R. HERNANDEZ-PEON AND H. BRUST-CARMONA 407 



there were no indications that the conditioned motor response could be re- 

 estabhshed by the buzzer-shock associations. 



Moreover, the general behaviour of all these cats with such mesencepha- 

 lic lesion was strikingly changed. The most notorious teaturc was impair- 

 ment of their vision and pain reactions. When walking, they bumped into 

 objects and the walls of the laboratory, i.e. they behaved as though they 

 were completely blind They were also unresponsive to painful stimuli 

 which, both in the intact and in the decorticate cats, evoked crawling and 

 aggressive behaviour. 



From the foregoing results it is evident that whereas extensive cortical 

 removals did not prevent conditioning, a small lesion in the mesen- 

 cephalic reticular formation eliminated visual and acoustic conditioned 

 respc^nses. It must be emphasized tliat the mesencephalic lesion impaired 

 various sensory discriminations to different degrees. While vision was 

 most impaired, olfaction seemed to be the modality least affected. It is 

 difficult to offer a satisfactory explanation for the surprising blindness 

 producec4 by a lesion placed tar bcliind the specific visual pathway. But the 

 apparent difficulty may be disposed of, if one remembers [a) that the 

 mesencephalic reticular formation receives photic impulses from the 

 retina (French, Vcrzeano and Magoun, 1953) as well as from cortical 

 visual areas (French, Hcrnandcz-Pcon and Livingston, 1955), and (/>) that 

 electrical stimulation of the same area modifies the excitability of the 

 retina (Granit, 1955; Hernandez-Peon, Scherrer and Velasco, 1956), the 

 lateral geniculate body (Hernandez-Peon, Scherrer and Velasco, 1956), 

 and the visual cortex (Bremer and Stoupel, 1959). The impairment of 

 vision might be the result of an interference with the centrifugal mechan- 

 isms which regulate transmission along the specific visual pathway, and/or 

 with the final integration of the photic impulses which result in conscious 

 vision. This explanation falls in line with the view proposed by Penfield 

 (Penfield, 1958, 1959) about the hij^lu'sr level of sensory iiite^^ratioii. Accord- 

 ing to this author, the final integration of sensory impulses is not achieved 

 at the cortical level but at the 'centrencephalon' where they arc finally 

 conveyed. The anatomical substratum of the 'centrencephalic system' is 

 the higher brain stem which includes the diencephalon, the mesencephalon 

 and probably the metencephalon. Our results indicate the important role 

 played by the brain stem in the integration of sensory impulses which 

 accompanies learning processes, and thus lend experimental support to 

 Penfield's hypothesis. 



The unavoidable conclusion that plastic association during conditioning 

 takes place at subcortical levels docs not preclude cortical participation in 



