Purpose: We previously reported that a single visual flash presented with multiple beeps with short intervals appears to be multiple flashes (Shams, Kamitani, & Shimojo, Nature 2000). Do the beeps produce mere impression of brightness fluctuation, or rather independent visual tokens? To demonstrate that the illusory flashes can be perceived independently at different spatial locations, a visual apparent motion display was combined with beeps. Methods: Two vertical bars (13 ms duration each) were flashed 3 deg horizontally apart with an interval of 106 ms, creating clear apparent motion. Each bar was accompanied by a beep (10 ms duration, simultaneous onset with the bar). Another beep was presented at a various timing between the flashes/beeps. In a condition without sound, a real additional bar was flashed at an in-between timing at the same position as either of the two bars. Subjects reported the perceived location of the illusory bar associated with the second beep (if any), or that of the additional real bar. The direction of apparent motion was randomized across trials. Results: All subjects (3) reported that an illusory bar was perceived with the second beep at a location between the real bars (difference from the real bars, p < .05). This is analogous to the cutaneous "rabbit" illusion where trains of successive cutaneous pulses delivered at a few widely separated locations produce sensations at many in-between points. The illusory bar appeared closer to the first (second) bar, as the second beep was presented temporally closer to the first (third) beep. The perceived location of the additional real bar was not significantly different from the actual location, thus purely visual "rabbit" was not observed. Conclusion: The sound-induced visual flashing creates visual tokens that can be spatially displaced, indicating visual proper, as opposed to cognitive, alteration, which nonetheless is uniquely induced by sound.

Supported by Caltech and NSF/ERC.