Ocular Compensation due to Labyrinthine Input during Natural Motion

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Abstract
With recent technical developments, it has become possible to study eye and head movements in all angular and linear degrees of freedom during natural activities such as standing and treadmill ambulation. Such studies have revealed that normal human gaze stabilization during ambulation is accomplished not only by ocular rotation in response to head rotation, but also by head translation coordinated with head rotation as appropriate to viewing distance. Typically, head translation in a given direction is coupled via an active mechanism with antiphase rotation in the opposite direction, accomplishing some gaze stabilization for near targets and supplementing the vestibulo-ocular reflex (VOR). Subunity angular VOR gain (eye velocity/head velocity) not only is observed during most natural activities, but also minimizes the disturbance to retinal image stability so that excellent visual acuity can be obtained. The generation of eye movements compensating for head motion during ambulation can be quantitatively modeled as an additive combination of semicircular canal responses, with otolith responses scaling by inverse viewing distance, so angular VOR gain correlates poorly with gaze stability. Compensatory eye movements during self-generated head rotations appear driven to a significant extent by nonvestibular mechanisms since they cannot be modeled as an additive combination of otolith and canal responses. People with unilateral vestibular deafferentation have normal gaze stability during natural activities such as standing, walking, and running, although gaze stability is impaired in bilateral deafferentation. Limitation of head motion during natural activities is a common strategy in those with reduced peripheral vestibular sensitivity.