Modeling the nonlinear context dependency of the neural integrator in the vestibuloocular reflex.

A neural integrator (NI) is presumed to exist in the oculomotor system to assist in numerous tasks such as maintaining gaze on imaginary targets in the dark. It is shared by all ocular reflexes including the vestibuloocular reflex (VOR). It has been widely accepted that the NI acts as a "perfect" integrator even in the dark with time constants as large as 1950s. However, the NI time constant is often less than ideal and its value can also be dependent on context [W. W. P. Chan and H. L. Galiana, "Integrator function in the oculomotor system is dependent on sensory context," J. Neurophysiol., vol. 93, pp. 3709--3717, Feb. 2005.]. In this paper, a nonlinear feedback model is postulated to model the context-dependent properties of the NI. Algorithms are first developed and validated to fit both linear and nonlinear NI models to experimental data in the presence of ocular nystagmus. Preliminary results indicate that even normal subjects can have a nonlinear VOR and NI.