A nonlinear model for context-dependent modulation of the binocular VOR

Gaze holding in healthy subjects

Eccentric gaze in darkness evokes minor centripetal eye drifts in healthy subjects, as cerebellar control sufficiently compensates for the inherent deficiencies of the brainstem gaze-holding network. This behavior is commonly described using a leaky integrator model, which assumes that eye velocity grows linearly with gaze eccentricity. Results from previous studies in patients and healthy subjects suggest caution when this assumption is applied to eye eccentricities larger than 20 degrees. To obtain a detailed characterization of the centripetal gaze-evoked drift, we recorded horizontal eye position in 20 healthy subjects. With their head fixed, they were asked to fixate a flashing dot (50 ms every 2 s)that was quasi-stationary displacing(0.5 deg/s) between ±40 deg horizontally in otherwise complete darkness. Drift velocity was weak at all angles tested. Linearity was assessed by dividing the range of gaze eccentricity in four bins of 20 deg each, and comparing the slopes of a linear function fitted to the horizontal velocity in each bin. The slopes of single subjects for gaze eccentricities of ±0−20 deg were, in median,0.41 times the slopes obtained for gaze eccentricities of ±20−40 deg. By smoothing the individual subjects' eye velocity as a function of gaze eccentricity, we derived a population of position-velocity curves. We show that a tangent function provides a better fit to the mean of these curves when large eccentricities are considered. This implies that the quasi-linear behavior within the typical ocular motor range is the result of a tuning procedure, which is optimized in the most commonly used range of gaze. We hypothesize that the observed non-linearity at eccentric gaze results from a saturation of the input that each neuron in the integrating network receives from the others. As a consequence, gaze-holding performance declines more rapidly at large eccentricities.

A re-examination of the time constant of the oculomotor neural integrator in human

We studied the horizontal oculomotor neural integrator in healthy human subjects during gaze holding in darkness. We found large variability among subjects with respect to the estimated time constants and the integrator's null position. We also found that individual subjects could demonstrate significantly nonlinear drift velocities as a function of eye position. Nevertheless, a consistent trend did not emerge. Consequently, cross subject averaging eliminates idiosyncratic nonlinear patterns and the average can be approximated by a linear function inside the range that was tested.

Primate disconjugate eye movements during the horizontal AVOR in darkness and a plausible mechanism

Disconjugate eye movements during the horizontal angular vestibulo-ocular reflex (AVOR) evoked in response to steps or pulses of head velocity have been previously reported in lateral eyed animals. In this study, we measured binocular responses to sustained sinusoidal and pseudo-random vestibular stimuli in yaw, delivered in darkness, in both human and monkey. The vestibular stimuli used in our experiments had peak velocities in the range of 120-200 degrees /s, frequencies in the range of 0.17-0.5 Hz, and durations between 60 and 75 s. Our results show a large vergence component to the AVOR response that systematically modulated with head velocity. We also examined our results for temporal-nasal preponderance in slow eye velocity. Although each subject showed some degree of directional preference, we did not find a systematically greater eye velocity for temporal-nasal direction across all subjects. Here, we present these findings and discuss that at least two possible sources could result in disconjugate eye movements during the horizontal rotational VOR in darkness: peripheral and central mechanisms.

Implications of gain modulation in brainstem circuits: VOR control system

Gain modulation is believed to be a common integration mechanism employed by neurons to combine information from various sources. Although gain fields have been shown to exist in some cortical and subcortical areas of the brain, their existence has not been explored in the brainstem. In the present modeling study, we develop a physiologically relevant simplified model for the angular vestibulo-ocular reflex (VOR) to show that gain modulation could also be the underlying mechanism that modifies VOR function with sensorimotor context (i.e. concurrent eye positions and stimulus intensity). The resulting nonlinear model is further extended to generate both slow and quick phases of the VOR. Through simulation of the hybrid nonlinear model we show that disconjugate eye movements during the VOR are an inevitable consequence of the existence of such gain fields in the bilateral VOR pathway. Finally, we will explore the properties of the predicted disconjugate component. We will demonstrate that the apparent phase characteristics of the disconjugate response vary with the concurrent conjugate component.

A nonlinear model of the neural integrator improves detection of deficits in the human VOR

A nonlinear model has been proposed to describe the set-point-dependent characteristics of the neural integrator (NI) in the oculomotor system. It was shown to yield improved prediction of slow-phase eye position in the vestibulo-ocular reflex (VOR) of normal subjects, when compared to the classical linear model of the NI. In this paper, we compare the parameters of this nonlinear NI model fitted to VOR data from: 1) compensated subjects diagnosed with vestibular deficiencies such as vestibular neuronitis and Meniere's disease and 2) normal (symptom-free) subjects. The identified models exhibit more severe nonlinearity in VOR patients than the normal controls. Several of the identified parameters in patients unmask asymmetries and more context dependence in the NI and in the VOR gain that are consistent with the lesioned side and could serve to support detection of lesions even after compensation.

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.

Neural computation in the binocular VOR circuit: a theoretical study

We present here a bilateral model for the horizontal rotational vestibulo-ocular reflex (VOR) that integrates sigmoidal nonlinearities in the response of VOR interneurons. This model realistically links the systems level approach to the underlying neural mechanisms. It is capable of producing VOR modulations with viewing context using only the efference copies of eye position signals to make novel predictions.