COVERT ORIENTING OF ATTENTION IN MACAQUES .2. CONTRIBUTIONS OF PARIETAL CORTEX

1. To understand some of the contributions of parietal cortex to the d ynamics of visual spatial attention, we recorded from cortical cells o f monkeys performing attentional tasks. We studied 484 neurons in the intraparietal sulcus and adjacent gyral tissue of two monkeys. We meas ured phasic responses to peripheral Visual stimuli while the monkeys a ttended toward or away from the stimuli or when attention was not cont rolled. Neurons were tested while the monkeys gazed at a spot of light (simple fixation task), actively attended to a foveal target (foveal attention task), performed a reaction time task. (cued reaction time t ask), made saccadic eye movements to visual targets (saccade task), or responded to a repetitious peripheral target (probability task). 2. I n a previous paper we demonstrated that monkeys, Like humans, responde d more quickly to visual targets when the targets followed briefly fla shed visual cues (validly cued targets) (Bowman ct al. 1993). It has b een hypothesized that the cue attracts attention to its locus and resu lts in faster reaction times (Posner 1980). In the present physiologic al studies, visual cues consistently excited these neurons when they w ere hashed in the receptive held. Such activity might signal a shift o f attention. Visual targets that fell within the receptive field and t hat immediately followed the cue evoked relatively weak responses. Thi s response was due to a relative refractory period. 3. Next we tested attentional processes in these tasks that were independent of the visu al response to the cue. We placed the cue outside of the receptive fie ld and the target within the receptive field. We found that 23% of the se cells had a significant decrease in their firing rate to validly cu ed targets in their receptive fields under these conditions. Strong re sponses were evoked by the same target when the cue was flashed in the opposite hemifield (invalidly cued targets). Thus this group of neuro ns responded best when attention was directed toward the opposite hemi field. 4. For another group of parietal cells (13%) there was an enhan ced response to targets in the visual receptive field when the cue was in the same hemifield. For the remaining 64% of the cells there was n o significant modulation in this task. 5. The cued reaction time task involved exogenous control of attention; the sensory cue gave spatial and temporal direction to attention. We used several other tasks to te st for endogenous control of attention. For some cells, when a monkey simply gazed at a spot of light there was only a modest response to pe ripheral visual stimuli; when the monkey performed the foveal attentio n task there was an increase in the intensity of response of the same cell to the same peripheral stimulus. Thus, when attention was directe d away from the Visual receptive held by endogenous control, there was a similar augmentation of response. 6. When an animal responded repet itiously to targets outside of the visual receptive field (probability task), there was a strong response evoked when the stimulus appeared unexpectedly within the receptive field. Weak responses were elicited at expected locations. The modulations in the cued reaction time, fove al attention, and probability tasks were quantitatively similar. These observations are also consistent with other data showing that a group of parietal cells responded best when attention was not directed into the visual receptive field. This was true whether attention was manip ulated exogenously or endogenously. 7. Approximately 45% of the neuron s tested discharged in relation to saccadic eye movements, and the lar gest number of such cells was located in the posterior bank of the int raparietal sulcus. 8. We conclude from these experiments that parietal cells participate in attentional processes. All respond to the visual cue that directs attention, and this may signal a shift of attention. When the visual cue was positioned near the receptive held, different ial activity was produced, all of which may signal attentional shifts. Certain of these cells also had modulations in endogenous tasks that augmented the response when attention was away from the receptive held ; such activity could signal a shift of attention to the receptive fie ld. These data provide some mechanisms for contributions of parietal c ortex to the dynamics of visual attention.