Language evolution: examining the link between cross-modality and aggression through the lens of disorders.

We demonstrate how two linguistic phenomena, figurative language (implicating cross-modality) and derogatory language (implicating aggression), both demand a precise degree of (dis)inhibition in the same cortico-subcortical brain circuits, in particular cortico-striatal networks, whose connectivity has been significantly enhanced in recent evolution. We examine four cognitive disorders/conditions that exhibit abnormal patterns of (dis)inhibition in these networks: schizophrenia (SZ), autism spectrum disorder (ASD), synaesthesia and Tourette's syndrome (TS), with the goal of understanding why the two phenomena altered reactive aggression and altered cross-modality cluster together in these disorders. Our proposal is that enhanced cross-modality (necessary to support language, in particular metaphoricity) was a result, partly a side-effect, of self-domestication (SD). SD targeted the taming of reactive aggression, but reactive impulses are controlled by the same cortico-subcortical networks that are implicated in cross-modality. We further add that this biological process of SD did not act alone, but was engaged in an intense feedback loop with the cultural emergence of early forms of language/grammar, whose high degree of raw metaphoricity and verbal aggression also contributed to increased brain connectivity and cortical control. Consequently, in conjunction with linguistic expressions serving as approximations/'fossils' of the earliest stages of language, these cognitive disorders/conditions serve as confident proxies of brain changes in language evolution, helping us reconstruct certain crucial aspects of early prehistoric languages and cognition, as well as shed new light on the nature of the disorders. This article is part of the theme issue 'Reconstructing prehistoric languages'.

Wellbeing differences in children with synaesthesia: anxiety and mood regulation.

Synaesthesia is a neurodevelopmental trait that causes unusual sensory experiences (e.g., perceiving colours when reading letters and numbers). Our paper represents the first evidence that synaesthesia can impact negatively on children's well-being, and that there are likely to be important mental health co-morbidities for children with synaesthesia. We recruited 76 synaesthetes aged 6-10 years who had one of two types of synaesthesia (grapheme-colour synaesthesia and sequence-personality synaesthesia), and compared them to almost one thousand matched non-synaesthete controls. We tested children's wellbeing with two different measures, and found a significant relationship between synaesthesia and affect (i.e., mood), and also between synaesthesia and anxiety. Children with synaesthesia showed evidence suggesting significantly higher rates of Anxiety Disorder, and also displayed a type of mood-moderation in demonstrating fewer extremes of emotion (i.e., significantly fewer negative feelings such as fear, but also significantly fewer positive feelings such as joy). We discuss our results with reference to the emotional moderation of alexithymia (the inability to recognize or describe one's own emotions), and to a set of known links between alexithymia, anxiety, autism and synaesthesia.

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.

Grapheme-color associations can transfer to novel graphemes when synesthetic colors function as grapheme "discriminating markers".

Synesthesia is a condition in which the perception of a stimulus in one modality automatically triggers a secondary sensation in another modality or processing stream. Our study focused on grapheme-color synesthesia, in which the visual perception of letters or numbers (graphemes) induces a specific color sensation (the synesthetic color). Grapheme-color synesthetes do not typically experience colors for novel graphemes. However, synesthetic colors associated with familiar graphemes can be transferred to graphemes learned later, even in adulthood. A previous study has shown that such a transfer can take place after only a 10-min writing exercise. In this study, we found that this immediate transfer occurs only when the synesthetic colors for familiar graphemes contribute to the discrimination of the graphemes to be learned. Synesthetes learned six novel graphemes, each of which was arbitrarily associated with one of six familiar graphemes. Half of the synesthetes were assigned to the heterogeneous condition, in which the synesthetic colors of one group of familiar graphemes were different from one another. The other half of the synesthetes were assigned to the homogeneous condition, in which the various colors of a whole group of familiar graphemes were categorically the same. After this learning session, less transfer of synesthetic colors to novel graphemes from the corresponding familiar graphemes occurred in the homogeneous condition than in the heterogeneous condition. These results support the view that synesthetic colors for graphemes may function as a grapheme acquisition aid.

Personality profile of child synaesthetes.

