The staging area for commands that drive motoneurons, which move the eyes, resides in the pontine and medullary reticular formations of the brainstem. When the head is prevented from turning, rapid eye movements, called saccades, shift the direction of gaze with the help of brainstem neurons that discharge a burst of action potentials. These burst neurons are held in check by inhibition from neurons that usually fire steadily but cease firing or pause with all saccades. During the past year we investigated whether, when the head is free to turn and a gaze shift is composed of an eye and a head movement, the pause of these omnipause neurons (OPNs) is still best correlated with eye movement or instead controls head duration or the duration of the overall gaze shift, i.e., eye position + head position. We found that pause duration was poorly correlated with head duration. Moreover, by correlating pause duration with movement duration, we could not ascertain whether pause duration was better correlated with eye or overall gaze duration. However, when we aligned the end of the pause with the end of the saccade, the pause was almost always better aligned with the end of the eye movement than the end of the gaze movement. Therefore, we conclude that the OPNs, which lie in the pons, are involved with the eye component of a gaze shift and not with the overall gaze shift nor the head movement component. In contrast, the firing patterns of various of the aforementioned burst neurons may be better correlated with either the eye movement, the gaze movement, or the overall gaze shift. Pause and burst neurons with these characteristics fit nicely into our model for a separate control of eye and head movements during a gaze shift.
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