562 



BRAIN MECHANISMS AND LEARNING 



that even the normal animals initially make more errors by responding 

 when they should not respond, than by not responding when they should. 

 The only deterrent to this strong generalized tendency to respond to the 

 negative cue is the absence of reward. In line with the principle stated 

 earlier the monkeys with frontal lesions exhibited a marked impairment 

 which, in the early part of training at least, may justifiably be labelled a 

 loss of inhibition. 



300-1 



200- 



100- 



LOSS of INHIBITION ? 



4 5 6 7 



200- TRIAL BLOCKS 



Fig. rt 

 Effect of frontal lesions on inhibiting responses to the negative stimulus in a 'go-no go' 

 situation. 



How can such an impairment be accounted for theoretically? The 

 answer, unfortunately, is that there are numerous possibilities as can be 

 seen in Fig. 7. An inhibitory defect can be postulated at any point in the 

 stimulus-response sequence and it will still provide a reasonable explana- 

 tion of the frontal animal's deficiency on discrimination tasks. For example, 

 if an inhibitory defect were present at the response and ot the chain, the 

 neural model could be a loss of suppression of the cortical motor systems. 

 Specifically, an hypothesis could be developed on the basis of avilable 

 ncurophysiological data (Mettlcr, 1935; Freeman and Krasno, 1940; 

 Dusser de Barenne, Carol and McCuUoch, 1942) that frontal cortex, 

 acting through the cauciate nucleus, normally exerts a suppressive influence 



