Place Theory

High frequency sounds selectively vibrate the basilar membrane of the inner ear near the entrance port (the oval window). Lower frequencies travel further along the membrane before causing appreciable excitation of the membrane. The basic pitch determining mechanism is based on the location along the membrane where the hair cells are stimulated.

A schematic view of the place theory unrolls the cochlea and represents the distribution of sensitive hair cells on the organ of Corti. Pressure waves are sent through the fluid of the inner ear by force from the stirrup .

The place theory is the first step toward an understanding of pitch perception. But considering the extreme pitch sensitivity of the human ear, it is thought that there must be some additional "sharpening" mechanism to enhance the pitch resolution.

Role of place theory in distinguishing vocal sounds

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Place theory concepts

Hearing concepts

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Sharpening of Pitch Perception

The high pitch resolution of the ear suggests that only about a dozen hair cells, or about three tiers from the four banks of cells are associated with each distinguishable pitch. It is hard to conceive of a mechanical resonance of the basilar membrane that sharp. So we look for enhancements of the basic place theory of pitch perception.

There must be some mechanism which sharpens the response curve of the organ of Corti, as suggested schematically in the diagram. Several such mechanisms have been suggested.

Index

Place theory concepts

Hearing concepts

Sound

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Mechanisms for Sharpening

Since it seems unlikely that the basic place theory for pitch perception can explain the extraordinary pitch resolution of the human ear, some sharpening mechanism must be operating. Several of the proposed mechanism have the nature of lateral inhibition on the basilar membrane. One way to sharpen the pitch perception would be bring the peak of the excitation pattern on the basilar membrane into greater relief by inhibiting the firing of those hair cells which are adjacent to the peak. Since nerve cells obey an "all-or-none" law, discharging when receiving the appropriate stimulus and then drawing energy from the metabolism to recharge before firing again, one form the lateral inhibition could take is the inhibition of the recharging process since the cells at the peak of the response will be drawing energy from the surrounding fluid most rapidly. Inhibition of the lateral hair cells could also occur at the ganglia, with some kind of inhibitory gating which lets through only those pulses from the cells which are firing most rapidly. It is known that there are feedback signals from the brain to the hair cells, so the inhibition could occur by that means.

Index

Place theory concepts

Hearing concepts

Sound

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