672 WILLIAM J. ARNOLD 



with no response. After reaching the criterion, each rat was tested for spon- 

 taneous recovery. 



Results 



The measures of acquisition and retention did not show significant difTer- 

 ences between experimental and control groups. Neither the dose of head 

 irradiation nor the postirradiation time interval afTected instrumental learn- 

 ing with relatively short postirradiation time intervals. However, later experi- 

 ments suggested that with longer postirradiation intervals time is an impor- 

 tant parameter. 



Maze Learning and Retention 



Of the series of experiments dealing with maze learning, all used the 

 same apparatus and essentially the same training procedure. They differed 

 only in radiation dosage. The apparatus was a 14-unit Tolman-Honzik T- 

 maze. The rats were run under a 23/2 hour hunger drive. They were given 

 1, 3, or 5 trials a day, until they reached the criterion of 3 successive trials 

 with a total of no more than 3 errors. 



In two experiments (Arnold, 1952), acquisition and retention were tested 

 after 300 r and 800 r to the whole head. There were no significant differ- 

 ences between irradiated and control animals. Also, no symptoms of radia- 

 tion sickness were observed, although within 60 to 90 days after irradiation, 

 the dark hair on the heads of the pied animals turned gray. 



A third, unpublished experiment tested the efTects of 2,000 r whole-head 

 irradiation on maze acquisition. The performance of the irradiated animals 

 was consistently better than that of the controls, but the differences were 

 not statistically significant. 



In a fourth experiment (Blair and Arnold, 1956), the efTects of 2,500 r 

 whole-head irradiation on retention of maze learning were tested. The rats 

 learned the maze, were irradiated, and then were tested on postirradiation 

 days 3, 12, 25, 40, 60, and 80. The control rats tended to jjerform better than 

 the irradiated rats on the day 3 test; but, by the day 25 test, there was a 

 reversal, and the irradiated rats were superior to the controls. This su- 

 periority was maintained on subsequent tests. These unexpected results 

 were accounted for in terms of motivational factors rather than in terms 

 of direct effects of irradiation on brain tissue. The hypothesized factors 

 were increased hunger and reduced exploration resulting from sickness 

 following irradiation. 



A fifth experiment (Blair, 1958) tested the effects of 5,000 r brain irradia- 

 tion on maze acquisition with training beginning 1, 30, and 60 days after 



