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1. Eye Movements
Five types of eye movements are generally recognized. These movements fall into two broad categories: gazestabilization movements and gaze-shifting movements. Gaze-stabilization movements rotate the line-of-sight to compensate for movement of the head and body, stabilizing the visual world on the retina. In the absence of these movements, visual acuity is severely compromised because visual images move across the retina whenever the observer moves through the visual environment. In almost all vertebrates two neural systems work together to achieve gaze stabilization: the vestibulo-ocular system and the opto-kinetic system. Gaze-shifting movements, unlike gaze-stabilization movements, shift the fovea into alignment with objects of interest in the visual world. The three classes of movements that achieve gaze shifts are saccades, smooth pursuit eye movements, and vergence eye movements. Saccades are rapid shifts of the line-of-sight from one point in the visual world to another. Smooth pursuit eye movements allow the line-of-sight to track moving objects, slowly rotating the eye to keep a visual stimulus which is moving across the visual ﬁeld aligned with the fovea. Together saccades and smooth pursuit movements are referred to as versional movements because they cause both eyes to move in a similar direction and at the same speed. Vergence eye movements, which occur only in binocular animals like humans, shift the lines-of-sight of the two eyes with regard to each other (either converging or diverging the angle between the two eyes) so that a visual stimulus can be aligned with the same point on each retina, even if that stimulus moves towards or away from the observer. In the nineteenth century Herring proposed that only vergence movements involve the independent control of both eyes. For all other movements, he proposed that the two eyes always receive identical movement commands. This hypothesis, which still informs our basic view of oculomotor control, is known as Herring’s law of equal innervation.
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2. The Musculature Of The Eyes
In humans, all eye movements are rotations of the eye (never linear motions or translations) and are accomplished by six extra-ocular muscles that operate in three antagonistic pairs. One pair, the medial and lateral rectus muscles, is located on either side of each eyeball and controls the horizontal orientation of the eye. A second pair, the superior and inferior rectus muscles, controls the vertical orientation of the eye and a third pair, the superior and inferior oblique muscles, controls rotations of the eye around the line-of-sight. Although most people are unaware of them, rotations of the eye about the line of sight are quite common, though usually less than 10 in amplitude.
3. Three Cranial Nerve Nuclei Control Tension In The Extra-Ocular Muscles
The tension generated in each muscle is regulated by the activity of pools of alpha-motor neurons located in the oculomotor brainstem. The pools of neurons regulating the superior rectus, inferior rectus, medial rectus, and inferior oblique muscles each lie within separate subdivisions of the oculomotor, or third cranial nerve, nucleus. Neurons innervating the superior oblique muscle lie within the trochlear, or fourth cranial nerve, nucleus. Neurons innervating the lateral rectus muscle lie within the abducens, or sixth cranial nerve, nucleus. Because these six pools are the only source of input to the extraocular muscles, they serve as a ﬁnal common pathway through which all eye movement control must be implemented. Physiological studies of these neurons made while monkeys produced each of the ﬁve types of eye movements described above indicate that all of these motor neurons participate in the generation of all of the ﬁve classes of eye movements; there is no specialization amongst the alpha-motor neurons for any particular class of movement.
Engineering models of the eye and its muscles developed in the 1970s (largely by David Robinson and colleagues at Johns Hopkins University, USA; Robinson 1981) indicate that each of these motor neurons must generate two types of muscle force to accomplish any of the ﬁve classes of movements: a pulsatile burst of force produced during the movement, which regulates the instantaneous velocity with which the eye rotates, and a long-lasting increment or decrement in maintained force, which persists after the movement is complete. This maintained step in force holds the eye stationary by resisting the elastic forces of the muscles and connective tissues surrounding the eye. Forces which act to passively pull the eye back towards a straight ahead position. Physiological experiments have demonstrated that each motor neuron produces both of these two types of force. In summary then, the control circuits associated with gaze stabilizing and gaze shifting movements all must activate the same set of alpha motor neurons and all of these circuits must produce signals which regulate both the velocity and position of the eyes in a manner appropriate for that type of eye movement.
4. Gaze Stabilization Movements
Gaze stabilization movements achieve image stabilization when the head rotates in space by rotating the eyes at a velocity equal to that of the head but in an exactly opposite direction. Gaze stabilization thus requires very precise information about the speed and direction of head rotation across a broad range of head speeds. At high speeds of head rotation the vestibulo-ocular system obtains information from the semicircular canals of the inner ear which accurately encodes the velocity of head rotation. However, as the velocity of head rotation decreases, the vestibular system begins to underestimate velocity. At low speeds the optokinetic system obtains information about the rate at which the whole visual ﬁeld slides across the retina, information which accurately encodes the velocity of head rotation. In this system as the velocity of head rotation increases head velocity is underestimated. Thus either system working alone could not fully compensate for all movements of the head and body, but working together these two systems can provide an accurate estimate of head velocity that can be used by the oculomotor control circuitry to counterrotate the eyes in the head across a broad range of head speeds. Of course, regardless of the speed of eye rotation, as the eyes move towards far eccentric positions in the orbit, it eventually becomes impossible for the eyes to move any farther. The gaze stabilization systems respond to this challenge by very quickly moving the eyes in the opposite direction; this gives rise to a repetitive pattern of quick reset movements and slow stabilization phases usually called nystagmus.
