A Mind Is a Stable Unity, But a Brain Is Not

Having lived in this world for quite a few decades, I am more impressed than ever by the unity and stability of the human mind. I think back on several decades of my life and I recall a great unity of my  self. There have been gradual changes in my opinions slowly caused by things I have learned more and more about, and also things I have experienced. But this has all felt very much a gradual change in a single mind. Never once did I ever feel that I was in some sense a different person than the person I was a day ago or a week ago. 

So it is for the average person. There may be rare mental illnesses that involve something like a split personality, but for almost all of us nothing like that occurs. Human beliefs are remarkably stable, often to the point of stubbornness. The stability and unity of the human mind comes into play when trying to get people to pay attention to a blog site like this one. I have on this site very many arguments and very much evidence contradicting the claim that the brain is the source of the human mind. But there are so many people who refuse to examine such arguments and evidence. It seems that there are so many people in their fifties and sixties and seventies who keep believing decade after decade what they were told in college, and who kind of are saying to themselves, "I've been believing this doctrine for decades, so I'm sure not going to change my mind now."  

Such inertia of belief is a difficulty for someone like myself trying to challenge old dogmas. But at least such inertia helps show the reality of what I am arguing for at this particular moment: that human minds are remarkably stable things, and typically show very great stability and unity over decades. The unity of the human mind is something we take for granted, because it has been such a stable feature of our lives, as stable as the feature of vision or hearing. But with some imagination, we can imagine human life as a very different affair. 

For example, people might flip-flop around in their political views from one month to the next. People might change their views or religion from one month to the next, one month being an atheist, the next month being a Christian, and the next month being a Buddhist. Or people might always be switching the objects of their affection from week to week, like some fickle ever-changing Lothario who switches his beloved four times a month . In that case we would probably never have marriages lasting for decades. Or, whenever people made decisions it might usually be a very zig-zag affair, like illustrated in the diagram below:

Such is not the human reality we observe. In general, human minds are remarkably stable and unified, and the persistence of beliefs, attitudes and emotions over a period of 40 years or 50 years is very common. But in the brain do we have a physical reality matching this unity and stability? We do not.

A person examining brain scans of an adult at different decades in his life might claim to observe stability, with little change. But at the microscopic level, it is an entirely different story. Brains are made mainly of neurons and units called synapses that connect neurons. The proteins that make up neurons and synapses are not at all stable. Instead they have average lifetimes of only a few weeks. 

A scientific paper states, "Recent studies have revealed that most proteins, including synaptic proteins, have half-lives that range between 5 and 7 days (Cohen et al., 2013, Dörrbaum et al., 2018)."  The average lifetime of a protein is about twice its half-life. 

The 2018 paper here is entitled "Brain tissue plasticity: protein synthesis rates of the human brain." It tells us the astonishing fact that proteins in the human brain are replaced at a rate of 3% to 4% per day. We read this:

"Where skeletal muscle tissue has been shown to turnover at a rate of 1–2% per day, here we show that brain tissue turns over much faster at a rate of 3–4% per day. This would imply complete renewal of brain tissue proteins well within 4–5 weeks. From a physiological viewpoint this is astounding, as it provides us with a much greater framework for the capacity of brain tissue to recondition. Moreover, from a philosophical perspective these observations are even more surprising. If rapid protein turnover of brain tissue implies that all organic material is renewed, then all data internalized in that tissue are also prone to renewal. These findings spark (even) more debate on the interpretation and (long-term) storage of data in neural matter, the capacity of humans to consciously or unconsciously process data, and the (organic) basis of our own personality and ego. All of this becomes quite remarkable in light of such rapid protein turnover rates of the human brain." 

A scientific paper states this:

"Experience-dependent behavioral memories can last a lifetime, whereas even a long-lived protein or mRNA molecule has a half-life of around 24 hrs. Thus, the constituent molecules that subserve the maintenance of a memory will have completely turned over, i.e. have been broken down and resynthesized, over the course of about 1 week."

Research on the lifetime of synapse proteins is found in the June 2018 paper “Local and global influences on protein turnover in neurons and glia.” The paper starts out by noting that one earlier 2010 study found that the average half-life of brain proteins was about 9 days, and that a 2013 study found that the average half-life of brain proteins was about 5 days. The study then notes in Figure 3 that the average half-life of a synapse protein is only about 5 days, and that all of the main types of brain proteins (such as nucleus, mitochondrion, etc.) have half-lives of 15 days or less.  The 2018 study here precisely measured the lifetimes of more than 3000 brain proteins from all over the brain, and found not a single one with a lifetime of more than 75 days (figure 2 shows the average protein lifetime was only 11 days). 

