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1 CIHR Group in Action and Perception, University of Western Ontario, London, Canada; Physiology & Pharmacology, University of Western Ontario, London, Canada
2 CIHR Group in Action and Perception, University of Western Ontario, London, Canada; Graduate Program in Neuroscience, University of Western Ontario, London, Canada
3 Otolaryngology Head and Neck Surgery, University of Toronto, Toronto, Canada
4 CIHR Group in Action and Perception, University of Western Ontario, London, Canada; Physiology & Pharmacology, University of Western Ontario, London, Canada; Graduate Program in Neuroscience, University of Western Ontario, London, Canada; Psychology, University of Western Ontario, London, Canada
* To whom correspondence should be addressed. E-mail: bcorneil{at}uwo.ca.
We recorded neck EMG activity following stimulation of the primate FEF. We assess our results in light of four forwarded mechanisms regarding movements evoked by FEF stimulation. Two indirect mechanisms propose that movements are generated in response to either a percept or an eccentric orbital position. Two other mechanisms propose that movements are evoked directly via a gaze command or separate eye and head commands. FEF stimulation evoked short-latency neck EMG responses from >95% of stimulation sites. Evoked responses usually preceded the gaze shift by ~20 ms, even for small gaze shifts not typically associated with head motion. Responses began earlier and attained larger magnitudes when accompanied by larger gaze shifts. We observed robust responses even when stimulation failed to evoke a gaze shift, and occasionally observed head-only movements when the head was unrestrained. EMG response latencies approached the minimal conduction time to the motor periphery, and hence are inconsistent with both indirect mechanisms. The widespread drive from the FEF, the scaling of neck EMG with gaze magnitude, and the earlier generation of EMG versus gaze responses are difficult to reconcile with the separate channels mechanism. The most parsimonious interpretation is that a gaze command issued by the FEF is decomposed into eye and head commands downstream of the SC. The relative timing of the neck EMG and gaze shift responses, and the presence of neck EMG responses on trials without gaze shifts, implies that head premotor elements are not subjected to the same brainstem control mechanisms governing gaze shifts.
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