306 AUDITORY BIOPHYSICS 



aural membrane and not by the frequency of nerve impulses in the 

 auditory nerve. 



The resolving power of the asymmetrical aural membrane is thus 

 limited by the distance between the maxima of two adjacent stimulated 

 patches to which two hair cells can respond. Thus a just-noticeable 

 change in pitch represents a minimum shift of the region of activity 

 along the aural membrane, which in the 1000-to-2000 pitch region cor- 

 responds very closely with the distance between two internal hair cells. 



Loudness, which is the experience of the intensity of the acoustic 

 disturbance, is probably directly related to the total number of nerve 

 impulses reaching the brain from a stimulated patch of the aural mem- 

 brane. 



Evidence for this hypothesis may be obtained from the work of 

 Derbyshire and Davis [1935]. They have shown that the voltage of the 

 action potential recorded from the auditory nerve increases as the 

 intensity of the stimulating tone is increased. As the intensity is 

 increased, it is found that the number of nerve impulses passing along a 

 nerve per unit of time also increases. Since the nerve-fiber impulses 

 are of an all-or-none nature, an increase in voltage of the overall action 

 potential of the acoustic nerve must be due to an increase in the number 

 of fibers activated. A reasonable conclusion is that an increase of activ- 

 ity in the auditory nerve is correlated best with an increase of active 

 fibers as a physiological basis of loudness. However, there is need for 

 more extensive experimental data to determine the functional relation 

 between loudness and the activity of the fibers in the auditory nerve. 



In the meantime, the response to intensity increases can be pictured 

 as an increased distortion of a narrow strip of aural membrane, which at 

 high intensities spreads enough to produce the experienced change in 

 pitch. The nerve endings lying at the position of maximal pressure 

 gradient stimulate the nerve fibers to discharge impulses oftener than 

 those farther away. As the distortion increases, the patch of excitation 

 spreads both laterally and across the aural membrane, thus increasing 

 the number of hair cells and associated nerve endings responding to 

 the mechanical distortion. Owing to the asymmetrical structure of the 

 aural mechanism and aural membrane, the change in the experienced 

 loudness does not duplicate the change in intensity but has a functional 

 relation to the intensity, thus probably accounting for the form of the 

 loudness function shown in Fig. VII-20. 



Auditory Patterns of the Aural Membrane 



Electrical response studies (aural microphonics) have demonstrated 

 that the sensation of pitch is determined by the position of the pressure- 



