EXCITATION OF AUDITORY RECEPTORS 



581 



tion has begun and increases up to about 250 msec. 

 Rather rapid stimulation, 30 to 40 shocks per sec, 

 is required. The optimal frequency is 100 cps. The 

 long latency and the need for repetitive stimulation 

 show quite clearly that this efferent inhibitory action 

 is not related to the temporal priority of nearly simul- 

 taneous bilateral signals. It is apparently an expression 

 of a rather general principle, namely central regula- 

 tion of the sensitivity of sense organs. The functional 

 relationships of this reduction in sensitivity are com- 

 pletely unknown. 



THEORY OF AURAL ACTION''' 



This author has suggested elsewhere a series of 

 possible mechanisms and interrelationships that, 

 taken together, offer a presently tenable working 

 hypothesis. This theory will be presented here in 

 brief for its value in unifying many varied experi- 

 mental observations, but the reader must recognize 

 that several assumptions, interpretations and opin- 

 ions, more or less plausible, are now added to the 

 experimental facts. 



Acoustic energy is delivered to the inner ear by the 

 external and middle ears. The frequency characteris- 

 tics of the external and middle ear determine to a 

 large extent the shape of the curve of auditory sensi- 

 tivity. The impedance match provided by the tym- 

 panic membrane and ossicles between air and intra- 

 cochlear fluid is nearly perfect, except perhaps for 

 frequencies below 500 cps, and contributes to the 

 great absolute sensitivity of the ear. Other aspects of 

 the middle ear structure and function are chiefly 

 protective. 



Acoustic pressure on the tympanic membrane 

 causes movement of the foot-plate of the stapes and 

 reciprocal movement of the round window membrane. 

 The fluid movements between these two windows 

 carry with them the elastic cochlear partition, but the 

 pattern of movement of this partition, determined 

 primarily by the graded stiffness of the basilar mem- 

 brane, is complicated. The pattern is a sequence of 

 traveling waves that move very rapidly at first, then 

 more and more slowly, as they travel toward the apex. 

 The amplitude increases gradually with travel to a 

 rather flat maximum and then falls off" quite sharply. 

 The positions of this maximum and of the cut-off 

 beyond it move toward the apex as the frequency is 

 reduced and toward the ba.se as the frequency is 



" See especially tlie papers of Davis (3, 4). 



raised. In this way the cochlea acts as a mechanical 

 frequency analyzer and the 'place principle' is estab- 

 lished as one element contributing to frequency dis- 

 crimination. 



The traveling wave pattern is an expression of 

 pha.se differences in the movements of different seg- 

 ments of the cochlear partition. It is a necessary con- 

 sequence of the graded stiffness of the cochlear parti- 

 tion, of the varying ma.ss of fluid that moves with it 

 and of the rather clo.se coupling inherent in a continu- 

 ous membrane such as the cochlear partition. The 

 energy is transmitted in part through the fluid as an 

 acoustic wave and in part along the membrane from 

 segment to segment. The stiffer basal region, which 

 for' middle and low frequencies moves almost in 

 phase, tends to drive the more flexible apical portion. 

 The nearly-in-phase movements of the partition in the 

 basal turn in response to low-frequency sounds cause 

 nearly synchronous stimulation of impulses in many 

 nerve fibers. Thus the frequency principle ('volley 

 principle') contributes to the space-time pattern of 

 nerve impulses in spite of the large phase differences 

 that are a.ssociated with the fundamental traveling 

 wave pattern. 



The movements of the partition in the traveling 

 wave pattern involve a bending of the basilar mem- 

 brane in two dimensions, both across and lengthwise. 

 The crosswise bending or bulging is sharpest at the 

 position of maximal amplitude, but it is also signifi- 

 cant for a considerable distance basally from the 

 ma.ximum. The longitudinal l)ending is sharpest in 

 the 'cut-off' region on the apical side of the maximum 

 and is probably negligible on the basal side. 



As the basilar membrane, and the organ of Corti 

 with it, bulge one way or the other, there is a shearing 

 action between the stiff reticular lamina of the organ 

 of Corti and the stiff and viscous tectorial membrane 

 that lies in contact with it becau.se the tectorial mem- 

 brane pivots around a different axis, as illustrated in 

 figure 12. The shearing action bends the hairs of the 

 hair cells, which are attached both to the organ of 

 Corti and to the tectorial membrane. This bending 

 is the mechanical movement that is critical for stimu- 

 lation. Protection against too great bending probably 

 is provided by the attachment of tectorial membrane 

 directly to the outer and inner borders of the organ 

 of Corti. 



The longitudinal bending causes longitudinal 

 vibratory mov-ements among the cells of the organ of 

 Corti and presumably bends lengthwise the hairs of 

 the cochlear partition. The external and internal hair 

 cells are not equally sensitive to radial and longi- 



