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MWDBOOK OF PHYSIOLOGY — NEUROPHYSIOLOGY III 



would predict loo per cent transfer. 16 Apparently the 

 visuallv deprived birds choose to place themselves 

 midway between these theoretical positions. 



The most serious effects of early visual deprivation, 

 however, have been reported for cats (401-403) and 

 chimpanzees (88, 363, 400). The earliest accounts of 

 visual deficits in the latter [two animals studied by 

 Riesen (400)] should perhaps be disregarded, since 

 the optic atrophy induced after 16 mo. in darkness 

 was severe and progressed in one of the two cases to 

 complete blindness. Later studies (403), however, 

 employed diffuse light as a condition of early rearing, 

 or a combination of darkness and diffuse light, which 

 was thought to prevent any atrophy of disuse. These 

 animals were at first unable to distinguish such simple 

 visual patterns as horizontally vs. vertically striped 

 fields, they failed to recognize their feeding bottle 

 until it touched some part of their body, and they 

 showed genera] neglect of their visual environment. 



alternative INTERPRETATIONS. Still, the interpreta- 

 tion of these fascinating experiments is problematic. 

 There are at least three other mechanisms besides the 

 one usually advanced that might account for the 

 visual difficulties after early visual deprivation: 

 atrophy of structure, suppression of function, or 

 general behavioral impairment. 



a Some atrophy of disuse may occur in spite of the 

 precautions taken. Brattgard (59) has found that 

 rabbits reared in darkness for 10 weeks from birth 

 showed complete lack of the pentose nucleoprotein 

 fraction in their retinal ganglion cells. Animals 

 brought out into light afterward did not develop 

 pentose nucleoprotein at the same rate as did con- 



16 Normally reared birds show mo per cent interocular 



transfer under c parable conditions, i.e. one eye knows 



what the othi 1 has been taught, at least .is long .is the patterns 

 are presented in the lower part of the visual fields (.265, 266, 

 317-jKi 1 A negative result (failure of interocular transfer 

 Im .1 color discrimination in the pigeon) lias been reported l>\ 

 Beritov & Chichinadze (37). Interocular transfer is present 

 but less consistent in fish 1 it'. 1 1'" In the goldfish, for instance, 

 transfer from eye to eye may fail if the index ol transfer is an 

 overt ,..: ! : ../iim' id electric shock (341). The animal 



is warned prioi to the onset ol the shock In the presentation 

 ol .1 visual pattern .1 vertically sniped field) to one eve After 

 the animal has learned to make .1 forward movement (avoiding 

 the shock each time the pattern the second lun- 



n .imed eye is tested under the same conditions. The avoidance 

 faih but di' animal does show a transfei in terms 

 ol .1 conditioned change in heart rate Vnthropomorphically 



lii e. that thi pattern does look 'threatening' 



to (lie untrained eye, but the animal dues not know what to do 

 about the threat. 



trols. In kittens reared in the dark, Zetterstrom 

 (558) noted that the elcctroretinogram (ERG) did 

 not appear until the third week from birth; normally 

 reared kittens show some ERG activity by the end of 

 the first week. Finally, in the most detailed histologic 

 study of this question to date, Weiskrantz (529) has 

 demonstrated significant losses of fibers (especially 

 the so-called Miiller fibers) in the neurorctinae of 

 kittens reared under the same conditions as those 

 employed by Riesen et al. (403). He points out, 

 rightly, that these findings do not rule out the hy- 

 pothesis that perceptual learning must occur as a 

 necessary part of development. However, the evidence 

 of atrophy diminishes the value of early sensory 

 deprivation as a lest of this hypothesis. It may be 

 convenient, as Weiskrantz notes (529), for psy- 

 chological theory that deprivation should produce 

 changes limited to higher neural structures, but per- 

 haps it is less convenient "for the nervous system 

 itself to make such a distinction."' 



b) Another potentially complicating factor is 

 suppression of function in the deprived sensory 

 modality. Such a process is perhaps not as hypo- 

 thetical as it was prior to the demonstrations of 

 suppression of afferent visual or tactile input upon 

 midbrain reticular stimulation [see Granit (168)], or 

 the transient changes in evoked potentials at lower 

 levels of the cat auditory system during 'distraction* 

 [see Hernandez-Peon et al. (204)], or during con- 

 ditioning [see Galambos et al. (140)]. In man, sup- 

 pression is said to play a role in the curious impairment 

 of pattern vision in the deviated eye in cases of uni- 

 lateral squint. This strabismic amblyopia is often 

 called 'ex anopsia,' as if disuse were the cause of the 

 perceptual deficit. For the neoempiricist, however, 

 the squinted eve dues not 'get amblyopic, 1 but 'stays 

 amblyopic," i.e. it does not build up those normal 

 relations between eye movements and patterned 

 visual stimuli which (for Hebb) are prerequisites for 

 acquisition of shape perception. 



Studies of basic visual thresholds (siuii as dark 

 adaptation, spectral sensitivity and absolute brightness 

 threshold I reveal essentially normal function in such 

 squinting eves in which form perception is lacking 

 (514). The condition has therefore been interpreted 

 bv Wald & Burian (514) as a selective impairment 

 of a 'higher* level of vision, i.e. of pattern perception, 

 since, in their opinion, the 'entire apparatus of light 

 perception" remains intact in the affected eye. More 



letentlv, however, Feinberg (123) has demonstrated 

 that there are, alter all, demonstrable changes in 

 more elemental") visual functions, e.g. a drastic 



