Editing Visual acuity (section)

== Measurement considerations ==
Visual acuity measurement involves more than being able to see the optotypes. The patient should be cooperative, understand the optotypes, be able to communicate with the physician, and many more factors. If any of these factors is missing, then the measurement will not represent the patient's real visual acuity.

Visual acuity is a subjective test meaning that if the patient is unwilling or unable to cooperate, the test cannot be done. A patient who is sleepy, intoxicated, or has any disease that can alter their consciousness or mental status, may not achieve their maximum possible acuity.

Patients who are illiterate in the language whose letters and/or numbers appear on the chart will be registered as having very low visual acuity if this is not known. Some patients will not tell the examiner that they do not know the optotypes, unless asked directly about it. Brain damage can result in a patient not being able to recognize printed letters, or being unable to spell them.

A motor inability can make a person respond incorrectly to the optotype shown and negatively affect the visual acuity measurement.

Variables such as pupil size, background adaptation luminance, duration of presentation, type of optotype used, interaction effects from adjacent visual contours (or "crowding") can all affect visual acuity measurement.

=== Testing in children ===
{{Main|Infant vision}}

The [[newborn]]'s visual acuity is approximately 6/133, developing to 6/6 well after the age of six months in most children, according to a study published in 2009.<ref>{{Cite journal |last1=Pan |first1=Y |last2=Tarczy-Hornoch |first2=K |last3=Cotter |first3=SA |name-list-style=vanc |date=June 2009 |title=Visual acuity norms in pre-school children: the Multi-Ethnic Pediatric Eye Disease Study |journal=Optom Vis Sci |volume=86 |issue=6 |pages=607–12 |doi=10.1097/OPX.0b013e3181a76e55 |pmc=2742505 |pmid=19430325}}</ref>

The measurement of visual acuity in infants, pre-verbal children and special populations (for instance, disabled individuals) is not always possible with a letter chart. For these populations, specialised testing is necessary. As a basic examination step, one must check whether visual stimuli can be fixated, centered and followed.

More formal testing using [[preferential looking]] techniques use ''Teller acuity'' cards (presented by a technician from behind a window in the wall) to check whether the child is more visually attentive to a random presentation of vertical or horizontal [[Spatial frequency|gratings]] on one side compared with a blank page on the other side – the bars become progressively finer or closer together, and the endpoint is noted when the child in its adult carer's lap equally prefers the two sides.

Another popular technique is electro-physiologic testing using [[visual evoked potential|visual evoked (cortical) potentials]] (VEPs or VECPs), which can be used to estimate visual acuity in doubtful cases and expected severe vision loss cases like [[Leber's congenital amaurosis]].

VEP testing of acuity is somewhat similar to preferential looking in using a series of black and white stripes ([[Spatial frequency|sine wave gratings]]) or checkerboard patterns (which produce larger responses than stripes). Behavioral responses are not required and brain waves created by the presentation of the patterns are recorded instead. The patterns become finer and finer until the evoked brain wave just disappears, which is considered to be the endpoint measure of visual acuity. In adults and older, verbal children capable of paying attention and following instructions, the endpoint provided by the VEP corresponds very well to the psychophysical measure in the standard measurement (i.e. the perceptual endpoint determined by asking the subject when they can no longer see the pattern). There is an assumption that this correspondence also applies to much younger children and infants, though this does not necessarily have to be the case. Studies do show the evoked brain waves, as well as derived acuities, are very adult-like by one year of age.

For reasons not totally understood, until a child is several years old, visual acuities from behavioral preferential looking techniques typically lag behind those determined using the VEP, a direct physiological measure of early visual processing in the brain. Possibly it takes longer for more complex behavioral and attentional responses, involving brain areas not directly involved in processing vision, to mature. Thus the visual brain may detect the presence of a finer pattern (reflected in the evoked brain wave), but the "behavioral brain" of a small child may not find it salient enough to pay special attention to.

A simple but less-used technique is checking oculomotor responses with an [[optokinetic nystagmus]] drum, where the subject is placed inside the drum and surrounded by rotating black and white stripes. This creates involuntary abrupt eye movements ([[nystagmus]]) as the brain attempts to track the moving stripes. There is a good correspondence between the optokinetic and usual eye-chart acuities in adults. A potentially serious problem with this technique is that the process is reflexive and mediated in the low-level [[brain stem]], not in the visual cortex. Thus someone can have a normal optokinetic response and yet be [[Cortical blindness|cortically blind]] with no conscious visual sensation.