Surfaces of forced vergence disparity

Abstract

Purpose: To introduce the concept of surfaces offorced vergence disparity using measurementsof fixation disparity from three young, healthy individuals.

Method: Fixation disparities were measured in three individuals in relation to variation in stimulus distance and vergence demand.  All measurementswere obtained by means of the Sheedy disparometerwhich can be used to measure not only fixation disparity but also associated phorias.  For each individual, consecutive measurements of fixation disparity only were determined over a short time period at four stimulus distances (0.25 m, 0.4 m, 1 m and 2 m) and under five different vergence demands as produced with the application of different amounts of prism (either base in or base out) and also without any prism.  Parametric and non-parametric statistical methods are used to understand short-term variation of fixation disparity and pseudo-3D and stereo-pairs represent thesurfaces of forced vergence disparity with which this paper is mainly concerned.

Results:  Surfaces of forced vergence disparity are very useful to study variation of fixation disparity inrelation to change in stimulus distance and vergence demand.  They are effectively 3-dimensional equivalents of 2-dimensional Ogle curves of forced
vergence disparity.

Conclusion: Surfaces of forced vergence disparity may be useful in many contexts - both in relation to normal or unusual binocular behaviour.  This paper introduces such surfaces in relation to
three individuals who were considered as having satisfactory binocular and accommodative-vergence function.  The surfaces assist one in understanding complicated three-dimensional or trivariate data that involves fixation disparities, different stimulus
locations and different accommodative-vergence demands upon the oculo-motor system.  (S Afr Optom 2013 72(1) 25-33)