SPECTRAL RESPONSE PATTERNS OF AUDITORY-CORTEX NEURONS TO HARMONIC COMPLEX TONES IN ALERT MONKEY (MACACA-MULATTA)

1. The auditory cortex in the superior temporal region of the alert rhesus monkey was explored for neuronal responses to pure and harmonic complex tones and noise. The monkeys had been previously trained to recognize the similarity between harmonic complex tones with and without fundamentals. Because this suggested that they could perceive the pitch of the lacking fundamental similarly to humans, we searched for neuronal responses relevant to this perception. 2. Combination-sensitive neurons that might explain pitch perception were not found in the surveyed cortical regions. Such neurons would exhibit similar responses to stimuli with similar periodicities but differing spectral compositions. The fact that no neuron with responses to a fundamental frequency responded also to a corresponding harmonic complex missing the fundamental indicates that cochlear distortion products at the fundamental may not have been responsible for missing fundamental-pitch perception in these monkeys. 3. Neuronal responses can be expressed as relatively simple filter functions. Neurons with excitatory response areas (tuning curves) displayed various inhibitory sidebands at lower and/or higher frequencies. Thus responses varied along a continuum of combined excitatory and inhibitory filter functions. 4. Five elementary response classes along this continuum are presented to illustrate the range of response patterns. 5. 'Filter (F) neurons' had little or no inhibitory sidebands and responded well when any component of a complex tone entered its pure-tone receptive field. Bandwidths increased with intensity. Filter functions of these neurons were thus similar to cochlear nerve-fiber tuning curves. 6. 'High-resolution filter (HRF) neurons' displayed narrow tuning curves with narrowband widths that displayed little growth with intensity. Such cells were able to resolve up to the lowest seven components of harmonic complex tones as distinct responses. They also responded well to widthband stimuli. 7. 'Fundamental (F0) neurons' displayed similar tuning bandwidths for pure tones and corresponding fundamentals of harmonic complexes. This response pattern was due to lower inhibitory sidebands. Thus these cells cannot respond to missing fundamentals of harmonic complexes. Only physically present components in the pure-tone receptive field would excite such neurons. 8. Cells with no or very weak responses to pure tones or other narrowband stimuli responded well to harmonic complexes or wideband noise. Such cells are classified as 'wideband (WB) neuorns'. 9. 'Narrowband (NB) neurons' responded well to pure tones or other narrowband stimuli but not to harmonic complexes or wideband noises. 10 Neurons with high-frequency inhibitory sidebands, with more than one best-frequency (BF) region or other complicated response patterns have also been observed. 11. Neuronal selectivity for various tones with distinct pitches has never been reported. It is unlikely that pitch is tonotopically represented in A1 and other surveyed cortical regions. It remains to be determined if tonal pitch depends on large scale parallel processing in the principal auditory fields or a topographic representation in different cortical areas.