570 



BRAIN MECHANISMS AND LEARNING 



fact that, according to Rosvold and Mishkin, it is not observed when the instru- 

 mental reaction to the inhibitory stimukis is punished, speak against this supposition 

 and support the view that perhaps we have to do here with the impairment of the 

 inhibitory processes themselves. Therefore, we must consider the question of what 

 sort of inhibition is affected by prefrontal lesions and in which way it is affected. 

 Here we must inevitably revert to the classical Pavlovian notions of external and 



.before prefrontal lobectomy — 



after prefrontal lobectomy — 



Fig. s 



Examples ot disinhibition of salivary food conditioned reflexes after prefrontal ablations. 



Each graph represents the voluminograph of salivation to a positive conditioned stimulus 

 and to an inhibitory stimulus following it. Note that after prefrontal lobectomy positive 

 CRs are unchanged, while negative ones are more or less disinhibited (After Brutkowski, 

 1957)- 



internal inhibition and to the immense experimental evidence on which these 

 notions are based. It is quite clear that what is impaired after a prefrontal lesion is 

 only internal (or conditioned in the wider sense of the word) inhibition, wliile 

 external (i.e. antagonistic) inhibition is not affected at all or may even be increased 

 after this lesion. 



For our further analysis we shall accept here, as a working hypothesis, a concep- 

 tion put forward some time ago by Konorski (1948), according to which internal 

 inhibition is based on the formation of inhibitory connections between the central 



