Modeling fixation-distance-related modulations in the vestibulo-ocular reflex (VOR)

The vestibulo-ocular reflex (VOR) serves to stabilize retinal images of targets during head angular and/or linear displacements by moving the eyes at an appropriate speed and in a direction that compensates for head movements. Contrary to traditional understandings, we now know that the VOR function is not hard-wired; rather it is deeply modified by the context in which eye movements occur. Among these contexts, the fixation context is the main focus of this thesis. It has been proven, both experimentally and through geometric relationships, that the VOR gain (the ratio of eye velocity to head velocity) modulates with fixation distance. Furthermore, depending on the target distance from each eye, the VOR gain can be different for each eye. We present a bilateral model that accounts for VOR response modulations with fixation distance and is also compatible with anatomical and physiological facts.All the previous models that have been proposed for viewing-distance-related VOR response modulations use black-box mathematical representations to modulate the reflex, providing no insight into the underlying anatomy and physiology. This thesis is an attempt to show that VOR fixation-distance-dependent modifications can be achieved automatically through the physiological characteristics of the second-order neurons in the vestibular nuclei (VN), without having to apply any specific vergence encoding signals. The model presented here relies on the nonlinear behavior of the specific cells in the VN, which has been observed in prior experimental studies. The model reproduces the VOR gain that is observed at different vergence levels. It also allows for modifications in VOR dynamics by applying changes to the sensitivities of the VN cells that lie within the system loops