CENTRAL AUDITORY MECHANISMS 



59' 



in the observation of retrograde chromatolysis in that 

 nucleus as a consequence of destruction of the vermis. 

 The tectopontile tract described by Rasmussen (77), 

 which provides communication from the inferior 

 colHculus to the pons, would also seem a possible 

 avenue from auditory pathway to the cerebellum. 



What may be the functional significance of such a 

 system is a proper but as yet unanswered question. 

 The further information that stimulation of the 

 audiovisual" area of the cerebellum may evoke re- 

 sponse in the cerebrocortical auditory area (94) and 

 that stimulation of the latter elicits response from the 

 former (42) may offer some help in answering the 

 question. One suggestion, which has been made 

 repeatedh' by one of the more ardent advocates of a 

 cerebellar contriljution to audition, is that the coch- 

 leocerebellocerebral pathwas pro\ides an alternative 

 pathway of auditory projection or integration (or 

 both) to the cerebral cortex which inay be implicated 

 in the preservation of auditory function after inter- 

 ruption of the regular cortical projection pathway. 

 This is neither a necessary nor a likely hypothesis. 

 The fact that the cerebellum receives an auditory 

 projection does not imply that it is implicated in the 

 psychological phenomenon of audition per se. 



The cerebral connections to the cerebellum are 

 presumably those described b\' Mettler (62) as pro- 

 jecting from the cat's cerebral auditory area to the 

 pons froiTi which a pontocerebellar relay would be 

 the expected pattern. A cerebellocerebral pathway 

 from the cerebellar cortex to the cerebellar nuclei to 

 the thalamus to the cortex would be a plausible or 

 even probable explanation of the functional evidence 

 that the auditory area of the cerebellum projects to 

 that of the cerebral cortex. It is the more plausible 

 when we recall the similar type of anteriorly directed 

 projection of the brachium conjunctivum. The pattern 

 of interprojection of cerebellar and cerebral areas is 

 thus probably no different in relation to the auditory 

 than to any of several other functional systems. The 

 most likely explanation of cerebellar auditory (and 

 visual) connections, therefore, would seem to be that 

 these add the information of distance receptors to that 

 of contact and proprioceptive receptors as these may 

 modulate the cerebellar contribution to regulation of 

 motor patterns. 



sumably at brain-stem levels. Certainly, it has been 

 demonstrated that arousal can be induced by auditory 

 stimuli in animals in which the standard acoustic 

 projection pathway has been bilaterally interrupted 

 (30, 57, 61, 96, 97). Thus, there is through the retic- 

 ular system another route from brain-stem acoustic 

 mechanisms to cerebral cortex, though this is of 

 general rather than specifically auditory distribution. 

 The ascending reticular system .seems to be a diffuse 

 and multisynaptic route, so the auditory and other 

 specific modalities of input tend to be swallowed up 

 in the more comprehensive functions of the ascending 

 reticular system. It is impossible to say to what ex- 

 tent, if any, this system may serve a specific sensory 

 function, though it would appear that this could not 

 be extensive in the light of what we know about 

 reticular function. 



Descending Fibers in the Auditory Pathway 



Fibers which proceed from rostral to caudal re- 

 gions, that is from higher to lower stations in the 

 auditory projection pathway, have been described at 

 all levels from the cerebral cortex to the cochlea. In 

 general these closely parallel the ascending system, 

 although they may bypass nuclei with greater free- 

 dom. They are better known anatomically than 

 physiologically with the possible exception of the 

 olivocochlear tract which was described by Rasmus- 

 sen (78-80). For a review of the available evidence 

 and current studies of the descending auditory path- 

 ways, the reader is referred to Chapter XXXI by 

 Livingston in this volume and to Galambos' recent 

 review (32, p. 502). He indicates that, for the first 

 time, a systematic anatomic study of these by ade- 

 quate degeneration methods is under way. While the 

 information is as yet meager, it is clear that a neural 

 system which provides a possible mechanism by which 

 the sensory system to which it belongs can achieve 

 some degree of .self-regulation may be of the utmost 

 importance in providing explanation of complex 

 functions, the means for which are not obviously 

 available in the organization of the afferent pathway. 



.AUDITORY CORTEX 



Reticular Activating System 



Like other sensory systems, the acoustic makes its 

 contribution to the reticular activating system pre- 



The development of knowledge of the cortical ter- 

 mination of the auditory projection system may be 

 said to have begun with the observations of Ferrier 

 (29) reported as part of a more general work in 1876. 



