428 BRAIN MECHANISMS AND LEARNING 



discuss a priori the possibility that these influences may act upon the non- 

 specific systems. 



Numerous axons have been recently described which originate in the 

 medulla and are distributed across the midbrain tegmentum, grey peria- 

 queductal matter, superior colliculus and pretectal area, non-specific 

 thalamic nuclei and sub-thalamic region (Nauta and Kuypers, 1958). 



From these diffused axonal projections those concerned with the non- 

 specific nuclei seem worthy of special mention, since there is evidence that 

 intralaminar stimulation facilitates specific visual responses in the 'cerveau 

 isole' animal (Jasper and Ajmone-Marsan, 1952; Jung, 1958). 



We ascribe similar importance to periaqueductal and tegmental projec- 

 tions of the reticular formation. They might be the structural bases of 

 functional antagonistic interrelations between different levels of the 

 reticular formation of the brain stem according to inferences from the 

 results discussed, although there is no direct demonstration. 



It is also possible that these inhibitory influences may operate at a sub- 

 cortical level of the specific pathway. 



From data obtained by other authors, these inhibitory influences might 

 be exerted on the retina by a centrifugal pathway (Granit, 1955; Dodt, 

 1956; Hernandez-Peon, Guzman, Alcaraz and Fernandez-Guardiola, 

 1957; Palestini, Davidovich and Hernandez-Peon, 1959), in the geniculate 

 lateral nucleus (Hernandez-Peon, Scherrer and Velasco, 1956). 



Although we do not disregard these last possibilities, our findings lead 

 us to consider them of less importance. In fact, as we pointed out before, 

 potentiation of the primary response in the pretrigeminal animal has a 

 marked and almost exclusive effect on waves 3, 4 and 5 which most authors 

 believe to have an intracortical origin. 



There is another point to which we should like to refer briefly. As we 

 have noted, potentiation of the primary and secondary cortical photic 

 responses was obtained either w^ith a desynchronized (MPP) or synchron- 

 ized EEG pattern (rostropontine animal). This observation induces us to 

 consider that there may be no causal relationship between the facilitating 

 and the synchronizing and desynchronizing influences. 



To sum up, we can now say that the midpontine cat has the following 

 characteristics: 



1. A predominantly desynchronized EEG, generally correlated with a 

 wakeful pattern, although we are not in a position to say that its alert 

 state is greater than that of a normal animal. 



2. Primary cortical potentials evoked by a flash do not show habitua- 



