Multisensory processes in the synaesthetic brain — An event-related potential study in multisensory competition situations

In synaesthesia certain external stimuli (e.g., music) trigger automatically internally generated sensations (e.g., colour). Results of behavioural investigations indicate that multisensory processing works differently in synaesthetes. However, the reasons for these differences and the underlying neural correlates remain unclear. The aim of the current study was to investigate if synaesthetes show differences in electrophysiological components of multimodal processing. Further we wanted to test synaesthetes for an enhanced distractor filtering ability in multimodal situations. Therefore, line drawings of animals and objects were presented to participants, either with congruent (typical sound for presented picture, e.g., picture of bird together with chirp), incongruent (picture of bird together with gun shot) or without simultaneous auditory stimulation. 14 synaesthetes (auditory–visual and grapheme-colour synaesthetes) and 13 controls participated in the study. We found differences in the event-related potentials between synaesthetes and controls, indicating an altered multisensory processing of bimodal stimuli in synaesthetes in competition situations. These differences were especially found over frontal brain sites. An interaction effect between group (synaesthetes vs. controls) and stimulation (unimodal visual vs. congruent multimodal) could not be detected. Therefore we conclude that multisensory processing works in general similar in synaesthetes and controls and that only specifically integration processes in multisensory competition situations are altered in synaesthetes.