6 ms·

I wonder how much of this is directly related to age (biological aging), and how much is just someone's brain becoming "full" due to more memories getting added

I wonder how much of this is directly related to age (biological aging), and how much is just someone's brain becoming "full" due to more memories getting added every year? It seems that memories must be stored as embeddings with single multi-neuron assemblies (cortical columns?) storing multiple embeddings as a kind of contents-addressable memory that is able to keep memories distinct due to the very high dimensional space (# neurons per assembly) being used. However, you'd expect that at some point if you store too many memories in a single assembly the recall accuracy is going to go down. You'd expect that with big brains being so costly, evolution has only equipped us with brains big enough to store a lifetime of memories, so it would be odd if memory didn't suffer as we get old. To make a computer analogy, it's a bit like a hash table getting too full. Say you had a hash table without any overflow mechanism... up to a point recall may still be pretty good, but as the table gets closer to full there will be more hash collisions and likelyhood of "false recall". Obviously the brain is not a computer, but the analogy may hold up reasonably well if you consider the hash table keys and values as embeddings and the store operation being an embedding merge rather than overwrite.

cheschire 2mo ago
I am not a biologist so I cannot say this authoritatively, but I’m fairly certain that’s not how neuroplasticity works.
- HarHarVeryFunny 2mo ago
  I only mentioned neural assemblies storing multiple embeddings - what about that contradicts your understanding of neuroplasticity?
- dboreham 2mo ago
  How so?
randomImmigrant 2mo ago
The brain does not have the Von Neumann bottleneck. Unlike most current digital systems, the brain doesn’t have a separate memory registry it needs to pull from. Engrams, that is, the physical trace of a memory, are not stable through life. They start out in the hippocampus, but as the stimulus recedes in time without reinforcement, it moves away. No evidence exists though that the memory is encoded in one set of cells. This spatial segregation of memory is the worst hangover from the “brain is a computer” analogy. Even if it is, why in the world would it be like our digital devices which specifically have the Von Neumann bottleneck? In biology, memory and processing are not segregated. There’s growing evidence the memory is much more distributed over the network, and is recomposed based on salience overlap with a new stimulus. Another factor to keep in mind is circadian rhythms. There’s growing evidence for how much the memory system and timekeeping system overlap, at a molecular level. Every neuron (and other cell) has an intrinsic clock that ticks at roughly 24 hours, and continues to do so even in total darkness. When you encode the memory has a lot to say, based on your chronotype, on how and how well you will remember it. Same with learning: there’s a time of day based variation. Sleep, and dreaming, is when these memories seem to get replayed and critical features and connections are incorporated into the system and its regime, awaiting the right triggers to access a state similar to when the memory formed. I’m stitching across a lot of different research, and I want to be clear many aspects of this system are not yet fully worked out. But what we do know points to a system that works with different physical and algorithmic priors, and the dynamics are sharply distinct from current digital computers.
- a_conservative 2mo ago
  This is really fascinating. The actual mechanisms of the human mind are distinct from computer systems, yet there are some parallels. There are some hints that increased memory access times scale with the amount of information the brain has stored vs the more typical narrative that aging decreases the capabilities of the brain. > Our results indicate that older adults'; performance on cognitive tests reflects the predictable consequences of learning on information-processing, and not cognitive decline. We consider the implications of this for our scientific and cultural understanding of aging. [0] https://pubmed.ncbi.nlm.nih.gov/24421073/ https://pubmed.ncbi.nlm.nih.gov/24421073/
- HarHarVeryFunny 2mo ago
  > The brain does not have the Von Neumann bottleneck Obviously not, which is why I didn't say it did! However, if you want to identify where long-term memories are stored, then that is in the cortex, but it should go without saying that this doesn't make the cortex the storage component of a von-Neumann architecture! > No evidence exists though that the memory is encoded in one set of cells I'm not sure what you are trying to say. Memories are presumably stored as embeddings - a distributed representation, and an episodic memory may well be stored as "chained together" episodic "scenes/chunks" where each chunk recalls the next. However, a distributed representation isn't the same as a holographic one, and any redundancy may well still be localized within given cortical columns, so I think you may be wrong if you are saying that individual memories/chunks are not confined to one set of cells (some localized neural assembly such as a cortical column).
  - randomImmigrant 2mo ago
