymmetric representation of moral and immoral concepts. The first experiment showed a distinct metaphorical association between morality and vertical space, consistent with earlier research. The second experiment showed that moral and immoral words are processed differently depending on whether they are used as metaphorically congruent or incongruent vertical cues. 'Moral-up' association modulated the amplitudes of the N1, P2, and late positive-going potential during the processing of moral words, whereas the 'immoral-down' association only modulated the amplitudes of the late positive-going potential induced during the processing of immoral words. These results suggest that asymmetry in the processing of vertically represented morality metaphors is reflected in the time course of the representation of these bipolar concepts, with the 'moral-up' association having an earlier effect than the 'immoral-down' association.

Chromatic Dimensions Earthy, Watery, Airy, and Fiery

In our study, for a small number of antonyms, we investigate whether they are cross-modally or ideaesthetically related to the space of colors. We analyze the affinities of seven antonyms (cold-hot, dull-radiant, dead-vivid, soft-hard, transparent-chalky, dry-wet, and acid-treacly) and their intermediate connotations (cool-warm, matt-shiny, numb-lively, mellow-firm, semi-transparent-opaque, semi-dry-moist, and sour-sweet) as a function of color. We find that some antonyms relate to chromatic dimensions, others to achromatic ones. The cold-hot antonym proves to be the most salient dimension. The dry-wet dimension coincides with the cold-hot dimension, with dry corresponding to hot and wet to cold. The acid-treacly dimension proves to be transversal to the cold-hot dimension; hence, the pairs mutually span the chromatic domain. The cold-hot and acid-treacly antonyms perhaps recall Hering's opponent color system. The dull-radiant, transparent-chalky, and dead-vivid pairs depend little upon chromaticity. Of all seven antonyms, only the soft-hard one turns out to be independent of the chromatic structure.

Distributed encoding of spatial and object categories in primate hippocampal microcircuits

The primate hippocampus plays critical roles in the encoding, representation, categorization and retrieval of cognitive information. Such cognitive abilities may use the transformational input-output properties of hippocampal laminar microcircuitry to generate spatial representations and to categorize features of objects, images, and their numeric characteristics. Four nonhuman primates were trained in a delayed-match-to-sample (DMS) task while multi-neuron activity was simultaneously recorded from the CA1 and CA3 hippocampal cell fields. The results show differential encoding of spatial location and categorization of images presented as relevant stimuli in the task. Individual hippocampal cells encoded visual stimuli only on specific types of trials in which retention of either, the Sample image, or the spatial position of the Sample image indicated at the beginning of the trial, was required. Consistent with such encoding, it was shown that patterned microstimulation applied during Sample image presentation facilitated selection of either Sample image spatial locations or types of images, during the Match phase of the task. These findings support the existence of specific codes for spatial and numeric object representations in primate hippocampus which can be applied on differentially signaled trials. Moreover, the transformational properties of hippocampal microcircuitry, together with the patterned microstimulation are supporting the practical importance of this approach for cognitive enhancement and rehabilitation, needed for memory neuroprosthetics.

Cross-modal associations between materic painting and classical Spanish music

The study analyses the existence of cross-modal associations in the general population between a series of paintings and a series of clips of classical (guitar) music. Because of the complexity of the stimuli, the study differs from previous analyses conducted on the association between visual and auditory stimuli, which predominantly analyzed single tones and colors by means of psychophysical methods and forced choice responses. More recently, the relation between music and shape has been analyzed in terms of music visualization, or relatively to the role played by emotion in the association, and free response paradigms have also been accepted. In our study, in order to investigate what attributes may be responsible for the phenomenon of the association between visual and auditory stimuli, the clip/painting association was tested in two experiments: the first used the semantic differential on a unidimensional rating scale of adjectives; the second employed a specific methodology based on subjective perceptual judgments in first person account. Because of the complexity of the stimuli, it was decided to have the maximum possible uniformity of style, composition and musical color. The results show that multisensory features expressed by adjectives such as "quick," "agitated," and "strong," and their antonyms "slow," "calm," and "weak" characterized both the visual and auditory stimuli, and that they may have had a role in the associations. The results also suggest that the main perceptual features responsible for the clip/painting associations were hue, lightness, timbre, and musical tempo. Contrary to what was expected, the musical mode usually related to feelings of happiness (major mode), or to feelings of sadness (minor mode), and spatial orientation (vertical and horizontal) did not play a significant role in the association. The consistency of the associations was shown when evaluated on the whole sample, and after considering the different backgrounds and expertise of the subjects. No substantial difference was found between expert and non-expert subjects. The methods used in the experiment (semantic differential and subjective judgements in first person account) corroborated the interpretation of the results as associations due to patterns of qualitative similarity present in stimuli of different sensory modalities and experienced as such by the subjects. The main result of the study consists in showing the existence of cross-modal associations between highly complex stimuli; furthermore, the second experiment employed a specific methodology based on subjective perceptual judgments.

