Haptic perception and synaesthesia

Jamie Ward, Michael J. Banissy and Clare N. Jonas

Synaesthesia involving haptic perception has been less well documented than other forms of synaesthesia. There are several possibilities why this might be. Firstly, it may well be less common than other types of synaesthesia. Day [1] reports that only 4.0% of synaesthetes report coloured touch and 0.8% report vision-to-touch, compared to 68.8% reporting coloured graphemes (note: these are percentages of synaesthetes, not percentages of general population). A second reason is that researchers don’t always ask about touch and synaesthetes don’t always volunteer it. We made a chance discovery of someone who experiences tactile sensations on her own body when watching someone else being touched as a result of an email request about other forms of synaesthesia. We have since found that other synaesthetes have it too but they didn’t report it until prompted because they considered it ‘normal’ (i.e., they assumed everyone else had it). Preliminary findings suggest that this could be just as common as grapheme-colour synaesthesia, once considered to be the most prevalent type of synaesthesia [2]. This chapter will review research on these types of synaesthesia and consider their neural and cognitive basis. We will draw particular attention to similarities between synaesthetic perception and multi-sensory perception involving vision and touch.
Synaesthetic perceptions have three defining features. They are conscious percept-like experiences that are involuntary and are elicited by a stimulus that is not normally associated with this experience [3]. The synaesthetic percept coexists with the percept of the inducing stimulus rather than over-riding it. Thus, a tactile stimulus may be perceived as a tactile sensation plus a colour sensation. As such, some have regarded synaesthesia as a special instance of the ‘binding problem’ in which two stimulus features (one veridical and one illusory) are combined into a unitary experience [4]. The stimulus that elicits the experience has been termed the ‘inducer’ and the experience itself has been termed the ‘concurrent’. Here we use the terminology of referring to different types of synaesthesia in terms of inducer-concurrent pairs separated with a hyphen. Thus, touch-colour synaesthesia refers to tactile inducers eliciting a concurrent experience of colour, and vision-touch synaesthesia refers to a visual inducer eliciting a tactile experience. We have also colloquially referred to the latter as ‘mirror touch’ synaesthesia given its similarity to mirror systems for action.