Memory for objects and places following lesions of the hippocampus or perirhinal cortex in rats

Retrograde memory for object discrimination problems and spatial information was assessed in rats that had received either bilateral aspiration lesions of the perirhinal cortex or bilateral NMDA lesions of the hippocampus. Rats learned three different object discrimination problems: They learned one problem approximately 72 hours prior to surgery, another problem approximately 24 hours prior to surgery, and a third problem approximately 1 hour prior to surgery. The rats also learned a place-memory problem, in which they were required to learn the location of a submerged platform in a pool of water, either 72 or 3 hours prior to surgery. Following recovery from surgery, rats were tested for (1) their ability to remember the presurgery object discrimination and place-memory problems, as well as (2) their ability to acquire new object discrimination problems. Neither lesion caused significant retention deficits for the object discrimination problems. Rats with perirhinal cortex lesions were impaired at acquiring the first postsurgery object discrimination problem but unimpaired at acquiring a second postsurgery object discrimination. Rats with hippocampal lesions were unimpaired at acquiring either postsurgery object discrimination problem. Rats with hippocampal lesions, but not those with perirhinal cortex lesions were impaired on the place-memory task that had been learned prior to surgery. These findings suggest that: (1) the hippocampus, but not the perirhinal cortex, plays an essential role in spatial memory, and (2) neither the hippocampus, nor the perirhinal cortex play an essential role in long-term memory for object discrimination problems