We live in a complicated world, with far more visual information available to us at any given time than the brain can handle. Attention is the selection process by which the brain chooses certain objects in the world in order to deal more effectively with them. In order to make accurate movements toward or away from attended objects the brain must have a spatially accurate representation of them. The parietal cortex has long been thought to be important in solving both of these problems, describing the visual world in an accurate way for the generation of attention and movement. One area of the parietal cortex, the lateral intraparietal area (LIP) has been described as being important in both the generation of eye movements and the generation of visual attention. We have discovered that LIP describes a spatially accurate salience or priority map which can be used simultaneously by the visual system to describe the locus of attention and by the oculomotor system to choose a goal for a rapid eye movement (saccade) when a saccade is appropriate. Two broad questions arise about this map: 1) what is its coordinate system and 2) how does it remain spatially accurate despite a moving eye? This proposal has three specific aims arising out of recent discoveries in our laboratory, which attack these general questions.
Aim 1) when the brain analyzes the visual world it does so in a coordinate frame which can be visual, e.g. the location an object on the retina or in the world, or motor, the amplitude of the saccade necessary to acquire that object.
Aim 1 uses the technique of saccadic adaptation, which enables us to separate the amplitude of a saccade from the target which evoked it. One way in which the brain maintains spatial accuracy is by using the metrics of a planned or recently completed saccade to remap the visual representation in LIP areas. We will also use saccadic adaptation to evaluate the metrics of the eye movement vector that shifts the receptive field.
Aim 2) The other way that the brain can generate a spatially accurate representation is by actually measuring eye position. We have discovered a proprioceptive representation of eye position in monkey primary somatosensory cortex. However, if this signal is too slow it could not be used for the description of the world for action.
Aim 2 is to study the time course of this signal, and see how it relates to eye position signals in LIP.
Aim 3) Humans and monkeys can make accurate eye movements to targets which appear and disappear before an intervening eye movement, but they also slightly mislocalize saccade targets flashed immediately around saccades.
Aim 3 is to study the spatial and temporal courses of the remapping process and the effects of inactivation of LIP to ascertain if the remapping mechanism can provide the mechanism by which humans can make fairly accurate but not perfect saccades to stimuli which appear and disappear around the time of a saccade.
Spatial processing is impaired in human patients with parietal and frontal lesions. Answering the questions posed in these specific aims will lead to a greater understanding of how the cerebral cortex orders the processes of visual attention and spatial perception. This in turn will lead to insight into the visual and oculomotor deficits that are so devastating in humans with lesions of frontal and parietal cortex, and will aid the designing of diagnostic, prognostic, and rehabilitative strategies.
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