In monkeys, bimodal neurons in premotor and parietal areas are thought to underlie a multisensory representation of the space around the body, i.e. peripersonal space (PPS; Graziano & Cooke, Neuropsychologia, 2006). In humans, neuroimaging data show that neural activity in the ventral premotor (VPM) and in the posterior parietal (PPC) cortex is modulated by poli-sensory stimuli approaching the body (see Makin et al., Behav Brain Res, 2008). In the present study, we directly tested in humans whether VPM and PCC have a necessary role in a multisensory representation of the PPS.
We used low frequency r-TMS to induce a transient virtual lesion of these two areas and of V1, serving as a control site, and tested whether this experimental manipulation affected audio-tactile integration in the space around the hand. Subjects performed a speed response task to a tactile stimulus on their right (controlateral to TMS stimulation) hand while concurrent task-irrelevant sounds were presented either near the hand (at 5 cm) or in the far space (at 100 cm).
In a Baseline condition, when no TMS stimulation was delivered, reaction time to tactile targets was speeded up if a sound was presented near the hand rather than in the far space, thus showing multisensory integration within the boundaries of the PPS. The speeding effect associated to near sounds disappeared when the same task was performed after inhibitory TMS over VPM and over PPC, thus suggesting a disruption of the multisensory integrative mechanism around the hand. Such result was not a generic consequence of TMS, because when the stimulation was administered over the control site V1, the response to tactile targets was faster when near rather than far sounds were presented.
These findings show that audio-tactile integration around the hand depends on the function of both the ventral premotor cortex and of the posterior parietal cortex.