| accommodation | The variation in the eye's optical power brought about by temporary changes in the shape of the lens.
|
 |
|
|
| afterimage | A visual sensation that persists after exposure to some intense stimulus; also, an illusory color produced by exposure to an intense stimulus.
|
|
|
|
| amacrine cell | A type of cell in the retina that transmits electrical signals from the bipolar cells to the ganglion cells, in the process influencing the rate at which those signals are generated over time. See bipolar cell and horizontal cell.
|
|
|
|
| anomalous myopia | A temporary myopia induced by lack of adequate stimuli for accommodation.
|
|
|
|
| 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.
|
|
|
|
| astigmatism | An error in refraction caused by variation in optical power along various meridians of the cornea.
|
|
|
|
| bipolar cells | Type of cell in the retina that transforms electrical signals from the photoreceptors into action potentials; these cells play a crucial role in generating ON and OFF responses. Together with amacrine and horizontal cells, bipolar cells form a network that shapes the receptive field properties of ganglion cells. See amacrine cell, horizontal cell.
|
|
|
|
| cataract | Clouding that reduces the lens's transparency and, hence, degrades the quality of the retinal image.
|
|
|
|
| choroid | A dark, spongy structure containing blood vessels that supply nourishment to the retina; because of its heavy pigmentation, the choroid absorbs scattered light.
|
|
|
|
| ciliary body | Located in the eye, a spongy network of tissue that manufactures aqueous humor.
|
|
|
|
| cones | Photoreceptors that are specialized for daylight and color vision. See rods.
|
|
|
|
| cornea | The transparent portion of the eye's front surface, which refracts light and allows it to pass into the eyeball.
|
|
|
|
| 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.
|
|
|
|
| depth of field | The range of distances over which the image of a scene remains sharply focused; varies with pupil size.
|
|
|
|
| divergent | Referring to light whose wavefronts spread outward, usually as the light proceeds away from its source.
|
|
|
|
| electromagnetic radiation | Energy that is produced by oscillation of electrically charged material; light encompasses a small portion of the electromagnetic spectrum.
|
|
|
|
| emmetropic | Referring to an eye whose focal point, in the absence of accommodation, coincides exactly with the retina.
|
|
|
|
| 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.
|
|
|
|
| fibrous tunic | The strong, leathery outermost layer of the eyeball.
|
|
|
|
| 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.
|
|
|
|
| fovea | Pit or depression in the retina; the region of sharpest vision.
|
|
|
|
| glaucoma | A relatively common ocular disorder in which fluid pressure builds up within the eyeball, eventually causing blindness if not corrected.
|
|
|
|
| horizontal cell | A type of cell in the retina that modulates the strength of signals generated by neighboring photoreceptors. Together with bipolar and amacrine cells, horizontal cells form a network responsible for shaping the receptive field properties of ganglion cells. See amacrine cell, bipolar cell.
|
|
|
|
| hyperopic | Referring to an abnormally short eyeball, in which the image is blurred because the eye's focal point lies behind the retina.
|
|
|
|
| image | The spatial distribution of light energy produced by the action of some optical system.
|
|
|
|
| inverse optics | The problem of determining what objects in the world generated a given image on the retina.
|
|
|
|
| iris | The two-layered ring of tissue that gives the eye its characteristic color.
|
|
|
|
| macula | The small, circular central region of the retina where vision is most acute.
|
|
|
|
| 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.
|
|
|
|
| nanometer | A unit of length in the metric system corresponding to one-billionth of a meter, used for specifying wavelength of light.
|
|
|
|
| ophthalmoscope | An optical device used to visualize the inside of the eye.
|
|
|
|
| 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.
|
|
|
|
| orbit | A cavity in the skull that houses an eyeball and its supporting structures.
|
|
|
|
| outer segment | The distal part of a photoreceptor that contains photopigment.
|
|
|
|
| photopigment | Light-sensitive molecules within a photoreceptor; light causes the photopigment to isomerize, releasing energy that alters the photoreceptor's electrical potential.
|
|
|
|
| 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.
|
|
|
|
| phototransduction | The process by which absorption of light by photopigment triggers an electrical signal within photoreceptors.
|
|
|
|
| pigment epithelium | A layer of the retina that helps to dispose of cellular debris.
|
|
|
|
| presbyopia | A significant decline in accommodative ability beginning in middle age.
|
|
|
|
| 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.
|
|
|
|
| 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.
|
|
|
|
| refraction | The bending of light by an optical element such as a lens.
|
|
|
|
| retina | The innermost layer of the eyeball, where light is detected by photoreceptors and transduced into neural signals that are processed by retinal ganglion cells.
|
|
|
|
| retinal ganglion cells | Cells within the retina with center/surround receptive field organization; axons of the retinal ganglion cells constitute the optic nerve. See bipolar cell, amacrine cell, horizontal cell.
|
|
|
|
| retinal image | The distribution of light falling on the retina; the quality and overall intensity of this image influence visual perception.
|
|
|
|
| rods | Photoreceptors that are specialized for vision under dim light. See cones.
|
|
|
|
| sclera | The tough, dense material that forms the eye's outermost coat; seen from the front, the sclera is the white of the eye.
|
|
|
|
| sclerosis | The hardening of any living tissue; hardening of the eye's crystalline lens may play a role in presbyopia. See presbyopia.
|
|
|
|
| synapse | A region where release of neurochemical transmitter substance from one neuron spreads to alter the electrical properties of nearby neurons. See transmitter substance.
|
|
|
|
| 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.
|
|
|
|
| vascular tunic | The middle layer of the eyeball; responsible for much of the eye's nourishment.
|
|
|
|
| vergence | Referring to eye movements in which the two eyes move in opposite directions. See conjunctive.
|
|
|
|
| visual angle | Unit of measure expressing the size of an image on the retina; the visual angle subtended by an image is determined by the size of the object casting that image and the viewing distance from eye to object.
|
|
|
|
| vitreous | The thick transparent fluid that fills the eye's largest chamber.
|
|
|
|
| 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.
|