T. PINTO HAMUY 



591 



factors determining the incapacity observed in our rats. This might depend 

 on: {a) the location and size of the lesion; (/)) the experimental situation; 

 (f) the animals' lack of pre-operative experience. The fnst two of these 

 factors {a) and {b) are closely related, since the complexity of the problem 

 can determine the critical level of intact cortex that the learning of a given 

 habit requires. 



10 



15 



TEN TRIAL UNITS 



Fig. 2 



Learning curves of a visual conditioned response (VCR) correspond- 

 ing to a group of normal rats (pre-operative learning) and neo- 

 decorticated rats (post-operative learning). 



{a) We tried to answer the first point by training two groups of animals 

 on the lACR to light (Pinto-Hamuy, Rojas and Araneda, 1959). One 

 group had an ablation in the rostral half of the cerebral cortex ('anterior 

 group') (Fig. 4); another had an ablation of the caudal half ('posterior 

 group') (Fig. 5). 



Each group consisted of thirteen animals; they were operated on and 

 subsequently trained. If the integrity of the visual cortex were essential for 

 this learning, we should expect the performance of the 'posterior group' 

 to be inferior. If, however, quantity was the determining factor, both 

 groups should have a similar level of performance. The curves of the 

 average performance per session for each group are represented in Fig. 6. 

 Here we can compare the performance of both groups with partial lesions 

 with the results obtained in fourteen normal rats as well as in a group 

 of fourteen decorticated animals (the term decorticated is used to mean the 

 group with more extensive lesions (i.e. 62 per cent)). It may be noted that 

 both the 'posterior group' in which the visual area is involved and the 



