Relative contributions of optical and neural limitations to human contrast sensitivity at different luminance levels

The relative contributions of optical and neural constraints on human contrast sensitivity as a function of spatial frequency were assessed for a range of luminances. First, the overall two-dimensional contrast sensitivity function (CSF) was obtained for three eyes at three luminances (0.05, 0.5 and 20 cd/m2) using a standard two-alternative forced-choice procedure. Second, the two-dimensional modulation transfer function (MTF) of the optics was measured for the same eyes and pupil sizes using a hybrid optical-digital method. The two-dimensional neural transfer function (NTF) was obtained as the ratio between the CSF and the MTF. Minor factors, such as the effect of cone aperture, were also considered in the computations. Our results show that at all luminances, the fall off in contrast sensitivity at high spatial frequencies is mainly due to optical factors. The losses in contrast sensitivity with decreasing luminance are, however, primarily due to noise limitations.