SPECTRAL ENVELOPE CODING IN CAT PRIMARY AUDITORY-CORTEX - LINEAR AND NONLINEAR EFFECTS OF STIMULUS CHARACTERISTICS

Electrophysiological studies in mammal primary auditory cortex have de monstrated neuronal tuning and cortical spatial organization based upo n spectral and temporal qualities of the stimulus including: its frequ ency, intensity, amplitude modulation and frequency modulation. Althou gh communication and other behaviourally relevant sounds are usually c omplex, most response characterizations have used tonal stimuli, To be tter understand the mechanisms necessary to process complex sounds, we investigated neuronal responses to a specific class of broadband stim uli, auditory gratings or ripple stimuli, and compared the responses w ith single tone responses, Ripple stimuli consisted of 150-200 frequen cy components with the intensity of each component adjusted such that the envelope of the frequency spectrum is sinusoidal. It has been demo nstrated that neurons are tuned to specific characteristics of those r ipple stimulus including the intensity, the spacing of the peaks, and the location of the peaks and valleys (C. E. Schreiner and B. M. Calho un, Auditory Neurosci., 1994; 1: 39-61). Although previous results sho wed that neuronal response strength varied with the intensity and the fundamental frequency of the stimulus, it is shown here that the relat ive response to different ripple spacings remains essentially constant with changes in the intensity and the fundamental frequency. These fi ndings support a close relationship between pure-tone receptive fields and ripple transfer functions. However, variations of other stimulus characteristics, such as spectral modulation depth, result in non-line ar alterations in the ripple transformation. The processing between th e basilar membrane and the primary auditory cortex of broadband stimul i appears generally to be non-linear, although specific stimulus quali ties, including the phase of the spectral envelope, are processed in a nearly linear manner.