Seeing sounds and tingling tongues: Qualia in synaesthesia and sensory substitution

In this paper we wish to bring together two seemingly independent areas of research: synaesthesia and sensory substitution. Synaesthesia refers to a rare condition where a sensory stimulus elicits not only the sensation that stimulus evokes in its own modality, but an additional one; a synaesthete may thus hear the word “Monday”, and, in addition to hearing it, have a concurrent visual experience of a red color. Sensory substitution, in contrast, attempts to substitute a sensory modality that a person has lost by transforming the information it provided so that it can be accessed through another, intact sensory modality. To make visual information accessible to a blind person, for example, data taken by a camera would be transformed into tactile or auditory information. What do synaesthesia and sensory substitution have in common? Research in both of these areas contributes to our understanding both of cross-modal cooperation and of sensory sensations or qualia, asking under what circumstances these can arise in a modality that is not stimulated. Synaesthesia reveals that this “sensory cross-activation” is possible, and sensory substitution research hopes to induce it. In this article, we will review briefly the literature on synaesthesia, and discuss the issue of qualia for this domain of research. We will then address the evidence for synaesthesia and visual qualia in the blind, and the research on sensory substitution, to finally ask whether sensory substitution may induce a ‘synthetic’ form of synaesthesia by taking advantage of the nervous system’s capacity for generating visual images in the absence of retinal input.