population mean (mean = 21 pd, SD = 6.0 pd)12 All patients had at least 660 sec arc of stereopsis on a random dot stereogram (RDS test)’3 and normal [6/6 (20/20)] visual acuity. All testing was conducted with refractive errors corrected. Assessment of asthenopia was determined using a written rating scale (Appendix A).

RDS’s were projected either with increasing base-out vergence demand in the experimental condition, or constant vergence demand in the control condition.9 The stimuli were either identical right and left eye views of a RDS (i.e., the flat fusion stimulus) or an RDS with an inner stereoscopic square (660 sec arc of crossed disparity). Responses for the identification of the stereoscopic and flat fusion stimuli were made on two illuminated push panels located below the projection screen. Programming and sequencing of stimuli, changes in vergence demand, delivery of reinforcement feedback for correct responses, and recording of trials and responses were controlled by solid state logic and electromechanical relay circuitry.

Forced vergence fixation disparity curves were measured at near (40 cm) using a standard research model fixation disparity device.’4 Two polarized nonius lines, one seen by each eye, subtending 1.4 deg in total height, were situated in a square blank field which was viewed binocularly. The borders of this square, which subtended an angle of 2.9 deg on a side, were centered about the nonius lines and provided the first fusable contour. Immediately surrounding the square was a field (9 deg horizontal X 14.5° vertical) of printed text of various sizes. Normal room illumination was used. All patients wore their habitual refractive correction.

Commercially available Vectograms (Bernell Corp.) which subtended an angle of 20° were presented at 40 cm to measure base-out fusional ranges.

All patients received an initial optometric examination which included measures of the following: refractive state; distance and near phoria, cover test, base-out and base-in prism vergence ranges; positive and negative relative accommodation; nearpoint of convergence; and accommodative facility.’2 Additional testing included measures of fixation disparity, fixation disparity forced vergence curves, vergence ranges at near using polarized vectograms, and a baseline of vergence performance using the RDS projection system. All patients also completed a seven item questionnaire to rate their asthenopia. Each item was scored on a scale of one to five. The higher the total score, the less the asthenopia (see Appendix A). All clinical tests, the fixation disparity tests, and the ques

tionnaire were administered at each phase of the experiment.

During baseline RDS testing, the patient wore polarizers and was instructed to look at a large screen where he would see either a dot pattern with a central inner square “popping out” or a similar dot pattern without the inner square. The stereoscopic and flat fusion stimuli were presented in random order separated by a 6 sec inter-stimulus interval in which the screen was dark. Stimuli remained in view either until a patient response occurred or for a maximum of 10 sec. A correct response in the presence of either stimulus resulted in the presentation of a 3 sec positive feedback cue light and termination of the stimulus. Correct responses made to the presence of the RDS containing the inner square led to an increase in the convergence demand of 0.66 base-out on the next scheduled trial. However, no increase occurred after a correct response to the flat fusion RDS. Incorrect re sponses in the presence of either stimulus, or lack of response within the 10 sec period, resulted in termination of the trial and led to a decrease in convergence demand by 1.32 base- in on the next trial. Maximum convergence performance during baseline testing was noted as that prism diopter value achieved before two consecutive errors.

After RDS baseline testing, forced vergence fixation disparity curves were measured. Patients were instructed to fixate the top of the lower nonius line as the experimenter slowly reduced the horizontal offset of the upper line with a calibrated vernier knob. The patient reported when vertical alignment was attained. Direction of offset was alternated and magnitude was randomized before each measure. During actual testing, the vergence demand was set with Risley prisms. At least four measures were taken and averaged at each prism setting. Prism values were alternated between base-in and base-out in 2 to 4 increments until the patient reported either persistent diplopia or blur. Each patient was also instructed to close the eyes between settings to minimize prism adaptation effects.’5 Before the commencement of testing, measurements were taken without prisms or polarizing lenses in place (each eye viewing both nonius lines) to acquaint the patient with the task and to establish the subjective zero reference point (i.e., constant error) for all subsequent measures. Fixation disparity curves were measured after each phase of the experiment.

After their test phases, patients were matched in order to form two groups on the basis of both severity of asthenopia and convergence performance achieved during baseline testing. Each