Synesthesia, Then and Now

Puzzling in its diversity and resistant to simple theoretical accounts, synesthesia has been a subject of scrutiny and investigation for more than a century. Over 30 years ago, the present author treated synesthesia as a perceptual, crossmodal phenomenon, in which a stimulus presented in one modality produces an additional sensation in another, and sought to understand synesthesia in light of principles of multisensory processing, under a broad framework of the 'unity of the senses.' Research over the ensuing three decades has highlighted the role of learning and cognition in most kinds of synesthesia, many of which are not cross-modal, while pointing to some of the neural processes associated with synesthetic experience. One approach to understanding synesthesia, monism, treats synesthesia as an end-point of a continuous trait. Another approach, dualism, aims to distinguish synesthesia from non-synesthesia and searches for the common denominators that underlie synesthesia in all of its manifestations. An alternative to both monism and dualism is pluralism, which posits several distinct categories of synesthesia, not all necessarily equal: One category (or more) may be prototypical, a good candidate being cross-modal synesthesia. The principles that characterize cross-modal perceptual synesthesia also characterize cross-modal perception in non-synesthetes, and the mechanisms that underlie prototypical cross-modal synesthesia may serve as the wellspring for the development of synesthesia's diverse other forms.