Evidence for sensory-trace comparisons in spectral shape discrimination.

The ability of experienced observers to discriminate changes in the shapes of complex sound spectra was studied for three conditions. In one condition, the reference spectrum or "background" was fixed in spectral shape across each block of trials; in a second condition, the reference spectrum differed in spectral shape from trial to trial within each block of trials but was the same for the two presentations within a trial; and, in a third condition, the reference spectrum differed in spectral shape on every stimulus presentation. The variation in the reference spectrum was a random perturbation in the amplitudes of the tonal components comprising the complex sounds. The signal was an intensity increment to the center component (1000 Hz) of the complex and was present in one interval of each two-interval, two-alternative, forced-choice trial. The principal experimental manipulations were the degree of amplitude perturbation of the reference spectrum and the interval of time between the two stimulus presentations of each trial (interstimulus interval, ISI). The theory proposed by Durlach and Braida [J. Acoust. Soc. Am. 46, 372-383 (1969)] describing memory processes involved in the perception of sound intensity was used to explain the experimental results. As that theory was applied in this study, only the condition in which the stimuli were perturbed between trials showed evidence for comparisons based on "sensory traces," while the conditions in which the stimuli were fixed or were perturbed within trials were best explained by comparisons based on judgments relative to the stimulus context.