BEHAVIORAL EFFECTS OF CRANIAL IRRADIATION 681 



the brain area. With higher doses, the mortaUty rate was so great that post- 

 irradiation testing was precluded, especially long-term testing. 



Both short-term effects (less than about 150 days after irradiation) and 

 long-term effects (more than about 250 days after irradiation) were tested. 

 Thus far, the results of the latter are inconclusive, and the following sum- 

 mary applies primarily to the short-term effects of irradiation. 



Whole head irradiation with 2,500 r increased emotionality in male rats 

 but not in females. 



Instrumental learning and retention were not significantly affected by 

 2,500 r whole head irradiation. 



A 2,500 r dose to the whole head and a 5,000 r dose to the brain area pro- 

 duced better maze learning and retention in irradiated animals than in 

 controls. It appears likely that this improvement in performance was due 

 primarily to increased hunger motivation and to reduce exploratory activity 

 following radiation sickness. Rats starved to match the weight loss of the 

 5j000 r brain-irradiated animals performed consistently better than the 

 irradiated animals. This finding suggests that brain irradiation may have 

 detrimental effects on maze learning which are masked by the facilitative 

 effects of increased hunger motivation and reduced exploration. 



In learning set formation, the performance of 5,000 r irradiated animals 

 was inferior to that of their controls. Again, this finding may be interpreted 

 in terms of increased hunger motivation. The irradiated animals may have 

 been so strongly motivated that they responded too quickly, with the result 

 that their accuracy was impaired. 



The effects of irradiation on discrimination seem to be a function of the 

 postirradiation time interval. The relatively short postirradiation time inter- 

 vals revealed no significant differences. Even a 221-day-postirradiation time 

 interval had no effect. However, a 323-day-postirradiation interval revealed 

 detrimental effects. These findings suggest that there may be a critical period 

 between 200 and 300 days after irradiation. 



General (locomotor) activity was depressed both by 2,500 r and by 5,000 r 

 except during an interval between the 10th and 15th days after irradiation. 



Hunger motivation was increased between 20 and 100 days after 2,500 r 

 whole head irradiation but not after 5,000 r brain irradiation. 



Thirst motivation was depressed for about 75 days after 5,000 r brain 

 irradiation. Maximum depression occurred 30 days after irradiation. 



Visual exploration was depressed during at least the first 50 days after 

 irradiation. Later tests at 190 to 200 days after irradiation showed no signifi- 

 cant difference between irradiated and control animals. 



It should be pointed out that, in our recent work, a new irradiation tech- 

 nique is being used. Until now, a vertical beam was applied to the top of the 

 animal's head. As a result, radiation that was not absorbed in brain tissue 



