CENTRAL AUDITORY MECHANISMS 



587 



over a slower system, perhaps one not hitherto con- 

 sidered as auditory (36). A similar phenomenon has 

 been demonstrated in the cerebral cortex (25). In the 

 case of the ccrebelhim, the very fact that response to 

 sound may be evoked requires some addition to the 

 classical definition of central auditory mechanisms. 

 Finally, while they have been mentioned from time to 

 time for manv years, fibers coursing within the 

 classical pathway, but running perversely from rostral 

 to caudal instead of ascending, have been until re- 

 cently consistently ignored as functional units. 



In considering the tracts and nuclei which we 

 classify as 'central auditory mechanisms', we should 

 keep in mind several functional requirements, some 

 of which are specificalK' auditory but others are of 

 more general neural significance, that is they are 

 functional requirements of any neural system. Taking 

 up the general requirements first, they have to do with 

 two closely related characteristics of neural systems: 

 /} the tendency for feed-back devices to occur, such as 

 recurrent collaterals by which any neuron mav by its 

 own discharge feed back into itself, or similar mecha- 

 nisms on a recurrent nucleus-to-nucleus basis, this 

 sort of device apparently serving to amplify the effect 

 of input into the system; and 1^) the apparent ability of 

 the system, by its own activity, to modify, modulate 

 or control that activity, a function which could be 

 served by the same kind of recurrent feed-back circuits. 



The more specifically auditory requirements have 

 to do /) with the mechanisms by which sound is 

 analyzed with respect to frequency and intensity, and 

 the combinations and permutations of these, and the 

 manner in which these are impressed upon the 

 brain, and 2) with the arrangements by which the 

 activities of the auditory system impress themselves on 

 integrative mechanisms of the brain and ultimately on 

 motor systems through which responses to auditory 

 stimulation may be mediated. It has been common to 

 speak of this kind of function in terms of ' levels of 

 integration', as though integration of auditory infor- 

 mation could be classified as to its own complexity 

 and to the complexity of response called for, and each 

 category relegated to a particular rostrocaudal level. 

 This concept may have a very general kind of validity, 

 but it will become increasingly evident that its useful- 

 ness is questionable. It must be applied with great 

 caution, and the delegation of degrees of perceptual 

 judgment to one or another of the cell masses of the 

 central auditory pathway represents a pattern of 

 thinking which is more dangerous than helpful. 



There appear to be significant differences between 

 the pathway of projection from sensory end organ to 



cerebral cortex in the auditory system as compared 

 with other afferent systems, for example the somatic 

 sensory systems. The latter are characterized by a 

 second-order link with a thalamic nucleus, whereas 

 in the auditory the fibers reaching the thalamus are 

 at least third order, and there are probably relatively 

 few that are not of fourth, fifth or higher order. There 

 is, in other words, in the auditory pathway a more 

 complex and devious system of nuclear interruptions. 

 One factor leading to this situation must have been 

 that, since the cochlea and its central connections de- 

 veloped phylogenetically late as compared with other 

 sensory .systems, the ascending pathway had to be 

 constituted from such scattered elements as were still 

 open to modification in a neural matrix otherwise too 

 fixed in pattern to permit of a new through pathway. 

 Figure i shows diagrammatically the main features 

 of the known connections of the auditory pathway. 

 Cerebellar connections are not shown. Connections 

 with the reticular system are shown schematically. 

 These are actually not known in anatomical detail, 

 but some .such connections must be present according 

 to physiological evidence. 



Cochlear Nuclei 



The course and terminations of two types of den- 

 dritic processes of cells of the spiral ganglion of Corti 

 have been studied and described by several authors 

 (e.g. 28, 59, 75). Despite the rather elaborate differ- 

 entiation thus revealed in the end organ of hearing, 

 the course and terminations of the axons of the 

 ganglion cells show no such corresponding differenti- 

 ation; rather, the terminations display a pattern of 

 organization of a different sort. For practical purposes 

 then, our story of the central auditory pathway may 

 begin with a group of fibers, showing little differenti- 

 ation, entering the medulla at the inferior border of 

 the pons as the cochlear portion of the eighth cranial 

 nerve. Immediately the fibers begin to bifurcate and 

 the resultant branches to pass to their terminations in 

 dorsal and ventral cochlear nuclei (28, 75). Each 

 fiber is said to terminate on 75 to too cells of the coch- 

 lear nuclei. This being true, it must also follow that 

 each cell of the nucleus receives terminations from 

 many incoming fibers because the total number of 

 cochlear nucleus cells is only about 2.9 times the num- 

 ber of cells of the ganglion of Corti (22). 



The cochlear nuclei are divisible each into several 

 parts. The organizational pattern in the dorsal coch- 

 lear nucleus is laminar, that in the ventral is not but 

 shows a similar degree of complexity and differenti- 



