Measurements of objective and subjective fixation disparity with and without a central fusion stimulus.

Background: Fixation disparity is the condition in which the images of a binocularly fixated object are not imaged on exactly corresponding retinal points, but are still within Panum's fusional areas. Measurements of objective and subjective fixation disparity have indicated that subjective fixation disparity does not indicate the true eye position. However, the effect of the foveal fusional lock (FFL) on the relationship between objective and subjective fixation disparity is still not clear.

Material/methods: This relationship was investigated in five subjects. The objective measurements were made using scleral search coils, and at the same time the Sheedy distance disparometer indicated the subjective fixation disparity. Measurements were obtained both with and without an FFL.

Results: It was found that without an FFL there was a significant difference (p<0.05) between subjective and objective fixation disparity, whereas when an FFL was added, the difference was no longer significant. Additionally, the absolute sizes of both SFD and OFD were found to be significantly smaller with the FFL.

Conclusions: These results indicate that the presence of an FFL makes subjective fixation disparity a more accurate indicator of the objective eye position. An instrument with an FFL should therefore be used clinically. The difference between objective and subjective fixation disparity presumably indicates a change in correspondence.