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Neural Mechanisms of Hearing /6 : 4 



submultiples of the stimulus frequency. Somehow the brain is thought 

 to carry out a frequency analysis on the over-all electrical signal. In 

 other words, the ear carries out a crude frequency analysis in terms of 

 exciting preferentially certain nerve fibers. The central nervous 

 system then carries out a finer frequency analysis. 



Acoustic 

 Pressure 



Axon 

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I 



Spikes in 

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minimi mm iiiiiiiiiii i i 



lllllllllll I I I I Integrated 



Figure 8. Illustration of the volley principle which allows 

 axons to fire once per cycle and still reproduce the shape of the 

 sound wave. 



Although it is clear that the detailed frequency analysis occurs by 

 sharpening within the central nervous system, it is by no means under- 

 stood exactly how or where this takes place. Part of the difficulty is in 

 distinguishing neural impulses from cochlear potentials. For example, 

 if a click is presented to the ear, two types of electrical potentials result. 

 The first, essentially simultaneous with the click, is the cochlear potential. 

 It remains even if the acoustic nerve is destroyed. The second is the 

 true nerve potential; it occurs after a slight time delay and is abolished 

 if the acoustic nerve is not functioning. For acoustic signals other than 

 clicks, it is difficult to distinguish between these two types of potentials. 

 Accordingly, most studies of the neural responses have been carried out 

 past the first synapse in the diffuse spiral ganglion and usually have been 

 restricted to the central nervous system. This has made it impossible 

 to determine at what level detailed frequency analysis occurs. 



