Independent mechanisms for ventriloquism and multisensory integration as revealed by theta-burst stimulation

The visual and auditory systems often concur to create a unified perceptual experience and to determine the localization of objects in the external world. Co-occurring auditory and visual stimuli in spatial coincidence is known to enhance performance of auditory localization due to the integration of stimuli from different sensory channels (i.e. multisensory integration). However, auditory localization of audio-visual stimuli presented at spatial disparity might also induce a mislocalization of the sound towards the visual stimulus (i.e. ventriloquism effect). Using repetitive transcranial magnetic stimulation we tested the role of right temporo-parietal (rTPC), right occipital cortex (rOC) and right posterior parietal (rPPC) in an auditory localization task in which indices of ventriloquism and multisensory integration were computed. We found that suppression of rTPC excitability by means of continuous theta-burst stimulation (cTBS) reduced multisensory integration. No similar effect was found for cTBS over rOC. Moreover, inhibition of rOC, but not of rTPC, suppressed the visual bias in the contralateral hemifield. In contrast, cTBS over rPPC did not produce any modulation on ventriloquism or integrative effects. The double dissociation found in the present study suggests that ventriloquism and audio-visual multisensory integration are functionally independent phenomena and may be underpinned by partially different neural circuits