WHAT IS VISUAL ACUITY?

FOUR KINDS OF VISUAL ACUITY

WHAT DETERMINES A PERSON'S VISUAL ACUITY?

HOW IS VISUAL ACUITY DETERMINED?

HOW IS VISUAL ACUITY TESTED IN CHILDREN WHO DO NOT KNOW LETTERS?

HOW IS VISUAL ACUITY TESTED IN CHILDREN WHO CANNOT MATCH OR NAME SYMBOLS?

WHAT IS CONTRAST SENSITIVITY?

WHY IS CONTRAST SENSITIVITY IMPORTANT TO MEASURE?

WHAT IS VISUAL ACUITY?

Visual acuity is the ability to see fine detail, for example, reading signs on the road, seeing separate stars in the night sky, and seeing a small white spot on a dark surface.

Visual acuity is expressed as a number representing the smallest object or feature that the person can see.

For example, when the eye doctor tests an adult's visual acuity, lines of letters of different size are shown at a fixed distance, usually 20 feet. If the person can read letters made of strokes of 1 minute arc (60 min arc = one degree) of visual angle, their acuity is 20/20. One minute of arc, or 20/20, is considered "normal" acuity.

If the person can read letters only of 2 min arc stroke width, their acuity is 20/40 (2 times wider stroke). 20/200 acuity means the person can read letters of only 10 min arc strokes (10 times wider than normal). So, the poorer the acuity, the larger the bottom number of the fraction. This is called the "Snellen fraction."

There are four distinct types of visual acuity: Detection, Resolution, Recognition, and Localization.

Detection Acuity means the smallest line width or dot size you can detect

Resolution Acuity is the smallest separation between dots or between bars in a grating you can resolve.

Recognition Acuity is the smallest letter size you recognize and discriminate.

Localization Acuity is the smallest offset of abutting lines that you see.

Cone receptors in the fovea of the retina (Picture of posterior pole of eye, side view of foveal depression)

Pupil size (determines the amount of light reaching the retina)

Quality of the optics of the eye

Amount of light falling on the retina

Contrast between elements of the test objects

THINGS THAT INTERFERE WITH NORMAL VISUAL ACUITY

Refractive error (e.g. need for glasses)

Clarity of the media of the eye (cornea, lens, vitreous)
(e.g. cataracts)

Damage to the macula (fovea)

Damage to the optic nerve

Damage to the visual cortex area corresponding to the macular projection

Behavioral or performance factors

In doctor's offices, acuity in adults is tested using letters (recognition acuity).

Symbol naming or matching is used.

Show Patti Pics or LEA Symbols - matching card for child to hold and single symbol held by examiner.

If the child does not easily name the symbols, matching is used. The child points to the picture that looks like the one the examiner holds.

Visual acuity is the smallest set of symbols the child names or matches. Acuity cannot always be tested in small children at 20 ft. The Snellen fraction is different, depending upon the test distance. For example, an acuity of 20/40 at 10ft is noted 10/20 and at 5 ft 5/10.

With grating resolution tests, e.g. preferential looking test using acuity cards, or a test using visually evoked potentials.

coming soon:

DEMONSTRATION OF HOW ACUITY IS TESTED

THE NUMBER REPRESENTING ACUITY

GRAPHING THE RESULT ON THE NORM GRAPH

ILLUSTRATION OF STRIPE WIDTHS NORMAL FOR DIFFERENT AGES

Contrast sensitivity is the ability to see differences in contrast between objects or parts of objects.

Visual acuity is the measure of high contrast sensitivity. Visual acuity tests use black letters or symbols on a white chart, or black and white gratings.

As the light-dark difference (contrast) becomes less visible between features and objects, they have to be larger in order to see them.

Examples:

1. High contrast letters
   very small sizes can be seen when black letters are shown on a white background
   
2. Lower contrast letters
   Letters must be enlarged when the letters are dim against the same white background

In all conditions where visual acuity is reduced, contrast sensitivity is reduced as well.

However in some conditiions that reduce acuity, contrast sensitivity is reduced more than expected based upon the visual acuity alone. This means that if visual acuity only is tested, the visual disability of the person with relatively reduced contrast sensitivity will be underestimated. Examples of changes in the CS curves, types of eye disorders in which they occur...