A pitch for decoding frequency more simply.

The music of Mozart is surely inspiring. However, one seldom hears even the most avid music enthusiast exclaim, "Ah, the 800-hertz fundamental in those violins is spectacular!"

One reason is that the human brain translates the numerical frequency of a sound into a qualitative characteristic called pitch--middle C, for example. How and where this handy translation takes place has long been murky. Now, researchers in Spain and Italy have proposed a new explanation for the brain's ability to sort out complex sounds--those that contain tones of different frequencies--such as music or voices. If the theory takes hold, it may challenge scientists' basic understanding of how the brain organizes hearing.

The aspect of a complex sound that largely defines its pitch is its lowest-frequency tone, or fundamental. Yet, people easily perceive a sound's pitch from the overtones when its fundamental is missing, as sometimes happens, for example, when they listen to a small radio.

Since the 1960s, researchers have developed several models to account for pitch perception, but all have shared the idea that the brain sorts through incoming overtones one by one until it has enough information to infer a missing fundamental. This process demands a lot of resources, and scientists believe it could only take place in the auditory cortex, the final and most powerful part of the brain's hearing system.

In the June 28 Physical Review Letters, physicist Julyan H.E. Cartwright of the Higher Council for Scientific Research in Granada, Spain, and his colleagues propose an alternative theory, in which pitch perception could occur earlier in the auditory process.

The team suggests that somewhere in the hearing system, two or more incoming overtones of different frequencies stimulate a specialized bundle of nerve cells called an oscillator, similar to structures found elsewhere in the brain. The oscillator then resonates at a frequency derived from the incoming tones. The researchers theorize that the brain uses that resonance frequency to determine the missing fundamental.

So far, Cartwright says, the theory appears to explain studies of how people perceive shifts in pitch as well as more elaborate models do. The new model, however, hasn't yet been applied to other data, some of which may provide tougher tests of the theory, he notes.

The researchers believe that if further data support the theory, the new model may lead to medical applications, such as better hearing aids. "The more we know about how we process sound, the more we can do to correct things when defects occur in the auditory system," Cartwright says.

Some scientists are skeptical of the theory, however. William M. Hartmann of Michigan State University in East Lansing observes that the model requires interaction between overtones. In experiments, though, researchers have observed that people perceive missing fundamentals best when the accompanying overtones travel through the auditory system along separate channels.

Study coauthor Diego L. Gonzalez of the Lamel Institute in Bologna, Italy, however, notes that signals that travel along separate nerve pathways through most of the auditory system may still converge and mingle at specialized sites along the way.

Acoustic psychologist Frederic L. Wightman of the University of Wisconsin-Madison argues that the new model doesn't explain how representations of musical tones could interact in this way in the neural depths of the auditory system, where signals operate not as waves but as all-or-nothing electrical spikes.

Cartwright responds that the visual, olfactory, and memory systems use neural oscillators to emit pulses at varying frequencies. "It's not saying anything outrageous to say these nerve cells would oscillate in response to incoming auditory signals," he says.