CENTRAL AUDITORY MECHANISMS 



607 



more useful by virtue of wide comparability with 

 others.) Hind found areas showing predilection for 

 higher and lower frequencies. There is general 

 agreement with Licklidcr's frequency map but, 

 whereas the latter's highest tested frequency was 8 

 kc. Hind's study goes as high as 50 kc. Furthermore, 

 Hind found two high frequency areas, namely an- 

 terior A I and posterior AH, and two low frequency 

 areas, namely posterior A I and anterior A H. On 

 both A I and A H, the area between high and low- 

 could be spoken of only as middle frequency range 

 area, the data permitting no finer gradation. Hind's 

 findings seem to agree with those of Woolsey & 

 Walzl (113). Together, they indicate a broad cor- 

 respondence between cochlea and cortical projection 

 on the one hand and stimulus frequency and cortical 

 frequency .sensitivity on the other. Let us note that 

 the data do not permit us to think here of a finely 

 tuned system. 



It is interesting to note that Hind was able, at each 

 cortical point studied, by varying both frequency 

 and intensity of stimulus, to outline areas of response 

 which look very much like those of single units in the 

 microelectrode studies. The focus of threshold fre- 

 quency is not as sharp, and the response area widens 

 rapidly both up and down the scale. 



Similar studies to those in the cat are available for 

 the monkey and the results, which will not be pre- 

 sented in detail, are similar. Woolsey (112) and Walzl 

 (107) repeated on monkeys their earlier experiments 

 on cats with similar results (cf. preceding section). 

 Licklider & Kryter (54) described a pattern of fre- 

 quency representation showing low frequencies to- 

 ward the anterior part of the auditory area of the 

 supratemporal plane (refer to fig. 8 for orientation) 

 grading to high frequencies most posteriorly. This 

 general arrangement was confirmed by Bailey et al. 

 (11). Kennedy (47) explored the monkey's temporal 

 region according to the same general plan as in Hind's 

 study on the cat. She found no widespread response 

 to tonal stimuli comparaijle to that responsive to 

 clicks described by Pribram et al. (73), although she 

 confirmed their findings with click stimulation. 

 Kennedy did find the presumptive monkey A I 

 area of the supratemporal plane respon.sive to tonal 

 onset which, enhanced by strychnine, yielded fre- 

 quency intensity thresholds for each point similar 

 to those of Hind. Her composite map of frequency 

 representation generally confirms but also extends 

 (with respect to both area and frequency range) 

 the study of Licklider & Kryter. It also considerably 

 sharpens the picture for the monkey and shows the 



pattern to conform to a plan of concentric octave 

 bands, each oriented from medial to lateral. As in 

 the cat, the overlapping of frequency range areas is 

 at least as impressive as their .separation, but the gen- 

 eral trend is perhaps more cleAr-cut than in the cat; 

 howe\'er, Kennedy shows no A II and this helps to 

 to make the results look cleaner as compared to 

 Hind's. 



Summary and Discussion of Tupalogic 

 and Tonotopic Projection 



It can be taken as settled that a degree of frequency 

 specificity is characteristic of some of the neurons of 

 the central acoustic system. In numbers, the.se vary 

 from a great many elements in the cochlear nerve, 

 through progressively diininishing percentages of 

 the total at intermediate recording stations, to an 

 undetermined but certainly small proportion of 

 elements in the auditory cortex. We must also reduce 

 the term 'specificity' to its real proportions, a quali- 

 fication which has often not been made in interpre- 

 ting this kind of data. The term really applies well 

 only if we are talking about threshold intensity and, 

 even with this qualification, it applies best to the 

 more caudally situated recording stations rather than 

 to the thalamus or cortex. A second aspect of this 

 specificity has to do with the manner and degree of 

 expansion of the respon.se area with increasing in- 

 tensity. At the nerve, and to an only slightly lesser 

 degree in the cochlear nuclei, the direction of the ex- 

 pansion is strictly toward the lower part of the scale 

 and in degree is so wide as to make us think that some 

 fibers are stimulated by high tones, many by inter- 

 mediate tones and virtually all by low tones, given a 

 stimulus of sufficient intensity. As we ascend this 

 changes, so that among those cortical elements which 

 are sensitive to tone each, though less sharply tuned 

 at threshold, is comparatively greatly restricted in 

 range of frequency sensitivity even at quite high 

 intensity and expansion of response area is both 

 up and down the scale. Thus, while 'attention' to the 

 parameter of frequency is evident froin cochlea to 

 cortex, the original coding of this information, im- 

 posed by the mechanical characteristics of the coch- 

 lea, is changed, perhaps in the cochlear nuclei, per- 

 haps aided by the superior olivary complex, perhaps 

 even more gradually, so that in the more rostral 

 parts of the pathway, tone-sensitive elements are in 

 one way even more frequency-selective than those in 

 the nerve. 



We have spoken of progressive dispersion of tone- 



