The Primate Hippocampal Formation: Evidence for a Time-Limited Role in Memory Storage

TLDR

Clinical and experimental studies show that the hippocampal formation and related medial temporal lobe structures are crucial for learning and memory, but their role diminishes over time as more permanent memory develops independently, likely in neocortex. The study prospectively examined retrograde amnesia in monkeys to determine the hippocampal formation’s contribution to memory function. Monkeys learned 100 object pairs at five time points before hippocampal removal, and two weeks post‑surgery, memory was tested with a single‑choice trial of all pairs. Normal monkeys showed typical forgetting, remembering recent objects better than older ones, whereas hippocampally damaged monkeys were severely impaired on recent memories but retained long‑term memories as well as controls, indicating the hippocampal formation is needed only for a limited time after learning.

Abstract

Clinical and experimental studies have shown that the hippocampal formation and related structures in the medial temporal lobe are important for learning and memory. Retrograde amnesia was studied prospectively in monkeys to understand the contribution of the hippocampal formation to memory function. Monkeys learned to discriminate 100 pairs of objects beginning 16, 12, 8, 4, and 2 weeks before the hippocampal formation was removed (20 different pairs at each time period). Two weeks after surgery, memory was assessed by presenting each of the 100 object pairs again for a single-choice trial. Normal monkeys exhibited forgetting; that is, they remembered recently learned objects better than objects learned many weeks earlier. Monkeys with hippocampal damage were severely impaired at remembering recently learned objects. In addition, they remembered objects learned long ago as well as normal monkeys did and significantly better than they remembered objects learned recently. These results show that the hippocampal formation is required for memory storage for only a limited period of time after learning. As time passes, its role in memory diminishes, and a more permanent memory gradually develops independently of the hippocampal formation, probably in neocortex.