Social perception in synaesthesia for colour

Face-Processing Differences Present in Grapheme-Color Synesthetes

Grapheme‐color synesthesia is a heterogeneous neurological phenomenon whereby the experience of a grapheme automatically and involuntarily elicits an experience of color. While the majority of synesthesia research has focused on inducer‐specific influences of synesthetic associations, more recent efforts have examined potential broader differences. Based on spontaneous reports from synesthetes detailing problems with face recognition, in conjunction with the geographical proximity of neurological regions relevant to both synesthesia and face processing, we sought to examine whether synesthetes demonstrated atypical face‐processing abilities.

A total of 16 grapheme‐color synesthetes and 16 age‐and‐gender matched controls (±3 years) completed the Cambridge Face Memory Test (CFMT; Duchaine & Nakayama, 2006) of face memory, the Vanderbilt Holistic Face Processing Task (VHPT‐F; Richler, Floyd, & Gauthier, 2014) of holistic face processing, as well as a standardized self‐report questionnaire the Faces and Emotions Questionnaire (Freeman, Palermo, & Brock, 2015). The results revealed significantly poorer performance in synesthete's ability to recognize faces in the CFMT that was driven by a reduction in upright advantage. Results also revealed a significant reduction in overall accuracy on the VHPT‐F for synesthetes, who despite this displayed a comparable holistic processing advantage compared to matched controls. Finally, synesthetes also rated themselves as significantly worse at face recognition. We suggest that this pattern may reflect differences in the development of individualized perceptual strategies.

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.

Reduced perceptual narrowing in synesthesia

Synesthesia is a neurologic trait in which specific inducers, such as sounds, automatically elicit additional idiosyncratic percepts, such as color (thus "colored hearing"). One explanation for this trait-and the one tested here-is that synesthesia results from unusually weak pruning of cortical synaptic hyperconnectivity during early perceptual development. We tested the prediction from this hypothesis that synesthetes would be superior at making discriminations from nonnative categories that are normally weakened by experience-dependent pruning during a critical period early in development-namely, discrimination among nonnative phonemes (Hindi retroflex /d̪a/ and dental /ɖa/), among chimpanzee faces, and among inverted human faces. Like the superiority of 6-mo-old infants over older infants, the synesthetic groups were significantly better than control groups at making all the nonnative discriminations across five samples and three testing sites. The consistent superiority of the synesthetic groups in making discriminations that are normally eliminated during infancy suggests that residual cortical connectivity in synesthesia supports changes in perception that extend beyond the specific synesthetic percepts, consistent with the incomplete pruning hypothesis.

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.

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'.

Emotion perception improvement following high frequency transcranial random noise stimulation of the inferior frontal cortex

Facial emotion perception plays a key role in interpersonal communication and is a precursor for a variety of socio-cognitive abilities. One brain region thought to support emotion perception is the inferior frontal cortex (IFC). The current study aimed to examine whether modulating neural activity in the IFC using high frequency transcranial random noise stimulation (tRNS) could enhance emotion perception abilities. In Experiment 1, participants received either tRNS to IFC or sham stimulation prior to completing facial emotion and identity perception tasks. Those receiving tRNS significantly outperformed those receiving sham stimulation on facial emotion, but not identity, perception tasks. In Experiment 2, we examined whether baseline performance interacted with the effects of stimulation. Participants completed a facial emotion and identity discrimination task prior to and following tRNS to either IFC or an active control region (area V5/MT). Baseline performance was a significant predictor of emotion discrimination performance change following tRNS to IFC. This effect was not observed for tRNS targeted at V5/MT or for identity discrimination. Overall, the findings implicate the IFC in emotion processing and demonstrate that tRNS may be a useful tool to modulate emotion perception when accounting for individual differences in factors such as baseline task performance.

Social perception and aging: The relationship between aging and the perception of subtle changes in facial happiness and identity

Previous findings suggest that older adults show impairments in the social perception of faces, including the perception of emotion and facial identity. The majority of this work has tended to examine performance on tasks involving young adult faces and prototypical emotions. While useful, this can influence performance differences between groups due to perceptual biases and limitations on task performance. Here we sought to examine how typical aging is associated with the perception of subtle changes in facial happiness and facial identity in older adult faces. We developed novel tasks that permitted the ability to assess facial happiness, facial identity, and non-social perception (object perception) across similar task parameters. We observe that aging is linked with declines in the ability to make fine-grained judgements in the perception of facial happiness and facial identity (from older adult faces), but not for non-social (object) perception. This pattern of results is discussed in relation to mechanisms that may contribute to declines in facial perceptual processing in older adulthood.