(Glossary under construction)

abducting nystagmus Jerk nystagmus that appears in the abducting eye on attempted lateral gaze when there is a lesion in the opposite medial longitudinal fasciculus (MLF).

abduction Horizontal rotation of one eye outward from the midline around the vertical axis.

abnormal retinal correspondence (ARC, abnormal correspondence, anomalous correspondence) A condition that exists when the two foveae no longer have a common visual direction. The fovea of one eye and a peripheral retinal element of the other eye have acquired a common visual direction.

accommodation The ability of the eye to increase the dioptric power of the crystalline lens to obtain a clear image of near objects on the retina.

adduction Horizontal rotation of one eye inward toward the midline around the vertical axis.

Adie's pupil A dilated slowly reacting pupil usually associated with hypoaccommodation. It occurs most frequently in females and constricts to the instillation of weak solutions of methacholine (no longer available) or pilocarpine that have no effect on normal pupils.

agonist The eye muscle whose contraction is responsible for eye movement.  (The contracting muscle receiving primary innervation (prime mover), opposed in action by another muscle called the antagonist.)

alternating strabismus A condition where first one eye and then the other fixates with no real preference one for the other.

alternating sursumduction (dissociated double hyperdeviation) A condition in which either eye deviates upward under cover but returns to its original position when the cover is removed. The eye usually extorts on elevation and intorts on its slowly floating return to neutral position. This condition is usually bilateral. Latent nystagmus is a frequently associated finding.

amblyopia Reduced vision that is not associated with any abnormality or disease of the fundi and that is uncorrectable by optic means;  May be due to organic lesions or, as the term is most frequently used in patients with strabismus, the reduction in vision that occurs secondary to suppression.

ametropia A condition where a refractive error is present.

amplitudes Total amount of convergence and divergence measured in binocular single vision.

angle alpha The angle formed between the visual line and optic axis.

angle gamma The angle formed between the optic axis and the fixation axis at the center of rotation of the eye.

angle kappa The angle formed between the visual line and the optic axis at the nodal point of the eye.

angle lambda The angle formed between the visual line and optic axis at the center of the pupil.

angle of anomaly The difference between the objective and subjective angle of deviation. This occurs in abnormal retinal correspondence.

aniseikonia A condition where there is a difference in the size of the images as seen by either eye.

anisocoria Inequality of the diameter of the pupils.

anisometropia Unequal refraction, the refraction differing in the two eyes.

antagonist The eye muscle that has the opposite action to the agonist muscle; the eye muscle that is inhibited while the agonist muscle is working.

antimetropia Opposing states of refraction in the two eyes-for example, myopia in one eye and hyperopia in the other eye.

aphakia Absence of a crystalline lens of the eye.

Argyll-Robertson pupil A miotic pupil that reacts much better to accommodation than to light. It is often seen in neurosyphilis.

asthenopia A term used to describe weakness, speedy tiring, or ocular fatigue set up by the use of the two eyes.

astigmatism Irregularity in the curvature of one or more refractive surfaces (cornea or anterior and posterior surfaces of the lens) of the eye. When such a condition occurs, rays emanating from a point are not brought into focus at a point on the retina but appear to spread as a line in various directions, depending upon the curvature.

ataxic nystagmus An older term for abducting nystagmus.

atropine A cycloplegic drug produced from belladonna (see cycloplegia).

Bell's palsy Facial paralysis.

Bell's phenomenon Bilateral and symmetrical upward and outward movement of the eyes upon closure of the lids.

Bielschowsky head tilting test Test designed to help determine the primary paretic muscle when there is a limitation of the superior rectus of one eye and the superior oblique of the other eye.

