Heterophoria: a vergence adaptive position.

Previous experiments have shown that as the adaptive change becomes more complete with prolonged wear of a prism, the adaptive decay during monocular occlusion (MO) becomes slower; at the same time, adaptation to a further prism disparity becomes faster. If phoria is an adapted position of the oculomotor system then, as with prism adaptation, it should demonstrate an adaptive decay during MO. The rate of this decay should be inversely related to the rate of adaptation to a prism. In the first part of the paper, changes in the horizontal and vertical phorias were demonstrated at distance and near during 4 hr of MO. In the second part, adaptation to various prism disparities (horizontal and vertical) was monitored. The rates of adaptation were found to be inversely related to the rates of phoria decay during MO. The phoria position is, therefore, an adapted position of the vergence system.