Response to stimulus change following observation or exploration by the rat: differential effects of hippocampal damages.

The tendency of the rat to approach the place that had been changed between two successive trials was studied in a total of 118 rats. The experiment was conducted in an enclosed T-maze under two different conditions of stimuli presentation on trial 1. In the "passive" test the rat was allowed to inspect the white-black maze arms but prevented from entering them by transparent partitions, in the "active" test the rat was permitted to explore the entire T-maze. On trial 2 of both tests, the color of one arm was changed, so that the arms were either both white or both black. Sham operated controls showed a preponderance of choosing the changed arms in 75-80 percent in both tests. Rats with lesions of the anterodorsal or the posteroventral hippocampal region showed no arm preference in the passive test, while in the active test, the same groups displayed significant preference for the changed arm. Since performance in both tests relies m memory required for detection of the place of change, the behavioral dissociation following the hippocampal damage cannot be explained by recent hypotheses postulating involvement of the hippocampus in recognition memory, working memory or construction and execution of cognitive spatial maps.