Effects of Fornix Transection on Spontaneous and Trained Non-Matching by Monkeys

Three monkeys with fornix transection and three normal control monkeys performed a series of tasks which were variations of delayed non-matching. Experiment 1 showed that even at short retention intervals fornix transection impaired the spontaneous tendency to explore novel objects. Experiment 2 provided differential reward for non-matching and showed that the fornix-transected monkeys learned and performed non-matching normally even though the sample-match retention intervals were long throughout the experiment. Experiment 3 showed that non-matching performance was transiently more disrupted in fornix-transected than in normal monkeys when the testing procedure was changed, in a variety of ways, while maintaining the basic non-match rule. Experiment 4 required the monkeys to discriminate objects they had displaced from objects they had seen but not displaced; fornix transection produced in this task a substantial and stable impairment. These four experiments require a revised interpretation of the effects of fornix transection upon recognition memory and exploration. Particularly they contradict the hypothesis, suggested by previous experiments, that fornix transection produces a defect in discrimination of stimulus familiarity in long-term but not in short-term memory. They suggest rather that fornix transection impairs memory of instrumental responses.