An electrographic study of activity of the brain was made with dogs during the course of formation and abolition of the conditioned avoidance reflex. Special attention was paid to background EEG activity which was observed during intervals of stimulation in the course of experiment.
1. In the experiment of habituation which preceded the experiment of conditioning, a sound stimulus was repeatedly applied to the animal. It was found that the stimulus became ineffective in causing arousal responses of EEG when the background EEG showed a deactivated pattern. EEG deactivation occurred in the course of experiment with increasing rapidity as the same experiment of habituation was conducted repeatedly.
2. The sound stimulus which had been neutralized through the experiment of habituation was used as the conditioned stimulus (CS), and an electric shock to the hindleg was the unconditioned stimulus (UCS). CS, paired or unpaired with UCS according to the principle of instrumental conditioning, was given in session of 15 to 25 trials per day over several days. The performance of the conditioned reflex (CR) in each session was measured in terms of score of CR, i.e., a number of correct CRs of a percentage of the total number of trials in one session. In earlier sessions of conditioning where scores of CR were low, deactivation of the background EEG occurred in about the same manner as observed in the experiment of habituation. The extent to which the background EEG was deactivated in one session of conditioning was gradually reduced as the animal passed through several sessions of conditioning. In final sessions having scores of CR as high as 100%, the background EEG was kept at a high level of activation throughout the session, though there occurred a slight degree of deactivation as trials of presenting CS were repeated.
3. CR was found to disappear only when the background EEG was deactivated to a certain extent. This was true with the abolition of CR by a procedure of experimental extinction as well as with that due to an injection of a small amount of barbiturate.
This work was done under the supervision of Dr. Kitsuya Iwama, to whom I am very much indebted. Thanks are due to Dr. Tomoaki Asano and Mr. Chosaburo Yamamoto who kindly read the manuscript. A part of the expense of this work was defrayed by a grant of the Ministry of Education awarded to Dr. Iwama.
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