Heterophoria.

With the large number of factors that contribute to the "position of rest" or fusion-free posture, it might be expected that heterophoria would be normally distributed. Data from a large sample of symptom-free subjects (N = 925) show that this is not so. Furthermore, the difference in the prism adaptation response in orthophoria, esophoria, and exophoria suggests that the presence of heterophoria in normal subjects is due to the partial saturation of prism adaptation.