Biases and regularities of grapheme-colour associations in Japanese nonsynaesthetic population

Associations between graphemes and colours in a nonsynaesthetic Japanese population were investigated. Participants chose the most suitable colour from 11 basic colour terms for each of 40 graphemes from the four categories of graphemes used in the Japanese language (kana characters, English alphabet letters, and Arabic and kanji numerals). This test was repeated after a three-week interval. In their responses, which were not as temporally consistent as those of grapheme-colour synaesthetes, participants showed biases and regularities that were comparable to those of synaesthetes reported in past studies. Although it has been believed that only synaesthetes, and not nonsynaesthetes, tended to associate graphemes with colours based on grapheme frequency, Berlin and Kay's colour typology, and colour word frequency, participants in this study tended in part to associate graphemes with colours based on the above factors. Moreover, participants that were nonsynaesthetes tended to associate different graphemes that shared sounds and/or meanings (e.g., Arabic and kanji numerals representing the same number) with the same colours, which was analogous to the findings in Japanese synaesthetes. These results support the view that grapheme-colour synaesthesia might have its origins in cross-modal association processes that are shared with the general population.

The semantics of biological forms

This study analyses how certain qualitative perceptual appearances of biological forms are correlated with expressions of natural language. Making use of the Osgood semantic differential, we presented the subjects with 32 drawings of biological forms and a list of 10 pairs of connotative adjectives to be put in correlations with them merely by subjective judgments. The principal components analysis made it possible to group the semantics of forms according to two distinct axes of variability: harmony and dynamicity. Specifically, the nonspiculed, nonholed, and flat forms were perceived as harmonic and static; the rounded ones were harmonic and dynamic. The elongated forms were somewhat disharmonious and somewhat static. The results suggest the existence in the general population of a correspondence between perceptual and semantic processes, and of a nonsymbolic relation between visual forms and their adjectival expressions in natural language.

Serotonergic hyperactivity as a potential factor in developmental, acquired and drug-induced synesthesia

Though synesthesia research has seen a huge growth in recent decades, and tremendous progress has been made in terms of understanding the mechanism and cause of synesthesia, we are still left mostly in the dark when it comes to the mechanistic commonalities (if any) among developmental, acquired and drug-induced synesthesia. We know that many forms of synesthesia involve aberrant structural or functional brain connectivity. Proposed mechanisms include direct projection and disinhibited feedback mechanisms, in which information from two otherwise structurally or functionally separate brain regions mix. We also know that synesthesia sometimes runs in families. However, it is unclear what causes its onset. Studies of psychedelic drugs, such as psilocybin, LSD and mescaline, reveal that exposure to these drugs can induce synesthesia. One neurotransmitter suspected to be central to the perceptual changes is serotonin. Excessive serotonin in the brain may cause many of the characteristics of psychedelic intoxication. Excessive serotonin levels may also play a role in synesthesia acquired after brain injury. In brain injury sudden cell death floods local brain regions with serotonin and glutamate. This neurotransmitter flooding could perhaps result in unusual feature binding. Finally, developmental synesthesia that occurs in individuals with autism may be a result of alterations in the serotonergic system, leading to a blockage of regular gating mechanisms. I conclude on these grounds that one commonality among at least some cases of acquired, developmental and drug-induced synesthesia may be the presence of excessive levels of serotonin, which increases the excitability and connectedness of sensory brain regions.

Affect-related synesthesias: a prospective view on their existence, expression and underlying mechanisms

The literature on developmental synesthesia has seen numerous sensory combinations, with surprisingly few reports on synesthesias involving affect. On the one hand, emotion, or more broadly affect, might be of minor importance to the synesthetic experience (e.g., Sinke et al., 2012). On the other hand, predictions on how affect could be relevant to the synesthetic experience remain to be formulated, in particular those that are driven by emotion theories. In this theoretical paper, we hypothesize that a priori studies on synesthesia involving affect will observe the following. Firstly, the synesthetic experience is not merely about discrete emotion processing or overall valence (positive, negative) but is determined by or even altered through cognitive appraisal processes. Secondly, the synesthetic experience changes temporarily on a quantitative level according to (i) the affective appraisal of the inducing stimulus or (ii) the current affective state of the individual. These hypotheses are inferred from previous theoretical and empirical accounts on synesthesia (including the few examples involving affect), different emotion theories, crossmodal processing accounts in synesthetes, and non-synesthetes, and the presumed stability of the synesthetic experience. We hope that the current review will succeed in launching a new series of studies on "affective synesthesias." We particularly hope that such studies will apply the same creativity in experimental paradigms as we have seen and still see when assessing and evaluating "traditional" synesthesias.

