EFFECTS OF RADIATION ON PSYCHOLOGIC PROCESSES 735 



manifest, on the subsequent transfer of a single learned discrimination 

 along a peripheral cue gradient. The results of the study indicated that the 

 chronic irradiated male monkey, at the dosage used, is less susceptible 

 to proactive inhibition than is the normal male monkey. Initial training on 

 a peripheral cue discrimination, during which no learning was manifest, 

 interfered less with the subsequent transfer of a single learned discrimina- 

 tion along a peripheral cue gradient by the chronic irradiated subjects 

 than by the normal subjects. 



The proactive effect appears to have been due to failure to learn the 

 initial problem. The consequence of the failure to learn the initial discrimi- 

 nation was apparently an association of the stimulus events of peripheral 

 cue testing with a 50% reinforcement schedule. An hypothesis of decreased 

 distractibility and narrowed scope of attention for the irradiated animal 

 would predict that fewer such stimulus events would be associated by the 

 irradiated animal than by the normal animal. Any change, then, in one of 

 the stimuli associated by both normal and iiTadiated monkeys would consti- 

 tute a proportionately greater alteration of the situation for the irradiated 

 animal. Thus, the irradiated animal should more easily discriminate a 

 change in the situation and redefine the problem, as appeared to be the 

 case in the present study. 



While the results of all of the aforementioned studies could be accounted 

 for in terms of decreased distractibility, another study conducted by Mc- 

 Dowell (1960) with some of the same subjects suggests that decreased dis- 

 tractibilty alone cannot account for all of the learning performance differ- 

 ences between controls and irradiated subjects. In that study, subjects were 

 tested for transfer of a single learned discrimination along a peripheral cue 

 gradient. When the subgroups were compared with respect to efficiency of 

 transfer performance, they were found to differ significantly as a nonlinear 

 function of radiation dosage. The author interpreted the results as reflecting 

 differential increments in incentive motivation for the subgroups during the 

 initial focal cue discrimination training. To explain these differential incre- 

 ments, he hypothesized that ratio of response rate to range of effective stim- 

 uli defines incentive motivation when the other determiners of incentive 

 motivation, as given by Spence (1956), are held constant. 



These same subjects were tested by Brown and McDowell (1960) on each 

 of eight visual acuity problems presented in order of increasing difficulty. 

 Each problem required the subject to choose between circles and circles 

 with breaks in order to procure a food reward. Previous visual acuity testing 

 (Davis et al., 1957) had shown a deficit only in the high dose irradiated 

 subjects (616 rep) during the 1st year following the radiation exposure 

 given 3 years prior to the initiation of the present study (Fig. 5). The 

 visual acuity deficit noted during the 1st year after exposure in the high 



