Background: In the evaluation of therapies aiming at binocular vision, for instance by the use of prisms or orthoptic training in the case of heterophoria, stereoacuity is often the primary outcome measure. To assess therapeutic effects it is necessary to separate them from perceptual learning with repeated testing. Learning stereoacuity has been investigated only in a few studies with up to six subjects.
Methods: To ascertain the interindividual variability of learning in stereoacuity we examined 24 subjects, 12 with and 12 without experience in psychophysical experiments. In a two-alternative forced-choice paradigm, subjects reported whether a vertical bar appeared in front of or behind a reference frame. Estimates of stereo threshold were obtained using an adaptive staircase procedure ("best PEST").
Results: We found a highly significant learning effect ( P<0.0001) with a marked interindividual variability. In some subjects the stereoacuity improved by a factor of >30 and in others it did not improve at all. The median of the learning factor was 1.7. There was no significant difference between novices and experienced subjects.
Conclusion: The great interindividual variability of learning in stereoacuity has important implications for therapeutic tests that use stereoacuity as an outcome measure: To distinguish therapeutic effects from improvements due to repeated testing, each subject's individual learning behaviour has to be taken into account, for example by starting out with an adequate training phase. The number of test repetitions required to reach a fairly constant level appears to be similar among individuals: in our paradigm most of the learning occurred within the first six blocks with 100 target presentations each.