PERCEIVING MUSICAL PITCHES may require much less neural processing and occur at a lower level of the nervous system than previously thought, according to a new explanation, offering possible insights into designing better hearing aids. A musical note is defined mainly by its lowest pitch, known as its "fundamental frequency," but a note also typically contains higher-pitched "overtones" with frequencies that are some multiple of the fundamental. Even when the fundamental frequency is completely removed from a note, the overtones often allow listeners to perceive the missing fundamental anyway. Being able to perceive missing frequencies may explain why hearing a classical symphony through a tiny radio, which cannot satisfactory reproduce the lowest-frequency pitches, sounds reasonably faithful to a live version heard in a concert hall. Recent explanations of how we perceive "residue tones" require extensive amounts of neural processing, which can only take place in the cerebral cortex. However, researchers in Spain and Italy (Julyan Cartwright, Higher Council for Scientific Research, Spain, 011-34-958-243360, julyan@galiota.uib.es) propose that residue perception may result from a "nonlinear" process, involving the generation of frequencies that are not multiples of the original signal. Much more efficient than previous linear models, their proposed mechanism can take place at neural centers much earlier than the cerebral cortex. Specifically, they propose a "three-frequency resonance" that takes place in some neural processing center before the cerebral cortex, in which the electrical signals generated by two overtones stimulate a population of nerve cells to fire electrical signals at a third frequency different from those of the two overtones. Better understanding of pitch perception may lead to applications in medicine; it is already known, for example, that hearing aids which concentrate on making the fundamental frequencies more intelligible produce better results than simple amplification alone. (Cartwright et al., Physical Review Letters, 28 June 1999; sound samples at http://www.imedea.uib.es/~piro/PitchPage/).