Bielschowsky phenomenon or sign A response seen in alternating sursumduction where the elevated, occluded eye depresses on placing a filter before the fixing eye.

bifocal spectacles Glasses in which the lenses are divided into two segments. The lower segment is used for near vision and the upper segment for distance vision.

bifoveal fixation The alignment of the visual axis of both eyes so that the object of regard falls on the foveae under binocular conditions.

binocular Refers to two eyes.

binocular single vision The ability to use corresponding points of the two retinae simultaneously in obtaining single vision, without the aid of a binocular instrument.

binocular visual acuity The level of visual acuity obtained in binocular single vision.

blind.spot mechanism A "coincidental" type of esotropia in which the image of the fixated object falls on the blind spot of the deviated eye.

blind-spot syndrome An "adaptive" type of esotropia that occurs when the esotropia is of a small to moderate degree with annoying diplopia and without sufficient divergence to regain foveal fixation. A motor response tends to move the eye farther to the esotropic side, projecting the image seen by the deviating eye onto the blind spot (optic disc).

Bruns' nystagmus Nystagmus in both directions of gaze; vestibular in one direction and gaze paretic in the opposite direction, due to cerebellopontine-angle lesions.

Bruns' syndrome Consists of association of vertigo, nausea, and headache with sudden movements of the head; originally described as associated with cysticercus infestation of the fourth ventricle.

caloric testing The introduction of hot or cold water into the external ear canal to set up convection currents or pressures in the membranous labyrinth. Usually the lateral semicircular canal is selectively stimulated by placing it in the vertical plane. Normally water is used at 7 C above or below body temperature in a standardized way, namely at least 8 ounces for 40 seconds.

cardinal directions of gaze The six positions of gaze used to test the primary field of action of the six pairs of ocular muscles. These include the tertiary oblique positions and right and left gaze.

cardinal positions of gaze Primary, up, down, right, and left gaze.

centrad/prism diopter A term used in prism nomenclature. A prism of 1 centrad strength produces a deviation of 1 cm of arc at a 1 meter distance.

central fixation A uniocular condition where the fovea is used for fixation.

central scotoma A localized area of decreased visual perception involving central vision.

cochlear hearing loss End organ hearing loss characterized by recruitment, that is, abnormally rapid growth in loudness for increases in intensity of sound.

cogwheeling (saccadic pursuit) Interruption of smooth pursuit movements by small saccades giving a jerky, uneven eye movement.

comitant (concomitant) Condition of heterophoria/tropia in which the angle of deviation remains the same with either eye fixing and in all the directions of gaze.

concave lens A lens with a depressed or hollow surface that diverges rays of light.

confusion A condition occurring at the onset of a manifest strabismus where dissimilar objects stimulate both foveae.  The foveae have the same visual direction and the images of the two objects are appreciated as being superimposed.

conjugate eye movement Binocular eye movements in which the eyes move the same distance and at the same speed and maintain the same relative position.  (Parallel movements of the two eyes in the same direction.)  Saccades and pursuit movements are normally conjugate.

convergence Disjugate movement of both eyes toward the midline; normally occurs when looking at a near object.  (Eye movement that causes the visual axes to turn in relative to each other.)

convex lens A lens with a rounded surface that converges rays of light to a point.

corresponding retinal elements (points) Two retinal points, one in each eye, which normally receive the same visual stimulus.  (Points on the retina of each eye that have the same visual direction.)

crista The end organ of the semicircular canals of the inner ear.

cyclophoria A tendency of the visual axes to deviate around the anteroposterior axis, kept latent by fusion.

cycloplegia Paralysis of the ciliary muscle of the eye.

cycloplegic  Drugs that cause cycloplegia; that is, paralysis of the ciliary muscle and paralysis of accommodation.

cyclotropia A manifest deviation of one visual axis around the anteroposterior axis.

degree The unit of measuring arcs or angles; one degree is 1/360 of a circle.

depressor A muscle whose action is to turn the eye downward.

deviation See heterotropia; strabismus.

dextroversion Conjugate movements of both eyes to the right.  (Opposite of levoversion)

diagnosis Determination of the nature of the disease.

