April 2000
Volume 41, Issue 5
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Eye Movements, Strabismus, Amblyopia and Neuro-ophthalmology  |   April 2000
Effect of Sustained Cyclovergence on Eye Alignment: Rapid Torsional Phoria Adaptation
Author Affiliations
  • Matthew J. Taylor
    From the Departments of Neurology and
  • Dale C. Roberts
    From the Departments of Neurology and
    Ophthalmology, The Johns Hopkins University School of Medicine, Baltimore, Maryland.
  • David S. Zee
    From the Departments of Neurology and
    Ophthalmology, The Johns Hopkins University School of Medicine, Baltimore, Maryland.
Investigative Ophthalmology & Visual Science April 2000, Vol.41, 1076-1083. doi:
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      Matthew J. Taylor, Dale C. Roberts, David S. Zee; Effect of Sustained Cyclovergence on Eye Alignment: Rapid Torsional Phoria Adaptation. Invest. Ophthalmol. Vis. Sci. 2000;41(5):1076-1083.

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Abstract

purpose. To describe adaptive changes in torsional alignment that follow sustained cyclovergence in healthy humans.

methods. Eye movements were recorded binocularly from four healthy subjects using dual-coil scleral annuli. Cyclovergence movements were evoked over periods of 30 to 150 seconds using a stereoscopic display, presenting gratings of lines arranged horizontally, vertically, or at 45°, subtending angles of up to 48°. In- and excyclodisparities of 5° were introduced and removed in a single-step fashion. After stimulation, the time course and magnitude of the decay in cyclovergence was compared with the subject either in darkness or viewing a baseline stimulus of zero cyclodisparity.

results. As reported previously, the cyclovergence response to incyclodisparities was greater than to excyclodisparities. After sustained excyclovergence, however, in all subjects and in response to all orientations of the gratings, the decay in darkness was incomplete, implying an adaptive change in torsional alignment. In response to the horizontal gratings, for incyclovergence there was also an incomplete decay in darkness but to a lesser degree than in response to excyclovergence, and in only three of four subjects. The incyclovergence evoked by the oblique and vertical gratings was of small magnitude, and its decay was unaffected by the presence or absence of a visual stimulus.

conclusions. After sustained cyclovergence, its decay in the absence of a visual stimulus may be incomplete. The residual component may be interpreted, by analogy with horizontal and vertical vergence, as reflecting so-called phoria adaptation for torsional alignment.

