Fornix Transection Impairs Learning of Randomly Changing Object Discriminations

The hippocampus has a well established role in spatial memory, but increasing evidence points to a role in nonspatial aspects of memory. To investigate such a role, six macaque monkeys received a bilateral transection of the fornix to disconnect subcortical inputs and outputs of the hippocampus. An additional six macaque monkeys constituted an unoperated control group. To test the involvement of the hippocampus in nonspatial aspects of memory, both groups were trained postoperatively on four concurrent visual object discrimination problems, each problem having one rewarded object and one unrewarded. After acquisition to criterion of these discriminations, the monkeys learned five subsequent stages of discriminations using the same objects. In each of these stages, both the pairings of objects one with another, and the reward assignments for the objects, were randomly reassigned. In the initial acquisition stage, control and fornix animals were equally proficient in learning the discriminations. In the five reassigned stages, however, monkeys with fornix transection made on average three times as many errors as the controls in learning the discriminations. This impairment was noted even in trials where the reward assignments from the previous stage were maintained in the new stage. These findings are consistent with other recent evidence for a role beyond the spatial domain for the fornix in monkeys.