Place and response learning of rats in a Morris water maze: Differential effects of fimbria fornix and medial prefrontal cortex lesions

The question examined in this study is concerned with a possible functional dissociation between the hippocampal formation and the prefrontal cortex i n spatial navigation. Wistar rats with hippocampal damage (inflicted by a b ilateral lesion of the fimbria fornix), rats with damage to the medial pref rontal cortex, and control-operated rats were examined for their performanc e in either one of two different spatial tasks in a Morris water maze, a pl ace learning task (requiring a locale system), or a response learning task (requiring a taxon system). Performance of the classical place learning (al locentric) task was found to be impaired in rats with lesions of the fimbri a fornix, but hot in rats with damage of the medial prefrontal cortex. whil e the opposite effect was found in the response learning (egocentric) task. These findings are indicative of a double functional dissociation of these two brain regions with respect to the two different forms of spatial navig ation. When the place learning task was modified by relocating the platform , the impairment in animals with fimbria fornix lesions was even more prono unced than before, while the performance of animals with medial prefrontal cortex lesions was similar to that of their controls. When the task was aga in modified by changing the hidden platform for a clearly visible one (visu al cue task), the animals with fimbria fornix lesions had, at least initial ly, shorter latencies than their controls. By contrast, in the animals with medial prefrontal cortex damage this change led to a slight increase in es cape latency. (C) 2001 Academic Press.