Visual adaptation changes the susceptibility to the fission illusion.

Sound-induced flash illusion (SiFI) is the illusion that participants perceive incorrectly that the number of visual flashes is equal to the number of auditory beeps when presented within 100 ms. Although previous studies found that repetition suppression can reduce an individual's perceptual sensitivity to the SiFI, there is not yet a consensus as to how visual adaptation affects the SiFI. In the present study, we added prolonged adapting visual stimuli prior to the presentation of audiovisual stimuli to investigate whether the bottom-up factor of adaptation affects the SiFI. The adapting visual stimuli consisted of one or two of the same visual stimuli that lasted for 2 minutes in succession, followed by the audiovisual stimuli. Both adaptation conditions showed SiFI effects. The accuracy of adapting double-flashes was significantly lower than that of in adapting a single flash for the fission illusion. Our analyses indicated that such a pattern could be attributed to a lower d' in adapting double-flashes than in adapting a single flash. However, the accuracy, discriminability and criterion were not significantly different between the two adaptation conditions because of the instability of the fusion illusion. Thus, the present study indicated that the reduced perceptual sensitivity based on visual adaptation could enhance the fission illusion in multisensory integration.