diagnostic positions of gaze The six positions of gaze: right, left, up and right, up and left, down and right, down and left, which represent the fields in which action of the individual ocular muscles are most purely represented.

diopter (D) Unit of measurement of the power of a lens; a lens of 1 diopter strength brings parallel rays of light to a focus at 1 meter distance.

diplopia Double vision; a condition in which a single object is seen double as a result of stimulation of noncorresponding retinal areas in the two eyes, often due to a deviation.

disconjugate eye movement Binocular eye movements in which the position of one eye changes relative to the other. Vergence movements are normally disconjugate. When one eye moves more slowly or a lesser distance, the movement is disconjugate.

disequilibrium Disturbance of balanced locomotion.

disinsertion Procedure in which the tendon of a muscle is cut free from its insertion and allowed to retract; commonly used to describe a weakening procedure of the inferior oblique muscles.

disjugate (disconjugate) Nonparallel movements of the two eyes in which the visual axes move in opposite directions.  (See disconjugate eye movement)

dissociated double hyperdeviation See alternating sursumduction.

divergence An eye movement that causes the visual axes to turn out with respect to each other.  (Disjugate movements of both eyes outward from the midline.)

doll's head reflexes Movement of the eyes when the head is turned from the side to side or the head is flexed, in the conscious patient while fixation is maintained, and in the unconscious patient without fixation.

drift Involuntary slow micro eye movements that occur between flicks or micro saccades.

Duane's classification of squint

  • convergence excess A convergent deviation greater at near than at distance. 
  • convergence insufficiency A divergent deviation greater at near than at distance. 
  • divergence excess A divergent deviation greater at distance than at near. 
  • divergence insufficiency A convergent deviation greater at distance than at near.

duction Movement of one eye.  (Monocular rotation of an eye.)

eccentric fixation A uniocular condition in which a retinal point other than the fovea is used for fixation.

electromyogram (EMG) Recording of the electncal activity of a muscle.

electronystagmogram (ENG) Recording of eye movements utilizing the resting potential of the eye.

electrooculogram (EOG) The recording of the resting potential of the eye. When recorded in relation to a visual stimutus, it is usually termed an EOG; when used as a means of recording eye movement, it is usually referred to as an ENG.

elevator A muscle whose action is to turn the eye upward.

emmetropia A condition in which rays of light coming from infinity are focused clearly on the retina of a normal eye without exerting any accommodative effort.

enophthalmos Recession of the eyeball into the orbit.

enucleation Excision (surgical removal) of the eyeball.

epicanthus A fold of skin over the inner canthus of the eye; the primary cause of pseudostrabismus.

eso Turning in.

esophoria A tendency for the visual axes to deviate inward, kept latent by fusion.

esotropia (convergent strabismus) A manifest deviation of one visual axis inward around the vertical axis.

etiology Causation of disease.

excyclophoria/tropia, extorsion Outward rotation of the globe from the 12 o'clock meridian around the anteroposterior axis.

exo Turning out.

exophoria A tendency for the visual axes to deviate outward, kept latent by fusion.

exophthalmos Protrusion of the eyeball beyond the orbital rim.

exotropia (divergent strabismus) A manifest deviation of one visual axis outward around the vertical axis.

extrinsic ocular muscles (extraocular muscles) The muscles situated outside the globe that rotate the eyeball.

fast eye movements Saccadic eye movements, including refixation movements and the fast phases of optokinetic and vestibular nystagmus.

fixation The act of looking directly at an object, making an effort to see it as clearly as possible; alignment of the fovea with the object of regard.

fixation disparity Term coined by Ogle and others (1949) who found "that in spite of the compulsion to register the dioptric images of the two eyes as near as possible on corresponding retinal points, there occurs an actual small deviation of the eyes and a corresponding slipping of the cortical images, although fusion is still maintained."  This deviation does not exceed 6 to 10 minutes of arc. The term describing a physiologic process has been used incorrectly as a synonym for various pathologic conditions such as small angle esotropia, monofixational phoria, and microstrabismus.

fovea A small pit in the macula that is the point of most distinct vision.  (The most sensitive part of the retina; the very center of vision.)

