Retrograde amnesia following combined hippocampus-amygdala lesions in monkeys

Abstract

To assess the retrograde amnesia associated with medial temporal lesions, 8 cynomolgus monkeys were trained on object discrimination problems at different times prior to surgery. Different sets of 20 two-choice object discriminations were trained beginning 32, 16, 8, 4, and 2 weeks prior to surgery. Additionally, 6 weeks prior to surgery, all monkeys were trained on a motor skill task. Four monkeys then sustained bilateral removal of the hippocampus, amygdala, and overlying allocortex. The other 4 monkeys served as unoperated controls. Three weeks after surgery, memory for the 100 preoperatively learned object discriminations was assessed by presenting each of the object pairs for a single trial. The control group evidenced little forgetting of the discriminations: They performed as well on object pairs they had last seen 9 months earlier as on object pairs they had learned a few weeks earlier. The operated group demonstrated a profound loss of memory for all the discriminations: They performed at or near chance on the object pairs from all of the five preoperative time periods. Essentially the same results were obtained in four additional retention tests conducted during the following 8 months. Thus, retrograde amnesia was not temporally graded across the 8-month preoperative period sampled by the study. In contrast to these findings, retention of the preoperatively learned motor skill was excellent in both groups. Finally, additional tests showed that the learning of new object pairs was impaired after surgery, whereas a new motor skill was acquired at a normal rate. © 1987, Psychonomic Society, Inc.. All rights reserved.