CENTRAL AUDITORY MECHANISMS 



603 



than it previously had seemed. It is possible this 

 region may prove to be, as Kiang suggests, a transi- 

 tional area or a fringe portion of A I. 



TOPOLOGIC AND TONOTOPIC PROJECTION 



An investigator entering upon the serious stud\ 

 of the neural aspects of audition 25 years ago in- 

 evitably found that the single most engrossing topic 

 of study and speculation was that of the neurological 

 basis of pitch perception. This was not at that time 

 a new tendency., for von Helmholtz was in large 

 part responsible for initiating it many years before 

 by expressing the idea that the basilar membrane 

 of the cochlea resonated in different, narrowly re- 

 stricted regions to different frequenceis of .sound. It 

 followed that if the cochlea is thus an analyzer of 

 frequency, it must be reflected faithfully in the brain 

 in order to make the results of its analytical efforts 

 available to conscious processes. There was little 

 opportunity to test this hypothesis rigorously until 

 the advent, during the 1930's, of instruments which 

 would reliably measure the neural results of stimula- 

 tion by sounds. When this occurred, there was a 

 rapid increment of interest in auditory neurophys- 

 iology and, in natural consequence, in study of the 

 anatomy of the auditory pathway and of behavior 

 as related to audition. It was quickly established that 

 different parts of the cochlea do indeed respond dif- 

 ferently to different stimulus frequencies, though not 

 for the reasons nor in the manner which von Helm- 

 holtz thought, and this served to whet interest in the 

 central reflection of the phenomenon. 



Another factor in the rapid acceleration of interest 

 in auditory neuroanatomy and neurophysiology was 

 the then recent demonstration of a very preci.se point- 

 to-point projection of the retina through the optic 

 tract and lateral geniculate body to the occipital 

 cortex. It was conceived that this sort of anatomical 

 arrangement might be characteristic of sensory projec- 

 tion systems in general. Certainly a similar orderliness 

 and topologic precision could be di.scerned in the 

 somesthetic system, in which the functional counter- 

 part of the visual field map was the body surface 

 map. Why not a projection of the organ of Corti 

 and a tonal map for the acoustic pathway? 



The central auditory pathway, unfortunately for 

 those theories, is more complex in its multinucleate 

 interconnections than the visual or somesthetic path- 

 ways and it has not yielded easily to being fitted 

 into the same general scheme. Nevertheless, the nor- 



mal anatomy of the system is not without some indi- 

 cations of orderliness and a variety of experimental 

 techniques has revealed even more. Likewise, on the 

 functional side, no easy uncomplicated scheme of 

 matching frequencies and fibers has presented itself. 

 It has gradually become evident that this system is 

 unicjue among sen.sory systems and presents problems 

 peculiar to itself; however, it has also become more 

 apparent that some relationship exists between ana- 

 tomical location and location in the audible spectrum. 

 Although it is somewhat awkward to do so, it will 

 be best to consider structural and functional localiza- 

 tion together, and the story will ije more coherent 

 if we largely ignore chronological sequence. 



One of the more conspicuous features of the nerve 

 supply of the organ of Corti and the termination of 

 the cochlear nerve fibers in the cochlear nuclei is 

 their orderly anatomical dispositions. At their en- 

 trance into the cochlear nuclei, the cochlear nerve 

 fibers bifurcate along a curving line such that the 

 linear relationship of their origins in the cochlea from 

 apex to base is preserved (59, 75) with those from the 

 apex bifurcating lateroventrally, those from the base, 

 dorsomedially. The two branches of each fiber then 

 pass respectively to dorsal and ventral cochlear 

 nuclei and multiple terminations among the cells 

 of these nuclei. Single unit responses in the dorsal 

 cochlear nucleus to pure tone stimuli were shown by 

 Galambos & Davis (34, 35) to respond selectively 

 to tones of different frequency, each having its charac- 

 teristic frequency. Very recently. Rose et al. (84), 

 applying a similar microelectrode technique, ex- 

 plored the dorsal nucleus more systematically and 

 demonstrated an orderly pattern of frequency re- 

 sponse, the basic feature of which is that characteris- 

 tic frequencies of the single units vary systematically 

 from high at the medial (dorsal) edge to low at the 

 lateral (ventral) edge. Less complete data indicate 

 further that a similar arrangement is repeated in 

 each of the two divisions of the ventral nucleus. 

 If we accept for the moment that the base of the 

 cochlea is concerned with high frequency reception 

 and the apex with low (a concept which will need to 

 be qualified presently), the frequency distribution 

 in the cochlear nuclei corresponds to the pattern of 

 nerve terminations. This study affords the first step 

 toward an explanation of the as yet inexplicable mean- 

 ing of the elaborate organization of the cochlear 

 nuclei. 



The story so far appears to be simple straightfor- 

 ward testimony in favor of the uncomplicated hy- 

 pothesis that each narrow segment of the organ of 