Frenzel's lenses High plus (+20 diopter) lenses, which are used in looking for small-magnitude nystagmus. The high plus lenses eliminate fixation and therefore accentuate nystagmus. They also provide magnification so that nystagmus is more obvious to the observer. Most frequently, Fresnel lenses are used in conjunction with electronystagmographic recording.

Fresnel press-on prism A series of small plastic prisms lying adjacent to each other on a thin platform of plastic. It has the same deviating power of the conventional prism but is only 1 mm thick, one-tenth the thickness of the conventional prism.

fusion The mental blending of the two images of an object falling on corresponding retinal points of the two eyes.  (Cerebral integration of the images from the two eyes.)

fusional reserves The range of convergence, divergence, and vertical vergence through which binocular single vision can be maintained. This may be measured in degrees or prism diopters (().

gaze movements Voluntary conjugate eye movements, saccades and pursuit movements.

Gunn pupil A peculiar pupillary reaction in which, although both the direct and consensual reactions are present, the contraction is not maintained under bright illumination so that the pupil slowly dilates again while the light is still kept upon the affected eye. This pathologic redilation occurs much more rapidly than the normal return in pupil size brought about by retinal adaptation to the light stimulus.

hemianopsia (hemianopia) Blindness in one-half of the visual field of both eyes.

Hering's law of equal innervation When an impulse goes to a muscle to contract, an equal impulse goes to its contralateral synergist (yoke muscle).

heteronymous diplopia Crossed diplopia.

heterophoria, phoria A tendency for the visual axis to deviate from parallelism, which is kept latent by fusion.

heterotropia, tropia A manifest deviation of the visual axes, a squint, strabismus.  (Any manifest deviation of the visual axes.)

homonymous diplopia Uncrossed diplopia.

horizontal ampulla The dilated anterior end of the horizontal (lateral) semicircular canal of the inner ear, which contains the vestibular end organ concerned with rotation about the vertical axis.

horopter Sum total of objects in physical space, the images of which stimulate corresponding retinal elements in the two eyes.

hydrops See Meniere's disease.

hyper Turning up.

hyperopia (hypermetropia) Farsightedness; condition in which parallel rays of light from infinity are brought to a focus behind the retina in the unaccommodated eye.

hyperphoria A tendency for the visual axes to deviate upward, kept latent by fusion.

hypertropia A manifest deviation of one visual axis upward around the horizontal axis.

hypo Turning down.

hypophoria A tendency for the visual axes to deviate downward, kept latent by fusion.

hypotropia A manifest deviation of one visual axis downward around the horizontal axis.

incomitant (nonconcomitant) Condition where the angle of deviation in a heterophoria/tropia differs according to which eye is fixating or in which direction of gaze the fixating eye is looking.

incyclophoria/tropia, intorsion Inward rotation of the globe from the 12 o'clock meridian around the anteroposterior axis.

inhibit With reference to the EMG, to drop out motor activity.

innervation Nerve supply.

internuclear ophthalmoplegia (INO) A combination of eye movement abnormalities attributed to a lesion of the medial longitudinal fasciculus (MLF). In the eye on the side of the lesion, adduction is limited. The other eye shows a jerk nystagmus on gaze to the side opposite the lesion. Convergence may or may not be decreased. if convergence is involved, it is thought that the medial rectus subnucleus of the third nerve is involved. Vertical nystagmus, especially on upgaze, and skew deviation are frequently associated.

interpupillary distance (IPD) The distance between the two pupillary centers.

intrinsic ocular muscles The muscles situated inside the eyeball, consisting of the ciliary muscle, the sphincter pupillae, and the dilator pupillae.

lens A refractive medium with at least one curved surface.

levoversion Conjugate movements of both eyes to the left.  (Opposite of dextroversion, which is a conjugate movement of both eyes to the right.)