    > Obviously not, which is why I didn't say it did! But you sneak it into your assumptions on what a memory must be. > However, if you want to identify where long-term memories are stored And why do you assume there is a specific “where” for the memory? > then that is in the cortex There’s good deal of evidence disproving this in the way you’re stating this. The cortex is involved in sensing, and yes, the sensory information associated with a memory will recruit appropriate cortical cells. This doesn’t mean the memory resides in the cortex. And detailed episodic recall keeps recruiting the hippocampus even for old memories, which is odd if the memory were somehow only in the cortex. > I'm not sure what you are trying to say. Let me restate what I’m saying then: There are four claims bundled together in the way you were describing biological memory: that a specific ensemble is activated when a given memory forms; that it’s the same ensemble that gets activated over time when that memory is retrieved; that it’s spatially compact, like a column in region of the brain; and that this ensemble it’s dedicated to that memory, or similar memories . The first is well supported. An engram, a network of neurons, is indeed activated when a memory first forms, and gets stabilized due to repeated stimulus. Re-activating these neurons in a different context can make the subject (a mouse) behave as it would if it had contextual signals to evoke said memory. However: 1. This engram is not in one particular part of the brain. There’s a cortical part that overlaps the sensory regions that were involved. But plenty of other regions are part of the engram 2. There’s turnover, over the course of weeks, when the specific cells involved in the engram drift, while the behavior remains stable. 3. The same synapses participate in many memories. > Memories are presumably stored as embeddings No. Let’s consider songbirds, which are an excellent worked out example (in an animal without the complex columnar cortical architecture mammals show, by the way). What’s learned is a temporal sequence, with neurons in a nucleus in their brains each firing one brief burst at a fixed point in the motif, so the content of the memory is its dynamics rather than any value. The circuit that evaluates the match against the tutor template is the same circuit generating the output being evaluated. Song degrades overnight during sleep replay and recovers the next day, and in seasonal species the song nuclei change size across the year with neurons added and lost while the song persists. There’s no read that leaves the item untouched, no persistent address, and no substrate holding still. “Stored as” imports all three. And it goes below the neuron or synapse. Hearing a tutor song drives immediate early gene expression that habituates with familiarity, and singing drives large transcriptional changes in the song nuclei that differ by social context for the same motor output. Since transcription runs on minutes to hours and the proteins turn over, any persistent state has to be actively regenerated rather than deposited. TLDR: the memory isn’t a static store. There’s no persistent “location” for it, distributed or otherwise, though specific locations can be in the chain that’s activated for retrieval/production. Instead, memory, over time, is driven by a dynamical regime that adjusts its dynamics to account for the temporal pattern in the salient stimulus. Nothing, down to the epigenetic changes in the chromatin of these neurons, can be seen as “the” location of “a” memory, especially over time. > if you are saying that individual memories/chunks are not confined to one set of cells (some localized neural assembly such as a cortical column). That is indeed the case.
thesz 2mo ago
A fascinating story of "Patient Sh." [1], the man who did not forget. [1] https://en.wikipedia.org/wiki/Solomon_Shereshevsky https://en.wikipedia.org/wiki/Solomon_Shereshevsky "His memory was so powerful that he could still recall decades-old events and experiences in the smallest details. After he discovered his own abilities, he performed as a mnemonist; but this created confusion in his mind. He went as far as writing things down on paper and burning it, so that he could see the words in cinders, in a desperate attempt to forget them. Some later mnemonists have speculated that this was a mentalist's technique for writing things down to later commit to long-term memory. Reportedly, in his late years, he realized that he could forget facts with just a conscious desire to remove them from his memory, although Luria did not test this directly." I believe that one need to have superhuman memorization abilities to have definite confusion due to too much remembered. More trivial explanation of this effect in normal ageing persons is age-related brain shrinkage. > Obviously the brain is not a computer, Our brain consists of approximately 86 billions quantum computers [2] controlling tens-of-thousands chemical neural networks with at least 10 coefficients, communicating [4] using lasers [5] (coherent light is laser light). [2] https://www.nature.com/articles/s41598-024-62539-5 [3] https://pmc.ncbi.nlm.nih.gov/articles/PMC11655932/ [4] https://pmc.ncbi.nlm.nih.gov/articles/PMC12230014/ [5] https://pubmed.ncbi.nlm.nih.gov/6204761/ Live with that. ;)
- safeimp 2mo ago
  The book Moonwalking with Einstein covers Patient S and memory in general. It's an enjoyable and quick read on the topic, would definitely recommend.
- adleyjulian 2mo ago
  I suspect the anxiety of being unable to forget things has an inherent selection bias. You will only stress over the things you can't forget, the things you have forgotten you won't stress over... Generally claims of eidetic memories are overstated, doubly for older claims, but Nigel Richards memorized a French dictionary in 9 weeks, over 6k words a day, x2 including the alphagram. I consider myself to have a decent memory, but that is 200x what I'd think myself capable of, assuming I want to retain it all at the end of the 9 weeks.
- 2mo ago
LorenPechtel 2mo ago
I don't think there is a "full" state, but watching the changes as the years go by I have thought of collisions. Not really like a hash table, though--most collisions are with things that would be filed together. I think it's more like the brain returns a random item matching the category. Brains are expensive, memory that gets fuzzy with age is probably the better choice than trying to feed a brain that does better.
- HarHarVeryFunny 2mo ago
  Yes, evolution is not going to penalize everyone with a "100-year brain" when the rest of our body seems optimized for something more like a 30-40 year lifespan. Evolution is all about propagating your genes, and not too many 100-year olds are going to be doing that! In any case, fuzzy associative recall is really the "design spec" for a brain, so semi-graceful degradation and fuzzy recall comes for free. I'm not sure if old-age memory unreliability is totally random - I would not expect it to be. I'd expect these associative recall failures to happen when there are too many memories partially matching the same "key" (activation pattern), which presumably happens mostly when they do have something in common.
