Figure 2. Analysis of closed-loop and individual ele-

ments of vergence responses to step stimuli created by a

prism. Fast or disparity vergence supplies most of the

initial response. As the disparity vergence response de-

cays, slow vergence makes up the lost difference. Re-

moval of prism eliminates the remaining fast disparity

vergence signal. Because slow vergence or adaptation has

a longer time constant, its demise is much longer. The fina

endpoint of decay is equal to the level of tonic vergence.

Henson and Dharamshi postulated that a cortical

motor memory map developed with each point in-

fluenced by its neighboring points. Sethi and

North’6 showed that adaptation was improved im-

mensely if the prismatic steps were small.

Schor’s8 model, incorporating a slow vergence

system with very long decay time constant (greater

than 30 s) into Maddox’s model, is very useful in

explaining a multitude of clinical observations. It

also has implications in diagnosis and treatment of

various oculomotor anomalies. Fig. 3 presents a

block diagram to depict the vergence feedback

based upon the findings of Schor,’7 Ciuffreda,’8 and

Ciuffreda and Hung.19

CLINICAL IMPLICATIONS OF VERGENCE

ADAPTATION

Cover Test

The most commonly performed measurement of

oculomotor integrity is the cover test. Testing is

usually done while the patient fixates an accom-

modative target at both 6 m and 40 cm. It is often

noted that the initial measurement of the angle of

deviation is not stable. With repeated alternate

occlusion the angle of deviation often increases.

The increase in angular measurement seems to be

dependent on: (1) the size of the initial latent

deviation; (2) the duration of occlusion;.and (3) the

strength of vergence adaptation. The increase in

the angle is a result of a rapid decay of fast fusional

302

OPTOMETRY & VISION SCIENCE

vergence by occlusion followed by a longer decay of

the slow fusional vergence response.

Stated another way, a measurement which in-

creases with repeated alternate occlusion represents

an initial elimination of fast fusional vergence fol-

lowed by a subsequent elimination of slow fusional

vergence. Conversely, removal of an occiuder during

a unilateral cover test permits fusion to reoccur.

This results in stimulation of the fast fusional

vergence system which feeds into the slow fusional

vergence system. Repeat occlusion with unilateral

cover testing results in the elimination of fast fu-

sional vergence signals with minimal effect on slow

fusional vergence signals because slow fusional ver-

gence has a long time constant. Therefore, the

deviation measured with the alternate cover test is

Figure 3. Block diagram depicting the interaction of the

various components of accommodation and vergence to

create a closed-loop negative feedback system. Disparity

vergence has a rapid rise and slower decay. As disparity

vergence decays, slow fusional vergence increases its

output and sustains the vergence response. The slow

fusional vergence system is driven by a signal created by

the difference between the signal to the fast fusion system

and the output from the fast fusion system. The vergence

response is also supplemented by proximal vergence and

the cross-links from the ACA ratio. Accommodation has a

similar servomechanism driving It.