BEHAVIORAL EFFECTS OF RADIATION ON PRIMATES 



723 



by the percentage of correct responses. Out of the 14 tests, 11 engendered 

 no loss in accuracy on the part of the irradiated animals; 3 tests (4-choice 

 oddity, visual acuity, and size discrimination) produced significant loss. 



By way of illustrating tests which produced no differences between normal 

 and irradiated subjects, we may consider the discrimination of pairs of 

 stimulus objects presented in consecutive and in serial order. Under the 

 former, the two stimuli of a given problem are presented 10 times in succes- 

 sion. Then they are exchanged for new ones, which are also presented for 

 10 trials. Ten such problems are given each day, making a total of 100 trials. 



When the problems are presented in serial order, the stimulus pairs are 

 changed on every trial. Each animal, therefore, goes through a list of prob- 

 lems 10 times. Thus, there is maximal opportunity for the different problems 

 to serve as distractions for the other problems, and we anticipate poorer 

 performance by the normal animals. The performance of the irradiated 

 animals should remain fairly high, since irradiation has long been thought 

 to make animals resistant to distraction (Nemenov and Yakovleva, 1944; 

 Harlow and Moon, 1956; McDowell, 1958). 



The data of Fig. 2 are based on averages of 3 weeks of testing at the end 



Fig. 2. Performance of normal and irradiated chimpanzees on visual discrimination 

 learning problems presented in serial and in consecutive order. 



of training. The performances of the two groups, now consisting of all irradi- 

 ated animals living until 1960 and their controls, are virtually identical. 



As an example of a test involving a loss of accuracy of response, consider 

 the data on 4-choice oddity. In this test, 4 square plaques are suspended by 

 their upper edges in such a way that pushing back a plaque uncovers a food 

 well. On each plaque is pasted a complex design cut from wallpaper. Three 

 of the plaques are identical and the 4th is different. The animal can obtain 

 food only by depressing the unique stimulus. Each combination of stimulus 



