Effects of hippocampal lesions on maze learning and retention in rats.

Abstract Clinical observations suggest that bilateral damage of the hippocampal region and fornix may result in severe disturbances of recent memory. Experiments were undertaken to determine whether hippocampal and fornical lesions in rats have deleterious effects on maze learning and retention. A fourteen-choice mutiple-T alley maze was employed. The effect on postoperative initial learning was studied in fifty-two rats. One group of animals sustained bilateral and another group unilateral hippocampal or fornical lesions. A third group served as operated controls and received lesions in the same neocortical areas as the hippocampectomized animals. A fourth group of one hundred four rats were used as nonoperated normal controls. The animals with hippocampal or fornical lesions showed a significantly defective maze learning compared with the operated and nonoperated controls in terms of total errors and time required to run the maze. Deficit in maze performance also followed removal of the mamillary bodies. Thirty-eight rats were used in retention experiments and sustained either bilateral or unilateral hippocampal lesions, or removals of the neocortex overlying the hippocampus. All animals in this series were incompletely trained preoperatively and retrained postoperatively. The hippocampectomized rats suffered significant impairment in maze retention and relearning compared with the operated controls. It was concluded that lesions of the hippocampus, fornix, and mamillary bodies seem to interfere with maze learning and retention, when incompletely trained preoperatively. The data obtained do not allow any conclusions as to how these structures are involved in learning and retention, neither as to their relative importance compared with other brain structures like neocortex or thalamus, both of which are previously known to influence learning and retention.