Abstract

Rats have a natural tendency to spend more time exploring novel objects than familiar objects, and this preference can be used as an index of object-recognition memory. Rats also show an exploratory preference for objects in locations where they have not previously encountered objects (an index of place memory), and for familiar objects in contexts different from those in which the objects were originally encountered (an index of context memory). In Experiment 1, rats were subjected to either 10 minutes of global-cerebral-ischemia (ischemia)--which resulted in significant neuropathology in the hippocampus with no observable damage to other areas--or sham ischemia. Rats were then tested on all three versions of the novelty-preference paradigm (NPP), with retention intervals, between the familiarization and test phases, of either 5 minutes or 24 hours. Both ischemic and sham rats displayed a novelty preference on all three trial types when the retention interval was 5 minutes. Rats that received sham surgery, but not ischemic rats, displayed this preference on OBJECT and PLACE trials with a 24-hour interval. Rats that received sham surgeries displayed a novelty preference. Neither group discriminated between objects on CONTEXT trials with a 24-hour interval. In Experiment 2, rats with cytotoxic lesions of the hippocampal formation were tested on the OBJECT version of the NPP with retention intervals of either 15 minutes or 24 hours. Both sham and rats with hippocampal ablation discriminated between novel and familiar objects in both delay conditions. The combined findings of these two experiments suggest that object-recognition deficits following ischemia are not due to hippocampal damage.