[Electrical activity of the dorsal hippocampus during DRL learning in the rabbit (author's transl)]

The change of electrical activity of the dorsal hippocampus during differential reinforcement of low rate (DRL) learning was examined in six rabbits. Bipolar electrodes were implanted stereotaxically into the dorsal hippocampus (3 mm posterior to bregma, 4-4.5 mm lateral and 7-7.5 mm ventral from bregma). After completion of continuous reinforcement learning, the animals learned DRL on a 10 sec. schedule. Rhythmic theta waves (6-8 Hz, 0.6-1.5 mV) were predominant in the early sessions of the DRL schedule. With further sessions, regular theta waves decreased and desynchronized waves or slightly irregular theta waves increased especially during the 10 second waiting period. Contrary to the report of Bennett et al., synchronized waves were always seen from 1-2 sec. before lever-press until completing of lever-press. Moreover, these preceding synchronized waves became more pronounced and of higher frequencies (8-10 Hz.) as learning proceeded. These high frequency regular waves (alpha-bursts) were seen in 5.6-35.8% of reinforced lever-press in the 10th session, and 79.5-93.8% in the 60th session. Alpha bursts were sometimes seen after 8-9 sec. waiting in the later sessions, but never seen within five secs.. During the extinction of DRL learning, the occurrence rate of alpha-bursts on adequate but non-rewarded lever-pressing decreased. These results cannot be explained satisfactorily in terms of an association between synchronized waves and voluntary movement. Frequency increase was seen when rabbits expected reinforcement after waiting for about 10 sec. or more. These results indicate that frequency increase of hippocampal synchronized waves is seen in association with higher central processing, most likely with memory.