The torsional orientation of the eye is usually determined solely by the position of the eye in its horizontal and vertical dimensions: Listing’s law constrains the degrees of freedom of movement of the eyes to two. Exclusively torsional movements, however, can be elicited in certain circumstances. In 1861, Nagel 1 first proposed that disjunctive torsional movement around the visual axis, cyclovergence, takes place when horizontal lines, observed in the stereoscope, are rotated in opposite directions. There was much controversy about the existence of these movements because subjective methods, none of which was entirely satisfactory, 2 had to be used to imply the presence of cyclovergence. It was not until 1975 that Crone and Everhard–Halm 3 demonstrated the existence of cyclovergence objectively using a photographic technique. Since then, cyclovergence has been well characterized (see Howard and Rogers 2 for a recent review). Many of its dynamic characteristics are qualitatively similar to those of horizontal and vertical vergence 4 5 6 7 although cyclovergence, similar to vertical vergence, is much slower than horizontal vergence 8 9 and usually 10 is not under voluntary control. 
Horizontal vergence can be elicited by horizontal disparity between the images on the retinas. If this retinal disparity signal is removed, either by occluding one eye 11 or by placing the subject in darkness, 8 the eyes return to their initial, resting horizontal position. The time taken for this decay is greater than the rise time in response to a disparity, 8 11 and if horizontal disparity vergence is maintained for periods of hours, then the decay on removal of retinal disparity is incomplete for several hours or until binocular fixation is reinstated. 12 The portion of the vergence response that is sustained after removal of the disparity stimulus has been attributed to the output of a slow fusional vergence response that leads to so-called prism or phoria adaptation. 13 It can be observed after exposure to disparities for periods as brief as 30 seconds. A similar phenomenon may be observed after exposure to vertical disparity. 14 15 16  
Investigation of the behavior of the cyclovergence system, by using subjective methods of estimating cyclovergence, suggests that, as is the case for the horizontal vergence system, the return to the initial alignment occurs more slowly in the dark than if a zero disparity stimulus is provided. In contrast to the horizontal system, the decay of torsion to its initial alignment is reportedly almost complete, even after prolonged (tens of minutes 17 ) exposure. Van Rijn et al. 18 reported a systematic difference in cyclovergence, measured using an objective technique, while a subject fixed on a dot with or without a structured background. This difference implies cyclophoria in the absence of disparity cues. Adaptation of cyclophoria, related to head roll, has been described recently in abstract form. 19  
In the present study, using an objective technique to measure the torsional position of the eye, the time course of decay of cyclovergence after a sustained exposure to cyclodisparity is described. The present study also provides an objective demonstration of rapid phoria adaptation in the cyclovergence system. 
Methods
Subjects
Four normal subjects (ages 22–54) who had no disorders of ocular motility and had normal stereopsis, indicated by identifying all elevated circles on the Randot test, participated in this study. Informed consent was obtained according to a protocol conforming to the Declaration of Helsinki and approved by the Johns Hopkins Joint Committee on Clinical Investigation. No spectacles were worn during testing. 
Display
Subjects were seated in the dark, their heads stabilized by a bite bar. Stimuli were presented dichoptically using a virtual-reality display mounted on the head (ProView 60; Kaiser Electro-optics, Carlsbad, CA), with a display brightness of 25 foot lamberts and display area of 36° (V) × 48° (H), composed of 480 × 640 pixels, the boundary of which was visible during display. This display was calibrated using a small video camera mounted on a calibrated gimbal to determine the angle subtended between lines forming a grid of known pixel spacing. The virtual-reality display was shown in preliminary experiments not to confound the measures of eye position. Recorded eye position was unaffected by whether the video display was on or off, and translation of the headset over many centimeters caused a variation in recorded eye position of, at most,± 0.16°. In the experiments, any headset movement was minimized by supports for the eyepieces that were fixed to the bite bar, and the head itself was stabilized with the bite bar. 
Three kinds of stimuli were used, all light-on-dark gratings rendered with antialiasing (3D Studio, Autodesk, San Rafael, CA). Horizontal gratings were used in all subjects: subtending 48° horizontally and 36° vertically, composed of eight equidistant horizontal, parallel lines, each subtending 0.4°, and a central fixation spot (a disc subtending 0.7°), located between the parallel lines. The horizontal disparity of the central fixation spot was such that 1.0° of vergence was required for binocular fixation. This was determined by comparing the vergence angle during viewing of the display and during viewing a distant target (124 cm) outside the display. The orientation of the grating presented to each eye could be altered independently, to allow control of cyclodisparity—defined as the relative orientation of the dichoptic images designated positive or negative according to whether the images are rotated top toward (in-) or top away (ex-), respectively, from each other. 
In additional experiments performed in two of the subjects, circular gratings of diameter 34°, composed of 10 equally spaced parallel lines of width 0.3°, with the lines either lying vertically or obliquely (running from left inferior to right superior, forming an angle of 45° to the vertical) were used. 
Eye Movements
