6:1/ Neural Mechanisms of Hearing 105 



mechanical energy into electrical spikes on nerve fibers. It was only 

 in the 1940's that a reasonable understanding of this action was developed. 

 Before considering the modern studies, the ideas firmly believed not very 

 many years ago will be briefly examined. Two general types of theories 

 were developed: the resonator theory, and the telephone theory. Al- 

 though neither can be supported any longer, the theories were both very 

 successful in one sense. Each correlated many of the known facts and 

 inspired scientists to carry out further experiments. Then, additional 

 studies showed that neither theory was correct and led to the present 

 concepts of cochlear action. 



The resonator theory was developed by Helmholtz. He had studied 

 musical instruments and found that they all resonated. Moreover, he 



Figure I. Helmholtz resonators. The exact shape and sym- 

 metry are not important. The resonant frequency depends 

 on the volume of the cavity and the cross sectional area and 

 length of the neck. Some glass Christmas tree ornaments 

 make excellent Helmholtz resonators. 



carried out frequency analyses of sound with specially built resonators. 

 They are still known as Helmholtz resonators. Their form is shown in 

 Figure 1. The resonant frequency depended on the geometrical pro- 

 perties of the resonator. By using a series of these, Helmholtz could 

 analyze the harmonics (that is, overtones) in a piano note and could 

 even analyze some of the frequency components of speech. Helmholtz 

 resonators were widely used until the advent of electronic analyzers. 

 The latter are more convenient and much more precise but in all cases 

 depend on an electrical resonant circuit. 



Helmholtz knew only mechanical resonators and so he looked for these 

 in the cochlea. The most promising structure seemed to be the basilar 

 membrane. This membrane separates the central cochlear duct from 

 the tympanic duct. The basilar membrane supports the organ of Corti 

 with its histologically complex structure and many nerve endings. 

 The basilar membrane has a fiber-like character, and it gets broader and 

 thicker as it proceeds along the spiral to the apex. This resembles the 

 general form of a piano, a collection of strings going from short, thin 



