Abstract
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.
The vergence system is responsible for the convergence and divergence movement of the eyes, allowing the visual system to fuse stimuli moving in depth. In normal circumstances, the horizontal vergence system reduces foveal retinal disparity rapidly, and we seldom experience diplopia or visual confusion.
1 Unlike the ballistic nature of saccadic eye movements, vergence is modeled as a dual-mode control system. The vergence response can be dissected into transient and sustaining components.
2 3 4 5 The transient portion—the initial component—is assumed to be an open-loop control component of enhanced speed of a vergence response. The sustaining portion—the slow component—is assumed to be driven by a visual feedback, closed-loop control system; the latter provides fine-tuning of the response and enables the extraordinary accuracy seen in binocular fixation.
6 7 Therefore, vergence movements have identifiably different temporal and dynamic properties (e.g., latency, velocity, and duration) from other kinds of eye movements (e.g., saccades, smooth pursuit, vestibular ocular reflex). A dual mode of control also exists for the initiation of vergence because latency can be regular (>120 ms) or express (80–120 ms). In a previous study, we reported that regular latency of vergence increased with aging but that the capacity to produce “express divergence” was still preserved in the elderly.
8 In this study, we continue to report aging effects on other parameters of vergence, such as gain, velocity, and duration.
The horizontal fusional vergence system maintains correspondence of images on the retina with precision, but not perfection.
1 Collewijn et al.
9 reported that some subjects showed ambiguous vergence movements that achieved only half the required vergence to stimuli of 5° or 10° along the median plane. Cornell et al.
10 showed that the amplitude of the vergence tends to be hypometric, resulting in underconvergence for near fixations and overconvergence for distance fixations. However, Rambold et al.
11 reported that healthy adults can make accurate vergence (e.g., average amplitudes 6.8° ± 0.5° for a change in vergence amplitude of 7°). In our earlier studies, making use of light-emitting diodes (LEDs) in three-dimensional space to elicit vergence changes of 15° also showed good accuracy in children (14° ± 1.5°) and adults (14° ± 1.8°).
12 The accuracy of horizontal vergence was not influenced by initial stimulus position
7 or age.
12 13
The peak velocity of vergence increases with its amplitude (main sequence), as does peak velocity of saccades when it increases with saccadic amplitude.
1 9 14 15 16 Several investigations have shown faster peak velocity for convergence than for divergence.
7 9 17 18 Hung et al.
19 reported that the slope of the peak velocity compared with the amplitude curve was approximately twice as high for convergence as for divergence. Zee et al.
17 demonstrated that for the entire range of vergence amplitudes elicited between 2.5° and 10°, the mean values of peak velocities of pure convergence movements were faster than for pure divergence; the authors, however, did report an exception (for 1 of 4 subjects, for vergence steps of 5°). Alvarez et al.
7 also reported that convergence responses were generally faster than divergence responses; this was the case for three of their four subjects. However, Patel et al.
20 21 reported that peak divergence velocity was significant higher than peak convergence velocity after durations of sustained convergence of 6° (vergence changes between 4° and 6°). Erkelens et al.
15 studied four subjects and found that the differences between maximum speed of convergence and divergence were not systematic. Thus, the difference of velocity between convergence and divergence may be dependent on the subject. In addition, divergence peak velocity is influenced by the starting angle of convergence; if divergence starts from highly convergent angle, the divergence velocity is faster.
7 Interestingly, convergence movements do not show such dependence on the starting convergence angle, which may account for the apparent discrepancies between convergence and divergence.
7 Studies of peak velocity of objective vergence in the elderly are scarce. Until now, only one study
13 showed that the peak velocity of vergence response to a sudden change in target disparity was decreased in elderly compared with young subjects.
The last important parameter is the duration of vergence movements, or the total lasting time between the onset and the offset of vergence. As with the other parameters mentioned, there is some difference in duration between convergence and divergence movements. Hung et al.,
19 using step amplitudes of 2° to 16°, reported shorter durations for convergence (389–709 ms) than for divergence (489–1141 ms). In the study of Leeuwen et al.,
22 the duration of vergence along the median plane ranged between 150 and 650 ms, was slightly shorter for convergence than for divergence, and was not dependent on the vergence peak velocity. In our previous studies, adults and children experienced longer durations for divergence than for convergence.
12 Children younger than 8 years experienced longer durations of vergence than older children and adults but no difference in peak velocity.
The effects of aging on peak velocity and duration of vergence are unclear. The present study examines the gain, velocity, and duration of convergence and divergence, starting from different initial vergence angles corresponding to different distances in elderly and young adults. The ensemble of findings allows a more comprehensive physiologic characterization of vergence performances in elderly compared with young adults.
Each trial started with the lighting of a fixation LED at the center. The fixation LED stayed on for a random period between 1.5 and 2 seconds. In the gap task, there was an interval of 200 ms between the offset of the fixation point and the onset of the vergence target. The target LED was kept on for 1.5 seconds
(Fig. 1B) . In the overlap task, the fixation point remained illuminated for 200 ms after the target LED appeared. The target stayed on for another 1.5 seconds
(Fig. 1C) . Subjects were required to make a vergence to the other central target LED as rapidly and accurately as possible. A 500-ms period of darkness separated the trials. Subjects were instructed to use this period for blinks. The total mean length of each trial was approximately 4 seconds. In each block, divergence (20–40 cm or 40–150 cm) and convergence (150–40 cm or 40–20 cm) were interleaved randomly at equal rates. Each block contained 60 trials and lasted 4 minutes; each subject performed two blocks, one in the gap condition and another in the overlap condition.
A calibration sequence was performed at the beginning and the end of each block; the target made the following predictive sequence for each viewing distance: center, 5° to left, center, 10° to left, center, 5° to right, center, 10° to right, center; the target stayed at each location for 2 seconds From these recordings, we extracted calibration factors.
From the two individual calibrated eye position signals, we derived the disconjugate signal (left eye − right eye). Onset and offset of vergence eye movements were defined as the points at which the vergence velocity exceeded or dropped 5°/sec (
Fig. 2 , points “i” and “e”, respectively). The process was performed automatically by computer, and verification was made by visual inspection of individual eye position and velocity trace.
For convergence and divergence, we measured the peak velocity (value “v” at the velocity trace;
Fig. 2 ), the gain (ratio of the amplitude of vergence [“i” to “e” at position trace;
Fig. 2 ] over the amplitude of the target excursion in depth), the duration, the time between “i” and “e,” the amplitude, and the duration of acceleration phase and deceleration phase (“i” to “v” and “v” to “e,” respectively, on the position trace;
Fig. 2 ).
Eye movements in the wrong direction, with latency shorter than 80 ms (anticipation), longer than 800 ms, or contaminated by blinks were rejected. Seven percent of the trials for young adults and 9% of the trials for older subjects had to be rejected based on these criteria.
Multiple analysis of variance (ANOVA) was performed on individual mean values of each parameter with the between-subjects factor age (young, elderly) and the within-subjects factors target distance (near, far) and type of vergence (convergence, divergence). Post hoc comparisons were made with the least significant differences test.