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 forcedvergence 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)
Highlights
Fixation disparity (FD) and associated phorias (AP) are probably under-utilized clinical parameters in general optometric and ophthalmologic practice
Surfaces of forced vergence disparity are very useful to study variation of fixation disparity in relation to change in stimulus distance and vergence demand
Surfaces of forced vergence disparity may be useful in many contexts - both in relation to normal or unusual binocular behaviour
Summary
Fixation disparity (FD) and associated phorias (AP) are probably under-utilized clinical parameters in general optometric and ophthalmologic practice. Different authors differ in terms of their understanding and definitions of fixation disparity and some[1,2,3] may regard a FD as an anomaly of binocular function or vision whereas others[4,5,6] consider small FD (and smallish horizontal FD) as being a normal adaptive measure to small inter-ocular retinal disparities or differences of position of the two oculo-centric axes of the right and left eyes of an individual In the latter approach[4,5,6] the FD and its variation are considered to reflect instantaneous changes in eye, and retinal image, position and FD is not generally regarded as an anomaly unless its value exceeds some particular amount (that typically ranges from 4-10 minutes of arc in the relevant literature4-8) and where symptoms of binocular discomfort are present. Whether in the form of vision therapy, lenses or prisms, or even occasionally extra-ocular muscle surgery or medications will naturally differ according to the clinical meaning and relevance that different clinicians and others may ascribe to FD and so the issue of properly understanding the real significance of FD is not a simple or trivial one
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
