Proximal vergence and perceived distance.

Proximal vergence was measured haploscopically in response to a change in physical distance of a large screen of blurred horizontal lines. These responses were compared to proximal vergence measurements made using a clinical method. Perceived distance of the horizontal stripes was estimated according to the perceived size of a projected afterimage. For 18 normal observers the change in vergence measured haploscopically (in prism diopters per meter angle change in test distance) averaged 2.59 (SD = 1.44) binocularly and 2.11 (SD = 1.22) monocularly. Clinical measures averaged 2.88 (SD = 1.57) binocularly and 2.10 (SD = 1.27) monocularly. Haploscopic and clinical measures of proximal vergence correlated only when the clinical technique was modified by viewing through pinholes. Taking estimated perceived distance into consideration only partially reduced the differences between proximal vergence measured haploscopically and clinically as well as binocularly and monocularly. Both haploscopic and clinical measures of proximal vergence were sizeable, indicating that this vergence component can contribute significantly to the normal vergence response.