MODELS FOR SPECIFIC NERVES FOR SPECIFIC FREQUENCIES 51 



replication of the incoming frequencies was found up to several thousand 

 cycles per second ! 



The explanation of the apparent discrepancy lies in the fact that 

 although individual nerve fibers cannot transmit above 1000 cycles/sec, 

 the groups of fibers making up the auditory nerve may cooperate to 

 transmit higher frequencies. In this so-called volley principle explana- 

 tion, nerve fiber #1 fires, say, every 0.001 second, neuron #2 fires every 

 0.001 second, but starting 0.00033 second later. Neuron #3 fires every 

 0.001 second, but starting after 0.00066 second. The combination of the 

 three neurons then fires three times every one-thousandth of a second, 

 thereby reproducing a frequency of 3000 cycles/sec. It is not known 

 where this "volley" action takes place, although the originator of the 

 theory, E. G. Wever, has proposed that it occurs in the neurons which 

 are connected to the inner ear membrane. According to Wever, hearing 

 takes place by a combination of frequency representation (using the 

 volley principle) and place representation (with neural sharpening). 



The kind of place theory still has to be specified. We return to the 

 previously mentioned fact that the canals are filled with an incom- 

 pressible fluid. The properties of the fluid are such that waves travel 

 much faster through the fluid than through the membrane, which is 

 composed of reasonably stiff elements not very tightly coupled to each 

 other. When pressure is exerted by the stapes, the liquid wave reaches 

 the helicotrema and in part goes through to travel down to the round 

 window long before any appreciable part of the membrane can have be- 

 gun to move. Because the pressures above and below the membrane are 

 not precisely in phase, there is a pressure difference across the membrane 

 which displaces the membrane, producing a bulge. The place where this 

 bulge occurs depends on the frequency of the incoming sound; the 

 higher the frequency, the closer it is to the stapes. The positioning of the 

 bulges seems to depend on a surprising property of the waves which can 

 be predicted by the complicated mathematical theories now available. 

 The property is that almost all of the wave is confined to a thin region 

 of the fluid at the periphery, where the fluid is in contact with the canal 

 material and with the membrane. Thus the situation is somewhat similar 

 to that of surface waves coming in to a beach. Depending on the fre- 

 quency and amplitude of the waves, they will "break" at various dis- 

 tances from the shore. 



Many of these aspects of the rapidly developing field of hearing re- 

 search are due to von Bekesy, who was awarded a 1961 Nobel Prize. 



