| accommodation | The variation in the eye's optical power brought about by temporary changes in the shape of the lens.
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| afterimage | A visual sensation that persists after exposure to some intense stimulus; also, an illusory color produced by exposure to an intense stimulus.
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| anomalous myopia | A temporary myopia induced by lack of adequate stimuli for accommodation.
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| aqueous humor | A watery fluid, produced by the ciliary body, that nourishes structures within the eye's anterior chamber and helps maintain the eye's shape.
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| astigmatism | An error in refraction caused by variation in optical power along various meridians of the cornea.
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| cataract | Clouding that reduces the lens's transparency and, hence, degrades the quality of the retinal image.
center frequency The frequency lying at the center of the range of frequencies contained in bandpass noise.
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| choroid | A dark, spongy structure containing blood vessels that supply nourishment to the retina; because of its heavy pigmentation, the choroid absorbs scattered light.
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| ciliary body | Located in the eye, a spongy network of tissue that manufactures aqueous humor.
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| collector cells | Specialized neurons within the eye that receive and process information from the photoreceptors.
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| cones | Photoreceptors that are specialized for daylight and color vision. See rods.
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| conjunctive | Referring to those movements of the eye in which both eyes move in the same direction. See vergence.
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| cornea | The transparent portion of the eye's front surface, which refracts light and allows it to pass into the eyeball.
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| crystalline lens | The elliptical optical element located immediately behind the iris of the eye. Temporary variations in thickness alter the eye's accommodation, or optical power.
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| depth of field | The range of distances over which the image of a scene remains sharply focused; varies with pupil size.
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| divergent | Referring to light whose wave-fronts spread outward, usually as the light proceeds away from its source.
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| electromagnetic radiation | Energy that is produced by oscillation of electrically charged material; light encompasses a small portion of the electromagnetic spectrum.
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| emmetropic | Referring to an eye whose focal point, in the absence of accommodation, coincides exactly with the retina.
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| extraocular muscles | In humans, six large muscles attached to the globe of the eye; by rotating the eyeball within the orbit, the coordinated contractions of these muscles control the direction of gaze. See rectus muscles.
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| fibrous tunic | The strong, leathery outermost layer of the eyeball.
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| floaters | Debris that drifts about within the eye's vitreous casting shadows on the retina and producing dark spots that appear to move along with the eye.
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| fovea | Pit or depression in the retina; the region of sharpest vision.
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| glaucoma | A relatively common ocular disorder in which fluid pressure builds up within the eyeball, eventually causing blindness if not corrected.
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| hyperopic | Referring to an abnormally short eyeball, in which the image is blurred because the eye’s focal point lies behind the retina.
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| image | The spatial distribution of light energy produced by the action of some optical system.
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| iris | The two-layered ring of tissue that gives the eye its characteristic color.
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| macula | The small, circular central region of the retina where vision is most acute.
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| myopic | Referring to an abnormally long eye, in which the retinal image is blurred because the eye's focal point lies in front of the retina.
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| nanometer | A unit of length in the metric system corresponding to one-billionth of a meter, used for specifying wavelength of light.
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| ophthalmoscope | An optical device used to visualize the inside of the eye.
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| optic disk | The region of the eye where the optic nerve penetrates the retina; also, the region where major blood vessels enter and exit the eye's interior.
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| orbit | Cavity in the skull that houses the eyeball and its supporting structures.
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| photopigment | Light-sensitive molecules within a photoreceptor; light causes the photopigment to isomerize, releasing energy that alters the photoreceptor's electrical potential.
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| photoreceptors | Specialized nerve cells (rods and cones) in the eye that contain photopigment; absorption of light by these cells triggers changes in the cells' electrical potential.
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| pigment epithelium | A layer of the retina that helps to dispose of cellular debris.
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| presbyopia | A significant decline in accommodative ability beginning in middle age.
primitives In theories of vision, simple elements that are thought to be building blocks for perception.
profile analysis The process by which the relative activity of various neurons registers some property of a stimulus.
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| pupil | The aperture in the eye formed by two sets of concentric bands of muscle; the constriction and dilation of these muscles vary the diameter of the pupil.
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| rectus muscles | Four of the extraocular muscles; largely responsible for moving the eyeball back and forth horizontally (medial rectus and lateral rectus) and up and down vertically (superior rectus and inferior rectus). See extraocular muscles.
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| refraction | The bending of light by an optical element such as a lens.
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| retina | The innermost layer of the eyeball, where light is detected by photoreceptors and transduced into neural signals that are processed by collector cells.
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| retinal ganglion cells | The collector cells that are responsible for the last stage of visual processing within the retina; axons of the retinal ganglion cells constitute the optic nerve.
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| retinal image | The distribution of light falling on the retina; the quality and overall intensity of this image influence visual perception.
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| rods | Photoreceptors that are specialized for vision under dim light. See cones.
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| sclera | The tough, dense material that forms the eye's outermost coat; seen from the front, the sclera is the white of the eye.
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| sclerosis | The hardening of any living tissue; hardening of the eye's crystalline lens may play a role in presbyopia. See presbyopia.
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| synapse | A tiny gap between adjacent nerve cells. See transmitter substance.
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| transmitter substance | One of several different neurochemicals that diffuse across synaptic gaps between adjacent nerve cells, allowing cells to communicate with one another. See synapse.
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| vergence | Referring to eye movements in which the two eyes move in opposite directions. See conjunctive.
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| vitreous | The thick transparent fluid that fills the eye's largest chamber.
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| wavelength | The distance from the peak of one wave to the peak of the next. For electromagnetic radiation, such as light, wavelength is determined by the rate at which the emitting substance oscillates. This physical property of light, specified in nanometers, is related to the perceptual experience of hue.
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2002 McGraw-Hill Higher Education