The Sense of Sight / All About the Eyes

submitted by Dr. Gary Farr Last Updated June, 14, 2002

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Page: 14

Movements of the eyes

Reflex pathways

In examining any reflex movement one must look for the sensory input—i.e., the way in which messages in sensory nerves bring about discharges in the motor nerves to the muscles; this study involves the connections of the motor nerves or nuclei with other centres of the brain.

When a subject is looking straight ahead and a bright light appears in the periphery of his field of vision, his eyes automatically turn to fix on the light; this is called the fixation reflex. The sensory pathway in the reflex arc leads as far as the cerebral cortex because removal of the occipital cortex (the outer brain substance at the back of the head) abolishes reflex eye movements in response to light stimuli. If the occipital cortex is stimulated electrically, movements of the eyes may be induced, and in fact one may draw a pattern of the visual field on the occipital cortex corresponding with the directions in which the gaze is turned when given points on the cortex are stimulated. This pattern corresponds with the pattern obtained by recording the visual responses to light stimuli from different parts of the visual field.

The remainder of the pathway—i.e., from the occipital cortex to the motor neurons in the brainstem—has long been considered to involve the superior colliculi as relay stations, and they certainly have such a role in lower animals; but in human beings a pathway from the cortex to the eye-muscle nuclei independent of the superior colliculi of the midbrain is now generally assumed.

Continual movements of the eyes occur even when an effort is made to maintain steady fixation of an object. Some of these movements may be regarded as manifestations of the fixation reflex; thus, the eyes tend to drift off their target, and, because of this, the fixation reflex comes into play, bringing the eyes back on target.

Experimentally, the fixation reflex can be studied by observation of the regular to-and-fro movements of the eyes as they follow a rotating drum striped in black and white. (Such movements of the eyes directed at a moving object are called optokinetic nystagmus; nystagmus itself is the involuntary movement of the eye back and forth, up and down, or in a rotatory or a mixed fashion.) While the eyes watch the moving drum, they involuntarily make a slow movement as a result of fixing their gaze on a particular stripe. At a certain point, fixation is broken off, and the eyes spring back to fix on a new stripe. Thus, the nystagmus consists of a slow movement with angular velocity equal to that of the rotation of the drum, then a fast saccade, or jump from one point of fixation to another, in the opposite direction; the process is repeated indefinitely.

Another type of nystagmus reveals the play of another set of reflexes. These are mediated by the semicircular canals—i.e., the organs of balance or the vestibular apparatus. Such a reflex may be evoked by rotating the subject in a chair at a steady speed; the eyes move slowly in the opposite direction to that of rotation and, at the end of their excursion, jump back with a fast saccade in the direction of rotation. If rotation suddenly ceases, the eyes go into a nystagmus in the opposite direction, the postrotatory nystagmus.

During rotation, certain semicircular canals are being stimulated, and the important point is that any acceleration of the head that stimulates these canals will cause reflex movements of the eyes; thus, acceleration of the head to the right causes a movement of the eyes to the left, the function of the reflex being to enable the eyes to maintain steady fixation of an object despite movements of the head. The reflex occurs even when the eyes are shut, and, when the eyes are open, it obviously cooperates with the fixation reflex in maintaining steady fixation. In many lower animals this connection between organs of balance and eyes is very rigid; thus, one may move the tail of a fish, and its eyes will move reflexly. In man, not only do the semicircular canals function in close relation to the eye muscles but so also do the gravity organ—the utricle—and the stretch receptors in the muscles of the neck. Thus, when the head is turned upward, there is a reflex tendency for the eyes to move downward, even if the eyes are shut. The actual movement is probably initiated by the reflex from the semicircular canals, which respond to acceleration, but the maintenance of the position is brought about by a reflex through the stretch of the neck muscles and also through the pull of gravity on the utricle, or otolith organ, in the inner ear.

Voluntary centre

The eyes are under voluntary control, and it is thought that the cortical area subserving voluntary eye movements is in the frontal cortex. Stimulation of this in primates causes movements of the eyes that are well coordinated, and a movement induced by this region prevails over one induced by stimulation of the occipital cortex. The existence of a separate centre in man is revealed by certain neurological disorders in which the subject is unable to fixate voluntarily but can do so reflexly; i.e., he can follow a moving light.

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