The capacity of the human nervous system for processing information is limited in comparison to the quantity of sensory formation ordinarily received by the body's receptors. Attention mechanisms enable the brain to cope with this sensory overload by electively filtering unwanted information. The difficulty in attending or responding properly to visual cues experienced by subjects with lesions of the posterior parietal region of the cerebral cortex suggests an important role of this area in directed attention. The long-range goal of this research program is to understand the brain mechanism that subserves selective attention. This proposal will pursue by examining effects of shifts of attention on the neuronal responses in the posterior parietal cortex in monkeys performing covert attention task. It is expected that new knowledge from these experiments will provide a better understanding of the mechanisms of higher-order sensory processing in the brain, and of the complex syndrome observed in patients with parietal lesions. Psychophysical studies and single neuron recordings in behaving monkeys have demonstrated the importance of the posterior parietal cortex for a variety, of visuospatial operations including attentive fixation, visual tracking, guiding saccades and reaching motions, and for the analysis and memory of spatial aspects of visual scenes. Although neurons with different functional properties are not precisely segregated, mounting evidence suggests some degree specialization within recently defined subdivisions of the parietal cortex. The activity of neurons in area 7a of the parietal cortex is powerfully modulated by directed attention. During directed attention, neurons respond more strongly to stimuli that appear at unattended locations than to stimuli at attended locations or to stimuli presented in nonattending states. These and other data suggest that area 7a plays a role in redirecting attention to unattended stimuli. The aim of this proposal is to study the role of area 7a in visuospatial memory and directed attention by recording and analyzing neuronal responses in monkeys performing a covert attention task. The first objective of this proposal is to determine if neurons in area 7a participate in the process of filtering sensory information, by encoding the spatial location of the most salient of current visual stimuli. This hypothesis will be tested by recording and comparing neuronal responses when a stimulus either appears by itself or as a pop-out stimulus in a multistimulus array. A class of neurons in the inferotemporal cortex have been identified in which the responses to stimuli are weaker if a stimulus matching in form had been presented previously in the same behavioral trial. It has been proposed that these neurons function as passive filters and have an important role in object recognition. It has also been suggested that this mechanism may provide templates that bias attention toward novel stimuli. The second objective of this proposal is to determine if the lack of response observed in the parietal cortex to stimuli that appear at attended locations is a result attention being directed to that location or whether it represents a spatial equivalent of the passive filtering mechanism observed in the inferotemporal cortex. This will be accomplished by recording neuronal responses to repeated stimuli in the parietal cortex as monkeys perform a spatial version of a delayed match-to-sample task. Neuronal activity recorded in the posterior parietal cortex during the delay periods of behavioral tasks has also been correlated with plans for motor action. In area LIP, for example, discharges during delay periods is associated with the direction of a pending saccadic eye movement. A third objective of this proposal is to determine if the delay period activity of area 7a neurons is related to remembered spatial location of visual stimuli independent of any requirements for spatially directed responses.