Neural correlates of induced grapheme-color synesthesia

Abstract

Synesthesia is a perceptual anomaly where stimulation of one sensory modality elicits sensation of a concurrent stimulus from another modality. A previous study has shown that it is possible to establish such an alteration of perception in non-synesthetes by means of hypnotic suggestion. We tried to extend these results to gain further insight in the underlying neural processes using magnetoencephalography (MEG) to record neural activity. Hypnotic suggestion has been proven to be a particular powerful tool to change even highly automatic processes, at least in a small population of individuals that are highly susceptible. Participants were administered a formal hypnotic induction and then suggested to perceive specific digits in previously assigned colors. Subsequently, they had to complete a simple digit detection task, where grey digits were shown on a colored background that was either congruent or incongruent to the beforehand learned associations. After the task a second hypnosis session was conducted to cancel the suggestions for synesthetic perception. MEG of the neural activity was recorded in a baseline session at the beginning, during the two hypnosis sessions and throughout the digit detection task. The results indicate that there was a significant alteration of perception, however not strong enough to modulate behavior in a measurable way. Nevertheless, the study yielded proof for hypnotically induced trance being an altered state of consciousness with significant changes in neural activity. Furthermore we were able to derive some conclusions on neural correlates of hypnotic suggestion, which are however not as clear as the evidence for the altered state. Much further research will be needed to explore both of the phenomena more precisely. For not only is hypnotic suggestion of practical importance to the therapist, but it also has broad implications for especially neuroscientific research as well. And given the fascinating nature of synesthesia, it is needless to say that it certainly would be of great interest to continue research in this field, too.