Electrical microstimulation of primate posterior parietal cortex initiates orienting and alerting components of covert attention

The posterior parietal cortex (PPC) is implicated in the control of visuospatial orienting, including both overt saccadic eye movements and covert shifts of attention (i.e., attention to a location other than at visual fixation). Some studies have suggested that the attentional system is part of the premotor processing in the brain, while others suggest they are separate. Here, we test how the PPC controls covert attention shifts in the absence of executed eye movements. Electrical microstimulation was applied to the right PPC while monkeys performed a spatial, cued target detection task, in which they were not allowed to move their gaze. At high currents, contralateral saccades were evoked. With currents below the thresholds for eliciting saccades, microstimulation produced a purely attentional shift (as indexed by decreased target reaction time) when a cue and target were presented in the contralateral visual field. This suggests that microstimulation can move attention specifically in the absence of any overt movements of the eyes or limbs. In addition, there was a reduction in reaction times in trials that did not evoke attentional orienting, suggesting a more general alerting effect of microstimulation These data provide direct evidence that the PPC may be a source of both attentional modulation of neuronal responses and saccadic eye movements to peripheral visual stimuli.