Permanent inner-ear tones develop under influence of descending neurons
Before 1996 it was unknown that the development of permanent weak tones in the human inner ear is influenced by descending signals from the auditory brain.
These tones, the so-called spontaneous otoacoustic emissions (SOAEs), have been measured since 1979 with sensitive microphones in the ear canal, and they appear in the majority of healthy ears. For the persons who have them they are normally not audible. As a rule, only a few tones are measured per ear, but in rare cases there can be about 20. The tone frequencies are stable over many years. They range from 0.5 to 10 kHz, the majority lying between 1.5 to 3 kHz.
The tones are generated by highly specialized cells in the inner ear, which act as both acoustic sensors and sound generators. Their function is to pick up weak sound and then to amplify it by cell body motion. Each of these cells is tuned to a specific frequency. For the sake of maximal sensitivity they have a self-regulated threshold that keeps them permanently on the brink of a spontaneous cell body motion.
Currently it is still unknown if such spontaneous cell body motion and tone generation are only harmless side effects of an extreme sensitivity, or perhaps even functional elements for a further increase in sensitivity. The lowering of sensation threshold by weak background noise, called stochastic resonance, is known from other systems of signal detection.
It could now be shown that the frequency distribution of these tones is not random. It is significantly biased in a way that can only be explained by the unique characteristics of the auditory midbrain (inferior colliculus).
Apparently the constant neural
signaling from this brain area down to the motor cells in the inner
ear is typically biased by the architecture and frequency map of this
central auditory nucleus. Such an anatomically determined descending influence
upon the inner ear is able to support the hearing of pitch of complex
tones, such as in speech and music.
Braun, M. (1997) Frequency spacing of multiple spontaneous otoacoustic emissions shows relation to critical bands: A large-scale cumulative study. Hear. Res. 114, 197-203. Abstract
Recent results on central auditory implications of SOAEs
Interesting links: Rémy Pujol