What is fixation disparity?
Fixation disparity is defined as the difference between the target vergence angle (binocular parallax) and the ocular converence angle during binocular fixation, as shown in Fig. 1. Fixation disparity occurs in the presence of binocular feedback, so it is a closed-loop error.
What causes fixation disparity?
Fixation disparity has been taken as a sign of stress on binocular vision because it is established that prism stress creates fixation disparity. This paper looks at the effect on fixation disparity of the stress caused by requiring subjects to read in inadequate illumination.
What is the normal range of horizontal fixation disparity at near?
Near vision is an important test condition in clinical optometry. Subjects with stronger asthenopic complaints tend to have a larger exo subjective fixation disparity in near vision (40 cm), but not in far vision (5 m) [24, 40, 41].
What is binocular disparity in psychology?
the slight difference between the right and left retinal images. When both eyes focus on an object, the different position of the eyes produces a disparity of visual angle, and a slightly different image is received by each retina.
What is dissociated phoria?
Dissociated phoria is defined as a deviation from the orthovergence position that occurs when no fusionable contours are provided. Associated phoria is a deviation of the eyes that appears under prism correction of fixation disparity: associated phoria equals the “aligning prism” [1] that nullifies fixation disparity.
What does Mallet Unit measure?
Background. The Mallett Unit is a clinical test designed to detect the fixation disparity that is most likely to occur in the presence of a decompensated heterophoria. It measures the associated phoria, which is the “aligning prism” needed to nullify the subjective disparity.
What is a Decompensating phoria?
Double vision that is only when both eyes are open is coming from a problem with the eye muscles. Many times true double vision is happening due to the eye muscles getting weaker as we age. We call this a decompensating phoria.
What is Sheards criterion?
Sheard’s criterion describes whether a phoria is likely to become decompensated (symptomatic) in reference to the opposing fusional reserves, which should be at least twice the value of the phoria.
Why does binocular summation improve sensitivity?
By combining the information received in each eye, binocular summation can improve visual acuity, contrast sensitivity, flicker perception, and brightness perception. Though binocular summation generally enhances binocular vision, it can worsen binocular vision relative to monocular vision under certain conditions.