CENTRAL AUDITORY MECHANISMS 



595 



confirmed by experiments reported by the same au- 

 thors (io8) in which the deficits in cortical response 

 to click stimulation were noted after local lesions in 

 the cochlea. These experiments then introduce in the 

 auditory cortex an organization based on an internal 

 integrity of the auditory projection pathway such that 

 the anatomical pattern of the cochlea seems to be 

 faithfully represented in the cortical receiving station. 

 A fuller discussion of this 'point-to-point' feature of 

 anatomical projection and its functional implications 

 will be found in another section of this chapter. 



Woolsey & Walzl introduced the terminology by 

 which the two strips of auditory cortex noted in the 

 foregoing paragraph were designated respectively 

 'A r (the superior or dorsal strip) and 'A IF (the 

 inferior or lateral strip). When this organization of 

 the cortical auditory field was subjected to further 

 study involving, in addition to electrophysiological 

 methods, cytoarchitecture and retrograde degenera- 

 tion in the medial geniculate body following selective 

 extirpation of parts of the auditory cortex by Rose 

 (83) and Rose & Woolsey (85), a further revision of 

 the terminology became necessary. For, as their re- 

 sults showed, A I occupies a more limited area than 

 originally designated by Woolsey & Walzl (fig. 5) and 

 is the only part whose destruction leads to widespread 

 degeneration in the pars principalis of the medial 

 geniculate. Cytoarchitectonic study shows that A I 

 and A II differ from each other and the anterior parts 

 of both diflPer from the posterior. These findings lead 

 to the map presented by Rose & Woolsey (fig. 5) in 

 which the auditory area is now divided into A I, A 

 II, and EP, the latter being compounded of the pos- 

 terior parts of the original A I and A II. It will be 

 noted that these areas now show varying degrees of 

 correspondence to those of earlier studies. For ex- 

 ample, A I now is closely similar to the more restricted 

 electrically responsive area shown by Ades (i) and 

 Kormiiller (50) and to the cytoarchitectonic maps of 

 Bremer & Dow (17) and Waller (106). It also cor- 

 responds to the posteroinferior portion of Vogt's (104) 

 old map based on myelination time. A I plus A II 

 now resembles the electrical map area of Bremer & 

 Dow (17), the geniculocortical projection area of 

 Woollard & Harpman (iio), and corresponds with 

 somewhat lesser fidelity to the anterior part of the 

 Campbell (21) map. EP corresponds clo.sely to the 

 posterior ectosylvian 'secondary area' of Ades (2). It 

 would appear that the restriction of responsive area 

 shown in the earlier o,scilloscopic studies may have 

 been due to the relative weakness of responses in A II 

 (except at its anterior end) and in EP which were 



not detected by the comparatively poor instruments 

 then available. The re-emergence of the EP area, 

 as it is now commonly called, plus the reaffirmation 

 by Kiang (49) that EP is to .some extent functionally 

 dominated by A I revives the question of the func- 

 tional significance of such a cortical interrelationship. 



At this point, while it has become apparent that 

 the limits of the cortex which can be activated by 

 acoustic stimulation may not have been completely 

 and finally defined, it will be useful to depart briefly 

 from the development of this essentially anatomical 

 concept to consider some functional studies. These 

 are of interest not only as they contribute to correla- 

 tion of structure and function, but also as they reflect 

 on the extent and internal organization of the auditory 

 cortex. 



The history of functional studies of the auditory 

 cortex is, to a great extent, a history of increasing 

 complexity of stimulus and experimental learning 

 situations. It is also a study in progression of con- 

 ceptualization of auditory function. It begins with the 

 experimenter striving for valid criteria to show simply 

 whether or not the experimental animal hears and 

 continues at present as a search for ways in which 

 auditory discriminative ability of animals can be 

 accurately assessed. 



Some of the earlier efforts to estimate the cortical 

 contribution to hearing in animals took the form of 

 hearing tests of greater or lesser refinement, following 

 total or hemidecortication (12, 38). It was demon- 

 strated that the decorticate dog can still acquire a 

 crude conditioned response to sound although not 

 nearly as readily as an intact animal. Although the 

 animal could acquire the habit, his absolute intensity 

 threshold was higher by 70 db (38). Other workers 

 (12) were less impressed by the auditory deficit in 

 decorticate cats. The decorticate animal, however, 

 shows a general debility and inattentiveness which is 

 more impressive than an auditory or any other specific 

 sensory defect. This leads one to suspect that any test 

 of hearing in such a preparation may be contaminated 

 to a considerable degree by other deficits which have 

 more to do with general integrative capacity than 

 with hearing per se. 



To avoid this difficulty, several workers resorted 

 to extirpations of, as they thought, specifically audi- 

 tory cortex. The theory was that if the cortical audi- 

 tory projection area is removed, then the entire 

 cerebral cortex is effectively eliminated from partici- 

 pation in any learning or conditioning process that 

 involves stimulation by sound. If this were so, then 

 any auditory function present before but absent after 



