Does the subjective response during the measurement of fusional reserves affect the clinical diagnosis?

Introduction: Fusional reserves differ with the method of measurement. The goal of this study was to compare the subjective and objective responses during the measurement of positive and negative fusional reserves using both step and ramp methods.

Methods: A haploscopic system was used to measure fusional reserves. Eye movements were recorded using an EyeLink 1000 Plus eye tracker (SR Research). The stimulus disparity was changed to either mimic a prism bar (steps) or a Risley prism (ramp). Subjective responses were obtained by pressing a key on the keyboard, whereas objective break and recovery points were determined offline using a custom algorithm coded in Matlab.

Results: Thirty-three adults participated in this study. For the ramp method, the subjective and objective responses were similar for the negative (break and recovery points (t(32) = -0.82, p = 0.42) and (t(32) = 0.42, p = 0.67), respectively) and positive fusional reserves (break and recovery points (U = -1.34, p = 0.18) and t(19) = -0.25, p = 0.81), respectively). For the step method, no significant differences in positive fusional reserves were observed when measured subjectively and objectively for the break (t(32) = 1.27, p = 0.21) or the recovery point (U = -2.02, Bonferroni-adjusted p = 0.04). For the negative fusional reserve, differences were not significant for either the break or recovery points (U = -0.10, p = 0.92 and t(19) = 1.17, p = 0.26, respectively).

Conclusion: Subjective and objective responses exhibited good agreement when measured with the ramp and step methods.