Optometry - Journal of the American Optometric Association
Volume 81, Issue 9 , Pages 469-475, September 2010

Vergence adaptation in clinical vergence testing

  • Catherine McDaniel, O.D., M.S.

      Affiliations

    • University of Houston College of Optometry, Houston, Texas
    • Corresponding Author InformationCorresponding author: Catherine McDaniel, O.D., University of Houston College of Optometry, 505 J Davis Armistead Bldg., Houston, Texas 77204-2020.
    • ,
  • Nick Fogt, O.D., Ph.D.

      Affiliations

    • The Ohio State University College of Optometry and the Department of Ophthalmology, Nationwide Children's Hospital, Columbus, Ohio

published online 26 July 2010.

Article Outline

Abstract 

Background

The purposes of this investigation were to determine whether vergence adaptation occurs after vergence range testing and vergence facility testing and to determine whether vergence adaptation correlates with the results of these tests.

Methods

Thirty subjects participated in 3 testing sessions on different days. During each session 1 of the following was tested: base-out prism bar vergences, vergence facility (12 base-out/3 base-in binocular prism flippers for 1 minute), and 5 minutes viewing with 6 prism diopters of base-out prism. Before and after each test, the near phoria was measured using the modified Thorington method.

Results

There was no correlation between the amplitude of the vergence ranges and the amplitude of vergence facility. Significant vergence adaptation as indicated by an esophoric shift of approximately 3 prism diopters occurred in all testing sessions. The amplitude of vergence adaptation did not correlate with either the amplitude of the blur vergence range or vergence facility. There was a significant correlation between the amplitude of vergence adaptation and the amplitude of the break vergence range.

Conclusions

The lack of correlation between the blur vergence range and the vergence facility is not likely because of vergence adaptation. The lack of correlation between the break vergence range and the vergence facility may be in part caused by vergence adaptation.

Keywords: Prism adaptation, Disparity vergence, Vergence facility, Vergence ranges

 

Vergence movements rotate the eyes in opposite directions to allow for bifoveal fixation. Vergence eye movements often are classified into 4 subtypes.1, 2, 3 Accommodative vergence is the response of the vergence system to blur-driven accommodation. Proximal vergence is the vergence response to the perceived nearness of a target. Disparity vergence is driven by retinal disparity, and tonic vergence is the angle of vergence in the absence of any visual stimuli.2

Disparity vergence is an important source of vergence innervation.2 The disparity vergence system is made up of 2 components: fast disparity vergence and slow disparity vergence.4, 5 Fast disparity vergence is driven by retinal disparity and is made up of 2 portions. First, there is a burst movement that moves the eyes to the vergence position necessary to attain binocular fusion. This portion of the fast disparity system is termed the transient system.4 The fast disparity vergence system contains a neural integrator to maintain the eyes at a new vergence posture (sustained system).4, 5, 6, 7, 8 However, this integrator is inefficient, and neural innervation is leaked over time. As the innervation decreases, the vergence position cannot be maintained, and disparity is increased. This retinal disparity results in another fast disparity vergence movement.

Slow disparity vergence is neural innervation that builds up in response to the effort put forth by the fast disparity vergence system.8, 9, 10, 11, 12 Slow disparity vergence is more efficient at maintaining a particular vergence position than is the fast disparity vergence system. Thus, utilization of slow disparity vergence to maintain the eyes in a convergent posture is said to relieve stress on the disparity vergence system.

When slow disparity vergence increases, the heterophoria (fusion-free) position of the eyes may move toward the angle at which vergence is being maintained. This phenomenon is referred to as vergence adaptation, or prism adaptation.8, 9, 10, 11, 12, 13

North and Henson14 and Schor and Horner15 studied individuals with nonstrabismic vergence disorders. Both groups concluded that vergence adaptation was reduced in many patients with these disorders. North and Henson14 went on to state that orthoptic exercises could result in improved vergence adaptation, and in those cases the asthenopic symptoms associated with nonstrabismic vergence disorders were likely to be relieved.

