384 BRAIN MECHANISMS AND LEARNING 



impulses as the basis of the changed electrical behaviour and makes it 

 it necessary to search for structural alterations. 



The material presented thus far indicates that a region of chronic 

 epileptiform discharge in one hemisphere can induce a similar pattern of 

 neuronal activity in cells of the opposite hemisphere which w^ere untouched 

 by the initial experimental procedure and which arc related to the primary 

 region by massive commissural pathways. After a time these contralateral 

 alterations become permanently fixed in local cellular organization and 



Fig. 9 

 Section tlirougli the region of the mirror focus. Note the collection of densely 

 stained cells to the right of the photomicrograph compared with the character- 

 istic staining of normal cortex to the left. Methyl green pyronin stain. 

 Magnification ■: 75. 



persist independently of the neural system which originally established 

 them. In addition to the spontaneous behaviour of these cells, we have 

 traced some measures of evoked activity which also indicate a reorganiza- 

 tion of the synaptic properties within the altered population. Of special 

 interest is the fact that these changes are long-lasting and that they survive 

 the chronic neuronal isolation of the involved neurones, an isolation which 

 prevents the spontaneous manifestation of the alteration which has taken 

 place. 



I should like to conclude by reporting some preliminary experiments 