lid lag A lag of the upper lid as the eye moves from up to down gaze; seen most frequently in thyroid disease.

lid retraction Retraction of one or both upper lids with exposure of the superior sclera when the eyes are in primary position or in attempted upgaze.

macula Shallow oval depression about the same size as the optic disc containing the fovea in its center.  (The area of the retina that in the normal eye has the most acute vision.)

malprojection (past pointing) A directional misinterpretation of the position of an object.  When looking uniocularly in the direction of the main action of a paralyzed muscle, an object is thought to be farther away from the midline than it actually is.  It occurs in cases of ocular palsy of fairly recent onset.

medial longitudinal fasciculus (MLF) A nerve fiber tract that interconnects the oculomotor and vestibular systems. It extends from the upper midbrain to the cervical region. The portion most related to the control of eye movements is that part connecting the vestibular, oculomotor (III), trochlear (IV), and abducens (VI) nuclei. This portion ascends in the brainstem, decussating slightly caudal to the sixth nerve nuclei.

Meniere's disease Increased fluid content within the membranous labyrinth, giving rise to Meniere's syndrome consisting of hearing loss, tinnitus, and attacks of vertigo.

meter angle Unit of measuring convergence. One meter angle of convergence is brought about when the eye moves to fix from infinity to a point at 1 meter. It is formed by a line from the center of rotation of either eye and the median line between the two eyes and the object of regard.

micro eye movements Small eye movements utilized in fixation and in position maintenance.

micro saccades Involuntary rapid micro eye movements of about l?.

microstrabismus (microtropia) Term coined by J. Lang in 1966 to indicate a "monolateral squint of less than 5? with harmonious anomalous correspondence and amblyopia of varying degree in the deviating eye."  The angle of anomaly may or may not coincide with the distance between the fovea and the point used for eccentric fixation. The fixation may be central or eccentric, and consequently it may or may not be detected with the cover test. The central scotoma present may be either the primary cause or secondary to the deviation.

microtropia Term used by Helveston and von Noorden in 1967 to describe a small deviation (of about 5?) that cannot be detected with the cover test because it is associated with eccentric fixation and harmonious anomalous correspondence. The size of the tropia equals the distance in the amblyopic eye between the fovea and the area used for fixation. (This is really one of the three possibilities described by Lang in 1966.) The central scotoma present is considered as the primary cause.

miosis Constriction of the pupil to smaller than normal.

miotic A drug that causes constriction of the pupil.

monocular (uniocular) Refers to one eye.

monocular diplopia A condition that occurs in abnormal retinal correspondence when an object stimulating a retinal area is projected in two different visual directions; the normal one and the abnormal one.

monofixation syndrome Generic term coined by Parks (1969) for a group of patients with or without a small deviation (of maximum 8( horizontally and 2( to 3( vertically), showing peripheral fusion and a stereoacuity from 3,000 to 67 seconds of arc but lacking bifoveal fusion because of a central scotoma in the nondominant eye. Theoretically a very small angle of ARC should be associated with it in the patients with a deviation, but Parks questions this.

monofixational phoria/tropia See monofixation syndrome.

mydriasis Dilation of the pupil.

mydriatic A drug causing dilation of the pupil.

myectomy Excision of part or all of a muscle; commonly used to describe a weakening procedure on an inferior oblique muscle in which the muscle is transected and a segment removed.

myopia Nearsightedness; condition in which parallel rays of light from infinity are brought to a focus in front of the retina in an unaccommodated eye.

myotomy Incision into or across the belly of a muscle to weaken its action; commonly used to describe a weakening procedure on the inferior obliques.

near point of accommodation (NPA) The nearest point at which the eye can see clearly while exerting the maximum amount of accommodation.

near point of convergence (NPC) The nearest point for which convergence is possible.

near synkinesis The normal reaction to a near object, consisting of accommodation, pupillary constriction, and convergence.

