Perceptual processing links autism and synesthesia: a twin study

Abstract

Background. Synesthesia occurs more commonly in individuals diagnosed with Autism Spectrum Condition (ASC) and is associated with autistic traits and autism-related perceptual processing characteristics, including a more detail-focused attentional style and altered sensory sensitivity. Furthermore, autistic traits and autism-like sensory sensitivity show a synesthesia-dosage effect, since they increase with the amount of synesthesia types in synesthetes and with the degree of synesthesia (how consistently colors are associated with graphemes) in non-synesthetes.Methods. Here we investigated a predominantly non-synesthetic twin sample, enriched for ASC and other neurodevelopmental disorders (n=65, 14-34 years, 60% female). We modelled the linear relationships between the degree of synesthesia and autistic traits, sensory sensitivity, and visual perception, both within-twin pairs (22 pairs) where all factors shared by twins are implicitly controlled (including 50-100% genetics), and across the entire cohort.Results. Our results indicate that the degree of synesthesia is associated with autistic traits only within the attention to details domain and with sensory hyper-, but not hypo-sensitivity. These associations were stronger within-twin pairs compared to across the sample. Further, twins with a higher degree of synesthesia were better than their co-twins in identifying fragmented images in the Fragmented Pictures Test (FPT).Conclusions. This study is the first twin study on the association between synesthesia and autism-related perceptual and clinical features, and the results suggest that a twin design can be more sensitive for detecting these associations. Consistent with previous findings, the results support an association between the degree of synesthesia and autism-related perceptual features, while utilizing a different self-report measure for sensory sensitivity. The novel finding of enhanced feature integration in the FPT in twins with a higher degree of synesthesia challenges the view of a generally more detail-focused attentional style in synesthesia and might be related to enhanced memory or mental imagery in more synesthetic individuals.

Genetic and environmental architecture of synesthesia and its association with the autism spectrum - A twin study

“…A genetic link between synesthesia and autism or autistic traits is in line with the nding of high prevalence of both synesthesia and autism in a family study (9). Our results also correspond to the outcomes of a previous study conducted by our group in a different, smaller twin cohort, showing that twins with more consistent grapheme-color associations assessed with an objective synesthesia test scored higher on the attention to detail domain of the AQ, compared to their co-twins (32). This latter study indicated an association between the degree of (sub-threshold) synesthesia and the attention to detail domain of the AQ when implicitly controlling for all factors shared by twins, underlining the importance of factors that are not shared by twins in this association, but without being able to differentiate non-shared genetic from non-shared environmental factors.…”

“…We used dimensional de nitions of both synesthesia and autism, rather than binary de nitions based on diagnostic criteria. This approach is justi ed a) because it allows modelling more variance (also between people not ful lling the criteria for synesthesia or autism), b) because autism is de ned as the extreme end of continuously distributed autistic traits (30) and c) because continuous de nitions of synesthesia have also been used successfully in previous research on the association between synesthesia and autism (21,22,31,32). For example, the number of different synesthesia types a synesthete has correlates positively with autistic traits, especially in the domain of attention to detail (21,22).…”