Horizontal, vertical, and torsional eye movements were recorded using the magnetic field search coil technique with dual-coil annuli. 20 The field coil system consisted of a cubic coil frame of welded aluminum that produced three orthogonal magnetic fields with frequencies of 55.5, 83.3, and 42.6 kHz and intensities of 0.088 Gauss. The side length of the coil frame was 1.02 m. Amplitude-modulated signals were extracted by synchronous detection. The bandwidth of the system was 0 to 90 Hz. Maximum levels of peak-to-peak noise signals, measured behaviorally for a period of 5 seconds of steady fixation, were approximately 0.05° for horizontal eye position, 0.08° for vertical eye position, and 0.1° for torsional eye position. The voltage offsets of the system were zeroed by placing the dual search coils in the center of a metal tube that shielded the coils from the magnetic fields. Thereafter, the relative gains of the three magnetic fields were determined with the search coils on a gimbal system placed in the center of the coil frame. Further details of the calibration procedure are as described previously. 7 21 The positions of the eyes were calculated in rotation vectors, but eye movement data are expressed here in a Fick coordinate reference frame, so that torsion reflects rotation of the eye around its line of sight. For practical purposes, because the eyes were kept near the straight-ahead position, the choice of coordinate frame does not affect the results. 
Possible Coil Artifacts
It has been suggested that long-term drifts (>20 seconds) of torsional coil signals are mainly due to torsional slippage of the annulus around the line of sight. 18 21 Short-term torsional changes (in the range 10–20 seconds), however, more closely reflect actual torsional eye position. 22 This is supported by photographic recordings of eye position. 23 Accordingly, most analysis of changes in eye position restricted comparisons to within short periods (see description later). Another type of artifact in torsional eye position occurs when the coil abruptly slips during large saccades and blinks. We could usually identify these changes and ensure the data were interpreted appropriately. 
Protocol
In each trial, the subject was initially presented with a stimulus at zero cyclodisparity. After a period of 10 seconds, the stimulus was stepped to a 5° cyclodisparity, either in- or ex-, which was maintained for 30 seconds. At the end of this period the stimulus either returned to zero cyclodisparity (baseline response), or was extinguished so that the subject was in darkness without any disparity cues (open-loop response) for 40 seconds before reappearance of the target at zero cyclodisparity. Thus, there were four permutations (two cyclodisparities × two final states). Subjects were presented with the four-permutation sequence of (short) trials three times, with brief breaks approximately every 5 to 8 minutes. Pairs of (long) trials (one cyclodisparity × two final states) in which the cyclodisparity was maintained for 150 seconds were interleaved between the three shorter sequences: incyclodisparity trials first, excyclodisparity second. 
At the start (twice) and the end (once) of the experiment, subjects were shown the stimulus at zero cyclodisparity for 60 seconds, interrupted at 20 seconds by 40 seconds of darkness, to allow assessment of baseline cyclophoria. 
Two subjects also performed trials with two other designs using the horizontal gratings. In the first, after 10 seconds of zero cyclodisparity, the 5° cyclodisparity (either in- or ex-) was maintained for 30 seconds, after which the targets were extinguished for only 5 seconds before reappearing at zero cyclodisparity. In the second, 10 seconds of zero cyclodisparity was followed by periods of cyclodisparity (either in- or ex-) lasting in sequence, 5, 10, 30, and 60 seconds, separated from one another by 10-second periods of darkness, finishing with 10 seconds of darkness followed by target reappearance at zero cyclodisparity. 
Auditory cues were used to provide a 5-second warning period before changes in cyclodisparity. During this period, the subject was encouraged to blink gently to reduce the necessity for blinking in the period subsequent to the change in stimulus. Subjects could identify the onset of fusion and periods of diplopia by pressing a button. 
Analysis
Cyclovergence was calculated by subtracting the torsional position of the right eye from that of the left. Therefore, a relative incyclovergence is positive and a relative excyclovergence, negative. Vergence in horizontal and vertical axes was calculated similarly. Version was taken as the average of the positions of the two eyes. The eye position was sampled at 500 Hz, and commercial software (Matlab, The Mathworks, Natick, MA; Excel, Microsoft, Redmond, WA) used for analysis. The magnitude of the cyclovergence response to the 5° cyclodisparity was calculated by the difference between mean cyclovergence in the 100-msec period just before the offset of cyclodisparity and in a 100-msec period 5 seconds later in the period of exposure to the zero cyclodisparity stimulus (when cyclovergence had returned its baseline value). If the latter period was unrepresentative (e.g., a blink obscured the eye position measures) then the first preceding 100-msec window without artifact was used. The use of this method, rather than simply comparing the initial and final eye positions in the period of cyclodisparity, has the advantage of a briefer time between compared positions and thus less likelihood that coil slip would have occurred in the interim (as described earlier). 
A measure of adaptation was calculated, using a similar method, by recording the residual cyclovergence after 5 seconds in darkness and expressing this as a proportion of the magnitude of cyclovergence in the baseline condition. Thus, if the cyclovergence decayed equally in darkness and with the target present, it would have a residual of zero; if it did not decay at all, a residual of 1; and if some part of the cyclovergence response were sustained in darkness, it would have a residual between zero and 1. Because the decay of cyclovergence may not be complete within this 5-second period, this method may tend to overestimate the degree of adaptation, but other measures (see the Results section) support the idea that much of the sustained response after 5 seconds in darkness is due to adaptation. 
For clarity of presentation only, in Figures 1 2 and 3 relative cyclovergence is shown, in that the value of cyclovergence at the disappearance of the stimulus is offset so that the value in response to a zero cyclodisparity stimulus tends to zero over time. Likewise, the cyclovergence at time 0 is equal for open-loop (dark) and baseline conditions. For display of the data in the figures, eye position data were filtered digitally with a 3-dB point of 12.5 Hz. 
Results
Response to Cyclodisparity
Results are means ± SE. Subjects MT, DZ, and VJ showed similar magnitudes of response to the cyclodisparity stimuli. Subject SA showed a qualitatively similar pattern, although the amplitude of responses was usually less than 1.0°. For subjects MT, DZ, and VJ, both individually and as a group, the magnitude of the cyclovergence responses to the horizontal gratings was significantly greater (P < 0.05, two-tailed t-test) for incyclodisparity (2.65 ± 0.20°) than for excyclodisparity (1.70 ± 0.25°; also, see Fig. 4 ). For subject MT, the magnitude of the incyclovergence response to the horizontal grating (2.27 ± 0.35°) was significantly greater than to either oblique (1.24 ± 0.08°) or vertical (0.70 ± 0.12°) gratings (P < 0.05, Bonferroni t-test), but there were no significant differences among the magnitudes of excyclovergence. For subject DZ, the magnitude of incyclovergence to the horizontal grating (2.60 ± 0.25°) was significantly greater than that to the vertical (1.05 ± 0.19°; P < 0.05, Bonferroni), but not the oblique (1.70 ± 0.23°). 
