Effects of Defocus on Monkey ( Macaca nemestrina ) Contrast Sensitivity

An observer's contrast sensitivity function (CSF) can be affected by degradation of either the optical or neural components of the visual system. In a continuing effort in this laboratory to separate these two components in infant and mature monkeys, we have measured behavioral CSF's with the retinal image defocused by as much as 6 D. With increased defocus, sensitivity decreases much more rapidly at high spatial frequencies than at low frequencies. Furthermore, we find that calculations of retinal image contrast, based upon available optical parameters of the monkey eye, underestimate the amount of optical defocus which still allows pattern detection by our monkeys at high spatial frequencies. Several possible explanations for this apparent conflict are discussed, including optical aberrations present in the primate eye, and the “spurious resolution” of defocused sinusoidal gratings.