As described earlier, a cornea with normal curvature provides much of the refractive power needed to focus distant objects on the retina without the need for accommodation. This normal condition, called emmetropia, will be altered if the curvature of the cornea (or, more rarely, the lens or retinal shape) is irregular, causing portions of the visual field to appear blurred (astigmatism). Visual problems are also encountered if the eyeball itself is too short from front to back. In this case, the cornea focuses the image in a plane behind the retina (hyperopia, or farsightedness). If the eyeball is too long, the image will be focused in front of the retina (myopia, or nearsightedness). These last two conditions can be corrected by convex or concave lenses, respectively. Contact lenses are commonly used to compensate for most forms of astigmatism, as the spherical surface of the contact lens optically replaces the distorted corneal surface.
Another condition that affects virtually all individuals sometime after the age of 40 is presbyopia, a condition caused by hardening of the lens that accompanies the aging process. Because the lens generates new lens cells throughout life and none is lost, the lens becomes compacted and loses its elasticity. As a result, the accommodation reflex is greatly attenuated. Bifocal lens are used to compensate for presbyopia by incorporating a concave lens on the top to assist in far vision and a convex lens on the bottom to assist in near vision.
brain stem, whereas loss of the direct reflex may involve only more distal components of the visual or oculomotor pathways.
In addition to simple autonomic reflexes, a number of complex visual motor reflexes can be automatically triggered when seeking and tracking a visual target. All eye movements occur in a coordinated fashion in both eyes, in the form of vergence movements (when the two eyes move toward each other as part of the accommodation reflex) or in the form of conjugate movements (in which the two eyes move in the same direction). These coordinated movements are achieved by joint action of yoked pairs of agonist and antagonist motor nuclei. The coordination is achieved by groups of interneurons located in areas of the reticular formation that surround the motor nuclei of cranial nerves IV and VI. Two specific areas, called gaze centers, have been identified. Fibers connect the gaze centers and the appropriate motor nuclei through a midline fiber bundle, the medial longitudinal funiculus (MLF).
Horizontal eye movements are coordinated through the horizontal gaze center (the paramedian pontine reticular formation, or PPRF) located near cranial nerve IV. Vertical eye movements are coordinated in the vertical gaze center, which is thought to be located near cranial nerve VI.
Eye movements are very stereotyped and consist of either saccades or smooth pursuit movements. Saccades are quick, jerky movements useful in searching for visual targets. This type of movement appears to be preprogrammed by the brain based on the distance the eye has to move to position the desired target on the fovea. Saccades are ballistic in that no corrections in the speed or direction of the movement can be made after the movement is initiated. Smooth pursuit movements are maintained movements that are capable of moving the eyes to continuously follow a moving visual target or moving the eyes across a series of stationary visual targets, as in reading. These movements are not ballistic in that their speed and direction can be modified continually. Saccades and smooth pursuit movements often occur in combination. A saccade may be used to find a novel target; smooth pursuit movements would then keep it focused on the fovea, adjusting for movement of the object or movement of the observer.
Rhythmic, repeating combinations of a long smooth pursuit followed by a saccade are called nystagmus, which is named after the direction of the fast (saccade) component (e.g., left beating nystagmus). Nystagmus would normally be observed in an ice skater who is spinning on the ice. The eyes of the skater would move in smooth pursuit to keep some targeted object focused on the fovea as long as possible and then a saccade would be used to reset the eye position. This reflex movement is called vestibular nystagmus, and it requires input from vestibular pathways.
Another condition in which nystagmus normally occurs is when the observer is stationary but the visual field is continuously moving in one direction, as happens when one looks out the window of a moving train. The eyes follow the visual field with a smooth pursuit movement until they reach the limit of the movement range, then a saccade is used to reset. This is called opticokinetic nystagmus.
An additional cortical area has been identified that plays an important, although not completely understood, role in initiating and/or influencing smooth pursuits, saccades, and opticokinetic nystagmus. This area is called the frontal eye field (Brodmann's area 8) and is located immediately frontal to the premotor area. There is strong evidence that the frontal eye field is necessary for ipsilateral voluntary saccades, but it may also play additional roles in other eye movements, as well. The cerebellum provides additional input for the fine-tuning of saccades and smooth pursuit movements.
Pathologic nystagmus occurs when inappropriate movements are observed that do not match the velocity or direction of the intended visual target. Vestibular
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