March 2009
Volume 50, Issue 3
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Eye Movements, Strabismus, Amblyopia and Neuro-ophthalmology  |   March 2009
Aging Effects on the Visually Driven Part of Vergence Movements
Author Affiliations
  • Qing Yang
    From the IRIS Physiopathologie de la vision et de la motricité binoculaire, Service d’Ophtalmologie-ORL-Stomatologie, Hôpital Européen Georges Pompidou, Paris, France.
  • Thanh-Thuan Le
    From the IRIS Physiopathologie de la vision et de la motricité binoculaire, Service d’Ophtalmologie-ORL-Stomatologie, Hôpital Européen Georges Pompidou, Paris, France.
  • Zoi Kapoula
    From the IRIS Physiopathologie de la vision et de la motricité binoculaire, Service d’Ophtalmologie-ORL-Stomatologie, Hôpital Européen Georges Pompidou, Paris, France.
Investigative Ophthalmology & Visual Science March 2009, Vol.50, 1145-1151. doi:10.1167/iovs.08-2474
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      Qing Yang, Thanh-Thuan Le, Zoi Kapoula; Aging Effects on the Visually Driven Part of Vergence Movements. Invest. Ophthalmol. Vis. Sci. 2009;50(3):1145-1151. doi: 10.1167/iovs.08-2474.

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      © ARVO (1962-2015); The Authors (2016-present)

