CENTRAL AUDITORY MECHANISMS 



60 I 



cortex concealed within the Sylvian sulcus, part of 

 it, in fact, facing inward toward the insula. This 

 makes necessary some special preparation in order 

 to gain access for the exploring electrode. Earlier 

 workers usually accomplished this by extensive re- 

 movals of the overhanging frontal and parietal 

 operculum, the latter of which, as later events have 

 shown, actually contains some auditory responsive 

 cortex. This was therefore missed until more re- 

 cently Pribram and his coworkers (73) were able to 

 expose the areas in question without major destruc- 

 tion of tissue. 



The first electrophysiological demonstration of the 

 simian auditory cortex, bv Ades & Felder (6), used 

 click stimulation and the usual exploration for 

 cortical response. An area on the posterior part of 

 the superior temporal plane was found to Ije re- 

 sponsive; this is shown in figure 7. This area is larger 

 than those outlined by Cllark and by Walker but 

 confirms the general location. It is somewhat smaller 

 than the area shown by Poliak (72) to receive genic- 

 ulocortical fibers. 



Licklider (53) and Licklider & Kryter (54), as- 

 suming the Ades-Felder definition of the auditory 

 area to be correct, explored it while stimulating with 

 short bursts of pure tone. They were able to demon- 

 strate a degree of specificity of various parts of the 

 area referable to frequency of stimulation. Bailey 

 et al. (i i) defined similar auditory areas from monkey 

 and chimpanzee, in each case confined to the supra- 



FIG. 7. View of monkey brain with operculum cut away to 

 expose supratemporal plane. Horizontal shaded area shows 'click 

 map' of Ades & Felder (6); crosshaiched area within 'click map' 

 shows area determined by Walker by retrograde degeneration 

 to be medial geniculate projection area. 



temporal plane, and confirmed the tonotopic dis- 

 tribution suggested by Licklider & Kryter. Walzl 

 (107) and Woolsey (112), using different methods, 

 also demonstrated a specificity of cochleocortical 

 projection in the same area but found, in addition, 

 a region of reversed order of projection on the upper 

 (parietal) bank of the sylvian fi.s.sure, thus extending 

 the boundaries of auditory cortex. The aspects of 

 these and other studies which relate to topical pro- 

 jection, localized response to different stimulus fre- 

 quencies or both will be considered in more detail 

 in a section dealing specifically with that aspect of 

 auditory projection. 



The most extensive auditory area yet described 

 for the monkey is that of Pribram et al. (73). They 

 mapped the cortical areas from which electrical re- 

 sponse could be evoked by clicks, exposing the depth 

 of the Sylvian fissure and the insula by gently sepa- 

 rating the lips of the fissure and wedging them 

 apart in various ways. They do not relate in detail 

 the means by which damage to the rich vascular 

 tree of the middle cerebral artery was avoided; 

 however, this surgical tour de Jorce must have been 

 accomplished because the effects of severe heinor- 

 rhage and ischemia in the region supplied by this 

 vascular tree are not e\'ident in the results. The 

 corte.x of the posterior supratemporal plane, superior 

 temporal gyrus, insula and inferior parietal lobe all 

 yielded responses to clicks (fig. 8). On the basis of 

 latency of initial positive deflection and other cri- 

 teria, the authors identify (by inference or direct 

 statement) subdivisions of the total responsive area 

 with those of the cat as follows: /) the posterior 

 supratemporal plane with A I; j?) the anterior margin 

 of responsive area of supratemporal plane, posterior 

 insula and posterior inferior parietal operculum 

 with 'secondary' area of Ades & Bremer and, hence, 

 EP of Rose & Woolsey; and 3) the parietal opercu- 

 lum with S II. This analysis omits most of the re- 

 sponsive area of the posterior insula which corre- 

 sponds roughly to the 'second' auditory area (or 

 simian A II) of Walzl (107) and Woolsey (112). In 

 this regard the data of Pribram et al. furnish no 

 parallel to the Walzl & Woolsey data because the 

 definition of A I and A II in the lexicon of the latter 

 two authors hinges upon the presence in each of 

 cochlear projections of mutually opposite orienta- 

 tion. 



Pribram et al. include data on retrograde degenera- 

 tion after lesions of the posterior supratemporal 

 plane but not of any other part of their responsive 

 area. So far as this goes, it confirms the impression 