Pathways to seeing music: enhanced structural connectivity in colored-music synesthesia

Synesthesia, a condition in which a stimulus in one sensory modality consistently and automatically triggers concurrent percepts in another modality, provides a window into the neural correlates of cross-modal associations. While research on grapheme-color synesthesia has provided evidence for both hyperconnectivity-hyperbinding and disinhibited feedback as potential underlying mechanisms, less research has explored the neuroanatomical basis of other forms of synesthesia. In the current study we investigated the white matter correlates of colored-music synesthesia. As these synesthetes report seeing colors upon hearing musical sounds, we hypothesized that they might show unique patterns of connectivity between visual and auditory association areas. We used diffusion tensor imaging to trace the white matter tracts in temporal and occipital lobe regions in 10 synesthetes and 10 matched non-synesthete controls. Results showed that synesthetes possessed hemispheric patterns of fractional anisotropy, an index of white matter integrity, in the inferior fronto-occipital fasciculus (IFOF), a major white matter pathway that connects visual and auditory association areas to frontal regions. Specifically, white matter integrity within the right IFOF was significantly greater in synesthetes than controls. Furthermore, white matter integrity in synesthetes was correlated with scores on audiovisual tests of the Synesthesia Battery, especially in white matter underlying the right fusiform gyrus. Our findings provide the first evidence of a white matter substrate of colored-music synesthesia, and suggest that enhanced white matter connectivity is involved in enhanced cross-modal associations.

Synesthesia, pseudo-synesthesia, and irritable bowel syndrome

BACKGROUND AND AIMS

Synesthesia is a sensory disorder where the stimulation of one sensory modality can lead to a reaction in another which would not usually be expected to respond; for instance, someone might see a color on hearing a word such as a day of the week. Disordered perception of sensory information also appears to contribute to the pathophysiology of irritable bowel syndrome (IBS). The purpose of this exploratory study was to ascertain whether these two conditions might be linked in any way.

METHODS

Two hundred consecutive IBS outpatients were screened for synesthesia and compared with 200 matched healthy volunteers (controls). Positive responders were tested for two types of synesthesia (grapheme-color and music-color/shape) using a questionnaire which was repeated after 3 months to test for reproducibility.

RESULTS

Of the 200 IBS outpatients screened, 26 (13%) patients and six (3%) controls claimed to be synesthetic (P < 0.001). Reproducibility was more variable in IBS patients than controls but despite this variability, 15 (7.5%) patients compared with 5 (2.5%) controls had greater than 75% consistency (P = 0.036), and 19 (9.5%) patients and 6 (3%) controls had greater than 50% consistency (P = 0.012). A reproducibility of less than 50% was observed in seven (3.5%) patients and no controls (0%) (P = 0.015), and these individuals were classified as having pseudo-synesthesia.

CONCLUSION

IBS patients clearly differ from controls in terms of claiming to have synesthetic experiences. These results justify additional studies on the relationship between IBS and synesthesia to further understand the neural mechanisms underpinning these two conditions and to establish whether they may be linked.

Understanding grapheme personification: a social synaesthesia?

Much of synaesthesia research focused on colour, but not all cross-domain correspondences reported by synaesthetes are strictly sensory. For example, some synaesthetes personify letters and numbers, in additional to visualizing them in colour. First reported in the 1890s, the phenomenon has been largely ignored by scientists for more than a century with the exception of a few single-case reports. In the present study, we collected detailed self-reports on grapheme personification using a questionnaire, providing us with a comprehensive description of the phenomenology of grapheme personification. Next, we documented the behavioural consequences of personifying graphemes using a congruity paradigm involving a gender judgement task; we also examined whether personification is associated with heightened empathy as measured using Empathy Quotient and found substantial individual differences within our sample. Lastly, we present the first neuroimaging case study of personification, indicating that the precuneus activation previously seen in other synaesthesia studies may be implicated in the process. We propose that frameworks for understanding synaesthesia could be extended into other domains of cognition and that grapheme personification shares more in common with normal cognition than may be readily apparent. This benign form of hyper-mentalizing may provide a unique point of view on one of the most central problems in human cognition - understanding others' state of mind.

