FUNCTION -- Vision

TEST I -- Visual Acuity
Visual acuity refers to the visual ability to discriminate and recognize subtle geometric variations. The most common standardized test of visual acuity involves the reading of a wall-mounted or hand-held Snellen Eye Chart. To begin the test, the patient should cover one eye under conditions of favorable lighting. The Snellen Chart, which comprises rows of black alphanumeric characters (of decreasing size) on a white background, is placed before the patient, who is asked to identify the characters row by row. A standardized number corresponding to the smallest line from which the patient can correctly identify the characters should be recorded. Then, the test must be repeated using the other eye.

INTERPRETATION
The numbers recorded for each eye serves as the denominator in a ratio for which the numerator is fixed at 20. The ratio then indicates that, for a given eye, the patient can discern printed characters at a distance of 20 feet (corresponding to the numerator) that members of the neurologically intact population should be able to discern at a distance given by the denominator. For example, an index of 20/200 indicates that, for the tested eye, the patient can discern at a distance of 20 feet what neurologically intact persons should be able to discern at a distance of 200 feet. In this specific instance, we would conclude that the patient is deficient with respect to visual acuity in the tested eye. Defects may involve the cornea, pupil, lens, fluidic media of the eye, central retina, optic nerve, optic chiasm, optic tract, lateral geniculate body of the thalamus, optic radiations, or occipital pole.

PITFALLS AND SAFEGUARDS
The test may be entirely invalid for patients unfamiliar with the alphanumeric characters that may be presented on the chart. For such individuals (e.g., young children, those for whom English is a foreign language; those whose vision may be intact yet, by virtue of neurological impairment, are unable to recognize or report that seen), alternative tests must be deployed. Alternatives may also be necessary when assessing clinicians or other patients sufficiently experienced with a given form of the test as to have the relevant sequences of alphanumeric characters committed to memory.

TEST II -- Color Vision
Snellen Charts may include colored spots (red, green, blue, yellow), which the clinician may ask the patient to identify. In lieu of this, the clinician may ask the patient to articulate the color of other objects that may be available (e.g., paper, pens, books, clothes, instruments). More rigorous testing of color vision may involve the use of Ishihara Plates, special graphical instruments designed specifically for the assessment of color vision.

INTERPRETATION
When visual acuity fails, color vision also often suffers, as, from the anatomical perspective, mechanisms that process visual detail overlap considerably with those that process color. On the other hand, color vision may suffer entirely independently of deficits in visual acuity, usually in association with sex-linked genetic defects. Specifically, individuals may fail to express certain retinal photopigments, leading to bilateral color blindness.

PITFALLS AND SAFEGUARDS
For tests of color vision, it is best to use objects, the colors of which are richly saturated and unambiguous (e.g., use red, blue, green, and yellow rather than orange, blue-green, or purple). Also ensure that lighting is adequate, as color vision utilizes retinal cells with relatively high thresholds for signal transduction.

TEST III -- Visual Fields
Visual fields reflect the portions of space that one should be able to see with each eye. Rapid assessments of the visual fields are best achieved through the method of confrontation. To begin, the clinician and patient should face each other at close proximity. Next, the patient should be instructed to close one eye, using the other to stare directly into the corresponding eye of the clinician. While the patient maintains a fixed gaze, the clinician will move objects (e.g., clinicians wiggling fingers) from points behind the patient directly forward and thus into what should correspond to portions of the visual fields of the neurologically intact patient. The patient should be instructed to indicate verbally when the moving objects become visible. For each eye, the clinician should assess at least six trajectories, corresponding to clock positions of 12:00, 2:00, 4:00, 6:00, 8:00, and 10:00.

INTERPRETATION
The failure of the patient to observe the moving objects in peripheral portions of the visual field may reflect pathology at virtually any point throughout the visual system, from the cornea to the anterior portions of primary occipital cortex in the vicinity of the calcarine sulcus.