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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. 
Materials and Methods
Subjects
Thirteen subjects between 60 and 93 years of age (mean, 70 ± 11 years) and 10 young adults between 20 and 32 years of age (mean, 25 ± 3 years) participated in the study. All young subjects were students from the laboratory and had normal visual acuity with or without correction. Elderly subjects were recruited from among senior laboratory workers or from the ophthalmology service in the hospital as they came for routine examination and spectacle correction. They had corrected visual acuity greater than 5/10, which is normal for their age. Cognitive performance was measured with the Mini Mental State Examination. 23 Their scores were better than 26 of 30 points. No subject showed visual, neurologic, or psychiatric disorders or received medication. For both groups, binocular vision was assessed with the TNO test of stereoacuity; all individual scores were normal (60 minarc or better). The investigation adhered to the tenets of the Declaration of Helsinki and was approved by the institutional human experimentation committee. Informed consent was obtained from all subjects after the nature of the procedure had been explained. 
Visual Display
The visual display on a horizontal table consisted of LEDs (each LED on 2.9 mm) placed at three viewing distances in the middle line, one at 20 cm from the subject, and the others at 40 cm and 150 cm from the subject. Fixation to 1 of 3 LEDs at the center required vergence angles of 17.1°, 8.6°, and 2.3° when the interpupillary distance was 6 cm for targets at 20, 40, and 150 cm, respectively (Fig. 1A) . In a dark room, the subject was seated in an adapted chair with a chin and frontal rest. The subject viewed binocularly and faced the visual display of the LEDs. Vertically, all target LEDs were placed at eye level. 
Fixation and Oculomotor Tasks: Gap and Overlap Conditions
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. 
Eye Movement Recording
Horizontal movements from both eyes were recorded simultaneously with an infrared eye movement device (IRIS; Skalar, Delft, Netherlands), installed in one hospital, for six young and four elderly subjects. Eye position signals were low-pass filtered with a cutoff frequency of 200 Hz and were digitized with a 12-bit analog-to-digital converter, and each channel was sampled at 500 Hz. The optimal resolution was approximately 0.1°. For the other four young adults and nine elderly subjects, eye movements were recorded with the rapid video eye tracker (installed in laboratory; Chronos, Berlin, Germany), which is based on high-frame rate CMOS sensors. 24 With the video eye tracker, eye position data were sampled at 200 Hz and stored on the hard disc for off-line analysis. The measurement resolution was less than 0.1°. The head was always stabilized by a chin rest, a forehead rest, or both. 
Statistical Analysis
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. 
Results
Because there was no difference for any parameter between the oculomotor tasks (gap and overlap), the data from these two conditions were regrouped. 
Gain
For convergence starting from 40 cm and for divergence starting from 20 cm, the required vergence change was 8.6°, whereas for convergence starting from 150 cm and for divergence starting from 40 cm, the required change was 6.3°. The gain of vergence was calculated by the ratio of vergence amplitude over the required amplitude. Figure 3shows the individual mean gain values of vergence with the standard error for each type of vergence, starting from near or far, in young and elderly conditions, respectively. Multiple ANOVA applied on the mean gain values showed a statistically significant effect of target distance (F 1,21 = 18.30; P < 0.001 [the gain was higher for vergence of farther targets than of nearer targets]) and a significant effect of vergence type (F 1,21 = 30.37; P < 0.001 [the gain was higher for convergence than for divergence]) but no effect of age (F 1,21 = 0.001; P = 0.97). A significant interaction was found between starting point and vergence (F 1,21 = 8.9; P < 0.01). The effect of starting point was greater for convergence than for divergence. 
Peak Velocity
Figure 4shows the individual mean peak velocity of vergence with the standard error for vergence starting from near or far in young and elderly subjects, respectively. In general, almost all subjects showed higher peak velocity for convergence than for divergence except a few subjects in some conditions (Fig. 4 , arrows). Multiple ANOVA applied on the mean values of peak velocity showed a statistically significant effect of the type of vergence (F 1,21 = 51.34; P < 0.001; higher peak velocity for convergence than for divergence) but no effect of age (F 1,21 = 0.02; P = 0.90) or of target distance (F 1,21 = 0.0005; P = 0.98). Further post hoc comparisons showed that the effect of the type of vergence was significant in young and elderly adults in all conditions (all P < 0.05). Thus, convergence showed higher peak velocity than divergence in young and elderly subjects, but there was no effect of aging on the peak velocity of vergence. Peak velocities from two elderly subjects (89 and 93 years) were comparable to those of the remaining elderly and young subjects. 
Duration
Figure 5shows the individual mean duration of vergence with the standard error for vergence starting from near or far in young and elderly subjects, respectively. Multiple ANOVA applied on the mean duration showed a statistically significant effect of age (F 1,21 = 6.76; P < 0.05 [longer duration for elderly than for young subjects]), the effect of target distance (F 1,21 = 4.57; P < 0.05) [longer duration for vergence of farther targets than that of nearer targets]) but no effect of task (F 1,21 = 0.12; P = 0.73) or of vergence type (F 1,21 = 2.35; P = 0.14). Further post hoc comparisons showed that the effect of age was statistically significant in all conditions (all P < 0.05); the effect of target distance was statistically significant for divergence only in elderly subjects (P < 0.05). A significant interaction was found between target distance and vergence (the effect of target distance on duration was statistically significant for divergence only [F 1,21 = 6.37; P < 0.05]). 
Asymmetry between Acceleration and Deceleration
For vergence eye movements, the acceleration phase (time to reach peak velocity) was shorter than the deceleration phase (time from peak velocity to the end of vergence), particularly for vergence requirements less than 10°. 9 In this study, the percentages of acceleration phase to total duration were approximately 32% and 28% for young and elderly, respectively. Multiple ANOVA applied on the mean duration of acceleration phase showed no statistically significant effect of age (F 1,21 = 1.5; P = 0.23), whereas the mean duration of deceleration phase showed a statistically significant effect of age (F 1,21 = 8.26; P < 0.01; Fig. 6 [longer duration of deceleration phase for elderly than for young subjects]). Further post hoc analysis showed such longer duration of deceleration phase for elderly than for young subjects in all conditions (all P < 0.05). In addition, multiple ANOVA applied on the amplitude of vergence showed no statistically significant effect of age for the amplitude of the acceleration phase (F 1,21 = 0.31; P = 0.59) or for the amplitude of the deceleration phase (F 1,21 = 0.61; P = 0.45). Thus, the prolongation of vergence duration for elderly subjects was caused by the prolongation of duration for the deceleration phase only. 
