DELAY PERIOD MICROSTIMULATION IN THE FRONTAL EYE FIELDS UPDATES SPATIAL MEMORIES
R.L. White*; L.H. Snyder
Dept Anatomy & Neurobiology, Washington University, St Louis, MO, USA
The firing of neurons in the frontal eye fields (FEF) encodes the location being remembered during a memory-guided saccade task. Microstimulation in FEF will drive a saccade to a site-specific movement field (MF). We were interested in whether stimulation below saccade threshold would influence the storage and processing of spatial memories in FEF. We performed subthreshold stimulation while an animal planned a saccade to a remembered target and asked whether stimulation influenced the subsequent saccade endpoint. We hypothesized that stimulation would bias memory-guided saccades towards the MF.

Significant deviations between stimulated and control saccades (p<0.05) were observed at 38 of 55 FEF sites in 3 monkeys. Surprisingly, most of these effects were in the direction away from the MF. Deviations were more likely and larger when motor but not visual activity was recorded at the site. Effects did not depend on whether significant memory activity was recorded.

Why are saccades directed away from the MF? The circuitry may respond to subthreshold stimulation as if a saccade had actually been evoked. This suggests that subthreshold stimulation can introduce a small (~1 degree) corollary discharge signal without driving a saccade. Three lines of evidence support this explanation: (1) the systematic effect of stimulation across the population was best explained by a vector antiparallel to the preferred direction; (2) when stimulation was applied outside of the memory period, it produced no effects; (3) stimulation only produced effects when combined with an instruction to compensate for eye movements (update). When an animal was instructed to remember a target as fixed to the center of gaze, requiring the suppression of updating, effects were virtually absent.

We conclude that subthreshold stimulation in FEF appears to introduce a corollary discharge signal into the oculomotor spatial memory system.

Support Contributed By: NEI; McDonnell & EJLB Foundations

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