CEREBRAL CORTICAL MECHANISMS FOR EYE MOVEMENTS AND VISUAL ATTENTION

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Two lines of inquiry were followed to determine how the cerebral cortex and its efferent regions control eye movements and visuospatial attention. Single neuron recording was used to probe the mechanisms whereby the parietal cortex of the monkey analyzes space. Previous experiments in this laboratory demonstrated that neurons in the parietal cortex describe the visual environment in a coordinate system whose origin is the center of gaze. Before a monkey makes a saccade, however, stimuli that will enter the receptive field of a neuron by virtue of the impending saccade can excite that neuron, even though they are currently not in a retinal location in which stimuli can normally excite the neuron. This is not due to a nonspecific enlargement of the receptive field. Instead, the excitability of the current receptive field decreases, as the excitability of the future receptive field increases. Both excitability changes begin more than 150 ms before the saccade. The existence of two populations of neurons, each describing the same stimulus, suggests that stimuli should be localized inaccurately if they flash only in the brief interval before a saccade. Monkeys can make accurate saccades to a stimulus flashed for 100 ms, 500 ms before an intervening saccade. If the stimuli are flashed for 100 ms, beginning 150 ms before a saccade, they mislocalize the target, as if they were averaging the two populations of neurons. Single neuron recording was used to analyze how the parietal cortex of the monkey participates in the processes underlying visual attention. Parietal neurons respond to the onset of flashed stimuli in their receptive fields. To see if this activity is related to the entry of the stimulus into the receptive field, or the attention evoked by a flashed light, a paradigm was developed in which a circular eight-element array appeared and remained on the screen for 10 minutes or more. The monkey made a saccade to the center of the array and then was instructed by the appearance of an exogenous cue, which matched one of the symbols, to make a saccade to the matching symbol in the stable array. When the monkey made the saccade to the center of the array, bringing one or more array members into the receptive field of the neuron, many neurons in the parietal cortex did not respond, although they did when the stimulus flashed in the receptive field. When the cue instructed the monkey to make a saccade to the stimulus in the array, the neurons began to discharge after the appearance of the cue with a latency longer than that of a response to a flash and then continued until the monkey made the saccade. If the cue appeared in the receptive field, the neurons discharged even if the saccade was made to a target outside the receptive field. These results suggest that parietal neurons require both that a visual stimulus lie in the receptive field of the neuron and that the monkey attend to the stimulus. Flashed stimuli ineluctably draw some attention, so the neurons respond to them whether or not the stimuli are relevant to the animal's task. Conversely, the neurons respond when, by virtue of a behavioral manipulation, the monkey attends to a target that has been in the neuron's receptive field but has not evoked a response because the monkey did not attend to it.