272 



BRAIN MECHANISMS AND LEARNING 



could be heterogeneous) (Figs. 6, 8, lo, 1 1) (Adrian, 1941; Bartley and 

 Heinbecker, 1938; Bremer and Bonnet, 1950; Brookhart and Zanchctti, 

 1950; Buser and Albe Fessard, 1957; Dempsey and Morison, 1943; 

 Dcmpsey, Morison and Morison, 1941 ; Eccles, 195 1 ; Forbes and Morison, 

 1939; Li, Cullen and Jasper, 1956; etc.). 



In each ainmal, responses exhibited marked "spontaneous' variations 

 that, as noticed by earher investigators, predominantly involved the 

 slower, later phenomena (waves N2, P3, N3 and, to a lesser extent, Ni) 

 (Adrian, 1941; Dcmpsey and Morison, 1941; 1943; Eccles, 1951; Forbes 



EFFECT OF T ON POT. EV. BY SS ON CSC 



Tl 



lV«¥/^*lUl*M^I 



Tn 



jW^fF'W^^'^ 



r- 



CESS. SS 

 200 



,i^4^t*r^^ 



Fig. 5 

 effect of tone on potentials evoked by ss on cxjntralateral sensory cortex. tl (first 

 application): T produced a drop in amplitude and duration of slow waves induced by each 

 peripheral shock. Tn (habituation): T produced little effects if any. T-CESS. 200 (after being 

 followed 200 times by cessation of SS) : T produced notorious 'masking" effects. EV., evoked. 

 FOT., potentials. 



and Morison, 1939). These more variable waves were also the most suscep- 

 tible to T effects (see below). It is necessary to point out that modifications 

 induced in evoked potentials by effective agents (including tones) occurred 

 always within the range of possible 'spontaneous' oscillations. Relative 

 heights and duration of individual waves differed also from one prepara- 

 tion to another. 



(B) SS iiitaruptioii. When SS was cut off, cats reacted in a variable 

 manner; the list of movements appearing at this instant was limited, how- 

 ever, and included head elevation or rotation, ear movements, eye 

 opening or blinking, body straightening (or general shift) and interruption 



