Synaesthesia and comorbidity
Carmichael, Duncan Andrew
Synaesthesia is a hereditary, neurological condition in which common stimuli trigger unexpected secondary sensations. For example, reading letters may result in the visualisation of colour, a variant known as grapheme-colour synaesthesia. While synaesthesia is thought to confer a range of benefits such as improved memory, empathy, visual search and creativity to the synaesthete, there is a small, yet growing, body of evidence that suggests synaesthesia may also be associated with more clinical conditions. This thesis investigates potential associations between synaesthesia and a range of clinical conditions, identifying a set of cormorbidities, and exploring the possible genetic roots of these associations. First, I identified an increased prevalence of multiple sclerosis (MS) and its clinical precursor, radiologically isolated syndrome (RIS) in synaesthetes self-referring for participation in scientific studies. Furthermore, I identified an increased occurrence of anxiety disorder in randomly sampled synaesthetes. In addition, I show that synaesthetes with anxiety disorder experience reduced luminance in their synaesthetic colours. I also conducted an association study into the genetic origins of synaesthesia and propose the immune hypothesis of synaesthesia, which provides a theoretical basis for comorbidities (linked to the altered cortical connectivity thought to underlie the development of synaesthesia). Finally, in phenotyping synaesthesia in individuals, I also validated the most widely used online test for synaesthesia, and use this test to provide a reliable prevalence of grapheme-colour synaesthesia in the general population. Such baselines are important for establishing whether other (e.g., clinical) populations are showing rates of synaesthesia higher than otherwise expected. I also demonstrate there is no significant difference in grapheme-colour synaesthesia prevalence between the sexes and discuss its implications for genetic theories of synaesthesia.