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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 



bellished but remains basic. The cellular and fiber 

 elements we traditionally consider as components of 

 the central auditory system with few exceptions are 

 the same as those which appear in Ramon y Cajal's 

 diagrams. 



The second phase of research on the central audi- 

 tory pathway has concerned itself with the discovery 

 of correlates between the characteristics of sound 

 stimuli and the anatomicophysiologic mechanisms 

 activated by them. From the beginning, two qualities 

 of sound, frequency and intensity, have been the focal 

 points of efforts to discover the neural correlates of 

 hearing. There appear to be several interrelated 

 reasons for this and for the consequent preoccupation 

 with the pure tone stimulus in auditory research. 

 Perhaps the most compelling force has been the in- 

 fluence of von Helmholtz who, soon after the middle 

 of the nineteenth century, proposed the idea that the 

 cochlea functions as a selectively resonant system in 

 which tones of given frequency produce localized 

 resonance of the basilar membrane. The implication 

 of this is that the end organ functions as an analyzer 

 of sound and delivers to the brain patterns of excita- 

 tion which are already analyzed with respect to 

 stimulus frequency. Besides the influence of von Helm- 

 holtz is that resulting from the relative ease with 

 which pure tone stimuli can be controlled in terms of 

 the standard parameters, frequency and intensity. 

 As will be seen, the experimentalist's bemusement 

 with pure tone has had both an advantageous and 

 disadvantageous effect upon the course of central 

 auditory research. However the relative good and 

 bad may be evaluated, it is impossible to discuss 

 central auditory function either historically or cur- 

 rently without giving a great deal of attention to pure 

 tone studies. 



There has been in recent years a growing trend 

 away from pure tone studies of the central auditory 

 system (at least in their simplest form). There are 

 three reasons for this: /) 'Click' stimulation has been 

 frequently used (where widespread rather than selec- 

 tive cochlear stimulation is desired) in order to avoid 

 the experimental consequences of the frequency 

 specificity which appears, at least to some degree, to 

 be characteristic of the projection pathway. 2) There 

 is difficulty in studying the higher auditory centers 

 by postablational hearing tests to separate the 

 operated from the unoperated animals unless sound 

 patterns more complex than pure tones or auditory 

 functions requiring more than acuity and frequency 

 discrimination are used. 3) There is a feeling among 

 some workers in audition that pure tone studies ignore 



an essential temporal clement in hearing which is 

 introduced when clicks or other complex stimuli are 

 used. 



With this introduction, we can proceed to examine 

 the available data relating more or less directly to 

 the beginnings of neurological explanation of some of 

 the simpler aspects of hearing as these are defined in 

 psychophysical terms. As we do so, it will become 

 apparent that few specific questions of this sort can be 

 authoritatively answered at this time. It will be 

 further apparent that, while considerable progress in 

 thinking about central auditory problems has occurred 

 in the last 25 years, few of even the earliest studies 

 during this period are altogether obsolete, although 

 interpretations and conclusions may be. Therefore, 

 perhaps the best approach to the problem will be a 

 semihistorical one in which we will attempt to de- 

 velop, summarize and evaluate current trends of 

 thinkine. 



CENTR.-\L AUDITORY PATHWAY 



To define as central auditory mechanisms all 

 neural elements which are activated by stimulation 

 of the organ of Corti would impose an impossible task 

 of description because sound stimulation may trigger 

 neural activity of far-reaching systems eventuating 

 finally in the activation of muscles. Consequently, 

 arbitrary limits must be imposed on the description, 

 if possible without causing a corresponding limitation 

 on our thinking of the con.sequences of sound stimula- 

 tion. One might choose, for example, to limit the 

 definition of central auditory mechanisms to the 

 classical pathway of Ramon y Cajal. This would 

 include fibers and nuclei through which may be 

 traced, anatomically, a clearly sequential series of 

 connections from the ganglion of Corti to the cerebral 

 cortex: cochlear nuclei, trapezoid body, superior 

 olivary nucleus, lateral lemniscus and its nuclei, 

 inferior colliculus. Inferior quadrigeminal brachium, 

 medial geniculate body and its fibers radiating to 

 the cortical auditory projection area. However, 

 strict adherence to the classical pathway would make 

 it impossible to explain .several phenomena observed 

 in experiments on the response of medial geniculate 

 body, cerebral cortex and cerebellum to soimd stimu- 

 lation. In the former case, for example, single neural 

 elements of the medial geniculate body may be found 

 with a latency of response far too long to be accounted 

 for by impulses which are transmitted via the tradi- 

 tional pathway; hence, these impulses must be carried 



