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FIXATION MOVEMENTS OF THE EYES
Perhaps the most important movements of the eyes are those that cause the eyes to "fix" on a discrete portion of the field of vision.
Fixation movements are controlled by two different neuronal mechanisms. The first of these allows the person to move his eyes voluntarily to find the object upon which he wishes to fix his vision; this is called the voluntary fixation mechanism. The second is an involuntary mechanism that holds the eyes firmly on the object once it has been found; this is called the involuntary fixation mechanism.
The voluntary fixation movements are controlled by a small cortical field located bilaterally in the premotor cortical regions of the frontal lobes, as illustrated in Figure 51-8. Bilateral dysfunction or destruction of these areas makes it difficult or almost impossible for the person to "unlock" the eyes from one point of fixation and then move them to another point. It is usually necessary for the person to blink the eyes or put a hand over the eyes for a short time, which then allows the eyes to be moved.
On the other hand, the fixation mechanism that causes the eyes to "lock" on the object of attention once it is found is controlled by secondary visual areas of the occipital cortex - mainly Brodmann are 19 located anterior to visual areas V -1 and V - 2 (Brodmann areas 17 and 18 ). When this area is destroyed bilaterally, an animal has difficulty keeping its eyes directed toward a given fixation point or becomes completely unable to do so.
To summarize, the posterior eye fields automatically "lock" the eyes on a given spot of the visual field and thereby prevent movement of the image across the retina. To unlock this visual fixation, voluntary impulses must be transmitted from the "voluntary" eye fields located in the frontal areas.
Mechanism of the Involuntary Locking Fixation - Role of the Superior Colliculi. The involuntary locking type of fixation discussed in the previous section results from a negative feedback mechanism that prevents the object of attention from leaving the foveal portion of the retina. The eyes even normally have three types of continuous but almost imperceptible movements: (1) a continuous tremor at a rate of 30 to 80 cycles per second caused by successive contractions of the motor units in the ocular muscles, (2) a slow drift of the eyeballs in one direction or another, and (3) sudden flicking movements that are controlled by the involuntary fixation mechanism. When a spot of light has become fixed on the foveal region of the retina, the tremorous movements cause the spot to move back and forth at a rapid rate across the cones, and the drifting movements cause it to drift slowly across the cones. However, each time the spot of light drifts as far as the edge of the fovea, a sudden reflex reaction occurs, producing a flicking movement that moves the spot away from this edge back toward the center. Thus, an automatic response moves the image back toward the central portion of the fovea. These drifting and flicking motions are illustrated in Figure 51-9, which shows by the dashed lines the slow drifting across the retina and by the solid lines the flicks that keep the image from leaving the foveal region.
This involuntary fixation capability is mostly lost when the superior colliculi are destroyed. After the signals for fixation originate in the visual fixation areas of the occipital cortex, they pass to the superior colliculi, probably from there to reticular areas around the oculomotor nuclei, and thence into the motor nuclei themselves.
Saccadic Movement of the Eyes - a Mechanism of successive Fixation Points. When the visual scene is moving continually before the eyes, such as when a person is riding in a car or turning around, the eyes fix on one highlight after another in the visual field, jumps per second. The jumps are called saccades, and the movements are called opticokinetic movements. The saccades occur so rapidly that not more than 10 per cent of the total time is spent in moving the eyes, 90 per cent of the time being allocated to the fixation sites. Also, the brain suppressed the visual image during the saccades so that one is completely unconscious of the movements from point to point.
Saccadic Movements During Reading. During the process of reading , a person usually makes several saccadic movement of the eyes for each line. In this case the visual scene is not moving past the eyes, but the eyes are trained to scan across the visual scene to extract the important information. Similar saccades occur when a person observes a painting except that the saccades occur in one direction after another from one highlight of the painting to another, then another, and so forth.
Fixation on Moving Objects - "Pursuit Movements." The eyes can also remain fixed on a moving object, which is called pursuit movement. A highly developed cortical mechanism automatically detects the course of movement of an object and then gradually develops a similar course of movement of the eyes. For instance, if an object is moving up and down in a wavelike form at a rate of several times per second, the eyes at first may be completely unable to fixate on it. However, after a second or so the eyes begin to jump coarsely in approximately the same pattern of movement as that of the object. Then after a few more seconds, the eyes develop progressively smoother and smoother movements and finally follow the course of movement almost exactly. This represents a high degree of automatic, subconscious computational ability by the cerebral cortex.