DISCRIMINATION OF CHANGES IN THE SLOPES OF THE AMPLITUDE SPECTRA OF NATURAL IMAGES - BAND-LIMITED CONTRAST AND PSYCHOMETRIC FUNCTIONS

Thresholds were measured for discriminating changes in the slopes of t he amplitude spectra of stimuli derived from photographs of natural sc enes and from random-luminance patterns. The variety and magnitudes of the thresholds could be explained by a model based on the discriminat ion of the changes in band-limited local contrast. Different spatial s cales of local contrast (or different spatial-frequency bands of about 1 octave) were implicated for different reference spectral slopes; th e model implicated a lower frequency-band for stimuli with shallower a mplitude spectra. The implications of the model were tested experiment ally by using stimuli in which the spectra were changed within restric ted spatial-frequency bands. When the amplitude spectra of the test an d reference stimuli differed only within the implicated frequency band s, thresholds were affected little. However, when the test and referen ce spectra differed at all frequencies except those in the implicated bands, the thresholds were elevated markedly. The forms of the psychom etric functions for the discrimination task were entirely compatible w ith the hypothesis that the task relies upon the ability to discrimina te changes of contrast. The Weibull functions fitted to the data had s lope parameters (beta) in the range 1 to 3, compatible with discrimina tion of low (but suprathreshold) contrasts.