The auditory 'C-process': analyzing the spectral envelope of complex sounds

Objective: To examine the hypothesis that auditory evoked potentials (AEPs) to pitch and timbre change of complex harmonic tones reflect a process of spectral envelope analysis. Methods: AEPs were recorded to: (1)continuous tones of 'clarinet' timbre wh ose pitch abruptly rose or fell by 1 or 7 semitones every 0.5 or 1.5 s; (2) a cycle of 6 pitches changing every 0.5 s; (3) tones of constant pitch who se timbre (spectral envelope shape) changed periodically: (4) pitch change of high- acid low-pass filtered 'clarinet' tones. Results: The amplitudes of the 'change-N 1 (CN1) potential peaking at ca. 9 0 ms and the following CP2 were influenced to a far greater degree by the t ime interval between changes, than by the magnitude of the change or by the time interval between occurrences of the same pitch. Amplitudes were also strongly dependent on the number of partials present, irrespective of wheth er they were increasing or decreasing in energy. The algebraic sum of the r esponses to pitch change of high- and low-pass filtered tones closely appro ximated the response to the unfiltered tone. Conclusion: The rate-sensitivity of the responses cannot be explained by th e refractoriness of Frequency-specific 'feature detector neurones, but rath er of a process (termed 'C-process') which analyzes amplitude modulations a cross the spectral envelope, the contribution of different frequency bands combining linearly in the scalp-recorded activity. On-going computation of the spectral envelope shape may be an important factor in maintaining the p erceptual constancy of timbre. (C) 2001 Elsevier science Ireland Ltd. All r ights reserved.