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Neural Mechanisms of Hearing/ 6 : 2 



amplitude. The general shape of these maxima is shown in Figure 4. 



Experiments with intact and excised ears in humans and laboratory 

 rodents, at low frequency and high intensity, showed similar maxima. 

 For a given sound pressure level, the lower the frequency, the greater 

 is the displacement. These measurements, when extrapolated to the 

 limit of audibility at 1 ,000 cps, show that the maximum displacements 

 of the basilar membrane may be smaller than a nuclear radius, 10 ~ 12 cm. 



Both the theoretical analyses and the model experiments agree that 

 all that is essential for the maxima of hydrodynamic waves, separated 

 according to frequency, are rigid walls, two parallel tubes separated by 

 an elastic membrane, and two windows, one driven and the other "open" 



High Frequency Maximum 



Low Frequency Maximum 



Figure 4. Maxima of the displacement of the rubber dam for 

 the model in Figure 3. Lower frequencies have maxima 

 nearer the windows. 



to the air of the middle ear. The maxima of these hydrodynamic waves 

 give only a crude place localization of different tones. The maxima 

 are narrow enough to account for the experiments with lesions and 

 cochlear potentials but are far too broad to explain pitch discrimination 

 by themselves. One must invoke a neural mechanism for the extremely 

 sharp pitch discrimination which the human ear can perform. This 

 pattern is discussed further in Section 3. 



The over-all action of the cochlea is, then, to convert (transduce) a 

 hydrodynamic wave into electrical spikes on nerve axons. In an 

 attempt to find the details of how this occurred, Bekesy and his co-workers 

 studied the electrical properties of the cochlea. Although the over-all 

 goal of describing the cochlear mechanism of neural excitation is still 

 incomplete, many interesting facts have been uncovered. They have 

 shown that in the intact animal the tympanic and vestibular ducts act 

 as an electrical shield around the cochlear duct. The fluid in the 

 tympanic and vestibular ducts is a good conductor. It is, however, 

 electrically insulated from the cochlear duct by the basilar membrane 

 and Reisner's membrane. Thus, the basilar membrane plays an import- 

 ant role both as the elastic membrane for mechanical vibrations and also 

 as an electrical insulator. In a phonograph cable, it is necessary to 

 surround the inner conductor with an insulator, which in turn is covered 

 by a second conductor. The outer conductor is called a shield and is 



