AFTER well over a century of argument about how our ears and brain discriminate pitch, one team claims to have made a significant step towards resolving the debate.
Our ears convert sound waves into vibrations in the basilar membrane inside a snail-shaped structure called the cochlea. Here hair cells detect the vibrations and send signals to the brain via the auditory nerve. One theory is that the brain works out pitch simply from the timing of signals from nerves, which varies depending on the frequency of a sound. Models based on this timing theory are currently in vogue.
The competing theory says that perceiving pitch depends on where in the cochlea nerve signals come from as well as the timing of nerve signals, based on the fact that different nerve cells detect different frequencies. The start of the cochlea picks up very high-frequency sounds, with more distant areas responding best to progressively lower frequencies.
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A clever trick that fools a high-frequency area of the cochlea into producing low-frequency signals has now been used to test which theory is correct. The method involves using a high-frequency carrier wave to stimulate the part of the basilar membrane tuned to such vibrations. But the wave is broken into tight pulses to make the nerves fire as if hit by a low-frequency tone, an effect that is supported by as-yet unpublished experiments.
And in recent experiments with such 鈥渢ransposed tones鈥� some groups have found that volunteers can determine the direction the tones came from, even when the carrier frequency is higher than the range within which we can detect the direction of a sound. This suggests transposed tones really do trigger nerve signals characteristic of a much lower frequency. The finding also shows that when it comes to detecting direction, it does not matter exactly where in the cochlea the signals originate, says Andrew Oxenham of the Massachusetts Institute of Technology. But what about determining pitch?
The MIT team asked volunteers to carry out a variety of tasks, such as determining the higher of two tones or tuning a note to match a reference tone. With pure tones the subjects performed very well. But with transposed tones their performance plummeted (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0306958101). 鈥淚t was a complete mess,鈥� says Oxenham. The results are all the more surprising given that people say the transposed tones 鈥渟ound鈥� like low-frequency ones, albeit with a high buzzy background. 鈥淲hatever they鈥檙e hearing, the brain doesn鈥檛 integrate it as pitch at all.鈥�
鈥淭he beauty of this is how cleanly it establishes that location plays a role in pitch perception,鈥� says Shihab Shamma of the University of Maryland, College Park. The work does not prove that location is all 鈥� both place and timing may play a role 鈥� but it should help focus attempts to define how the brain discriminates different pitches.
