Comparison of symmetrical prism adaptation to asymmetrical prism adaptation in those with normal binocular vision

Abstract

This study sought to determine whether symmetrical compared to asymmetrical horizontal prisms (base-out or base-in) evoked different rates of phoria adaptation. Sixteen young adults with normal binocular vision participated in a symmetrical phoria adaptation experiment using a 3Δ base-out or 3Δ base-in binocular prism flipper and an asymmetrical phoria adaptation experiment using a 6Δ base-out or 6Δ base-in monocular wedge prism. The experiments were randomized and counterbalanced to reduce the influence of the prism stimulation order. Asymmetrical base-out prism adaptation was significantly faster than symmetrical prism adaptation for subjects with normal binocular vision. Asymmetrical phoria adaptation with base-in prism was not significantly different from symmetrical phoria adaptation implying that there are directional asymmetries (convergent versus divergent eye movements) in the slow fusional component of vergence. Data suggest that a potential interaction between the version system and the slow fusional vergence system may exist. Results have clinical relevance because patients with convergence or divergence insufficiency/excess may potentially show more pronounced differences between symmetrical and asymmetrical phoria adaptation compared to binocularly normal controls. These differences might also be relevant to clinical measurements such as vergence fusional range, which can be measured symmetrically (with Risley prisms in a phoroptor) or asymmetrically (with prism bar).

Keywords: Heterophoria; Phoria adaptation; Prism adaptation; Vergence adaptation.

Figure 1.

The two types of prisms used to test phoria adaptation. Left-hand side (A): wedge prism used to test asymmetrical phoria adaptation. In this condition, the vergence disparity was introduced by placing a 6Δ wedge prism in front of the right eye. Right-hand side (B): flipper prisms used to test symmetrical phoria adaptation. In this condition, a vergence disparity of 3Δ was introduced to the left and to the right eye for a total vergence disparity of 6Δ.

Figure 2.

Phoria measurements plotted over time (minutes) were fitted with an exponential function for each subject. The time (horizontal axis) represents the cumulative time that subjects spent using binocular vision to look through the single wedge prism or the binocular flipper prisms. The vertical axes represent the change in phoria level in Δ. Each color line represents a different subject. The four experimental conditions were symmetrical phoria adaptation (one 3 Δ prism in front of each eye) with base-out (a) and base-in prisms (b), and asymmetrical phoria adaptation (one 6Δ prism in front of the right eye) with base-out (c) and base-in (d) vergence disparities.

Figure 3.

Mean symmetrical (a and b) and asymmetrical (c and d) phoria adaptation in response to either base-out (a and c) or base-in (b and d) prisms. The abscissa indicates the cumulative time (in minutes) that subjects spent using binocular vision to look through the single wedge prism or binocular flipper prisms. The ordinate axes are the change in phoria level in Δ. Negative phoria levels in the ordinate axes indicates exophoria range (a and c), while positive phoria levelsindicate esophoria range (b and d). Mean phoria adaptation measures were fitted with an exponential function. Error bars represent standard error (SE) of the mean.

Figure 4.

Each bar in the bar graphs represent the final phoria level (Δ) of each of the participants after 7 minutes of prism adaptation. The symmetrical phoria adaptation experiement is plotted in blue and the asymmetrical phoria adaptation experiment is plotted in red. The group mean of the change in final phoria level is shown at the top of the bar graphs for the four experimental conditions with ± one standard deviation (SD). The participants who reached an adaptation of 6 Δ were able to completely adapt to the prism.