The Vestibular Ocular Reflex

The vestibular-ocular reflex (VOR) is a second form of motor learning whose neural substrate has been largely identified within the cerebellum. The VOR causes movements of the eyes that are intended to compensate for movements of the head and body so that visual images remain stable on the retina. When we move our heads, our eyes rotate effortlessly so that the world does not appear to move in the opposite direction. If magnifying spectacles are worn, head movements lead to larger displacements of the visual scene, and the VOR adapts so that the eye movements have the appropriate magnitude.

Evidence from single-unit recordings and ablation studies shows that this adaptation requires the floc-cular complex of the cerebellum. Animal studies of the VOR are typically performed in the dark, with the animal placed on a turntable so that visual signals and body movements do not provide a potential source of the neural and behavioral responses. Rotations of the head cause changes in the rate of simple spike discharges for the Purkinje cells within the floccular complex. When vision is provided and experimental manipulations cause the VOR to incorrectly compensate for the movement, the magnitude of these changes is modulated in a manner that is consistent with the appropriate alteration of the VOR gain.

In some respects it is surprising that similar circuits should be involved in adaptation-like tasks, such as adjusting the VOR, and eye-blink conditioning. Eye-blink conditioning involves discrete movements whose magnitude and trajectory require little on-line adjustment. In contrast, the VOR is a continuous behavior that involves the coordination of multiple muscles whose degree of activation must be precisely modulated. However, there is considerable overlap in the computational requirements for the two tasks. For either behavior to be successful, the organism must learn to precisely time specific muscle commands with sensory events to minimize an unwanted stimulus. For eye-blink conditioning, the stimulus is the puff of air on the animal's eye; for the VOR, the stimulus is movement of the image on the retina.

In the VOR, the initial motor reflex and vestibular inputs are represented along the parallel fibers and the Purkinje cells learn to produce the pattern of activation to tune the motor command so that the eye movement correctly compensates for the change in the head's position. Climbing fibers provide signals about the slippage of the image on the retina. Thus, there is another parallel between VOR adaptation (and normal function) and eye-blink conditioning. The simple spikes provided by the mossy fibers provide a representation of the current environmental context—a representation that likely includes an efference copy of the animal's own movements but also sensory information relevant for these actions. The climbing fibers provide an error signal, a representation of the unexpected and aversive air puff or of a mismatch between expected and actual eye position.

All About Alzheimers

All About Alzheimers

The comprehensive new ebook All About Alzheimers puts everything into perspective. Youll gain insight and awareness into the disease. Learn how to maintain the patients emotional health. Discover tactics you can use to deal with constant life changes. Find out how counselors can help, and when they should intervene. Learn safety precautions that can protect you, your family and your loved one. All About Alzheimers will truly empower you.

Get My Free Ebook

Post a comment