PERCEPTION 



•633 



with parietal lesions (right, left or bilateral), and 

 appeared equally on the visual and tactile-kinesthetic 

 versions of the task. In fact, when the results of the 

 parietal group were eliminated from those of the total 

 population with brain injuries tested, no differences 

 between those with brain injuries and control sub- 

 jects remained. The deficit thus was specific for 

 parietal lesions, but independent of the sense modality 

 through which the information was received. In this 

 respect then, the disturbance was not an agnosia, since 

 the term implies, in its traditional sense, a disorder 

 limited to 'higher functions' within a single sensory 

 modality. 



Curiously, the group with maximal deficit on the 

 route-finding task (in both its visual and tactile forms) 

 also showed maximal impairment of tactile two-point 

 discrimination (526), as if the underlying functions 

 shared, at least in part, a common neural substrate. 

 (The same patients, apparently, also show abnormally- 

 elevated thresholds for the detection of duality of 

 dichotic clicks.) These unexpected patterns of symp- 

 toms suggest that seemingly 'simple' sensory deficits 

 may be correlated with alterations in performance 

 that in traditional terms would have been considered 

 as 'complex.' For an experimental critique of agnosia, 

 these results mean that we should be cautious before 

 we assume that the organism should fall apart accord- 

 ing to subdivisions of our textbooks. Defects in the 

 nervous system may fail to divide the presumably 

 lower and higher aspects of perception as neatly as 

 can be done by a priori classifications. 



Deprivation Studies 



DEPTH PERCEPTION AFTER EARLY VISUAL DEPRIVATION. 



Although the evidence is fragmentary, perception in 

 subhuman forms may be increasingly less dependent 

 on any particular perceptual experience as one de- 

 scends the phylogenctic scale. We have mentioned 

 the early appearance of depth perception after hatch- 

 ing in chicks (21 1 ); there is additional evidence that 

 their pecking responses involve binocular vision from 

 the start, in spite of the limited overlap of their mo- 

 nocular fields (212). Rats reared in darkness appar- 

 ently show little perceptual deficit, since they can 

 jump with nearly normal accuracy from platform to 

 platform, adjusting the force of their jump to the 

 visually perceived distance (310). In the most recent 

 experiments of this type [by Walk et al. (515)], 

 possible artifacts in earlier studies were minimized by 

 placing dark-reared rats, on first exposure to light, 

 near a 'visual cliff.' These tests, which eliminate the 



pretraining needed in the study by Lashley & Russell 

 (310), are based on the assumption that, given a choice, 

 an animal will not descend over a vertical edge toward 

 a surface that appears to be far away. A 'near' and a 

 'far' surface was provided by placing two identical 

 checkered fields, one to each side of a narrow hori- 

 zontal runway. Horizontal glass plates of equal height 

 extended to either side of the runway, but on one 

 side the checkered field was immediately below the 

 glass, and on the other side 53 in. below it. Nearly 

 all of the experimental rats (reared for 90 days in the 

 dark) descended on the 'near" side on first trial, 

 just as did their normally reared litter mates. Such 

 strong evidence for 'nativism' in depth perception is 

 in apparent contrast i<> the disabling effects of rearing 

 in darkness or unpatterned light for carnivores and 

 primates (see above, p. 1622). The resolution may lie 

 in genuine species differences (188) or in the greater 

 side effects of restricted rearing in higher forms. 



TACTILE DEPRIVATION: EFFECTS ON BODY SCHEME, 

 Onlv one stud) has extended the "early deprivation' 

 |),n . u 1 1 ■_■ 1 1 1 in the tactile-kinesthetii sphere Nissen 

 ft al. (363) encased all lour limbs of a ^-wk.-old 

 chimpanzee in cardboard cylinders which extended 

 from elbows .md knees to beyond the fingers and toes. 

 Interrupted only for occasional testing and for 

 refitting of the cylinder .1-- the animal grew, the 

 deprivation extended for 30 months. After removal 

 of the cylinders, the chimpanzee was .iblc to perform 

 visual size, form and depth discriminations within 

 normal limits, but had abnormal difficulties in 

 learning such ,1 simple tactile-kinesthetic task as 

 turning its head to the side on which its fingers had 

 been stimulated by firm pressure, 



The results of this smdv (unless explained by some 

 atrophy of disuse 1 recall the common belief that 

 tactile and postural orientation is acquired in the 

 form of 'schemata' as tin- result of use of the various 

 body parts [see Head (186) and Oldfield & Zan«>will 

 (366, 367)]. Sudden removal of a part (as by ampu- 

 tation) uncovers the schema in the form of a per- 

 sistent phantom limb. In keeping with these interpre- 

 tations would be the absence of phantom limbs after 

 removal of limbs that had been anesthetized or 

 dencrvated for long periods prior to their loss ( 1 77, 

 178, 186, 475). Additional evidence on these phenom- 

 ena is needed, particularly with respect to the con- 

 sistent finding that tactile functions (touch-pressure, 

 point localization, two-point discrimination) are 

 better on an amputation stump than in the homol- 

 ogous portions of the intact limb (177, 178, 475)- 