4.1 Vestibulo-Ocular Response
In the vestibulo-ocular system (often called the vestibulo-ocular reﬂex or VOR) the rotational velocity of the head is transduced directly by the semicircular canals of the inner ear (even in total darkness) and enters the brain via the vestibular nucleus. Robinson and colleagues recognized that because this signal already encodes the velocity of head rotation, in order to compute the desired direction and velocity for a gaze stabilization movement it would only be necessary for the brain to take the inverse of this signal. Recalling that all oculomotor alpha-motor neurons produce both a pulsatile burst of force that regulates the velocity of the eye movement and a long-lasting step-like increment or decrement in maintained force that holds the eye stationary, Robinson proposed that the vestibular signal could be channeled directly to the alpha-motor neurons to produce the pulsatile burst of force. He then went on to suggest that changes to the long-lasting step in force required after each eye rotation could be computed from this pulse, or velocity, signal by the mathematical operation of integration. In the 1980s the lesion of a discrete brain area, the nucleus prepositus hypoglossi, was shown to eliminate only the long-lasting force change from the motor neurons without eﬀecting eye velocity during movements, suggesting that the nucleus prepositus hypoglossi is a neural integrator employed by the oculomotor system (Leigh and Zee 1991).
4.2 Optokinetic Response
In the optokinetic response (often called optokinetic nystagmus or OKN) head velocity is computed based on the global motion of the retinal image. This computation, determining the speed and direction of image motion, is performed in the pretectum and yields a signal that also speciﬁes the rotational velocity of the eyes necessary to achieve image stabilization. This visually derived signal passes to the vestibular nucleus where it has been shown to combine with the vestibular signal used by the VOR to yield a neural estimate of head velocity that is accurate across the entire range of speeds for which gaze stabilization can be accomplished. This combined signal is then processed by the same circuitry that Robinson and his colleagues described for the vestibulo-ocular system. Gaze stabilization thus appears to involve two sensory systems that are combined and passed to a single control circuit that governs eye rotation.
5. Gaze Shifting Movements
In some sense, gaze-shifting movements oppose gaze stabilization. While the VOR and OKN stabilize images on the retina to maximize visual acuity, gaze shifting movements redirect the line-of-sight, temporarily reducing the acuity with which some or all the visual world is perceived.
The saccadic system, in order to achieve a precise gazeshift, must supply the alpha-motor neurons with a command that controls the amplitude and the direction of the desired change in the line-of-sight. Current evidence indicates that this command can originate in either of two brain structures: the superior colliculus of the midbrain or the frontal eye ﬁelds of the neocortex (Sparks 1986). Both of these structures contain laminar sheets of neurons that code all possible saccadic amplitudes and directions in a topographic map-like organization and both project to brainstem regions that supply control signals to the oculomotor neural integrators and the motor neurons described above. Activation of neurons in these collicular and cortical maps has been shown to precede the onset of saccadic eye movements by about 30 milliseconds. One group of theories proposes that these cortical and collicular signals govern a brainstem feedback loop (incorporating the neural integrator) which accelerates the eye to a high velocity and keeps the eye in motion until the desired eye movement is complete while other theories make similar predictions but place this feedback loop outside the brainstem. In either case, it seems clear that the superior colliculus and frontal eye ﬁelds are important sources for these signals because if both of these structures are removed, no further saccades are possible. The superior colliculus and frontal eye ﬁelds, in turn, receive input from many areas within the visual and parietal corticies, the basal ganglia, and brain structures involved in audition and somatosensation. These areas are presumed to participate in the processes that must precede the decision to make a saccade, processes like attention and target selection.
5.2 Smooth Pursuit
In the smooth pursuit system, signals carrying information about target motion are extracted by motion processing areas in the visual cortex and then passed to the dorso-lateral pontine nucleus of the brainstem. There, neurons have been identiﬁed which code either the direction and velocity of pursuit eye movements, the direction and velocity of visual target motion, or both. These signals are passed to the cerebellum where neurons have been shown to encode speciﬁcally the velocity of pursuit eye movements. These neurons, in turn, make connections with cells known to be upstream of the nucleus prepositus hypoglossi (the oculomotor integrator described above). As in the other oculomotor systems, the brainstem integrator appears to compute the longterm holding force from this signal and then to pass the sum of these signals to the motor neurons.
A number of diﬀerent cues are used by primates to determine the distance to a visual stimulus and from this information to compute the convergence angle necessary to ﬁxate a target. The relative positions of a stimulus falling on the two retinas (the binocular disparity of a stimulus) can be used to compute how much additional convergence or divergence is required to cause a stimulus to fall on the same position in each retina. The state of the lens in the eye can also be used to determine the distance to objects both in and out of focus and thus to compute the required vergence angle. For that matter, even perspective cues can be used to compute a desired vergence angle. In any case, all of these cues are used to activate brainstem circuitry which appears to be very similar to the brainstem circuitry employed by the saccadic system. This brainstem circuit appears to generate a vergence velocity command similar to the saccadic velocity command which is in turn integrated and passed on to the alpha motor neurons. Of course, unlike the commands for versional movements, in this case each eye receives a very diﬀerent command for both velocity and holding force.
While many important questions remain the focus of active research, the oculomotor system is a comparatively well understood set of neural mechanisms that serves as an excellent model for understanding movement control in general.
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