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HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY I 



sensitive elements in tiie upper reaches of the audi- 

 tory pathway. This may be a misleading term. What 

 we really mean is that such elements become less and 

 less predominant from the standpoint of numbers, 

 or to state it another way, there is a progressively 

 increasing proportion of elements which have no 

 direct concern with the parameter of stimulus fre- 

 quency. Many of these, however, do respond (as do 

 all the tone-sensitive units) to complex sounds (e.g. 

 clicks, noise) encompassing wide bands of the audible 

 spectrum. We do not know why this is true but, within 

 the bounds of known facts, it is not difficult to en- 

 vision a theoretical switching system by which tone 

 and noise sensitive and only noi.se-sensitive elements 

 would come to exist in parallel. We know that the 

 cochlear nuclei contain at least three projections or 

 replicas of the organ of Corti. According to the micro- 

 electrode evidence (84), these retain the original 

 coding of the cochlea; however, in the output of the 

 nuclear neurons, there is no inherent constraint to 

 preserve the original coding pattern. If, in the ensu- 

 ing relays, we picture one group of cells each of which 

 receives approximately equal synaptic terminations 

 from all parts of one cochlear replica, it would pre- 

 sumably be responsive to summated volleys from a 

 sufhciently wide band of the replica, regardless of 

 position on the replica, and would therefore be re- 

 sponsive to stimuli of frequency bands of a given 

 minimal width, regardless of position on spectrum, 

 but not to narrow bands or pure tones. A second group 

 of cells, each receiving sufficiently concentrated synap- 

 tic terminations from a restricted part of the cochlear 

 replica, would respond to stimuli of frequency bands 

 of narrow proportions or e\en to pure tones. If in 

 succeeding relays the units of the second group began 

 to overlap each other somewhat, we would expect to 

 find the kind of changes which in fact ha\e been found 

 in the response area of successively higher single 

 auditory units, namely some widening at threshold 

 but without a corresponding widening at higher in- 

 tensity where small differences are insignificant. 



That the units showing sensitivit\' to the various 

 segments of the audible spectrum retain, in the main, 

 their positions relative to each other in the ascent to 

 the cortex is evident, although there is also reason 

 to believe there is some degree of dispersion. This is 

 in accord with the observation, direct or incidental 

 by a variety of methods of many in\estigators on 

 several segments of the auditory pathway and on the 

 whole pathway from cochlea to cortex, that the ar- 

 rangement of auditory elements, though often intri- 

 cate with respect to nuclear organization, is always 



orderly and maintains spatial relationships quite 

 faithfully. Macroelectrode studies of relative sensi- 

 tivity of diflTerent regions of the auditory cortex con- 

 firm the impression that, relatively at least, tone- 

 sensitive elements of similar frequency range tend 

 roughl) to group, though not to segregate themselves, 

 and maintain an orderlv relationship to elements of 

 different frequency characteristics. These studies 

 also accord well with those which demonstrate projec- 

 tion of the cochlea to the cortex in a recognizable 

 pattern. 



The less careful reader might, at this ]3oint, feel 

 we have established a good case for the primary rela- 

 tionship of frequency specificit}' and anatomical order 

 and for these conjointly as the prime organizational 

 feature of the auditory system. It must be re-empha- 

 sized that frequency tuning of auditory neural ele- 

 ments and of the o\erall grouping of these as meas- 

 ured by electrical response is relatively fine only at 

 threshold intensity and at higher intensity is an even 

 less con\incing feature when compared to the pre- 

 cision of the psychophysical phenomenon of pitch 

 discrimination to which, presumably, we must relate 

 it. We can only suppose that the neurophysiological 

 facts so far known reveal to us only a part of the pic- 

 ture. 



Efforts to translate the anatomicophysiological 

 phenomenon of tonotopic projection into terms of 

 hearing in animal experiments have been discourag- 

 ing but possibly needlessly so. There is good reason 

 to believe the meager success of such ventures is the 

 consequence of having asked the wrong questions, 

 these in turn growing out of unwarranted assumptions. 

 One doulile assumption of this sort is that pitch dis- 

 crimination, of necessity, must depend upon intact 

 auditory cortical function because it is a 'complex' 

 auditory function. Neither part of this is necessarily 

 true. Compared to deficits of human auditory func- 

 tion resulting from temporal lobe lesions, pitch dis- 

 crimination would be on the simple side. Deficits 

 in human subjects are not conspicuous unless they 

 involve actual auditory aphasia or unless the patient 

 is subjected to rigorous testing which goes far beyond 

 routine audiometry. It might also be pointed out 

 that it is impossible to prove the presence of a com- 

 plex auditory deficit such as auditory aphasia unless 

 one can first establish that basic perception is essen- 

 tially intact. The only auditory function which seems 

 to have been clearly tied to the cortex in aniinals, 

 namely discrimination between two three-tone pat- 

 terns, would appear off hand to be of a very diflTerent 

 order of integrati\c complexity than aphasia. Sup- 