SkyPuncher 2mo ago
I feel like I feel this increasingly as I age. I used to be able to remember nuanced details of API and computer systems. Now, so many of those memories simply get stored as “FLAG: double check the docs” (because I’ll forget). I’m a lot better at assessing things from a high level, knowing where to find the information I need, and knowing how to assess it for accuracy - but I can’t remember details anymore. Books become summaries. Trips become a few key snapshots representing the trip.
WhyNotHugo 2mo ago
I see it differently: as a teen, I could remember the exact word for lots of things, I could recall numbers and data with high precision. Nowadays, I'm pretty bad at all those things, but I can very well understand metaphors and connects concepts from wildly different topics — which I couldn't back then. I see this kind of "interconnecting ideas" of my current years as somewhat connected with "fuzzy memory". It's like, when I retrieve data, it's not as precise, and sometimes I get neighbouring data, but that can actually of use.
- avadodin 1mo ago
  You may have lost your superhuman ability to recall the script of every single episode of a pop cartoon with 20 seasons but you have gained the superhuman ability to make dad jokes.
Someone 2mo ago
> You'd expect that with big brains being so costly, evolution has only equipped us with brains big enough to store a lifetime of memories It hasn’t, and rightfully so. I don’t think there’s any evolutionary advantage in remembering the first steps you did or the first time you drank milk, for example, and indeed, nobody remembers those. A brain that remembers where the berries grew last year, where that cosy cave is, or where that angry tribe lives has evolutionary advantages. Most of the rest such as remembering specifics about events we visited, I think, is collateral non-damage, plus, oftentimes, reinforcement learning (many childhood memories get reinforced by your (grand)parents telling you repeatedly what they remember happened)
- HarHarVeryFunny 2mo ago
  Right, I really meant as an upper bound, not as sufficient to memorize everything you ever see or do.
timster6442 2mo ago
Am neurobiologist and I don't buy the "full" argument, its much more likely a brain aging thing. The brain, before your born, in GW25 (gestational week 25) has finished growing all the neurons you're basically [we can talk about this later perhaps] going to have for the rest of your life. At GW25 current estimates say you have ~86 billion neurons. Now the timing for this next part is a bit unsure but your body doesn't need anywhere close to 86 billion neurons, so at some point as early as early adulthood you start to lose 85,000 neurons per day pretty steadily until you die. Now neurons are not the end all be all because connections are potentially what really matter but that is to say that the neurons that you have after GW25 are the neurons you basically have. Now going onto synaptic connections where things start to matter more. Now synapses form and then get pruned all the time its natural. But the rate of formation and the rate of pruning is not the same at all times of life. Now from a raw number of synapse scale we see a tipping point at around 16-26 years old (debated hence the big range) where the number of synapses start to go down, indicating that the rate of pruning is now outpacing the rate of formation. [It does seem however that the dysfunction of the rate of pruning i.e. not enough ends up with consequences like schizo or asd {autism}]. There is another factoid that the rate of decline seems to stay relatively stable until you hit ~60 and then synapse related decline becomes much more noticeable and we start to think of synapse loss as exponential. Now about new neurons after GW25 is a whole topic in of itself... heavily debated but I won't get into that history, the most accepted viewpoint is that it does exist in the hippocampus [other regions as well] (really cool work with c14 carbon dating from atomic bomb {2010} and perhaps less cool new sequencing methods give evidence {2025/2026}). Note estimates of how many are born are comparatively low-ish, 500-1000 neurons per day. That begs another question however which is why and what do they do? Final interesting part is that when you stop the mouse hippocampus neurons from dividing, [unethical to do in humans :( ] distinct representations of experiences start to look similar and overlap. I also know if you ablate neurogenesis in mouse nasal cortex I think mice lose the ability to form new sensory sensations all together. Disclaimer, the evidence for function of new born neurons is actually pretty low only a few studies have tried this so its no where close to accepted and thus far far from textbook standard so take it as you will.
- HarHarVeryFunny 2mo ago
  But couldn't, and wouldn't you expect, both things to be true? Existing memories may be degrading as a result of ongoing neuron death and synapse pruning, but at the same time our "storage capacity" (neurons+synapses) is essentially fixed or decreasing, and new memories are going to increasingly be competing with existing ones. Presumably there is at least some connection between our neuron + synapse counts and species longevity. If we had evolved to live longer then one might naively expect us to have more neurons/synapses to support life-long learning and memories, although you could also argue that, for example, episodic memories too far back are not useful to remember, so maybe there is a cap to how much episodic memory capacity we need, and the complexity of the evolutionary niche we inhabit also limits the amount of declarative knowledge we need to store. It's been noted in birds that hippocampus size seems highly correlated with the species memory capacity - but specifically for memory of seasonal seed hiding locations, where the bird only needs memory capacity for the number of seeds (can be 10's of thousands) hidden in a single season - they presumably reuse this hippocampal memory capacity each season, and forget the last season. Have there been any studies relating brain size / memory capacity to the need of a species for lifelong memory?

