Facilitation of bottom-up feature detection following rTMS-interference of the right parietal cortex

In visual search tasks the optimal strategy should utilize relevant information ignoring irrelevant one. When the information at the feature and object levels are in conflict, un-necessary processing at higher level of object shape can interfere with detection of lower level orientation feature. We explored the effects of inhibitory trains of transcranial magnetic stimulation (rTMS) on the right and left parietal cortex in healthy subjects performing two visual search tasks. One task (Task A) was characterised by an object-to-feature interference. The other task (Task B) was without such interference. We found that rTMS of the right parietal cortex significantly reduced reaction times (RTs) in Task A, where object recognition interferes with detection of orientation. This significant RT reduction was present only for the first 10 trials. Interestingly, right parietal rTMS had no effect on Task B. Moreover, rTMS of the left parietal cortex did not modify subjects' RTs in either task. Subjects' accuracy was equally affected by rTMS in both tasks over time. We suggest that inhibition of the right parietal cortex by means of rTMS facilitates feature-based visual search by inhibiting the interfering feature binding and spatial attentional processes. This allows subjects to accomplish Task A faster.