The prism that aligns fixation disparity does not predict the self-selected prism Joanna M. N. Otto, Michael Bach and Guntram Kommerell Universita¨ ts-Augenklinik, Killianstr. 5, 79106 Freiburg, Germany Abstract Background: Associated phoria is defined as the prism that aligns monocular markers in a fixation disparity (FD) test. We examined whether the ÔFD-aligning prismÕ predicts the Ôself-selected prismÕ (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 potentio- meter. 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 (£15.9 cm m )1 = prism dioptre). In five instances, the FD-aligning prism indicated a vergence position further in the exo direction (£3.6 cm m )1 ). From the first to the second session, changes of ‡1.0 cm m )1 occurred for the FD-aligning prism in 8/20 observers (£11.5 cm m )1 ), and for the SSP in 11/20 observers (£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. Keywords: aligning prism, associated phoria, fixation disparity, heterophoria, Mallett unit Introduction Since the work of Ogle et al. (1949), dissociated phoria has been distinguished from associated phoria. Dissoci- ated phoria is defined as a deviation from the orthover- gence position that occurs when no fusionable contours are provided. Associated phoria is a deviation of the eyes that appears as the prism correction of fixation disparity (FD). FD is commonly examined according to the observerÕs directional perception of monocular markers, usually Nonius lines (Petrus Nonius, 1502–1578), embedded in binocular contours. The associated phoria equals the Ôaligning prismÕ (Evans, 2007) that nullifies any offset between the monocular markers. The deter- mination of associated phoria takes advantage of the fact that paracentral monocular markers, added to a binoc- ular fixation target, loosen the fusional accuracy and allow the eyes to deviate from bifoveal fixation [search coil recordings (Gerling et al., 1998)]. This deviation (usually only a fraction of 1°), perceived by the observer as a misalignment of the monocular markers, tells the examiner that the associated phoria has not been fully corrected. In the following, we use the term ÔFD-aligning prismÕ as a synonym for Ôassociated phoriaÕ. Received: 28 February 2008 Revised form: 20 May 2008, 7 July 2008 Accepted: 19 July 2008 Correspondence and reprint requests to: Prof. Guntram Kommerell, MD. E-mail address: guntram.kommerell@uniklinik-freiburg.de Ophthal. Physiol. Opt. 2008 28: 550–557 doi: 10.1111/j.1475-1313.2008.00599.x ª 2008 The Authors. Journal compilation ª 2008 The College of Optometrists