cheschire 2mo ago

I am not a biologist so I cannot say this authoritatively, but I’m fairly certain that’s not how neuroplasticity works.

HarHarVeryFunny 2mo ago
I only mentioned neural assemblies storing multiple embeddings - what about that contradicts your understanding of neuroplasticity?
dboreham 2mo ago
How so?

randomImmigrant 2mo ago

The brain does not have the Von Neumann bottleneck. Unlike most current digital systems, the brain doesn’t have a separate memory registry it needs to pull from. Engrams, that is, the physical trace of a memory, are not stable through life. They start out in the hippocampus, but as the stimulus recedes in time without reinforcement, it moves away. No evidence exists though that the memory is encoded in one set of cells. This spatial segregation of memory is the worst hangover from the “brain is a computer” analogy. Even if it is, why in the world would it be like our digital devices which specifically have the Von Neumann bottleneck? In biology, memory and processing are not segregated. There’s growing evidence the memory is much more distributed over the network, and is recomposed based on salience overlap with a new stimulus. Another factor to keep in mind is circadian rhythms. There’s growing evidence for how much the memory system and timekeeping system overlap, at a molecular level. Every neuron (and other cell) has an intrinsic clock that ticks at roughly 24 hours, and continues to do so even in total darkness. When you encode the memory has a lot to say, based on your chronotype, on how and how well you will remember it. Same with learning: there’s a time of day based variation. Sleep, and dreaming, is when these memories seem to get replayed and critical features and connections are incorporated into the system and its regime, awaiting the right triggers to access a state similar to when the memory formed. I’m stitching across a lot of different research, and I want to be clear many aspects of this system are not yet fully worked out. But what we do know points to a system that works with different physical and algorithmic priors, and the dynamics are sharply distinct from current digital computers.

