When Sounds Make You See Double: Brain Mechanisms of Illusory Versus Veridical Visual Perceptions
Poster Presentation

Raphaël Meylan
The Functional Electrical Neuroimaging Laboratory Division de Neuropsychologie, Centre Hospitalier Universitaire Vaudois, Lausanne, Switzerland

Micah M. Murray
The Functional Electrical Neuroimaging Laboratory, Neuropsychology Division and Radiology Service, Centre Hospitalier Universitaire Vaudois, Lausanne, Switzerland

Eric Tardif
The Functional Electrical Neuroimaging Laboratory, Neuropsychology Division,Centre Hospitalier Universitaire Vaudois, and Physiology Institute, University of Lausanne, Lausanne, Switzerland

Sandra Lehmann
The Functional Electrical Neuroimaging Laboratory Division de Neuropsychologie, Centre Hospitalier Universitaire Vaudois, Lausanne, Switzerland

     Abstract ID Number: 100
     Full text: Not available
     Last modified: March 21, 2005

Abstract
Multisensory events can affect subsequent unisensory processing. An auditory stimulus presented shortly after an auditory-visual multisensory pair can result in illusory visual perceptions (Shams et al., 2000 Nature). The rate of seeing this illusion is modulated by the inter-stimulus-interval, making it possible for subjects to have two different perceptions of the same physical stimulus. We investigated if different brain mechanisms generate these two different perceptions and whether illusory and externally-driven perceptions engage similar brain mechanisms, using electrical neuroimaging methods. Subjects completed a 2-alternative forced-choice task, judging the number of visual flashes on each trial where either 1 or 2 flashes (F) were presented with 0, 1, or 2 beeps (B). Subjects were accurate on all conditions, except the 2B1F condition. Here, two flashes were reported instead of one ~60% of the time. Electrical neuroimaging showed that at 90ms post-stimulus different electric field topographies (i.e. generator configurations) accounted for responses to the same stimulus when perceived as two flashes versus one flash. Moreover, this topography was indistinguishable from that elicited by retinally-driven visual perceptions. LAURA Source estimations further implicate both lower- and higher- level visual cortices in this illusion. Sound-induced visual perceptions thus modulate visual responses early in cortical processing.