Differential effect of double-pulse TMS applied to dorsal premotor cortex and precuneus during internal operation of visuospatial information

Human neuro-imaging studies have often reported co-activation of the dorsal premotor cortex (PMd) and the posterior parietal cortex (PPC) during internal operation of visuospatial information, referred to here as "visuospatial mental operation". However, the functions assigned to the PMd and PPC during these tasks are still unclear. Here, we examined the significance of these two areas for a visuospatial mental operation using the transcranial magnetic stimulation (TMS) technique. Subjects performed a task in which a visuospatial mental operation was required. A localization study conducted prior to the TMS experiment using functional magnetic resonance imaging (fMRI) revealed that the PMd and the medial part of the PPC, precuneus (PCu), were specifically activated during the visuospatial mental operation. Then, we impeded the activities of the PMd and the PCu in the right hemisphere during the same task using double-pulse TMS to determine whether these activities were necessary for the task. The TMS was applied at different times in relation to the visuospatial mental operation cue. Consequently, only the TMS applied at 300 ms after the cue affected the task performance. Furthermore, we found that the TMS at this time to each area differentially affected the performance: TMS to the PMd hindered the performance of the task whereas TMS to the PCu facilitated it without a speed/accuracy trade-off. These effects were not found in the control condition that lacked a visuospatial mental operation. These findings suggest that the PMd and the PCu are involved in differential aspects of visuospatial mental operations.