SENSORY DISCRIMINATION 



I 46 I 



Bilateral ablation of the cortical areas for taste in the 

 rat has been shown to result in a deficit in discrimina- 

 tory ability (21). A similar deficiency in taste dis- 

 crimination has been found in the monkey after bi- 

 lateral ablation of the cortex of the anterior part of 

 the island of Reil, the operculum and the anterior 

 supratemporal plane (13). The cortical areas for taste 

 in the monkey have not been defined by electro- 

 physiological methods. 



Of all the sensory systems the investigation of the 

 olfactory system has been particularly unrewarding. 

 (Present knowledge is summarized by Adey in Chap- 

 ter XXI of this Handbook.) Schemes to classify stimuli 

 which arouse sensations of odor have not been satis- 

 factory even from a subjective standpoint. While man 

 can discriminate different odors on a qualitative basis, 

 there are no particular qualities which appear to be 

 distinctive in the same sense that sweet, sour, salty 

 and bitter are for taste. As in taste, substances which 

 arouse similar sensations are not necessarily similar 

 in chemical composition. 



It is difficult to expose the sensory receptors for 

 olfaction and also difficult to isolate and record from 

 single neural uniis of the peripheral olfactory system. 

 Adrian (4) has been successful in recording activity 

 of units of the olfactory bulb and the results of his 

 experiments are of special interest in thai it appears 

 that different patterns of response arc elicited by 

 different stimulating substances. 



Lack of accurate knowledge of the central connec- 

 tions of the olfactory system, as well as inadequate 

 control of the parameters of stimulation, has made 

 impossible anything but the crudest kind of experi- 

 mentation through the use of the ablation method. 

 About all that can be said at present is that the only- 

 lesion which has been shown to have an effect upon 

 capacity' for olfactory discrimination is bilateral sec- 

 tion of the olfactory tracts (7, 8, 200, 201). 



space discrimination. 1 '- For in. in and for many of the 

 higher mammals, the visual and somesthetic systems 

 are usually thought of as being the senses of primary 

 importance in discriminations involving the localiza- 

 tion of objects in space. This is true, vision is the sense 

 used in most instances for localization of objects at 

 a distance; touch as well as vision, for objects near the 

 body. The auditory system is of less importance. 

 Sources of sound can be localized as to angular 

 position with reasonable accuracy, although some 



12 For an interesting discussion of spatial and temporal 

 events in the central nervous system as related to the space and 

 time dimensions in the external world, see Davis (48). Space 

 discrimination is discussed by Teuber, Chap. LXV, this volume. 



confusions arise; only crude estimates can be made 

 of distances. Considered not as a system providing 

 accurate localization of objects in space but as a 

 system which provides a background of information 

 about environment, it may be argued that the auditory 

 system is not of secondary importance in space dis- 

 crimination. This function of auditory cues has been 

 stressed by Ramsdell (172) and by Myklebust (150) 

 in discussing the problems of the deafened individual. 



Other sensory systems also play significant roles in 

 space discrimination. Visual and tactual discrimina- 

 tions of space can be made because postural adjust- 

 ment and orientation of the body are maintained. 

 The kinesthetic and vestibular systems are essential 

 for the maintenance of posture and orientation. 

 Moreover, visual and tactual perceptions of distance 

 and position of objects are in part at least learned; 

 tactual and particularly kinesthetic cues are critical 

 in this learning. The remaining sensory systems, 

 taste and olfaction, do not, as far as we know, con- 

 tribute significantly, to space discrimination and will 

 not be considered further in the present discussion. 



At the periphery, visual and somesthetic space are 

 represented as two-dimensional maps. Objects at a 

 distance project upon the retina in an organized 

 spatial pattern. Objects broughl into contact with 

 the body produce spatial patterns of excitation in the 

 skin receptoi - 



Evidence from anatomic, il and electrophysiological 

 studies shows clearly that for all species of animals 

 that have been studied, there is topographic projec- 

 tion of the retina upon higher centers up to and in- 

 cluding the cerebral cortex (jj, 33, 34, 42, 138, 160, 

 1711, 202) Likewise, for the somesthetic system, 

 different regions oi the body are topographically 

 projected upon the cerebral cortex and this organiza- 

 tion is maintained in pathways and centers inter- 

 vening between skin and cortex (7,4, 55, 139, 149, 226). 



In the auditory end organ, outer space is nol 

 represented l>\ a spatial map. As we have seen (p. 

 147)8), it is frequency of the sound stimulus that is 

 represented spatially along the basilar membrane. 

 Sounds at different angles from the axis through the 

 two ears differ in their time of arrival, in their rela- 

 tive intensity and in their complexity at the end 

 organs. Psychophysical studies (126) have shown 

 that for tones of low frequency (1000 cps and below), 

 time differences (time of arrival or phase) at the two 

 ears provide the principal cues for space localization; 

 intensity differences are more important for tones of 

 high frequency (above approximately 4000 cps). 

 There is a range of frequencies between 1000 and 4000 



