Impairments in recognition memory for object and for location after transient brain ischemia in monkeys.

Using an object recognition or location memory test employing either small or large sets of training stimuli, we examined the effects of selective damage to the hippocampus after ischemia in Japanese monkeys. Ischemic (ISC) monkeys were significantly impaired in learning a delayed matching-to-sample (DMS) task (10 sec) when the sample and test objects were drawn from a set of 45 possible stimuli, but not in learning a DMS when a set of 300 possible stimuli was used ISC monkeys were also impaired in a delayed matching-to-location (DML) task that employed a 3-well tray but not in one that employed a 10-well tray. After criterion learning was attained, ISC monkeys were impaired significantly only in the 300-stimuli version and at the longest delay tested (10 min). These results suggest that the hippocampus might be involved in processing comparisons and forming relationships between current and recent stimuli, but not in the remembering of familiar objects, and in long-term maintenance (more than 10 min) of stimulus memory.