Gaze-related activity of putative inhibitory burst neurons in the head- free cat

1. Previous studies in the cat have demonstrated that output neurons of the superior collicular as well as brain stem omnipause neurons have discharges that are best correlated, not with the trajectory of the eye in the head but, with the trajectory of the visual axis in space (gaze = eye- in-head + head-in-space) during rapid orienting coordinated eye and head movements. In this study, we describe the gaze-related activity of cat premotor 'inhibitory burst neurons' (IBNs) identified on the basis of their position relative to the abducens nucleus. 2. The firing behavior of IBNs was studied during 1) saccades made with the head stationary, 2) active orienting combined eye-head gaze shifts, and 3) passive movements of the head on the body. IBN discharges were well correlated with the duration and amplitude of saccades made when the head was stationary. In both head-free paradigms, the behavior of cat IBNs differed from that of previously described primate 'saccade bursters'. The duration of their burst was better correlated with gaze than saccade duration, and the total number of spikes in a burst was well correlated with gaze amplitude and generally poorly correlated with saccade amplitude. The behavior of cat IBNs also differed from that of previously described primate 'gaze bursters'. The slope of the relationship between the total number of spikes and gaze amplitude observed during head- free gaze shifts was significantly lower than that observed during head- fixed saccades. 3. These studies suggest that cat IBNs do not fit into the categories of gaze-bursters or saccade-bursters that have been described in primate studies. The discharge characteristics of cat IBNs during active gaze shifts are consistent with a gaze feedback control system in which an upstream burst generator signal, which modulates the firing behavior of the IBNs, is shared between the head and eye motor circuits. A given burst reflects the summed movement of eye and head, and would lead to a larger displacement of the visual axis head-free than head-fixed because the head contributes to the overall gaze displacement.