nonoptic reflex system The reflex system related to postural and vestibular reflexes.

normal retinal correspondence (NRC) A condition that exists when the two foveae have a common visual direction and all other retinal areas of the two eyes also correspond secondarily.

nystagmus Short, rhythmic, oscillatory movements of the eyes; may be horizontal, vertical, or rotary; may occur constantly or only when the gaze is in a definite direction; may be pendular or jerk in configuration. Pendular nystagmus has movements of equal amplitude and velocity; jerk nystagmus may be resolved into a slow and fast phase.

objective vertigo The sensation that the environment is spinning without the individual.

occluder A device for covering one eye to oblige the other eye to fixate.

occlusion A method of obscuring the vision of one eye so as to ensure the use of the other eye (for example, an elastoplast patch over the eye or nailpolish on the lens).

ocular torticollis A condition dating from early infancy in which an abnormal head posture is adopted in order to overcome an anomaly caused by a congenital palsy of one or more vertically acting extraocular muscles.

oculocephalic reflexes (proprioceptive head-turning reflex, doll's head eye phenomenon) The eye movement obtained as the head is rotated in a patient who is obtunded, unconscious, or unable to fixate.

ophthalmoplegia Paralysis of one or more of the ocular muscles.

opsoclonus Chaotic saccadic eye movements in many directions, which are conjugate and related to disorders of the cerebellar pathways.

optic axis A line passing through the centers of curvature of all the refracting surfaces, on which lie the principal foci, the principal points, and the nodal points of the system.

optokinetic nystagmus (OKN) Jerk nystagmus elicited by repetitive movement of stimuli across the visual field. The slow phase is a pursuit movement and the fast phase is a saccade. A defect in the fast phase of OKN is frequently of diagnostic help in locating a deep parietal lobe lesion.

orthophoria A condition where there is no deviation of the visual axes from parallelism even on prolonged dissociation.

oscillopsia The subjective sensation that the environment is oscillating; related to disorders of the vestibular system.

otolith One of the receptive elements associated with the vestibular apparatus of the inner ear. The otoliths respond to linear acceleration of the head and body.

Panum's fusional area An area around corresponding retinal points, including slightly disparate retinal elements, whose stimulation will result in binocular fusion with stereopsis rather than diplopia.

Panum's fusional space Spatial representation of Panum's fusional area surrounding the horopter.

paradoxic diplopia Diplopia occurring because of abnormal projection; one not expected in a particular type of deviation.

paralysis Total lack of function of a muscle (complete inability of action of a muscle).

paramedian pontine reticular formation (PPRF) A specific region of the reticular formation at the level of the sixth nerve nuclei, in the paramedian area, concerned with the final integration of fast horizontal eye movements.

paresis A weakness of a muscle or group of muscles (weakness or incomplete/partial paralysis of a muscle).

penalization A method of treating amblyopia without occlusion by forcing the patient to use one eye for distance and the other eye for near. This can be achieved by optical means, atropinization, miotics, or any combination of these.

phi phenomenon Apparent movement of the fixation object occurring on dissociation when covering one eye and then the other. Occurs in the presence of heterophoria or heterotropia with normal retinal correspondence.

phoria See heterophoria.

photophobia Intolerance of or sensitivity to light.

physiologic diplopia Occurs in binocular single vision where objects situated nearer than the fixation object or beyond the fixation object, and thus falling on noncorresponding retinal points, are seen double.

pleoptics A systematic method of analyzing and treating functional amblyopia, especially amblyopia with eccentric fixation.

pontine gaze center (PGC) An area in the pons near the sixth nerve nucleus, responsible for conjugate movements to the same side.

position maintenance system The eye movement system concerned with maintaining the eyes in position with respect to a specific object or direction. In this function it also coordinates the other eye movement systems.

presbyopia Deficiency in the accommodative ability of the eyes caused by sclerosis of the crystaline lens.

