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Pairing two brief auditory beeps with a single flash can evoke the percept of a second, illusory, flash. Investigations of the underlying neural mechanisms are limited to post-stimulus effects of this sound-induced illusory flash. We investigated whether touch modulates the visual evoked potential in a similar vein, and also looked at pre-stimulus activity. Electroencephalogram (EEG) was recorded over occipital and parieto-occipital areas of 12 observers. We compared bimodal EEG to its unimodal constituents (i.e., the difference waves) and found significant positive deflections around 110 ms and 200 ms and negative deflections around 330 ms and 390 ms from stimulus onset. These results are similar to those reported for the sound-induced illusion, albeit somewhat later. Furthermore, comparison of the EEG activity between those trials in which the illusion was perceived and those in which it was absent revealed that the phase of pre-stimulus alpha was linked to perceiving the illusion or not. We conclude that touch can modulate activity in the visual cortex and that similar neural mechanisms underlie perception of the sound- and touch-induced illusory flash and that the phase of the alpha wave at the moment of presentation that affects perception., (Copyright © 2014 Elsevier Ireland Ltd. All rights reserved.)

In the illusory flash paradigm, a single flash may be experienced as two flashes when accompanied by two beeps or taps, and two flashes may be experienced as a single flash when accompanied by one beep or tap. The classic paradigm restricts responses to '1' and '2' (2-AFC), ignoring possible qualitative differences between real and illusory flashes and implicitly assuming that illusory flashes are indistinguishable from real flashes. We added a third response category 'different from that of either 1 or 2 flashes' (3-AFC). Eight naïve and 6 experienced observers responded to 160 real and 160 illusory flash trials. Experienced observers were exposed to 1,200 trials before the experiment but without receiving feedback on their performance. The third response category was used for only 4 % of the real flash trials and for 44 % of the illusory flash trials. Experienced observers did so more often (78 %) than naïve observers (18 %). This shows that observers can reliably identify illusory flashes and indicates that mere exposure to illusory flash trials (without feedback) is enough to detect and classify potential qualitative differences between real and illusory flashes.

In Illusory Flash (IF) experiments, congruent multisensory presentation has no effect on the mean estimate of the number of events, but decreases the variance in comparison with unisensory presentation. In contrast, congruent multisensory presentation in other Temporal Numerosity Judgement (TNJ) tasks affects the mean estimate (i.e., it often results in a reduction in underestimation) and increases the variance. In three experiments, we investigated the differences between both paradigms as possible causes of this discrepancy: the presence or absence of incongruent stimuli (Experiment 1), the instruction to the observer to either count flashes, beeps or multisensory events (Experiment 2), and the range of pulses presented (Experiment 3). We found significant differences between the mean numerosity estimate of multisensory and unisensory series in Experiment 3, but not in 1 and 2. This suggests that the difference in the range of pulses presented in IF (1-3 pulses) and TNJ (1-10 pulses) is the primary cause of the discrepancy. In the discussion we propose that this result may be explained by the use of two different strategies and their susceptibility to multisensory presentation. For small pulse numbers, observers can accurately count both unisensory and multisensory pulses. For larger numbers, observers can no longer count but will estimate the number based on the pulse series duration which is improved for multisensory stimuli.

Irrelevant events in one sensory modality can influence the number of events that are perceived in another modality. Previously, the underlying process of sensory integration was studied in conditions in which participants knew a priori which sensory modality was relevant and which was not. Consequently, (bottom-up) sensory interference and (top-down) selective attention were confounded. We disentangled these effects by measuring the influence of visual flashes on the number of tactile taps that were perceived, and vice versa, in two conditions. In the cue condition, participants were instructed on which modality to report before the bimodal stimulus was presented. In the no-cue condition, they were instructed after stimulus presentation. Participants reported the number of events that they perceived for bimodal combinations of one, two, or three flashes and one, two, or three taps. Our main findings were that (1) in no-cue conditions, the influence of vision on touch was stronger than was the influence of touch on vision; (2) in cue conditions, the integration effects were smaller than those in no-cue conditions; and (3) irrelevant taps were less easily ignored than were irrelevant flashes. This study disentangled previously confounded bottom-up and top-down effects: The bottom-up influence of vision on touch was stronger, but vision was also more easily suppressed by top-down selective attention. We have compared our results qualitatively and quantitatively with recently proposed sensory-integration models.