a_conservative 2mo ago
This is really fascinating. The actual mechanisms of the human mind are distinct from computer systems, yet there are some parallels. There are some hints that increased memory access times scale with the amount of information the brain has stored vs the more typical narrative that aging decreases the capabilities of the brain. > Our results indicate that older adults'; performance on cognitive tests reflects the predictable consequences of learning on information-processing, and not cognitive decline. We consider the implications of this for our scientific and cultural understanding of aging. [0] https://pubmed.ncbi.nlm.nih.gov/24421073/ https://pubmed.ncbi.nlm.nih.gov/24421073/
HarHarVeryFunny 2mo ago
> The brain does not have the Von Neumann bottleneck Obviously not, which is why I didn't say it did! However, if you want to identify where long-term memories are stored, then that is in the cortex, but it should go without saying that this doesn't make the cortex the storage component of a von-Neumann architecture! > No evidence exists though that the memory is encoded in one set of cells I'm not sure what you are trying to say. Memories are presumably stored as embeddings - a distributed representation, and an episodic memory may well be stored as "chained together" episodic "scenes/chunks" where each chunk recalls the next. However, a distributed representation isn't the same as a holographic one, and any redundancy may well still be localized within given cortical columns, so I think you may be wrong if you are saying that individual memories/chunks are not confined to one set of cells (some localized neural assembly such as a cortical column).
- randomImmigrant 2mo ago
  > Obviously not, which is why I didn't say it did! But you sneak it into your assumptions on what a memory must be. > However, if you want to identify where long-term memories are stored And why do you assume there is a specific “where” for the memory? > then that is in the cortex There’s good deal of evidence disproving this in the way you’re stating this. The cortex is involved in sensing, and yes, the sensory information associated with a memory will recruit appropriate cortical cells. This doesn’t mean the memory resides in the cortex. And detailed episodic recall keeps recruiting the hippocampus even for old memories, which is odd if the memory were somehow only in the cortex. > I'm not sure what you are trying to say. Let me restate what I’m saying then: There are four claims bundled together in the way you were describing biological memory: that a specific ensemble is activated when a given memory forms; that it’s the same ensemble that gets activated over time when that memory is retrieved; that it’s spatially compact, like a column in region of the brain; and that this ensemble it’s dedicated to that memory, or similar memories . The first is well supported. An engram, a network of neurons, is indeed activated when a memory first forms, and gets stabilized due to repeated stimulus. Re-activating these neurons in a different context can make the subject (a mouse) behave as it would if it had contextual signals to evoke said memory. However: 1. This engram is not in one particular part of the brain. There’s a cortical part that overlaps the sensory regions that were involved. But plenty of other regions are part of the engram 2. There’s turnover, over the course of weeks, when the specific cells involved in the engram drift, while the behavior remains stable. 3. The same synapses participate in many memories. > Memories are presumably stored as embeddings No. Let’s consider songbirds, which are an excellent worked out example (in an animal without the complex columnar cortical architecture mammals show, by the way). What’s learned is a temporal sequence, with neurons in a nucleus in their brains each firing one brief burst at a fixed point in the motif, so the content of the memory is its dynamics rather than any value. The circuit that evaluates the match against the tutor template is the same circuit generating the output being evaluated. Song degrades overnight during sleep replay and recovers the next day, and in seasonal species the song nuclei change size across the year with neurons added and lost while the song persists. There’s no read that leaves the item untouched, no persistent address, and no substrate holding still. “Stored as” imports all three. And it goes below the neuron or synapse. Hearing a tutor song drives immediate early gene expression that habituates with familiarity, and singing drives large transcriptional changes in the song nuclei that differ by social context for the same motor output. Since transcription runs on minutes to hours and the proteins turn over, any persistent state has to be actively regenerated rather than deposited. TLDR: the memory isn’t a static store. There’s no persistent “location” for it, distributed or otherwise, though specific locations can be in the chain that’s activated for retrieval/production. Instead, memory, over time, is driven by a dynamical regime that adjusts its dynamics to account for the temporal pattern in the salient stimulus. Nothing, down to the epigenetic changes in the chromatin of these neurons, can be seen as “the” location of “a” memory, especially over time. > if you are saying that individual memories/chunks are not confined to one set of cells (some localized neural assembly such as a cortical column). That is indeed the case.

