628 HARRY F. HARLOW 



mals at 5,000 r and noted that much smaller doses have produced severe 

 changes in the CNS as reported by Haymaker et al. (1954). 



Arnold et al. (1954), using a 23 Mev betatron source, reported that doses 

 between 4,200 and 8,000 r destroyed all neural components, but left larger 

 blood vessels intact, whereas doses between 1,800 and 3,000 r produced only 

 slight neural or glial cell damage. 



In general accordance with these results are data given by Settlage and 

 Bogumil (1955). They implanted needles holding Co*'" seeds at the surface 

 of the cortex of cats and monkeys. The needles were maintained in place for 

 such a time as to deliver a dose of 4,000 r 5 mm from the radiation source, 

 producing a hemispheric lesion of 8-10 mm in diameter. The transition from 

 totally necrosed cells to apparently normal cells was abrupt, taking place 

 within approximately a millimeter. These investigators also noted resistance 

 of at least the larger blood vessels to damage, and the same phenomenon in 

 man has been reported by O'Connell and Brunschwig (1937) and Wa- 

 chowski and Chenault (1945). Thus, there is reason to question earlier in- 

 terpretations that neurologic changes following brain irradiation were the 

 indirect result of vascvilar damage. It would appear that radiation doses must 

 attain a level of 3,500 r or more to produce widespread or total destruction 

 of the cerebral neurons. 



As might be expected, physiologic and biochemical measures of CNS 

 function indicate radiation effects at doses lower than those necessary to 

 produce cell degeneration. Brooks (1956) has shown that the threshold 

 radiation dose necessary to produce prompt depression in the EEG approxi- 

 mated 1,000 r in a group of 12 monkeys subjected to 1,000 r per min from 

 a Ba^*° and La^*" gamma radiation source. Caster et al. (1958) demon- 

 strated an acute depression of low frequency EEG 12 hours after irradiation, 

 and subsequently throughout test days 4 through 12. This change and some 

 other changes in EEG pattern occurred after a single 700 r dose and were 

 correlated with depressed concentration of deoxyribonucleic acid. 



Arnold et al. (1954) reported abnormal EEG records 1 week after a dose 

 of 1,800-3,000 r and 3 months after doses of 900—1,800 r; even doses as low 

 as 375 r produced spiking in the EEG after delay. Recently, Gangloff and 

 Haley (1960) have reported changes in spontaneous and evoked potentials 

 from the hippocampus following 200 and 400 r doses of whole body and 

 head irradiation. They point out that electrical changes from hippocampus, 

 brain stem, and the diffuse thalamic nuclei are more susceptible to low 

 radiation doses than are such changes recorded from the cortex, caudate 

 nuclei, and posterior hypothalamus. 



There is other evidence to indicate that the cortex may be more radio- 

 resistant than some of the other cerebral centers. Arnold et al. (1954) found 

 damage to the brain stem and moderate damage to the hypothalamus at 



