Vergence position of rest: spontaneous variability

We determined the vergence position of rest (horizontal component) in two sessions, 1 to 6 weeks apart. Each session contained 8 trials. Twenty observers with normal eyes looked through Risley prisms that allowed a continuous modulation of their strength. We asked the observers to look at a fully fusionable picture, and to adjust the prisms such that viewing appeared most relaxing. The viewing distances were 400 cm and 50 cm.

The standard deviation among the 8 trials of each session, averaged over the two sessions, the viewing distances of 400 cm and 50 cm, and the 20 observers, was + or -1.4 prism dioptres (cm/m). From the first session to the second, many observers changed the preferred prism significantly. At a viewing distance of 400 cm, 10 of 20 observers changed their preferred prism by more than 1.0 prism diopter, up to 3.3 prism diopters. At a viewing distance of 50 cm, 11 of 20 observers changed their preferred prism by more than 1.0 prism diopter, up to 5.1 prism diopters.

The prismatic strength estimated as being most relaxing can be very variable. Before considering a prescription, the practitioner should determine whether the preferred prismatic strength remains reasonably stable. To ascertain this consistency, more than one session seems to be necessary.