The temporal theory of

hearing Hearing, or auditory perception, is the ability to perceive sounds through an organ, such as an ear, by detecting vibrations as periodic changes in the pressure of a surrounding medium. The academic field concerned with hearing is audit ...

states that human perception of sound depends on temporal patterns with which neurons respond to

sound In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' b ...

in the

cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory or ...

. Therefore, in this theory, the pitch of a pure tone is determined by the 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 Place theory is a theory of hearing that states that our perception of sound depends on where each component frequency produces vibrations along the basilar membrane. By this theory, the pitch of a sound, such as a human voice or a musical tone, ...

of hearing, which instead states that pitch is signaled according to the locations of vibrations along the

basilar membrane The basilar membrane is a stiff structural element within the cochlea of the inner ear which separates two liquid-filled tubes that run along the coil of the cochlea, the scala media and the scala tympani. The basilar membrane moves up and down ...

. Temporal theory was first suggested by August Seebeck.

Description

As the basilar membrane vibrates, each clump of

hair cell Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates, and in the lateral line organ of fishes. Through mechanotransduction, hair cells detect movement in their environment. ...

s along its length is deflected in time with the sound components as filtered by

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

cochlear nerve The cochlear nerve (also auditory nerve or acoustic nerve) is one of two parts of the vestibulocochlear nerve, a cranial nerve present in amniotes, the other part being the vestibular nerve. The cochlear nerve carries auditory sensory informatio ...

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 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 within the range of frequencies we can hear. 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 some frequencies are too high to see any synchrony in the

firings.

The random firing solution

Beament outlined a potential solution. He noted that in two classic studies 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 Place theory is a theory of hearing that states that our perception of sound depends on where each component frequency produces vibrations along the basilar membrane. By this theory, the pitch of a sound, such as a human voice or a musical tone, ...

may be dominant for higher frequencies. However, it is also suggested that place theory may be dominant for low, resolved frequency harmonics, and that

temporal theory The temporal theory of hearing states that human perception of sound depends on temporal patterns with which neurons respond to sound in the cochlea. Therefore, in this theory, the pitch of a pure tone is determined by the period of neuron firing ...

may be dominant for high, unresolved frequency harmonics.

Experiments to distinguish rate and place effects on pitch perception

Experiments to distinguish between place theory and rate theory using subjects with normal hearing are difficult to devise, because of the strong correlation between rate and place: large vibrations at a low rate are produced at the apical end of the

while large vibrations at a high rate are produced at the basal end. The two stimulus parameters can, however, be controlled independently using

cochlear implant A cochlear implant (CI) is a surgically implanted neuroprosthesis that provides a person who has moderate-to-profound sensorineural hearing loss with sound perception. With the help of therapy, cochlear implants may allow for improved speech unde ...

s: pulses with a range of rates can be applied via different pairs of electrodes distributed along the membrane and subjects can be asked to rate a stimulus on a pitch scale. Experiments using implant recipients (who had previously had normal hearing) showed that, at stimulation rates below about 500 Hz, ratings on a pitch scale were proportional to the

log Log most often refers to: * Trunk (botany), the stem and main wooden axis of a tree, called logs when cut ** Logging, cutting down trees for logs ** Firewood, logs used for fuel ** Lumber or timber, converted from wood logs * Logarithm, in mathe ...

of stimulation rate, but also decreased with distance from the round window. At higher rates, the effect of rate became weaker, but the effect of place was still strong.{{cite journal , vauthors=Fearn R, Carter P, Wolfe J , title=The perception of pitch by users of cochlear implants: possible significance for rate and place theories of pitch , journal=Acoustics Australia , volume=27 , issue=2 , pages=41–43 , year=1999 , url=http://www.phys.unsw.edu.au/jw/Fearnetal.html

References

Psychoacoustics Theories