6o4 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



Corti responds to a correspondingly narrow frequency 

 band and is connected by its own group of nerve 

 fibers to an isolated part of the cochlear nuclei. 

 True, upon arrival at the latter stadon, a rather dis- 

 quieting multiplication of the end organ seems to 

 take place such that the organ of Corti is projected 

 not once but several times to the nuclei, this being 

 accomplished by the systematic terminal branching 

 of the cochlear axons. There are, however, aspects 

 of this and related studies which are more fundamen- 

 tally disturbing to the hypothesis in its .simplest 

 form. In both the microelectrode studies cited in the 

 preceding paragraph, while the frequency response 

 band of a given nuclear element was extremely 

 narrow or punctate when the intensity of the stimu- 

 lus was near threshold, as the intensity was increased 

 the band became progressively wider, especially 

 toward the low end of the scale. (There was little or 

 no expansion of the response band to higher fre- 

 quencies, even at \ery high intensity.) 



When information on microelectrode studies of 

 cochlear nerve fibers became available (99, 100, loi), 

 one of the more striking features was the exaggeration 

 of the principle just described for the second-order 

 units in the cochlear nucleus. The individual units 

 in the nerve also showed sharply restricted frequency 

 specificity at threshold intensities but, upon increasing 

 intensity, each fiber responded to a wider and wider 

 range of lower frequencies but not to higher. Instead 

 of responding each to its own frequency, therefore, 

 it would be more correct to say that each fiber re- 

 sponds to all frequencies up to its high frequency 

 limit and to none higher. This fits well with direct 

 observation by von Bekesy of cutoff points of vibra- 

 tion of the basilar membrane which vary with fre- 

 quency of stimulus and involve all of the membrane 

 up to the cutoff point (99, too). 



If the feature of auditory nerve function just de- 

 scribed is true, then random partial lesions of the 

 nerve should not, as was once supposed, result in 

 hearing loss in the form of tonal islands but instead 

 in losses at the highest frequencies with smaller 

 lesions and a progressive high frequency loss as more 

 and more fibers are involved. Relatively few fibers 

 are stimulated by high tones, and so high frequencies 

 are most vulnerable since in the spiral course of the 

 nerve bundles also, it is inconceivable that any 

 appreciable lesion could miss these. Hence, in lesions 

 sparing only a few fibers, hearing should be preserved 

 only for tones at the low end of the spectrum (since 

 most or all fibers are sensitive to low frequencies, 

 and the lower tones are therefore relatively invul- 



nerable to any but complete section of the nerve). 

 This is, in fact, the common finding in both animal 

 and human studies of this kind (41, 66, 91, 92, 93). 



Studies of localized frequency response of neural 

 elements in stations lying between the cochlear nuclei 

 and the auditory cortex are relatively few in number, 

 although they are increasing currently. They rely 

 mainly on the microelectrode techniques. Such studies 

 have been made of the superior olivary nuclear com- 

 plex (33, 99), the inferior colliculus (99, 102) and 

 the medial geniculate body (31, 40). All of these 

 share with each other and with the studies on cochlear 

 nerve and nuclei the finding of elements which can 

 be activated by tonal stimuli; of others, already 

 discharging spontaneously, whose rate of (spike) 

 discharge is increased by tonal stimuli; and of still 

 others, already discharging, whose activity is in- 

 hibited by stimulation. An exception is seen in the 

 work of Tasaki & Davis (100, loi) who found no 

 fibers in the cochlear nerve whose acti\ity was 

 inhibited by stimulation. 



The response band or area seems to undergo some 

 change in shape at successively higher stations. In 

 the nerve, it is characterized by a sharp high fre- 

 quency cutoff and a long extension into lower fre- 

 quency range. The cochlear nuclear elements show 

 high frequency cutoff nearly as sharp as nerve ele- 

 ments but with a lesser expansion of the area into 

 lower frequencies at higher intensities (31, 99, loi). 



An abstract report of single unit recording from the 

 several subdivisions of the superior olivary complex 

 has just appeared (33), and a brief account of simi- 

 lar though less extensive experiments is included 

 together with those on other nuclei (99). There is a 

 greater variety of responsive units in these cell masses 

 than in the cochlear nuclei, their relati\e numbers 

 varving with location in the subdivisions and also 

 with other factors. Some units (from the reports it is 

 not clear what percentage) respond differentially to 

 tonal stimuli. The response area of a given unit 

 resembles closely those of cochlear nuclear units in 

 that the high frequency cutoff is still sharp and the 

 degree of extension into low frequency range about 

 the same as for the nucleus. Sumi et al. (99) report 

 that they found trapezoid elements responding to 

 tones over 20 kc situated rostrally, those to tones be- 

 low 300 cps caudally, and between these, 5000 and 

 3000 cps elements side by side. Thus there is indica- 

 tion of tonotopic localization of the projection thus 

 far. 



The inferior colliculus (99, 102) shows some dif- 

 ferences and some similarities to the lower centei 



