Reflexive Fusional Vergence and Its Plasticity Are Impaired in Convergence Insufficiency

Abstract

We compared the adaptive capacities of reflexive fusional convergence and divergence in 10 participants with untreated convergence insufficiency (CI) to 10 agematched binocularly normal controls (BNCs) in an effort to elucidate the functional basis of CI. METHODS. Vergence responses were monitored binocularly at 250 Hz using video-based infrared oculography, while single and double-step disparity stimuli were viewed dichoptically. The double-step stimuli were designed to induce an adaptive increase in the convergence or divergence reflexive fusional response dynamics. RESULTS. As expected, convergence responses in the CI population were significantly slower at baseline (BNC 12.0 ± 1.8°/s vs. CI 7.4 ± 2.5°/s; P < 0.001), but divergence response velocities were similar between groups (P = 0.38). Critically, we observed an impaired adaptive change in convergence peak velocities in the CI group when compared to BNCs (-18.2% ± 27.3% vs. 25.4% ± 9.8%; P < 0.001). Adaptive changes in reflexive fusional divergence responses were similar between groups (P > 0.5) and significantly less robust when compared to BNC convergence. CONCLUSIONS. The results support the hypothesis that the adaptive capacities of vergence are related to the strength of the underlying reflexive fusional response. Combined, the evidence suggests that the clinical condition of convergence insufficiency is underpinned by an underdeveloped or perturbated reflexive fusional vergence response mechanism. We relate these observations to different clinical guidelines for the management and treatment of this condition.

Underlying neurological mechanisms associated with symptomatic convergence insufficiency

“…Our group completed a pilot study demonstrating that young adult CI participants had significantly slower peak velocity compared to BNC 28 . Slower peak velocity in CI participants has also been observed by others in the young adult population 34 , the pediatric population 35 , concussed pediatric with CI population 36 , and mild traumatic brain injury with CI participants 37 . The novelty of this current work is that it is the first to demonstrate a correlation between vergence peak velocity and the right cuneus functional activity.…”

When virtual and real worlds coexist: Visualization and visual system affect spatial performance in augmented reality

“…Specifically, it was shown that the induced stress resulted in a deficit of binocular stability after only 10 minutes of exposure to images with conflicting binocular and focus cues ( Mon-Williams, Wann, & Rushton, 1993 ). Observed shifts of horizontal phorias ( Mon-Williams, Wann, & Rushton, 1993 ; Karpicka & Howarth, 2013 ) and altered near point of convergence ( Wee et al, 2012 ; Yoon et al, 2020 ) might be proposed as indicators of the increased load on the convergent fusional reserves ( Wann, Rushton, & Mon-Williams, 1995 ; Erkelens & Bobier, 2020 ). Moreover, it was demonstrated that the accuracy of spatial judgments in the stereoscopic environment correlated with convergent fusional reserves, near point of convergence, and stereoscopic acuity thresholds ( McIntire et al, 2014 ), and fusional reserves allowed the prediction of the realism of depth in stereoscopic displays ( Hibbard, Haines, & Hornsey, 2017 ).…”

Convergence Insufficiency Neuro-Mechanism Adult Population Study: Phoria Adaptation Results

“…The ability to adapt the fast fusional is correlated to the ability to adapt the slow fusional vergence system in BNCs, meaning those who can exhibit larger amounts of modification or change to the fast fusional system also exhibit larger amounts of change to the slow fusional vergence system and vice versa. 23,71 The ability to adapt the fast fusional system 72,73 and the slow fusional system 40,41 is reduced in patients with CI and improves post orthoptics/vision therapy. 30,31,43,55,[74][75][76][77] Prior studies of phoria adaptation report similar trends as those reported here.…”

“…11 , 22 , 23 Others suggest that binocular vision disorders, like CI and convergence excess, may result from underlying adaptive disorders, such as abnormal vergence or phoria adaptation. 23 – 32 …”