In: Hallucinations. Research and Practice, ed. by Blom, J. D., & Sommer, I. New York, Springer, chap. 7, pp. 91-104. (ISBN: 978-1-4614-0958-8).
Synaesthesia is an atypical neurological condition in which an individual experiences an ancillary affective, cognitive, or perceptual event in addition to that normally elicited by a stimulus. It manifests in a wide variety of different forms with a substantial number of synaesthetes experiencing multiple forms. However, it remains unclear whether these diverse forms collectively represent a uniform phenomenon. Synaesthesias display considerable phenomenological similarities across individuals, including automaticity and consistency, but there are a number of dimensions on which synaesthesias vary, even amongst individuals with the same form. There is consensus that synaesthesia results from greater functional coordination between cortical regions supporting the processing of the stimulus and the concurrent experience, but it remains uncertain whether this effect is best explained by excess structural connectivity or cortical disinhibition. Studying the characteristics of synaesthesias and individual differences between and among its different forms may provide insights into the nature of hallucinations and subjective experience.