Motor properties of peripersonal space in humans

Abstract

Background: A stimulus approaching the body requires fast processing and appropriate motor reactions. In monkeys, fronto-parietal networks are involved both in integrating multisensory information within a limited space surrounding the body (i.e. peripersonal space, PPS) and in action planning and execution, suggesting an overlap between sensory representations of space and motor representations of action. In the present study we investigate whether these overlapping representations also exist in the human brain.

Methodology/principal findings: We recorded from hand muscles motor-evoked potentials (MEPs) induced by single-pulse of transcranial magnetic stimulation (TMS) after presenting an auditory stimulus either near the hand or in far space. MEPs recorded 50 ms after the near-sound onset were enhanced compared to MEPs evoked after far sounds. This near-far modulation faded at longer inter-stimulus intervals, and reversed completely for MEPs recorded 300 ms after the sound onset. At that time point, higher motor excitability was associated with far sounds. Such auditory modulation of hand motor representation was specific to a hand-centred, and not a body-centred reference frame.

Conclusions/significance: This pattern of corticospinal modulation highlights the relation between space and time in the pps representation: an early facilitation for near stimuli may reflect immediate motor preparation, whereas, at later time intervals, motor preparation relates to distant stimuli potentially approaching the body.

Figure 1. Experimental set up.

The main panel represents the experimental set up and a typical subject during Experiment 1. The small upper panel represents the sequence of events in each trial. The small lower panel represents a typical subject during Experiment 2, when participants placed their right arm to the side, with the hand pointing backwards (far from the source of near sounds).

Figure 2. Mean MEP amplitude with respect to baseline (MEPi) recorded when sounds were presented NEAR (red lines) and FAR (blue lines) from the subjects' right hand (Experiment 1).

(A) MEPi recorded with lower (120% rMT) TMS pulse intensity. (B) MEPi recorded with higher (140% rMT) TMS pulse intensity. Error bars denote s.e.m. Asterisks indicate a significant NEAR-FAR comparison (p<.05).

Figure 3. Raw MEPs amplitudes recorded from the FDI (top) and the ADM muscle (bottom) in one representative subject from Experiment 1 (only 120% rMT blocks are shown).

Figure 4. Mean MEP amplitude with respect to baseline (MEPi) recorded when sounds were presented NEAR (red lines) and FAR (blue lines) from the subjects' body (Experiment 2).

(A) MEPi recorded with lower (120% rMT) TMS pulse intensity. (B) MEPi recorded with higher (140% rMT) TMS pulse intensity. Error bars denote s.e.m.