Fixation disparity and stereopsis

The effect of fixation disparity on stereopsis was investigated. The changes in stereopsis and fixation disparity were studied by using a clinical test to determine which individuals possess a fixation disparity and then administering appropriate corrective lenses. Ten experimental subjects and five control subjects were Investigated over a six week period. Those manifesting fixation disparity wore corrective prism lenses from the second through the fifth week. An apparatus was constructed that allowed data for curves relating the magnitude of fixation disparity as a function of forced vergence to be taken. Stereopsis measurements were taken twice a week on a Howard-Dolman apparatus. It was found that all fixation disparity curves could be categorized into two of the types reported previously by Ogle, i.e., type 1 and type 2. Also, most subjects' fixation disparity data remained unchanged during one month of wearing prism corrective lenses. Slight Improvement in stereopsis after correcting fixation disparity occurred for some Individuals but the changes were temporary. The stereotest performance of the subjects with convergent fixation disparity showed greater variability following prism correction than the performance of subjects with divergent fixation disparity. It was concluded that fixation disparity has little effect on stereoscopic depth perception.