For the horizontal gratings, all subjects were able to fuse the images when either the in- or the excyclodisparity was maintained for 150 seconds. Failure to fuse the excyclodisparity within the shorter 30-second exposure occurred on one (subjects MT and VJ), two (subject DZ), and six (subject SA) of eight occasions. Subjects MT, DZ, and VJ were also able to fuse the short incyclodisparity, but subject SA failed to fuse the short incyclodisparity on seven of eight occasions. When fusion occurred, the time taken to fuse the images was in each subject significantly less for incyclodisparities than for excyclodisparities (Bonferroni t-test, P < 0.05). Overall, the fusion times for the horizontal gratings were for incyclodisparities, 12.4 ± 1.0 seconds and for excyclodisparities, 25.8 ± 1.6 seconds. The vertical and oblique stimuli were fused readily by both subjects in whom they were used. 
Decay of Cyclovergence
The time course of restoration of zero cyclovergence when the stimulus stepped back from cyclodisparity to the baseline zero cyclodisparity was relatively rapid, smooth, and stereotyped; qualitatively symmetrical for recovery from in- and excyclovergence; and similar among subjects (see Fig. 1 , BL, baseline traces). This was not true, however, for the decay observed when the subjects were in darkness (Fig. 1 , OL, open-loop traces). Figure 1 shows the time course of decay over a 5-second period for two subjects, two durations of preceding cyclodisparity, and three grating types. 
For preceding excyclovergence (excy), for both subjects, and for all three gratings, when the subject was in darkness, a substantial proportion of the excyclovergence was maintained (see Table 1 ), implying cyclophoria adaptation. In contrast, adaptation after incyclovergence was more idiosyncratic. In both subjects, the cyclovergence response evoked by the oblique and vertical gratings decayed with the same time course and to the same extent, whether they were exposed to the baseline zero cyclodisparity stimulus or were in darkness (open-loop). The response to the horizontal grating varied between the two subjects. For subject MT, the decay after the short cyclodisparity (Fig. 1B , top left panel) was slower in the dark, and a sustained component of the cyclovergence remained at 5 seconds. For subject DZ, (Fig. 1A , top left panel) the rates of decline in the dark and to zero cyclodisparity were the same. After the long cyclodisparity, (Figs. 1A 1B , bottom left) for both subjects some cyclovergence persisted after 5 seconds in darkness. 
Table 1 summarizes the residual cyclovergence at 5 seconds after the short cyclodisparity with the horizontal gratings for all four subjects. Considering the responses for subjects VJ, SA, and MT there was residual cyclovergence at 5 seconds after both in- and excyclovergence, suggesting some cyclophoria adaptation. For subject DZ, residual cyclovergence was only present after excyclovergence. For subjects MT and DZ responses are also tabulated for the oblique and vertical gratings. Some residual cyclovergence can be seen after the exposure to the excyclodisparity but not to the incyclodisparity. 
This residual cyclovergence swiftly decayed on presentation of targets at zero cyclodisparity. Figure 2 shows, for subject MT, the changes in cyclovergence after moving from sustained cyclodisparity, through 5 seconds of darkness, to zero cyclodisparity. For stimuli in which there was no apparent residual cyclovergence (incyclovergence, oblique, and vertical grating stimuli) the appearance of the targets at zero cyclodisparity caused little or no change in cyclovergence. For stimuli to which there appeared to be adaptation (incyclovergence to the horizontal grating, and all excyclovergence responses), the presentation of the zero-cyclodisparity stimulus evoked a corrective cyclovergence. The same experiment was performed in subject DZ with similar results. 
As a control for the changes in cyclovergence measured in darkness after sustained exposure to a cyclodisparity, we looked for spontaneous changes in cyclovergence when subjects were simply maintained in darkness for 40 seconds without prior cyclodisparity and then exposed to the zero cyclodisparity stimulus. Table 2 shows the magnitude of changes in cyclovergence on the reappearance of the target at zero cyclodisparity. These data are a measure of the baseline cyclophoria. Changes in horizontal vergence and vertical version are also included because, depending on the orientation of Listing’s plane in a given subject, changes in horizontal vergence and the vertical position of the eyes in the orbit can themselves be accompanied by cyclotorsional components of opposite directions in each eye. 24 As Table 2 shows, in subject DZ, who exhibited the largest change in cyclovergence on reappearance of the zero cyclodisparity stimulus, there was some associated upward movement. On subsequent, more detailed re-examination of this subject’s phoria using repetitions of the same trial design, there was a change in cyclovergence of −0.8°, in horizontal vergence −0.2°, and in vertical version −3.3° (n = 9). These results suggest that at least some of the changes in cyclovergence in darkness after sustained exposure to a cyclodisparity are the result of a cyclophoria aftereffect, and cannot be solely attributed to changes in cyclophoria associated with changes in vergence or vertical eye position. 
The data in Figure 3 , from subject MT, also bear on the issue of the underlying cyclophoria by showing the changes in cyclovergence on presenting a zero cyclodisparity stimulus after 40 seconds of darkness for different prior cyclodisparities (short or long, in- or excyclodisparity) or no prior cyclodisparity. With a prior short incyclodisparity (IN), the change in cyclovergence on target reappearance is no different from Z (no prior cyclodisparity). In response to the short excyclodisparity, however, after 40 seconds in darkness there is still a sustained component of the excyclovergence, of approximately 0.5°. Similarly, after the long incyclodisparity (Fig. 3 , bottom) there is evidence of phoria adaptation as reflected in the difference between the IN and Z traces. 
