What is normal binocular vision?

The vergence position of the eyes is determined by the near fixation-accommodation-miosis synkinesis and the fusion mechanism. The contribution of both systems was analysed in 30 normal subjects and 16 subjects with abnormal binocular vision. Prism fixation disparity curves were determined in three different experimental situations: the routine method according to Ogle, a method to stimulate the synkinetic convergence (Experiment I, with one fixation point as sole binocular stimulus) and a method to stimulate the fusion mechanism (Experiment II, with random dot stereograms). Experiment I produced flat curves and Experiment II steep curves. The mean diameter of the horizontal Panum area was 5 minutes of arc in Experiment I and 2 degrees in Experiment II. On the basis of these findings, it was postulated that the synkinetic system operates in the absence of fixation disparity and the fusion system in the presence of fixation disparity. In Experiment II, esodisparities of 100 minutes of arc occur in a number of normal subjects. The dividing line between normal and abnormal binocular vision therefore is blurred. Normal persons can display disparities, the order of magnitude of which is equal to that of the angle of squint in micro-strabismus.