Retrograde amnesia and selective damage to the hippocampal formation: memory for places and object discriminations

Using a within-subjects design, rats were trained on two place-memory probl ems and five object-discrimination problems at different intervals prior to receiving either ibotenate lesions of the hippocampal formation or sham su rgery. Places # 1 and 2 were fixed-platform water-maze tasks that were run in different rooms and they were learned during the 14th and 2nd week befor e surgery, respectively. Object-discrimination problems #1-5 were learned d uring the 13th, 10th, 7th, 4th, and Ist week before surgery, respectively. Rats with hippocampal lesions displayed impaired retention of both Place pr oblems with no evidence of a temporal gradient to the impairment. In contra st to their retrograde place-memory deficits, the hippocampal rats displaye d normal retention of the five object-discriminations that were learned bef ore surgery. Hippocampal lesions had similar consequences for anterograde l earning, as the lesioned rats were impaired in acquisition of a new water-m aze problem that was run in a third room (Place #3), whereas they showed no rmal acquisition of two new object-discriminations. The findings indicate t hat the hippocampal formation is not required for long-term consolidation o f information underlying accurate performance of object-discriminations, an d that its critical role in memory for places persists for at least 14 week s, and probably for as long as those memories exist. (C) 1999 Elsevier Scie nce B.V. All rights reserved.