Eyeli

Binocular Vision A-Z

OUR GUIDE TO BINOCULAR VISION TERMINOLOGY

A

B

C

D

E

F

G

H

I

J

K

L

M

N

O

P

Q

R

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Y

Z

A

AC/A RATIO

The AC/A ratio, refers to the relationship between the convergence response and the accommodative stimulus.  We calculate how many dioptres of convergence is produced for every dioptre of accommodation that is stimulated.

ACCOMMODATION

The ability of the crystalline lens to change its focus when looking from distance to near objects, in an effort  to maintain clear vision by focusing the image on the retina.

ACCOMMODATION FLEXIBILITY OR FACILITY

When we measure the accommodation flexibility or facility, we measure the responsiveness of the crystalline lens to change accommodation quickly and accurately when shifting focus from far to near targets, and vice versa.  The test involves alternating negative and positive lenses to induce and relax accommodation and is measured in cycles per minute.

ACCOMMODATION AMPLITUDE OR RESERVE

When testing the amplitude of accommodation, we measure the maximum amount of accommodative power that can be produced by a particular crystalline lens.

ACCOMMODATIVE SPASM

When ciliary muscles contract to focus on a near object,  the muscles can go into a spasm, making it impossible to relax resulting in blurred vision at both distance and near.

AMBLYOPIA

Also called lazy eye, is reduced vision typically in one eye due to the brain suppressing input from the affected eye because of unequal visual signals from each eye caused mainly by strabismus or anisometropia which then leads to poor development of visual acuity in the affected eye.

ANISOMETROPIA

This is when your two eyes have different refractive powers which causes the eyes to focus unevenly.

ANISEIKONIA

This is a difference in the perceived size or shape of images between eyes, and can arise from a variety of physiological, neurological, retinal, and optical causes. Aniseikonia is associated with anisometropia, as both anisometropia itself and the optical correction for anisometropia can cause aniseikonia.

ASSOCIATED PHORIA

When your eyes do not align together perfectly when looking at a target. We can measure how far the eyes still needs to move to achieve perfect alignment. The amount of movement that is required to achieve alignment is called the associated phoria.

B

BINOCULAR ACUITY

The maximum visual acuity obtainable while maintaining binocular single vision.

C

CONVERGENCE

The ability of the two eyes to move simultaneously inward while maintaining a single image when focusing on a near object.

CONVERGENCE INSUFFICIENCY

This is when the two eyes struggle to work together to keep focus on a near object. This results in one eye turning outward causing blurred or double vision.

D

DIVERGENCE

The ability of the eyes to simultaneously turn outward while maintaining a single image when focusing on objects further away.

E

ESOPHORIA

This is the inward turning of the eyes when fusion is broken, usually when covering one eye. When fusion is present with both eyes and esophoria is present, the eyes need to constantly work to keep a single focus. This could be due to a combination of anatomical, mechanical and innervation factors.

ESOTROPIA

This is when one or both eyes turn inward. Both eyes are not focusing on the same object.

EXOPHORIA

This is the outward deviation of the eyes when fusion is broken, usually when covering one eye. When fusion is present with both eyes focusing and there is an exophoria present, the eyes and brain constantly needs to work harder than usual to maintain this single vision which can lead to eye strain.

EXOTROPIA

This is when one or both eyes turn outward. Both eyes are not focusing on the same object.

EYE TEAMING

This is how efficient the two eyes work together in a precise and coordinated manner. Good eye teaming allows sustained, single and comfortable vision and is the basis of depth perception.

F

FIXATION DISPARITY

This is the amount of heterophoria possible when fusion is present or the distance between two images which is so close together it seems single.

FUSION

This is the ability of the brain to take the two separate images of each eye and form a single, unified image. If the eyes don’t align precisely, it would cause eye strain, blur or double vision.

FUSION COMPULSIVE REFLEX

The extra ocular muscles move the eyes in a way that helps to place the image on corresponding retinal points.

FUSION LOCK

A target that both eyes can focus on simultaneously to achieve single vision.

FUSIONAL RANGES

This is the extent to which a patient can maintain binocular single vision in the presence of increasing vergence demands.

FUSIONAL RESERVES

This is the maximum amount the eyes can converge or diverge while still maintaining a single image.  As the image doubles, the break point is reached.

N

NEAR POINT OF COVERGENCE

This is the point where the visual axes intersect under the maximum effort of the convergence.

NEGATIVE RELATIVE ACCOMMODATION

This is the measure of the maximum ability to relax accommodation while maintaining clear, single binocular vision.

P

PHORIA

This is a latent or hidden eye deviation. The eyes appear to be straight, but when covering an eye and breaking fusion, the eyes assume a position away from normal alignment.

POSITIVE RELATIVE ACCOMMODATION

This is the measure of the maximum ability to stimulate accommodation while maintaining clear, single binocular vision.

S

STEREO ACUITY

The accuracy and sharpness of images acquired with binocular depth perception.

STRABISMUS

When the eyes are not lined up properly and pointing in different directions when focusing at an object.

SUPPRESSION

This is a subconscious adaptation by a person’s brain to eliminate the symptoms of disorders of binocular vision such as strabismus, convergence insufficiency, and aniseikonia. The brain can eliminate double vision by ignoring all or part of the image of one of the eyes.