Rate theory (hearing)

Rate theory is a theory of hearing which states that our perception of sound depends on the rate at which neurons signal the frequency of each component. Therefore, the pitch of a pure tone would be determined by the rate at which the neurons fire. It was first suggested by A. Seebeck. Rate theory competes with the place theory of hearing, which instead states that pitch is signaled by viberations along the basilar membrane.

Description
As the basilar membrane vibrates, each clump of hair cells along its length is deflected in time with the corresponding resonant frequency for its position. The more intense this component frequency is, the more the hair cells are deflected and the more likely they are to signal. Rate theory supposes that it is these signal rates and their number which dominate our perception of sound.

High amplitudes (volume)
Above a certain threshold nerve fibers, whose characteristic frequency do not exactly match the stimulus, will still respond simply because of the intensity of motion induced in larger areas of the basilar membrane by loud sounds. Maintaining the ability to discriminate frequencies at high dB levels is complicated by this loss of specificity of responding. Rate theory (temporal theory) can help explain how we maintain this discrimination. Even when a larger group of nerve fibers are all firing there is a periodicity to this firing which corresponds to the frequency of the stimulus. So for example, every 10th peak in a loud 4 kHz tone, will be represented by a maximal rate of firing of whichever nerves are responding to the tone regardless of the nerves normal characteristic frequency.

High frequencies
Neurons have a maximum firing frequency which falls within the range of frequencies we can hear. In order to be complete, rate theory must somehow explain how we distinguish pitches above this maximum firing rate.

The random firing solution
In his book, "How We Hear Music"1, Cambridge professor Sir James Beament outlined a potential solution. He noted that in two classic studies2,3individual hair cell neurons did not always fire at the first moment they were able to. Though they would fire in time with the vibrations, the neurons would not fire on every vibration. The number of skipped vibrations was seemingly random. The gaps in the resulting train of neural impulses would then all be integer multiples of the period of vibration. For example, a pure tone of 100 Hz has a period of 10 ms. The corresponding train of impulses would contain gaps of 10 ms, 20 ms, 30 ms, 40 ms, etc. Such a group of gaps can only be generated by a 100 Hz tone. The set of gaps for a sound above the maximum neural firing rate would be similar except it would be missing some of the initial gaps, however it would still uniquely correspond to the frequency. The pitch of a pure tone could then be seen as corresponding to the difference between adjacent gaps.

Another solution
Research using modern cochlear implants suggests that the perception of pitch may depend on both the neurons' location and rate at which they fire. Place theory may be dominant for frequencies above the maximum firing rate.