Fig. 2. Total divergence fusional reserve (phoria/tropia plus divergence fusional convergence amplitude measurements minus prism in the spectacles) is presented for each orthoptic therapy session. Measurements are for disparity stimuli moving with a constant velocity (ramp) which is depicted in the upper curve, and jump duction (step) which is depicted in the lower curve. As amplitudes improved, therapy was increased in difficulty by making the vergence stimuli smal ler and by increasing the velocity of the target during ramp therapy.

continued to improve, additional step-wise reductions in the total amount of prism correction were made, i.e., 30B0, 25B0, 2oBO, i6BO, I2BO, ioBO, and 8B0. Initial prismatic changes were large, while subsequent changes were made progressively smaller. With progress, fusional amplitude therapy was made more difficult by slowly increasing the velocity of the vergence stimuli to 5k/sec and/or by reducing the stimulus size to 6 degrees.’2

As smooth divergence amplitudes improved, more difficult step (jump duction) stimuli were then introduced. Stimulus presentation, vergence demand, and reinforcement contingencies were presented with the VTS3 computerized method of training. Fig. 2 presents both the maximum total fu sional divergence reserve and maximum total step amplitude measured at each session. It is readily apparent that both ramp and step fusional ampli tudes progressively improved over time.

Over a one-year period of weekly therapy, horizontal divergence fusional amplitudes improved at distance to x/- I0/- 14 and at near to x/4/2; the more positive post-therapy values demonstrated moderate improvement of fusion ability. Initial occlusion with an alternate cover test resulted in an esophoric deviation of approximately 7^ at distance and orthophoria at near. Upon pro longed repeated alternate cover testing for two minutes, the magnitude of the deviation gradually increased, until it equalled the magnitude found at the initial examination, i.e., 40^ esotropia. Three years after orthoptic treatment, the patient is presently wearing progressive addition lenses without any prism incorporated into the spectacle lens before each eye. After a stressful day, however, the patient often has to revert to his prismatic glasses, which have 2^ BO in each eye.

Towards the end of the orthoptic therapy, both horizontal versional (monocular and binocular) and vergence eye movements were recorded using a commercially-available infrared eye movement system (Gulf and Western, Eye Trac Model 200). This system has a bandwidth from DC to 250 Hz, a resolution of 0.2 degrees, and a linear range of +/- 10 degrees; however, the frequency response of the eye movement traces was limited by the bandwidth of the strip chart recorder (DC to 8o Hz). The target consisted of a small (5 mi arc), bright spot of light presented on a display monitor 57 cm from the subject which was controlled by a function generator.

Representative eye movement recordings are shown in Fig. 3. Midline