Abstract

Synaesthesia has been described as a perceptual experience in which a stimulus presented through one modality will spontaneously evoke a sensation experienced in an unrelated modality. While synaesthesia can occur in response to drugs, sensory deprivation, or brain damage, research has largely focused on individuals with the heritable trait, amounting to roughly 4% of the population. These experiences typically occur from increased connectivity between associated modalities, and synaesthesia is known to be involuntary, automatic, and stable over time. Furthermore, research suggests all individuals show similar cross-modal interactions to those of synaesthetes, begging the question of how different developmental synaesthesia is from acquired, drug-induced, and even sub-threshold synaesthetic associations in the normal population. This review examines the physiological basis of synaesthesia as well as the impact of learning, then works to draw parallels with synaesthesia and other similar phenomena, and closes with our current understanding of hereditary in synaesthesia and directions for future research.