P 51. Spatial remapping in the audio-tactile ventriloquism effect: A TMS investigation on the role of the ventral intraparietal area

Abstract

Question Previous studies have suggested that the putative human homologue of the ventral intraparietal area (hVIP) is crucially involved in the remapping of touch into external spatial coordinates and in the realignment of tactile and visual maps. However, it is yet unclear whether hVIP is critical for the remapping process during audio-tactile spatial interactions as well. The audio-tactile ventriloquism effect, where the perceived location of a sound is shifted toward the location of a synchronous but spatially disparate tactile stimulus, was used to probe spatial interactions in audio-tactile processing. Behavioral studies of the audio-tactile ventriloquism effect have shown that with hands crossed over the body midline, auditory localization is biased toward the external location of the tactile stimulus, rather than toward the anatomical side of the hand that was stimulated. Therefore, we hypothesized that interference with the tactile remapping process from anatomical to external coordinates by targeting the hVIP with TMS-induced transient virtual lesions would reverse this effect. Methods Eighteen healthy young adults participated in the study. Participants were asked to report the perceived location of brief auditory stimuli presented from three different locations (left, −9°; center, 0°; right, +9°). Auditory stimuli were presented either alone (unimodal stimuli) or together with a synchronous but spatially discrepant tactile stimulus applied to the left or right index fingers (bimodal stimuli), which were located to the left and right side of the speaker array at −18.5° and +18.5°. Crucially, arm posture was alternated between experimental blocks, with participants either adopting a parallel hand posture or crossing the hands over the body midline. Single TMS pulses (with an intensity of 120% of the resting motor threshold) were delivered 80 ms after the onset of each stimulus, either to the right hVIP or the right primary somatosensory cortex (SI, control site) using online neuro-navigation based on individual high-resolution MRI scans. For a subgroup of ten participants, additional control conditions were run, with either sham stimulation of the hVIP or no TMS at all. Results In agreement with the hypothesis that hVIP–TMS would interfere with the tactile remapping process, we indeed found an anatomically defined shift of sound localization in the crossed posture condition when TMS was applied to the hVIP. More specifically, when the left hand was crossed over the body midline, we observed a localization shift toward the left (rather than the right) external side selectively when the right hVIP was targeted with TMS. Neither stimulation of the control site (SI) nor sham stimulation of the hVIP led to an anatomically defined ventriloquism effect. Conclusions Thus, hVIP–TMS interfered with the remapping of touch into an external reference frame. This finding suggests that hVIP is crucially involved in transforming spatial reference frames across audition and touch. Acknowledgments The study was supported by the European Community‘s Seventh Framework Programme (grant agreement no 228916) through the NOMS project, by the Werner-Otto-Foundation (to F.H.) and by the SFB 936 C4 (to F.H.).