The effects of neurotoxic lesions of the perirhinal cortex combined to fornix transection on object recognition memory in the rat

Abstract

The effects of lesions centred in the perirhinal cortex region (Prh) or in both the perirhinal cortex region and the fornix (Prh+Fx) were assessed in two different working memory tasks, one spatial the other nonspatial. For the spatial task the rats were tested in an eight arm radial maze, using a standard procedure in which they were rewarded for avoiding previously visited arms. The Prh+Fx, but not the Prh, rats produced significantly more errors (re-entries) and these started significantly earlier in each session when compared with a surgical control group. The nonspatial task was a test of spontaneous object recognition in which rats were tested on their ability to discriminate between a familiar and a novel object. For the initial tests the Prh group failed to discriminate between the objects, but the Prh+Fx group showed a clear preference for the novel object. Observation of the test showed, however, that the Prh+Fx group were spending a greater length of time initially exploring the sample (familiar) object. When the amount of exposure to the sample object was limited to either 20 or 40 s (i.e. was the same for all three groups), the Prh+Fx group now failed to discriminate between the two objects. This change was especially evident for shorter sample duration (20 s). The Prh group did, however, show an amelioration of their deficit with this further testing. The present results support previous dissociation between spatial and nonspatial working memory, and indicate that there may be some recovery of function following perirhinal cortical damage.