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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



operation couW be said to depend upon mediation 

 through the auditory cortex. (Of course, as we now 

 know, the geniculotemporal radiation is not quite the 

 bottleneck it was then supposed. It is not the only 

 avenue through which excitation aroused by sound 

 may reach the cerebral cortex, and it may not ije 

 the only effective avenue by which such excitation 

 can produce a specifically auditory cortical sign.) A 

 series of studies in which various aspects of hearing 

 were tested before and after auditory cortical extirpa- 

 tion were carried out beginning about 20 years ago. 

 These yielded results which were surprising because 

 of the difficulty encountered in seriously impairing 

 auditory function. Several specific studies are con- 

 sidered in the following paragraph. 



The earlier workers in this area were convinced 

 that the intensity threshold of hearing for pure tones 

 constituted the proper initial criterion of cortical 

 auditory function. This led immediateh to apparent 

 discrepancies in results between different laboratories 

 and even different individuals in the same laboratories 

 (51, 58, 74) because, as ultimately became clear, the 

 factor of recovery time between extirpation of the 

 auditory cortex and retesting of thresholds is crucial. 

 Those who were retested within a very short time 

 showed varying degrees of threshold elevation, those 

 who waited several days or weeks before being re- 

 tested demonstrated little or no loss of acuity for pure 

 tones. Finally Girden (37) demonstrated that in the 

 dog, after incomplete lesions of the auditory cortex, 

 initial losses in acuity gave way with continued testing 

 and the thresholds returned nearly to preoperative 

 levels. Even then, the blame for discrepant results 

 tended to be fixed on differences in testing methods 

 and on the degree of completeness of destruction of 

 auditory cortex, the latter factor being complicated 

 further by differences in understanding of extent of 

 auditory cortex and by this kind of experiment it.self 

 being used as a criterion of determining that extent. 

 Kryter & Ades (51) demonstrated that ab.solutc in- 

 tensity threshold to pure tones does not rise appreci- 

 ably due to extirpation of auditory cortex in the cat, 

 even when the cortical lesions in some instances 

 extended considerably beyond the widest boundaries 

 suggested for the area. By this time, workers were 

 despairing of the intensity threshold to pure tone as a 

 reliable indicator of cortical auditory function. It be- 

 came apparent, in retrospect, that the confusion of 

 previous studies had occurred, at least in part, becau.se 

 simple acuity as measured in this way is simply not 

 dependent on cortical participation. It appeared logi- 

 cal then to .seek .some more complex manifestation of 



auditory function which could be tested by a condi- 

 tioning method and which might prove to be de- 

 pendent on auditory cortex. 



A series of studies directed toward that end began 

 in 1946 with the report of Raab & Ades (74) indicat- 

 ing that, while of interest in other respects, the func- 

 tion of discrimination of differences in intensity of 

 sound, measured in terms of difference limens, was 

 not the cortex-bound function .sought. This impression 

 was confirmed by Rosenzweig (87). The next obvious 

 point of attack was the ability of the animal to 

 discriminate between small differences in frequency 

 before and after extirpation of auditory areas. This 

 kind of study has been done by Butler et al. (20), by 

 Meyer & Woolsey (64) and by Allen (9). Before 

 discussing these studies, it is necessary to digress 

 briefly to note the addition of still another cortical 

 area which shows .signs ol auditory function. 



In 1945, Tunturi (103) described in the dog an 

 area in which electrical response to auditory stimula- 

 tion could be evoked. This area lies outside any of 

 those previously described as auditory in the dog or 

 as homologous areas in the cat. It lies in fact partly 

 in the second somatic area (43). Also in 1945 Allen 

 (9) using Tunturi's map found that, whereas aljlation 

 of the traditional auditory areas temporarily impaired 

 but failed to destroy permanently the ability of dogs 

 to discriminate widely different frequencies, this 

 ability was permanently lost if the third auditorv area 

 of Timturi were also destroyed. Later studies on the 

 cat have confirmed the fact that auditory stimulation 

 elicits response in the second somatic area (15, 16, 



65, 70)- 



Meyer & Woolsey (64) trained cats to respond to 

 change in frequency of a gi\en tone at irregularly 

 spaced intervals in a series of 2 sec. tones which were 

 otherwise alike. Once the cats were trained, a rough 

 difference limen for frequency was determined. They 

 then extirpated, symmetrically, in \arying combina- 

 tion the following cortical areas: A I, A II, EP, 

 suprasylvian gyrus, temporal region (see fig. 6) and 

 the cerebellar tuber vermis. Following operation, the 

 animals were retrained and retested. It was foimd 

 that if A I, A II, EP and S II (second somatic area) 

 were completely destroyed on both sides, the animals 

 could no longer achie\e the frequenc\- discrimination. 

 No other combination of lesions had this efl'cct and 

 if remnants of A I and A II escaped damage, fre- 

 quency discrimination was maintained. Butler el al. 

 (20) used a basically similar plan but with what they 

 felt was a more reliable and critical method of testing. 

 In addition, they carefulK' analyzed the retrograde 



