50 PHYSICAL ASPECTS OF HEARING 



An alternative resonance theory requires the existence of fibers which 

 are tuned (like the strings in a musical instrument) according to their 

 length and tension to produce a single note or frequency. Then, an in- 

 coming sound would cause the corresponding fibers to vibrate and the 

 vibrations would stimulate the nerves connected to them. But there 

 arises the purely experimental question of whether such fibers exist, and 

 if fibers can be found, there is the additional question of whether they 

 have sufficiently different lengths and tensions to respond to the range 

 of frequencies of human hearing (30 to 15,000 cycles/sec). 



Studies of the membrane have shown that there are indeed structural 

 fibers in the membrane and that there are also hair cells in an organ 

 lying directly above the membrane, but that neither set of possible 

 resonators has a sufficient range of frequencies to be responsible for 

 human hearing. 



Both kinds of theories discussed up to this point are included in the 

 general category of place theories, so called because the excitation of a 

 particular place in the cochlea is supposed to inform the brain of the 

 arrival of a particular frequency. It was a great disappointment that the 

 place theories didn't work, because there existed impressive experimental 

 evidence for their validity. 



The kinds of evidence for the validity of place theories came from 

 direct examination of the membranes. The ears of individuals known to 

 be deaf to certain frequencies were examined after death, and lesions 

 were found in certain places along the membrane; the places found in a 

 study of many ears presented a consistent picture for the places of 

 various frequencies. It was also found that deafness to certain fre- 

 quencies could be induced in animals by subjecting an animal to such a 

 great intensity of a particular frequency that the ear was injured and 

 the animal subsequently no longer responded to the frequency. Examina- 

 tion of the resultant lesions in the membranes of such animals gave a 

 place representation of frequencies along the membrane which coincided 

 quite well with that derived from naturally occurring lesions. Thus there 

 would seem to be no question that there is at least a significant amount 

 of validity to the place theories, despite the inability of researchers to 

 find a suitable model. 



The resonance theories discussed to this point stemmed from the 

 proposals of the German scientist Helmholtz, who began his studies in 

 the middle of the 19th century. You recall that we began our examina- 

 tion of place and resonance theories because of the finding that individual 

 nerve fibers cannot transmit impulses more often than at the rate of 

 1000 cycles/sec. Yet in recent years, it has become possible to measure 

 the impulse frequencies directly on the auditory nerve, and faithful 



