Combining TMS and tDCS to investigate peripersonal space representations in the Human motor system

Interacting with the external world requires the integration of multisensory cues about environmental stimuli with information about the body in a coherent representation of the Peripersonal Space (PPS). A network of fronto-parietal regions, involving the ventral premotor cortex (vPMc) and the posterior parietal cortex (PPc), might support this function, since neurons in these areas integrate somatosensory with visual and acoustic stimuli near the body. Although recent TMS studies have shown that stimuli presented near or far from the hand are capable of modulating the excitability of the hand motor representation, information on the causal influence of PPS network in this motor modulation is lacking. Here we test whether motor modulation contingent upon the processing of stimuli within PPS relies on the activity of two key nodes of the putative human PPS network, namely the vPMc and the PPc. To this aim we used a ‘perturb-and-measure’ paradigm combining tDCS and TMS techniques. Cathodal tDCS (15 min, 1mA) was applied to transiently suppress cortical excitability in two target areas (vPMc and PPc) or in V1, serving as control site. Single-pulse TMS (120% of rMT) was used to measure hand corticospinal excitability and motor-evoked potentials (MEPs) from the right FDI muscle were recorded and compared when an acoustic stimulus was presented either near the hand or in the far space. TMS pulses were delivered at 3 different intervals (50, 175, and 300 ms) after sound presentation. In Experiment 1 (16 subjects), MEPs were recorded after Real-tDCS over vPMc (test) or after Sham-tDCS over the same site (sham control). After the sham stimulation, we found a space- and temporal-specific modulation of the hand motor representation: MEP amplitudes recorded at 300 ms after the onset of near sounds were lower than those recorded after far sounds, confirming that stimuli near the hand may reduce hand corticospinal excitability. This motor modulation disappeared after Real-tDCS over vPMc. In Experiment 2 (12 subjects), we compared the effect of Real-tDCS over PPc (test) or over V1 (active control). We found that stimulation of these two regions did not disrupt motor modulation due to stimuli within PPS: MEPs recorded at 300ms from sound onset were again lower when a near, compared to a far sound, was presented. PPS representation implies both (multi)sensory (processing external stimuli potentially approaching the body) and motor functions (preparing appropriate motor responses). By combining tDCS and TMS we showed that inhibitory motor response to sounds presented near the body was disrupted by suppression of vPMc, but not of PPc or V1. Our findings suggest a crucial role of vPMc in the motor representation of the PPS. Thus, the two nodes of the putative PPS network have partially dissociable functions, being vPMc, rather than PPc, critically involved in transforming sensory representations of space into motor responses.