On the existence of a channel based mechanism for spectral envelope perception

Auditory processing appears to consist of a series of domain-specific filtering operations that separate complex stimuli into different auditory channels. This begins with tuning in the audio-frequency domain, followed by tuning in the temporal frequency (modulation) domain, and perhaps tuning in the spectral frequency domain. While a great deal is known about temporal envelope perception, relatively little is known about spectral envelope perception. One theory of spectral shape perception posits that the spectral envelope is processed by a physiologically-based filter bank. The idea of a spectral modulation filter bank is similar in concept to the revolutionary idea of spatial frequency channels in the visual system proposed in the early 1960's. However, to date, there is no behavioral evidence indicating that there are spectral modulation frequency channels in the auditory system. In this dissertation, several inter-related aspects of spectral envelope perception were examined. The preliminary studies reported here provide the first psychophysical evidence in support of a channel-based mechanism for spectral envelope perception. Further evidence of a channel-based mechanism for spectral envelope perception is obtained from Experiment 1 (the phase sensitivity experiment) which determined the influence of component phase on the detectability of two-component complex spectral envelopes. This experiment was modeled after a classic experiment performed by Graham and Nachmias (1971) in the visual domain using the concept of phase sensitivity. In Experiment 2, the phenomenon of spectral modulation detection interference was examined, in which the influence of spectral modulation in one audio-frequency region on the detection of spectral modulation in another audio-frequency region was explored. The results revealed that the presence of the interfering modulation produced maximal interference when the signal and interfering modulation were similar in frequency. This spectral modulation detection interference (SMDI) was similar in many respects to temporal modulation detection interference (MDI). SMDI may reflect a perceptual fusion of spectral envelopes and is consistent with an interaction within and/or among monaural and binaural domain-specific spectral envelope channels. In Experiment 3, spectral modulation masking period patterns were obtained for various suprathreshold contrast levels using simultaneous and forward masking paradigms. The results indicated that the auditory system selectively amplifies the spectral contrast for modulation frequencies near 1 and 2 cyc/oct.;The data reported here are consistent with the hypothesis of a channel-based mechanism for spectral envelope perception. Furthermore, the results obtained in the different experiments reported here indicate that the perception of spectral modulation is similar in many respects to the perception of temporal modulation in the auditory system and spatial modulation in the visual system.