CHAPTER XXIV 



Central auditory mechanisms 



HARLOW" W" . A D E S U. S. .\'aval School of Aviation Medicine, Pensacola, Florida 



C: H A P T E R CONTENTS 



INTRODUCTION 



Introduction 



Central Auditory Pathway 



Cochlear Nuclei 



Efferent Fibers from Cochlear Nuclei 



Lateral Lemniscus and Its Nucleus 



Inferior CoUiculus 



Inferior Quadrigeminal Brachium 



Medial Geniculate Body 



Auditory Connections with Cerebellum 



Reticular Activating System 



Descending Fibers in Auditory Pathway 

 Auditory Cortex 



Auditory Cortex in Primates 

 Topologic and Tonotopic Projection 



Summary and Discussion of Topologic and Tonotopic Pro- 

 jection 

 Other Aspects of Central Auditory Function 



Loudness 



Laterality of Projection 



Dispersion of Excitation, Recurrent Pathways and Inhibition 



THE ASSIGNMENT IN THIS CHAPTER is to give an account 

 of the cell and fiber groups of the brain which are 

 more or less directly related to hearing. The coverage 

 of this area is not necessarilv uniform and probably 

 expresses a certain degree of the author's bias, not only 

 in choice of material but perhaps also in interpreta- 

 tion. The author, adinitting to an unspecified degree 

 of bias in both respects, finds it futile to offer apology 

 but instead suggests to the reader the additional study 

 of a recent review (32) and volume (log) to which 

 text and bibliographic references will be found. The 

 reference listing for this chapter is likewise not com- 

 plete, especially in a historical sense, and the reader 

 may remedy this deficiency by reference to the works 

 just mentioned and also to the recent Bibliography on 

 Hearing (98). 



The history of research on the central auditory path- 

 way goes back only a few years into the last century, 

 not even as far as the study of other sensory systems. 

 The greater volume of significant work has been done 

 in the last three decades only. This history can be 

 divided into two phases which, while they began at 

 different times, have been largely concurrent. The first 

 necessarily concentrated on defining the neural struc- 

 tures which are primarily concerned with sound 

 stimulation and those showing fiber connections, more 

 or less direct, with the input from the cochlear nerve. 

 The early functional studies suffered, as we now know, 

 from the fact that the pathway of projection from ear 

 to cerebral hemisphere is bilateral with only slight 

 contralateral preference; consequently, clinical studies 

 of one-sided damage to the brain produce only meager 

 auditory deficits, and these can be demonstrated only 

 by very sensitive tests. For this reason, studv of central 

 auditory inechanisms lagged behind studies on other 

 sensory systems in which there is a great preponder- 

 ance of contralateral projection. It is only with the 

 advent of increasingly reliable physiological appara- 

 tus and the development of more sensitive behavioral 

 testing in the past 25 to 30 years, that tracts, nuclei 

 and cortical areas responsive to auditory stimulation 

 have been reliably defined. This definition continues 

 to be studied and revised, although attention to it 

 has gradually given way to studies of the qualities of 

 response rather than mere presence or absence of 

 response. 



The anatomical aspect of the first phase of central 

 auditory study fared better than the functional and 

 as well as that of other neural systems. The original 

 work of Ramon y Cajal (75, 76), as far as the auditory 

 pathway is concerned, has been amplified and em- 



585 



