Several studies have suggested that there is a link between synaesthesia and autism but the nature of that link remains poorly characterised. The present study considers whether atypical sensory sensitivity may be a common link between the conditions. Sensory hypersensitivity (aversion to certain sounds, touch, etc., or increased ability to make sensory discriminations) and/or hyposensitivity (desire to stimulate the senses , or a reduced response to sensory stimuli are a recently introduced diagnostic feature of autism spectrum conditions (ASC). Synaesthesia is defined by unusual sensory experiences and has also been linked to a typical cortical hyper-excitability. The Glasgow Sensory Questionnaire (GSQ) was administered to synaesthetes and people with ASC. Both groups reported increased sensory sensitivity relative to controls with a large effect size. Both groups also reported a similar pattern of both increased hyper- and hypo-sensitivities across multiple senses. The AQ (Autism-Spectrum Quotient) scores were elevated in the synaesthetes, and one subscale of this measure (attention to detail) placed synaesthetes within the autistic range. A standard laboratory test of visual stress (the Pattern Glare Test), administered online, corroborated the findings of increased sensitivity to aversive visual stimuli in synaesthetes. We conclude that atypical sensory sensitivity is an important shared feature between autism and synaesthesia.

Savant syndrome is a condition where prodigious talent co-occurs with developmental difficulties such as autism spectrum conditions (ASC). To better understand savant skills, we previously proposed a link with synaesthesia: that savant syndrome may arise in ASC individuals who also happen to have synaesthesia. A second, unrelated claim is that people with autism may have higher rates of synaesthesia. Here we ask whether synaesthesia is indeed found more often in autism per se,o r only in cases where autism co-occurs with savant skills. People with autism in previous studies when tested for synaesthesia were not differentiated into those with and without savant abilities. Here we tested three groups: people with autism who also have savant skills (n = 40), people with autism without savant skills (n = 34), and controls without autism (n = 29). We used a validated test to diagnose grapheme-colour synaesthesia. Results show a significantly higher prevalence of synaesthesia in people with ASC, but only those who also have savant skills. This suggests that synaesthesia in autism is linked to those with savant abilities rather than autism per se. We discuss the role of synaesthesia in the development of prodigious talent.

BackgroundSavant syndrome is a condition where prodigious talent can co-occur with developmental conditions such as autism spectrum conditions (autism). It is not yet clear why some autistic people develop savant skills while others do not.MethodsWe tested three groups of adults: autistic individuals who have savant skills, autistic individuals without savant skills, and typical controls without autism or savant syndrome. In experiment 1, we investigated the cognitive and behavioural profiles of these three groups by asking participants to complete a battery of self-report measures of sensory sensitivity, obsessional behaviours, cognitive styles, and broader autism-related traits including social communication and systemising. In experiment 2, we investigated how our three groups learned a novel savant skill—calendar calculation.ResultsHeightened sensory sensitivity, obsessional behaviours, technical/spatial abilities, and systemising were all key aspects in defining the savant profile distinct from autism alone, along with a different approach to task learning.ConclusionsThese results reveal a unique cognitive and behavioural profile in autistic adults with savant syndrome that is distinct from autistic adults without a savant skill.Electronic supplementary materialThe online version of this article (10.1186/s13229-018-0237-1) contains supplementary material, which is available to authorized users.

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.

Cognitive benefits associated with grapheme-colour synaesthesia in adults are well documented, but far less is known about whether such benefits might arise in synaesthetes as children. One previous study on a very small group of randomly sampled child synaesthetes found cognitive benefits in short-term memory and processing speed (the ability to quickly scan an array of images and discriminate between them), but was inconclusive for a test of receptive vocabulary. Using a stratified population sample ( Growing Up in Scotland Project, Edinburgh, UK: Scottish Executive, 2007), we investigated the performance of a large cohort of child grapheme-colour synaesthetes using four literacy measures taken at age 10 years. These were three verbal comprehension measures (expressive vocabulary, receptive vocabulary and sentence comprehension) and one measure of academic self-concept in reading (plus one measure of academic self-concept in numeracy as a comparison). After controlling for demographic differences between groups, synaesthetes showed significantly enhanced performance for expressive and receptive vocabulary compared to their peers, but no benefits in sentence comprehension. Child synaesthetes also reported significantly higher academic self-concept for reading, but not for numeracy. Finally, we found that synaesthetes made significantly more progress than controls across the primary school years, although they began school with no a priori advantage. Our study provides powerful new evidence that children with grapheme-colour synaesthesia show vocabulary and literacy differences, which we contextualize within a theory of synaesthetic development. This article is part of a discussion meeting issue ‘Bridging senses: novel insights from synaesthesia’.