M. R. COVIAN, C. TIMO-IARIA AND R. F. MARSEILLAN 435 



arc shown in Fig. i. At the end of the experiment an electrolytic lesion was 

 made in the brain stem reticular formation with a direct current ot 2-3 mA 

 during 15 sec. The animal's head was perfused with formalin through one 

 carotid artery and electrode position was later determined from serial 

 sections stained by Toluidine blue. 



RESULTS 



The stimulation of the right forepaw evoked (Fig. 2) the characteristic 

 surface positive-negative primary responses in the corresponding area of 

 the somatosensory cortex. With previous stimulation of the brain stem 



Fig. 2 

 Cortical evoked potential in somatosensory area by 

 tactile stimulation. Time 200 cps. Calibration : see uV 

 — same calibration for all other records. 



reticular formation at 100 msec, interval and 1.5 volts, the negative com- 

 ponent disappeared and a slight reduction of the positive one was observed 

 (Fig. 3). In Figs. 4 and 5 the same phenomenon is presented when the 

 stimulation was made at 50 msec, interval and 1.5 volts. 



It can be seen in Fig. 6 in superimposed sweeps, in the lower record the 

 evoked potential and in the upper one the effect of reticular formation 

 stimulation, at 100 msec, interval and 4 volts. 



FF 



