Defocused contrast sensitivity function in peripheral vision

Purpose Human peripheral detection performance is affected by optical factors such as defocus and higher order aberrations. From optical theory, we would expect defocus to produce local depressions (notches) in the contrast sensitivity function (CSF). However, such notches have not been observed in peripheral vision, and it is unknown whether human peripheral vision can detect local depressions (notches) in the CSF, such as those produced by monochromatic defocus when all monochromatic ocular aberrations are corrected. The purpose of the study was to identify such notches. Methods Participants were three adult emmetropes. Following full adaptive optics correction, on-axis and 20 degrees nasal visual field detection CSFs in monochromatic light were measured for the right eye with a 7 mm diameter pupil, both without and with +/- 2 D defocus, and with separate determinations for horizontal and vertical gratings. Defocused CSFs were compared with predictions based on theoretical modulation transfer functions. Results Notches in the monochromatic defocused CSFs were identified for peripheral vision at optically predicted spatial frequencies with other monochromatic ocular aberrations corrected, provided that there was adequate spatial frequency sampling. The spatial frequencies of notches were similar to those predicted from optical theory, but their depths (0.3 to 0.9 log unit) were smaller than predicted. Conclusion With fine spatial frequency sampling, notches were identified in defocused monochromatic CSFs when all other monochromatic ocular aberrations were corrected, both on-axis and at 20 degrees eccentricity. Unless recognised as such, notches may contribute to noise in through-focus detection measurements of peripheral visual performance.