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Carlos Cassanello, Abhay Nihalani, Vincent Ferrera; The role of the frontal eye fields in velocity compensation during saccades to moving targets. Journal of Vision 2008;8(6):231. doi: 10.1167/8.6.231.
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Saccadic eye movements are programmed based on a position error signal that represents the difference between the current and desired eye position. Saccades based on position error alone should always undershoot a moving target. However, monkeys and humans make accurate saccades to moving targets, and there is good evidence that this accuracy results from the use of target velocity in saccade programming. The FEF receives input from visual motion-processing areas MT and MST, and sends output to oculomotor centers, and is therefore likely to be involved in saccade target velocity compensation. To decide whether neurons in the FEF code for position error (PE) or saccade amplitude (SA), and whether their responses are modulated by target velocity (TV), we recorded 96 neurons from awake-behaving rhesus monkeys making saccades to moving targets using a step-ramp paradigm. By comparing the neuronal responses preceding the onset of saccades to stationary targets to the responses to moving targets, we could determine whether firing rate is better correlated with PE or SA. We found that one third of the cells coded better for SA in saccades to moving targets, suggesting that they integrate velocity signals to compensate for the target displacement during the last 100 msec before the onset of the saccade. In addition, 55 (57%) cells showed significant modulation of firing rate by target velocity during an interval of 100 msec centered on saccade onset. The modulation in firing rate can be fitted well with a quasi-linear or a quadratic dependence on the variable SA-PE, which is in turn behaviorally linearly correlated to TV. This is different from the neuronal responses in the deeper layers of the superior colliculus that have been shown to encode PE alone. The results suggest a role for FEF in integrating target velocity to program accurate saccades to moving targets.
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