A comparison of two horopter criteria.

Both the binocular nonius and classical nonius criteria were applied to a group of subjects in order to compare the resulting horopters. Results indicated that monocular visual directions shifted along with changing binocular disparities when using the binocular nonius technique. This method constitutes a modification of the frontal plane criterion, since binocular portions still can be set equidistant to the fixation object. Comparing equidistance (binocular nonius) with equidirection (nonius) indicated that the 2 loci were essentially the same. Shifts in the nonius horopter induced by a small overall magnifier could not be predicted for 2 out of 7 subjects; one of these with aniseikonia and the other with intermittent exotropia. Irregularities in all horopter data could be accounted for by shifts in the vergence angles during the experiment.