Positive relative accommodation (PRA) in
biology
Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary i ...
, is a measure of the maximum ability to stimulate
eye accommodation while maintaining clear, single
binocular vision
In biology, binocular vision is a type of vision in which an animal has two eyes capable of facing the same direction to perceive a single three-dimensional image of its surroundings. Binocular vision does not typically refer to vision where an ...
. This measurement is typically obtained by an
orthoptist
Orthoptics is a profession allied to the eye care profession. Orthoptists are the experts in diagnosing and treating defects in eye movements and problems with how the eyes work together, called binocular vision. These can be caused by issues with ...
,
ophthalmologist
Ophthalmology ( ) is a surgery, surgical subspecialty within medicine that deals with the diagnosis and treatment of eye disorders.
An ophthalmologist is a physician who undergoes subspecialty training in medical and surgical eye care. Followin ...
or
optometrist
Optometry is a specialized health care profession that involves examining the eyes and related structures for defects or abnormalities. Optometrists are health care professionals who typically provide comprehensive primary eye care.
In the Uni ...
during an
eye examination
An eye examination is a series of tests performed to assess vision and ability to focus on and discern objects. It also includes other tests and examinations pertaining to the eyes. Eye examinations are primarily performed by an optometrist, op ...
using a
phoropter
A phoropter or refractor is an ophthalmic testing device. It is commonly used by eye care professionals during an eye examination, and contains different lenses used for refraction of the eye during sight testing, to measure an individual's re ...
. After the patient's distance correction is established, she or he is instructed to view small letters on a card 40
cm from the
eyes. The examiner adds lenses in −0.25 diopter increments until the patient first reports that they become blurry. The total value of the lenses added to reach this point is the PRA value.
High PRA values (>= 3.50
diopter
A dioptre (British spelling) or diopter (American spelling) is a unit of measurement with dimension of reciprocal length, equivalent to one reciprocal metre, 1 dioptre = 1 m−1. It is normally used to express the optical power of a lens or cur ...
s) are considered to be diagnostic of disorders involving
accommodative excess.
Those with
accommodative insufficiency
Accommodative insufficiency (AI) involves the inability of the eye to focus properly on an object. accommodation (eye), Accommodation is the adjustment of the curvature of the lens (eye), lens to focus on objects near and far.
In this condition, ...
typically have PRA values below −1.50 diopters.
See also
*
Accommodation in fish
*
Adaptation (eye)
In visual physiology, adaptation is the ability of the retina of the eye to adjust to various levels of light. Natural night vision, or scotopic vision, is the ability to see under low-light conditions. In humans, rod cells are exclusively resp ...
*
Amplitude of accommodation
The amplitude of accommodation is the maximum potential increase in optical power that an eye can achieve in adjusting its focus. It refers to a certain range of object distances for which the retinal image is as sharply focussed as possible.
Amp ...
*
Convergence insufficiency
Convergence insufficiency is a sensory and neuromuscular anomaly of the binocular vision system, characterized by a reduced ability of the eyes to turn towards each other, or sustain convergence.
Symptoms
The symptoms and signs associated with con ...
*
Mandelbaum Effect
*
Negative relative accommodation Negative relative accommodation (NRA) was proposed by Joseph Kearney of Oxford University in 1967 as a measure of the maximum ability to relax accommodation while maintaining clear, single binocular vision. This measurement is typically obtained by ...
References
{{DEFAULTSORT:Positive Relative Accommodation
Ophthalmology