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

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.

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.

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).

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.