CENTRAL AUDITORY MECHANISMS 



6X1 



Dispersion of Excitation, Recurrent 

 Pathways and Inhibition 



In this final section, the author would like to take 

 up as a group certain considerations which have 

 been touched upon in earlier sections of this chapter 

 and by other authors. Presumably, these should ul- 

 timately apply to the elucidation of central auditory 

 functions but are at present speculativ-e and little 

 supported by evidence. They must for the present 

 be spoken of in general terms. The evidence which 

 will be cited is more by way of justifying the specula- 

 tion than of supporting a theory; indeed, we have no 

 complete coherent theory to offer but only a number 

 of facts, some connected, some possibly connected, 

 and some suggestions of things which should be con- 

 sidered in future investigations. 



One of the distinguishing features of the auditory 

 system is the multiplication of elements at succes- 

 sively higher nuclear stations in the pathway (22). 

 Coupled with this are the twin physiological phe- 

 nomena of temporal dispersion of electrical response 

 to brief (click) stimuli and amplification of response 

 (5, 25, 32, 36). In this case, amplification takes the 

 form of increased amplitude of response and tem- 

 poral dispersion of extending the duration of response 

 if, for example, we compare these factors from coch- 

 lear nuclei to medial geniculate body. The simplest 

 way of accounting for these phenomena is by assum- 

 ing the amplification to be the consequence of the 

 larger number of units available to be fired in the 

 larger, more rostral nucleus, and the temporal dis- 

 persion to be the consequence of the multisynaptic 

 connections which result in a lateral lemniscus com- 

 posed of several different orders of fibers. There are 

 additional possibilities which might contribute to 

 both phenomena. Each nucleus traversed contains 

 within itself the neural matrix requisite (quantitatively 

 speaking) to an internal temporal and spatial dis- 

 persion process in addition to that we are attributing 

 to the pathway as a whole. The inferior colliculus 

 constitutes, in one sense at least, a tract parallel to 

 the lemniscal fibers which pass directly to the medial 

 geniculate; lemniscal fibers diverge to the colliculus 

 and fibers from it reconverge on the main path at 

 the medial geniculate, providing a possible feed-back 

 device which might augment the dispersion processes. 



We also know, howe\'er, from many electrophysio- 

 logical studies that amplification and temporal dis- 

 persion are only two of the things which may happen 

 to a burst of activitx' in the auditorv system aroused 



by so simple a stimulus as a click. The response may 

 also be reduced in amplitude or obliterated by pre- 

 ceding or simultaneous auditory stimuli of the same 

 or different type, depending upon where, when and 

 how strong the two are relative to each other and upon 

 where the recording is done. We may, at this point, 

 recall the recurrent fibers mentioned in the early 

 description of the pathway as still another feed- 

 back portion of a complex input to a given nucleus. 

 When we recall that stimulation and inhibition are 

 equally possible consequences of the firing of one 

 neuron into another (depending on conditions at a 

 given instant), we must realize the term "feed back' 

 as used here may be positive or negative with respect 

 to the whole or a part of the pattern being processed 

 in a given nucleus at a given time. 



To make concrete the rapidly growing po.ssibilities 

 in the foregoing paragraphs, there is evidence, cited 

 by Galambos (32) in a discussion of inhibition, that 

 inhibition, intranuclear or that provided by recur- 

 rent fibers or both, may in the cochlear nuclei act 

 to restrict the frequency range to which a given unit 

 responds. He further indicates that other studies are 

 in progress, both anatomical and physiological which 

 we can hope will lead to an expansion of this factual 

 nucleus. 



We are not constrained to limit the possibilities of 

 selective inhibition or other forms of modulation to 

 the elements of the traditional projection pathway. 

 It has already been pointed out in discussing the retic- 

 ular formation that the auditory system feeds in some 

 way into the brain-stem reticular system and thereby 

 exerts its influence, in common with other sensory 

 systems, upon the status of general cortical activity 

 quite independently or indeed in the absence of an 

 intact auditory projection pathway. Adey et al. (8) 

 have recently demonstrated also that various cortical 

 areas (including, as it happens, A I in the cat) fire 

 freely back into the brain-stem ascending reticular 

 system. There are many indirect suggestions in the 

 literature which would lead one to believe that this 

 system also may fire into the specific afTerent path- 

 ways. This would provide a reciprocal arrangement 

 at brain-stem levels which would permit interaction 

 not only, in this ca.se, between the auditory system 

 and the cortex via the reticular system but also be- 

 tween the auditory and other sensory systems. 



Any, all or none of the foregoing connections may 

 exist and work in any, all or none of the ways sug- 

 gested. But one thing is clear, namely that the central 