The paper here states, "Experiments indicate in absence of activity average life times ranging from minutes for immature synapses to two months for mature ones with large weights."

 A neuron in the brain may be compared to some house held by a remodeling enthusiast who keeps tearing down rooms and replacing them with different rooms. The synapses in the brain may be compared to something even more unstable.  The average width of a synapse (only about 500 nanometers) is about 1000 times less than the average width of a neuron. Given how small synapses are (1000 times less wide than neurons), and given the very rapid turnover rate of synapse proteins (which have average lifetimes of a few weeks or less), it is very unlikely that any particular synapse lasts for years.  

There is another way in which the brain fails to physically qualify as a stable unity. Neurons in the brain are connected by dendrites and synapses, and the connection is not a continuous material connection. Every synapse has a physical gap. Below is a  schematic diagram of a synaptic gap:

The number of these synaptic clefts (also called synaptic gaps) in the brain is enormous. It has been estimated that the human brain has about 90 billion neurons, and about 100 trillion synapses. So there are something like 1000 synapses for every neuron. Each one of these synapses is a break in the physical unity of the brain. 

The brain isn't like a continent. It's more like some huge archipelago, consisting of countless little islands. Synapses are like the gaps between the islands in the visual below. 

Clearly the brain is not a stable unity. There is another reason for thinking that the brain is not a stable unity: the fact that the brain consists of two separate hemispheres:

It is true that there are fibers connecting these hemispheres, but the physical organization of having two hemispheres is another physical reason for thinking that the brain has no great unity. 

There are many humans that exist as stable unified minds even though there is the most dramatic physical disunity in the brain. These are the people who have brains existing as two separate hemispheres that are not connected. 

A normal human brain consists of two halves called hemispheres, which are connected by a set of fibers called the corpus callosum. However, the corpus callosum fails to develop in many people. Such a condition is called agenesis of the corpus callosum. People lacking a corpus callosum have basically two separate brains inside their skull. Does this result in two minds, as we would expect under the theory that the brain makes the mind? No, people lacking a corpus callosum have a single, unified mind.  

Agnesis means a failure of something in a developing child during a mother's pregnancy. The paper here is entitled "Outcomes Associated With Isolated Agenesis of the Corpus Callosum: A Meta-analysis." The paper distinguishes between what it calls "cACC" and "pACC," defining cACC as complete agenesis of the corpus callosum (in other words, the total failure of the corpus callosum to appear) and defining pACC as partial aegnesis of the corpus callosum (partial  failure of the corpus callosum to appear). The paper gives us these statistics, telling us that in only a relatively small minority of cases of a complete congenital absence of the corpus callosum are there cognitive problems:

"In cACC, chromosomal anomalies occurred in 4.81% (95% confidence interval [CI], 2.2–8.4) of the cases. Gross and fine motor control were abnormal in 4.40% (95% CI, 0.6–11.3) and 10.98% (95% CI, 4.1–20.6) of the cases, respectively, whereas 6.80% (95% CI, 1.7–14.9) presented with epilepsy. Abnormal cognitive status occurred in 15.16% (95% CI, 6.9–25.9) of cases. In partial ACC, the rate of chromosomal anomalies was 7.45% (95% CI, 2.0–15.9). Fine motor control was affected in 11.74% (95% CI, 0.9–32.1) of the cases, and 16.11% (95% CI, 2.5–38.2) presented with epilepsy. Cognitive status was affected in 17.25% (95% CI, 3.0–39.7) of cases."

So in only about 16% of the subjects with a complete lack of a corpus callosum was there an abnormal cognitive status. These cACC (complete agenesis of the corpus callosum) people were basically people with two separate brains in their skulls. But most of them apparently had normal unified minds, since the paper says that only 15% of them had an abnormal cognitive status. 

Below is a quote from a book written by a medical doctor:

"Case I. — Dr. Alexander Bruce reports a man of ordinary intelligence and good character, and who, for thirteen years, did the work of a porter with perfect satisfaction, and exhibited no notable peculiarities. Yet in this man the corpus callosum was completely absent (23).

Case II. — Malinverni reports a soldier, aged 30 years, of ordinary intelligence, but with a slight tendency to melancholia, who had complete absence of the corpus callosum (24).

Case III. — Eichler reports a laborer, 43 years of age, married, diligent and capable, a good husband, sober and quiet, and could read and write, yet he had no corpus callosum, and had other important malformations of the brain (25)."

