444 BRAIN MECHANISMS AND LEARNING 



electric shock applied in the central sensory pathway or by using a physiological 

 stimulus applied to the receptor. I would like to add that it was reported some time 

 ago by Dr Purpura that dendritic potentials which are supposed to arise in the 

 cortex are depressed during reticular stimulation or during arousal. 



Magoun. Local cortical responses, along with other surface-negative phases of 

 evoked cortical potentials, are most susceptible to EEG arousal by reticular 

 stmiulation or by natural alerting to attention. The conclusion that an inhibition of 

 dendritic depolarization is the key feature of EEG arousal needs some qualification, 

 however. Drs Eidelberg and Feldman tested this with thioscmicarbazide which 

 blocks the formation of GAB A to the point of generating seizure discharge in the 

 nervous system. In this circumstance, surface-negative recruiting responses became 

 verv much augmented, as do negative phases of other evoked cortical potentials. 

 However, these augmented surface-negative phases could still be abolished by 

 reticular stimulation. This suggests that if an active inhibition is involved here, it is 

 one that is independent of GAB A biochemistry. Additionally, in this connection, 

 when the surface-negative phase is blocked in EEG arousal, one does not see an 

 inverted, surface-positive potential, as in the situations in which Drs Purpura and 

 Grundfest proposed a hyperpolarizing potential to be unmasked. 



EccLES. The GAB A is not just blocking an excitatory surface dendritic potential 

 and leaving an inhibitory one. Grundfest has published records which show that a 

 better interpretation is that GABA is simply eliminating surface excitatory poten- 

 tials and leaving the excitatory potentials deeper down on the apical dendrites and 

 somas, which of course would give rise to surface-positive potentials. 



