Complexity of visual stimuli affects visual illusion induced by sound

• We examine the effect of complexity of visual stimulus on fission illusion.
• Complex visual stimulus is difficult to induce fission illusion.
• The effects of complexity of visual stimulus depend on SOA.
• The processing speed of stimulus influences audio-visual integration.

The fission illusion is induced by multisensory (audio-visual) integration. In the present study, we assume that perceptual efficiency affects the fission illusion’s rate because this illusion occurs in a short temporal range through the integration of visual and auditory information. The present study examined the effect of perceptual efficiency on the fission illusion by presenting visual patterns with various degrees of complexity. The results indicated that it was more difficult to induce the fission illusion when more complex visual patterns were used. The effect of pattern on the illusion differed according to the stimulus onset asynchrony between the first visual stimulus and the second auditory stimulus. These results suggest that the fission illusion has a higher probability of occurring when the perceptual process of the first visual stimulus is completed and integrated with the first beep before the presentation of the second beep. Thus, the audio-visual integration is affected by the perceptual efficiency of the physical stimuli.