We have three cases of people with no corpus callosum, meaning that they had two separated hemispheres of the brain. But in each case their mind seemed normal. 

Sometimes to prevent seizures, the entire corpus callosum is surgically severed. This is sometimes called a split brain operation. The operation leaves patients with a single unified self. Insinuations to the contrary sometimes made by materialists are an example of severe deception. 

Physician Michael Egnor states the following about Sperry's research:

"The neuroscientist Roger Sperry studied scores of split-brain patients. He found, surprisingly, that in ordinary life the patients showed little effect. Each patient was still one person. The intellect and will – the capacity to have abstract thought and to choose – remained unified. Only by meticulous testing could Sperry find any differences: their perceptions were altered by the surgery. Sensations – elicited by touch or vision – could be presented to one hemisphere of the brain, and not be experienced in the other hemisphere. Speech production is associated with the left hemisphere of the brain; patients could not name an object presented to the right hemisphere (via the left visual field). Yet they could point to the object with their left hand (which is controlled by the right hemisphere). The most remarkable result of Sperry’s Nobel Prize­–winning work was that the person’s intellect and will – what we might call the soul – remained undivided. The brain can be cut in half, but the intellect and will cannot. The intellect and will are metaphysically simple."

The actual facts about split-brain surgery are related here by a surgeon who has performed such an operation. He states this about split-brain patients:

"After the surgery they are unaffected in everyday life, except for the diminished seizures. They are one person after the surgery, as they were before."

The surgeon states: "In a rational scientific community in which evidence and reason held sway, split-brain surgery would be hailed as compelling evidence for dualism and the immateriality of the intellect and will."

They Memorized Many Times Faster Than a Brain Could Ever Do

 The main theory of a brain storage of memories is that people acquire new memories through a strengthening of synapses. There are many reasons for disbelieving this claim. One is that information is generally stored through a writing process, not a strengthening process. It seems that there has never been a verified case of any information being stored through a mere process of strengthening. Another reason for rejecting the claim is that human memories can last 1000 times longer than the average lifetime of proteins in the brain. A scientific paper states, "Recent studies have revealed that most proteins, including synaptic proteins, have half-lives that range between 5 and 7 days (Cohen et al., 2013, Dörrbaum et al., 2018)."  The average lifetime of a protein is about twice its half-life. 

The 2018 paper here is entitled "Brain tissue plasticity: protein synthesis rates of the human brain." It tells us the astonishing fact that proteins in the human brain are replaced at a rate of 3% to 4% per day. We read this:

"Where skeletal muscle tissue has been shown to turnover at a rate of 1–2% per day, here we show that brain tissue turns over much faster at a rate of 3–4% per day. This would imply complete renewal of brain tissue proteins well within 4–5 weeks. From a physiological viewpoint this is astounding, as it provides us with a much greater framework for the capacity of brain tissue to recondition. Moreover, from a philosophical perspective these observations are even more surprising. If rapid protein turnover of brain tissue implies that all organic material is renewed, then all data internalized in that tissue are also prone to renewal. These findings spark (even) more debate on the interpretation and (long-term) storage of data in neural matter, the capacity of humans to consciously or unconsciously process data, and the (organic) basis of our own personality and ego. All of this becomes quite remarkable in light of such rapid protein turnover rates of the human brain."

Such rapid replacement of brain proteins is utterly inconsistent with claims that brains store old memories of what someone learned decades ago. A person like me remembers many things he learned 50 years ago, but if my brain was storing my memories, I would not be able to remember back more than a few months, given a 3% per day replacement of brain proteins.  A 2022 scientific paper confesses this:

"Conclusive evidence that specific long-term memory formation relies on dendritic growth and structural synaptic changes has proven elusive. Connectionist models of memory based on this hypothesis are confronted with the so-called plasticity stability dilemma or catastrophic interference. Other fundamental limitations of these models are the feature binding problem, the speed of learning, the capacity of the memory, the localisation in time of an event and the problem of spatio-temporal pattern generation."

If it were true that memories were stored by a strengthening of synapses, this would be a slow process. The only way in which a synapse can be strengthened is if proteins are added to it. We know that the synthesis of new proteins is a rather slow effect, requiring many minutes of time. In addition, there would have to be some very complicated encoding going on if a memory was to be stored in synapses. The reality of newly-learned knowledge and new experience would somehow have to be encoded or translated into some brain state that would store this information. When we add up the time needed for this protein synthesis and the time needed for this encoding, we find that the theory of memory storage in brain synapses predicts that the acquisition of new memories should be a very slow affair, which can occur at only a tiny bandwidth, a speed which is like a mere trickle. Do a Google image search for "speed of protein synthesis" and you will see charts that look like this (with data point dots scattered across the lines):

Don't make the mistake of thinking that a brain storage of new memories would occur as quickly as the speed of protein synthesis.  Such a storage would rely on three things that would be slow:

(1) Protein synthesis itself, which would require an average of multiple minutes. 

