The effect of contrast randomisation on the discrimination of changes in the slopes of the amplitude spectra of natural scenes

It has been suggested (Tadmor and Tolhurst, 1994 Vision Research 34 541-554 ) that the psychophysical task of discriminating changes in the slope of th e amplitude spectrum of a complex image may be similar to detecting differe nces in the degree of blur. It has also been suggested that human observers may perform this discrimination by detecting changes in the effective cont rast within single narrow spatial-frequency bands, rather than by detecting changes in the slope per se which would involve the use of contrast inform ation across many different frequency bands. To distinguish between these t wo possibilities, we have developed an experiment where observers were aske d to discriminate changes in the spectral slope while different amounts of random contrast variation were introduced, with the purpose of disrupting t heir performance. This disruptive effect was designed to be particularly ma nifest if the observer really was performing a single-frequency-band contra st discrimination but to be unnoticeable if the observer was discriminating the change of slope per se. Our results imply that the observers do not us ually detect changes in contrast in just one narrow spatial-frequency band when they discriminate changes in the slope of the amplitude spectrum. Rath er, they must compare contrast between bands or, at least, they use contras t information from more than one band. However, for edge-enhanced (whitened ) pictures, there is some evidence to suggest that observers rely on contra st changes in only a limited low-spatial-frequency band.