bject familiarization and novel-object preference in rats

Weinvestigatedwhetherobject familiarizationwas related tonovel-object preference in thenovel-object preference (NOP) test in rats. In Experiment 1, we found that no significant correlation existed between the time spent investigating 2 identical copies of a sample object and the degree of preference for a novel object. In Experiment 2, rats investigated 2 identical sample objects for a total of 5, 30, 60, 90 or 120 s. Investigatory preference for the novel object was compared to chance expectancy as well as between the groups. Only the 90-s group and the 120-s group displayed above-chance investigatory preference for the novel object, but novel-object preference for these 2 groups did not differ from each other, suggesting that a minimal amount of sample object investigation is necessary for rats to develop a novel-object preference, beyond which no increase in novel-object preference was found. In Experiments 3 and 4, esions normal rats and rats with hippocampal lesions were given repeated test trials, with the same sample object presentedwith a different novel object, at 24-h and (Experiment 3) and 35-s intervals (Experiment 4). In both experiments, novel-object preference did not increase in magnitude with repeated sample object exposures, suggesting that increased familiaritywith the sample object doesnot result in increased novel-object preference. Ratswith lesions of the dorsal hippocampus showed an unreliable investigatory