SENSORY DISCRIMINATION 



[463 



In experiments involving ablation of the somatic 

 areas of the cortex in animals, the tests used have not 

 as a rule required localization of tactual stimulation 

 of the skin. An exception is a study by Peele (160) who 

 measured, among other things, localization of touch 

 and pinprick. Unilateral ablations were made of 

 Brodmann"s areas 1 and 2, 3, 5 or 7 separately or of 

 1, 2, 5 and 7 together. Peele reported that localization 

 of tactile and painful stimuli '"was persistently im- 

 possible after all ablations." In other investigations, it 

 has been shown that the monkey and chimpanzee 

 show a deficit in ability to discriminate factually be- 

 tween such objects as a cone and pyramid. Discrimi- 

 nations of this kind probably depend in part upon 

 tactual localization. Ruch & Kasdon (187) in com- 

 paring effects of cortical ablation on placing and 

 hopping reactions and on weight discrimination have 

 suggested that the greater effect on the former may 

 be due to the fact that spatial localization is to some 

 extent required whereas it is not in weight discrimi- 

 nation. 



Another kind of experimental evidence as to space 

 localization for somesthesis comes from the experi- 

 ments of Dusser de Barenne (54, 55). He showed thai 

 strychnine applied to the cortex of animals led to 

 scratching at a particular part of the body or to 

 hypersensitivity of an area of the skin. By observing 

 the skin areas affected, he was able to produce maps "I 

 the sensory cortex similar to those obtained later b\ 

 the evoked potential method. From Dusser de Ba- 

 renne's experiments, it may be inferred that arousal of 

 neural activity in a restricted part of die somato- 

 sensory cortex gives rise to sensations localized in a 

 particular part of the body. 



Although the evidence from animal studies is 

 sparse, it tends to substantiate the more abundant 

 evidence from clinical observations. Head & Holmes 

 were the first to use the careful techniques of the 

 psychophysics laboratory in the examination of pa- 

 tients with brain damage. Because they had early 

 become convinced that one should look for different 

 qualities of sensation which might be separably 

 affected by cortical lesions, the observations made b\ 

 Head & Holmes are particularly relevant here. 

 Among other deficits in somesthetic sensation re- 

 sulting from damage to the cortex of the parietal 

 lobes, they did find in many cases disturbance in 

 tactual localization (90, 96). One of the three main 

 functions of the somatic cortex according to Head 

 was "the appreciation of relationships in space." 



Critchley (46) has recently summarized clinical 



research on the parietal lobes. He cites numerous 

 studies in addition to those of Head & Holmes in 

 which deficits in somesthetic localization have been 

 observed after parietal lobe damage. Complete ab- 

 sence of localization of tactual, pain or temperature 

 stimuli is not reported, but errors in accuracy of 

 localization are often increased. 



The findings of clinical studies in which the surface 

 of the cortex has been explored by electrical stimula- 

 tion provide a most convincing kind of evidence that 

 topographical projection of the surface of the body 

 upon the cerebral cortex is of functional significance 

 in spatial discrimination. Penfield and his collaborators 

 have shown that in unanesthetized patients stimula- 

 tion of points in both postcentral and precentral gyri 

 by mild electric current gives rise to reports of somes- 

 thetic experience (161 — 163). The sensations described 

 by the patients are localized in particular regions of 

 tin- body and a systematic relationship is found to 

 exist between the points on the cortex excited and the 

 parts of the body to which sensations are referred. 

 The kinds of sensation which the patients report are, 

 in most cases according to Penfield & Rasmussen 

 (163), feelings such as numbness, tingling or electric 

 shock. Feelings of movement in parts of the body also 

 occur, but less frequently. 



Partial loss in capacity to localize sound in space 

 after ablation of auditory cortex has been found in 

 experiments on the rat, dog and cat. The earlier 

 experiments on the rat (164, 165) and dog (74) are 

 difficult to evaluate because of incomplete anatomical 

 information on locus and extent of lesions. It is clear 

 from the studies on the cat (155, 157) that partial 

 bilateral ablation of the auditory projection areas 

 results in a decrement in performance in a discrimina- 

 tion requiring localization of sound in space. 



A survey of the clinical literature on effects of 

 temporal lobe damage in man does not bring to light 

 much evidence bearing upon the problem of localiza- 

 tion of sound in space Accurate tests of ability to 

 localize have usually not been made. A recent report 

 by Longo it til. ( 132 1 suggests that temporal lobe 

 lesions in man may produce disturbances in sound 

 localization when the latter is measured carefully; 

 they state that "cases with lesions involving the 

 temporal lobe showed marked impairment of localiza- 

 tion of sound in the contralateral auditory field while 

 lesions in other areas of the cortex showed no such 

 impairment." 



For the kinesthetic and vestibular senses, studies 

 have not been done in which spatial discrimination 



