CENTRAL AUDITORY MECHANISMS 



609 



posing, for argument, that pitch discrimination is a 

 'complex function', it still does not follow that the 

 auditory cortex is the only or even the best neural 

 matrix in which the discrimination may be made. 

 If we consider the possibility that a kind of signal to 

 noise ratio operates between frequency-sensitive and 

 nonfrequency-sensitive elements, then the auditory 

 cortex affords the poorest ratio of any part of the sys- 

 tem. Finally, the animal behavioral experiments may 

 be clouded to the extent that the learning and reten- 

 tion and conditioned response aspects of the method 

 are neurologically inseparable from the purely audi- 

 tory aspects. 



OTHER .ASPECTS OF CENTRAL AUDITORY FUNCTION 



If it is apparent that the auditory system contains 

 a tonotopic organizational pattern, it is equally ap- 

 parent it is not filled by this pattern. Like many paths 

 through a jungle, this tonotopic path through the 

 auditory system has been found only because it was 

 suspected and sought. It is also well to consider the 

 jungle where other matters may be equally signifi- 

 cant. By far the greater number of neural elements 

 in the system cannot be demonstrated to have any- 

 thing to do with this parameter (frequency) of the 

 acoustic stimulus. Such has been the preoccupation 

 with it, however, that any discussion of audition is 

 inevitably dominated by it. Nevertheless, some in- 

 vestigative attention has, in fact, been otherwise 

 directed and more should be. The remainder of the 

 chapter will be devoted to several other aspects of 

 central auditory function which have received some 

 and require more attention. 



Loudness 



The neurophysiological correlates of loudness 

 probably cannot be altogether divorced from those 

 of pitch, althoua;h the subject was avoided almost 

 entirely in the preceding section. The main reason 

 for this was the desire to a\oid confusion of issues in 

 an area where much more is known of one side of the 

 issue than of the other. A second, and hardly less 

 compelling reason, howeser, is the status of our ideas 

 about the neural mediation of loudness which is 

 currently as or more confused than at any time in 

 recent years. 



Traditionally, loudness has been regarded, rather 

 vaguely, as being expressed in terms of quantitv of 

 excitation. Whereas frequency was supposed to in- 



volve the appropriate restricted group of fibers, 

 loudness was supposed to be expressed in terms of a 

 greater or lesser proportion of the total cross .section 

 of pathway excited. With the realization that, at 

 least with respect to the cochlea, the total amount of 

 end organ being stimulated and total number as well 

 as site of origin of nerve fibers are involv^ed in the 

 analysis of frequency, it became apparent that the 

 same device could not be used simultaneously for the 

 factor of loudness, at least in a simple way. The pos- 

 sibility exists, however, that some interaction be- 

 tween inner and outer hair cells may lie at the bottom 

 of the mechanical aspect of loudness. Whatever may 

 be the cochlear, nerve and cochlear nuclear corre- 

 lates of loudness, if the hypothesis proposed in the 

 preceding discussion relative to a frequency recoding 

 function of the nuclei is at all correct, the mediation 

 of loudness might also take a different form in the 

 ascending pathway. 



It would probably be a mistake to suppose that 

 loudness could be subserved by any of the possible 

 upward projecting patterns without the addition of a 

 factor of selective neural inhibition by recurrent ele- 

 ments. There is little evidence to call upon in this 

 respect, and the possible significance of such elements 

 will be discussed below in more general terms. For 

 the moment, we can only exercise caution in theoriz- 

 ing about the mediation of loudness, bearing in mind 

 that there is a large factor of relativity inherent in 

 the concept and hoping that further investigation 

 of the cochlear end organ will yield some suggestion 

 as to direction. 



Laterality of Projection 



One of the more distinctive features of the central 

 acoustic system, as contrasted to other .sen.sory sys- 

 tems, is its tendency to bilateral reduplication and 

 its bewildering array of commi.s.sural opportunities. 

 It is obvious the system begins with two ears which 

 are situated on opposite sides of the head, the open- 

 ings of the external auditory meatuses 180° apart 

 in terms of direction of orientation, plus or minus 

 what few degrees of bias may theoretically be im- 

 posed by the presence of the pinna. This immediately 

 suggests the possibility that source and direction of 

 sound may be perceived in part as a consequence of 

 the relative time of arrival or loudness or both of 

 signal for the two ears. From psychophysical studies, 

 it is clear that directionality is one of the properties 

 of sound perception. It is not the function of this 

 chapter to go extensively into the analysis of direc- 