Discussion
We found no effect of age on vergence eye movements except on vergence duration, when the elderly showed a longer deceleration phase than the young adults. 
Good Vergence Gain and Speed in Elderly
Our data show no aging effect on the gain and peak velocity of vergence eye movements; similar observations have been made for saccades. 25 Our findings contrast those of Rambold et al., 13 who reported in elderly subjects a decrease of peak velocity for vergence eye movements. Perhaps this was related to the differences in experimental paradigms and methods used by the two studies. For example, Rambold et al. 13 used target steps of 7° between 3° and 10° vergence angle, whereas we used steps of 6.3° between 2.3° and 8.6° vergence angle and steps of 8.5° between 8.6° and 17.1° vergence angle. In addition, our data showed that convergence has higher peak velocity than divergence. In Rambold et al., 13 no significant difference was found in peak velocity between convergence and divergence. All these factors together could influence evaluation of the aging effect on the peak velocity of vergence. Moreover, our findings are consistent with physiological evidence for no aging effect of brain stem and peripheral structures on which peak velocity depends. Indeed, midbrain vergence burst cells display a discrete burst of activity just before and during vergence eye movements. 26 The profile of the burst (firing rate) is correlated with instantaneous vergence velocity, and the number of spikes (including burst-tonic cells) in the burst is correlated with the size of the vergence movements. Until now, histologic studies of human senescent brains have failed to identify neuronal degeneration in the midbrain or the motoneurons of the oculomotor nucleus, trochlear nucleus, or abducens nucleus. 27 28 29 Thus, similarity for gain and peak velocity of vergence between elderly and young adults is supported by histologic results that the vergence-burst generators and the nuclei containing motor neurons of the extraocular muscle remain relatively unchanged with aging. 
Longer Duration of Vergence in Elderly
To our knowledge, this is the first report of longer duration of vergence for elderly than for young adults. Interestingly, a developmental study of vergence showed longer duration in young children (younger than 8 years) compared with older children (8–12 years) and adults. 12 This may appear paradoxical because no similar development or degeneration change was observed for the amplitude or the peak velocity of vergence in children 12 or in the elderly (this study). Note that, in the case of vergence movements, peak velocity is an instantaneous parameter, but duration is long because the execution of vergence is under visual feedback control. Van Leeuwen et al. 22 thought that the duration of vergence is not dependent on the vergence peak velocity. Consistently, our studies in elderly (present study) and in young children 12 suggest independent control mechanisms for the two parameters. Hung et al. 30 proposed that vergence responses contain two components. One is an initial large, fast, open-loop component that is directly related to the underlying motoneuronal controller signal and that produces relatively high velocities. The other is a smaller, slower, closed-loop component that reduces the residual disparity to within neurosensory fusional tolerance. Analysis of our results showed that the prolongation of vergence duration in elderly subjects came from the prolongation of duration of the deceleration phase only. This suggested that the hypothetical first component (begin under open-loop control) is not changed; it is the execution of the second component (closed-loop control, after 150 ms in our study) that is slowed by aging. One possible interpretation of this is that execution of the second-phase movement is controlled in a closed loop by sensory processing of residual disparity. Such disparity processing could become slower because of reduced visual acuity, thereby leading to longer duration. Prolongation of duration could be the way to achieve good final accuracy similar to that in young adults. 
Difference in Peak Velocity between Convergence and Divergence
The difference in peak velocity between convergence and divergence is still a controversial issue. Our data showed that convergence has higher peak velocity than divergence for young and elderly subjects, in accordance with many previous studies, 7 17 18 19 though those studies used different stimuli for producing vergence eye movements. In fact, Hung et al. 30 assessed dynamics using the peak velocity-amplitude relationship, or main sequence, and clearly demonstrated that vergence eye movements are remarkably similar under a variety of stimulus conditions: in the instrument space using disparity-only stimulation or in the free space environment, where the disparity, blur, proximity, and other monocular and binocular depth cues interact and reinforce each another. Convergence showed higher peak velocity than divergence for disparity and natural stimulus conditions. Hung et al. 19 reported that the peak velocity could be approximately twice as high for convergence as for divergence under disparity-only stimuli. Kumar et al. 18 reported that under “natural” conditions, convergence shows higher speed than divergence. These results are also consistent with neurophysiologic findings. For instance, most neurons, lying dorsal and lateral to the oculomotor nucleus, which exhibit changes in tonic levels of activity proportional to changes in convergence, 26 31 were classified as convergence neurons because they showed increased activity during convergent eye movements. Divergence cells, which increase their activity during divergent eye movements, were found in smaller numbers in the same region. 31 32 Moreover, examination of the time courses of signal from the medial rectus motoneurons presented by Gamlin and Mays 33 revealed that the slope was higher for convergence than for divergence. 
Yet, some studies 13 15 reported that the difference in maximum speed of convergence and divergence were not systematic. These differences among investigators might have resulted from variation in the experimental protocol or target configuration. 19 The study by Alvarez et al. 7 showed that divergence peak velocity was influenced by the starting angle of convergence. If the eyes start from a highly convergent angle, divergence velocity is faster. Convergence movements, however, do not appear to show such dependence on starting vergence angle, which may account for the apparent discrepancies between convergence and divergence. In our study, we did not find such dependence of peak velocity of divergence on the initial position. Instead, we found that dependence on initial vergence angle varies from study to study. 
 
