Localizing and reacting to tactile events on the skin requires the coordination of several spatial maps. In particular, since the primary somatosensory cortex represents skin location independently of body posture, the brain must re-align tactile coordinates in order to retrieve the location of events in external space. We have investigated the temporal course of this process as well as its physiological bases.

In a first (psychophysical) experiment, participants held their arms crossed and performed a discrimination task on lateralised visual targets presented near the hands, after receiving a non-predictive tactile cue on the same or different hand. During the first hundred milliseconds after the tactile cue, reaction times were speeded up for anatomically congruent but spatially incongruent visual targets. This pattern reversed at longer cue-target intervals, so that tactile cues now produced a facilitation of visual targets presented at the same external location. These results reveal the time course of the dynamic remapping of tactile space, with early somatotopic representations, reflecting the neural organization in SI, giving way to the later external frame of reference that characterizes our everyday life experience. From the time course of the cueing effects as well as previous findings, we reasoned that some structures in the posterior parietal cortex (PPC) may be critical for this tactile remapping.

In a second experiment we therefore aimed to disrupt processing in the PPC during remapping using TMS. We designed a tactile remapping task involving unilateral stimulation, appropriate for unilateral TMS pulses. Participants held their left arm flexed with the forearm in vertical position, lateral to the face. They were asked to judge whether a tap at one of several forearm locations was higher or lower than a preceding tap at one of several possible locations on their face. Location of the taps on arm and face varied randomly and the arm was passively moved between different vertical positions. Therefore, integration of spatial information from touch and proprioception was necessary to achieve the task. Single pulse TMS was applied at the scalp, targeting the right ventral intraparietal area (VIP) or a control site (vertex), 60 or 250 ms after the arm tap. The Just Noticeable Difference in the elevation judgement task was poorer following VIP TMS than control TMS, suggesting disruption of tactile remapping process. Participants also consistently underestimated the elevation of arm taps (i.e. increased proportion of “down� responses) after TMS in the VIP condition, producing a shift in the point of subjective equality (PSE). A control experiment showed that perceived position of the hand was not affected by VIP TMS, ruling out explanations based on simple proprioceptive masking. Our TMS results indicate that parietal disruption selectively impairs remapping of touch into egocentric space. The effects of PSE in the VIP condition are discussed in terms of reversion of the body schema towards a canonical arm position.