Cortical processes underlying sound-induced flash fusion

When two brief flashes presented in rapid succession (< 100 ms apart) are paired with a single auditory stimulus, subjects often report perceiving only a single flash [Andersen, T.S., Tiippana, K., Sams, M., 2004. Factors influencing audiovisual fission and fusion illusions. Brain Res. Cogn. Brain Res. 21, 301–308; Shams, L., Iwaki, S., Chawla, A., Bhattacharya, J., 2005a. Early modulation of visual cortex by sound: an MEG study. Neurosci. Lett. 378, 76–81, Shams, L., Ma, W.J., Beierholm, U., 2005b. Sound-induced flash illusion as an optimal percept. Neuroreport 16, 1923–1927]. We used event-related potentials (ERPs) to investigate the timing and localization of the cortical processes that underlie this sound induced flash fusion, which is complementary to the sound-induced extra flash illusion that we analyzed previously [Mishra, J., Martinez, A., Sejnowski, T.J. and Hillyard, S.A., Early cross-modal interactions in auditory and visual cortex underlie a sound-induced visual illusion. J. Neurosci. 27 (2007) 4120–4131]. The difference ERP that represented the cross-modal interaction between the visual (two flashes) and auditory (one sound) constituents of the bimodal stimulus revealed a positive component elicited 160–190 ms after stimulus onset, which was markedly attenuated in subjects who did not perceive the second flash. This component, previously designated as PD180 [Mishra, J., Martinez, A., Sejnowski, T.J. and Hillyard, S.A., Early cross-modal interactions in auditory and visual cortex underlie a sound-induced visual illusion. J. Neurosci. 27 (2007) 4120–4131], was localized by dipole modeling to polysensory superior temporal cortex. PD180 was found to covary in amplitude across subjects with the visual evoked N1 component (148–184 ms), suggesting that inter-individual differences in perceiving the illusion are based at least in part on differences in visual processing. A trial-by-trial analysis found that the PD180 as well as a subsequent modulation in visual cortex at 228–248 ms was diminished on trials when the two flashes were perceived as one relative to trials when two flashes were correctly reported. These results suggest that the sound induced flash fusion is based on an interaction between polysensory and visual cortical areas.