that before the prisms were worn (see Fig. 1). The time course for adaptation was exponential, with most of the adaptation occurring within a few minutes of wearing the prism. Adaptation to horizontal prism has been reported to be logarithmic by Ogle et al.6 The time course for adaptation is related to both the amount of prism and the duration of exposure to the adapting prism.

Carter7 showed that prolonged wearing of prisms not only altered the phoria position and shifted the fixation disparity curve but also shifted the fusional reserves for their subjects. His subjects completely adapted to horizontal prisms as large as 10pd base- in and 32 pd base-out in 15 min. Most of the adaptation occurred within the first few minutes. After adaptation, fixation disparity curves, fusional amplitudes (including distance divergence amplitudes which are extremely reliable), and phoria measurements taken through the prisms were similar to the measurements before wearing the prism. These findings imply that wearing prisms causes an actual shift in the oculomotor position of the eyes. These adaptive changes remained stable for as long as the prisms were worn.

Removal of the adapted prism, when it is of a large magnitude, rarely results in immediate motor fusion because the aftereffects of adaptation decay over time. The recovery after adaptation upon removal of the prism generally takes from 15 min to 8 h if fusion is not, allowed, i.e., dark, sleep, or diplopia. Carter7 noted that esophoria, which was induced by wearing a base-out prism, was still evident after 8 h of sleep. “However, the esophoria disappeared after only 20 min of single binocular vision.” Thus, sensory fusion with a compensatory motor movement is necessary for adaption or vergence aftereffects to occur. Carter has suggested that the previously described changes in tonicity

FIgure 1. Forced vertical fixation disparity curves for a hyperphoric subject without prism and after wearing 2, 4, and 6à of vertical prism. While wearing this prism, the subject demonstrates a forced fixation disparity curve similar to that found before wearing prisms. The subject has shifted his forced fixation disparity curve by adapting to the prism. (Reprinted by permission of the publisher from Ogle KN, Prangen A. Observations of vertical divergence and hyperphorias. Arch Ophthalmoi 49:325. Copyright 1953, Am Med Assn.)

from adaptation serve to relieve the stress on the fusional or disparity vergence system. Carter stated that his subjects who demonstrated significant adaptation did not report discomfort from prolonged vergence. Carter concluded from these findings that patients with good sensory fusion adapt, whereas those with poor sensory fusion do not adapt and show symptomatic heterophoria.

Schor8 incorporated the results of the previously described studies and his research findings into a model which explained the interactions of the various components of both the accommodative and vergence system. Specifically, Schor9 described two types of vergence: a fast, reflex fusion system driven by retinal disparity vergence and a slow, adaptive system which received its input from the fast fusional disparity vergence system. According to this model the phoria is a vergence error which is corrected by fusional vergence. The slow vergence system or adaptor “reduces the stress or load placed on the vergence system by the heterophoria under binocular conditions.” The sum of the slow and fast systems equals the total fusional vergence. According to Schor, fixation disparity is the result of incomplete prism adaptation and represents a purposeful error necessary to sustain vergence.

Schor8’9 has suggested that prism adaptation varies from individual to individual, varies with time of adapting prism, and varies with the direction of the adapting prism. Rapid prism adaptation, according to Schor,’° is highly correlated with the shape of the fixation disparity curve. The steeper the curve the poorer the prism adaptation. According to Schor’s model the output from the fast fusion system decays similar to a leaky neural integrator over a 10- to 15-s period of time. On the other hand, the slow disparity vergence system increases its output to relieve the fast fusion mechanism which maintains a constant total output (see Fig. 2).

The phenomenon of vergence adaptation also occurs with nonconcomitant vergence disparity. Cusick and Hawn” and Pascal12 measured the phoria in both primary and in depressed gaze in spectacle-corrected anisometropes. They found that the induced hyperphoria was eliminated by adaptation. Ellerbrock and Fry13 measured vertical phorias in 42 anisometropes in both primary and depressed gaze. Orthophoria was measured regardless of the induced prismatic error created by the anisometropia. They postulated that there was a continuous adaptive process which occurred over the entire oculomotor field. Similar findings have been reported by Allen14 and Henson and Dharamshi 15

Henson and Dharamshi15 measured the time course of adaptation with 1.00, 3.00, and 4.00 D of induced anisometropia. They reported that adaptation rates were similar for both concomitant vs. noncomitant stimuli and’ spread to areas where visual experience was not allowed. Interestingly, adaptation was more complete in downgaze (where there was more visual experience) than in upgaze.

Implications of Vergence Adaptation—Cooper 301