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TL:DR A brain is turned into mush, destroyed, then 98% of the organ is replaced with other brain tissue. Can that 2% account for memories shared between both brains? Can the brain remember things that happened to the other brain? The new brain is 2% old brain and 98% new brain.

Story:

Character A is a self made immortal organism, a girl who learned the magic needed to modify her body both consciously and automatically.

Character A has the ability to modify every cell of her body, 300 years ago she learned bio magic and covered herself with nearly indestructible and regenerating solid plate armor, she has no face and no mouth. Her head looks like an obsidian helmet but is actually made of light sensitive cells which work as eyes. Character A can sometimes grow bio weapons out of her body, like toxic smoke, explosives and flammable acid that burns win contact with water. Most importantly she doesn't age, doesn't get cancer or any disease.

Character A does not have a face or facial muscles, she could build one at any time but for amusement when she is asked what she looks like, she opens her face helmet showing exposed veins, fat tissue and nerves, no bones, she doesn't have bones in her face or skull, just the shell armor.

Character A is living in another more primitive world but comes from our modern world, she has deep knowledge of political and military history, that's why she has worked for kings and emperors as a military commander over the centuries.

Character A was alive for more than three hundred years, but let herself be killed in battle for personal reasons.

The Empire needs Character A alive for their military agenda, Character A is a strong warrior, a good tactician and the best propaganda ever.

Character A has the same importance as Jesus, a living Jesus made of flesh.

The Empire's magicians have learned how to activate the subconscious magic powers that regenerate character A and bring her back to life, After the damage which killed Character A the regeneration takes 2 years to bring her back to life, as compared to the 3 weeks needed to regenerate or modify 45kg of tissue while she was alive.

For my story I need character A to wake up with the mentality of a newborn baby who needs to learn everything from the beginning, a dangerous and lethal newborn baby in the body of an immortal murder machine with centuries worth of muscle memory of killing people.

As character A grows to the mental age of 7, I need some of her old memories to come back, is it possible or do I need to handwave it with something?

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    $\begingroup$ Despite the name, muscle memory is encoded in the brain, not the muscles. Someone with the brain of a newborn baby has the motor skills of a newborn baby. If they really "need to learn everything from the beginning", they won't have centuries' worth of muscle memory. $\endgroup$ – Nuclear Hoagie Jun 10 at 17:11
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    $\begingroup$ @NuclearHoagie actually it's both at the same time. The brain activates a pattern, then a set of neurons both in the spine and nearby the muscles take over to execute the pattern. Hold a pingpong ball in your hand and the neurons will constantly let different muscle fiber groups contract and relax to maintain the illusion of a constant force being exerted without tiring individual muscle fibers too much. A proper muscle memory has the timings of all these processes streamlined for the best effect. $\endgroup$ – Demigan Jun 10 at 17:27
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    $\begingroup$ Yes, but as soon as they recover they'll be overcome by an irresistible urge to run for public office. $\endgroup$ – Mon Jun 11 at 0:38
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    $\begingroup$ there's a simple solution to this: brain is (intentionally) still 100% intact - just kept on life support (again, intentionally). memories are locked behind trauma (it doesn't matter whether it's intentional or not - whichever suits your agenda really. if you normally don't feel pain, extreme pain will probably create trauma). "personal reasons" doesn't mean she wanted to die. if she really wanted to die she could have self destructed every cell in her body just as easily as she made herself immortal $\endgroup$ – somebody Jun 11 at 7:44
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    $\begingroup$ a girl who learned the magic - Done. You're already including magic in this scenario, you can explain anything you want to explain with magic. $\endgroup$ – Darrel Hoffman Jun 11 at 13:42
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Better practice your handwaving.

Scientific knowledge of exactly how memories are stored is incomplete at best, but it appears to be spread through significant parts of the cortex and distributed. There was an old joke among neurosurgeons, when they'd make an incision or otherwise touch the brain, "There goes yesterday's ham sandwich," because so often they'd find very minor deficits after such small damages.

It's very unreasonable, however, to think that a mere 2% surviving tissue from the original brain will store any significant memories, or with any completeness.

Also, it's worth noting that "muscle memory" is no such thing -- it's conditioned reflex, which resides mainly in the motor centers of the brain and to a much lesser extent in the reflex processing in the spinal cord. Your immortal killing machine of a person wouldn't even be a real baby -- a baby has several months of in-womb neural connection forming before birth, such that they can at least mostly control "autonomic" systems like digestion and breathing, where a freshly recreated brain, if truly grown from "blank" tissue, would lack even that.

