Synaesthesia: The existing state of affairs

Abstract

In synaesthesia one type of stimulation evokes the sensation of another, such as when hearing a sound produces photisms—that is, mental percepts of colours. In the past, the idiosyncrasy of this phenomenon, as well as the natural mistrust of scientists towards the subjective, consigned synaesthesia to the periphery of scientific interest. However, the landscape has changed radically in the last two decades. The labour of many researchers, inside as well as outside of cognitive neuroscience, has transformed synaesthesia into a scientific reality whose existence can be demonstrated and studied empirically. The present paper summarizes and reflects on our current knowledge concerning synaesthesia in all its aspects (cognition, behaviour, neurology, genetics, and demographics).

Keywords:

• Synaesthesia
• Cross-modal perception
• Neurocognitive models of synaesthesia

Edward M. Hubbard provided helpful comments on the manuscript. We would also like to thank three anonymous reviewers for their comments. Matej Hochel's research activities are supported by a scholarship from Agencia Espanola de Cooperacion Internacional (AECI), Ministry of Foreign Affairs (Spain). This study was supported by grant (SEJ2006–09029) from the Spanish Ministry of Education and Science to E. G. Milán.

Notes

¹Typically synaesthetes claim they have been synaesthetic from as early as they can remember. No cases of spontaneous remission of synaesthesia have been reported so far even though synaesthetic capacity can sometimes be lost as a consequence of cerebral trauma. (See Sacks, Waserman, Zeki & Siegel, 1988; Spalding & Zangwill, 1950.)

²It must be noted that even in this study (Simner et al., 2006), testing of certain subtypes of synaesthesia was excluded for ethical and/or technical reasons: namely, the variants triggered by, or inducing, pain and emotional states.

³For instance, when hearing phonemes leads to the perception of photisms, we are dealing with a cognitive intermodal subtype of synaesthesia.

⁴It is not clear whether this classification reflects a dichotomic dimension, which corresponds to qualitative differences between the synaesthetes, or whether there is a continuum with cases laying “halfway” between the “lower” and the “higher” synaesthesia.

⁵Ward et al. (in press) suggest that the projector–associator dichotomy needs to be further subdivided, in order to include all types of frames of reference reported by synaesthetes. See the original paper for a full account on this topic.

⁶It must be noted that in both studies the authors did not check for (or do not report on) the presence of subtypes of synaesthesia other than the grapheme–colour variety.

⁷In theory, the opposite result is also conceivable—that is, the participant could experience graphemes in response to colours. See the discussion by Hubbard and Ramachandran (2005), where they argue why such a possibility is much less plausible.

⁸A similar mechanism was proposed earlier by Armel and Ramachandran (1999) to explain acquired synaesthesia, observed in a patient who became blind as a consequence of suffering retinitis pigmentosa.

⁹Nonetheless, Ramachandran and Hubbard (2005) argue that the phenomenon of congenital synaesthesia can differ from the drug-induced experiences. Superficial similarity of the phenomena should not be straightforwardly interpreted in terms of identical neuronal mechanisms.