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Purpose: To examine gain, speed, and temporal characteristics of initial and closed-loop components of vergence eye movements in young and elderly subjects., Methods: Vergence eye movements in 13 elderly and 10 young adults were examined. A table with light-emitting diodes was used to elicit vergence starting from near (convergence, 40-20 cm; divergence, 20-40 cm) or from far (convergence, 150-40 cm; divergence, 40-150 cm). Vergence eye movements were recorded with a video eye tracker or an infrared eye movement device., Results: There were no aging effects on the gain or peak velocity of vergence. Vergence duration was longer in elderly than in young adults, but only for the second, closed-loop components, driven by visual feedback. Elderly and young adults showed higher peak velocity and gain for convergence than for divergence., Conclusions: This observation is discussed in the context of physiological evidence of a robust convergence, rather than a divergence, generator at the brain stem level. Such a specific effect of aging on the duration of the closed-loop component is attributed to the reduced capacity of cortical processing of visual binocular disparity; slowing of vergence would allow good final accuracy.

Prior studies indicate that postural stability under binocular viewing is not better than under monocular viewing. This was tested at the distances of 145 cm [Fox, C.R. (1990). Some visual influences on human postural equilibrium: binocular versus monocular fixation. Perception and Psychophysics, 47 (5), 409-422] and 90 cm [Isotalo, E., Kapoula, Z., Feret, P.H., Gauchon, K., Zamfirescu, F., & Gagey, P.M. (2004). Monocular versus binocular vision in postural control. Auris Nasus Larynx, 31 (1), 11-17]. On the other hand, postural stability is known to decrease with distance increase. We re-examined the effect of binocular versus monocular viewing on postural stability at near and far distances (40 and 200 cm), and for both young (25.7+/-2.7 years), and old subjects (61.2+/-4.6 years). For both groups of subjects, proximity decreased the area of CoP, the standard deviation of antero-posterior sway (SDy) and the variance of speed. The group of elderly presented increased variance of speed at far distance in comparison with young subjects. The novel finding is the interaction between distance and viewing condition. Under binocular viewing, the area of CoP was significantly higher at far distance than at near; in contrast, monocular viewing produced similar CoP values at both distances. Increased instability at far distance when both eyes are viewing is attributed to decreased sensitivity to binocular disparity cues and to visual motion in depth resulting from body sway. Monocular viewing would provide similar stability at far and at near distance, because sensitivity to lateral visual motion, detected monocularly, decreases less with distance than sensitivity to binocularly detected motion in depth. Alternatively, such monocular viewing could increase subject's attention and lead to tighter postural control regardless of the distance.

Visual stabilization of posture is known to improve when the distance to target fixation decreases; this is attributed to increased angular size of retinal slip induced by body sway. At near distance, however, the eyes converge and efferent or afferent oculomotor signals could also be involved in posture stabilization. The goal of this study is to test whether the distance effect exists for both young and elderly and to test the role of vergence itself and of gaze position. Eighteen young (25.3 years) and 17 elderly (61.6 years) subjects were asked to fixate a target in quiet stance presented either at close (40 cm) or at far distance (200 cm); the vergence angle was 9 degrees and 2 degrees , respectively. For each distance, three gaze positions were studied straight-ahead (0 degrees ), 15 degrees up or down. We found a decrease in the surface of center of pressure (CoP), of standard deviation of antero-posterior and lateral body sway and of speed variance at near distance that occurs for both young and elderly. At far distance, the surface of CoP is smaller for 15 degrees up or down gaze in comparison with straight-ahead position, but at near distance there is no such gaze position effect. In an additional experiment, subjects fixated a target at far distance (200 cm) but prisms were used to cause the eyes to converge by an amount similar to that required for 40 cm viewing distance. The use of prisms decreased surface of CoP to values similar to those for natural near viewing distance. The effect of gaze position and of convergence (experiment with prisms) leads us to suggest that in addition to retinal slip, the ocular motor signals and perhaps related neck muscle activity are involved in postural stabilization. Finally, the elderly presented higher speed variance of CoP than the young subjects even though the surface itself was similar to adult values. We suggest that increment of speed variance is the first sign of senescence in postural control.