Figure 4 shows the results of exposure to progressively longer cyclodisparities separated by periods of darkness for subjects MT and DZ. As described, incyclodisparities evoked quite large cyclovergence responses, which decayed in the intervening dark periods, although in the case of MT there appeared to be a proportion of the cyclovergence response sustained in darkness, particularly after the 30-second presentation of the incyclodisparity. Excyclodisparities evoked smaller vergence responses, but these were largely sustained in darkness, only correcting on the reappearance of the zero cyclodisparity stimulus at the end of the trial (145 seconds). In each subject, this experiment was performed twice with incyclodisparities preceding excyclodisparities and twice with ex- before in-, but the responses were similar in both permutations, providing no evidence of an effect of order on the responses. 
Discussion
The main finding of this investigation is the persistence, in darkness, of some of the change in cyclovergence induced by a preceding short period of sustained exposure to a cyclodisparity. This is a form of phoria adaptation for the torsional orientation of the eyes. In this section, we review the features of the initial cyclovergence response to the cyclodisparity, the subsequent change in cyclophoria, and the potential functional and clinical implications of cyclophoria adaptation. 
For the horizontal gratings, the amplitude and the asymmetry (being higher for incyclodisparity) of the cyclovergence response to the cyclodisparity observed here are in accord with responses to similar stimuli reported previously. 9 17 25 The time to fusion was also less, by approximately 50% on average, for the incyclodisparity stimuli, perhaps reflecting the larger motor response, and consequent smaller residual disparity, in this direction. We could not directly compare the responses to the three different orientations of the gratings, because the field of view of the vertical and oblique gratings was slightly smaller than that of the horizontal gratings, but the finding of a lower amplitude of response to the vertical 26 and oblique stimuli, given their reduced vertical-shear component, 27 and also the smaller angle they subtended, 28 29 are compatible with that described in the literature. The decay of cyclovergence on return to the zero cyclodisparity stimulus was also similar to that seen previously, with the return from both incyclovergence and excyclovergence being initially rapid and decreasing in velocity asymptotically as the eyes approached the position of rest. 9 17  
A detailed analysis of the decay of cyclovergence in darkness after sustained cyclovergence stimulation has not been reported previously using an objective method. Previous investigators, using subjective methods to assess retained cyclovergence at discrete times, reported that after long (30 minutes) exposure to cyclodisparity, decay was slower in open-loop (absence of disparity cues) than in baseline (zero cyclodisparity stimulus present) conditions, but still almost complete. 17 For all excyclovergence stimuli presented here, some cyclovergence persisted in open-loop (dark) conditions until the visual stimulus was restored. We observed a variable response for incyclovergence, however. In some circumstances its decay in open-loop (dark) and baseline (zero disparity) conditions was equal in both extent and rapidity, whereas in others its decay in darkness was slower. These differences between the pattern of decay in darkness of the responses to excyclovergence and incyclovergence could not be explained solely by any preexisting cyclophoria, which was much smaller than the changes in cyclovergence measured in darkness after the sustained exposure to the cyclodisparity stimulus. It is certainly possible, however, that any inherent cyclophoria may influence the relative response to in- and excyclodisparities in a given subject. Nevertheless, it can be speculated that the more persistent cyclovergence aftereffect in darkness after sustained ex- rather than incyclovergence could supplement the relatively less robust immediate, visually driven cyclovergence response to ex- compared with incyclodisparities. 
In the horizontal vergence system, incomplete decay of vergence in open-loop conditions after prolonged vergence has been attributed to a slow fusional vergence system, producing phoria or prism adaptation, and may be conceptualized as a slow integral controller in parallel with the output of a fast fusional controller. 30 It would be attractive by analogy to consider a similar model for the control of the cyclovergence system, but any satisfactory model of the cyclovergence system would also have to explain the dependence of gain on amplitude 5 and the marked asymmetry between incyclovergence and excyclovergence, seen not only in the amplitude at onset, 9 but also, as described here, in their decay in the dark. 
The presence of phoria adaptation in the cyclovergence system is consistent with the finding that cyclovergence is tightly controlled in normal subjects. 18 Although torsional diplopia—which has little influence on foveal function—does not present the same degree of subjective disturbance of binocular vision as does horizontal or vertical diplopia, torsional misalignment still can have perceptual consequences for the localization and orientation of objects (e.g., the perception of slant). 
There are also findings in the clinical literature relevant to maintaining torsional alignment. Acquired cyclotropia, for example, due to fourth nerve palsy, presents the retinas with disparate images of the world. Such patients exhibit adaptive responses to cyclotropia, although the responses are variable in nature and extent. 31 Torsional diplopia in casual viewing is far less frequent than that which can be elicited with Maddox Rods, 32 which in turn underestimates the presence of anatomic cyclotropia. 33 34 Much of the relief from torsional diplopia is achieved by sensory mechanisms. 31 32 Patients may retain a disparity-driven cyclovergence response, 35 yet it is reported that on monocular occlusion in the clinic setting, no decay of any adaptive cyclovergence is observed, whether on fixation photographs obtained with either eye fixating, or during scotometry. 31 Preoperative monocular occlusion maintained over days, however, may reveal some decay of a corrective cyclovergence. 36  
In summary, we have shown that phoria adaptation occurs to help sustain changes in torsional alignment just as it does for vertical and horizontal alignment. The error signal or signals that drive torsional phoria adaptation (cyclodisparity, monitoring of cyclovergence, or both), its relationship to vertical phoria adaptation, and its anatomic and physiological substrate remain to be determined. 
 
