Eye Movements: Physiological

Maintaining the Line of Sight

Two types of eye movements maintain the line of sight: the vestibulo-ocular reflex (VOR) and the optokinetic response (OKR). The VOR uses signals from the bony labyrinth of the inner ear that indicate how the head is moving in space. A direct three-neuron pathway relays these signals quickly (in less than 1 one-hundredth of a second!) to counter-rotate the eyeballs to keep the retinal images of the world stable during head movements. For example, as you look at this text, try shaking your head back and forth, and notice that your eyes automatically and rapidly move in the opposite direction, so that your ability to see the text remains unperturbed. As your head moves to the right, sensory neurons for the right inner ear increase their activity, whereas those in the left ear decrease their activity. These changes in activity are relayed by interneurons in the brain stem to the motor neurons that control the six muscles that move each eye. In addition, a copy of the signals from the inner ear are summed over time and stored in a process akin to mathematical integration; this integrated signal is also applied to the eye motor-neurons, so that your eyes are held in position when you stop moving your head.

The OKR also stabilizes the eyes, but uses visual signals about the movement of images across the retina. Because sensory transduction takes longer for vision than for the vestibular system, the OKR reacts slower than the VOR does (after 100 milliseconds rather than 10 milliseconds). However, unlike the signals from the inner ear, which decay after a few seconds of head movement, visual signals persist as long as images move across the retina. Thus, the OKR can compensate for low speeds and constant motions, even when there are no signals detected by the inner ear. For example, as you look at this text, try holding your head still and slowly move the page; the OKR will move your eyes to automatically follow the motion of the page, so that your ability to see the text again remains intact. The signals for this behavior come from areas of the brain that are specialized for processing visual motion. Some of these areas are located in the mantle of the cerebral cortex that covers most of the brain in primates, and others [Page 428]are located in nuclei underneath the cortex. Neurons in these areas provide inputs to the same interneurons and motor neurons in the brain stem that are used for the VOR. This shared brain stem circuit for holding and moving the eyes, which is used by all eye movement systems, is referred to as “the final common pathway.”

...