Learning-related representational changes reveal dissociable integration and separation signatures in the hippocampus and prefrontal cortex.

Margaret L Schlichting,Jeanette A Mumford,Alison R Preston

doi:10.1038/ncomms9151

Abstract

The episodic memory system enables accurate retrieval while maintaining flexibility by representing both specific episodes and generalizations across events. Although theories suggest that the hippocampus (HPC) is dedicated to represent specific episodes while the medial prefrontal cortex (MPFC) generalizes, other accounts posit that HPC can also integrate related memories. Here we use high-resolution functional magnetic resonance imaging in humans to examine how representations of memory elements change to either differentiate or generalize across related events. We show that while posterior HPC and anterior MPFC maintain distinct memories for individual events, anterior HPC and posterior MPFC integrate across memories. Integration is particularly likely for established memories versus those encoded simultaneously, highlighting the greater impact of prior knowledge on new encoding. We also show dissociable coding signatures in ventrolateral PFC, a region previously implicated in interference resolution. These data highlight how memory elements are represented to simultaneously promote generalization across memories and protect from interference.

Highlights

The episodic memory system enables accurate retrieval while maintaining flexibility by representing both specific episodes and generalizations across events
As pattern completion shows a bias towards recent information[1], encoding AB and BC in close temporal proximity might result in A–C similarity increases
We investigated learning-related changes in the neural representations of indirectly related A and C items for evidence of separation and integration using a representational similarity analysis (RSA)[31] searchlight constrained to anatomical regions of interest (ROIs)

Summary

Introduction

The episodic memory system enables accurate retrieval while maintaining flexibility by representing both specific episodes and generalizations across events. We show dissociable coding signatures in ventrolateral PFC, a region previously implicated in interference resolution These data highlight how memory elements are represented to simultaneously promote generalization across memories and protect from interference. The tension between separation and integration stems from the conceptualization of the hippocampus (HPC) as a fast-learning system that rapidly encodes pattern-separated memories Under this model, even related events are coded by largely nonoverlapping populations of HPC neurons[1,2,3], a scheme thought to support detailed retrieval and protect from interference[1]. Pattern completion during new memory formation would result in overlapping populations of HPC neurons coding related events[1,9], termed integration. Integration via nodal coding is defined as learning-related increases in similarity of A and C due to their common association with node B

Methods

Results

Conclusion