Figure 1.
 
Cyclovergence decay over a 5-second period from incyclovergence (incy) or excyclovergence (excy) in either baseline (BL, zero cyclodisparity stimulus visible) or open-loop (OL, dark) conditions. Relative cyclovergence in degrees on the ordinate, time from the end of presentation of cyclodisparity (0 seconds) in seconds on the abscissa. Shown is the mean of the three short trials (top) or the result from a single long trial (bottom), for each of the three gratings: horizontal (left), oblique (middle), vertical (right). (A) Subject DZ; (B) subject MT. When the OL and BL traces are separated at 5 seconds, cyclophoria adaptation may be inferred.
Figure 1.
 
Cyclovergence decay over a 5-second period from incyclovergence (incy) or excyclovergence (excy) in either baseline (BL, zero cyclodisparity stimulus visible) or open-loop (OL, dark) conditions. Relative cyclovergence in degrees on the ordinate, time from the end of presentation of cyclodisparity (0 seconds) in seconds on the abscissa. Shown is the mean of the three short trials (top) or the result from a single long trial (bottom), for each of the three gratings: horizontal (left), oblique (middle), vertical (right). (A) Subject DZ; (B) subject MT. When the OL and BL traces are separated at 5 seconds, cyclophoria adaptation may be inferred.
Figure 2.
 
