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Updated: Jun 27, 2026
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Virtual Prism Adaptation Therapy: Protocol for Validation in Healthy Adults
Published on: February 12, 2020
The prism that aligns fixation disparity does not predict the self-selected prism.
Joanna M N Otto1, Michael Bach, Guntram Kommerell
1Universitäts-Augenklinik, Killianstr. 5, 79106 Freiburg, Germany.
Summary
The fixation disparity (FD)-aligning prism does not reliably predict the self-selected prism (SSP) for comfortable binocular vision. Discrepancies suggest visual conditions during testing influence prism prescription outcomes.
Area of Science:
- Ophthalmology
- Optometry
- Vision Science
Background:
- Associated phoria is defined by the prism needed to align monocular markers in a fixation disparity (FD) test.
- This study investigates if the FD-aligning prism predicts the self-selected prism (SSP), which is the prism an observer finds most comfortable for viewing fully fusionable images.
Purpose of the Study:
- To determine if the prism that aligns markers in an FD test predicts the prism an observer selects for comfortable viewing.
- To explore potential reasons for discrepancies between FD-aligning and self-selected prisms.
Main Methods:
- FD-aligning prism was measured using a test with a central fusion lock.
- SSP was determined by showing identical images to both eyes on a monitor, with observers adjusting counter-rotating prisms for comfort.
- Measurements were repeated 1-6 weeks later.
Main Results:
- In 16 out of 20 subjects, the FD-aligning prism indicated a greater eso vergence position than the SSP.
- Changes of at least 1.0 prism diopter occurred in 8/20 observers for the FD-aligning prism and 11/20 for the SSP between sessions.
- The FD-aligning prism showed greater variability and directional bias compared to the SSP.
Conclusions:
- Discrepancies between SSP and FD-aligning prism may arise from differing visual conditions during testing, specifically binocular rivalry in the FD test.
- Spontaneous changes in both prism measures over 1-6 weeks highlight the need for research into the repeatability of prism prescription methods.
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