Effects of ibotenate hippocampal and extrahippocampal destruction on delayed-match and -nonmatch-to-sample behavior in rats

The effects of ibotenate lesions of the hippocampus (HIPP) or hippocampus p lus collateral damage to extrahippocampal structures (HCX) were investigate d in rats trained to criterion on spatial versions of either a delayed-matc h (DMS) or delayed-nonmatch-to-sample (DNMS) task. After recovery from surg ery, animals were retrained at "0" sec delays, then assessed at 0-30 sec de lays for 15 d, retrained again at 0 sec delays, and retested for another 25 d on 0-30 sec delays. Pretrained HIPP lesioned animals showed marked delay -dependent deficits in both tasks that never recovered. Detailed examinatio n of within- and between-trial performance factors, including changes in re sponse preferences, length of previous trial delay, and sequential dependen cies, revealed important factors operating in lesioned animals that were ei ther absent or insignificant before the lesion. Pretrained HCX-lesioned ani mals showed deficits similar to those of HIPP animals, with the noticeable exception of a strong "recency" influence of the previous trial. Another gr oup of HIPP- and HCX-lesioned animals trained on the tasks after the lesion showed reduced impairments of the type described above, suggesting that ex trahippocampal structures trained after the lesion can assume the role of t he hippocampus to some degree. The findings indicate that both the type of lesion and the previous history of the animal determine the postlesion DMS and DNMS performance of animals suffering damage to the hippocampus and/or related structures.