Decay of cyclovergence from in- (incy) or excyclovergence (excy). Open-loop conditions (darkness) from time 0, with target reappearance at zero cyclodisparity at 5 seconds (arrow). Relative cyclovergence (degrees) against time (seconds). Subject MT. Note the change in cyclovergence on target reappearance, implying prior cyclophoria adaptation.
Figure 2.
 
Decay of cyclovergence from in- (incy) or excyclovergence (excy). Open-loop conditions (darkness) from time 0, with target reappearance at zero cyclodisparity at 5 seconds (arrow). Relative cyclovergence (degrees) against time (seconds). Subject MT. Note the change in cyclovergence on target reappearance, implying prior cyclophoria adaptation.
Figure 3.
 
Changes in cyclovergence over time for subject MT. Subject in darkness for 40 seconds before targets illuminated with zero cyclodisparity at time 0, previously short (top) or long (bottom) incyclodisparity (IN) or excyclodisparity (EX). Zero cyclodisparity (Z) is the mean response over three trials in which the darkness had been preceded by the zero cyclodisparity stimulus. Note the cyclophoria adaptation for the short EX and long IN trials. There are some artifacts in the long EX trace due to blinks, but the difference between Z and EX traces still implies cyclophoria adaptation.
Figure 3.
 
Changes in cyclovergence over time for subject MT. Subject in darkness for 40 seconds before targets illuminated with zero cyclodisparity at time 0, previously short (top) or long (bottom) incyclodisparity (IN) or excyclodisparity (EX). Zero cyclodisparity (Z) is the mean response over three trials in which the darkness had been preceded by the zero cyclodisparity stimulus. Note the cyclophoria adaptation for the short EX and long IN trials. There are some artifacts in the long EX trace due to blinks, but the difference between Z and EX traces still implies cyclophoria adaptation.
Figure 4.
 
Changes in cyclovergence with progressively longer periods of cyclodisparity separated by periods in darkness. Cyclovergence (continuous line) and stimulus cyclodisparity (broken line); (degrees) against time (seconds). Subjects MT and DZ, either incyclodisparities (incy, top two traces) or excyclodisparities (excy, bottom two traces). Periods without stimulus trace represent darkness. Cyclovergence relative to level at time 0 (onset of first cyclodisparity). Persistent cyclovergence in darkness implies cyclophoria adaptation.
Figure 4.
 
Changes in cyclovergence with progressively longer periods of cyclodisparity separated by periods in darkness. Cyclovergence (continuous line) and stimulus cyclodisparity (broken line); (degrees) against time (seconds). Subjects MT and DZ, either incyclodisparities (incy, top two traces) or excyclodisparities (excy, bottom two traces). Periods without stimulus trace represent darkness. Cyclovergence relative to level at time 0 (onset of first cyclodisparity). Persistent cyclovergence in darkness implies cyclophoria adaptation.
Table 1.
 
Cyclophoria Adaptation in Four Subjects
Table 1.
 
Cyclophoria Adaptation in Four Subjects
DZ MT VJ SA
Horizontal
IN 0.04 ± 0.13* , † 0.34 ± 0.08* 0.41 ± 0.12 0.67 ± 1.7
EX 0.70 ± 0.16* , † 0.44 ± 0.09* 0.35 ± 0.22 1.0 ± 0.21
Oblique
IN −0.01 ± 0.11 0.12 ± 0.22
EX 0.78 ± 0.25 0.65 ± 0.11
Vertical
IN −0.33 ± 0.35 −0.25 ± 0.48
EX 1.44 ± 0.32 0.78 ± 0.08
Table 2.
 
Baseline Cyclophoria, Horizontal Vergence, and Vertical Version in Four Subjects
Table 2.
 
Baseline Cyclophoria, Horizontal Vergence, and Vertical Version in Four Subjects
DZ MT VJ SA
Cyclovergence −1.59 0.03 0.39 −0.17
Horizontal vergence −2.1 0.91 1.43 0.44
Vertical version −8.01 −1.73 4.03 −2.52
The authors thank Heimo Steffen, who participated in preliminary aspects of these experiments. 
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Figure 1.
 
Cyclovergence decay over a 5-second period from incyclovergence (incy) or excyclovergence (excy) in either baseline (BL, zero cyclodisparity stimulus visible) or open-loop (OL, dark) conditions. Relative cyclovergence in degrees on the ordinate, time from the end of presentation of cyclodisparity (0 seconds) in seconds on the abscissa. Shown is the mean of the three short trials (top) or the result from a single long trial (bottom), for each of the three gratings: horizontal (left), oblique (middle), vertical (right). (A) Subject DZ; (B) subject MT. When the OL and BL traces are separated at 5 seconds, cyclophoria adaptation may be inferred.
Figure 1.
 
