The 13th Bielschowsky lecture: accommodation and convergence – ratios, linkages, styles and mental somersaults

A relatively fixed relationship between convergence and accommodation appears to be fundamental to binocular vision and its disorders. The accommodative convergence to accommodation ratio (AC/A) has traditionally been considered a major explanatory factor, but it fits only a small number of clinical diagnoses and fails to explain others. An alternative model, based on the different weights the visual system places on the main cues to target position in depth, fits concomitant strabismus, heterophoria and convergence and accommodation anomalies more comprehensively. Typical accommodation can be surprisingly variable and many intermittently-strabismic people still use binocular disparity as their primary visual cue, with blur and proximal/looming cues having less weight. The convergence-accommodation to accommodation (CA/C) linkage is therefore more important than the AC/A relationship in the majority of typical and atypical cases. Between-diagnosis style differences in the relative balance between these relationships can explain many clinical findings. Instead of “accommodation drives convergence”, or “convergence drives accommodation”, we should instead think of the visual and non-visual cues which drive both systems more independently and flexibly