Update on Fixation Disparity

Purpose

The aim of the paper is to summarise and discuss the current state of knowledge on the overall topic of fixation disparity (FD).

Material and Methods

A systematised PubMed literature search based on the term “fixation disparity” was conducted; in addition, the publication relies on the reference book on “Perceiving in Depth” written by I. P. Howard.

Results

Until the 1990s, the understanding regarding subjective FD was quite uniform. Studies with high-resolution, objective examination methods and simultaneous measurement of subjective and objective FD have since shown that the vergence error (= objective FD) is usually not very well correlated with the vernier error (= subjective FD). It can therefore be assumed that the two types of FD are two different phenomena. The objective vergence error can indicate how large the shift of the image position on the retina is within the central panum area. Subjective FD, on the other hand, must be seen as the result of multiple processing events in the brain.

Conclusion

There is a difference between subjective and objective FD. This finding does not contradict the clinical relevance of subjective FD. Aligning prisms to correct subjective FD are effective in reducing visual complaints in certain cases. It is possible that aligning prisms reduce a relevant proportion of heterophoria.