Abstract
BackgroundThe sound-induced flash illusion is an auditory-visual illusion – when a single flash is presented along with two or more beeps, observers report seeing two or more flashes. Previous research has shown that the illusion gradually disappears as the temporal delay between auditory and visual stimuli increases, suggesting that the illusion is consistent with existing temporal rules of neural activation in the superior colliculus to multisensory stimuli. However little is known about the effect of spatial incongruence, and whether the illusion follows the corresponding spatial rule. If the illusion occurs less strongly when auditory and visual stimuli are separated, then integrative processes supporting the illusion must be strongly dependant on spatial congruence. In this case, the illusion would be consistent with both the spatial and temporal rules describing response properties of multisensory neurons in the superior colliculus.Methodology/Principal FindingsThe main aim of this study was to investigate the importance of spatial congruence in the flash-beep illusion. Selected combinations of one to four short flashes and zero to four short 3.5 KHz tones were presented. Observers were asked to count the number of flashes they saw. After replication of the basic illusion using centrally-presented stimuli, the auditory and visual components of the illusion stimuli were presented either both 10 degrees to the left or right of fixation (spatially congruent) or on opposite (spatially incongruent) sides, for a total separation of 20 degrees.Conclusions/SignificanceThe sound-induced flash fission illusion was successfully replicated. However, when the sources of the auditory and visual stimuli were spatially separated, perception of the illusion was unaffected, suggesting that the “spatial rule” does not extend to describing behavioural responses in this illusion. We also find no evidence for an associated “fusion” illusion reportedly occurring when multiple flashes are accompanied by a single beep.
Highlights
In cases where multiple senses provide congruent information concerning the same external event, multisensory integration can result in perceptual advantages
The increase was strongest in the case where one flash was accompanied by two beeps compared to a single beep
In order to test the effect of beeps on the number of flashes reported, the mean responses for the five 1-flash trials were submitted to a repeated-measures Analysis of Variance (ANOVA), with a 5-level within-subjects factor Beep (0–4 beeps)
Summary
In cases where multiple senses provide congruent information concerning the same external event, multisensory integration can result in perceptual advantages. If an experimenter arranges incongruent information to be presented to each sensory modality, the senses can interfere with each other, causing altered or illusory percepts. Previous research has shown that the illusion gradually disappears as the temporal delay between auditory and visual stimuli increases, suggesting that the illusion is consistent with existing temporal rules of neural activation in the superior colliculus to multisensory stimuli. If the illusion occurs less strongly when auditory and visual stimuli are separated, integrative processes supporting the illusion must be strongly dependant on spatial congruence. In this case, the illusion would be consistent with both the spatial and temporal rules describing response properties of multisensory neurons in the superior colliculus
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