THE ROLE OF FRONTO-PARIETAL AREAS IN THE MOTOR REPRESENTATION OF THE PERIPERSONAL SPACE (PPS)

Introduction: In order to interact with the external world, our brain integrates 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 [1,2]. Previous TMS studies have highlighted the motor counterpart of PPS by showing that stimuli presented near or far from the hand are capable of modulating the excitability of the hand representation in the motor cortex (M1) [3,4]. Here we test whether M1 modulation due to PPS representation relies on the activity of two key nodes of the PPS fronto-parietal network, namely vPMc and PPc. To this aim we used a ‘perturb-and-measure’ paradigm [5]: transcranial direct current stimulation (tDCS) was applied to transiently suppress activity in vPMc or PPc; then single-pulse TMS was used to measure the excitability of the hand motor representation, when an acoustic stimulus was presented either near the hand or in the far space. Methods: 15 minutes of cathodal, inhibitory, tDCS were applied to target areas (vPMc and PPc) or to V1, serving as a control site. Single-pulse TMS was applied to the hand area of M1 and motor-evoked potentials (MEPs) from FDI muscle were recorded after presenting a sound either at ≈5 cm (NEAR sounds) or at ≈100 cm (FAR sounds) from the hand. TMS pulses were delivered at 120% of resting motor threshold (rMT), at 3 different intervals (50, 175, and 300 ms) after sound presentation. In Experiment 1 (16 subjects), we compared MEPs after Real-tDCS over vPMc (test) or after Sham-tDCS over the same site (sham control). In Experiment 2 (12 subjects), we compared the effect of Real-tDCS over PPc (test) or over V1 (active control). Results: After the sham stimulation, we found a space-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 suppress hand M1 excitability [3,4]. This motor modulation disappeared after Real-tDCS over vPMc. In contrast, Real-tDCS over PPc or V1 did not affect the space-specific modulation of M1: MEPs recorded at 300ms from sound onset were again lower when a near, compared to a far sound, was presented. Conclusions: PPS representation implies both (multi)sensory (processing external stimuli potentially approaching the body) and motor functions (preparing appropriate motor responses) [1-4]. By using a tDCS-TMS perturb-and-measure paradigm [5] 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 fronto-parietal network representing the PPS have partially dissociable functions, being vPMc, rather than PPc, mainly involved in transforming sensory representations of space in motor responses. References: 1. Graziano MS, Cooke DF (2006) Parieto-frontal interactions, personal space, and defensive behavior. Neuropsychologia 44, 845-859. 2. Bremmer F, Schlack A, Shah NJ, Zafiris O, Kubischik M, Hoffmann K, et al. (2001) Polymodal Motion Processing in Posterior Parietal and Premotor Cortex: A Human fMRI Study Strongly Implies Equivalencies between Humans and Monkeys. Neuron 29, 287-296 3. Serino A, Annella L, Avenanti A (2009) Motor Properties of Peripersonal Space in Humans. PLoS ONE 4, e6582. 4. Makin TR, Holmes NP, Brozzoli C, Rossetti Y, Farnè A (2009) Coding of visual space during motor preparation: Approaching objects rapidly modulate corticospinal excitability in hand-centered coordinates. Journal of Neuroscience 29, 11841-51. 5. Avenanti A, Bolognini N, Maravita A, Aglioti SM (2007) Somatic and motor components of action simulation. Current Biology 17, 2129-35.