Functional dissociation between fornix and hippocampus in spatial conditional learning.

Do lesions of the fornix or the hippocampus impair the performance of spatial conditional associative learning tasks, and to what extent does damage to these brain structures result in comparable deficits in this type of spatial behavior? The available evidence is not clear. In the present study, rats with lesions of the fornix, hippocampus, and normal control animals were trained on two spatial-visual conditional learning tasks in which they had to form arbitrary associations between visual stimuli and the context in which these stimuli were embedded. In one condition, rats were required to choose stimulus X in place A and stimulus Y in place B, and there was no overlap in the contents of the two scenes. In the other condition, the animal approached the same scene from two different directions and had to select stimulus X when the scene was viewed from perspective A and to select stimulus Y when the scene was viewed from perspective B. Rats with fornix transection were able to learn both conditional tasks at a rate comparable to that of normal control animals, but rats with hippocampal damage were severely impaired under both conditions. The findings extend the range of tasks known to be sensitive to damage of the hippocampus. In addition, the results argue that the fornix is not necessary for the acquisition of certain spatial conditional learning tasks and that this brain structure cannot be used as an indicator of hippocampal dysfunction under all learning situations.