Variation of Binocular Contrast Summation with induced vertical fixation disparity

Background: Previous studies have supported the notion that binocular summation varies with stimulus spatial frequency. In this study we hypothesize that the magnitude of the interocular phase shift induced by a fixation disparity has an indirect relationship with the magnitude of binocular contrast summation. Methods: Binocular and monocular contrast detection thresholds were measured in 6 (six) subjects for 3 (three) spatial frequencies (1 (one), 3 (three) and 9 (nine) cpd) and for various magnitudes of vertical fixation disparities induced by 5 (five) magnitudes of vertical prism (0 (zero) PD, 1.5PD BU/BD, 3 (three) PD BU/BD). Detection thresholds were measured using a 2 (two) interval forced choice method with a descending methods of limits. Vertical fixation disparity was measured at the beginning and end of each binocular contrast sensitivity measure for each spatial frequency condition. Results: The binocular contrast summation ratio was compared to the average phase shift produced by the vertical prism. The data trends provide suggestive indications that induced period shift due to vertical fixation disparity may influence the magnitude of binocular contrast summation, however there was no significant difference in this relationship. There was significant vergence adaptation to vertical prism during testing. Discussion: The current experimental paradigm is not effective at revealing a phase dependence, specifically given the confound of prism induced vergence adaptation. Thus, these results should be interpreted with caution.