Previous research into personality and synaesthesia has focused on adult populations and yielded mixed results. One particular challenge has been to distinguish traits associated with synaesthesia, from traits associated with the ways in which synaesthetes were recruited. In the current study we addressed recruitment issues by testing randomly sampled synaesthetes, and we looked particularly at synaesthesia in childhood. Our child synaesthetes were identified by a screening program across 22 primary schools in the South East of England (n = 3387; children aged 6 to 10 years old). This identified two types of synaesthete (grapheme-colour synaesthesia and sequence-personality synaesthesia), and we tested their personalities using both child-report and parent-report measures. We found strong support for synaesthesia being associated with high Openness to Experience, a personality trait linked to intelligence and creativity. Both synaesthesia subtypes showed this feature, supporting previous research in adults (1-3). We additionally found low Extraversion in grapheme-colour synaesthetes and high Conscientiousness in sequence-personality synaesthetes. We discuss our results with reference to earlier recruitment issues, and as to how perceptual differences such as synaesthesia might link to trait-differences in personality.

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.

Does synaesthesia protect against age-related memory loss?

Synaesthesia is known to be linked to enhanced episodic memory abilities, across a variety of stimuli and tests, but the evidence has tended to come from younger adults. This enhanced cognitive ability in early adult life, together with the known brain-related differences linked to synaesthesia (e.g., in both grey and white matter structure), makes it an ideal candidate for exploring the notion of 'reserve'. That is, synaesthetes may be able to utilize additional cognitive and/or neural resources to mitigate against the effects of age-related decline. This was explored in a 2 × 2 design contrasting age (young, old) against the presence/absence of synaesthesia in two different studies: recognition memory for digits, snowflakes, and music; and visual associative learning. Synaesthesia and age had independent, non-interacting, effects on memory ability suggesting that, while synaesthetes show a memory advantage and maintain this advantage in later life, the presence of synaesthesia is not able to act as a reserve to protect against the effects of ageing. On our tasks, the benefit of having synaesthesia (enhancing memory) was of a similar magnitude to the effects of age (impairing memory); in other words, elderly synaesthetes present with 'youthful' memory abilities. It is important for future research on elderly cohorts to consider the presence of synaesthesia as an individual difference.

Deepening understanding of language through synaesthesia: a call to reform and expand.

In this paper, I present arguments and suggestions for the improvement of the scientific study of synaesthesia, and particularly grapheme-colour synaesthesia in relation to psycholinguistic research, although the principles I advocate can be easily adapted to any subfield of synaesthesia study. I postulate that the current state of research on synaesthesia in general, and on grapheme-colour synaesthesia in particular, suffers from a lack of exploratory evidence and essential groundwork upon which to build hypothesis-testing studies. In particular, I argue that synaesthesia research has been artificially bounded by assumptions about the nature of synaesthetic experiences, which constrain both the questions that researchers ask and the way in which they go about answering those questions. As a specific example, I detail how much of the current research on grapheme-colour synaesthesia is built to accommodate two major assumptions about the nature of colours for letters and for words-assumptions which I will contend are not universally true, and the exceptions to which point to a much richer and heterogeneous understanding of synaesthetic experience than current research practices capture. The top-down predetermination of what is important or meaningful to measure, and what is not, has subsequently impeded a full understanding of what synaesthesia is and how it works. I argue that these assumptions must be carefully addressed and evaluated, both for the particular case of grapheme-colour synaesthesia and for the field as a whole, to move towards a holistic and fruitful understanding of synaesthesia as a phenomenon and as a tool to study language, thought and perception. To that end, I propose specific recommendations for synaesthesia researchers to solidify and expand their understanding and to capture the actual experience of synaesthetes. This article is part of a discussion meeting issue 'Bridging senses: novel insights from synaesthesia'.

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'.

Echoes from the past: synaesthetic colour associations reflect childhood gender stereotypes.

