NERVE RESPONSE AS RELATED TO LOUDNESS AND PITCH 305 



at a point one and one half turns from the base of the cochlea while the 

 response area due to a very low frequency was localized at the apex of 

 the cochlea. A composite picture of the location of various frequency 

 response areas on the aural membrane is given in Fig. VII-18 with an 

 indication of the width of the basilar membrane. 



That the aural membrane is subjected to intense localized stresses 

 as the impulse travels across the scala media is also supported by 

 Reboul's data shown in Fig. VII-24. The calculated positions of 

 maximum pressure disturbances are compared with the experimental 

 positions on the basilar membrane, located by lesions which were pro- 

 duced by long-continued acoustical stimulations. Various experiment- 

 ers agree that lesions can be found at 5, 11, and 17 mm, respectively, 

 from the basilar end of the cochlea, for frequencies 8192, 4096, and 2048 

 cycles. The calculated values are found at 3.6, 12, and 26 mm for fre- 

 quencies 8000, 4000, and 2000 cycles. These are in good enough agree- 

 ment with the experimental results to justify the hope that a place 

 theory of frequency reception due to localized stresses produced by 

 pressure gradients may eventually be supported by further experi- 

 mental evidence. 



Nerve Response as Related to Loudness and Pitch 



The average experienced pitch ranges from 20 to 20,000 cycles per 

 second. There are approximately 3500 hair cells in the inner row of 

 the organ of Corti and about 20,000 divided among the three outer rows. 

 They are quite evenly spaced along the aural membrane. Each inner 

 hair cell is innervated by one or two nerve fibers, and each nerve fiber 

 makes connections with one or two hair cells (Lorente, de N6 [1933]). 

 The maximum number of impulses per second that a nerve fiber can 

 carry is nearly 1000. This maximum frequency response is imposed, as 

 was shown in the previous chapter, by what is called the " refractory 

 period " of the nerve fibers. After a nervous impulse has passed along 

 a nerve, there is an absolute refractory phase, or short period of time, 

 during which the nerve is unable to transmit an impulse. If a single 

 nerve fiber cannot transmit more than 1000 impulses per second, how 

 can the identification of a 20,000-cycle tone be explained? To explain 

 this high pitch the idea of a group of cooperating fibers was introduced, 

 each able to conduct in rotation, so that while one fiber is in its active 

 state a second may be passing through its inactive or refractory phase. 

 Thus, if three nerve fibers end in a very small stimulated patch of the 

 aural membrane, a series of three out-of-phase discharges may travel to 

 a common terminal, arriving as a frequency pattern. The sensation 

 of pitch is determined by the position of the stimulated patch on the 



