For people with synesthesia, certain sensory stimuli like sounds, written letters or tastes lead to additional inner sensations such as color, texture or shape. The mechanisms underlying synesthesia are unknown but the brains of synesthetes are atypically connected. While synesthesia is regarded as non-pathological, it is more common for people with Autism Spectrum Disorder (ASD). ASD is characterized by impairments in social functioning, but is also commonly associated with altered sensory sensitivity and enhanced attention to details. People with synesthesia show a similarly altered profile in sensory processing but no deficits in social functioning. The co-occurrence of the two conditions might further be associated with special talents (savant skills) observed in a proportion of people with ASD. Using a twin design, we will investigate the impact of genes vs environment on the link between synesthesia and ASD and the role of altered sensory processing therein. Using brain imaging, we will further investigate patterns of altered brain connectivity - generally and in association with enhanced detail focus - overlapping between the two conditions. The results will inform us about the developmental and neural mechanisms of both synesthesia and ASD and their potentially common genetic and neuronal origins. They will also provide insights into the mechanism underlying enhanced detail focus and the role of synesthesia and sensory processing in the development of savant skills.