(2) Much additional time required for some act by which sensory information was encoded in some never-discovered storage format allowing sensory information to be translated into brain states or synapse states. 

(3) The time needed for signal transmission to occur across various parts of the brain, which would be quite an additional slowing factor, because of the relatively slow speed of transmission across synapses and dendrites, illustrated by the diagram below:

The synaptic gaps of chemical synapses and relatively slow dendrites (speed bumps for the brain) vastly outnumber myelinated axons, meaning for the brain the slowing parts vastly outnumber the fast parts. 

Memory contests show that some humans can actually acquire new memories at a speed very many times greater than the slow speed that would occur if brains were storing memories by protein synthesis. For example, according to a page on the site of the Guinness Book of World Records, "The fastest time to memorize and recall a deck of playing cards is 13.96 seconds, achieved by Zou Lujian (China) at the 2017 World Memory Championships held in Shenzhen, Guangdong Province, China, on 6-8 December 2017." Memorization speeds this fast utterly discredit claims that learning occurs by synapse strengthening, which would require the synthesis of new proteins, something which would require multiple minutes. 

The page here on www.wikipedia.org describes a competition called the World Memory Championships, which has the website here.  There are various different competitions, which are described in Chapter 7 (page 57) of the handbook you can read here:

Discipline 1:  a competition to memorize as many abstract images as possible, given 15 minutes to memorize, and 30 minutes to recall.  (Page 58.) 

Discipline 2:  a competition to memorize as many binary numbers as possible  given 5 minutes to memorize, and 15 minutes to recall (national level), or 30 minutes to memorize, and 60 minutes to recall (international level).  (Page 62.)

Discipline 3:  a competition to memorize as many random decimal digits ( such as 8, 9, and 2) as possible, given 15 minutes to memorize, and 30 minutes to recall (national level), or 30 minutes to memorize, and 60 minutes to recall (international level), or   60 minutes to memorize, and 120 minutes to recall (world  level). (Page 67.)

Discipline 4:  a competition to memorize as many name and face combinations as possible, given 5 minutes to memorize, and 15 minutes to recall (national level), or 15 minutes to memorize, and 30 minutes to recall (international level or world level).  (Page 70.) Competitors are asked to provide names when shown a face. 

Discipline 5:  a "Speed Numbers" competition to memorize as many random decimal digits ( such as 8, 9, and 2) as possible, given 5 minutes to memorize, and 15 minutes to recall. (Page 75.)

Discipline 6:  a competition to memorize as many pairs of dates and fictional events as possible, given 5 minutes to memorize, and 15 minutes to recall. (Page 80.)

Discipline 7:  a competition to memorize as many separate packs of shuffled playing cards as possible, given 10 minutes to memorize, and 30 minutes to recall (national level), or 30 minutes to memorize, and 60 minutes to recall (international level), or   60 minutes to memorize, and 120 minutes to recall (world  level). (Page 82.)

Discipline 8:  a competition to memorize as many random words as possible, given 5 minutes to memorize, and 15 minutes to recall (national level), or 15 minutes to memorize, and 30 minutes to recall (international level or world level).  (Page 87.) 

Discipline 9:  a 'Spoken Numbers" competition to memorize as many spoken numbers as possible, with the numbers being read at a rate of one number per second. (Page 92, complicated rules.)

Discipline 10:  a "Speed Cards" competition to commit to memory as many cards as possible, given 5 minutes or less for memorization, and only 5 minutes for recall.  (Page 98.)

Below is performance data recorded on the site and on the wikipedia.org page here.

Discipline 1, abstract images:  Two competitors in 2021 (Huang Jinyao and Xu Yangran) were able to memorize more than 1000 abstract images in only 15 minutes (or score more than 1000 points on such a competition, indicating similar ability).

Discipline 2, binary digits: Four competitors in 2021 were able to recall more than 600 binary digits memorized in a 30-minute period. Ryu Song I was able to recall 7485 binary numbers memorized in a 30-minute period (WMSC World Championship 2019).

