Distinct neural processes in grapheme-colour synaesthetes and semantic controls

In synaesthetes, stimulation of one sensory pathway provokes a sensory experience (e.g. a colour concurrent) in a different sensory modality or sub-modality. Results of synaesthetic Stroop and priming tests indicate that the perception of a colour concurrent interferes with the processing of a veridical colour in synaesthetes. We here examined the congruency between a stimulus' colour and the colour concurrent both in grapheme-colour synaesthetes and in non-synaesthetes trained on grapheme-colour associations. Electrophysiological (electroencephalogram) and behavioural measurements were collected during a priming task that included grapheme-grapheme and grapheme-colour patch pairs. To investigate covert bidirectional synaesthesia, an additional inverted colour patch-grapheme condition was included. Both groups of participants showed longer reaction time and more negative-going N300 and N400 event-related potential (ERP) components on incongruent trials. Whereas ERP effects in the non-synaesthetes were largely confined to the late cognitive components N300, P300 and N400, the synaesthetes also showed congruency-dependent modulation of the early sensory component N170. Our results suggest that early cognitive processes distinguish cross-modal synaesthetic perceptions from acquired associations. The involvement of both early- and late-stage cognitive components in bidirectional synaesthesia possibly indicates similar feature-binding mechanisms during processing of opposite flow directions of information, namely grapheme-colour and colour-grapheme.