Questions: Hippocampus: Memory Consolidation and Spatial Representation

Question 1

Patient H.M. had both hippocampi surgically removed. Which of the following abilities would you expect him to RETAIN?

• A. Remembering what he ate for breakfast that morning
• B. Recognizing faces of people he met after surgery
• C. Learning a new route through his neighborhood from memory
• D. Improving at a mirror-tracing motor skill through repeated practice

Answer: (D)

Procedural (motor skill) memory depends on the cerebellum and basal ganglia, not the hippocampus. H.M. could learn new motor skills — including mirror-tracing — even with no conscious memory of the training sessions. This dissociation between declarative memory (facts and episodes, hippocampus-dependent) and procedural memory (skills, hippocampus-independent) was one of the key scientific discoveries from studying H.M.

Question 2

A student studies for an exam and then stays up all night reviewing notes instead of sleeping. Based on hippocampal consolidation research, what is the MOST likely consequence for memory?

• A. No effect — memories are fully consolidated the moment they are encoded, regardless of sleep
• B. Impaired consolidation — sharp-wave ripple replay during sleep drives hippocampal-to-cortex memory transfer
• C. Enhanced consolidation — sustained waking activity reinforces hippocampal memory traces
• D. Only procedural memories are affected; declarative memories consolidate without sleep

Answer: (B)

During slow-wave sleep, hippocampal place cells replay waking activity patterns in compressed bursts (sharp-wave ripples), coordinated with cortical slow oscillations. This replay is thought to drive the transfer of memories from the hippocampal index to long-term cortical storage. Disrupting sleep disrupts this process. The intuition that 'staying awake reinforces memories' is wrong — the hippocampal-to-cortex transfer requires sleep-specific neural dynamics.

Question 3

Hippocampal damage that causes anterograde amnesia typically spares well-consolidated older memories more than recently formed ones.

• T. True
• F. False

Answer: True

This temporal gradient of amnesia is a key prediction of the hippocampal consolidation model. Older memories have been gradually consolidated to cortex and no longer depend on the hippocampal trace; recent memories still do. H.M. retained childhood memories from before his surgery but could not form new declarative memories. The gradient — older memories more protected than recent ones — is one of the strongest lines of evidence for the index-and-consolidate model.

Question 4

The hippocampus permanently stores memories the way a hard drive stores files — memories reside there indefinitely.

• T. True
• F. False

Answer: False

The hippocampus acts as a fast-learning temporary buffer or index, not permanent storage. It binds together the distributed cortical representations of an experience. Over time, through consolidation, the cortex learns the associations directly and the hippocampal trace becomes unnecessary. Permanent storage is distributed across neocortex. The hippocampus's role is temporary — critical early, but expendable once cortical consolidation is complete. This is why hippocampal damage doesn't erase all memories.

Question 5

Why does hippocampal damage typically produce anterograde amnesia (inability to form new memories) while leaving many older memories intact?

Model answer: The hippocampus serves as a fast-learning temporary index that binds together the distributed cortical components of a new experience. New memories initially depend on this hippocampal index for retrieval. Through consolidation — driven partly by sleep-based replay during sharp-wave ripples — the cortex gradually learns the associations directly, making the hippocampal link unnecessary. Old memories that completed this transfer before the damage are now stored in cortex and survive. New memories cannot be formed because the indexing mechanism no longer exists.

This question targets the most clinically important aspect of hippocampal function. The key distinction is between encoding/indexing (hippocampus-dependent) and long-term storage (cortex-distributed). Understanding this distinction explains both anterograde amnesia and the temporal gradient of retrograde amnesia — a two-for-one conceptual payoff.