616 



MARCEL MONNIER AND P. KRUPP 



On the other hand, potentials evoked in the neocortex by stimulation of 

 the midbrain reticular formation at low frequency tend to show increased 

 amplitude and lowered threshold during the first hours after weak irradia- 

 tion (600 r) . This is no longer the case after strong irradiation (900 r) 



(Fig. 8). 



Threshold Vonotron 



VOLT Stimulation of midbrain reticui.syst 3/sec- 



6 hours 



I Irradiation 

 Cobalt 60 



Fig. 8. .\ction of various gamma irradiations on the threshold of the cortical poten- 

 tials evoked by stimulation of the midbrain reticular system. 



These observations prove that weak gamma radiations slightly and transi- 

 torily activate the reticular arousal system, but that stronger radiations 

 diminish the effects of reticulo-thalamo-cortical projections, probably by a 

 depressing action at the cortical level. 



Potentials evoked in the hippocampus by low frequency stimulation of 

 the contralateral hippocampus show a marked increase in amplitude after 

 weak (400 r) and strong (900 r) gamma irradiation. There is no doubt 

 that gamma radiations activate the archicortex (Fig. 9). 



On the other hand, potentials e\oked in the cortex by stimulation of the 

 ipsilateral hippocampus at low frequency (3 cps ) increase slightly in ampli- 

 tude after weak doses (400 r), but decrease after strong doses (900 r). 



Similarly, the threshold of these evoked cortical potentials shows a slight 

 decrease after weak irradiation (400 r, 600 r) (Fig. 10); this facilitation 

 disappears after strong irradiation. 



This suggests again that weak gamma radiations activate the hippocampus 

 and the hippocampocortical projection activity. Strong irradiation, on the 

 contrary, moderates the eflfect of these projections, probably by depression 

 at the cortical level. 



The action of gamma radiations on the specific thalamocortical projection 

 systems is less consistent. Weak doses (400 r) slightly enhance the specific 

 evoked potentials, whereas stronger doses reduce their amplitude and raise 



