The neural basis of pitch and harmony in the auditory system

Abstract

Background in psychophysics Psychophysical experiments have shown that the human brain has a natural preference for harmonic sounds and relationships between musical tones. Even naïve listeners are able to distinguish harmonic from inharmonic musical intervals and, under many conditions, the pitches of octaves are confused even though their fundamental frequency differs by a factor of two. Our psychophysical investigations in Mongolian gerbils have shown that animals can also learn to differentiate musical intervals. Background in neurophysiology In order to understand pitch perception and the neuronal basis of harmonicity, we have to understand how neurons in the central auditory system process acoustic signals. It has been demonstrated that temporal processing mechanisms in the auditory brainstem are essential for these fundamental aspects of musical perception. Neurophysiological experiments have revealed that the periodicity of signal envelopes is crucial for the perception of pitch. Neuronal representations of envelope periods, which reflect the superposition of partials of harmonic sounds in the cochlea, are analysed by temporal correlation mechanisms in the central auditory system. These mechanisms also include processing of resolved partials. Aims In this paper we attempt to explain pitch perception and our preference for harmonic sounds on the basis of the temporal neuronal analysis and the spatial representation of pitch information in the brain. Conclusions As a result of cochlear analysis, frequency is mapped along a tonotopic axis in all auditory brain areas. Similarly, as a result of temporal analysis, periodicity is mapped from the midbrain to the auditory cortex. In each case, tonotopic and periodotopic axes are orthogonal to each other. One may say that spectral information, as an important aspect of timbre, is mapped along a first neural axis, periodicity (pitch) is mapped along the second neural axis of the auditory system. Finally, as a result of temporal analysis, neurons in the auditory midbrain show preferences for harmonically related sounds. The major function of the ventral nucleus of the lateral lemniscus (VNLL), which seems to have a structure reminiscent of the pitch helix of music psychology, appears to be to control and suppress these harmonic responses.