ACTION OF GAMMA RADIATION ON ELECTRICAL BRAIN ACTIVITY 619 



their threshold (Fig. 11). This is also the case with the unspecific thalamo- 

 cortical projections. The threshold of the recruiting responses evoked in the 

 cortex by stimulation of the medial thalamus increases slightly after stronger 

 irradiations (Fig. 12). 



The fact that all potentials evoked in the cortex by stimulation of reticu- 

 lar, hippocampal, or thalamic systems are reduced by stronger gamma ir- 

 radiation (900 r) indicates that the cortical neurons and synapses resp>onsible 

 for the evoked cortical potentials are depressed by such higher doses. 



Discussion and Conclusion 



In opposition to the previous conception that the brain shows a marked 

 resistance to ionizing radiations, the present results prove that weak gamma 

 irradiation produces transitory, reversible alterations in this organ. Weak 

 irradiation of the head in the rabbit (400 r, 600 r) is followed by hyperactive 

 motor behavior with oral patterns, activated EEG with desynchronization 

 in the neocortex, activation of the reticular ascending system, and activa- 

 tion of the archicortex with precritical discharge of spike potentials in the 

 hippocampus. These observations agree with results of whole body irradia- 

 tion (400 r) in the cat: transitory discharge of spike potentials in the hippo- 

 campus and amygdaloid nucleus, increased arousal rhythms in the cortex 

 (Gangloff and Haley, 1960) . 



On the contrary, a stronger gamma irradiation (900 r) of the rabbits 

 head moderates the cortical activity (high voltage slow wave syndrome), 

 whereas a precritical hyperactivity persists in the hippocampal archicortex. 

 Similarly, x-irradiation of the head (1,500 r ) in the monkey is followed 

 by slow brain activity (Ross et al., 1954) . 



The reversibility of the electroencephalographic symptoms, namely of 

 thalamocortical effects, observed after weak irradiation suggests an early 

 action of gamma radiations on neuronal and synaptic activities at cortical 

 level. 



References 



Gangloff, H., and Haley, T. J. 1959. Veranderungen der elektrischen Hirnaktivitat 



bei Katzen nach Rontgenbestrahlung. Experientia 15, 397-399. 

 Monnier, M., and Gangloff, H. 1961. "Atlas for Stereotaxic Brain Research on the 



Conscious Rabbit." Elsevier, Houston, Texas. 

 Ross, A. T., Leavitt, S. R., Hoist, E. A., and Clemente, C. D. 1954. Neurological and 



electroencephalographic effects of x-irradiation of the head in monkeys. A.M.A. 



Arch. Neurol. Psychiat. 71, 238-249. 



