570 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



100 



I I I 1 1 III 1 — I ll 



__ Bekesy ' 



""■^ .^"Pricking in middle ear" 10 



T I I II I I ij 



"I 1 — I I I I 1 11 



160 



|-140„ 



E 



120 T 



>. 



T3 



100 8 



O 

 O) 



-80 i 



c 



60 3 



« 



40 ■? 



.5? 



20 

 -0 



100 

 Frequency m cps 



10,000 



FIG. 6. The threshold of audibihty and the threshold of feeling. MAP, minimum audible pressure 

 at the eardrum. MAF, minimum audible pressure in a free sound field, measured at the place 

 where the Hstener's head had been. [From Licklider (g).] 



waves in a tube with a flexible wall. This velocity is 

 less than the velocity of sound in water but it is so 

 fast that the increase in pressure in scala vestibuli 

 relative to scala tympani is virtually simultaneous 

 throughout the length of the cochlea. The pressure 

 wave travels much faster than the phase velocity of 

 the traveling wave of mechanical movement that 

 occurs in response to this difference in pressure be- 

 tween the two scalae. An over-all net movement of 

 the cochlear partition towards scala tympani in re- 

 sponse to this differential pressure occurs because the 

 round window forms a flexible portion of the other- 

 wise rigid walls of the bony labyrinth. The round 

 window membrane bulges outward and thus allows 

 inward movement of the stapes. Inside the cochlea, 

 the cochlear partition bulges toward the round 

 window. 



When the movement is very slow, some fluid also 

 flows through the helicotrema. But all parts of the 

 cochlear partition do not move with equal prompt- 

 ness. The relatively stiff portion in the basal turn 

 moves very nearly in phase with the driving force, 

 but the more flexible apical portions, particularly 

 those with a resonant frequency lower than the fre- 

 quency of the acoustic wave that is driving the parti- 

 tion, tend to lag behind. As the acoustic wave reverses 



its pressure, the portion that is 'tuned' to lower 

 frequencies tends to overshoot and continues to lag 

 behind the driving force exerted on it by the acoustic 

 pressure in the fluid. Thus, because of the gradation 

 of stiffness, a traveling wave of displacement appears 

 on the cochlear partition (fig. 7). Furthermore, be- 

 cause of the continuity of the partition, the stifTer 

 portion, moving almost as a unit, drives the more 

 flexible portion. 



The traveling wave increases in amplitude as it 

 moves apically and reaches its maximum near the 

 region where the resonant frequency of the basilar 

 membrane corresponds to the frequency of the driving 

 waves (fig. 8). The amplitude of movement falls off 

 rather rapidly beyond this point; also the phase lag 

 increases rapidly as the traveling wave moves on 

 toward the apex. The velocity of travel therefore 

 diminishes, and consequenth the wavelength of the 

 displacement pattern becomes shorter. A little distance 

 beyond the position of maximum amplitude there is 

 no significant movement at all. In the region of rapid 

 diminution of amplitude the phase lag amounts to a 

 full cycle or more. 



If the driving frequency is increased, the position of 

 maximum amplitude moves toward the oval window; 

 if it is decreased, the maximum moves toward the 



