The involvement of posterior parietal cortex in feature and conjunction visuomotor search

Successful interaction with the environment often involves the identification and localization of an item. Right posterior parietal cortex (rPPC) is necessary for the completion of conjunction but not feature visual search, regardless of the attentional requirements. One account for this dissociation is that the rPPC is primarily involved in processing spatial information. For target identification, conjunction tasks require that spatial information is used to determine if features occur at the same location, whereas feature search does not require such a process. This account suggests that if the requirement to localize the target is made explicit, then rPPC may also be necessary for feature search. This was examined using TMS and by manipulating the response mode: Participants were either required to press a button indicating the presence/absence of the target or else had to point to the target. TMS over rPPC did not disrupt performance of the feature task when a button press was required but significantly increased response time and movement time for the same task in the pointing condition. Conjunction search in both response conditions was significantly impaired by TMS. Performance on a task that required pointing to a target in the absence of distractors and thus did not involve visual search was unaffected by rPPC stimulation. We conclude that rPPC is involved in coding and representing spatial information and is therefore crucial when the task requires determining whether two features spatially co-occur or when search is combined with explicit target localization via a visuomotor transformation.