The objective of system identification methods is to construct a mathematical model of a dynamical system in order to describe adequately the input-output relationship observed in that system. Over the past several decades, mathematical models have been employed frequently in the oculomotor field, and their use has contributed greatly to our understanding of how information flows through the implicated brain regions. However, the existing analyses of oculomotor neural discharges have not taken advantage of the power of optimization algorithms that have been developed for system identification purposes. In this article, we employ these techniques to specifically investigate the "burst generator" in the brainstem that drives saccadic eye movements. The discharge characteristics of a specific class of neurons, inhibitory burst neurons (IBNs) that project monosynaptically to ocular motoneurons, are examined. The discharges of IBNs are analyzed using different linear and nonlinear equations that express a neuron's firing frequency and history (i.e., the derivative of frequency), in terms of quantities that describe a saccade trajectory, such as eye position, velocity, and acceleration. The variance accounted for by each equation can be compared to choose the optimal model. The methods we present allow optimization across multiple saccade trajectories simultaneously. We are able to investigate objectively how well a specific equation predicts a neuron's discharge pattern as well as whether increasing the complexity of a model is justifiable. In addition, we demonstrate that these techniques can be used both to provide an objective estimate of a neuron's dynamic latency and to test whether a neuron's initial firing rate (expressed as an initial condition) is a function of a quantity describing a saccade trajectory (such as initial eye position).
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.(ABSTRACT TRUNCATED AT 250 WORDS)
The significance of a nystagmus-dependent, transient component in the overall slow-phase response of the vestibulo-ocular reflex (VOR) is brought into focus. First, a simulated example is presented that shows how this transient component can bias current algorithms for the estimation of VOR parameters. Second, new algorithms are proposed that are able to estimate VOR parameters regardless of the presence of transients. Third, the new algorithms are applied to experimental data, and the results are compared with those from current algorithms. The results clearly show that the transient component can significantly alter the apparent VOR time constant, particularly when the reflex has been lesioned. The algorithms open new areas of research on the possible role of nystagmus in enhancing the compensatory function of the VOR.
The significance of a nystagmus-dependent, transient component in the overall slow-phase response of the vestibulo-ocular reflex (VOR) is brought into focus. First, a simulated example is presented that shows how this transient component can bias current algorithms for the estimation of VOR parameters. Second, new algorithms are proposed that are able to estimate VOR parameters regardless of the presence of transients. Third, the new algorithms are applied to experimental data, and the results are compared with those from current algorithms. The results clearly show that the transient component can significantly alter the apparent VOR time constant, particularly when the reflex has been lesioned. The algorithms open new areas of research on the possible role of nystagmus in enhancing the compensatory function of the VOR.
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