482 



Comparative Animal Physiology 



Intensity Discrimination. The mechanism for the perception of intensity 

 is apparently dual." With increasing intensity of a given frequency there 

 is: (1) an increase of the frequency of impulses in single nerve fibers, espe- 

 cially for high frequencies; (2) an increase in the number of fibers which 

 carry impulses. As the vibrations increase in amplitude a wider and wider 

 band of sense cells responds. At low frequencies the impulses of a single 

 fiber have a one-to-one correspondence to the pressure waves of the stimulus. 

 This is true below 500 cycles for the initial waves and at the frequency of 

 200 cycles with equilibration. However, above these sound frequencies the 

 frequency of impulses varies with intensity. For example, at 7000 cycles 

 (Fig. 160) the frequency of discharge of a single fiber may increase from 

 less than 10 to as much as 135 per second as the intensity is increased from 

 -82 to -30 db below a given reference level. It can also be seen from Figure 



5000 



sooo 



6000 7000 



Sound Frequency 



Fig. 160. Curves showing the relation between frequency of spike potential discharge 

 of a single auditory nerve fiber and the intensity and frequency of the sound stimulus. 

 Note that with increasing intensity the fiber responds to a greater frequency band, and, 

 at each frequency, with a greater frequency of spike potential discharge. After Galambos 

 and Davis," from Fulton .^^ 



160 that although this one fiber was responsive only at frequencies very close 

 to 7000 cycles at -78 and -82 db it was responsive to all frequencies between 

 5000 and 8000 at -20 db, with, however, a maximal response still at 7000. 

 Sound Localization. One important function of the human ear, in addi- 

 tion to discrimination of frequency and intensity, is that of localization of 

 the. source of sound. A person with normal hearing is able to locate sounds 

 with a surprising degree of accuracy. This is possible even with very com- 

 plex sounds, such as that of a symphony orchestra, during which even a blind 

 listener, if he is close to the orchestra, is able to locate the various instruments. 

 Such localization is largely a function of the intensity differences and of the 

 phase differences of the sound waves as they strike the two ears. For all 

 tones above 500 cycles and especially for those above 5000 cycles, one ear may 

 be in the "shadow" of the head. For a tone of 10,000 cycles loudness diflfer- 



