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 one part of parietal cortex, the lateral intraparietal area (LIP) describe the visual environment in a coordinate system whose origin is the center of gaze. We have previously shown that this is true for saccades and slow eye movements. When the eye moves the spatial location of a recently vanished stimulus into the receptive field of the neuron, many parietal neurons discharge despite the fact that when the stimulus flashed it was not in their receptive fields. We used the Duncker illusion to see if this receptive field shift were locked to the eye movement or subject to perceptual distortion. In this illusion humans perceive a spot moving at right angles to a flow field to be moving diagonally. If they attempt a saccade to a target flashed early in the pursuit epoch, their saccades are inaccurate as if they had compensated for the illusory smooth pursuit that they did not make. Monkeys make similar saccadic errors. LIP neurons show the effects of this perceptual distortion: they respond when the trajectory of illusory pursuit moves the receptive field onto the spatial location of a briefly flashed stimulus which would not evoke a response if the identical smooth pursuit were not made across the flow field. This mechanism of receptive field shifts now explains how the brain can generate spatially accurate movements when a human or monkey moves through its environment, without having to wait for the retinal representation to reestablish itself in the cerebral cortex after the eyes have moved in space. Although neurons in the parietal cortex discharge before visually-guided accadic eye movements, it has not been clear if this response is related to the attention that the saccade target evokes, or the intention to make the saccade. To distinguish between this possibility, we investigated the effect of a visual distractor on the monkey's oculomotor performance and the response of neurons in the parietal cortex.If LIP were involved exclusively in saccade planning, one would expect that a saccade target flashed outside the receptive field would result in a suppressed response to subsequent stimuli irrelevant to the saccade plan. We used a distractor task to study the effect of saccade target location on the visual responses of neurons in LIP. We trained two monkeys to perform a standard memory-guided delayed saccade task and a distractor task in which a stimulus flashed for 200 ms late in the delay period. The distractor was flashed at the same spatial location as the target or at a different locations without significantly affecting saccade accuracy, latency, and velocity. The target and/or distractor location was placed in the neuron's receptive field. Neurons exhibited either a normal or an enhanced visual response to the distractor during the memory period when the target flashed outside the receptive field. When the distractor flashed at the location of the saccade target, the response to the distractor was either unchanged or diminished. The greatest response to a stimulus occurred at the first presentation of the stimulus at a given spatial location, regardless of its significance saccade planning. Presaccadic discharge was similar when only the distractor or target flashed in the receptive field. These results are consistent with a role of LIP in the generation of visuospatial attention, but inconsistent with the specific planning or generation of saccades.
