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Neural Mechanisms of Hearing /6 : 2 



removed within an hour after death.) In cases in which specific 

 frequencies were missing in the person's hearing, lesions occurred at 

 corresponding regions along the basilar membrane (as predicted by the 

 resonator theory!). The cochlear microphonics show maxima along the 

 basilar membrane (again at the place indicated by the resonator theory!). 

 Finally, if the basilar membrane of an experimental animal is destroyed 



Acoustic 

 Pressure 



Spikes 



Axon Spikes 



Time 



Figure 2. This illustrates that to reproduce a sine wave many 

 spikes per cycle are necessary. Even the 1 5 per cycle illustrated 

 integrates at best to a crude sine wave. 



in a narrow region, it is found by both behavioral and electrical studies 

 that the animal cannot hear in the corresponding frequency region. 



Thus, neither the resonator theory nor the telephone theory can be 

 maintained in the light of present knowledge of the ear and the nervous 

 system. Although the resonator theory is anatomically unsound, a 

 spatial localization of frequencies along the basilar membrane does 

 occur. Likewise, although the telephone theory per se cannot be 

 maintained, its prediction of the form of the integrated nerve potentials 

 in the brain stem is reasonably accurate. 



2. Cochlear Mechanism of Neural Excitation 



Present theories incorporate all of the positive evidence presented in the 

 last section. Needless to say, these theories are somewhat more complex. 

 At least 12 different variations exist around the basic hydrodynamic 

 theory proposed originally by Bekesy. He showed that, in addition to 

 the compressional waves traditionally treated in acoustics, there could 

 also exist certain slow hydrodynamic waves in a structure such as the 

 cochlea. These waves bear certain similarities to surface waves on a 

 large body of water or interfacial tension waves at an oil-water interface. 

 (Note, "bear certain similarities" does not mean "identical to!") 

 These hydrodynamic waves are in a dispersive medium, that is, one in 



