Phoria adaptation as a mechanism for maintaining good binocular coordination in normal subjects

Papers written about phoria adaptation often discuss the modification of eye alignment as evidence for mechanisms that compensate for changes in the oculomotor system due to development, disease and injury. While clinicians often encounter patients with diplopia or other signs of ocular misalignment, most people, in fact, have nearly perfect eye alignment even when viewing far targets with one eye occluded. Evidence that good eye alignment is the result of adaptive mechanisms can be seen with the binocular misalignment that results from the monocular patching of one eye in normal subjects for extended periods of time. We have tested extensively the ability of subjects to adapt the alignment of their eyes in various viewing conditions that present binocular disparities in the context of different version, vergence and head postures. Our conclusion from these studies is that one of the primary functions of phoria adaptive mechanisms is to manage the parsing of extraocular muscle forces during everyday viewing situations. The concept of Hering’s law of equal innervation is an oversimplification of a very complex process in which the relative participation of the twelve extraocular muscles needs to vary with the distance and eccentricity of the object of interest as well as the pitch and roll angles of the head. We note also that the problem of maintaining good coordination of the eyes is exacerbated with dynamic eye and head movements. I will review our work on the adaptation of vertical phoria and cyclophoria in relation to orbital eye position and head posture.