The role of the CA3 subregion of the dorsal hippocampus in spatial pattern completion in the rat.

Rats were trained on a delayed matching-to-sample for a spatial location task to examine spatial pattern completion. On the sample phase of the task, rats were trained to move a small black block covering a food well that could appear in one of five possible spatial locations. During the choice phase of the task, rats were required to find the same food well, with the block removed so as to receive reinforcement. After reaching stable performance in terms of accuracy to find the correct location, rats received neurotoxic injections into the CA3 subregion of the hippocampus. The control group received vehicle injections into the CA3 subregion. After surgery, four extramaze cues were always available during the sample phase, but during the choice phase zero, one, two, three, or four cues were removed. The results indicate that normal vehicle control injected rats display excellent pattern completion across all reductions in the availability of cues, whereas rats with CA3 lesions are impaired in pattern completion as indicated by a linear increase in errors as the number of available cues is reduced. It appears that the CA3 subregion of the hippocampus plays an important role in spatial pattern completion.