Nonlinear alteration of transient vergence dynamics after sustained convergence

Disparity vergence responses before versus after repetitive vergence therapy in binocularly normal controls

This study sought to determine whether significant changes would be observed between vergence eye movements before and after 12 hr of repetitive vergence therapy (1 hr per day on different days) in subjects with normal binocular vision compared to controls. Disparity vergence responses from 23 subjects were studied. An assessment protocol that minimized the influence of the near dissociated phoria on the disparity vergence system was designed. The following parameters were quantified for the responses: latency, time to peak velocity, settling time, peak velocity, and accuracy (difference between the response and stimulus amplitudes). The following outcomes were observed when comparing the results after vergence therapy to the baseline measurements: (a) near point of convergence and near dissociated phoria did not significantly change (p > 0.15); (b) latency, time to peak velocity, and settling time significantly decreased (p ≤ 0.01); and (c) accuracy significantly improved (p < 0.01). Results support that vergence peak velocity is dependent on the subject's near dissociated phoria. The accuracy and temporal properties of vergence eye movement responses from subjects with normal binocular vision can be improved after vergence therapy. These methods can be utilized within future studies to quantitatively assess vergence therapy techniques for patients with binocular dysfunction.

A pilot study of disparity vergence and near dissociated phoria in convergence insufficiency patients before vs. after vergence therapy

Purpose: This study examined the relationship between the near dissociated phoria and disparity vergence eye movements. Convergence insufficiency (CI) patients before vergence therapy were compared to: (1) the same patients after vergence therapy; and (2) binocularly normal controls (BNC).

Methods: Sixteen subjects were studied—twelve BNC and four with CI. Measurements from the CI subjects were obtained before and after 18 h of vergence eye movement therapy. The near dissociated phoria was measured using the flashed Maddox rod technique. Vergence responses were stimulated from 4° symmetrical disparity vergence step stimuli. The peak velocity of the vergence response and the magnitude of the fusion initiating component (FIC) from an independent component analysis (ICA) were calculated. A linear regression analysis was conducted studying the vergence peak velocity as a function of the near dissociated phoria where the Pearson correlation coefficient was computed.

Results: Before vergence therapy, the average with one standard deviation FIC magnitude of convergence responses from CI subjects was 0.29° ± 0.82 and significantly less than the FIC magnitude of 1.85° ± 0.84 for BNC (p < 0.02). A paired t-test reported that the FIC and near dissociated phoria before vergence therapy for CI subjects significantly increased to 1.49° ± 0.57 (p < 0.04) and became less exophoric to 3.5Δ ± 1.9 exo (p < 0.02) after vergence therapy. A significant correlation (r = 0.87; p < 0.01) was observed between the near dissociated phoria and the vergence ratio of convergence peak velocity divided by divergence peak velocity.

Conclusion: The results have clinical translational impact in understanding the mechanism by which vergence therapy may be changing the vergence system leading to a sustained reduction in visual symptoms.

Effect of the accommodation-vergence conflict on vergence eye movements

With the broader use of stereoscopic displays, a flurry of research activity about the accommodation-vergence conflict has emerged to highlight the implications for the human visual system. In stereoscopic displays, the introduction of binocular disparities requires the eyes to make vergence movements. In this study, we examined vergence dynamics with regard to the conflict between the stimulus-to-accommodation and the stimulus-to-vergence. In a first experiment, we evaluated the immediate effect of the conflict on vergence responses by presenting stimuli with conflicting disparity and focus on a stereoscopic display (i.e. increasing the stereoscopic demand) or by presenting stimuli with matched disparity and focus using an arrangement of displays and a beam splitter (i.e. focus and disparity specifying the same locations). We found that the dynamics of vergence responses were slower overall in the first case due to the conflict between accommodation and vergence. In a second experiment, we examined the effect of a prolonged exposure to the accommodation-vergence conflict on vergence responses, in which participants judged whether an oscillating depth pattern was in front or behind the fixation plane. An increase in peak velocity was observed, thereby suggesting that the vergence system has adapted to the stereoscopic demand. A slight increase in vergence latency was also observed, thus indicating a small decline of vergence performance. These findings offer a better understanding and document how the vergence system behaves in stereoscopic displays. We describe what stimuli in stereo-movies might produce these oculomotor effects, and discuss potential applications perspectives.

The changes in phoria and convergence to divergence peak velocity ratio are correlated

PURPOSE

Phoria is used in the diagnosis of binocular dysfunctions, such as convergence insufficiency. A common treatment for convergence insufficiency is vision therapy, yet it is controversial whether the phoria changes as a result of rehabilitation. Phoria can vary due to the duration and/or the type of prior visual tasks. The variability potentially observed within phoria measurements leads to difficulties in confidently assessing changes within longitudinal studies. Hence, we propose to measure phoria and vergence peak velocities on separate days to evaluate whether a more robust measurement can be attained.

METHODS

Eleven subjects with normal binocular vision participated in two identical experimental sessions. Four-degree convergence and divergence steps stimulated from targets at far, middle, and near initial vergence positions were recorded using a limbus tracking system. Near dissociated phoria was measured after a set of step stimuli. The vergence ratio was defined as the convergence peak velocity divided by the divergence peak velocity. Linear regression analyses calculated the correlation between the phoria and the vergence ratio, and the difference between phoria measurements and vergence ratio measurements, recorded on different days.

RESULTS

Near dissociated phoria measurements and vergence ratios were highly correlated for all three initial vergence positions (r > 0.85, p < 0.005). The change in phoria was significantly correlated to the change in vergence ratio (r > 0.94, p = 0.0001). The slope of the linear regression analysis between the phoria and the vergence ratio using individual subject data was repeatable between the sessions (r = 0.99, p < 0.00001) despite large changes in phoria observed in a few individuals.

