Examining tactile spatial remapping using transcranial magnetic stimulation

Previous research has provided evidence for two stages of tactile processing (e.g., Azanon and Soto-Faraco, 2008; Groh and Sparks, 1996). First, tactile stimuli are represented in a somatotopic representation that does not take into account body position in space, followed by a representation of body position in external space (body posture representation, see Medina and Coslett, 2010). In order to explore potential functional and neural dissociations between these two stages of processing, we presented eight participants with TMS before and after a tactile temporal order judgment (TOJ) task (see Yamamoto and Kitazawa, 2001). Participants were tested with their hands crossed and uncrossed before and after 20 min of 1 Hz repetitive TMS (rTMS). Stimulation occurred at the left anterior intraparietal sulcus (aIPS, somatotopic representation) or left Brodmann Area 5 (BA5, body posture) during two separate sessions. We predicted that left aIPS TMS would affect a somatotopic representation of the body, and would disrupt performance in both the uncrossed and crossed conditions. However, we predicted that TMS of body posture areas (BA5) would disrupt mechanisms for updating limb position with the hands crossed, resulting in a paradoxical improvement in performance after TMS. Using thresholds derived from adaptive staircase procedures, we found that left aIPS TMS disrupted performance in the uncrossed condition. However, left BA5 TMS resulted in a significant improvement in performance with the hands crossed. We discuss these results with reference to potential dissociations of the traditional body schema.