Temporal Aspects of Sound Induced Visual Fission and Fusion in the Double-Flash Illusion Paradigm

Abstract

The sound-induced flash illusion (SIFI) is thought to be due to early audiovisual connections which allow rapid interplay between early auditory and visual cortices. However, the temporal relationship between the perception of physically presented and additional illusory flashes has not yet been directly addressed. We presented participants with a stream of four brief auditory white noise bursts separated by gaps of 60ms (SOA=70ms) and one or two brief flashes which could appear in any of the gaps between noise bursts (positions 1–4) or before the auditory stream (position 0). All possible combinations were randomized and presented equally often. First, participants were asked to indicate the perceived number of flashes, and second, to indicate the perceived position of the flash(es). Thus we could compare the physical separation of the flashes with the perceived separation. We report the following results: 1) In illusory-fission trials the average perceived distance between the perceived flashes is ∼100ms. 2) SIFI is strongest when the flash is presented at position 1 and 2. 3) The extent of experienced fission-illusion depends on performance in aligning flashes with auditory gaps. 4) We observed auditory induced de-fusion which is strongest at position 1 and again correlated to overall task performance.