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Abstract:
The pedestal or dipper effect is the large improvement in the detectability of a sinusoidal grating observed when it is added
to a masking or pedestal grating of the same spatial frequency, orientation, and phase. We measured the pedestal effect
in both broadband and notched noiseVnoise from which a 1.5-octave band centered on the signal frequency had been
removed. Although the pedestal effect persists in broadband noise, it almost disappears in the notched noise. Furthermore,
the pedestal effect is substantial when either high- or low-pass masking noise is used. We conclude that the pedestal effect
in the absence of notched noise results principally from the use of information derived from channels with peak sensitivities
at spatial frequencies different from that of the signal and the pedestal. We speculate that the spatial-frequency components
of the notched noise above and below the spatial frequency of the signal and the pedestal prevent off-frequency looking,
that is, prevent the use of information about changes in contrast carried in channels tuned to spatial frequencies that are
very much different from that of the signal and the pedestal. Thus, the pedestal or dipper effect measured without notched
noise appears not to be a characteristic of individual spatial-frequency-tuned channels.
