EXCITATION OF AUDITORY RECEPTORS 



5«:5 



more than 5 mv at most. It is dangerous, therefore, to 

 ascribe to the endocochlear potential anything more 

 than an accessory function, namely to hyperpolarize 

 the cuticular surfaces of the hair cells and thereby 

 increase the sensitivity of the auditory detector. In 

 the utricle the negative intracellular potential of the 

 hair cells apparently must suffice as the ' pool of bio- 

 logical energy.' 



The unique chemical composition of endolymph 

 does not necessarily imply a high positive potential. 

 The high potassium is present in the utricle; the po- 

 tential is not. The two are probably unrelated. 

 Perhaps the high potassium merely serves to maintain 

 the proper colloidal state and consequent viscosity 

 of the tectorial membrane! 



The cochlear microphonic and the negati\e sum- 

 mating potential are believed to e.xcite directly the 

 nerve fibers in contact with the hair cells. Only a 

 passive role as electrical conductors is ascribed to the 

 nerve endings. There is no synapse-like delay in 

 excitation. The phase relation of neural excitation to 

 cochlear microphonic is correct for the electrical 

 theory. The current flows from hair cell into nerve 

 fiber and outward across the nerve membrane and 

 thus can e.xcite the nonmeduUated dendritic terminals 

 like one tremendous node of Ranvier or like non- 

 meduUated fibers elsewhere. Spatial .summation be- 

 tween the several hair cells attached to a given nerve 

 fiber is clearly possible, as is also a facilitating action 

 between summating potential and cochlear micro- 

 phonic. A neurohumoral step between hair cell and 

 nerve fiber is an acceptable addition to this simple 

 electrical theory. 



Transmission of Auditory Injorrnalion^'^ 



We can now summarize the best present answers 

 to the questions implied in the introduction concern- 

 ing frequency and intensity discrimination and time 

 differences. 



Frequency (pitch) discrimination, the core of 

 classical 'theories of hearing' (11, 23) is now con- 

 sidered to be a duplex function. We do not think of 

 either a place principle (von Helmholtz) or a periodic- 

 ity principle (Rutherford) but of a combined or du- 

 plex theory (Wever, Licklider). 



The position of maximal stimulation, or more 

 probably the cut-off boundary of strong stimulation, 

 is certainly one part of the mechanism for identifica- 



" See especially the papers of Davis (4), Licklider (8), von 

 Bekesy (21) and Wever (23). 



tion of frequency, particularly of high frequencies. 

 The organ of Corti of the basal turn is essential for the 

 hearing of high tones. Surgical injuries combined with 

 behavioral tests in animals and disease in humans have 

 established this fact firmly. Partial section of the 

 auditory nerve may cause a complete high-tone hear- 

 ing loss. Injuries to the apical end of the cochlea may 

 cause a restricted low-frequency hearing loss but 

 complete loss of sensitivity for the low frequencies does 

 not occur. There is nevertheless a clear relation be- 

 tween frequency and position along the organ of 

 Corti. Fine frequency discrimination is still a problem, 

 however. The inaxima of the ' resonance curves' of the 

 cochlear partition (fig. 8) are much too flat, and the 

 'response areas' of individual nerve fibers (fig 19) 

 are too asymmetrical to account for the known facts 

 of frequency discrimination without some additional 

 hypothesis. A model in which the skin of the forearm 

 is exposed to traveling waves of tactile stimulation is 

 surprisingly effective, however, in giving a sharp suId- 

 jective location of the tactile sensation and in dis- 

 criminating changes of frequency by changes in this 

 location (21). The model reinforces the general 

 opinion that a neural interaction, involving inhibition 

 of the impulses from less strongly stimulated areas 

 must be involved. Such inhibitory interaction at the 

 level of the cochlear nucleus is already familiar. 



Direct information as to the frequency of .sounds 

 below 4000 cps is al.so carried in the auditory nerve 

 by the volley principle. This information is believed 

 to contribute importantly to frequency discrimination 

 and to the sense of pitch (8, 23). Opinions differ as to 

 the upper frequency at which it ceases to be important 

 and as to how the space and the 'periodicity' prin- 

 ciples interact in the region of o\erlap. In any ca.se 

 the periodicity (volley) principle gains in importance 

 and the place principle loses as the frequency is 

 lowered. 



Intensity discrimination and .subjective loudness 

 are usually attributed to the number of nerve impulses 

 per second traversing the auditory nerve. Recruitment 

 of additional fibers as intensity is increased is certainly 

 one mechanism of increasing this number, and faster 

 average rate of discharge per fiber is another. It is 

 possiiale also that certain high-threshold fibers con- 

 tribute more per fiber to loudness than do others, and 

 it is by no means necessary to assume that loudness is 

 a simple linear function of the total number of im- 

 pulses per second. 



Temporal information and also the binaural differ- 

 ences in time utilized in auditory localization are 



