Stereoacuity is affected by induced phoria but returns toward baseline during vergence adaptation.

Purpose: In normal binocular single vision adaptation to an induced deviation occurs over a period of several minutes. This study investigates the effect on stereoacuity during vergence adaptation.

Methods: Stereoacuity, using the Frisby near stereotest, was measured in 20 participants aged 20.2 +/- 1.9 years with normal binocular single vision. Measurements were taken immediately on inducing a phoria with 12(Delta) base-out prisms (split), and after 3, 6, and 9 minutes of adaptation. A measure of stereoacuity was also taken with the same size prisms that were used concomitantly to control for the reduced visual acuity.

Results: Stereoacuity was found to decrease following introduction of the 12(Delta) base-out prismatic glasses (9.4 +/- 2.5 arcsec compared with 24.4 +/- 21.4 arcsec) and then increase over the 9 minute period of adaptation (ANOVA, p = 0.0002). Using post-hoc tests with Bonferroni correction, the decrease of stereoacuity on introduction of the prism was significant (p = 0.0039), and although an increase in stereoacuity appeared to occur after 3 and 6 minutes of wearing the 12(Delta) glasses (14.9 +/- 4.3 arcsec, 12.3 +/- 4.5 arcsec), this did not return to the baseline value until 9 minutes, when the stereoacuity had increased to 12.6 +/- 10.6 arcsec (p = 0.1982).

Conclusions: In these participants, inducing a deviation with base-out prisms negatively affected near stereoacuity. However, as adaptation occurred, the level of stereoacuity was seen to increase back toward the baseline measurement.