The extent of the overlap between the neural pathways for fast disparity vergence and slow disparity vergence is not known.16, 17 If this overlap is substantial, then it is unlikely that subcategories of vergence dysfunction exist. Subcategories of vergence dysfunction might include those with only rapid vergence deficits (which could involve the transient portion of fast disparity vergence, proximal vergence, and voluntary/accommodative vergence18, 19) or those with only sustained vergence deficits (which would involve sustained fast disparity vergence and slow disparity vergence). Another subcategory might include individuals with deficits of both rapid and sustained vergence.

Further, because vergence adaptation may occur after very short periods of fusion (15 seconds or less),10, 20 it seems that most clinical tests will be influenced by vergence adaptation. Thus, even if subcategories of vergence dysfunction exist, it might be difficult to detect these variations because vergence adaptation will in most cases affect the test result.

Two clinical tests that may, however, have the potential to differentiate patients with a rapid vergence deficit from those with a vergence sustaining deficit (should these subcategories exist) are heterophoria with compensating vergence ranges21 and vergence facility.22, 23, 24

Vergence ranges are said to reflect an individual's ability to compensate for the heterophoria.21 Vergence ranges are measured by having patients view a distant or near target through progressively increasing amounts of prism. The prism is increased in a smooth and continuous fashion using Risley prisms, or alternatively the prism is increased in small (2 prism diopter) steps using a prism bar. The accommodative demand remains constant throughout the test. Patients are told to maintain fusion and to report when the target blurs or doubles.

In the experiment described below, it is important to note that the vergence range values are taken from the heterophoria position rather than from the target vergence demand. Clinically, these values often are recorded from the target vergence demand.

Vergence ranges measured from the heterophoria position reflect the “true” amplitude of disparity vergence.25 Vergence ranges measured from the vergence demand are termed the relative disparity vergence ranges.25 Relative vergence ranges can result in an underestimation of the true vergence range. For example, if the patient has a 15–prism diopter exophoria, then (relative) base-out vergence range as measured from the target vergence demand will be significantly underestimated.

Finally, when vergence ranges are measured from the heterophoria, these measurements will not be influenced by changes in vergence adaptation induced by pre-experiment activity (e.g., reading).13 This is because shifts in the heterophoria resulting from convergent vergence adaptation are accompanied by equal shifts in the “true” vergence ranges.13 For example, an increase in convergent vergence adaptation after reading could lead to an esophoric shift in the heterophoria. The relative vergence range, however, would be affected by this pre-experiment vergence adaptation.

It is not clear which vergence subsystems might be assessed with vergence range testing. Changes in vergence demand are made in very small steps or continuously during these tests, so the vergence subsystems that respond rapidly (fast disparity vergence, accommodative convergence, proximal convergence) are not likely to be significantly stressed at least in the case of convergence. This is supported by the results of Alvarez et al.26 These investigators found that convergence eye movements made from increasingly convergent postures do not show a change in peak velocity. Vergence peak velocity is thought to reflect the more rapid vergence subsystems.27 The vergence “stress” experienced near the ends of the vergence ranges is most likely the result of dissociation of accommodation and convergence or stress on the more sustained elements of the vergence system.

The more sustained elements of the vergence system (the sustained portion of the fast disparity vergence system and the slow disparity vergence system) would be needed to maintain the increasingly convergent or divergent posture required during the test. Support for these ideas is provided by several investigators including Alpern,28 Goss,29 and Rosenfield et al.30 All of these researchers showed that vergence range testing induces at least small amounts of slow disparity vergence innervation.

Vergence facility is another method by which the efficiency of the vergence system is assessed.22, 23 A single split prism or a binocular prism flipper is used. The prisms are oriented base-in on one side and base-out on the other side. Typical prism combinations used for vergence facility testing (all in prism diopters) include 12 base-out/3 base-in, 8 base-out/8 base-in, and 15 base-out/5 base-in.22, 23

In vergence facility testing, patients are asked to fixate on a target at a constant accommodative demand while viewing the target through one of the prisms (split prism) or through one side of the prism flipper. The task is to continue viewing the target through that prism until the target is single. Once fusion is attained, patients are told to look through the other portion of the split prism and to wait until fusion is once again obtained. The patient switches between the 2 prisms in this fashion for 1 minute, and the number of cycles completed is recorded.

