236 BRAIN MECHANISMS AND LEARNING 



light upon the dynamic course of events ckiring acquisition according to 

 which responses grow and fade in one structure after another until at 

 last a stable pattern of brain activity associated with the fully elaborated 

 CR becomes established. Any concept, therefore, that holds that the 

 essential events in conditioning and learning occur at a particular instant 

 in time and exclusively in the cerebral neocortex struggles against an 

 already impressive and continuously growing body of contrary facts. 



As for the work in our own laboratory, the research effort in which 

 Drs Shcatz, Hearst, Bogdanski, Vernier and several others have colla- 

 borated has been directed towards a descriptive analysis of the electro- 

 physiological events associated with acquisition, retention and extinction 

 of learned responses. It is our feeling that much new information remains 

 to be collected before we can comprehend even the simplest basic events 

 in learning, and we would do this with minimum reference to the pre- 

 conceptions of the processes involved that we ourselves invent or that we 

 have inherited in the form of 'syntheses' achieved by students now long 

 dead. Our experimental plan and essential results have already been 

 described and need not be repeated here (ist Macy Conference); we have 

 been observing evoked EEG responses in simple learning situations. Our 

 modest progress to date may be summarized under several general 

 headings. 



I. hi normal cats and monkeys the EEG response evoked by a brief 

 acoustic or visual signal of constant strength is not constant in size or 

 duration. This variability in response is not restricted to the cortical 

 terminus of the specific analyser but can be measured in widespread 

 cortical and subcortical regions as well. Presumably the observed vari- 

 ability reflects the processes underlying the 'orienting response' intensively 

 studied in many laboratories, and is related to what others call 'attention' 

 (Galambos, Sheatz and Vernier). 



3. Reinforcement of these auditory or visual signals (CS) by shock, food 

 or mildly noxious stimuli like a puff of air in the fice (US), remarkably 

 increases the probability that the response to the CS will be large in 

 amplitude and prolonged in duration at all sites where it is recordable. 

 Fig. I illustrates this phenomenon as well as other relevant points. 



3. If, however, the CS is a signal that an operant response (e.g. pressing 

 a lever) will avoid shock or procure food, the brain activity evoked by the 

 CS tends to become reduced during acquisition and may disappear in the 

 fully trained animal (Hearst ct ah). The evoked activity returns, however, 

 when the behavioural response is prevented (e.g. bv moving the lever out 



