This shows a child's head.
For young children, however, memory is more general or “gist”-like, and these general recollections are typically not tied to a specific context. Credit: Neuroscience News

Memory Mechanics: How Young Brains Transition from Gist to Episodic Memory

Summary: Researchers discovered key molecular mechanisms that shift children’s memory formation from general or “gist-like” to event-based or “episodic”. This change in memory formation typically occurs between ages four and six.

The study identified that the maturation of inhibitory cells called parvalbumin-expressing (PV) interneurons, wrapped by a dense matrix called the perineuronal net, enables memory specificity and appropriately sized engrams.

This new understanding of memory development might unlock insights into conditions affecting the brain, such as autism spectrum disorder and concussion.

Key Facts:

  1. The study reveals that as inhibitory cells in the hippocampus mature, memory transitions from general to specific, forming appropriate-sized engrams.
  2. The research team was able to accelerate the development of the perineuronal net, creating specific episodic memories in juvenile mice.
  3. The findings could inform new insights into various conditions affecting the brain, from autism to concussion.

Source: Hospital for Sick Children

How do our brains become capable of creating specific memories? In one of the first preclinical studies to examine memory development in youth, a research team at The Hospital for Sick Children (SickKids) may have identified a molecular cause for memory changes in early childhood. 

Event-based memories, also known as episodic memories, are what people traditionally think of when they hear the word memory: a recollection tied to a specific context. For young children, however, memory is more general or “gist”-like, and these general recollections are typically not tied to a specific context.  

In a study published in Science led by Drs. Paul Frankland and Sheena Josselyn, both Senior Scientists in the Neurosciences & Mental Health program at SickKids, the researchers pinpoint the molecular mechanisms underlying the change from gist-like to episodic memory in mice.

The team notes that understanding this change, which generally occurs between four and six years old in children, may inform new insights in child development research and conditions which affect the brain, from autism spectrum disorder to concussion. 

A parvalbumin interneuron (blue) surrounded by the perineuronal net. Credit: Hospital for Sick Children.

“Researchers have studied how episodic memory develops for decades, but thanks to the development of precise cellular interventions we were now able to examine this question at the molecular level for the very first time,” says Frankland, who also holds a Canada Research Chair in Cognitive Neurobiology. 

Growth of perineuronal net may trigger change in memory 

In adults, memory traces (also known as engrams) are made up of 10 to 20 per cent of neurons, but the overall size of these engrams is doubled in young children, with 20 to 40 percent of neurons making up an engram supporting a memory. 

So why the change? The hippocampus, a part of the brain responsible for learning and memory, contains a variety of neurons including a type of inhibitory cell called a parvalbumin-expressing (PV) interneuron.

These inhibitory cells constrain the size of the engram and enable memory specificity. The research team identified that as these interneurons mature, memory transitions from general to more specific and engrams are formed at the appropriate size. 

Using viral gene transfer technology developed by Dr. Alexander Dityatev, head of the Molecular Neuroplasticity research group at the German Center for Neurodegenerative Diseases, the researchers decided to delve deeper and explore the reason for this change.

They found that as a dense extracellular matrix, known as the perineuronal net, develops around these interneurons in the hippocampus, the interneurons mature, shifting the way our brain creates engrams and stores memories.  

“Once we identified the perineuronal net as a key factor in interneuron maturation, we were able to accelerate the net’s development and create specific episodic, rather than general, memories in juvenile mice,” says Josselyn, who holds a Canada Research Chair in Circuit Basis of Memory.  

Informing new insights into brain function and cognition 

While the team was able to trigger this change in memory type by accelerating the development of the perineuronal net, they also note that the reasons for the age difference between gist-like and episodic memories should not be overlooked. 

“When you think about what purpose memory serves, it makes sense that a child’s memory would function differently from an adult,” explains Adam Ramsaran, a PhD candidate in the Frankland Lab and first author on the study.

“At three years old, you don’t need to remember the specifics. A gist-like memory helps children build a large knowledge base which can get more specific as they grow older and have more experiences.” 

Building on these molecular discoveries, the research team sped up the growth of the perineuronal net by providing an enriched environment to allow the formation of specific memories, a finding which is helping to inform child development research underway at SickKids and the University of Toronto. 

“Outside of memory development, we also found similar maturation-type mechanisms involved in different sensory systems of the brain,” says Frankland.

“The same brain mechanism may be used by several different brain regions for several different purposes, which presents exciting new opportunities for research and collaboration.”  

Funding: This study was funded by Brain Canada, the Canadian Institutes of Health Research (CIHR), University of Toronto, SickKids Research Institute, German Research Foundation, German Center for Neurodegenerative Diseases, National Institutes of Health (NIH), Natural Sciences and Engineering Research Council of Canada (NSERC), Ontario Graduate Scholarship program, Ontario Trillium Scholarship program and the Vector Institute. 

About this memory and neurodevelopment research news

Author: Jelena Djurkic
Source: Hospital for Sick Children
Contact: Jelena Djurkic – Hospital for Sick Children
Image: The header image is credited to Neuroscience News. The article image is credited to Hospital for Sick Children

Original Research: Closed access.
A shift in the mechanisms controlling hippocampal engram formation during brain maturation” by Paul Frankland et al. Science


A shift in the mechanisms controlling hippocampal engram formation during brain maturation

The ability to form precise, episodic memories develops with age, with young children only able to form gist-like memories that lack precision. The cellular and molecular events in the developing hippocampus that underlie the emergence of precise, episodic-like memory are unclear.

In mice, the absence of a competitive neuronal engram allocation process in the immature hippocampus precluded the formation of sparse engrams and precise memories until the fourth postnatal week, when inhibitory circuits in the hippocampus mature.

This age-dependent shift in precision of episodic-like memories involved the functional maturation of parvalbumin-expressing interneurons in subfield CA1 through assembly of extracellular perineuronal nets, which is necessary and sufficient for the onset of competitive neuronal allocation, sparse engram formation, and memory precision.

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