57^ 



BRAIN MECHANISMS AND LEARNING 



A clear example of locomotor disinhibition after large prefrontal lesions was 

 also found by Shumilina (1949) in Anokhin's laboratory in experiments in wliich 

 the animal received food at two ends of the stand to two different CSs. 



To summarize, we think that, to use convenient expressions introduced by 

 Rosvold and Mishkin, prefrontal lesions may produce either relatively pure drive 

 disinhibition, when the extent of the lesion is limited to the frontal poles, or, in 

 addition, motor-response disinhibition when the lesion encroaches upon the 

 pericruciate area. It remains to be elucidated whether or not it would be possible 

 also to obtain pure motor-response disinhibition (without drive disinhibition) if 

 only the pericruciate area were to be removed. 



As to the mechanism ot motor-response disinhibition it should also be considered 

 as some release phenomenon, analogous to many forms of hypcrmotility produced 

 by various subcortical lesions. 



ucs 



ucs 



cs» 



^•R CS» 



a b 



Fig. 4 

 Schemes of excitatory and inhibitory connections involved in 

 type II conditioned reflex-arcs, a, positive conditioned CR; b, 

 inhibitory CR. Continuous Hncs, excitatory connections, broken 

 hues, inhibitory connections. Further exphinations in text. 



According to the vast experunental evidence ot our laboratory, the 'reflex-arc 

 of instrumental CRs should be considered as consisting of two parallel pathways 

 (cf. Wyrwicka, 1952) : one pathway runs from the central representation of the CS 

 through the corresponding drive centre to the representation of the instrumental 

 reaction (CS-*UCS->-R); the second pathway runs directly from the CS 'centre' to 

 the motor 'centre' (CS^R). When an inhibitory CR is established, inhibitory 

 connections are formed between the CS and UCS centres ('drive inhibition') and 

 between the CS and R centres ('motor-response inhibition'), but not between the 

 UCS and R centres, since, to positive CSs, the movement R is performed (Fig. 4). 

 When as a result of prefrontal lesions drive inhibition is impaired, both classical 

 and instrumental inhibitorv reflexes are disinhibited, as the pathway- CS^-UCS is 

 no longer blocked. If motor-response inhibition is impaired the animal will be 

 hyperactive but drive inhibition may be left intact. 



