Figure 2. Analysis of closed-loop and individual elements 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 decays, slow vergence makes up the lost difference. Removal 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 final 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 influenced by its neighboring points. Sethi and North’6 showed that adaptation was improved immensely 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 accommodative 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

vergence by occlusion followed by a longer decay of the slow fusional vergence response.

Stated another way, a measurement which increases with repeated alternate occlusion represents an initial elimination of fast fusional vergence followed 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 fusional vergence signals with minimal effect on slow fusional vergence signals because slow fusional vergence 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.