508 BRAIN MECHANISMS AND LEARNING 



discriminations fMishkin and Pribram, 1954; Pribram and Barry, 1956; 

 Wilson and Mishkin, 1959). And this temporal lesion is specific to the 

 visual tasks; it docs not affect other learning problems, such as somesthetic 

 discrimination (Wilson, 1957; Pasik, ct a\., 1959). After establishing such a 

 functional localization, i.e. visual discrimination to temporal cortex, the 

 next step would be to analyse the neural mechanisms involved. Since one 

 does not know what ultimate cellular or molecular mechanisms to look 

 for, the studies conducted so far have dealt, at a gross level, with diverse 

 aspects of the phenomenon, each designed to answer a particular question. 

 For example, attempts to clarify the nature of the visual defect following 

 temporal ablations have been reported (Mishkin and Hall, 1955; Chow 

 and Orbach, 1957; Orbach and Fantz, 1958). 



The experiments here reported were designed to investigate the anato- 

 mical pathways necessary for the retention of visual discriminations, and 

 the function of isolated temporal and visual cortices in monkeys. Also, the 

 effect of local epileptic after-discharges on learning and retention, as well 

 as any consistent electroencephalographic (EEG) changes in the temporal 

 and visual areas during learning, have been studied. 



CORTICO-CORTICAL AND SUBCORTICO-CORTICAL PATHWAYS 



The finding that the temporal cortex is implicated in visual learning and 

 retention indicates the presence of more or less direct inter-communica- 

 tions between temporal and visual areas. In the monkey, the only known 

 cortico-cortical pathways of the striate area are short axon fibres to and 

 from the adjacent parastriate region (Mettler, 1935; Clark, 1941). From 

 there, post-synaptic fibres secondarily transmit visual impulses to other 

 cortical areas. Similarly, the anterior temporal cortex receives subcortical 

 fibres from the n. pulvinaris medialis, and it sends fibres back to this 

 nucleus (Chow, 1950; Whitlock and Nauta, 1956). Therefore, the tem- 

 poral cortex may receive visual impulses or may exert influences on the 

 visual area through either one of these two routes, or both. In earlier 

 studies, I interrupted the cortico-cortical pathways by bilaterally ablating 

 the parastriate cortex, and the subcortical pathways by bilaterally destroy- 

 ing the n. pulvinaris medialis (Chow, 1952, 1954). All these animals 

 retained pre-operatively learned visual discriminations as did normal 

 controls. In other words, these lesions did not duplicate the effect of 

 bilateral removal of the temporal cortex. I have not been able to destroy 

 completely in these animals the parastriate cortex or the pulvinar (it 

 should be noted that only the posterior part of the n. pulvinaris medialis 