Recently, Melville and Firth31 investigated the relationship between positive (convergent) fusional vergence ranges and vergence facility. They found no correlation between these values and speculated that the lack of correlation may have resulted because the 2 tests reflect different aspects of the vergence system.31

Specifically, they hypothesized that vergence ranges could reflect activity in the slow disparity vergence mechanism, whereas vergence facility might reflect the activity of the fast disparity vergence system. In any event, the lack of correlation between the vergence ranges and the vergence facility suggests that the rapid and sustained vergence systems do not overlap completely and that these clinical tests can be used to independently assess the vergence systems.

The purpose of this investigation was to determine whether vergence adaptation occurs after both vergence range and vergence facility testing. The second purpose was to determine whether vergence adaptation correlates with the results of these tests. If vergence adaptation correlates with vergence ranges but not vergence facility, then vergence adaptation might be at least partially responsible for the lack of correlation between the 2 clinical tests.

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Methods 

All subjects who participated in this experiment completed a consent form approved by The Ohio State University Biomedical Science Review Board and a Health Insurance Portability and Accountability Act (HIPAA) authorization form before the study. The entrance criteria were 20/20 visual acuity uncorrected or corrected with contact lenses in each eye, no strabismus or amblyopia, and no prior vision therapy training.

Thirty young subjects participated in 3 randomized testing sessions on different days. The average age of the subjects was 26.2 years. The age range was 23 to 35 years. Twelve of the subjects were men, and 18 were women.

The time during the day when testing was performed was variable. Therefore, the baseline level of vergence adaptation could have been higher if, for example, an individual had been reading before the testing session. However, by running each testing session on different days in random order, and by measuring the vergence ranges from the heterophoria, any influence of pre-experiment vergence adaptation was minimized.13

In each session, the modified Thorington was used to measure the heterophoria before and after clinical testing.32, 33 In this test, the 2 eyes are dissociated by placing a Maddox rod over 1 eye.32, 33 To measure the horizontal phoria, the patient is asked to tell the examiner at which number the red line (created by a penlight in the center of the test card) intersects the x-axis on the test card.32, 33

The heterophoria measurement after each clinical test was performed immediately upon completion of these tests. No binocular time was allowed between the end of the clinical test and the heterophoria measurement. The modified Thorington card was surrounded by a large white posterboard that eliminated surrounding fusional stimuli. The penlight was held in the center of the modified Thorington card while the subjects were seated and with the head secured in a chin and headrest set up 16 inches from the card.

The 3 testing sessions were as follows: base-out prism bar vergences (blur/break/recovery), vergence facility (12 base-out / 3 base-in binocular prism flippers), and 5 minutes viewing with 6 prism diopters of base-out prism. Studies show that at near, base-out vergence ranges induce similar amounts of vergence adaptation as do base-in vergence ranges.11, 34 Therefore, base-out vergence ranges were arbitrarily chosen for this study, but base-in vergence ranges are expected to yield similar results. The test target for the bar vergence and vergence facility sessions was a 20/30 vertical row of letters at 16 inches. A polarized bar reader was used as a suppression check for both tests.

For the bar vergence test, subjects were instructed to keep the target single and clear throughout the duration of the test. Instructions were to report when the target blurred (blur finding), doubled (break finding), and fused (recovery finding) into one again. Each of these values was recorded.

During vergence facility (tested for 1 minute), subjects were instructed to make the target single with each flip of the prism and to inform the examiner when they had done so. To monitor for ocular suppression, the subject was asked to report if any part of the row of letters disappeared or flashed on and off during either test. No instances of suppression were noted.

For the session in which the subject wore 6 base-out prism for 5 minutes, a video was viewed on a computer monitor at 16 inches. The subjects were instructed to keep the video image single and clear and to report to the examiner if they were unable to do so for the duration of the 5 minutes. The order of the sessions was generated randomly for each subject. Each testing session took place in the same laboratory room and was performed under the same lighting conditions.

