The vestibulo-ocular reflex (VOR) has been created with an incredible plasticity for the purpose of enabling change inevitable with disease and aging. In fact, without this adaptation, the normal effects of age or the constant attacks from disease would render us with significant functional impairments—namely gaze and gait instability. Some examples of the critical and extreme plasticity within the VOR include altered VOR gain with newly fitted glasses and complete reversal of the VOR when fitted with lenses that demand it. The focus of this chapter is on evidence for VOR adaptation from animal and human studies. In particular, I focus on the robustness of VOR plasticity, evidence for location sites of such plasticity, and how this adaptation occurs. For information concerning oculomotor (e.g., compensatory saccade) and vestibulospinal compensatory strategies that may assist a deficient VOR, please see Chapters 6 and 9.
Role of Vision and Head Motion in Adaptation—Overview
Two sensory stimuli are required for significant adaptation of the VOR: vision and head motion. Traditionally, adaptation paradigms designed to enhance the gain of the VOR have been done with short-term (less than 1 hour) or long-term (greater than 1 day) time exposure. A number of VOR adaptation studies have demonstrated a robust capability for changing the normal VOR by coupling head motion with target motion. When the viewed targets move in such a manner that the image slips on the retina, the VOR is recalibrated. The direction of the recalibrated VOR (increased or decreased) depends on the direction of the target motion in relation with the head motion. During head rotation, visual following of targets that move at a velocity different from the head rotation will create an adaptation (change) in the gain of the VOR. This is commonly achieved by one of two means: (1) targets are viewed wearing lenses that maximize the visual world (increases target velocity) or minimize the visual world (decreases target velocity) or (2) targets are viewed that move in a direction opposite the head rotation (increases target velocity) or move in the same direction of head rotation (decreases target velocity). The difference between the target velocity and the eye velocity is termed retinal slip and can be quantified. Retinal slip is therefore a velocity error signal, thought to be the most effective means for changing VOR behavior (e.g., VOR gain).1,2
Eye rotation—angular motion of the eye along a specified axis. For example, torsion refers to a clockwise or counterclockwise rotation around a cephalocaudal axis (the line of sight) when the eye is at its centered and primary position.
Critical Evidence of VOR Adaptation in Animals
Early and seminal studies, critical for establishing therapeutic principles of gaze and gait stability, investigated the role of vision and motion in animals that underwent a unilateral vestibular lesion.3–5 Data suggest when animals with a ...