The auditory C-process of spectral profile analysis

Objectives: We here review the findings of several experiments, aimed at clarifying the functional role of the human auditory cortex in the processing of complex sound mixtures. Methods: Long-latency auditory evoked potentials were recorded to abrupt changes in the pitch or timbre of continuous complex tones (synthesized musical instrument sounds). Changes were made at intervals of 0.5–4.5 s while the subjects read a magazine. Results: The main response was a P1/N1/P2 complex which was maximal at the vertex and symmetrically distributed, consistent with origin in the supratemporal cortices of both hemispheres. To distinguish them from the conventional responses to brief pure tones, the potentials were named CP1 (c. 55 ms), CN1 (90 ms) and CP2 (165 ms). Responses to changes of pitch, where all the spectral components changed frequency, and to changes of timbre, where the frequencies remained the same but their energy levels changed, were very similar to one another. The response amplitudes were little affected by the magnitude of frequency changes in the range 6–100%, but were strongly influenced by the rate at which changes occurred (requiring at least 4 s for full recovery) and by the breadth of the changing frequency spectrum (the upper partials of the tone in sum contributing more than the fundamental). When the C-potentials were made refractory by a high rate of pitch changes (16/s) within a narrow frequency range, responses could still be elicited by infrequently interspersed changes of timbre. When the tones were split into their high and low partials, the responses to change in the two frequency bands combined roughly algebraically. Conclusions: The responses appear to represent a cortical process concerned with analysing the distribution of sound energy across the frequency spectrum (‘spectral profile analysis’). This may be an important stage in the analysis of complex sound mixtures and in the perception of sound quality.