I. CENTRAL SYSTEMS FOR THE CONTROL OF EYE MOVEMENTS
A. The eyes move at two speeds, fast and slow (Dell’Osso and Daroff, 1999)
Physiologic fast eye movements, saccades, include:
All voluntary horizontal and vertical eye movements.
The kickback phase of jerk nystagmus, whether pathologic or physiologically induced by caloric or optokinetic stimuli.
Rapid eye movements in sleep.
Pathologic fast eye movements, opsoclonus and ocular flutter, are faster than any saccades that the person can produce by volitional eye movements and are described later.
Slow eye movements include:
The deviation phase of vestibular and optokinetic nystagmus.
B. The five eye-movement systems tested by the neurologic examination (NE)
Five central systems control voluntary and reflex eye movements (Bender, 1980; Bender et al 1998). Voluntary selection of a visual target requires saccadic action to move the eyes to the target. After that, fixation, fusion, following, and focusing (vergences and the control of refraction) proceed more or less automatically (Table 5-1).
TABLE 5-1The Five Major Eye-Movement Systems ||Download (.pdf) TABLE 5-1 The Five Major Eye-Movement Systems
|System ||Function or characteristic |
|Saccadic system ||Produces all volitional movements and the fast phase of reflex eye movements (frontal lobe) |
|Fixation (position maintenance system) ||Fixates and maintains eyes on target and locks them in unison to fuse the two retinal images into one visual image (occipital lobe) |
|Smooth pursuit system ||Keeps eyes on moving target (occipital lobe) |
|Vergence system ||Converges or diverges eyes for near or distant targets (occipital lobe) |
|Counter-rolling system ||Vestibular and neck proprioceptive system: counter-rolls the eyes to keep them fixed on the visual target in compensation for head movement |
Saccadic system: saccade = to jerk or rein in. Saccadic movement describes eye movements by increments or jerks, like a ratchet (Kennard et al., 1994).
Self-demonstration of saccades:
Look straight ahead. Then, while keeping your head completely still, move your eyes all the way to the right and hold them there.
With your head still, very slowly try to move your eyes as continuously and smoothly as possible all the way from the right to the left. Attend to how your eyes move. Do they move continuously and smoothly or by incremental jerks? ___________________________________________________
You will experience your eye movements as jerks, that is, saccades. Try this experiment on other people and observe their saccades.
You cannot move your eyes smoothly voluntarily. All volitional eye movements require saccades.
Fronto-tegmental corticobulbar pathways are thought to mediate all such saccades (Fig. 5-1).
The supplementary motor area, substantia nigra, superior colliculus, vermis, fastigial nucleus, reticular formation, and vestibular system play roles in saccadic production and accuracy.
To test for saccadic accuracy: Hold up your index fingers about 18 inches apart and ask the patient (Pt) to look at one and then the other. ...