Kenneth N. Ogle

Further Considerations of Fixation Disparity and the Binocular Fusional Processes

The fusion compulsive reflex through parxad ticular synergies to the extraocular muscles strives to direct the pointing of the eyes so that like images fall as near as possible on corresponding retinal points. Recently it has been emphasized that this pointing may not be exact if heterophorias* are present, but that, in spite of this inexactxad ness, there is fusion because the images will still fall within Panums areas of fusion. The deviation between the images is desigxad nated a fixation disparity. It occurs bexad cause the eyes are actually overconverged or underconverged by a small amount from the object of fixation, owing to the compulsion of an active heterophoria. Though fusion is maintained the images of the fixation point actually fall on disparate retinal points. The magnitude of this disparity, the anguxad lar error in the convergence of the eyes, is necessarily small, being limited by the size of Panums areas, and accordingly is measured in minutes of arc. Its magnitude depends on many factors, but remains relatively conxad stant for the same individual when measured under similar test conditions. The accumulation of many more data on this phenomenon since the preliminary pubxad lication permits us to interpret the results somewhat more broadly and to revise several of the earlier points of view.