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 min-

utes 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 adap-

tation occurred within the first few minutes. After

adaptation, fixation disparity curves, fusional am-

plitudes (including distance divergence amplitudes

which are extremely reliable), and phoria measure-

ments 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 re-

moval 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 evi-

dent 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 ver-

gence 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 diver-

gence and hyperphorias. Arch Ophthalmoi 49:325. Copy-

right 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 ad-

aptation 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 var-

ious 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 fu-

sional disparity vergence system. According to this

model the phoria is a vergence error which is cor-

rected 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. Accord-

ing to Schor, fixation disparity is the result of

incomplete prism adaptation and represents a pur-

poseful error necessary to sustain vergence.

Schor8’9 has suggested that prism adaptation var-

ies from individual to individual, varies with time

of adapting prism, and varies with the direction of

the adapting prism. Rapid prism adaptation, ac-

cording to Schor,’° is highly correlated with the

shape of the fixation disparity curve. The steeper

the curve the poorer the prism adaptation. Accord-

ing 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 regard-

less 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 adap-

tation 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