Consequences of monocular diplopia for the contrast sensitivity function

Though the human eye generally creates a single image on the retina, the literature contains many examples showing perceptual monocular diplopia. Previously, monocular diplopia resulting from astigmatic defocus has been demonstrated to cause a notch (local minimum) in the contrast sensitivity function (CSF). We examine Verhoeff's (1900) model which explains how monocular diplopia can occur through an interaction between defocus and common ocular aberrations. From the measured ocular transverse aberration function and from the measured monocular diplopia of three cyclopleged subjects we predicted multiple notches in the CSF with hyperopic spherical defocus. Monochromatic and polychromatic CSF were measured for vertical gratings with best refraction and with simulated myopia and hyperopia. Multiple notches in CSF were observed experimentally. Notches in the polychromatic CSF were smaller and broader than those found in the monochromatic CSF. Our aberration model was successful in predicting notches in the CSF with hyperopic spherical defocus. The implications for clinical measurement of CSF are discussed.