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L. Shams, Y. Kamitani, S. Shimojo; Sound modulates visual evoked potentials in humans. Journal of Vision 2001;1(3):479. doi: 10.1167/1.3.479.
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Purpose: We have previously shown that sound can alter the visual perception radically and qualitatively (Shams et al., Nature '00). It is not clear, however, at what level of perceptual processing the interaction between the signals of different modalities occurs. This interaction can occur anywhere from the early sensory processing to late modality-specific to polysensory stages. We aimed to investigate whether the cross-modal influences on visual perception occur at the level of visual pathway, or rather the visual sensory activity is independent of the signals of other modalities as is generally believed. To this end, we compared the visual evoked potentials (VEP) in the presence and absence of sound. Methods: We measured the VEPs of seven participants in three different conditions. Trials, 120 per condition, were randomized. Condition V consisted of visual stimulation (a white disk of 2 deg. size flashed at 5 degrees eccentricity on a black background on a CRT). Control condition A consisted of auditory stimulation (2 brief sounds lasting for 7ms each played from a speaker adjacent to the screen). Condition AV consisted of audio and visual stimulation; corresponding to the combination of the stimuli in conditions A and V. VEPs were recorded from electrodes MO, O1, and O2. For each electrode, the average of the difference wave AV-(A+V) was calculated across participants. This difference wave was taken as the measure of change in the activity of the visual areas by sound. Results: We found that difference wave had an amplitude which differed from zero significantly between 100 and 200 ms poststimulus. Conclusion: Considering that VEPs prior to 200ms are believed to be due to modality specific neural activity, these results suggest that the activity in visual cortex is modulated by sound. This implication challenges the general belief that the visual cortical processing is independent of other modalities.
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