Figure 1.
 
(A) Three LEDs along the median line were placed on a horizontal table at 20 cm, 40 cm, and 150 cm in front of the subject to elicit convergence and divergence. (B) The paradigm used for the gap condition, the central fixation point, disappears 200 ms before the appearance of the eccentric target. (C) For the overlap condition, the central fixation point remains illuminated when an eccentric target appears.
Figure 1.
 
(A) Three LEDs along the median line were placed on a horizontal table at 20 cm, 40 cm, and 150 cm in front of the subject to elicit convergence and divergence. (B) The paradigm used for the gap condition, the central fixation point, disappears 200 ms before the appearance of the eccentric target. (C) For the overlap condition, the central fixation point remains illuminated when an eccentric target appears.
Figure 2.
 
Typical recordings of convergence (upper) and divergence (lower) from one young (left) and one elderly (right) subject are obtained by subtracting the eye position of the right eye from that of the left eye (LE − RE). The corresponding velocity trace is shown in gray and is scaled on the right. Arrows (i, e): beginning and end of vergence; v, peak velocity.
Figure 2.
 
Typical recordings of convergence (upper) and divergence (lower) from one young (left) and one elderly (right) subject are obtained by subtracting the eye position of the right eye from that of the left eye (LE − RE). The corresponding velocity trace is shown in gray and is scaled on the right. Arrows (i, e): beginning and end of vergence; v, peak velocity.
Figure 3.
 
Individual and group mean values of gain, with standard errors for convergence or divergence for nearer or farther targets for young and elderly subjects.
Figure 3.
 
Individual and group mean values of gain, with standard errors for convergence or divergence for nearer or farther targets for young and elderly subjects.
Figure 4.
 
Individual and group mean values of peak velocity with standard errors for convergence or divergence for nearer or farther targets for young and elderly subjects.
Figure 4.
 
Individual and group mean values of peak velocity with standard errors for convergence or divergence for nearer or farther targets for young and elderly subjects.
Figure 5.
 
Individual and group mean values of duration with standard errors for convergence or divergence for nearer or farther targets for young and elderly subjects; significantly longer duration for elderly than for young adults in all vergence responses.
Figure 5.
 
Individual and group mean values of duration with standard errors for convergence or divergence for nearer or farther targets for young and elderly subjects; significantly longer duration for elderly than for young adults in all vergence responses.
Figure 6.
 
Significant difference of duration between young and elderly for the deceleration phase of vergence but not for the acceleration phase of vergence.
Figure 6.
 
Significant difference of duration between young and elderly for the deceleration phase of vergence but not for the acceleration phase of vergence.
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Figure 1.
 
(A) Three LEDs along the median line were placed on a horizontal table at 20 cm, 40 cm, and 150 cm in front of the subject to elicit convergence and divergence. (B) The paradigm used for the gap condition, the central fixation point, disappears 200 ms before the appearance of the eccentric target. (C) For the overlap condition, the central fixation point remains illuminated when an eccentric target appears.
Figure 1.
 
(A) Three LEDs along the median line were placed on a horizontal table at 20 cm, 40 cm, and 150 cm in front of the subject to elicit convergence and divergence. (B) The paradigm used for the gap condition, the central fixation point, disappears 200 ms before the appearance of the eccentric target. (C) For the overlap condition, the central fixation point remains illuminated when an eccentric target appears.
Figure 2.
 
Typical recordings of convergence (upper) and divergence (lower) from one young (left) and one elderly (right) subject are obtained by subtracting the eye position of the right eye from that of the left eye (LE − RE). The corresponding velocity trace is shown in gray and is scaled on the right. Arrows (i, e): beginning and end of vergence; v, peak velocity.
Figure 2.
 
Typical recordings of convergence (upper) and divergence (lower) from one young (left) and one elderly (right) subject are obtained by subtracting the eye position of the right eye from that of the left eye (LE − RE). The corresponding velocity trace is shown in gray and is scaled on the right. Arrows (i, e): beginning and end of vergence; v, peak velocity.
Figure 3.
 
Individual and group mean values of gain, with standard errors for convergence or divergence for nearer or farther targets for young and elderly subjects.
Figure 3.
 
Individual and group mean values of gain, with standard errors for convergence or divergence for nearer or farther targets for young and elderly subjects.
Figure 4.
 
Individual and group mean values of peak velocity with standard errors for convergence or divergence for nearer or farther targets for young and elderly subjects.
Figure 4.
 
Individual and group mean values of peak velocity with standard errors for convergence or divergence for nearer or farther targets for young and elderly subjects.
Figure 5.
 
Individual and group mean values of duration with standard errors for convergence or divergence for nearer or farther targets for young and elderly subjects; significantly longer duration for elderly than for young adults in all vergence responses.
Figure 5.
 
Individual and group mean values of duration with standard errors for convergence or divergence for nearer or farther targets for young and elderly subjects; significantly longer duration for elderly than for young adults in all vergence responses.
Figure 6.
 
Significant difference of duration between young and elderly for the deceleration phase of vergence but not for the acceleration phase of vergence.
Figure 6.
 
Significant difference of duration between young and elderly for the deceleration phase of vergence but not for the acceleration phase of vergence.
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