Grapheme-colour synaesthesia is a neurological phenomenon in which linguistic symbols evoke consistent colour sensations. Synaesthesia is believed to be influenced by both genetic and environmental factors, but how these factors interact to create specific associations in specific individuals is poorly understood. In this paper, we show that a grapheme-colour association in adult synaesthetes can be traced to a particular environmental effect at a particular moment in childhood. We propose a model in which specific grapheme-colour associations are 'locked in' during development in children predisposed to become synaesthetes, whereas grapheme-colour associations remain flexible in non-synaesthetes. We exploit Western gender-colour stereotypes to test our model: we found that young girls in general tend to associate their first initial with the colour pink. Consistent with our model, adult female synaesthetes are influenced by their childhood environment: they associate their first initial with pink. Adult female non-synaesthetes do not show this bias. Instead, in our study, non-synaesthetes tended to associate their first initial with their current favourite colour. The results thus support the 'locking in' model of synaesthesia, suggesting that synaesthetic associations can be used as a 'time capsule', revealing childhood influences on adult linguistic associations. Grapheme-colour synaesthesia may thus offer an extraordinary opportunity to study linguistic development. This article is part of a discussion meeting issue 'Bridging senses: novel insights from synaesthesia'.

How non-veridical perception drives actions in healthy humans: evidence from synaesthesia.

We continually perform actions that are driven by our perception and it is a commonly held view that only objectively perceived changes within the 'real' world affect behaviour. Exceptions are generally only made for mental health disorders associated with delusions and hallucinations where behaviour may be triggered by the experience of objectively non-existent percepts. Here, we demonstrate, using synaesthesia as a model condition (in N = 19 grapheme-colour synaesthetes), how objectively non-existent (i.e. non-veridical) but still non-pathological perceptions affect actions in healthy humans. Using electroencephalography, we determine whether early-stage perceptual processes (reflected by P1 and N1 event-related potential (ERP) components), or late-stage-integration processes (reflected by N2 component), underlie the effects of non-veridical perceptions on action control. ERP analysis suggests that even though the examined peculiarities and experimental variations are perceptual in nature, it is not early-stage perceptual processes, but rather higher-order executive control processes linking perceptions to the appropriate motor response underlying this effect. Source localization analysis implicates activation within medial frontal cortices in the effect of how irrelevant non-veridical perceptions modulate behaviour. Our results challenge common conceptions about the determinants of human behaviour but can be explained by well-established theoretical frameworks detailing the link between perception and action. This article is part of a discussion meeting issue 'Bridging senses: novel insights from synaesthesia'.

Distinct colours in the 'synaesthetic colour palette'.

In grapheme-colour synaesthesia, particular linguistic elements evoke particular colour sensations. Interestingly, when asked, non-synaesthetes can also associate colours to letters, and previous studies show that specific letter-to-colour associations have similar biases to those of synaesthetes. However, it is an open question whether the colours reported by synaesthetes and non-synaesthetes differ overall: is there a 'synaesthetic colour palette'? In this study, we visualize the overall distribution in colour space of colour concurrents in grapheme-colour synaesthetes, and colour associations in non-synaesthetic controls. We confirm the existence of a synaesthetic colour palette: colour concurrents in synaesthetes are different from colour associations in non-synaesthetes. We quantify three factors that distinguish the colour palette of synaesthetes and non-synaesthetes: synaesthetes have an increased over-representation of 'pure' (unmixed) hues, an increased presence of 'warm' (yellow, orange, brown) colours, and an increased presence of achromatic (grey, white, black) colours. Furthermore, we demonstrate that differences in the synaesthetic colour palette can be used to train a machine learning algorithm to reliably classify single subjects as synaesthetes versus non-synaesthetes without using test-retest consistency data. As far as we know, this is the first time an individual could be 'diagnosed' as a synaesthete, based only on his or her colours evoked by letters. 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'.

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'.

Exploring the relationship between grapheme colour-picking consistency and mental imagery.

Previous research has indicated a potential link between mental imagery and synaesthesia. However, these findings are mainly based on imagery self-report measures and recruitment of self-selected synaesthetes. To avoid issues of self-selection and demand effects, we recruited participants from the general population, rather than synaesthetes specifically, and used colour-picking consistency tests for letters and numbers to assess a 'synaesthete-like' experience. Mental imagery ability and mental rotation ability were assessed using both self-report measures and behavioural assessments. Consistency in colour-picking for letters (but not numbers) was predicted by performance on the visual mental imagery task but not by a mental rotation task or self-report measures. Using the consistency score as a proxy measure of grapheme-colour synaesthesia, we provide more evidence for the suggestion that synaesthetic experience is associated with enhanced mental imagery, even when participants are naive to the research topic. This article is part of a discussion meeting issue 'Bridging senses: novel insights from synaesthesia'.

