December 1983

Treating Convergence insufficiency—Cooper eta!.

983

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 iden-

tical 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 dis-

parity). 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 se-

quencing of stimuli, changes in vergence de-

mand, 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 re-

search 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 binocu-

larly. The borders of this square, which sub-

tended 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 ex-

amination which included measures of the fol-

lowing: 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 in-

cluded 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 com-

pleted 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, re-

sulted in termination of the trial and led to a

decrease in convergence demand by 1.32 base-

in on the next trial. Maximum convergence per-

formance 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. Pa-

tients 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 re-

ported when vertical alignment was attained.

Direction of offset was alternated and magni-

tude was randomized before each measure. Dur-

ing 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 refer-

ence point (i.e., constant error) for all subse-

quent 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 perform-

ance achieved during baseline testing. Each