pretectal area The general area at the junction of the mesencephalon and diencephalon, ventral and slightly rostral to superior colliculus. This area contains the centers for vertical gaze, pupillary reactions, and vergence movements.

primary deviation Term used when investigating paralytic deviations; refers to the amount of deviation measured when the nonparetic eye is fixating; always smaller than the secondary deviation.

primary position The position of eyes looking straight ahead and to infinity. It is at this position that the normal examination of eye movement starts.

prism A wedge-shaped lens that deflects traversing rays of light away from its apex.

prism diopter  A unit of measurement of a prism; a prism of 1 prism diopter strength displaces the image of an object 1 cm at 1 meter distance.  Also known as a "centrad".

progressive supranuclear palsy (PSP) A progressive supranuclear ophthalmoplegia, with pseudobulbar palsy, dysarthria, dystonic rigidity of the neck, and parkinsonlike habitus and facies. It is characterized by early loss of saccadic eye movements greatest in dowogaze. Pathologic examination usually reveals neural loss and gliosis in the subthalamic nuclei, substantia nigra, superior colliculi, and periaqueductal gray matter. The disease progresses to death within 5 years.

proprioception The ability to convey to the central nervous system an indication of the posture and movements of the body brought about by impulses (proprioceptive impulses) arising within the body from muscles, tendons, and joints and from the labyrinth of the inner ear.

ptosis Drooping of the upper eyelid as a result of paralysis or atrophy of the levator palpebrae superioris or M?ller's muscle.

pursuit A slow eye movement, utilizing the occipitomesencephalic pathways, in which the eyes track a slowly and smoothly moving target. Thus, tracking movements and the slow phase of optokinetic nystagmus are pursuit movements.

rapid eye movement (REM), sleep REM sleep is that part of the sleep cycle during which rapid eye movements of the same speed as saccadic movements are noted. It is accompanied by increased cerebral cortical blood flow, a rise in brain temperature, and an acceleration of spontaneous neural activity throughout the brain. The brain in many respects appears awake, but the sleeper remains limp and quiet. These periods of sleep are associated with dreaming.

recession An operation in which the insertion of a muscle is moved from its normal position to one situated nearer the origin of the muscle, weakening its action.

recruit With reference to the FMG, the addition of new motor activity with increased contraction or effort.

refixation movement Saccade utilized to move the fovea to a new object of attention.

refraction The phenomenon of the bending of light as it passes from one transparent medium to another of a different density.

refractive error Condition where parallel rays of light from infinity do not come to a focus on the retina in an unaccommodated eye.

refractive index The refractive power of a substance in comparison with that of air.

relative fusional convergence The act of exerting convergence while keeping accommodation fixed.

relative fusional divergence The act of exerting divergence while keeping accommodation fixed.

resection Excision of a portion of a muscle in order to shorten it, thus increasing its action.

reticular formation (RF) An area of the brainstem found in the medulla, pons and mesencephalon, with a reticular structure. It is vitally concerned with states of consciousness and the fine coordination of eye movements, particularly fast eye movements.

retinal element Any point on the retina.

retinal rivalry A conflict between the two retinae when two dissimilar images are superimposed; first one image is suppressed and then the other.

saccade A rapid eye movement that is dependent on the frontomesencephalic pathways. All fast eye movements are saccades. These include refixation movements, the fast phase of vestibular nystagmus, and the fast phase of optokinetic nystagmus. A saccade can attain velocities up to 700?/sec.

saccadic pursuit (cogwheeling) Interruption of smooth pursuit movements by small saccades, giving a jerky, uneven eye movement.

saccadic system The oculomotor system used in making fast eye movements, refixation movements, and the fast phase of optokinetic and vestibular nystagmus.

saccule The membranous sac adjacent to the cochlea, which Contains a sensory receptor that probably has gravitational functions.

scotoma An area of decreased or absent visual perception within the visual field.  (A blind spot in the visual field.)

secondary deviation Term used when investigating paralytic deviations referring to the amount of deviation measured when the paretic eye is fixating; always larger than the primary deviation.

