BEHAVIORAL EFFECTS OF CRANIAL IRRADIATION 675 



a high mortaHty rate (particularly at the higher dosage levels) made it im- 

 possible to obtain definitive data. The data that were obtained for animals 

 having doses up through 8,000 r were consistent with the previous findings 

 that irradiated rats tend to learn more rapidly than corresponding controls. 



Discrimination Learning and Formation of a Learning Set 



The effects of 5,000 r brain irradiation on discrimination and on learning 

 set were investigated in two experiments: 20 rats were subjects in the first; 

 and 24, in the second. The apparatus was a 5-choice discrimination box, 

 described fully in a previous publication (Koronakos and Arnold, 1957). 

 The task involved learning to discriminate visually which of 5 doors gave 

 entry from the choice to the reward compartment. Stimulus cards with dif- 

 ferent geometric figures were attached to each door and the correct door 

 was signified by the "odd," i.e., non-matching, pattern. A series of 9 such 

 visual discriminations were given, with 30 trials on each problem. 



/ Results 



The irradiated animals showed a slight superiority on the first two prob- 

 lems. However, on the remaining seven problems, their performance was 

 consistently inferior to that of the controls. Acquisition of the oddity learn- 

 ing set was acquired more slowly in the experimental group. At the same 

 time, the latencies of the experimental group were consistently lower than 

 those of the control group. This suggests that the experimental animals were 

 more strongly motivated than the controls; and, as a result, they tended to 

 respond too quickly to allow time for making a correct discrimination. 



Discrimination Learning — Long Term Effects 



The work reported thus far involved relatively short postirradiation time 

 intervals, but, because of histologic findings of damage after 9-12 months, we 

 became increasingly interested in the long-term effects of irradiation; and it 

 was decided to test discrimination learning 300 or more days after exposure. 



Subjects were 24 albino rats divided into an experimental and a control 

 group; there were 6 males and 6 females in each group. These animals had 

 previously learned the 14-unit T-maze. The apparatus was a two-choice 

 discrimination box. By pushing open either of two doors, the rat gained 

 access to a food reward. Black or white stimulus cards were attached to the 

 doors. The animals were irradiated with 5,000 r to the brain 323 days prior 

 to the beginning of the experiment. After preliminary adaptation to the ap- 

 paratus, the animals were given 5 trials per day on a black-white discrimina- 