CONCLUSIONS

The change in phoria is significantly correlated to the change in vergence ratio. For longitudinal studies including the efficacy of vision therapies for patients with binocular dysfunctions, we suggest assessing both phoria and vergence velocities to reduce the variability potentially observed from different days.

Sustained fixation induced changes in phoria and convergence peak velocity

PURPOSE

This study sought to investigate the influence of phoria adaptation on convergence peak velocity from responses located at different initial vergence positions.

METHODS

Symmetrical 4° convergence step responses and near dissociated phoria (measured at 40 cm from the subject's midline) were recorded from six subjects with normal binocular vision using an infrared limbus tracking system with a haploscope. Two different sustained fixations (1° and 16° convergent rotation along the subject's midline) were used to study whether phoria had an influence on the peak velocity of convergence responses located at two initial vergence positions (1° or 'far' steps and 12° or 'near' steps).

RESULTS

Phoria was significantly adapted after a sustained fixation task at near (16°) and far (1°) (p<0.002). A repeated measures ANOVA showed that convergence far steps were significantly faster than the near steps (p<0.03). When comparing convergence steps with the same initial vergence position, steps measured after near phoria adaptation were faster than responses after far adaptation (p<0.02). A regression analysis demonstrated that the change in phoria and the change in convergence peak velocity were significantly correlated for the far convergence steps (r = 0.97, p = 0.001). A weaker correlation was observed for the near convergence steps (r = 0.59, p = 0.20).

CONCLUSION

As a result of sustained fixation, phoria was adapted and the peak velocity of the near and far convergence steps was modified. This study has clinical considerations since prisms, which evoke phoria adaptation, can be prescribed to help alleviate visual discomfort. Future investigations should include a systematic study of how prisms may influence convergence and divergence eye movements for those prescribed with prisms within their spectacles.

Vision therapy in adults with convergence insufficiency: clinical and functional magnetic resonance imaging measures

PURPOSE

This research quantified clinical measurements and functional neural changes associated with vision therapy in subjects with convergence insufficiency (CI).

METHODS

Convergence and divergence 4° step responses were compared between 13 control adult subjects with normal binocular vision and four CI adult subjects. All CI subjects participated in 18 h of vision therapy. Clinical parameters quantified throughout the therapy included: nearpoint of convergence, recovery point of convergence, positive fusional vergence at near, near dissociated phoria, and eye movements that were quantified using peak velocity. Neural correlates of the CI subjects were quantified with functional magnetic resonance imaging scans comparing random vs. predictable vergence movements using a block design before and after vision therapy. Images were quantified by measuring the spatial extent of activation and the average correlation within five regions of interests (ROI). The ROIs were the dorsolateral prefrontal cortex, a portion of the frontal lobe, part of the parietal lobe, the cerebellum, and the brain stem. All measurements were repeated 4 months to 1 year post-therapy in three of the CI subjects.

RESULTS

Convergence average peak velocities to step stimuli were significantly slower (p = 0.016) in CI subjects compared with controls; however, significant differences in average peak velocities were not observed for divergence step responses (p = 0.30). The investigation of CI subjects participating in vision therapy showed that the nearpoint of convergence, recovery point of convergence, and near dissociated phoria significantly decreased. Furthermore, the positive fusional vergence, average peak velocity from 4° convergence steps, and the amount of functional activity within the frontal areas, cerebellum, and brain stem significantly increased. Several clinical and cortical parameters were significantly correlated.

CONCLUSIONS

Convergence peak velocity was significantly slower in CI subjects compared with controls, which may result in asthenopic complaints reported by the CI subjects. Vision therapy was associated with and may have evoked clinical and cortical activity changes.

The influence of vergence adaptation on open-loop vergence dynamics

The objectives of this experiment were to measure the effect of sustained convergence on the open-loop vergence peak velocity and open-loop vergence amplitude, and to assess the correlation between changes in the phoria and changes in open-loop vergence peak velocity induced by sustained convergence. Subjects sustained convergence on a target that required 12 degrees of convergence for 5 minutes. Convergence and divergence movements of 4 degrees from the 12 degrees convergent position were measured before and after sustained convergence. Following sustained convergence, the open-loop vergence peak velocity and vergence amplitude both increased for convergence (regression slope=3.68, r=0.47). Vergence velocity and vergence amplitude both decreased for divergence (regression slope=1.76, r=0.36). After sustained convergence, a convergent shift in the phoria was noted in most cases. This shift correlated with changes in open-loop peak vergence velocity more for convergence (regression slope=1.1, r=0.33) than for divergence (regression slope=0.71, r=0.22). The results might be due to shifts in disparity detection brought about by the period of sustained convergence.

Stereopsis From a Performance Perspective

PURPOSE

A patient can demonstrate a poor stereoscopic test or task performance for reasons inherent within the test/task itself for reasons dependent on normal physiology common to all human subjects, and also for reasons that are outside of normal physiology and are unique or idiosyncratic to a particular person's visual system. This article reviews the literature for the first two reasons, but emphasizes the pathophysiology involved in the idiosyncratic and abnormal reasons.

RESULTS

Using control systems analysis, it is shown that deficits in stereoscopic performance can be explained by reference to the quantitative aspect of stereoscopic threshold and qualitative aspects such as speed of response, reliability-robustness, and strength of percept. The relationship between fixation disparity and stereopsis is seen to be central to this explanation.

CONCLUSIONS

Proceeding from diagnosis to treatment, control systems analysis offers physiologically based explanations for the corrective procedures necessary to ameliorate abnormal conditions. Several topics for applied research in stereopsis are suggested.