6 : 1/ Neural Mechanisms of Hearing 107 



also knew that the cochlea acted as a transducer changing sound energy 

 to electrical energy, but, without electronic gadgets, they knew nothing 

 of the form of this electrical energy. They reasoned that the cochlea 

 acted as a microphone, transmitting along the nerve fibers a signal 

 whose form was that of the incoming pressure wave. In one of its 

 variations, the theory suggested that only the nerve fibers nearest the 

 windows to the middle ear were stimulated by weak sounds but that 

 the entire cochlea was activated by loud sounds. 



If the proponents of this theory had had more electronic instruments 

 available, they might have found additional evidence which could have 

 been misinterpreted to support the telephone theory. In one experi- 

 ment an electrode is placed at or near the cochlea. Definite electrical 

 potentials are discovered which do reproduce the form of the applied 

 pressure wave. These potentials are called cochlear potentials or 

 cochlear microphonics; they are small in magnitude, perhaps no bigger 

 than 100 /xv, but they definitely exist in the cochlea and not in the 

 measuring equipment. These cochlea'r microphonics were discovered 

 in the 1930's by Wever and Bray. Another experiment follows the 

 auditory pathways into the brain stem. If an electrode is placed in 

 these areas, a signal is picked up which is an integrated response of many 

 nerve fibers. This electrical potential reproduces the form of the applied 

 sound pressure, provided the frequency is below 3-4 kc. (Above 4 kc 

 a submultiple of the applied frequency is usually present.) 



Several lines of evidence show that the telephone theory cannot be 

 valid over most of the audible range. The most unequivocal of these is 

 that an individual nerve fiber cannot conduct more than 1,000 spikes 

 per second. This limitation occurs because there is a period of 1 milli- 

 second or more following a spike during which time another cannot be 

 generated. The occurrence of 1.000 spikes per second would not allow 

 the nerve fiber to reproduce a sound wave of 1,000 cps. As shown in 

 Figure 2, many spikes are necessary per cycle. Thus, the telephone 

 theory cannot be valid above about 60 cps. Moreover, whereas the 

 integrated spikes in the fiber tracts in the brain stem do reproduce the 

 form of the pressure wave, the potentials on the surface of the cerebral 

 cortex fail to follow above 200 cps. 



In addition to the impossibility of individual axons acting as telephone 

 lines, the telephone theory is refuted by a large body of experimental 

 evidence favoring a place theory of hearing. In other words, different 

 frequencies are represented at different places along the basilar membrane 

 of the cochlea, albeit not analyzed by a resonator mechanism. For 

 instance, lesions can often be observed in the inner ears of deafened 

 persons. (In order to observe this, the person's hearing had to be tested 

 in the hospital immediately before death. Then the ear had to be 



