The Foveation System

As in most primates, the human retina has a tremendously specialized central zone, the fovea, where visual acuity is 1000 times better than vision just 10° eccentric. Hence, to "look at'' something is effectively to foveate it. However, the fovea subtends only 1 ° of visual angle (equivalent to the full moon's subtend). Therefore, the second principal function of eye movements is to foveate important parts of the visual scene.

Foveation is accomplished by a triad of voluntary eye movements: saccades, fixation, and smooth pursuit. Of these, saccades are the most conspicuous because humans incessantly make saccades in order to maximize retrieval of high-quality visual information by foveal viewing. Although the saccadic movement is ballistic and stereotyped, saccades are voluntary with regard to the choice of whether or not to make a saccadic movement (when to look) and what saccadic vector to program (where to look). For example, reading is accomplished with a succession of voluntary saccades that march across each line (Fig. 1), and during each fixation (between saccades) the reader processes the foveated word(s) while simultaneously programming the next saccade.

Smooth-pursuit eye movements, the third member of the foveation triad, assists the viewing of objects moving in space by matching eye velocity to target velocity, up to 50-100°/sec. A record of smooth pursuit is shown in Fig. 2. Pursuit is sometimes confused with the OKR; however, they are very different movements, as described later, and pursuit often must override the combined vestibulooptokinetic reflexes (e.g., during combined head-eye tracking of a moving stimulus). Instead, pursuit is more equivalent to the continued fixation of a target that happens to be moving.

Whereas image stabilization is effected by a largely reflexive brain stem system, the more voluntary, cognitive foveation system and its constituent eye movements (saccades, fixation, and pursuit) involve many areas of neocortex, most notably the frontal eye field (FEF) region of the frontal lobe.

Smooth Pursuit

Figure 1 Eye movements during reading. Eye movement records showing the pattern of saccades and fixations made while reading the sentence shown in the abscissa. Records were obtained using the infrared photoelectric scleral reflection technique (at 100 Hz). Time increases along the ordinate; therefore, to recreate the eye's movement the traces should be read from bottom to top. Notice that the sentence/line was read with 10 progressive (rightward) saccades and one regressive (leftward saccade), and the majority of the time spent reading the line is during the 11 fixations between the saccades (adapted from J. K. O'Regan, Eye Movements and Their Role in Visual and Cognitive Processes (E. Kowler, Ed.), copyright 1990, with permission from Elsevier Science).

Figure 1 Eye movements during reading. Eye movement records showing the pattern of saccades and fixations made while reading the sentence shown in the abscissa. Records were obtained using the infrared photoelectric scleral reflection technique (at 100 Hz). Time increases along the ordinate; therefore, to recreate the eye's movement the traces should be read from bottom to top. Notice that the sentence/line was read with 10 progressive (rightward) saccades and one regressive (leftward saccade), and the majority of the time spent reading the line is during the 11 fixations between the saccades (adapted from J. K. O'Regan, Eye Movements and Their Role in Visual and Cognitive Processes (E. Kowler, Ed.), copyright 1990, with permission from Elsevier Science).

Smooth Pursuit System And The Brain

Figure 2 Smooth-pursuit eye movements. Horizontal eye position (top) and eye velocity (bottom) records of a monkey's smooth-pursuit tracking of a small spot with sinusoidal motion (amplitude 7 7.5°, frequency 0.5 Hz). Tracking sinusoidal motion is equivalent to tracking the bob of a pendulum, a classic pursuit test. Maximum target velocity is 23.6°/sec, and maximum pursuit velocity was ~25°/sec. Saccades are easily identified by their spikes in the eye velocity traces. A total of seven saccades were made during the 6 sec of sinusoidal motion, starting with a large catch-up saccade shortly after the start of the target motion. At the end of the record smooth pursuit continued for part of a fourth cycle (predictive pursuit), even though the target was extinguished after three cycles (C. J. Bruce, unpublished data).

Figure 2 Smooth-pursuit eye movements. Horizontal eye position (top) and eye velocity (bottom) records of a monkey's smooth-pursuit tracking of a small spot with sinusoidal motion (amplitude 7 7.5°, frequency 0.5 Hz). Tracking sinusoidal motion is equivalent to tracking the bob of a pendulum, a classic pursuit test. Maximum target velocity is 23.6°/sec, and maximum pursuit velocity was ~25°/sec. Saccades are easily identified by their spikes in the eye velocity traces. A total of seven saccades were made during the 6 sec of sinusoidal motion, starting with a large catch-up saccade shortly after the start of the target motion. At the end of the record smooth pursuit continued for part of a fourth cycle (predictive pursuit), even though the target was extinguished after three cycles (C. J. Bruce, unpublished data).

C. The Vergence-Accommodation System

As a consequence of frontally placed eyes (found in all primates and in predatory species of other vertebrate orders), the visual fields of the two eyes have considerable overlap. Stereoscopic depth can be extracted from the small differences in the these overlapping images, yielding detailed information about the three-dimensional (3D) structure of the visual world. However, good stereopsis requires that both eyes be directed at (i.e., foveate) the same object in visual space. Vergence (or disjunctive) eye movements provide such binocular alignment in response to changing fixation target distances, which necessitate that the two eyes point in different directions. In contrast, all of the aforementioned eye movement types are conjugate (or equiva-lently, conjunctive). Thus, the two eyes move as one during saccades, pursuit, VOR, etc. but not during vergence. The basic neural mechanisms of vergence and accommodation are in the brain stem, especially in midbrain regions immediately adjacent to the oculomotor nucleus (n. III). However, the vergence-accom modation system is also dependent on neocortex, especially primary visual (striate) cortex and the frontal lobe cortex.

III. HOW DO THE EYES MOVE? PHENOMENOLOGY OF EYE MOVEMENTS

The oculomotor response can be reduced to seven basic types of eye movements (Table I) that achieve the three principal functions of image stabilization, fovea-tion, and binocular alignment. Although these movements differ in many regards (e.g., whether they are fast or slow, voluntary or reflexive, as well as which of the three functions is served), the kinematics of eye movements are generic. All eye movements share a common final path represented by three cranial nerve nuclei and the six pairs of eye muscles that they control. This section describes the basic mechanics of eye movements and the muscles, nerves, and motor nuclei involved.

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