Retrograde and anterograde object recognition in rats with hippocampal lesions

Retrograde and anterograde object-recognition memory was assessed in rats with cytotoxic lesions of the hippocampal formation (HPC), using a paradigm based on the natural tendency of rats to spend more time exploring novel objects than familiar objects. The rats were allowed to explore a sample object for 5 min/day on 5 consecutive days, either 5 weeks or 1 week before surgery. After surgery, retrograde recognition was assessed by comparing the amount of time spent exploring the sample versus a novel object in a free-choice situation. Control rats spent more time exploring the novel object than the sample objects from both presurgery time periods, whereas rats with HPC lesions did not discriminate between the novel objects and sample objects from either presurgery time period. Despite their deficits on the retrograde recognition test, the rats with HPC lesions performed like control rats on anterograde recognition tests, displaying a strong exploratory preference for novel objects over sample objects, with retention delays of either 15 min or 24 h. The findings suggest that extrahippocampal circuitry is capable of supporting object recognition, but only if the HPC does not participate in encoding the original encounter with the object.