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Related Experiment Video

Updated: Nov 8, 2025

Brain Imaging Investigation of the Memory-Enhancing Effect of Emotion

Brain Imaging Investigation of the Memory-Enhancing Effect of Emotion

Published on: May 4, 2011

Engram Size Varies with Learning and Reflects Memory Content and Precision.

Jessica Leake^1,2, Raphael Zinn^1,2, Laura H Corbit³

¹Centre for Neuroscience and Regenerative Medicine, Faculty of Science, University of Technology Sydney, Sydney, New South Wales 2007, Australia.

The Journal of Neuroscience : the Official Journal of the Society for Neuroscience

|April 23, 2021

Summary

Poorly learned fear memories, often formed under stress, are encoded by fewer hippocampal cells. These memories overgeneralize because limited neuronal ensembles lack the detail for context discrimination.

Keywords:

c-fos context engram fear conditioning hippocampus memory

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Area of Science:

Neuroscience
Memory Research
Behavioral Biology

Background:

Memories formed under suboptimal conditions, like inadequate learning time, often exhibit errors and overgeneralization.
Previous research indicates that fear memories formed with insufficient learning time display maladaptive properties, such as context overgeneralization.
The precise neuronal underpinnings of imprecise fear learning and memory remain largely unexplored.

Purpose of the Study:

To investigate the neuronal basis of context fear memory imprecision.
To examine how learning-dependent changes in memory precision are encoded in hippocampal neuronal ensembles.
To understand the relationship between engram size and memory specificity.

Main Methods:

Utilized c-fos expression to track neuronal activity in male mice during context fear conditioning.
Employed a c-fos-driven tagging method (TRAP2 mouse line) to identify neurons active during both learning and recall.
Analyzed neuronal reactivation patterns in the hippocampus in response to conditioning and similar contexts.

Main Results:

The number of c-fos-encoding cells correlated with session length, not learning history.
Learning and memory specificity positively correlated with neuronal activity in a subset of dentate gyrus cells active during learning and recall.
Comprehensive memories (longer learning) showed more double-labeled cells and preferential reactivation in the conditioning context, indicating discrimination.
Impoverished memories (shorter learning) had fewer double-labeled cells and equal reactivation in both contexts, correlating with overgeneralization.

Conclusions:

Engram size is dynamic and directly influenced by learning duration.
Larger neuronal engrams are critical for supporting both neuronal and behavioral discrimination.
Imprecise fear memories may result from smaller engrams lacking detailed information for context discrimination, leading to overgeneralization.