730 W. LYNN BROWN AND ARNOLD A. McDOWELL 



precedent established by Davis et al. (1957) and have treated each set of 

 experimental results in terms of comparisons of subjects within untreated 

 control, low dose, and high dose subgroups. Treatment groups A and E 

 comprise the untreated control subgroup; treatment groups B, C, and F, 

 the low dose subgroup; and treatment groups D, G, and H, the high dose 

 subgroup. Most of the studies have dealt with a relative radiation dosage 

 variable, rather than with an absolute dosage variable. 



The initial suggestion of a radiation-induced change in distractibility came 

 from systematic observations of the free cage behavior of the subjects in the 

 three subgroups described (McDowell, 1958). 



These observations were made approximately 1 year after the cessation of 

 exposure of the treated animals. It was found that the frequency of responses 

 to cage parts as manipulanda, the prepotent stimulus class in this behavioral 

 setting, was significantly greater for the irradiated than for the control 

 subjects. The frequency of responses to uncontrolled auditory stimuli occur- 

 ring outside the test room, however, was significantly greater for control 

 than for irradiated subjects. 



These two findings suggested the hypothesis that the chronic irradiated 

 monkey is less distractible than is the normal monkey, i.e., that radiation 

 exposure reduces the probability of response to the weaker stimuli in the 

 environment, with the consequence that the relative effectiveness of the 

 strongest stimulus is increased. Subsequent tests supported the tenability of 

 this hypothesis. In these tests it was found that all experimentally induced 

 stimulus conditions which significantly affected the performance latencies of 

 the irradiated subjects on a simple repetitive task also significantly affected 

 the performance latencies of the controls. Some stimulus conditions, however, 

 which did not significantly affect the performance latencies of the irradiated 

 subjects, did significantly affect the performance latencies of controls. Visual 

 and auditory stimulus conditions were used. 



On the basis of the findings on distractibility of the irradiated and control 

 subjects, it was predicted that the learning performance of the irradiated 

 subjects would be superior to that of the controls on tasks such as spatial 

 delayed response and discrimination problems with reduced stimulus cues — - 

 tasks which are considered by many researchers to place a premium on at- 

 tentiveness to the locus of food reward during the trial-setting phase of 

 testing. 



A study conducted by McDowell and Brown (1960b) with normal sub- 

 jects clearly shows that the nature of sensory stimulation during the period 

 of delay, as well as learning during the trial-setting phase, is a factor in 

 spatial delayed response performance. Figure 1 shows the per cent errors per 

 day over successive days of testing for two groups of normals tested with 

 identical stimulus cues on spatial delayed response. One group had darkness 



