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Accommodation: adjustment of the eye for seeing at different distances, accomplished by contraction or relaxation of the ciliary muscle, changing the shape of the lens, thus focusing a clear image on the retina; accompanied by changes in pupil size

Amblyopia: reduced functional vision in one eye, uncorrectable by standard procedures, e.g. application of lenses

Astigmatism: refractive error which prevents light rays from coming to a point or focus on the retina, caused by unequal degrees of refractive power in the various meridians of the cornea

Binocular Fixation: ability to use the two eyes simultaneously to focus on a target and fuse the two images into a single one, giving a correct interpretation of its position in space and its solidity (depth perception)

Blindness: (Legal Definition): corrected visual acuity of 20/200 or less, or a visual field diameter of no more than 20 degrees in the better eye

Central Vision: ability of the eye to perceive a target in the direct line of vision

Ciliary Muscle: a ring of muscle around the lens, narrowing its diameter and thickening it to allow a stronger focus (accommodation) on a target

Cochleo-palpebral Reflex: blinking or eyelid tensing in response to sharp noise

Cocontraction: simultaneous contraction of agonist and antagonist muscles around a joint to provide stability

Compensatory Eye Movements: eyes staying on stationary target during active head movements

Cones: light sensitive cells located primarily in the central retina, sensitive to fine detail and color

Conjugate Gaze or Deviations: symmetrical movements of the eyes in the same direction at the same time

Consensual Reflex: light directed into one pupil resulting in similar constriction in the other

Convergence: process of using extraocular muscles to direct the visual axes of the two eyes to a near point (simultaneous turning of the eyes inward)

Cornea: the transparent outer coat of the eyeball through which light rays pass

Cortical Visual Impairment: visual loss caused by a disturbance of the posterior visual pathway and/or visual cortex

Countertorsion: apparent movement of the eyes in the opposite direction that the head is turned

Defensive Blink to Threat: reflexive closure of the eyelids in response to a stimulus presented abruptly

Divergence: process of using extraocular muscles to direct the visual axes of the two eyes to a point further away (simultaneous turning of the eyes outward)

Doll's Eye Response: countertorsion of eyes in opposite direction of head turned in a horizontal or vertical plane

Esophoria: tendency of the eyes to turn inward

Esotropia: inward turn of the eyes

Exophoria: tendency of the eyes to turn outward

Exotropia: outward eye turn

Extraocular Muscles: external eye muscles which move the eyeball

Eye-Neck Reflex: opisthotonos (arching back) in response to bright light directed into eyes when head is erect and unsupported

Fixation: direction of gaze whereby the image of the target falls on the fovea centralis

Focal Length: distance light rays travel after refraction to the point of focus

Focus: point at which light rays meet (noun); the ability to accommodate (verb)

Fovea Centralis: depression in the center of the macula where vision is most acute (highest concentration of cones)

Fundus: interior of the eye visible through an ophthalmoscope

Fusion: power of the brain to coordinate the two images received simultaneously by the eyes into a single mental image

Heterophoria: tendency of the eyes to deviate

Heterotropia: deviation of the eyes from the normal straight position

Hippus: constant small constrictions and dilations of the pupil caused by spontaneous movements of the iris rather than external stimulation

Hyperopia (farsightedness): refractive error in which the focal point for light rays from a distant object is posterior to the retina when accommodation is at rest

Hyperphoria: tendency toward upward deviation of one eye

Hypertropia: upward deviation of one eye

Iris: colored circular tissue suspended behind the cornea and immediately in front of the lens, perforated by the pupil; regulates the amount of light entering the eye by changing the size of the pupil

Lens: transparent structure suspended in the eyeball between the aqueous and vitreous, furnishing the additional focusing power needed to bring light rays to a focus on the retina

Light Perception: ability of the eyes to distinguish the presence or absence of light

Macula: area of the retina surrounding the fovea, and with the fovea comprises the area of most acute vision

McCarthy's Reflex: tapping supraorbital area of each eye (above eyebrow) results in homolateral blink

Monocular Fixation: ability to direct the fovea to a target and maintain the match

Myopia (nearsightedness): refractive error in which the focal point for light rays from a distant object is anterior to the retina when accommodation is at rest

Naso-palpebral Reflex: bilateral blink in response to tapping on the bridge of the nose

Nystagmus: involuntary oscillatory movements of the eyeballs in horizontal, vertical, rotatory, or mixed directions

Occlusion: covering one eye to encourage use of the other

OD: Oculus dexter (right eye)

Optic Nerve: the nerve carrying visual impulses from the retina to the brain, where images are formed

OS: Oculus sinister (left eye)

OU: Oculi unitas (both eyes)

Palpebral: pertaining to the eyelid

Perception: the ability to recognize, organize, and act upon information received through the senses: seeing, hearing, smelling, tasting, touching, moving, and body position in relation to space

Peripheral Vision: ability to perceive presence and motion of targets outside the direct line of vision

Photophobia: excessive sensitivity to light

Proprioception: Reception of sensory information from muscles, tendons, and joints, allowing internal awareness of body parts and their position in space

Pupil: opening in the center of the iris

Pupillary Reaction: constriction or dilation of the pupil in response to various changes in light

Refraction: 1) the bending of light rays in passing from one transparent medium into another of different density 2) determination of refractive errors of the eye and correction by prescriptive lenses

Retina: anterior coat of the eye, formed of light-sensitive nerve fibers and connected with the optic nerve

Rods: light sensitive cells located primarily in the peripheral retina, sensitive to movement and light, even low intensity

SC (Sine Correction): without corrective lenses

Scanning: eye movements used to explore visual stimuli

Strabismus: inability of the two eyes to simultaneously direct the visual axes to the same target

Tracking: ability to follow horizontal, vertical, circular, or diagonal movements of targets with smooth pursuit eye movements

Vision: the process of seeing (sight)

Visual Acuity: sharpness of vision (ability of the eye to distinguish detail, especially for reading)

Visual Field: the entire area that can be seen while the eye is fixing or gazing steadily at a target in the direct line of vision

Visual Memory: ability to remember a visual image no longer present

Visuo-Palpebral Reflex: blink or eyelid closure in response to bright light directed into eye

Visual Perception: the capacity to interpret visual sensory input and assign meaning to what is seen.

* Visual Function

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