Investigating genetic links between grapheme-colour synaesthesia and neuropsychiatric traits.

Synaesthesia is a neurological phenomenon affecting perception, where triggering stimuli (e.g. letters and numbers) elicit unusual secondary sensory experiences (e.g. colours). Family-based studies point to a role for genetic factors in the development of this trait. However, the contributions of common genomic variation to synaesthesia have not yet been investigated. Here, we present the SynGenes cohort, the largest genotyped collection of unrelated people with grapheme-colour synaesthesia (n = 723). Synaesthesia has been associated with a range of other neuropsychological traits, including enhanced memory and mental imagery, as well as greater sensory sensitivity. Motivated by the prior literature on putative trait overlaps, we investigated polygenic scores derived from published genome-wide scans of schizophrenia and autism spectrum disorder (ASD), comparing our SynGenes cohort to 2181 non-synaesthetic controls. We found a very slight association between schizophrenia polygenic scores and synaesthesia (Nagelkerke's R2 = 0.0047, empirical p = 0.0027) and no significant association for scores related to ASD (Nagelkerke's R2 = 0.00092, empirical p = 0.54) or body mass index (R2 = 0.00058, empirical p = 0.60), included as a negative control. As sample sizes for studying common genomic variation continue to increase, genetic investigations of the kind reported here may yield novel insights into the shared biology between synaesthesia and other traits, to complement findings from neuropsychology and brain imaging. This article is part of a discussion meeting issue 'Bridging senses: novel insights from synaesthesia'.

Neurophenomenology of induced and natural synaesthesia.

People with synaesthesia have additional perceptual experiences, which are automatically and consistently triggered by specific inducing stimuli. Synaesthesia therefore offers a unique window into the neurocognitive mechanisms underlying conscious perception. A long-standing question in synaesthesia research is whether it is possible to artificially induce non-synaesthetic individuals to have synaesthesia-like experiences. Although synaesthesia is widely considered a congenital condition, increasing evidence points to the potential of a variety of approaches to induce synaesthesia-like experiences, even in adulthood. Here, we summarize a range of methods for artificially inducing synaesthesia-like experiences, comparing the resulting experiences to the key hallmarks of natural synaesthesia which include consistency, automaticity and a lack of 'perceptual presence'. We conclude that a number of aspects of synaesthesia can be artificially induced in non-synaesthetes. These data suggest the involvement of developmental and/or learning components in the acquisition of synaesthesia, and they extend previous reports of perceptual plasticity leading to dramatic changes in perceptual phenomenology in adults. This article is part of a discussion meeting issue 'Bridging senses: novel insights from synaesthesia'.

Enhanced self-reported affect and prosocial behaviour without differential physiological responses in mirror-sensory synaesthesia.

Mirror-sensory synaesthetes mirror the pain or touch that they observe in other people on their own bodies. This type of synaesthesia has been associated with enhanced empathy. We investigated whether the enhanced empathy of people with mirror-sensory synesthesia influences the experience of situations involving touch or pain and whether it affects their prosocial decision making. Mirror-sensory synaesthetes (N = 18, all female), verified with a touch-interference paradigm, were compared with a similar number of age-matched control individuals (all female). Participants viewed arousing images depicting pain or touch; we recorded subjective valence and arousal ratings, and physiological responses, hypothesizing more extreme reactions in synaesthetes. The subjective impact of positive and negative images was stronger in synaesthetes than in control participants; the stronger the reported synaesthesia, the more extreme the picture ratings. However, there was no evidence for differential physiological or hormonal responses to arousing pictures. Prosocial decision making was assessed with an economic game assessing altruism, in which participants had to divide money between themselves and a second player. Mirror-sensory synaesthetes donated more money than non-synaesthetes, showing enhanced prosocial behaviour, and also scored higher on the Interpersonal Reactivity Index as a measure of empathy. Our study demonstrates the subjective impact of mirror-sensory synaesthesia and its stimulating influence on prosocial behaviour. This article is part of the discussion meeting issue 'Bridging senses: novel insights from synaesthesia'.