TEMPORAL ASPECTS OF VISUAL-SEARCH STUDIED BY TRANSCRANIAL MAGNETIC STIMULATION

Transcranial magnetic stimulation was applied over the parietal visual cortex of subjects while they were performing 'popout' or conjunction visual search tasks in arrays containing eight distracters. Magnetic stimulation had no detrimental effect on the performance of pop-out se arch, but did significantly increase reaction times on conjunction sea rch when stimulation was applied over the right parietal cortex 100 ms ec after the onset of the visual display for trials when the target wa s present. Target absent reaction times were elevated when stimulation was applied 160 msec after array onset. Stimulation had no effect on the number of errors made. The results suggest that a sub-region of th e right parietal lobe is important for conjunction search but not for preattentive pop-out. The result from target present trials is consist ent with timing data from studies of single cells in monkeys and the h ypothesis that parietal areas generate a signal that projects back to extrastriate visual areas to enhance the processing of features in a r estricted part of the visual field. The timing of the effect indicates that transcranial stimulation disrupts the mechanisms underlying the focal attention necessary for feature binding in conjunction search. T he effects of TMS on target absent trials are interpreted in terms of fronto-parietal connections and the role of frontal cortex in decision -making. The results also highlight the efficacy of transcranial magne tic stimulation as a complement to other spatial and temporal imaging techniques. (C) 1997 Elsevier Science Ltd.