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Results 

The results of the vergence range and vergence facility testing are shown in Table 1. Again, these values were taken from the heterophoria position rather than from the target vergence demand.

Table 1. Results of vergence range and vergence facility testing for each subject
SubjectVergence range – blurVergence range – breakVergence facility
111249.5
2172615.5
3203616
4182814.5
525359.5
6182315.5
7212617
8245115
994215.5
1010147.5
11∗408
12∗1510
13173315.5
14192413
15243917
16253513.5
17152415
18172720
1936.541.516
2016.520.516
21∗4417
22234814.5
2319.544.516
24262811.5
2517.521.510
26152417
2713.537.516
28203215.5
29204014
30182410

∗No blur finding was noted during vergence range testing.

In all of the statistical comparisons that follow, the blur finding was made equal to the break finding when no blur finding was measured. Further, the comparisons below were made using a nonparametric test statistic because these data were in some cases not normally distributed.

The magnitude of the base-out to blur vergence ranges and the magnitude of the base-out to break ranges were compared with the vergence facility results. There was no correlation between the base-out to blur findings and vergence facility (Spearman rank correlation coefficient, r = 0.03, P = 0.87). There was also no correlation between the base-out to break findings and the vergence facility (Spearman rank correlation coefficient, r = 0.27, P = 0.15).

For the vergence ranges, there was a significant correlation between the blur findings and the break findings (Spearman rank correlation coefficient, r = 0.57, P = 0.001) (see Figure 1). This comparison was performed to determine what factors (blur findings, vergence adaptation, or both) correlate with the break findings.

The difference in the heterophoria before and after vergence testing at each session was collected and analyzed. Any change in the heterophoria brought about during the test session represented the amplitude of vergence adaptation. The amount of vergence adaptation calculated in this way for all 30 subjects is shown in Table 2.

Table 2. Mean heterophoria and amount of vergence adaptation for each test condition for each subject
Change in heterophoria (esophoric direction)
SubjectMean heterophoria6 base-out prismVergence rangesVergence facility
1–1.00411
2–1.00123
3–6.33403
4–3.67343
5–5.00327
61.83632.5
73.00462
8–5.33487
92.33352
102.00534
110.00065
12–6.00414
13–3.675.575
140.67212
150.83022.5
16–4.67153
171.00145
185.00234
19–1.83333
20–2.50101
21–0.67412.5
22–2.83163
230.3328.53.5
24–8.33444
25–1.002.52.52
261.0020.51
272.3322.51
28–2.33485
29–0.502.564
30–8.17352.5
Mean ± SD–1.48 ± 3.382.78 ± 1.553.67 ± 2.473.25 ± 1.58

Post-test heterophorias were significantly different than pretest heterophorias for all conditions (Wilcoxon signed rank test, P<0.001 for all comparisons). These shifts in heterophoria are almost certainly not attributable to natural variation in heterophoria measurements. First, in all cases, the shift in heterophoria after the test was in the esophoric direction as expected. Second, when Howarth and Heron35 measured the distance phoria on 31 subjects on 5 separate occasions using a procedure very similar to that used in the current experiment, the mean of the standard deviations of these values was 0.499 ± 0.427 prism diopters; thus, the changes in this experiment were larger than those expected from natural variability in the measurements. There was no significant difference between the amplitude of vergence adaptation found in the 3 conditions (Friedman, P = 0.47).

The amplitude of vergence adaptation was compared with the vergence facility results and the vergence range results. There was no correlation between the vergence facility and the amplitude of vergence adaptation induced by vergence facility testing (Spearman's rank correlation coefficient, r = –0.30, P = 0.11). Further, there was no correlation between the base-out to blur vergence range and vergence adaptation induced by vergence range testing (Spearman's rank correlation coefficient, r = 0.25, P = 0.18). However, there was a significant positive correlation between the base-out to break vergence range and the amplitude of vergence adaptation brought about by vergence range testing (Spearman's rank correlation coefficient, r = 0.45, P = 0.013) (see Figure 2).