One possible way around is if the brain were regrown with the same neural connections that it had before the, um, accident. This would require a nearly molecular level 3D record of everything inside the skull, at a minimum, and technology far beyond anything we can even explain today -- or magic that works in mysterious ways -- but there's some reason to believe that at least some reflex and memory are encoded in the connections between brain cells. If those connections can be recreated, much of memory might return with them.

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  • $\begingroup$ You could pose as well that the brain is grown back according to existing DNA. That means breathing and the like will be functional, while memory will still be mostly shot. Also part of muscle memory is based in the spine, so some muscle responses can come back quicker when the brain is relearning them. $\endgroup$ – Trioxidane Jun 11 at 9:52
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    $\begingroup$ @Trioxidane DNA contains the construction plan, not the after-build detailing -- like having the blueprints to make a computer chip doesn't give you the operating system. I did note that a small fraction of "muscle memory" is in the reflex centers of the spinal cord, however. $\endgroup$ – Zeiss Ikon Jun 11 at 11:38
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Based on other answers, it seems impossible that the brain recovers. But your character has taken that into consideration, seeing as she can alter her biology at will. Unknown to everyone but herself, she made a secondary memory storage in her body, to be able to restore her memories in case of significant damage to her brain. The way they triggered her regeneration, the memory restoration is not activated, because they didn't know about it, but she later stumbles on these other memories stored somewhere in her body.

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    $\begingroup$ Great firs answer/post Marcelo, Please take our tour and refer to the help center for guidance, as and when needed. Enjoy the site. $\endgroup$ – A Rogue Ant. Jun 11 at 3:13
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Memory is a complex issue. It combines chemical composition, the amount and distance of linked neurons and constant feedbackloops that remain firing electric signals to maintain a type of behaviour/memory, like liking a particular soda.

To further complicate things the brain is constantly modifying itself and can have immense potential on small portions of it. This plasticity of the brain means that a mailman will have an increased area for navigation and area memory, an area that will shrink and give over neurons to other tasks if you stop being a mailman and start doing other tasks. A more extreme example I personally knew someone with a mental problem who died of cancer. Then when they did an autopsy they found out that during his cancer one half of his brain had basically rotten away completely, yet he never showed any signs that he was deteriorating mentally as somehow the remaining half of his brain took over functions.

We can try to look at forms of dementia, but the problem is that only a few key neurons need to disappear to disable an entire neuron chain linked to a particular set of memories. If the neuron mass is reconstructed with all links intact you may find that many memories are intact. So we can't say "if with dementia the brain shrinks by X% then X% of memories are gone".

We don't know exactly what neurons are responsible for exact memories, but what we do know is that one memory isn't just stored in one place. It is a dance between many neurons all over your brain to retrieve, process, visualize and eventually experience the memory. If your memory neurons are intact but the set of neurons that steer their activation aren't there the memory can't be retrieved and your brain will basically start overwriting the memory as it repurposes them for other tasks. Or if the connections to neurons that process it aren't there the memory can't be relived.

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The 2% regenerates the rest.

Your immortal invulnerable super can regenerate. Regeneration requires a template of what must be regenerated and that template includes memories. Having been dead for a while her regeneration is a little lacking, as you say. But she has 2% of the original brain and that 2% gradually grows back some of what was lost. It is not exactly growing back, since she has a newborn baby brain in that space. The 2% original brain colonizes that brain with regenerated cells based on the original.

The recreation is not perfect. Regenerated tissue is close to the original but not exact. Some memories are missing and many are hazy and muddled. She remembers allowing herself to die but cannot remember why she chose that. She remembers showing people her face of nasty veins, fat tissue and nerves but wonders why she ever thought that was amusing.

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As character A grows to the mental age of 7, I need some of her old memories to come back, is it possible or do I need to handwave it with something?

In these 7 years, the brain of Character A will have changed drastically. Children use and shape their brain a lot ! the 2% remnant memories you are talking about, precious military skills for the Empire, are completely outside the scope of a 7 year old ! These areas will not fit in and therefore fade out and become rewired. I'm afraid your reborn hero (who fled to a foreign primitive place to escape ?) not only looses her freedom again at 7, she'll have to be reprogrammed as well. And 7 years is very early. The painfull, torturous attempt may just fail. Chance is, history repeats ! The whole effort does not bring her back as a soldier for the Empire, but she retains all her powers, and she'll not be loyal.. she may flee again..

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Memories can be stored in all cells of the body, not only in brain cells.

