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Neuron study helps explain why we forget

Memories from early childhood are notoriously elusive but why can’t we recall our most formative experiences? New research…

Childhood memories seem few and far between – if they still exist at all. So why can’t we dig them up as adults? Rob./Flickr, CC BY-NC

Memories from early childhood are notoriously elusive but why can’t we recall our most formative experiences? New research suggests it could be a case of the old making way for the new – neurons, that is.

A study, published today in Science, has found that neurogenesis – the generation of new neurons – regulates forgetting in adulthood and infancy and could significantly contribute to the phenomenon of “infantile amnesia”.

Throughout life, new neurons are continually generated in the dentate gyrus, part of the brain’s hippocampus. This is one of only two areas in the mammalian brain that consistently generates neurons after infancy, aiding the formation of new memories of places and events.

These new neurons compete for established neuronal connections, altering pre-existing ones. By squeezing their way into these networks, new neurons disrupt old memories, leading to their degradation and thus contributing to forgetting.

Neurogenesis is particularly rampant in humans during infancy but declines dramatically with age. So researchers hypothesised that this increased disruption to hippocampal memories during childhood renders them inaccessible in adulthood.

Rodent recollections

To investigate the correlation between neurogenesis and forgetting, a team from the University of Toronto conducted a series of tests on mice, guinea pigs and a type of small rodent called degus.

First, a group of infant and adult mice were trained to fear a certain environment through the use of mild electric foot shocks.

Some of the adult mice were then provided access to running wheels, an activity that has been shown to boost neurogenesis. When returned to the initial environment, the adult mice who used the running wheels had largely forgotten their fear of the electric shocks, while those without the wheels maintained an association between the space and fear.

From the group of infant mice a number were given drugs to slow the rate of neurogenesis to see if decreasing the generation of new neurons mitigated the forgetting normally observed in infant mice. In accordance with the researchers’ hypothesis, the ability of these animals to retain memories improved in comparison to their untreated counterparts.

The study was then moved to rodents whose infancy period distinctly differs from mice – and humans – guinea pigs and degus. These rodents have a shorter postnatal hippocampal neurogenesis because they are more neurologically mature at birth. That means they have extended memory retention as infants so those animals were given drugs to artificially increase neurogenesis – which resulted in forgetting.

Psychologist Dr Amy Reichelt, from the University of New South Wales, said it was good the study used infant guinea pigs and degus.

“These animals are born in a ‘precocious’ way – they are basically miniature adults – able to run about independently, as opposed to mice, rats and humans who are vulnerable and dependent at birth,” she said.

“In young animals where neurogenesis is at a high level, memory circuits are constantly changing, so this supports that certain memories are ‘pruned’ out and thus forgotten – supporting the notion of infantile amnesia.”

How could you forget?

Previous studies have examined the relationship between hippocampal neurogenesis and memory, with a focus on its importance in the consolidation of memories in adult animals. But they have not considered how neurogenesis can also jeopardise memory retention.

Behavioural psychologist Dr Jee Hyun Kim, Head of the Developmental Psychobiology Lab at Melbourne’s Florey Institute of Neuroscience and Mental Health, said: “It has long been speculated that the ‘immaturity’ of the hippocampus may be responsible for infantile amnesia. Back in the days ‘immaturity’ was interpreted as dysfunctional, or low in function.

“However, recent studies speculated that immaturity can also occur in the form of hyper functionality. This study shows that the extreme plastic nature of our brains early in life can be the reason why we forget quickly episodic memories happening early in life.”

Infantile amnesia is not restricted to hippocampus-dependent memories in humans and animals. Dr Kim said it was likely that neurogenesis formed only a part of the story.

“I wouldn’t be surprised if we find undiscovered neurogenesis in other parts of the brain,” she said.

A spotless mind

But does this research hint at ways of improving memory retention in the future?

“It would not be feasible to discourage neurogenesis and reduce forgetting of existing memories,” Dr Kim said, “as adult neurogenesis has a well-established link to depression (low neurogenesis means high depression)”.

Surprisingly, it’s the other side of the coin that promises more potential opportunities. Harnessing neurogenesis to destabilise pre-existent memories could have its own benefits. Dr Kim said depressed or anxious people may want to forget and focus on creating better memories and/or thought patterns.

This can be especially constructive for children who experience trauma in early life, Dr Reichelt said.

“Increasing neurogenesis could be a useful therapy to treat or prevent the onset of post-traumatic stress disorder,” she said.