thesz 2mo ago

A fascinating story of "Patient Sh." [1], the man who did not forget. [1] https://en.wikipedia.org/wiki/Solomon_Shereshevsky https://en.wikipedia.org/wiki/Solomon_Shereshevsky "His memory was so powerful that he could still recall decades-old events and experiences in the smallest details. After he discovered his own abilities, he performed as a mnemonist; but this created confusion in his mind. He went as far as writing things down on paper and burning it, so that he could see the words in cinders, in a desperate attempt to forget them. Some later mnemonists have speculated that this was a mentalist's technique for writing things down to later commit to long-term memory. Reportedly, in his late years, he realized that he could forget facts with just a conscious desire to remove them from his memory, although Luria did not test this directly." I believe that one need to have superhuman memorization abilities to have definite confusion due to too much remembered. More trivial explanation of this effect in normal ageing persons is age-related brain shrinkage. > Obviously the brain is not a computer, Our brain consists of approximately 86 billions quantum computers [2] controlling tens-of-thousands chemical neural networks with at least 10 coefficients, communicating [4] using lasers [5] (coherent light is laser light). [2] https://www.nature.com/articles/s41598-024-62539-5 [3] https://pmc.ncbi.nlm.nih.gov/articles/PMC11655932/ [4] https://pmc.ncbi.nlm.nih.gov/articles/PMC12230014/ [5] https://pubmed.ncbi.nlm.nih.gov/6204761/ Live with that. ;)

safeimp 2mo ago
The book Moonwalking with Einstein covers Patient S and memory in general. It's an enjoyable and quick read on the topic, would definitely recommend.
adleyjulian 2mo ago
I suspect the anxiety of being unable to forget things has an inherent selection bias. You will only stress over the things you can't forget, the things you have forgotten you won't stress over... Generally claims of eidetic memories are overstated, doubly for older claims, but Nigel Richards memorized a French dictionary in 9 weeks, over 6k words a day, x2 including the alphagram. I consider myself to have a decent memory, but that is 200x what I'd think myself capable of, assuming I want to retain it all at the end of the 9 weeks.
2mo ago

LorenPechtel 2mo ago

I don't think there is a "full" state, but watching the changes as the years go by I have thought of collisions. Not really like a hash table, though--most collisions are with things that would be filed together. I think it's more like the brain returns a random item matching the category. Brains are expensive, memory that gets fuzzy with age is probably the better choice than trying to feed a brain that does better.

HarHarVeryFunny 2mo ago
Yes, evolution is not going to penalize everyone with a "100-year brain" when the rest of our body seems optimized for something more like a 30-40 year lifespan. Evolution is all about propagating your genes, and not too many 100-year olds are going to be doing that! In any case, fuzzy associative recall is really the "design spec" for a brain, so semi-graceful degradation and fuzzy recall comes for free. I'm not sure if old-age memory unreliability is totally random - I would not expect it to be. I'd expect these associative recall failures to happen when there are too many memories partially matching the same "key" (activation pattern), which presumably happens mostly when they do have something in common.

SkyPuncher 2mo ago

I feel like I feel this increasingly as I age. I used to be able to remember nuanced details of API and computer systems. Now, so many of those memories simply get stored as “FLAG: double check the docs” (because I’ll forget). I’m a lot better at assessing things from a high level, knowing where to find the information I need, and knowing how to assess it for accuracy - but I can’t remember details anymore. Books become summaries. Trips become a few key snapshots representing the trip.

WhyNotHugo 2mo ago

I see it differently: as a teen, I could remember the exact word for lots of things, I could recall numbers and data with high precision. Nowadays, I'm pretty bad at all those things, but I can very well understand metaphors and connects concepts from wildly different topics — which I couldn't back then. I see this kind of "interconnecting ideas" of my current years as somewhat connected with "fuzzy memory". It's like, when I retrieve data, it's not as precise, and sometimes I get neighbouring data, but that can actually of use.

avadodin 1mo ago
You may have lost your superhuman ability to recall the script of every single episode of a pop cartoon with 20 seasons but you have gained the superhuman ability to make dad jokes.

