Memory Consolidation

PSY 506A11.4.11Molly Bisbee

A Summary

Overview

• Definition

• Standard model

• MTT

• Differences

• Evidence

General Definition

• A process after initial encoding that is critical to stable long-term memory– Stabilizes memories– Incorporates new information with old

Temporal Characteristics

• Cellular or synaptic consolidation– Brief process lasting seconds or minutes

• Initial emphasis

• Systems consolidation– Lasts weeks, months, or years

• Recent emphasis

Human Memory Systems

The Standard Model (STM)

• dispersed neocortical sites linked together by hippocampal complex, where ensemble trace is rapidly created (via LTP or the like)

– H-trace, or “index”, “binds” the dispersed C-traces, allowing dispersed cortical sites to be reactivated together

• over time, C-traces link directly; intervention of H-trace no longer needed, it fades away

– information content is the same whether or not H-trace is involved in retrieval

– Hippocampus plays same role in episodic and semantic memory

STM

STM Problems

• Length of consolidation period

• Dissociations among episodic and semantic memory

• Reconsolidation

Multiple Trace Theory• H always involved in storage and retrieval of episodic memory• The H-C combination is the episode memory trace• H represents contextual component of episode• each re-activation/retrieval of a memory occurs in different

context and results in altered trace; expanded or strengthened H trace and H-C links

• episodic and semantic memory must be considered separately

MTTSimilar to Standard Theory

• hippocampus automatically encodes all attended information

• H- traces are sparsely encoded in distributed representations

• these ensembles act as pointers to the cortical ensembles storing attended information

Different from Standard Theory

• the H-C complex is the episode memory trace; H serves not only to bind elements in C, but also as the repository of critical episodic components

• each re-activation of a memory occurs in a different context and results in a new sparse, distributed trace in H

• each such trace shares some of the C representations

MTT asserts that H and C are both always involved in the storage and retrieval of episodic memory, and that episodic and semantic memory must be considered separately.

MTT Implications

• Aging episodic memories either forgotten or benefit from formation of stronger, expanded, memory traces

• HC active during retrieval of episode memories of any age

• HC preferentially active in processing spatial/contextual material

• H plays different role in episodic and semantic memory

• Partial HC damage affects memories proportional to age/strength; complete HC lesions will yield flat gradient of RA for episodic, but not semantic memories

• Remote memories in amnesics should be generic in nature

• Older memories will have stronger, more distributed, traces in H

• Reactivating a memory should lead to updating and reconsolidation

The evidence?

HC active during retrieval of episode

memories of any age

HC active during retrieval of recent episode memories

vs.

Older memories will have stronger, more distributed, traces in H

The evidence?

Older memories will have weaker, less distributed, (or nonexistent) traces in H

vs.

Remote events

Recent events

Foci of left hippocampal activations across subjects

Older memories are represented more diffusely within hippocampus

Reactivating a memory should lead to updating and reconsolidation

Reactivating a memory should not affect it

vs.

The evidence?

Reactivating a memory should increase H involvement

Reactivating a memory should reduce H involvement

vs.

The evidence?

R phg

L post HC L post HC

Recent 2 > Recent 1, p < 0.05 Remote 2 > Remote 1, p < 0.05

R phg

Repeated retrievals increase activation in hippocampus – equivalently for recent and remote memories

Reactivation

• Memories can be reactivated either online (during wake) or offline (during sleep)