6oo 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



assuming this to be true and predicating a new study- 

 on lliis assumption has, by his findings, introduced 

 some new confusion which has then required its 

 own gradual resolution. We have now arrived at an 

 interesting dilemma in which we recognize several 

 types of auditory thalamocortical projection to a 

 wide lateroventral extension of the auditory area 

 as first defined and, in addition, projection from 

 hitherto nonauditory thalamic nuclei to a cortical 

 area originally considered to belong to the somes- 

 thetic svstem but now known to be excitable also by 

 sound (and, indeed, Ijy the nonacoustic labyrinth 

 as well). Moreover, these latter-day auditory areas 

 seem to play essential roles in the mediation of 

 auditory-guided learned behavior. When we add to 

 this the findings of Lilly which tend to blur func- 

 tional if not anatomical boundaries, we begin to be 

 less impressed than we once were with the possi- 

 bility of singling out certain areas whose sole re- 

 sponsibility and exclusive prerogative lie in the 

 realm of auditory integration. On the other hand, 

 we must be equally careful to avoid the other horn 

 of the dilemma by keeping in mind that, however 

 dim the boundaries may become, the areas we call 

 auditory do respond differently to sound than do 

 other cortical areas and they do show differences 

 among themselves. 



Auditory Cortex in Primates 



The foregoing section was ba.sed almost entirely 

 on the brain of the cat. Comparable studies on the 

 primate brain are far fewer in number and com- 

 paratively lacking in the area of behavioral studies. 

 Otherwise, a history of developing knowledge of the 

 primate cortical auditory areas would parallel that 

 in the cat since neurophysiology traditionally uses 

 the cat for pilot experiments which, after trial, 

 modification and revision, can be applied to the 

 monkey. The history of monkey experiments reflects 

 the greater efficiency which is made mandatory by 

 the expense of buying monkeys out of the charac- 

 teristically meager operating budget for neurophys- 

 iological studies. 



The early development of knowledge of the 

 monkey auditory cortex is similar to that of the 

 cat, often appearing in the same accounts, such as 

 those of Ferrier, Munk and Campbell. It will suffice 

 here to say that by the beginning of the twentieth 

 century, inference, extrapolation and inspired guess- 

 work, based on some knowledge of human and 

 carnivore brains, had implanted firmly and widely 



the belief that the primate auditory area is located 

 somewhere in the superior temporal convolution. 

 Fortunately in view of this, the facts, as they subse- 

 quently accumulated, support this belief. 



Aside from the cytoarchitectural studies of the 

 earlier neurologists, the modern investigation of the 

 primate auditory area may be said to begin with the 

 studies of Poliak in 1932 (72) based on Marchi 

 studies of monkey brains after lesions in the medial 

 geniculate body. He described the course and 

 terminations of the auditory radiations, defining as 

 the cortical projection area thus delineated the 

 greater part of the superior surface of the superior 

 temporal gyrus. The concentration of terminations 

 was greater posteriorly than anteriorly, the focal 

 zone coinciding with an elevation toward the pos- 

 terior end of the concealed face of the gyrus which 

 Poliak likened to Heschl's convolution in man. 

 Poliak described a lesser concentration of fibers 

 which reaches the lateral face of the superior tem- 

 poral gyrus. 



Walker (105) and Clark (23), both using the 

 method of retrograde degeneration in the medial 

 geniculate body following lesions in the superior 

 temporal cortex, are in general agreement with 

 Poliak on the location of the projection area of the 

 medial geniculate; however, both outline a smaller 

 area confined to the posterior part of the superior 

 face of the gyrus. If the situation in the monkey is 

 similar to that found in the cat by Rose & Woolsey 

 (85), in which only A I of all the auditory region 

 receives essential projection, it would be expected 

 that only the corresponding area in the monkey 

 would be revealed by the retrograde degeneration 

 method. On the other hand, the Marchi method in 

 conjunction with medial geniculate lesions should in 

 addition demonstrate some of the fibers constituting 

 sustaining projections to a wider area. No study of 

 the primate auditory thalamocortical relationships 

 comparable to the Rose and \V'oolsey study of the 

 cat is available. There are, however, some hints de- 

 rived from several other studies that similar prin- 

 ciples may apply. 



Electrophysiological efforts to map the primate 

 auditory cortex, like the comparable studies of the 

 cat, show the same sort of progression. They begin 

 with a limited area and, with improvement of in- 

 struments and methodology, expand and become 

 subdivided. There is, in the monkey, an additional 

 handicap which limited the accuracy of the early 

 studies. This arises from the fact the primate audi- 

 torv area, unlike the feline, lies almost entirely in 