Someone 2mo ago

> You'd expect that with big brains being so costly, evolution has only equipped us with brains big enough to store a lifetime of memories It hasn’t, and rightfully so. I don’t think there’s any evolutionary advantage in remembering the first steps you did or the first time you drank milk, for example, and indeed, nobody remembers those. A brain that remembers where the berries grew last year, where that cosy cave is, or where that angry tribe lives has evolutionary advantages. Most of the rest such as remembering specifics about events we visited, I think, is collateral non-damage, plus, oftentimes, reinforcement learning (many childhood memories get reinforced by your (grand)parents telling you repeatedly what they remember happened)

HarHarVeryFunny 2mo ago
Right, I really meant as an upper bound, not as sufficient to memorize everything you ever see or do.

timster6442 2mo ago

Am neurobiologist and I don't buy the "full" argument, its much more likely a brain aging thing. The brain, before your born, in GW25 (gestational week 25) has finished growing all the neurons you're basically [we can talk about this later perhaps] going to have for the rest of your life. At GW25 current estimates say you have ~86 billion neurons. Now the timing for this next part is a bit unsure but your body doesn't need anywhere close to 86 billion neurons, so at some point as early as early adulthood you start to lose 85,000 neurons per day pretty steadily until you die. Now neurons are not the end all be all because connections are potentially what really matter but that is to say that the neurons that you have after GW25 are the neurons you basically have. Now going onto synaptic connections where things start to matter more. Now synapses form and then get pruned all the time its natural. But the rate of formation and the rate of pruning is not the same at all times of life. Now from a raw number of synapse scale we see a tipping point at around 16-26 years old (debated hence the big range) where the number of synapses start to go down, indicating that the rate of pruning is now outpacing the rate of formation. [It does seem however that the dysfunction of the rate of pruning i.e. not enough ends up with consequences like schizo or asd {autism}]. There is another factoid that the rate of decline seems to stay relatively stable until you hit ~60 and then synapse related decline becomes much more noticeable and we start to think of synapse loss as exponential. Now about new neurons after GW25 is a whole topic in of itself... heavily debated but I won't get into that history, the most accepted viewpoint is that it does exist in the hippocampus [other regions as well] (really cool work with c14 carbon dating from atomic bomb {2010} and perhaps less cool new sequencing methods give evidence {2025/2026}). Note estimates of how many are born are comparatively low-ish, 500-1000 neurons per day. That begs another question however which is why and what do they do? Final interesting part is that when you stop the mouse hippocampus neurons from dividing, [unethical to do in humans :( ] distinct representations of experiences start to look similar and overlap. I also know if you ablate neurogenesis in mouse nasal cortex I think mice lose the ability to form new sensory sensations all together. Disclaimer, the evidence for function of new born neurons is actually pretty low only a few studies have tried this so its no where close to accepted and thus far far from textbook standard so take it as you will.

HarHarVeryFunny 2mo ago
But couldn't, and wouldn't you expect, both things to be true? Existing memories may be degrading as a result of ongoing neuron death and synapse pruning, but at the same time our "storage capacity" (neurons+synapses) is essentially fixed or decreasing, and new memories are going to increasingly be competing with existing ones. Presumably there is at least some connection between our neuron + synapse counts and species longevity. If we had evolved to live longer then one might naively expect us to have more neurons/synapses to support life-long learning and memories, although you could also argue that, for example, episodic memories too far back are not useful to remember, so maybe there is a cap to how much episodic memory capacity we need, and the complexity of the evolutionary niche we inhabit also limits the amount of declarative knowledge we need to store. It's been noted in birds that hippocampus size seems highly correlated with the species memory capacity - but specifically for memory of seasonal seed hiding locations, where the bird only needs memory capacity for the number of seeds (can be 10's of thousands) hidden in a single season - they presumably reuse this hippocampal memory capacity each season, and forget the last season. Have there been any studies relating brain size / memory capacity to the need of a species for lifelong memory?