For my story I need character A to wake up with the mentality of a newborn baby who needs to learn everything from the beginning, a dangerous and lethal newborn baby in the body of an immortal murder machine with centuries worth of muscle memory of killing people.

"Muscle memory" may not be as unrealistic as it sounds. Although it's a figure of speech used to refer to procedural memory, it actually might exist in the literal sense - not necessarily that memory is preserved in muscle, but that certain memories can be stored in all cells of the body.

As a key point, there have been patients who received organ transplants (ex: kidney or heart), and later recalled memories from their donors' lives. These included their donors' hobbies, preferences, and even personalities. I remember hearing about a transplant recipient who developed a preference for fast food and motorcycles after his surgery... he later discovered that these were his donor's favorites. Do a quick Google search and you'll see many scientific studies on this. Below is just one such example.

https://pubmed.ncbi.nlm.nih.gov/31739081/

Memory preservation is seen in other instances as well. Research on monarch butterfly migration shows that these butterflies migrate over three generations. The memory of the places they've been as well as their destinations are passed down to their offspring in their DNA. Realize that DNA is encoded in every single cell in the body... not just brain cells. There also may be certain memories stored in mitochondrial DNA, assuming a person's nucleus gets damaged.

Whether or not there is another explanation for these memory transfers, you could certainly use them as the basis for a fictional work. Have your character regain memories that were stored in somatic (body) cells other than the destroyed brain tissue.

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  • $\begingroup$ Best answer so far $\endgroup$ – user85880 Jun 12 at 0:50
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For my story I need character A to wake up with the mentality of a newborn baby who needs to learn everything from the beginning, a dangerous and lethal newborn baby in the body of an immortal murder machine with centuries worth of muscle memory of killing people.

As character A grows to the mental age of 7, I need some of her old memories to come back, is it possible or do I need to handwave it with something?

Its a really good idea, but before asking how to achieve it, we need to check some of these points.

"Muscle memory" doesn't mean what you seem to think

Its a shorthand term for complex physical skills that don't (subjectively) appear to involve thought. For example, walking, riding a bicycle, throwing a dart at the bullseye. We kind of learn and trust our bodies, to "know" how to do things like this, or so it seems, even though the learning is really a brain skill. That's muscle memory.

Your characters muscle memory could make her have prodigious skill with physical objects (movement, weapons) but they won't choose how she uses those. That's more about her instincts - the instinct to react with lethal force, the instinct to hide or ambush, all her other choices.

If they wake up with the "mentality of a newborn baby", what exactly is retained and what isn't?

Do they retain knowledge of weapons for example, how to dismantle a gun? Can they dismantle a gun under pressure without understanding how they knew it? Do they remember tactics, and what instincts if any need to come back? Do they remember personal history/events, when you say "some of her old memories come back"?

Also, just as telling, what stuff doesn't come back?

How much influence does nurture have, to rewrite anything

Usually whatever ones instincts and memories, the person one grows up to be, is heavily influenced by role models, upbringing, and lessons learned during this life.

Your character may have old memories but they have to come back in the context of an entire life of new experiences and lessons.

For example, if they were brought up by peacelovers, or fanatics seeking war, would that influence them? If the old person had seen animals as utilities for devising weapons, but the new one had been given a puppy as a pet at age 3....?

The impact of experience, and returning abilities/memories, will be very different 2nd time around

Killing a person age 150 is very different from blasting your primary social connection into sludge at age 2 or age 9, because of an uncontrolled brief tantrum over ice cream. Will her outburstings, traumatise her? What lessons will they sear into her?

She has no idea what it means, either. For children, "Am I okay as a person" is a huge issue. (Think peer pressure and social media or bullying feedback, and that's for children virtually the same as any other by comparison. As children its a huge deal to be okay with oneself)

So the new person may develop negative beliefs about herself or her abilities, deep down, that were never in the old person. What happens?

So.....

If you can clarify some of these, I can expand this, and try to actually answer based on the new information. For now I can only point out what's needed, to try answering.

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"Character A is a self made immortal organism, a girl who learned the magic needed to modify her body both consciously and automatically."

So, magic.

Part of the magical spell(s) that gives her these abilities is to regenerate her memories as well as her body. Easy peasy.

OR

"The Empire's magicians have learned how to activate the subconscious magic powers that regenerate character A and bring her back to life..." AND they create a spell which helps her get her memories back too. How? Create an "ether" of some kind where all of mankind's memories are stored.

If you're already using magic, you're in handwave country and you're all good.

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