Representation of Spectral Profiles in the Auditory Systems
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This paper explores the question of how spectral profiles (such as spectral peaks) might be represented and perceived in the auditory system. Using profile analysis methods, we measured listeners' sensitivities to changes in spectral peak shapes that were uniquely described in terms of two parameters: a symmetry factor (SF) which roughly measure the local evenness or oddness of a peak, and a bandwidth factor (BWF) which reflects the tuning or sharpness of a peak. The effects of several manipulations on the perceptual thresholds were also tested; they include using different peak levels, varying spectral component densities, and randomizing the frequencies of the peaks. The basic result that emerges is that threasholds to changes in SF and BWF are constant regardless of peak shape. Thus, for the detection of SF changes, dSF thresholds are approximately constant regardless of a peak's SF and BWF. The only exception occurs towards the narrowest peaks where detection thresholds rise. For the detection of BWF changes, all dBWF/BWF thresholds remain constant regardless of peak shape. A fundamental conclusion arising from these data is that peak profiles are represented along two sensitive and largely independent axes: peak bandwidth and symmetry factors. More generally, however, it is argued that for an arbitrary spectral profile these two axes simply correspond to the magnitude and phase of a fourier transformation (or more precisely, of a Wavelet transformation) of the profile, closely analogous to the spatial frequency transformations described in the visual system. Further physiological and phychophysical evidence in support of this hypothesis is discussed.