PERCEPTION 



I 62 I 



but a preoperative discrimination between upright 

 and inverted V's is permanently lost. The animals 

 show impairment in everyday situations; they bump 

 into objects in a strange room and have trouble 

 finding food held in front of the affected eye. None 

 of these difficulties is found on testing the other eye. 



It is important to note that the impairment of 

 visual discrimination increases with increasing abla- 

 tion from supposedly nonvisual areas. When the 

 removal is done in two stages, and the first includes 

 the temporal lobe and all cortex adjacent to the 

 visual island, the deficit is present but less severe. 

 If one then removes the frontal region, the impair- 

 ment becomes approximately as severe as if the total 

 removal were made in one single stage. For instance, 

 it is only after the second removal that the animals 

 show the permanent inability to discriminate upridit 

 and inverted V's. Such results suggest that the most 

 anterior regions play some role in visual performance. 



Curiously, the results are different for somesthetic 

 discrimination. If an island consisting of sensorimotor 

 cortex is produced in a 'split-brain' cat (by decor- 

 ticating the rest of one hemisphere, as shown in fig. 

 igB), tactile discriminations trained in the cor- 

 responding (i.e. opposite) forepaw are retained; they 

 are permanently lost when the island of cortex is 

 removed. The greater functional efficiency of the 

 isolated somatic cortex, as compared with isolated 

 visual cortex in the cat, might suggest a different 

 mode of organization in these two sense modalities. 

 However, it may be important, as Sperry himself 

 points out (439), that the isolated somatic cortex 

 does include a 'motor' area, while the isolated visual 

 cortex does not. 



Isolation Studies 



The recent studies by Sperry (439) employ ablation 

 techniques to achieve various degrees of anatomical 

 isolation of primary receptor fields within the neo- 

 cortex. The effects on function are actually not as 

 drastic as those of another technique of isolation, 

 that of depriving an intact organism, early or later 

 in life, of normally patterned sensory input. 



PATTERN VISION AFTER REMOVAL OF CONGENITAL 



cataracts. In man, such sensory deprivation is often 

 thought to result from congenital cataracts which 

 prevent pattern vision until the cataracts are sur- 



gically removed (511). The problem was posed by 

 Molyneux (351) and discussed by Berkeley (38) and 

 Diderot (105). Would a man, born blind, whose 

 sight has been restored, manage to identify, visually, 

 patterns familiar to him from his earlier tactual 

 experience? The majority of clinical reports on such 

 cases has given a negative answer [see von Senden 

 (511)]. The formerly blind patients have to go through 

 prolonged periods of acquisition of pattern perception. 

 They may be able to distinguish colors before they 

 can discriminate shapes. They distinguish a square 

 from a hexagon by laborious and often erroneous 

 counting of corners, confuse a rooster with a horse 

 (because both have tails), or call a fish a camel 

 (because they confuse the dorsal fin of the fish with a 

 camel's hump) (524). 



These reports may indeed mean what von Senden 

 (511), their compiler, thought they meant, that 

 normal shape perception is acquired through early- 

 learning and that early isolation from patterned 

 visual stimuli retards or prevents the necessary 

 learning to perceive. Yet there are serious ambiguities 

 in this clinical evidence. The case reports are rarclv 

 adequate [see Michael Wertheimer's critique (538) 

 of von Senden's material], and there is the possibility 

 of other lesions in patients with cataracts which 

 makes the interpretation doubtful. 



EFFECTS OF EARl Y VISUAI DEPRIVATION ON SHAPE 



PERCEPTION in si urn \i\\ SPECIES. It is for tliis reason 

 that a number of recent experimental studies have 

 been done, primarily on Hebb's suggestion (188), 

 employing the paradigm of early visual deprivation 

 in lower forms; in birds (429, 430'. rats (187), cats 

 (401-403) and chimpanzees (88, 363, 400). Para- 

 doxically, Hebb himself had shown earlier ( 187 I that 

 rats reared in darkness were capable of practically 

 normal visual pattern discriminations when first 

 brought into light. In birds, such as ring doves (429, 

 430), rearing with translucent goggles (eliminating 

 patterned light stimulation) results in sonic retarda- 

 tion of shape discrimination learning when the goggles 

 are removed. However, the deficit is not great. In 

 fact, the animals can be given their first visual dis- 

 crimination training with one eye; when tested for 

 'transfer' of this learned discrimination to the other 

 (untrained) eye, they show some transfer (since they 

 need significantly fewer trials for the second eye). An 

 extreme form of Hebb's empiricist theory would 

 predict no interocular transfer for these visually 

 naive animals, while extreme forms of Gestalt theory 