Cyclovergence decay over a 5-second period from incyclovergence (incy) or excyclovergence (excy) in either baseline (BL, zero cyclodisparity stimulus visible) or open-loop (OL, dark) conditions. Relative cyclovergence in degrees on the ordinate, time from the end of presentation of cyclodisparity (0 seconds) in seconds on the abscissa. Shown is the mean of the three short trials (top) or the result from a single long trial (bottom), for each of the three gratings: horizontal (left), oblique (middle), vertical (right). (A) Subject DZ; (B) subject MT. When the OL and BL traces are separated at 5 seconds, cyclophoria adaptation may be inferred.
Figure 2.
 
Decay of cyclovergence from in- (incy) or excyclovergence (excy). Open-loop conditions (darkness) from time 0, with target reappearance at zero cyclodisparity at 5 seconds (arrow). Relative cyclovergence (degrees) against time (seconds). Subject MT. Note the change in cyclovergence on target reappearance, implying prior cyclophoria adaptation.
Figure 2.
 
Decay of cyclovergence from in- (incy) or excyclovergence (excy). Open-loop conditions (darkness) from time 0, with target reappearance at zero cyclodisparity at 5 seconds (arrow). Relative cyclovergence (degrees) against time (seconds). Subject MT. Note the change in cyclovergence on target reappearance, implying prior cyclophoria adaptation.
Figure 3.
 
Changes in cyclovergence over time for subject MT. Subject in darkness for 40 seconds before targets illuminated with zero cyclodisparity at time 0, previously short (top) or long (bottom) incyclodisparity (IN) or excyclodisparity (EX). Zero cyclodisparity (Z) is the mean response over three trials in which the darkness had been preceded by the zero cyclodisparity stimulus. Note the cyclophoria adaptation for the short EX and long IN trials. There are some artifacts in the long EX trace due to blinks, but the difference between Z and EX traces still implies cyclophoria adaptation.
Figure 3.
 
Changes in cyclovergence over time for subject MT. Subject in darkness for 40 seconds before targets illuminated with zero cyclodisparity at time 0, previously short (top) or long (bottom) incyclodisparity (IN) or excyclodisparity (EX). Zero cyclodisparity (Z) is the mean response over three trials in which the darkness had been preceded by the zero cyclodisparity stimulus. Note the cyclophoria adaptation for the short EX and long IN trials. There are some artifacts in the long EX trace due to blinks, but the difference between Z and EX traces still implies cyclophoria adaptation.
Figure 4.
 
Changes in cyclovergence with progressively longer periods of cyclodisparity separated by periods in darkness. Cyclovergence (continuous line) and stimulus cyclodisparity (broken line); (degrees) against time (seconds). Subjects MT and DZ, either incyclodisparities (incy, top two traces) or excyclodisparities (excy, bottom two traces). Periods without stimulus trace represent darkness. Cyclovergence relative to level at time 0 (onset of first cyclodisparity). Persistent cyclovergence in darkness implies cyclophoria adaptation.
Figure 4.
 
Changes in cyclovergence with progressively longer periods of cyclodisparity separated by periods in darkness. Cyclovergence (continuous line) and stimulus cyclodisparity (broken line); (degrees) against time (seconds). Subjects MT and DZ, either incyclodisparities (incy, top two traces) or excyclodisparities (excy, bottom two traces). Periods without stimulus trace represent darkness. Cyclovergence relative to level at time 0 (onset of first cyclodisparity). Persistent cyclovergence in darkness implies cyclophoria adaptation.
Table 1.
 
Cyclophoria Adaptation in Four Subjects
Table 1.
 
Cyclophoria Adaptation in Four Subjects
DZ MT VJ SA
Horizontal
IN 0.04 ± 0.13* , † 0.34 ± 0.08* 0.41 ± 0.12 0.67 ± 1.7
EX 0.70 ± 0.16* , † 0.44 ± 0.09* 0.35 ± 0.22 1.0 ± 0.21
Oblique
IN −0.01 ± 0.11 0.12 ± 0.22
EX 0.78 ± 0.25 0.65 ± 0.11
Vertical
IN −0.33 ± 0.35 −0.25 ± 0.48
EX 1.44 ± 0.32 0.78 ± 0.08
Table 2.
 
Baseline Cyclophoria, Horizontal Vergence, and Vertical Version in Four Subjects
Table 2.
 
Baseline Cyclophoria, Horizontal Vergence, and Vertical Version in Four Subjects
DZ MT VJ SA
Cyclovergence −1.59 0.03 0.39 −0.17
Horizontal vergence −2.1 0.91 1.43 0.44
Vertical version −8.01 −1.73 4.03 −2.52
Copyright 2000 The Association for Research in Vision and Ophthalmology, Inc.
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