Sherrington's law of reciprocal innervation When an impulse goes to a muscle to contract, an equal and opposite impulse goes to its ipsilateral antagonist to relax.

simultaneous foveal perception (SFP) The ability to perceive simultaneously and superimpose two dissimilar images, subtending 1 at the nodal point, with one image formed on each fovea.

simultaneous macular perception (SMP) The ability to perceive simultaneously and superimpose two dissimilar images, subtending 3,, at the nodal point, with one image formed on each macula.

simultaneous paramacular perception (SPP) The ability to perceive simultaneously and superimpose two dissimilar images, subtending 5,, or more at the nodal point, with one image formed on each retina.

simultaneous perception The ability to perceive simultaneously two dissimilar images, one formed on each retina, but not necessarily to perceive them as being superimposed.

SISI (short-increment sensitivity indices) A hearing test in which high scores are suggestive of cochlear dysfunction and probably dependent on phenomenon similar to recruitment.

skew deviation An abnormal eye position in which one eye remains lower than the other in all ranges of horizontal movement. It may occur bilaterally, in which case one eye is lower in gaze to the right of midline and the other is lower on gaze to the left of midline.

slow eye movements All nonsaccadic eye movements, including pursuit movements, vestibular slow phases, and vergence movements. Tracking eye movements can reach velocities up to 100?/sec and slow phases can reach velocities of 300?/sec in vestibular nystagmus.

smooth pursuit system The oculomotor system utilized in following slowly moving objects.

squint A manifest deviation of the eyes; a heterotropia, a tropia.

stereopsis The ability to see similar images falling on slightly disparate retinal points and to mentally blend them into one with the appreciation of depth or the perception of depth by parallax.

strabismus (squint) An abnormality of the eyes in which the visual axes do not meet at the point of fixation.  (A manifest deviation of the eyes, a squint, a heterotropia.)

superimposition The ability to perceive two similar images, one formed on each retina, as being superimposed but not mentally fused.

suppression Term applied to the mechanism in which reduced acuity occurs in an anatomically normal visual pathway. The reduced acuity is attributed to an active inhibitory process that turns off vision, at least partially, in strabismus and during saccadic eye movements.  (A condition in which the image of an object formed upon the retina is not perceived but is mentally ignored or neglected either partially or completely; an active cortical inhibition.)

supranuclear Refers to lesions or pathways above the level of the nuclei. In general, these pathways and lesions affect conjugate eye movements.

synergist A muscle aiding the primary action of the muscle under consideration.

tenectomy Segmental excision of the tendon of a muscle used to weaken its action; commonly used on the superior oblique muscles.

tenotomy Incision into or across the tendon of a muscle used to weaken its action; commonly used to weaken the superior oblique muscles.

tonic neck reflexes Postural reflexes generated by stimulation of vestibular pathways by neck movements.

torsion Rotation of the eye around the anteroposterior axis.

track To follow the movement of an object.

triplopia Condition in which three images of the same object are seen; occurs binocularly when monocular diplopia is present.

utricle A membranous sac connected to the membranous semicircular canals. It contains the macula, which senses linear accelerations including gravity.

vergence An eye movement in which the visual axes turn in or out with respect to each other; a disjugate binocular movement.

vergence system The oculomotor system utilized in changing the alignment of eyes with respect to each other.

vergences Disjugate movements of the eyes such as convergence and divergence.

version Parallel movement of both eyes.  (Conjugate movement of the eyes in the directions of gaze.)

vertical gaze center (VGC) An area in the upper mesencephalon-pretectal area concerned with conjugate vertical eye movements.

vertigo The Sensation of Spinning. Frequently associated with lesions of the vestibular apparatus.

visual axis Line drawn from the fixation point to the fovea through the nodal point of the eye.

visual direction (spatial value, local sign) A property of each retinal point when stimulated to localize the object in space.