Place Theory
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.
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Sharpening of Pitch PerceptionThe 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.
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Mechanisms for SharpeningSince 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. |
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