Auditory midbrain laminar structure appears adapted to f0 extraction: further evidence and implications of the double critical bandwidth

Abstract

The psychoacoustic 'critical bandwidth' (CB), e.g. approximately 2.6 semitones (= 0.22 octave) at 1.5-3 kHz, is known from many spectral integration phenomena. Cat data suggest that it is represented in the inferior colliculus (IC) (Ehret and Merzenich, 1985, Science 227, 1245-1247), where it is consistently related to the fibrodendritic laminae (Schreiner and Langner, 1997, Nature 388, 383-386). The recent discovery of the CB and the double CB (2CB) in the statistics of frequency spacing of spontaneous otoacoustic emissions (Braun, 1997. Hear. Res. 114, 197-203) has initiated further investigations of the novel phenomenon of 2CB. Meta-analysis of psychoacoustic valuation studies of pure-tone intervals again revealed the effects of CB and 2CB. Valuations showed a significant stepwise change with interval size: <CB unpleasant, CB-2CB pleasant, >2CB indifferent. Scrutiny of cat and human data indicated that for both species, at least in the midspectrum (1-3 kHz in humans), the tonotopic ranges within single IC laminae and the tonotopic distances between neighboring laminae may equal 1 CB (distances to second next laminae being 2CB). This unique architecture would provide the most economical neural convergence of period information from pairs of adjacent harmonic partials 3-6 of complex sound. The resulting summed postsynaptic potentials would thus contain a beat frequency equaling f0 of sound input and being detectable by the known neural behavior of characteristic periodicity response.