Short-term effects of aligning prisms on the objective and subjective fixation disparity in far distance

Abstract


 
 
 
 Fixation disparity (FD) refers to a suboptimal condition of binocular vision. The oculomotor aspect of FD refers to a misadjustment in the vergence angle between the two visual axes that is measured in research with eye trackers (objective fixation disparity, oFD). The sensory aspect is psychophysically tested using dichoptic nonius lines (subjective fixation disparity, sFD). Some optometrists use nonius tests to determine the prisms for constant wear aiming to align the eyes. However, they do not (yet) use eye trackers. We investigate the effect of aligning prisms on oFD and sFD for 60 sec exposure duration of prisms determined with the clinically established Cross test in far distance vision. Without prisms, both types of FD were correlated with the aligning prism, while with prisms the FD was close to zero (these analyses included all base-in and base-out cases). The effect of base-in prisms on oFD was proportional to the amount of the aligning prism for the present 60 sec exposure, similar as for the 2- 5 sec exposure in Schmid et al. (2018). Thus, within 1 minute of prism exposure, no substantial vergence adaptation seems to occur in the present test conditions. Further studies may investigate intra- individual responses to different exposure times of aligning prisms in both prism directions. 
 
 
 

Topical Review: Methodological Variables in Clinical and Laboratory Measurements of Fixation Disparity

“…In terms of objective and subjective fixation disparities, there is substantial evidence that significant differences between these fixation disparities can occur. 8,26,93,111,113,[116][117][118][119] When these differences occur, the objective fixation disparity is usually larger than the subjective fixation disparity. This suggests that, although it is difficult to determine under what conditions these differences occur, these differences are not entirely idiosyncratic.…”

“…Jaschinski and colleagues 93,[116][117][118][119] have published several relatively recent investigations in which objective and subjective fixation disparities were compared. In these studies, summarized hereinafter, the investigators have looked at a number of variables that may have an impact on measurements of objective and subjective fixation disparities.…”

“…Although they did not account for the pupil size change artifact in those studies, they concluded that the system was adequate to measure objective fixation disparities of approximately 10 arc minutes. Later articles by Jaschinski et al 93,[117][118][119]121 acknowledged the pupil artifact and applied a specific procedure in which the difference in pupil size between the monocular calibration and binocular test condition was determined and used to correct the objective fixation disparity. Notably, in the article in which Jaschinski 121 proposed his method to account for the pupil artifact in his eye tracker, he reported that the mean difference between the corrected and uncorrected fixation disparities was 3.6 arc minutes at a viewing distance of 40 cm and 11.7 arc minutes at a viewing distance of 24 cm.…”

“…In a more recent study by Schroth and colleagues, 119 subjective fixation disparities were assessed with the Cross test at 5 m, and objective fixation disparities were measured with an eye tracker for 16 subjects. Subjects viewed through the aligning prism as determined subjectively for 1 minute before the comparison of objective and subjective fixation disparities.…”

Do standard optometric measures predict binocular coordination during reading?

“…Furthermore, several studies in the last decades set out to show, which optometric measures best predicted asthenopic symptoms and suggested subjective fixation disparity as relevant predictor (Sheedy & Saladin, 1978;Yekta, Pickwell, & Jenkins, 1989). However, even though objective and subjective fixation disparity are correlated, they show a different overall pattern in reaction to prisms or training, for example (Jaschinski, 2018;Schroth, Joos, Alshuth, & Jaschinski, 2019), and thus, the question remains, whether objective fixation disparity is also a good predictor for asthenopic symptoms.…”