Study

1. Central vision
Vision is a complex human sense with many facets that cannot be measured. Visual thresholds
can be broadly classified into three group: light discrimination (LD), spatial discrimination,
temporal discrimination. LD – include brightness sensitivity (minimum visible), brightness
discrimination (minimum perceptible), brightness contrast and color discrimination.
There are two types of vision: central and peripheral vision. Central vision includes VA and color
vision, peripheral vision – VF and dark adaptation.
Visual Acuity (VA) - is ability of eye to discriminate two points with minimal distance between
them.
Two objects are so close that two adjacent cones are stimulated, between them is located one
unstimulated cone.
Ophthalmologists rely on a variety of psychophysical assessments and express vision as a
measure of visual acuity, although acuity is only one component of vision.
The term VA refers to an angular measurement relating testing distance to the minimal object
size resolvable at that distance. The most basic types of vision testing are the distance and near
visual acuity tests.
Ophthalmologists usually refer to the Snellen acuity as a measure of the resolving ability of the
eye. The traditional measurement of distance acuity refers to a visual test in which a target
subtends a visual angle of 5 minutes of arc when a subject is 20 feet (6 meters) away from that
target. The distance to the test target may be arbitrary, but the visual angle subtended for the
standard 20/20 test object must be 5 minutes of arc, while each part or stroke of the letters
subtends 1minuteof arc. The Snellen notation is the most common method of expressing visual
acuity measurement. A variety of test targets are used in visual acuity testing. Each individual
letter, number, or picture on a testing chart is referred to as an optotype. Soviet
ophthalmologic school uses Sivtceva-Golovina chart, the distance to the test target is 5 meters.
The near acuity test is usually performed at 16 inches, 40sm. 33 sm – SCh. Patients with
extremely low vision need special testing. We use the formula of Snellen, Vis=d/D when the
vision low then 1-st line; d- distance from patient can read first line, D- distance from patient
must read first line of chart (50m)
VA of the different places of retina is different. Highest VA in the macula and are decreased
with removal from it. If VA is less than 0,005 – point the distance from which patient can count
fingers. Then VA may be hand motion (HM), then – light perception (LP) with right or wrong
projection – 1/∞ proectio lucis certa.
An objective method of measuring VA includes definition involuntary optokinetic nystagmus.
Physiology of visual acuity
To resolve detail, the eye's optical system has to project a focused image on the fovea, a region
inside the macula having the highest density of cone photoreceptors (the only kind of
photoreceptors existing on the fovea), thus having the highest resolution and best color vision.
Acuity and color vision, despite of being done by the same cells, are different physiologic
functions that don't interrelate. Acuity and color vision can be affected independently without
affecting the other function.

2. Visual acuity expression.
Visual acuity is often measured according to the size of letters viewed on a Snellen chart or the
size of other symbols, such as Landolt Cs or Tumbling E. In some countries, acuity is expressed
as a vulgar fraction, and in Russia as a decimal number. Using the foot as a unit of
measurement, (fractional) visual acuity is expressed relative to 20/20. Otherwise, using the
meter, visual acuity is expressed relative to 6/6. For all intents and purposes, 6/6 vision is
equivalent to 20/20. In the decimal system, the acuity is defined as the reciprocal value of the
size of the gap (measured in arc minutes) of the smallest Landolt C that can be reliably
identified. A value of 1.0 is equal to 20/20.
Visual acuity is registered documenting if the test was for distant or near vision, the eye
evaluated and if corrective lenses (i.e., spectacles or contact lenses) were used:
1. Distance from the chart
- D (distant) for the evaluation done at 20 feet (or 6 meters).
- N (near) for the evaluation done at 14 inches (or 35 cm).
2. Eye evaluated
- OD (Latin oculus dexter) for the right eye.
- OS (Latin oculus sinister) for the left eye.
- OU (Latin oculi uterque) for both eyes.
3. Usage of spectacles during the test
- cc (Latin cum correctore) with correctors.
- sc: (Latin sine correctore) without correctors.
Name Abbreviation Definition
Counting Fingers CF Ability to count fingers at a given distance.
Hand Motion HM
Ability to distinguish a hand if it is moving or not in front of
the patient's face.
Light Perception LP Ability to distinguish if the eye can perceive any light.
No Light
Perception
NLP Inability to see any light. Total blindness.

3. Color vision
Objects reflect different wavelengths of light thereby producing the sensation of color. The
appreciation of color is a function of the cones of the eye, the three types of which are
maximally sensitive to either blue, green, or red. The two best-known theories are the Young-
Helmholtz theory and Herring theory. Young-Helmholtz theory: there are three kinds of cones,
each "tuned" to absorb light from a portion of the spectrum of visible light
- cones that absorb long-wavelength light (red)
- cones that absorb middle-wavelength light (green)
- cones that absorb short-wavelength light (blue)
Herring theory: trichromatic signals from the cones fed into subsequent neural stages and
exhibited two major opponent classes of processing:
1. spectrally opponent processes, which were red versus green and yellow versus blue
2. a spectrally non-opponent process, which was black versus white.
The most common test of color vision is the Ishihara pseudo- isochromatic color plate exam or
Rabkin’s plates. This is a series of test plates where a matrix of colored dots is arranged with a
number visible on the page. The numbers are visible to individuals with normal color vision but
are confused or invisible to those individuals who are red/green color deficient.
A normal individual, who requires all 3 primary colors (green, red, blue) is classified as a normal
trichromat.
Abnormal subjects who require only 2 of the 3 colors are dichromats.
Dichromats may be deuteranopes, protanopes, and tritanopes
Persons who require only 1 color – monochromats
Who require 3 colors, but in abnormal proportions are anomalous trichromats
Congenital color defects (such as red/green color "blindness") and optic nerve disorders tend to
produce red/green defects whereas acquired disorders of the macula tend to produce
blue/yellow color vision defects.
Acquired: Tritan (blue-yellow), Protan-deutan (red-green)