Functionally dissociating aspects of event memory: the effects of combined perirhinal and postrhinal cortex lesions on object and place memory in the rat

Abstract

Reciprocal interactions between the hippocampus and the perirhinal and parahippocampal cortices form core components of a proposed temporal lobe memory system. For this reason, the involvement of the hippocampus in event memory is thought to depend on its connections with these cortical areas. Contrary to these predictions, we found that NMDA-induced lesions of the putative rat homologs of these cortical areas (perirhinal plus postrhinal cortices) did not impair performance on two allocentric spatial tasks highly sensitive to hippocampal dysfunction. Remarkably, for one of the tasks there was evidence of a facilitation of performance. The same cortical lesions did, however, disrupt spontaneous object recognition and object discrimination reversal learning but spared initial acquisition of the discrimination. This pattern of results reveals important dissociations between different aspects of memory within the temporal lobe. Furthermore, it shows that the perirhinal-postrhinal cortex is not a necessary route for spatial information reaching the hippocampus and that object familiarity-novelty detection depends on different neural substrates than do other aspects of event memory.

Fig. 1.

Coronal sections illustrating the extent of the largest (gray) and smallest (black) lesions of the PPRH cortex. Thenumbers correspond to the approximate position from bregma (Paxinos and Watson, 1997).

Fig. 2.

Photomicrographs showing the extent of a typical PPRH lesion. Left, The lesion in the left and right hemispheres at approximately bregma −4.0 and −4.8 (Paxinos and Watson, 1997). Right, The lesion in the left and right hemispheres at approximately bregma −6.8 and −8.0 (Paxinos and Watson, 1997). Note the very small amount of CA1 damage in theleft section in the right panel. This is a typical amount of CA1 damage in cases in which hippocampal damage occurred.

Fig. 3.

Performance of PPRH and CONT animals on the Morris swim task. a, Mean escape latencies during acquisition. b, Mean swim path lengths during acquisition. c, Percentage of time spent in the four quadrants during a probe test conducted after the tenth acquisition session. There were no differences between the groups during either test.

Fig. 4.

Performance of PPRH and CONT animals on the radial arm maze task. a, Mean number of arms visited to obtain reward from all eight arms. b, Number correct in the first eight choices. c, Total number of errors committed across five sessions in which the maze was rotated during a 30 min delay period.

Fig. 5.

Performance of PPRH and CONT animals on the spontaneous recognition test. a, Difference in time spent exploring the novel and familiar objects (d1).b, Ratio of time spent exploring the novel object (d2). **p < 0.01, significantly poorer performance of PPRH animals relative to controls.