Place and Time Coding of Frequency in the Peripheral Auditory System: Some Physiological Pros and Cons

Abstract

Evidence for and against classical theories of place and period mechanisms for the coding of frequency, and the modifications of the theories invoked to account for the pitch of residue and other types of stimuli, were examined in the light of physiological data. These included new data on the temporal discharge patterns of cochlear nerve fibers [cat] under stimulation with 2-tone complexes, harmonic and inharmonic 3-tone complexes and 5-tone complexes of differing relative phase. They showed that certain arguments against period coding of residue pitch were invalid. The interspike intervals in the discharge patterns of cochlear fibers under these conditions were consistent with the pitches heard. The classical period theory needs to be modified to take into account the normally relatively sharp frequency selectivity of cochlear fibers, and requires certain inefficiencies on the part of the central processor for pitch. Comparison of measures of cochlear fiber frequency selectivity with analogous psychophysical data in man, including those on the existence region of residue pitch, suggested that residue-type stimuli judged to be tonal in quality could be sufficiently resolved spectrally at the cochlear fiber level to serve as input to any of the current spectral pattern recognition mechanisms proposed for the pitch extraction of complex signals, and also, could generate patterns of temporal discharge reflecting enough waveform interaction between the harmonics to convey the pitch heard, because of the shape of the cochlear filters. (This conclusion might have to be qualified in the light of further physiological experiments on the second effect of pitch shift.) The present evidence, psychophysical and physiological, suggested the following synthesis: muscial interval recognition and relatively crude frequency discrimination can be accomplished by trained observers on signals where the frequency appears to be coded exclusively in terms of temporal information. However, the pitch quality of these signals was judged to be poor or absent. Signals, apparently coded exclusively by place mechanisms, while having tonality, allowed relatively crude frequency discrimination and judgment of musical intervals. With the possible exception of psychophysical data on the phenomenon of diplacusis, the present evidence could not exclude the possibility that the central pitch extractor mechanism utilized the place and period cues produced by pure-tone signals (below 5 kHz) and residue-type signals, both signals evoking strong pitch and fine acuity of frequency discrimination. The degree of salience of a signal''s pitch could well depend on the coherence of the 2 types of cue.