256 BRAIN MECHANISMS AND LEARNING 



clcctroccrcbral response. The stages were defined b\- changes in the surface 

 electrical activity according to the scheme just outlined. Simultaneous recordings 

 were made from the visual cortex, nucleus ventralis anterior of the thalamus, the 

 periaqueductal portion of the mesencephalic reticular system, and the hippocampus. 

 The signal stimuli were bright intermittent light (UCS) and low intensity pure 

 tone (CS) delivered in the same pattern as illustrated in the previous figures, except 



STAGE I 



Vis. Cortex *^, 



UCS 



'mm>,Ji$^lt^^llfm0^ 



CS 



Vent. Aiit. .y- fft^f'-\' 



UCS 



CS 



Mes. Ret. 



UCS 



CS 



Hippocampus „^^ ^,., „ ..«».,^«4»^^^J,*^, 



UC^ 



CS 



Fig. 5 

 Pattern of response during Stage I of the conditioning prticedure. (See te.xt for further 



explanation.) 



that the duration was shortened so that the entire trial could be included on a 

 single sweep of the oscilloscope. Thirty-tour animals, both cats and rabbits, were 

 used. All observations were made during acute experiments. Ether anaesthesia 

 was used for tracheotomy and craniotomy. Animals were immobilized under 

 Flaxedil and placed in the Horsley-Clarke stereotaxic instrument after injection 

 of procaine into points ot pressure and contact. Tungsten microelectrodes of the 



