The Sense of Sight / All About the Eyes

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

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Page: 15

The nature of eye movements

So far, the relation of the movements of the eyes to the requirements of the visual apparatus and their control have been touched upon. To examine the character of the movements in some detail requires rapid, accurate measurement of the movements that the eyes undergo. Modern studies of this subject employ a contact lens fitting on to the globe; on the lens is a small plane mirror, and a parallel bundle of rays is reflected off this mirror onto a moving film.

By the use of refined methods of measuring the position of the eyes at any moment, it becomes immediately evident that the eyes are never stationary for more than a fraction of a second; the movements are of three types: (1) irregular movements of high frequency (30–70 per second) and small excursions of about 20 seconds of arc; (2) flicks, or saccades, of several minutes of arc occurring at regular intervals of about one second; and between these saccades there occur (3) slow irregular drifts extending up to six minutes of arc. The saccades are corrective, serving to bring the fixation axis on the point of regard after this has drifted away from it too far, and thus are a manifestation of the fixation reflex.

The significance of these small movements during fixation was revealed by studies on the stabilized retinal image: by a suitable optical device the image of an object could be held stationary on the retina in spite of the movements of the eye. It was found that under these conditions the image would disappear within a few seconds. Thus, the movements of the eye are apparently necessary to allow the contours of the image to fall on a new set of rods and cones at repeated intervals; if this does not occur, the retina adapts to their stimulus and ceases to send messages to the central nervous system. The small flicks mentioned above are essentially the same as the larger movement made when the two eyes fixate (fix on) a light when it suddenly appears in the peripheral field; this is given the general name of the saccade, to distinguish it from the slower movements occurring during convergence and smooth following. The dynamics of the saccade have been studied in some detail by several workers. There is a reaction time of about 120 to 180 milliseconds, after which both eyes move simultaneously; there is a definite overshoot and, with an excursion of 20°, the operation is completed in about 90 milliseconds. The maximum velocity increases with the extent of the movement, being 300° per second for 10° and 500° per second for 30°. A remarkable feature is the apparent absence of significant inertia in the eyeball, so that movement is halted, not by any checking action of antagonistic muscles but simply by cessation of contraction of the agonists; thus, the movement is not ballistic. Once under way, the saccade is determined in amount, so that the subject cannot voluntarily alter its direction and extent. The control mechanism for the saccadic type of movement can be described as a sampled data system, i.e., the brain makes discontinuous samples of the position of the eyes in relation to the target and corrects the error, in contrast to a continuous feedback system that takes account of the error all the time.

The movements of the eyes when they converge onto a near object are in remarkable contrast to the saccade; the angular velocity is only about 25° per second, compared with values as high as 500° per second in the saccade. The great difference in speed suggested to two investigators that the two movements are executed by different muscle fibres. In fact, the extraocular muscles do contain two types of muscle fibre with characteristically different nerve supplies, and some recent studies tend to support this view of a dual mechanism.

If a moving light suddenly appears in the field of view, and if its rate of movement is less than about 30° per second, the response of the eyes is remarkably efficient; a saccade brings the eyes on target, and they follow the motion at almost exactly the same angular velocity as that of the target; inaccuracies in following lead to corrective saccades. When the rate of movement of the target is greater than about 30° per second, these corrective saccades become more obvious because now smooth following is not possible; the eyes make constant-velocity movements, but the velocity rarely matches that of the moving target, so that there must be frequent corrective saccades. Studies have shown that the following movements are highly integrated and must involve a continuous feedback system whereby errors are used to modify the performance. Thus, the systems for control of saccades and tracking movements are fundamentally different.

Vision suppression during a saccade

If one looks into a mirror and fixates one of one's eyes and then fixates the other, one does not see the eyes moving; and it has been argued that, during an eye movement, vision is suppressed; if vision were not suppressed, moreover, it seems likely that the images of the external world would appear smeared during a movement. Experimental studies have shown that there is, indeed, a suppression of vision during a saccade.

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