MRVisualHandout - 1 Neural Bases of Speech & Language 1...

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1 ANATOMY AND PHYSIOLOGY OF THE VISUAL SYSTEM I. Image-Forming System of the Eye A. All vertebrates that use vision for survival have a highly-evolved eye that has one mechanism or another for forming a sharp image of the visual world on a “sheet” of photoreceptors. B. In vertebrates that live out of the water, the “basic eye” has two lenses. 1. The cornea is a strong (bends the light a lot) lens that has a fixed focal length. It cannot be adjusted to focus the image on the retina. 2. The lens of the eye (proper noun) is a weak lens, but its focal length is adjustable, and is normally used to focus the image of objects that are close to the eye (less than 20 feet). a. The process of changing the focal length of the lens of the eye to focus on close objects is called “accommodation .” b. Accommodation comes about by the contraction of the ciliary muscle. This muscle is effectively “in parallel” with the elastic zonule fibers that tend to pull on the edges of the lens capsule. When the ciliary muscle contracts, it takes the tension off the lens capsule. The lens capsule is naturally elastic, so tends to round the lens into a sphere. The lens capsule covering the anterior surface of the lens is weaker than the rest of the capsule, so the anterior part tends to bulge out when the lens is “rounded up.” This bulging increases the curvature of the anterior surface of the lens, and thus makes it bend the light more. C. There are a set of terms used to describe the usual imperfections of the focusing system of the eye. 1. Emmetropic : a normal eye that forms a sharp image of distant objects (greater than 20 feet) without accommodation. 2. Myopic : “near sighted” eye that can not focus on distant objects because the eyeball is too long, so the image is focused in front of the retina. Since accommodation can only make the lens system stronger yet, the eye cannot solve this problem by itself. The uncorrected myopic eye has an abnormally short “near point.” To correct a myopic eye, one has to decrease the optical power of the system, so diverging lens (that minify rather than magnify) are used in the glasses. 3. Hyperopic : “far-sighted” eye that can focus on distant objects by accommodation, but that has an abnormally long “near point,” so cannot read small print. Also, continuous muscular effort is needed to see clearly at a distance, so continuous active vision (as in long distance driving) can produce “eye strain,” which is the ciliary muscle complaining. The usual problem is that the eyeball is too short, so the 1
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2 unaccommodated system focuses the image behind the retina. Converging lenses, which add light-bending power to the dioptrics system, are used for correction. 4.
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This note was uploaded on 09/09/2009 for the course SPEECH AND 449 taught by Professor Rogers during the Summer '06 term at University of Washington.

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MRVisualHandout - 1 Neural Bases of Speech & Language 1...

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