disparity

disparity

The condition of being unequal or totally different. The word is used mainly to refer to non-corresponding points in the retina. See retinal disparity; retinal corresponding points.
binocular disparity See stereoscopic visual acuity; retinal disparity; depth perception.
crossed disparity Retinal disparity induced by an object nearer to the eyes than the point of fixation and focused on the temporal retina. Thus, the image received by the right eye appears to the left and that received by the left eye appears to the right. Syn. crossed retinal disparity.
fixation disparity See retinal disparity.
retinal disparity Binocular vision in which the two retinal images of a single object do not fall on corresponding retinal points, i.e. when the object lies off the horopter. If, however, the two retinal images still fall within Panum's area, the object will still be seen single. At the fixation point this may cause over- or under-convergence of the eyes. This particular case is called fixation disparity (or retinal slip). The presence of fixation disparity often indicates that binocular vision is under stress and the patient has an uncompensated heterophoria. Optical correction or orthoptic exercises usually eliminate the symptoms. Fixation disparity can be measured either (1) directly (e.g. Disparometer, Wesson Fixation Disparity Card, both consisting of targets with pairs of vernier lines of various angular separation, each line being seen by one eye through polarizing filters), or (2) indirectly as an associated phoria (e.g. Mallett fixation disparity unit). Syn. binocular disparity. See stereoscopic visual acuity; anaglyph; esodisparity; exodisparity; fusional movements; depth perception; random-dot stereogram; stereopsis; stereotest.
uncrossed disparity Retinal disparity induced by an object farther away from the eyes than the point of fixation and focused on the nasal retina. Thus, the image received by the right eye appears to the right and that received by the left eye appears to the left. Syn. uncrossed retinal disparity.
vergence disparity See motor fusion.