Distance adaptation depends upon plasticity in the oculomotor control system

Abstract

Maintaining binocular fixation on a target at 20 cm in the absence of secondary cues to distance produced changes in apparent distance and lateral phoria. Positive lenses of 0, .5, 2.0, 3.5, and 5.0 spherical diopters (SD) were used to manipulate the level of accommodative convergence in force during the period of maintained fixation. An inverse relationship was found between the stimulus to accommodation and the magnitude of the induced esophoria, the phoria being linearly related to an increase in apparent distance. The distance aftereffect obtained in the condition with the lowest net accommodative stimulus (Le., 0 D) equaled that typically produced by base-out prism adaptation with full secondary cues to distance available. In a second experi· ment, subjects walked through a well·lit hallway while viewing through a pair of 5 A base-out prisms. It was shown that increasing the stimulus to accommodation by adding negative lenses of 0, 1.5, 3.5, and 5.5 SD reduced the adaptive change in apparent distance, as well as the change in phoria produced by the conventional base-out prism adaptation paradigm. It was concluded that a change in the resting tonus of the disparity vergence system underlies such adaptation, rather than recalibration of the oculomotor cues to distance. Monocular exposure data indicated that a small change in the tonus control for the accommodative system may be present as well.

Components of prism adaptation in terminal and concurrent exposure: Organization of the eye-hand coordination loop

“…Previous studies of terminal versus concurrent exposure have also usually employed targets that were physically straight ahead but optically displaced (but see Freedman, 1968). Thus, the eyes could have assumed an asymmetric posture and at least part of the visual adaptation observed could consist in eye-muscle potentiation aftereffects (e.g., Ebenholtz, !974, 1976;Ebenholtz & Fisher, 1982) rather than mere central perceptual recalibration.f In the present experiments, when targets were used they were physically displaced so as to minimize asymmetric eye posture. Some of the previous studies (e.g., Uhlarik & Canon, 1971) measured adaptation with prisms in place (direct effects), rather than aftereffects with the prisms removed.…”

Does perceptual adaptation to telestereoscopically enhanced depth depend on the recalibration of binocular disparity?

“…DIStance tests were carried out with two types of targets: the threedimensional pyramid ( Figure 3) described above and a luminous two-dimensional target which was identical to that used by Ebenholtz and Fisher (1982) and which was seen in total darkness. A Green's refractor was adjacent to the table so that the refractor head could easily be positioned in line with a target mounted on the optical bench.…”

“…The subject's interocular axis, measured with a Titmus PO scope, and eye dominance were recorded prior to the pretests. Lateral phoria was then measured with the Green's refractor with a Maddox rod, using an intermittent exposure technique (Ebenholtz & Fisher, 1982). The target consisted of a red light-emitting diode (LED) with cross hairs, mounted at eye level at a distance of 33.3 em from the front surface of the cornea and seen in total darkness.…”

“…Accommodation and convergence therefore had to be dissociated to some degree in order to maintain both fusion and sharp focus." No methods currently exist for estimating how innervational balance between the accommodation and vergence controllers (see systems model, Ebenholtz & Fisher, 1982) is achieved under conditions in which the stimuli for accommodation and convergence are noncongruent and vary over time. It is entirely possible that the oculomotor demand created within these systems was conducive to a change in active innervation, such that the test form was experienced as both nearer and reduced in depth following exposure.…”

“…Changes within one or more of these systems may therefore be expected to lead to the modificationof stereoscopic depth perception. Given plasticity in the neural control of vergence and accommodation (Ebenholtz, 1983(Ebenholtz, , 1985Ebenholtz & Fisher, 1982), perceptual learning (Wallach, 1976) need not occur to account for adaptation to the telestereoscope.…”