A critical review of the neuroimaging literature on synesthesia

Abstract

Synesthesia refers to additional sensations experienced by some people for specific stimulations, such as the systematic arbitrary association of colors to letters for the most studied type. Here, we review all the studies (based mostly on functional and structural magnetic resonance imaging) that have searched for the neural correlates of this subjective experience, as well as structural differences related to synesthesia. Most differences claimed for synesthetes are unsupported, due mainly to low statistical power, statistical errors, and methodological limitations. Our critical review therefore casts some doubts on whether any neural correlate of the synesthetic experience has been established yet. Rather than being a neurological condition (i.e., a structural or functional brain anomaly), synesthesia could be reconsidered as a special kind of childhood memory, whose signature in the brain may be out of reach with present brain imaging techniques.

Keywords: DTI; VBM; connectivity; fMRI; synesthesia.

FIGURE 1

Methodology for searching the functional neural correlates of synesthesia. The letter A may trigger the experience of red for a synesthete but not a control subject (A). In order to isolate the neural correlates (represented as the brain icon of the CerCo lab logo) of this experience, one may compare the brain activations of synesthetes and controls to this stimulus (B). To ensure that any observed difference is due to the synesthetic experience, one may compare the same subjects for similar activations that do not trigger any synesthetic experience, like pseudo-letters or false fonts (C). In such a control experiment, the stimuli are therefore “similar” to those triggering the synesthetic experience, but the phenomenal experience is “different” than the synesthetic experience. One may also compare the activation by synesthetic and real colors (D).