Rats with lesions of the hippocampus are impaired on the delayed nonmatching-to-sample task.

Rats with ibotenic acid lesions of the hippocampus (H-IBO) were trained on the trial-unique delayed nonmatching-to-sample task (DNMS) using a short delay of 4 s. The H-IBO group learned the nonmatching rule as quickly as control animals. However, performance was impaired on the DNMS task when the delay between the sample and choice phase was increased to 1 or 2 min. The use of 4-s delay (probe) trials indicated that the H-IBO animals retained the nonmatching-to-sample rule throughout testing. In a second experiment, using the same groups of rats, extended training at the 1-min delay did not ameliorate the deficit produced by H-IBO lesions. The finding of impaired recognition memory in rats after hippocampal lesions is consistent with findings from humans and monkeys. Several methodological issues are considered that have complicated the interpretation of earlier studies of recognition memory in rats following hippocampal lesions. The capacity for recognition memory in humans, monkeys, and rodents is discussed as a straightforward example of hippocampus-dependent (declarative) memory.