Background: Associated phoria is defined as the prism that aligns monocular markers in a fixation disparity (FD) test. We examined whether the 'FD-aligning prism' predicts the 'self-selected prism' (SSP), i.e. the prism that the observer judges to be most comfortable when viewing fully fusionable images.
Methods: For the FD-aligning prism, we employed a test target with a central fusion lock. To determine the SSP, identical images were shown to both eyes. The images were displayed on a cathode-ray tube monitor at a distance of 50 cm, separated for the two eyes by means of liquid crystal shutter goggles. The observer adjusted counter-rotating prisms, recorded with a potentiometer. The procedure was repeated about 1-6 weeks later.
Results: Among the 20 non-strabismic subjects over two sessions, there were 16 instances in which the FD-aligning prism indicated a vergence position further in the eso direction than the SSP (< or =15.9 cm m(-1) = prism dioptre). In five instances, the FD-aligning prism indicated a vergence position further in the exo direction (< or =3.6 cm m(-1)). From the first to the second session, changes of > or=1.0 cm m(-1) occurred for the FD-aligning prism in 8/20 observers (< or =11.5 cm m(-1)), and for the SSP in 11/20 observers (< or =5.1 cm m(-1)).
Discussion: We hypothesise that the discrepancy between the SSP and the FD-aligning prism may be attributable to the fact that the SSP was determined while the eyes were presented with identical images, whereas the FD-aligning prism was determined while the images for the right and left eyes were different in the area of the Nonius lines. The unusual experience of binocular rivalry around the Nonius lines may have influenced the vergence of the eyes. Considering the spontaneous changes observed within an interval of 1-6 weeks, in both the FD-aligning prism and the SSP, we feel that research on the repeatability of methods for prescribing prisms is indicated.