Temporal theory (hearing)

The temporal theory of hearing states that our perception of sound depends on the temporal patterns with which neurons respond to sound in the cochlea. Therefore, the pitch of a pure tone would be determined by period of neuron firing patterns, either of single neurons or groups as described by the volley theory. Temporal or timing theory competes with the place theory of hearing, which instead states that pitch is signaled according to the locations of vibrations along the basilar membrane.

Temporal theory was first suggested by August Seebeck.

Description

As the basilar membrane vibrates, each clump of hair cells along its length is deflected in time with the sound components as filtered by basilar membrane tuning for its position. The more intense this vibration is, the more the hair cells are deflected and the more likely they are to cause nerve firings. Temporal theory supposes that the consistent timing patterns, whether at high or low average firing rate, code for a consistent pitch percept.

High amplitudes

At high sounds levels, nerve fibers whose characteristic frequencies do not exactly match the stimulus will still respond, because of the motion induced in larger areas of the basilar membrane by loud sounds. 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 periodicity of the stimulus.

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 volley theory, in which groups of neurons cooperate to code the temporal pattern, is an attempt to make the temporal theory more complete, but there are still frequencies too high to see any synchrony in the auditory nerve firings.

The random firing solution

Beament [cite book | author = James Beament | title = How We Hear Music | publisher = The Boydell Press | year = 2001 | isbn = 0-85115-813-7] outlined a potential solution. He noted that in two classic studies [cite book | author = Nelson Y. S. Kiang | title = Discharge Patterns of Single Auditory Fibers | publisher = MIT Research Monograph 35 | year = 1969 ] [cite journal | author = J. J. Rose, J. Hind, D. Anderson, and J. Brugge | title = Response of Auditory Fibers in the Squirrel Monkey | journal = J. Neurophysiol | volume = 30 | date = 1967 | pages = 769–793] individual 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

Modern research suggests that the perception of pitch depends on both the places and patterns of neuron firings. Place theory may be dominant for high frequencies. [cite book | chapter = Pitch Perception Models | author = Alain d'Cheveigné | title = Pitch | editor = Christopher J. Plack, Andrew J. Oxenham, Richard R. Fay, and Arthur N. Popper | publisher = Birkhäuser | year = 2005 | isbn = 0387234721 | url = http://books.google.com/books?id=n6VdlK3AQykC&pg=PT214&dq=place+pitch++volley+theory++believed&lr=&as_brr=3&ei=nQm-SKjEIom6tQPwv6XqBw&sig=ACfU3U2OVgellLwHhDDfIb0jiFYckfWq_A ]

References

External links

* [http://www.phys.unsw.edu.au/~jw/Fearnetal.html The dependence of pitch perception on the rate and place of stimulation of the cochlea: a study using cochlear implants.]