The mechanism of octave circularity in the auditory brain
Before 2004 it was unknown if the human brain is hard-wired for the perception of octave circularity. In all advanced musical cultures the names for tones in scales are repeated, when the octave interval (frequency ratio 2:1) is reached. For example, in the European music system, the octave-spaced tones of 110, 220, and 440 Hz are all called A. Only in technical descriptions an additional octave number is added, like A2, A3, and A4. The universal practice of tone name circularity indicates an equally universal circularity in pitch perception. Octave circularity in pitch perception was also observed in the monkey.
It had earlier been known from the cat and the rabbit that the ventral division of the medial geniculate nucleus (MGN) of the thalamus is the only part in the auditory brain of mammals that has an octave architecture. Here, stacked neuron layers show a frequency mapping that progresses in jumps of one octave across layers and in a fine-graded scaling along layers.
The fiber network between the neuron layers is such that all signals triggered by octave-spaced tones, such as A2, A3, and A4, are likely to be pooled. They could then be trans-coded into an additional signal for a general A. This would explain why all tones called A, regardless from which octave, have a common pitch quality, a so-called pitch chroma. Thus, the auditory thalamus can be considered as the anatomical basis of our internal chroma map.
For a short summary of the main findings: The Octave - History of a Discovery
Evolutionary advantage: Complex harmonic sounds, such as animal calls, vowels, or violin tones, easily become octave ambiguous. If one of partials 1, 3, or 5 has a low amplitude in the sound spectrum, partials 2, 4, 6 and 8 can be mistaken by the pitch extracting mechanism as partials 1, 2, 3, and 4 of a different complex sound, whose pitch is one octave higher. This is the reason why even experienced musicians easily make octave errors. Apparently octave errors were irrelevant during the evolution of hearing. However, animals gained in pitch stability within a one-octave frame by simply collapsing all simultaneous pitch candidates, as determined in the auditory midbrain, onto one single and general octave frame at a subsequent stage of processing, i.e., in the auditory thalamus.
Data on pitch shift caused by the medical drug carbamazepine and on
statistics of inner-ear tones (SOAEs) now revealed
the first physiological evidence of hard-wired octaves in the human
Braun, M. (2006) A retrospective study of the spectral probability of spontaneous otoacoustic emissions: Rise of octave shifted second mode after infancy. Hear. Res. 215, 39-46. Abstract, ask for PDF (Main result three years later replicated)