Temporal dynamics of parietal cortex involvement in visual search

The functional–neuroanatomic relationship that describes the involvement of the parietal cortex in visual search was investigated using repetitive transcranial magnetic stimulation (rTMS; 10 Hz, 500 ms in duration). Twelve adult participants performed feature-based visual search for a unique letter—without eye movements—under conditions that involved manipulations of search efficiency (efficient versus inefficient) and target-selection demands (set-size: 4 versus 10). rTMS was applied over the right posterior parietal cortex at the onset of the search array for all factorial conditions (0–500 ms); stimulation was additionally administered at 500 ms post-array onset (500–1000 ms) during inefficient search (set-size 10). Stimulation over the primary sensorimotor cortex served as a within-subjects control condition, and eye movements were monitored continuously. Significant increases in reaction time were restricted to parietal stimulation during inefficient search (set-size 10), with interference observed when rTMS was administered at the onset of the search array and at 500 ms post-array onset. The early effect was confined to target-present trials and the late effect was confined to target-absent trials, which may indicate temporally dissociable parietal involvement in target detection and response-based selection and/or search termination, respectively. Error rates did not vary significantly as a function of any of the independent variables. Taken together, these results are consistent with evidence from functional magnetic resonance studies indicating that inefficient feature-based visual search requires an intact parietal cortex, and also indicate that the parietal cortex is involved in inefficient search later than has been previously reported.