BEHAVIORAL EFFECTS OF CRANIAL IRRADIATION 677 



this was true even of the 6 month group. With respect to discrimination 

 learning, no significant deterioration occurred within 221 days after irradia- 

 tion. It will be recalled, however, that, in a previous experiment, we tested 

 rats 323 days after irradiation and found the irradiated rats to be inferior 

 to their corresponding controls in discrimination learning. This suggests that 

 there is a critical period between 200 and 300 days after irradiation. 



The scanty data on learning set formation allow no definitive conclusions. 

 Of the data obtained, there were no consistent diflferences between experi- 

 mental and control animals tested 92 days after irradiation. 



Motivation — General Level of Activity 



One experiment tested the effects of 2,500 r to the whole head, and three 

 others tested the effects of 5,000 r to the brain area. Fifty days prior to ir- 

 radiation, the rats were placed in automatically recording activity wheels 

 with attached living cages in order to establish a base-line activity lev^el. 

 Activity of the experimental animals exposed to 2.500 r whole head irradia- 

 tion was recorded for 120 days. Of the three 5,000 r experimental groups, 

 activity in one was recorded for 120 days; in another, for 300 days; and in 

 the third, until death. 



Results 



In all four experiments, irradiated rats were less active except for a brief 

 interval between the 10th and 15th days after irradiation. At the peak of this 

 activity burst, experimental rats made three times as many revolutions per 

 day as the controls (2,300 vs 700). Thereafter, the activity of the experi- 

 mental animals diminished to a level well below that of the controls. In both 

 the experimental and control groups, there w^as a gradual decline in activity 

 with time, a tendency which continued until death. 



Motivation — Hunger and Thirst 



In the first e.xperiment on this topic, the effects of 2,500 r to the whole 

 head on hunger motivation were tested; in the second and third, the effects 

 of 5,000 r to the brain area on hunger and thirst motivation were tested. 

 Measurements were made in a Jenkins-Warden obstruction apparatus before 

 and after irradiation. A 48 hour deprivation period was used in the hunger 

 tests and a 23 hour period in the thirst tests. 



