Distinct patterns of behavioural impairments resulting from fornix transection or neurotoxic lesions of the perirhinal and postrhinal cortices in the rat

The present study provides evidence that lesions of the fornix (FNX) and of the perirhinal/postrhinal cortex (PPRH), which both disconnect the hippocampus from other brain regions, can lead to distinct patterns of behavioural impairments on tests of spatial memory and spontaneous object recognition. For example, whereas FNX lesions impaired allocentric spatial delayed alternation in a T-maze but generally spared a test of spontaneous object recognition, PPRH lesions produced the opposite pattern of results. Indeed, on the T-maze task PPRH animals significantly outperformed controls when the retention delay was increased to 60 s. In addition, some evidence was found that contributions from both the fornix and perirhinal/postrhinal cortex may be required when object and spatial information must be integrated. In an object-in-place test, for example, PPRH animals failed according to two measures, and FNX animals failed according to one measure, to discriminate objects that had remained in fixed locations from those that had exchanged locations with other objects. Neither lesion, however, affected performance of a visuospatial conditional task, a Pavlovian autoshaping task, or a one-pair pattern discrimination task. It is suggested that the perirhinal/postrhinal cortex, rather than being specialised for a particular type of associative learning, is important for processing complex visual stimuli.