Episodic long-term memory in the rat: effects of hippocampal stimulation

Rats with electrodes implanted in dorsal hippocampus were trained to perform a delayed spatial matching-to-sample task on a radial arm maze. Subseizure level electrical stimulation of the dorsal hippocampus applied during the study phase disrupted retention of a specific arm when tested at a 20-min delay but had no effects at 1- and 12-min delays. There were no state-dependent or proactive effects of stimulation. Subseizure level stimulation of the hippocampus immediately after the study phase resulted in normal retention. In contrast, seizure level stimulation of the hippocampus applied either during or immediately after the study phase disrupted retention at all three (1, 12, 20 min) retention delays. The data support the interpretation that the hippocampus is involved in the encoding of critical information (spatiotemporal attributes) in long-term working memory, but not short-term memory.