This subproject is one of many research subprojects utilizing theresources provided by a Center grant funded by NIH/NCRR. The subproject andinvestigator (PI) may have received primary funding from another NIH source,and thus could be represented in other CRISP entries. The institution listed isfor the Center, which is not necessarily the institution for the investigator.This project concerns the neuronal mechanisms involved in controlling shifts in the direction of gaze in the monkey. Such control becomes more complicated if the head also turns when the eyes move and/or if the eyes start from different initial eye positions (IPs). First, we investigated how the frontal eye fields (FEFs), a cortical area that controls eye saccades with the head restrained, behave when gaze shifts occur with the head free to rotate. Microstimulation applied for 200 ms evoked large gaze shifts with substantial head movement components from most sites in the dorsomedial FEF, but small, predominantly eye-only gaze shifts from ventrolateral sites. The size and direction of gaze movements were strongly affected by the IP prior to stimulation. Stimulus-evoked gaze shifts and their eye and head components resembled those elicited naturally by visual target steps. When stimulus train durations exceeded 200 ms, the substantial head movement often grew for the entire stimulus duration. We conclude that the dorsomedial FEF generates a gaze command signal that can produce eye, head, or combined eye-head movements depending on the IP. Second, we studied the effects of IP on the saccade-related firing of putative abducens motoneurons. The bursts accompanying ipsiversive saccades with identical velocity profiles were quite similar for IPs above the threshold for steady firing. The excess burst rate above the threshold for steady firing was either constant or decreased with ipsiversive IP, and both the number of excess spikes in the burst and burst duration were nearly constant. However, below threshold, both peak burst rate and burst duration increased substantially with ipsiversive IPs. Moreover, the pause associated with contraversive saccades shortened considerably. These data suggest that the net force driving saccades appears to be generated by a combination of saturating and non-saturating burst commands and the recruitment of additional motoneurons.
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