Vergence adaptation and the clinical AC/A ratio

Abstract

The accuracy of the gradient technique for measuring the clinical accommodative convergence to accommodation (AC/A) ratio is dependent upon obtaining veridical heterophoria measurements. However, previous studies have demonstrated that the sustained output of slow fusional vergence, which may take several minutes or even hours to decay, can bias heterophoria assessment. In the clinical setting, it is usual to estimate the AC/A ratio after just a few seconds of dissociation. This study investigated whether the slow fusional vergence response alters this crosslink ratio by comparing values of AC/A measured both before and immediately after a 1-hr period of continuous monocular occlusion. Sustained occlusion produced a significant change in near heterophoria in 10 out of the 21 subjects examined, but no significant change in AC/A was observed in either the adapting or non-adapting subgroups. Accordingly, while the sustained output of slow fusional vergence will influence clinical measurements of heterophoria, its presence does not alter the stimulus AC/A ratio significantly.

Comparison of vergence mechanisms between university students with good and poor sleep quality

“…Fusional vergences are the result of a motor response to a sensory stimulus brought on by the images of the object of regard drifting off one fovea, causing disparity and a subsequent corrective movement of both eyes to maintain fusion and prevent diplopia [9] . Heterophoria is the locus of intersection or deviation of the visual axes, measured with respect to the object of regard, in the absence of a fusional vergence response [10] . Vergence system anomalies, which are one of the binocular vision anomalies, occur when the fusional vergence values are too low to compensate for the heterophoria.…”