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Yixing Xu, Carine Karachi, Michael E. Goldberg; The Oculomotor System Does Not Use LIP Visual Gain Fields To Calculate Saccade Target Positions. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4687.
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How the oculomotor system accurately tracks the spatial locations of saccade targets despite a constantly moving eye is a question of interest to neuroscientists, psychologists, and neuroscientific and opthalmologic clinicians. An almost universally held theory of how the brain solves this problem uses the gain fields, eye position modulated neuronal responses, in the posterior parietal cortex (PPC) to compute target positions in supraretinal coordinates (Zipser and Andersen, Nature, 1988). In order for the gain fields to subserve this function, they must accurately reflect eye position. We tested the accuracy of the gain fields in the lateral intraparietal area (LIP) in the PPC after a saccade in order to study the role of the gain fields in coordinating eye movements.
We trained two monkeys to perform two different oculomotor tasks. In both tasks, monkeys first made a conditioning saccade from an eye position where the visual response to a saccade stimulus was high to an eye position where the visual response to a saccade stimulus was low, or vice versa. Saccade stimuli flashed after a variable delay following the conditioning saccade. The monkeys then made memory-guided saccades to the remembered locations of the stimuli, either immediately or at some later time. We recorded from single LIP neurons using Tungsten electrodes and tracked the accuracy of the monkeys’ eye movements using scleral search coils.
The visual responses of LIP neurons were spatially inaccurate for at least 150 ms after the end of a conditioning saccade. The majority of these responses (61 of 89 cells, 69%) indicated the presaccadic eye position. The gain field model predicts that saccades to stimuli flashed immediately after a conditioning saccade should be mislocalized to the presaccadic eye position. Saccadic behavior was, however, largely accurate in both tasks, regardless of when stimuli flashed after the conditioning saccade.
Gain field theory cannot explain the accuracy of many eye movements, because the oculomotor system is spatially accurate at a time that the gain fields themselves are inaccurate. Additionally, the eye position signal that modulates visual responses to create the gain fields is thought to arise from a corollary discharge, but its slow time course is more consistent with that of the proprioceptive eye position signal in area 3a of somatosensory cortex.
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