Is A always red? Multisensory integration of synesthetic stimuli in synesthetes and non-synesthetes

Our research project aimed at investigating multisensory temporal integration in synesthesia and explore whether or not there are commonalities in the sensory experiences of synesthetes and non-synesthetes. Specifically, we investigated whether or not synesthetes are better integrators than non-synesthetes by examining the strength of multisensory binding (i.e., the unity effect) using an unspeeded temporal order judgment task. We used audiovisual stimuli based on grapheme-colour synesthetic associations (Experiment 1) and on crossmodal correspondences (e.g., high-pitch — light colours; Experiment 2) presented at various stimulus onset asynchronies (SOAs) with the method of constant stimuli. Presentation of these stimuli in congruent and incongruent format allowed us to examine whether congruent stimuli lead to a stronger unity effect than incongruent ones in synesthetes and non-synesthetes and, thus, whether synesthetes experience enhanced multisensory integration than non-synesthetes. Preliminary data support the hypothesis that congruent crossmodal correspondences lead to a stronger unity effect than incongruent ones in both groups, with this effect being stronger in synesthetes than non-synesthetes. We also found that synesthetes experience stronger unity effect when presented with idiosyncratically congruent grapheme-colour associations than in incongruent ones as compared to non-synesthetes trained in certain grapheme-colour associations. Currently, we are investigating (Experiment 3) whether trained non-synesthetes exhibit enhanced integration when presented with synesthetic associations that occur frequently among synesthetes. Utilizing this design we will provide psychophysical evidence of the multisensory integration in synesthesia and the possible common processing mechanisms in synesthetes and non-synesthetes.