Delay period microstimulation perturbs memory-guided saccade behavior

Robert L. White, III* and Lawrence H. Snyder

Neurons in the lateral intraparietal area (LIP) and the frontal eye field (FEF) are believed to play a role in the planning and execution of saccades. During a delayed saccade task, many neurons in these areas exhibit memory activity (increased firing rates above baseline during the delay period) after a target has been flashed in their visual response field (RF). To determine the relative contribution of LIP and FEF in planning saccades, we performed delay period microstimulation in each area to assess whether saccade behavior could be altered by specifically perturbing memory-related neural activity during the delay. Furthermore, we asked what type of effects microstimulation had on saccade behavior. We hypothesized that microstimulation in either area would systematically bias memory-guided saccades towards the RF of neuron(s) recorded at the stimulation site.

Statistically significant deviations from control were observed in 33 of 52 FEF sites, but only 8 of 43 LIP sites (two animals for each area). Suprisingly, most of these effects were in the direction away from the recorded RF. Effects were strongest for targets at adjacent directions (+/- 45 deg). In FEF sites, effects did not depend on whether significant (p<0.05) memory activity was recorded at the site (n=23). However, in LIP, the population of sites where significant memory activity was recorded (n=17) showed a significant deviation in a direction away from the RF that was also strongest for adjacent target directions. No effects were observed in the population of LIP sites where only visual responses were recorded.

Analysis of the direction of stimulation effects across all target locations revealed that the systematic effect of stimulation for the population was better explained by a vector antiparallel to the preferred direction observed at the site (or the suprathreshold evoked direction) than a vector antiparallel to the RF center. The fact that the deviation is not directly repulsive, but instead antiparallel, suggests that stimulation may counterfeit an efference copy signal. The circuitry may respond to subthreshold microstimulation as if a saccade has been executed in the suprathreshold evoked direction. As a result, the system 'remaps' the location of the target to compensate for the fictive eye movement. The result is a deviation of the remembered location that is antiparallel to the evoked vector. These results demonstrate that delay period microstimulation is a useful method for probing spatial information processing in the brain.

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Presented at the 2004 Annual Meeting on Computational & Systems Neuroscience.