Join the conversation

11 Comments sorted by

  1. Michael Bartlett

    PhD Candidate at ANU

    Really interesting news. Keep this kind of reporting coming!

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  2. Allan Gardiner

    Dr

    How many people do you know who will readily feed you the line "I forget" when asked something that they say they can't bring to mind? Many people are so focused on trying to remember these two simple words that just about all else seems to escape them at ti_mes'meric. They really ought to synaps..err..snap out of it and get a gr_ip'sissima verba.

    If everyone lost the ability to forget -- by simply forgetting to forget -- then they'd all have total recall...and be easily able to forge[t] themselves some new horizons.

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  3. Terry J Wall
    Terry J Wall is a Friend of The Conversation.

    Still Learning at University of Life

    “In young animals where neurogenesis is at a high level, memory circuits are constantly changing, so this supports that certain memories are ‘pruned’ out and thus forgotten – supporting the notion of infantile amnesia"

    I can see a couple of weaknesses in this. First is that MRI scans have proven we only use a very small percentage of the brain, so why would we need to prune memories when there is so much spare storage not being used?

    Secondly, there is a branch of "medicine" called NeuroLink that maintain that there may be over 300k of individual enzymes needed to make the brain tick over the way it was once designed. Or to put it another way, to allow the synapses to be fully functional.

    If that is the case, why are we not discussing the pluses and minuses of boosting the diet with readily available trace elements? After all they make up over half the ingredients needed to create an enzyme.

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    1. Michael Shand

      Software Tester

      In reply to Terry J Wall

      Pretty sure if you just eat a healthy balance of fresh fuit and veg you will be fine, no need to introduce trace elements into food

      Newton seemed okay with normal food, I'm sure Joe blogs will be fine with normal food

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    2. Adam Johnson

      n/a

      In reply to Terry J Wall

      Terry, you said that "I can see a couple of weaknesses in this. First is that MRI scans have proven we only use a very small percentage of the brain, so why would we need to prune memories when there is so much spare storage not being used?"

      That we only use a very small percentage of our brain indicates that we only have capacity to use that much - ie that we are unable for whatever reason to use more than that small percentage.

      Given that we can only use that small percentage then it follows that we need to move stuff out to fit new stuff in - otherwise the brain gets overcrowded and new stuff would be more than our brain could accommodate.

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    3. Terry J Wall
      Terry J Wall is a Friend of The Conversation.

      Still Learning at University of Life

      In reply to Adam Johnson

      Good comment Adam, but I am not sure that you are right.

      In the case where disease blocks to movement of fluid from the brain to the spinal column, the brain can become extremely compressed to a very thin layer around the inside of the skull. Interestingly such people do NOT loose mental skills, if fact I recall that they gain in some significant ways. Work that out.

      Anyway it proves that the neurons are very fluid and can switch on areas previously not picked up in MRI scans, as being active.

      So it does leave the possibility if you were to provide the brain with additional fuel, for want of a better description, it may indeed be capable or expanding into previously less areas.

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  4. Michael Shand

    Software Tester

    Good article

    I remember an article being posted on The Conversation a few years ago from a Neuro-scientists who explicitly said that neuro-science cannot tell us how the brain works only when it doesn't to try and justify a free will concept

    ohh how times change

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  5. Paul Prociv

    ex medical academic; botanical engineer at University of Queensland

    So, does this mean that politicians and shonky business operators have enhanced neurogenesis in parts of their brain?

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  6. Rod Andrew

    Editor, teacher, engineer

    Thanks,
    An interesting article. At what age does 'Neurogenesis' stop or slow down enough to stop causing the memory loss effect?

    Also, I've also wondered why I can't recall early events consciously, but they can be triggered by things like smell or certain times of the year. Is that connected?

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  7. Mike Roloff

    logged in via Facebook

    i am in the process of completing an autobiographical book entitled SCREEN MEMORIES. Screen Memories are concentrates of memories around traumatic events. These are very concretely recalled and can be explicated like dreams and generally a whole world of experience associates to them.
    Mine date to an abandnment experience at age 9 months, and then there is a whole series, I end at age 21 with the formation of a screen memory during a fire fighting experience in Alaska. However, I also did a psychoanalysis with a complete regression to it appears my birth during which these memories were intensely refreshed. Psychoanalyisis itself made for the poroduction of fresh neurons and neuronal connection - the famous "shrinking' which is actually a brain growth, and which you can feel crackling away in your head! That is also the lifting of what is called repression.

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