Grapheme-color and tone-color synesthesia is associated with structural brain changes in visual regions implicated in color, form, and motion

Synesthesia is a rare condition in which stimulation in one modality leads to a secondary experience in another sensory modality. Varying accounts attribute the condition to either neuroanatomical differences between the synesthetes and non-synesthetes or functional differences in how sensory brain regions interact. This study employed voxel-based morphometry to examine whether synesthetes who experience both grapheme-color and tone-color synesthesia as their evoked sensation show neuroanatomical differences in gray matter volume compared to non-synesthetes. We observed that synesthetes showed an increase in gray matter volume in left posterior fusiform gyrus (FG), but a concomitant decrease in anterior regions of left FG and left MT/V5. These findings imply that synesthesia for color is linked to neuroanatomical changes between adjacent regions of the visual system.

Visual identification of haptically explored objects in high and low hypnotizable subjects

Hypnotizability is associated with peculiar characteristics of sensorimotor integration, imaginal abilities, and preferences in the sensory modality of imagery. The visual recognition of haptically explored objects involves an interaction among these processes and is a proper tool to investigate their possible hypnotizability-related modulation. Sixteen high hypnotizables and 16 lows participated in the study. Higher frequencies of correct recognition (RF) were observed in highs. RF improved across both groups. As an effect of learning, shorter recognition times were found in males among highs and in females among lows. The findings are consistent with the literature suggesting that hypnotizability levels may be associated with specific modes of sensory integration and/or imagery.

Superior facial expression, but not identity recognition, in mirror-touch synesthesia

Simulation models of expression recognition contend that to understand another's facial expressions, individuals map the perceived expression onto the same sensorimotor representations that are active during the experience of the perceived emotion. To investigate this view, the present study examines facial expression and identity recognition abilities in a rare group of participants who show facilitated sensorimotor simulation (mirror-touch synesthetes). Mirror-touch synesthetes experience touch on their own body when observing touch to another person. These experiences have been linked to heightened sensorimotor simulation in the shared-touch network (brain regions active during the passive observation and experience of touch). Mirror-touch synesthetes outperformed nonsynesthetic participants on measures of facial expression recognition, but not on control measures of face memory or facial identity perception. These findings imply a role for sensorimotor simulation processes in the recognition of facial affect, but not facial identity.

Hypnotizability-related effects of vestibular impairment on posture and locomotion

Body sway and locomotion are differentially modulated in high (highs) and low (lows) hypnotizable subjects undergoing alteration of visual and neck/leg proprioceptive inputs. The study's aim was to investigate whether partial impairment of vestibular information due to backward head extension affects postural (Study 1) and locomotor behavior (Study 2) differentially in highs and lows. Results showed that, at variance with the visual and proprioceptive modalities, vestibular inactivation did not induce major differences between the 2 groups, with the exception of improvement in walking straight across consecutive trials, which was observed only in highs. The article presents an overview of the structures and mechanisms possibly involved in the observed hypnotizability-related differences in motor control and suggests that hypnotic susceptibility might be a relevant factor in neuro-rehabilitative treatments because it accounts for part of the variability in the sensorimotor self.

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.

Induced Cross-Modal Synaesthetic Experience Without Abnormal Neuronal Connections

Are the kinds of abnormal cross-modal interactions seen in synaesthesia or following brain damage due to hyperconnectivity between or within brain areas, or are they a result of lack of inhibition? This question is highly contested. Here we show that posthypnotic suggestion induces abnormal cross-modal experience similar to that observed in congenital grapheme-color synaesthesia. Given the short time frame of the experiment, it is unlikely that new cortical connections were established, so we conclude that synaesthesia can result from disinhibition between brain areas.

Number Forms in the Brain

Mental images of number lines, Galton's “number forms” (NF), are a useful way of investigating the relation between number and space. Here we report the first neuroimaging study of number-form synesthesia, investigating 10 synesthetes with NFs going from left to right compared with matched controls. Neuroimaging with functional magnetic resonance imaging revealed no difference in brain activation during a task focused on number magnitude but, in a comparable task on number order, synesthetes showed additional activations in the left and right posterior intraparietal sulci, suggesting that NFs are essentially ordinal in nature. Our results suggest that there are separate but partially overlapping neural circuits for the processing of ordinal and cardinal numbers, irrespective of the presence of an NF, but a core region in the anterior intraparietal sulcus representing (cardinal) number meaning appears to be activated autonomously, irrespective of task. This article provides an important extension beyond previous studies that have focused on word-color or grapheme-color synesthesia.

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.