The Vestibular and Optokinetic Systems

Abstract

The vestibulo-ocular and optokinetic reflexes are the earliest eye movements to appear phylogenetically. The vestibulo-ocular reflex (VOR) stabilizes retinal images during head motion by counter-rotating the eyes at the same speed as the head but in the opposite direction. Information about head motion passes from the vestibular sensors in the inner ear to the VOR circuitry within the brainstem, which computes an appropriate eye velocity command. The eyes, confined in their bony orbits, normally do not change position, and their motion relative to the head is restricted to a change in orientation. However, the head can both change position and orientation relative to space. Thus, the function of the VOR is to generate eye orientation that best compensates for changes in position and orientation of the head. Because the drive for this reflex is vestibular rather than visual, it operates even in darkness. To appreciate the benefits of having our eyes under vestibular and not just visual control, hold a page of text in front of you, and oscillate it back and forth horizontally at a rate of about two cycles per second. You will find that the text is blurred. However, if you hold the page still and instead oscillate your head at the same rate, you will be able to read the text clearly. This is because when the page moves, only visual information is available. Visual information normally takes about 100 msec to travel from the visual cortices, through a series of brain structures, to the ocular motoneurons that move the eyes. This delay is simply too long for the eyes to keep up with the oscillating page. However, when the head moves, both vestibular and visual information are available. Vestibular information takes only about 7–15 msec to travel from the vestibular sensors, through the brainstem, to the ocular motoneurons. With this short latency, the eyes can easily compensate for the rapid oscillation of the head. Thus, damages to the vestibular system often cause oscillopsia, an illusion of motion in the stationary environment, especially during head movements.