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Journal of Ophthalmology
Volume 2014 (2014), Article ID 274803, 13 pages
http://dx.doi.org/10.1155/2014/274803
Research Article

The Bifixation Field as a Function of Viewing Distance

1School of Psychology, The University of Queensland, St. Lucia, QLD 4072, Australia
2Centre for Vision Research and Department of Psychology, York University, Toronto, ON, Canada M3J 1P3

Received 30 November 2013; Accepted 23 April 2014; Published 28 May 2014

Academic Editor: Gernot Horstmann

Copyright © 2014 Philip M. Grove et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Hering reported that the area over which he could bifixate a target was smaller at near convergence distances than far convergence distances and predicted that in extreme horizontal gaze positions, the temporally directed eye lags behind the nasally directed eye. We tested these predictions using a subjective index of eye position. Experiment  1 confirmed that the bifixation field was significantly smaller at near convergence distances. When bifixation broke down at the near distance, the nasally directed eye lagged behind the temporally directed eye for all observers. At the far distance, the nasally directed eye preceded the temporally directed eye for four of six observers. Experiment  2 also confirmed that the bifixation field was smaller at near convergence distances but the nasally directed eye always lagged behind the temporally directed eye at the limits of the bifixation field. We confirmed Hering’s first prediction that the bifixation field is smaller at near convergence distances than at far ones. However, the majority of our results indicate that the nasally directed eye lags behind the temporally directed eye at the limits of the bifixation field, contrary to Hering’s prediction. We conclude that the eyes drift toward their tonic state of vergence when fusion breaks.