Purpose. 

To investigate the effect of repeated testing on adaptation to a 2 Δ vertical prism in subjects with normal binocular single vision.

Methods. 

Subjects were required to have good visual acuity and normal binocular single vision. The residual deviation was measured after a 1-min period of adaptation to a 2 Δ vertical prism using a modified Maddox Rod technique. This measurement was repeated 10 times, with a 5-min rest period between each trial.

Results. 

For eight young adult subjects, the mean residual deviation at 1 min reduced with consecutive trials, and this effect was statistically significant (p < 0.0001). No significant correlation was present between the amplitude of vertical vergence and the increase in adaptation with repeated demand.

Conclusions. 

A practice effect for vertical vergence adaptation appears to be present in young adult subjects with normal binocular single vision.