Segmented refraction of the crystalline lens as a prerequisite for the occurrence of monocular polyplopia, increased depth of focus, and contrast sensitivity function notches

Theoretical computations of modulation transfer functions (MTF’s) of the optical system of the human eye have shown that irregular aberration consisting of a small circular segment with refractive power slightly different from the surround introduces at higher spatial frequencies (>20 cpd) an enhancement of the retinal image contrast on flanks of the optimum-focus plane. When the pupil size is larger than 3 mm, enhancement is substantial; as a result, multiple foci appear at the affected, higher spatial frequencies and generate a greater depth of focus. The contrast enhancement also produces troughs on either flank of the optimum-focus plane. With slight coincident defocus (±0.5 diopter) of the retinal image of a sine-wave grating, notches in the MTF curves, with a contrast reduction in the intermediate frequency range of a factor of 2 to 3 and a low cutoff spatial frequency of ˜3 cycles/deg, are produced. In our theoretical study, multiple foci, monocular polyplopia, and increased depth of focus are implicated in the generation of contrast sensitivity function (CSF) notches. It is demonstrated that CSF notches of optical origin can extend to lower spatial frequencies (< 10 cycles/deg). As a result, before the presence of a CSF notch can be attributed to neurological abnormality, optical factors, including irregular aberrations, must be eliminated.