The reciprocal actions of accommodation and convergence

Abstract

In normal binocular vision the faculties of accommodation and convergence operate in unison. When attention is directed from a relatively distant object to a nearer point, convergence of the eyes is stimulated so that correspondence of the retinal images is maintained. At the same time the need to refocus the retinal images provides a stimulus to accommodation. Thus the basic stimuli to the binocular adjustment for near vision are (a) the temporal displacement of the retinal images relative to one another, (b) the change in the vergence ofthe light reaching the retina. (The term 'vergence' is used throughout to refer to the degree of divergence or convergence of the light: it is measured in dioptres.) Donders (1864) showed that some latitude of action is possible between accommodation and convergence; there is an amplitude of accommodation corresponding to each convergence value, while on the other hand convergence can vary relative to a fixed condition of accommodation. In determining such amplitudes each function in turn requires to be held in a fixed state of action, while the other is varied. The need to provide a stimulus, in order to fix either one of the two mechanisms while the other is changed, leads to the question how far either function, being unstimulated, responds in sympathy to stimulated changes in the other.As it is necessary to avoid the direct stimulation of the mechanism when its induced reaction is being studied, the response of convergence to accommodation can be detected only during monocular fixation, since the need for binocular fusion is the stimulus to convergence. This relationship, accommodative convergence, is well known, and is commonly measured in clinical practice, but there has always been a question whether a similar relation holds in the opposite direction, i.e. the reaction of accommodation to convergence. Thus Christoferson & Ogle (1956) wrote: 'To what extent convergence-accommodation exists is uncertain because the difficulties of measurement have prevented

Ocular vergence-induced accommodation and its relation to dark focus

“…However, confirming the results of pilot work mentioned in the introduction, the target exerted a greater influence on monocular accommodation as it came closer than 50 em. This was true despite the fact that the target light was much smaller than that used by either Owens and Leibowitz (1975) or Fincham and Walton (1957). Because of these findings, it became obvious that the target parameters should be further explored, and this was the purpose of Experiment 2.…”

“…Rather, when viewing such a target monocularly in an otherwise totally dark room, their subjects' accommodation tended to revert to the DF position regardless of whether the target was presented at a distance: of 50 or 400 em! Because of the findings of Fincham and Walton (1957) and Owens and Leibowitz (1975) using a small target, it appeared to the present author that such a stimulus would be ideal for controlling convergence without directly affecting accommodation. The idea was to present the target in otherwise total darkness at various distances binocularly, require subjects to converge on the target, and measure accommodation.…”

“…However, the effects of ocular vergence typically have not been examined in such studies, making conclusions regarding the voluntary nature of accommodation difficult to evaluate. If, as several writers (e.g., Fincham & Walton, 1957; see Morgan, 1968, for a review) have suggested, changes in ocular vergence automatically produce changes in accommodation, it is possible that the voluntary response is actually ocular vergence (which is known to have voluntary components) and that accommodation changes just follow along. If such were the case, it would hardly be correct to describe accommodation as directly under voluntary control.…”

“…Fincham and Walton (1957) described the "need to eliminate the direct stimulation of accommodation ... while at the same time presenting to each eye sufficiently well-defined images to stimulate binocular fusion and to control convergence" (p. 489). Indeed, Fincham and Walton developed a procedure for doing so.…”

“…One important question concerns the tendency of changes in ocular vergence to influence accommodation. It is assumed by some researches that convergence can drive accommodation (Fincham & Walton, 1957;Morgan, 1968). In fact, Fincham and Walton have maintained that convergence is the primary stimulus to dynamic accommodation in normal binocular vision.…”