Earl L. Green

Relationships Between Lateral Heterophoria, Prism Vergence, and the Near Point of Convergence

Numerous physiologic functions of the binocular neuromuscular mechanism have been measured for years with various kinds of testing equipment. Relatively few studies have been made to determine to what extent these various testing devices measure the same or different characteristics. This study was designed to investigate the relationships between lateral heterophoria, prism vergence, and the near point of convergence, as well as the relationship of these variables to age, sex, and refractive error. Other partially similar studies have been made previously. Haessler considered the relationship of heterophoria at 20 feet to refractive error and to the convergence angle. His sample consisted of 1,000 cases and was preselected to the extent that the following subjects were eliminated: (a) those with more than 0.5 prism diopters of hyperphoria; (b) those with more than 4.0 diopters of ametropia; and (c) those who had not worn correcting lenses for their ametropia for at least several months. Haessler employed the Maddox-rod test for heterophoria and justified his use of it on the basis of a study by Weymouth, who, with data from 12 subjects, concluded that the screen and parallax test, von Graefes prism diplopia test, and the Maddox-rod test were essentially equivalent at a testing distance of 20 feet. Weymouths interpretatation has since been confirmed. Haessler

A Center for Ocular Divergence: Does it Exist? *

The existence of a center for ocular convergence in the brain is well established. Evidence for the existence of a separate center for ocular divergence, presented by Bruce, is much less convincing. The convergence center has been located anatomically by Bender and Weinstein, but no anatomic location has been demonstrated for a divergence center. Arguments for the existence of a divergence center, short of its anatomic localization, come from many sources. Our purpose is threefold. First, we shall review these arguments and demonstrate that they do not necessarily require the existence of a divergence center. Second, we shall present an argument that the results of the disruption of fusion in the tests for heterophoria may be interpreted in terms of a single convergence center rather than in terms of two separate centers, one for convergence and the other for divergence. Third, we shall present the elasticity theory of divergence and arguments in its favor.