SENSORY DISCRIMINATION 



'459 



base of the cochlea and that as frequency of the stim- 

 ulus is lowered, the region of maximal excitation 

 moves towards the apex. It is also generally agreed 

 that the degree of cochlear analysis, particularly for 

 low tones, is not sufficient to account for the fineness 

 of pitch discrimination. If pitch discrimination de- 

 pends upon a place principle, then neural processes 

 must result in some 'sharpening' or 'channeling' so 

 that frequencies that can be discriminated activate 

 separate neural units. To demonstrate such sharp- 

 ening in a sensory system, Bekesy (20) has used a di- 

 mensional mechanical model of the cochlea with the 

 tactual receptors of the forearm of a human subject 

 serving as the receptor cells of the basilar membrane; 

 the traveling waxes which under stroboscopic light 

 may be seen moving along the surface membrane ol 

 the model are felt by the subject as a vibratory stimu- 

 lus applied to a fairly sharply localized region on the 

 forearm. The phenomena demonstrated by BeJcesy's 

 model imply that 'inhibitory' or 'suppressor 1 processes 

 in the afferent nerve pathways from the skin result 

 in a channeling of neural activit) . 



The fact that the impulses in peripheral and central 

 neural pathways are synchronous in frequency with 

 stimulating tones throughout a wide range of fre- 

 quencies m.iv also be used in accounting for pitch 

 discrimination. Wever (220) has suggested that fre- 

 quency of nerve impulses may provide the principal 

 cue for discrimination of tones in the lower range, 

 perhaps up to 400 cps, for intermediate frequencies, 

 a combined place and frequency principle may 

 operate (400 to 5000 cps) with place being of primary 

 importance for the highest audible frequencies (above 

 5000 cps). 



Several investigators have recently called attention 

 to the long-known but often disregarded fact that 

 there are at least two kinds ol subjective experience 

 which we tend to include under the single heading of 

 pitch perception. For example, we perceive tones such 

 as those produced by an oscillator or organ pipe and 

 we say that one tone is higher or lower in pitch than 

 another. We can also make similar judgements about 

 sounds which have an intermittent, rougher, noisier 

 characteristic; these sounds can be matched in pitch 

 with pure tones but they are by no means identical 

 to the observer (49, 125, 126, 146, 180). As Licklider 

 (125) notes, not only are there two attributes of pitch 

 sensation, there are also two characteristics of the 

 physical stimulus for pitch, namely frequency and 

 periodicity. He postulates two neural mechanisms to 

 account for the two attributes of pitch sensation. One 

 is the classical frequency analysis according to place; 



the second is a neuronal autocorrelation analysis based 

 on periodicity. The latter operates only for frequencies 

 in the lower range, perhaps up to 1000 cps. 



There is not only conclusive evidence for frequency 

 analysis in the cochlea but also for topographic pro- 

 jection of the cochlea in neural pathways and centers 

 up to and including the auditory areas of the cortex 

 (69, 71, 84, 94, 106, 107, 109, 124, 203, 205, 208, 210, 

 211, 227, 228). The fact of such projection does not 

 necessarily mean that it provides the basis for pitch 

 discrimination. As Lashley ( 120) has pointed out, the 

 maintenance of systematic spatial organization lrom 

 peripheral end organ to cortex max be the result of 

 the mechanics of embryonic development. Wires may 

 be strung side by side in a telephone cable, but mes- 

 sages sent over these wires ma) still be in a frequenc) 

 code. Nevertheless, there remains the fact that in a 

 system such as that formed by the auditory neural 

 pathways, spatial arrangement of axons in tracts and 

 of the cells upon which the) end must inevitably play 

 a role in determining the manner in which afferent 

 events produce the elletent activity which exenlu.ilh 

 results iii the final acts of sensor) discrimination. 



To the inxcstigatoi searching for an explanation in 

 neurophysiological terms of sense-quality discrimina- 

 tion, the rather beautiful pit lure ol' lonotopie organi- 

 zation in the auditor) system is, at first, both reassur- 

 ing and exciting, reassuring in that it suggests that a 

 simple principle, spatial organization, max be funda- 

 mental for discrimination of a sensory quality and 

 exciting because of the possibilities lor experimental 

 test. 



When the experimental lest is made, the results are 

 disappointing, at least at liist consideration. Complete 

 bilateral ablation of the tonotopicallx organized audi- 

 tory cortex produces little or no elleet upon the ca- 

 pacity of experimental animals to discriminate 

 changes in frequency of tones (36, 144, 156, 179). 

 Of course, the tonotopic organization exists in sub- 

 cortical centers and, as we have alread) seen, learned 

 responses to sound cues can be made in the absence 

 of all cortex, nexei theless, the results may seem some- 

 what surprising in \ iew of the fact that the auditory 

 cortex provides a better structural possibility for dis- 

 crete spatial differentiation than lower centers which 

 have a lesser number of nerve cells and consequently 

 lesser possibilities of discrete connections. At least, 

 it might be expected (if topographic organization is 

 important in sense-quality discrimination) that the 

 fineness of frequency discrimination might be altered 

 by ablation of the auditory cortex. 



Since the initial studies of Blix, Goldscheider and 



