6:4/ Neural Mechanisms of Hearing ||3 



of feedback. They also emphasize that many of the types of neural 

 action essential for hearing also occur in other senses. It has indeed 

 been possible for Bekesy to make an enlarged cochlear model, using the 

 forearm for a sensing organ. Most of the phenomena of hearing are 

 reproduced by this model. 



The model consists of a series of resonant vibrators of varying fre- 

 quency running along the arm. When these are electrically driven, 

 several neighboring ones respond, the central one most strongly. The 

 person senses the resonant frequency at a much more sharply located 

 spot than is indicated by the behavior of the vibrators. Phenomena of 

 masking, beats, harmonic distortion, and sharp frequency and intensity 

 discrimination are all shown by this "analog" of the ear. Thus, there 

 can be no doubt that the nervous system, in some fashion, does sharpen 

 neural impulses. Likewise, funnelling can be demonstrated for the arm 

 analog. Just how the nervous system goes about sharpening and 

 funnelling is not known. 



The arm models of the ear demonstrate that nonlinear and harmonic 

 distortion occurs in the nervous system, as well as in the tympanic 

 membrane and middle ear. It is quite probable that a similar distortion 

 also occurs within the inner ear. This is indicated by the cochlear 

 potentials. However, because the cochlear and neural potentials are so 

 difficult to separate, it is not certain whether distortion actually occurs 

 within the cochlea. 



The arm models strongly support the idea that pitch discrimination 

 is, to a large degree, a function of the central nervous system. The 

 details of this action are not known yet. Nonetheless, many experiments 

 with vertebrates, and even invertebrates, have shown that the central 

 nervous system can carry out complex actions such as "sensation 

 sharpening," "amplitude analysis," and "funnelling." Anatomical and 

 electrical studies of the central nervous system emphasize the possibilities 

 of such computerlike actions. 



4. Cortical Representation 



The spike potentials generated in the basilar membrane of the cochlea 

 travel along the fibers of the acoustic nerve. As has been stated, most 

 sensory nerve cell bodies are located in compact groups called ganglia. 

 The acoustic nerve, however, has a diffuse set of cell bodies spread out 

 along its path through the spiral bony partition which supports the 

 cochlea. These nerve cell bodies are called the spiral ganglion. The 

 pulses in the second set of axons in the acoustic nerve enter the brain. 

 The acoustic nerve is the eighth one (counting from the front end) to 



