Implications of gain modulation in brainstem circuits: VOR control system

Abstract

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.

The Horizontal Angular Vestibulo-Ocular Reflex: A Nonlinear Mechanism for Context-Dependent Responses

“…The model structure and connections are similar to the one developed in [9] with linear summing junctions except for nonlinear canals and nonlinear EHV cells. It should be noted that some known connections, i.e.…”

“…EHV cells are known to behave nonlinearly and also exhibit vergence dependent behavior [21], [5], [22]. In the former model [9], context dependency relied on a single nonlinearity applied after the sum of EHV afferent signals. Therefore, as different gains were generated with viewing context, the dynamics of the system also changed.…”

“…[7] suggested instead a multiplication of vestibular and eye position signals within the direct VOR pathway as the neural substrate that mediates VOR gain modulations. By using the concept of gain modulation [8], the theoretical work of Khojasteh and Galiana [9] suggested that imbedding an appropriate local nonlinearity in the response of bilateral VOR interneurons could generate context dependent responses. Their proposed mechanism uses monocular efferent copies of eye position that are known to converge on premotor cells in the medial VN as the signal to modulate VOR gain.…”

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

“…Previous nonlinear identification algorithms, as well as modeling studies by Galiana et al [7][8][9][10] have also proposed that healthy human subjects could exhibit nonlinear integration characteristics during VOR movements. They also suggested that perhaps the neural integrator time constant is highest closer to the origin and declines at lateral positions.…”

“…Magnetic search coil measurements in goldfish during spontaneous scanning of the surroundings also revealed nonlinear P-V plots, suggesting that at least in goldfish integrator function depends on eye position in the orbit [5]. There are also several modeling studies [6][7][8][9][10][11] NI network, although these predictions have not yet been verified by experimental studies. In this article we present the results of gaze-holding experiments in healthy human subjects, using videooculography technique.…”