Shifts in perceived spatial frequency of low-contrast stimuli: data and theory.

We studied shifts in perceived spatial frequency of foveally viewed stimuli as a function of contrast, using lower contrasts and/or a lower range of spatial frequencies than used previously by others. Reliable shifts in perceived spatial frequency were found, but the direction of the shift at low frequencies was not the same as at high frequencies for some observers. Models based on either light scatter or an early compressive nonlinearity (transducer function) probably cannot account for these results. A multiple spatial-frequency-channels model in which a nonlinear contrast-transfer function (CTF) follows the output of each channel, however, is consistent with most of the results for suprathreshold contrasts. We considered several versions of this latter model, differing in their assumptions about the peak frequencies and bandwidths of the underlying channels, the shape of the CTF, and the combination rule by which the outputs of the channels are labeled and combined according to a weighted average.