Finally, the difference between the blur and break was calculated. There was a significant relationship between this difference and the amplitude of vergence adaptation brought about by vergence range testing (r = 0.38, P = 0.044) (see Figure 3).

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Discussion 

As expected, vergence ranges did not correlate with vergence facility. This was true when the vergence facility result was compared with both the base-out to blur vergence range and to the base-out to break vergence range. The results suggest that 2 different, independent aspects of the vergence system will be tested with vergence ranges and vergence facility. The results provide more evidence that the neural pathways for fast and slow disparity vergence do not overlap completely and that clinical tests are available to independently probe each of these pathways.22, 31

A central question in this investigation was whether the presence of vergence adaptation could explain the lack of correlation between the vergence range and vergence facility results.

Equivalent amounts of vergence adaptation occurred with both vergence adaptation and vergence facility. However, the amplitude of vergence adaptation did not correlate with either the base-out to blur vergence ranges or the vergence facility results. Thus, the lack of correlation between these 2 tests is not likely to be the result of vergence adaptation. It is likely then, that at least a portion of the vergence range response is derived from the sustained portion of the fast disparity vergence system.4, 5, 6, 7, 8, 9, 10

On the other hand, the vergence subsystems with more rapid responses are likely to influence the results of vergence facility. One might expect vergence facility to primarily reflect the activity of the transient portion of the fast disparity vergence system.4, 5 This is because the test requires individuals to alternate between divergent and convergent positions and because fusion is not maintained for long periods at each vergence posture. Why then is it necessary to include all of the vergence systems that respond rapidly as potential inputs during vergence facility?

The answer lies in the assumption that the disparity detection range is large enough to respond to a vergence demand of 15 prism diopters or more.36 Such a large disparity detection range would be required for the vergence responses during vergence facility to be attributed entirely to the fast disparity vergence system.

The upper limit of disparity detection ranges is not known, raising the possibility that in addition to fast disparity vergence, patients use other types of vergence to complete the vergence facility task including proximal vergence18 and accommodative (voluntary) vergence.19

Of course, vergence adaptation did occur during vergence facility testing. It seems that vergence facility might ultimately be affected by vergence adaptation. It is known that vergence adaptation increases over time. Therefore, if the testing period for facility were increased beyond the 1-minute period usually used clinically, then the patient may begin to struggle to diverge through the base-in side of the flipper as convergent (base-out) vergence adaptation increases.

The amplitude of vergence adaptation did correlate to some extent with the base-out to break vergence range. The difference between the break and blur findings associated with the vergence ranges showed a mild (but statistically significant) correlation with the amplitude of vergence adaptation brought about by vergence range testing. Larger break findings are therefore associated to some extent with larger blur findings and with larger amounts of vergence adaptation.

The break finding may be influenced by vergence adaptation because individuals have maintained their increasingly convergent posture for a longer period than that associated with the blur finding. It is possible that vergence adaptation might contribute mildly to the lack of correlation between the base-out to break vergence range and vergence facility.

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Conclusion 

The vergence range and vergence facility measurements do not correlate. However, this lack of correlation cannot be attributed to vergence adaptation in the case of the blur vergence range finding. Instead, the blur finding associated with vergence ranges most likely reflects the efficiency of the sustained portions of the fast disparity vergence system, whereas vergence facility represents activity in the transient portion of the fast disparity vergence system or some combination of rapid vergence inputs.

The break vergence range finding was influenced to some extent by both the blur vergence range finding and the amplitude of vergence adaptation. It is possible that the lack of correlation between the break vergence finding and vergence facility is mildly related to vergence adaptation.

Patients with vision-related asthenopic symptoms who have normal compensating disparity vergence ranges should undergo vergence facility testing. The vergence facility testing should be limited to 1 minute, as previously recommended, to avoid confounding influences of vergence adaptation.24

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PII: S1529-1839(10)00277-0

doi:10.1016/j.optm.2010.01.012

Optometry - Journal of the American Optometric Association
Volume 81, Issue 9 , Pages 469-475, September 2010

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