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 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 phenomenon of the coordinate system shift has been extended to slow eye movements. Monkeys were trained to follow a slowly moving visual stimulus by smooth pursuit, or by suppression of the vestibuloocular reflex when the animal is rotated at the same speed as the stimulus. In both cases 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. 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, monkeys were trained on an anti-saccade task. Monkeys began the task by looking at a fixation spot that could be either red or green. When the fixation light was red, the monkeys subsequently made a saccade to acquire a flashed target. When the fixation light was green, the monkeys made a saccade in the opposite direction from the flashed target, but of the same distance as the target. The majority of parietal neurons responded to the visual stimulus whether it dictated a saccade toward or away from it. A few cells responded to the stimulus but also when an opposite stimulus dictated a saccade to the spatial location of the stimulus. The least common cell type only had information about the saccade direction. These data show that the majority of neurons in the parietal cortex describe a visual stimulus and the attention drawn to that stimulus. Few cells describe movement direction uniquely.