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 lumin ances (0.05, 0.5 and 20 cd/m2) using a standard two-alternative forced -choice procedure. Second, the two-dimensional modulation transfer fun ction (MTF) of the optics was measured for the same eyes and pupil siz es using a hybrid optical-digital method. The two-dimensional neural t ransfer function (NTF) was obtained as the ratio between the CSF and t he MTF. Minor factors, such as the effect of cone aperture, were also considered in the computations. Our results show that at all luminance s, the fall off in contrast sensitivity at high spatial frequencies is mainly due to optical factors. The losses in contrast sensitivity wit h decreasing luminance are, however, primarily due to noise limitation s.