EFFECTS OF RADIATION ON CNS AND ON BEHAVIOR 629 



doses of 1,800 r and less — approximately half the dose requhed to produce 

 cortical damage. Similar data have been presented by Clemente and Hoist 

 (1954), who reported localized damage in the hypothalamic and medullary 

 regions in 4 monkeys receiving 2,000-3,000 r. They emphasize the suscep- 

 tibility of the hypothalamus and medulla to radiation efTects, in contrast 

 with the cerebral cortex and cerebellum, and point out that this is true not 

 only in terms of cell degeneration, but also in terms of alteration of the 

 blood-brain barrier as measured by penetration of trypan blue. 



Since learning, particularly complex learning, is largely dependent on the 

 integrity of the cerebral cortex in mammals, and since the cortex is radio- 

 resistant, one should not be surprised to find that learned behaviors are not 

 adversely affected by whole body radiation. For the monkey the LD/50/30 

 is approximately 600 r, and the LD/ 100/30 is probably 700 r or less. Thus, 

 the lethal whole body dose is about 25% of that required to produce serious 

 damage to the cerebral cortex, and it is entirely possible that death from 

 medullary or hypothalamic damage and destruction would supervene before 

 cortical damage took place, even if higher doses could be given through the 

 use of protective agents. 



It is possible to demonstrate changes in cortical electrical activity and in 

 some cortical enzyme systems at whole body doses below the LD/50/30, but 

 it is extremely doubtful if changes of the magnitude demonstrated would be 

 associated with any behavioral disturbance. There is now extensive literature 

 on the efTects of whole body radiation on a wide range of learning tasks in a 

 fair sample of animal species, and improved performance after irradiation 

 has been reported as often as performance decrement. Thus, improved per- 

 formance has been found for monkeys by Harlow and Moon (1956), 

 Riopelle et al. (1956), and McDowell and Brown (1961) and for rats by 

 Blair and Arnold (1956) and by Blair (1958). The surprising fact is that 

 individual animals have actually shown essentially perfect learned perform- 

 ance a few hours before death in studies using either food or electric shock 

 as incentives. It is possible that some member of the symposium will disclose 

 new data incompatible with my comments, but I believe that the picture is 

 perfectly clear. 



There is evidence that the CNS, including the cortical neurons, may be 

 adversely affected by delayed radiation effects induced by sublethal doses. 

 Arnold et al. (1954) have demonstrated that monkeys given localized 

 cerebral doses of 3,000-5,000 r from a 23 Mev betatron source showed a 

 virtually complete recovery during an intermediate stage, and then 6 to 8 

 months latei-, they developed a fulminating course of delayed radionecrosis 

 which was strikingly selective for the white matter. These investigators em- 

 phasized the fact that the brain stem and hypothalamus were particularly 

 sensitive to delayed radiation damage even with doses as low as 1,500 r. 



