Evidence for “Pitch Neurons” in the Auditory Midbrain of Chinchillas
An important feature of our auditory system is its ability to detect voiced signals even under extremly noisy conditions (“cocktail party effect”). By spectral filtering, the cochlea improves signal-to-noise relations. However, this gives rise to the problem that information about a broadband signal gets scattered over various frequency channels in the auditory system. Psychophysic experiments show that for voiced or harmonic sounds the auditory system seems to make use of periodicity information to recombine this distributed information (Assman and Summerfield, 1990). In the auditory periphery, the envelope of complex tones is coded by phase locking (Shofner et al., 1996; Zhao & Liang, 1995; Ruggero, 1991). In the auditory midbrain this kind of temporal information is degraded and periodicity information is transformed into a rate-place code. In the inferior colliculus (ICC) best modulation frequencies (BMF = maximum of a modulation transfer function) of neurons are represented topographically, roughly orthogonal to the tonotopic organization (Langner, 1992). Modulation frequencies relevant for communication sounds (especially human speech) are in general below 1000 Hz. Therefore neurons in the inferior colliculus, that are tuned to low frequencies (characteristic frequency = CF < 1000 Hz) are likely candidates for spectral integrators of distributed activity representing a broadband signal. The aim of the present investigation was to look for such neurons with low CFs that may integrate particular periodicity information over a broad frequency range.
KeywordsModulation Frequency Auditory System Pure Tone Inferior Colliculus Dorsal Cochlear Nucleus
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