E. GRASTYAN 



259 



cent of the units recorded in a given area at a given stage ot conditioning showed 

 an increase in discharge frequency. The foUov^^ing figures are taken from a cat in 

 which the sequential pattern to be described was particularly prominent. 



On the initial presentation of the signal stimuli before conditioning (Fig. 3) the 

 visual cortex is observed to respond only to the unconditioned stimulus, i.e. 

 intermittent light. Units in ventralis anterior did not respond cither to the condi- 

 tioned or to the unconditioned stimulus, whereas units in the mesencephalic 

 reticular formation responded with increased discharge to both stimuli. At this 

 stage units in the hippocampus were observed to respond only to the unconditioned 

 stimulus. Habituation trials were then carried out in which the conditioned stimulus 

 was presented repeatedly without reinforcement. Habituation was considered 

 complete when no change in unit discharge occurred in any area on presentation 

 of the conditioned stimulus (Fig. 4). Paired trials were then reintroduced until the 



L Mot -L Som S«n5 



STAGE III 

 Mot -L Sotn S«ftl 



Som 5«ns - L Aud 



UWUM"M'l!jJ^- 



L 4ud - L Vis 



^'.V.V>\\V.'',V.M'.V,V.'.V,'vV*'i-' — ■• 



<.VA^/w^'■^;5'■''','*'^'• - 



R^d -RSomS^os ' ' „ V, R IVu« 



B Som Sens -H Moi R lud - R Mol 



Fig. 8 



Examples of EEG tracings from surface cortical electrodes for each of three stages of electro- 

 cerebral conditioning. Unanaesthetized animal. Derivations are from implanted electrodes at 

 the sites indicated. First upward deflection on the signal channel indicates onset of soo c/sec. 

 tone. A photocell records the stroboscopic flashes superimposed upon the signal line. The final 

 downward deflection indicates cessation of tone and light. Calibration : 50 uV and i second. 



first signs of conditioning were observed, hi Stage I (Fig. 5) which was character- 

 ized by generalized desynchronization in the cortical surface tracing, units in the 

 visual cortex responded to both the conditioned and unconditioned stimulus. Units 

 in the ventralis anterior showed no change in response to the conditioned stimulus 

 but had now developed distinct increase in discharge frequency to the uncondi- 

 tioned stimulus. Units in the mesencephalic reticular system again responded to 

 both stimuli. At this stage, however, it was noted that hippocampal discharge 

 occurred in response to the conditioned stimulus as well as the unconditioned 

 stimulus. Stage II (Fig. 6), characterized by the appearance of a repetitive response 

 in surface cortical tracings, was correlated with suppression of discharge in the 

 visual cortex when the conditioned stimulus was presented and also in ventralis 

 anterior during presentation of both of the signal stimuli. Spontaneously firing 

 units in the mesencephalic reticular formation were also suppressed by both stimuli, 

 while in contrast, those of the hippocampal formation were increased in discharge 

 frequency during both conditioned and unconditioned signals. The development 

 of Stage in (Fig. 7) was defined by the appearance oi desynchronization limited to 

 S 



