Reduced audio–visual integration in synaesthetes indicated by the double-flash illusion

It has been suggested that synaesthesia is the result of a hyper-sensitive multimodal binding-mechanism. To address the question whether multi-modal integration is altered in synaesthetes in general, grapheme-colour and auditory–visual synaesthetes were studied using the double-flash illusion. This illusion is induced by a single light flash presented together with multiple beep sounds, which is then perceived as multiple flashes. By varying the separation of auditory and visual stimuli, the hypothesis of a widened temporal window of audio–visual integration in synaesthetes was tested. As hypothesised, the results show differences between synaesthetes and controls concerning multisensory integration, but surprisingly other than expected synaesthetes perceive a reduced number of illusions and have a smaller time-window of audio–visual integration compared to controls. This indicates that they do not have a hyper-sensitive binding mechanism. On the contrary, synaesthetes seem to integrate even less than controls between vision and audition.

► Multimodal integration is examined in synaesthetes using the double-flash illusion.
► Synaesthetes perceive a reduced number of illusions compared to controls.
► Further, they show a smaller time-window of audio–visual integration.
► The results suggest that synaesthetes integrate less between vision and audition.
► They further indicate that they do not have a hyper-sensitive binding mechanism.