Discipline 3, random decimal digits: Ryu Song I was able to recall 4620 decimal digits  memorized in an hour-long  period (WMSC World Championship 2019). Seven competitors in 2021 were able to recall more than 600 binary digits memorized in a 30-minute period. 

 Discipline 4, face and name combinations:  Katie Kermode was able to recall the names of 224 previously unseen people from their images, having had only 15 minutes to memorize their names (IAM World Championship 2018). Similarly, the scientific paper here says someone identified as SM1 "memorized 215 German names to the corresponding faces within 15 minutes at the Memoriad in 2015 in Istanbul." (The paper stated that the super-memorizers it studied did not have increased hippocampal volumes.) Several Mongolian or Chinese contestants were able to recall the names of more than 600 previously unseen people from their images, having had only 15 minutes to memorize their names (2021 World Memory Championships). 

Discipline 5, Speed Numbers: Wei Quinru was able to recall 642 digits memorized in a 5-minute period (Korea Open Memory Championship 2024). Four  people were able to each recall more than 800 digits memorized in a 5-minute period (2021 World Memory Championships).

Discipline 6, Dates and Fictional Events:  Prateek Yadav memorized in 5 minutes dates corresponding to 154 fictional events (2019).  Several other contestants memorized in 5 minutes dates corresponding to 700+ fictional events (2021 World Memory Championships).

Discipline 7, "Hour Cards" Card Memorization:  Kim Su Rim memorized 2530 cards in 60 minutes.  

Discipline 8, Random Words:  Prateek Yadev memorized 335 random words in 15 minutes. Several others in 2021 memorized more than 500 random words in 15 minutes. 

Discipline 9, "Spoken Numbers":  Ryu Song I was able to recall 547 decimal digits that had been read at a rate of one per second (WMSC World Championship 2019).  Tenuun Tamir and several other Mongolian or Chinese contestants were able to recall more than 600 decimal digits that had been read to him at a rate of one per second. 

Discipline 10, "Speed Cards":  Munkhshur NARMANDAK memorized 981 cards in five minutes, and several others memorized more than 600 cards in five minutes. 

Below from the World Memory Championships site is a table showing some of the best performers (link).

What we have in the performance records above is what can be roughly describing as lightning-fast memorization ability. Such an ability has been demonstrated by many subjects, doing many different types of memorization. The performances listed above are many times faster than any conceivable result that could be produced if memories are stored in brains.  There does not exist any detailed credible theory that can explain fast memorization by neural or synaptic processes. When neuroscientists say something about how memories form, they typically engage in hand-waving that vaguely refers to processes that are known to be very slow, such as synaptic strengthening. 

Routinely displaying instant recall abilities utterly unaccountable by the activity of brains completely lacking in addresses, sorting or indexes (the things that make fast retrieval possible in computers), humans do not recall at the speed of brains. Humans recall at the speed of souls. And the fastest memorizers do not memorize at the speed of brains. Such memorizers memorize at the speed of souls. 

For other posts documenting the ability of some humans to memorize at a blazing fast speed, see my posts with a tag of "photographic memory" or "eidetic memory."  On page 29 of the nineteenth century book here, we have an interesting account of photographic memory obtained under hypnosis (with it apparently progressing to become photographic memorization that could occur outside of hypnosis). The author states that eventually outside of hypnosis "the duration of a single second or a mere

glimpse at the page was sufficient for the pupils to retain in their memory the whole contents of it."

Why We Should Not Think the Human Brain Can Store Very Old Memories

Neurologists like to assume that all your memories are stored in your brain. But there are actually quite a few reasons for doubting this unproven assumption, including the research of scientists such as Karl Lashley and John Lorber. Their research showed that minds can be astonishing functional even when large parts of the brain are destroyed, either through disease or deliberate surgical removal. Lorber documented 600 cases of people with heavy brain damage (mostly due to hydraencephaly), and found that half of them had above average intelligence. Some children with brain problems sometimes undergo an operation called a hemispherectomy, in which half of their brain is removed. An article in Scientific American tells us, “Unbelievably, the surgery has no apparent effect on personality or memory.”

Given such very astonishing anomalies, we should give serious consideration to all arguments against the claim that your brain is storing all your memories. I will now discuss such an argument. The argument can be summarized as follows: there is no plausible mechanism by which the human brain could store very long-term memories such as 50-year-old memories. Every neurological memory theory that we have cannot explain any memories that have persisted for more than a year.

The Fact That Humans Can Remember Things for 50 Years

First, let's look at the basic fact of extreme long-term memory storage. It is a fact that humans can recall memories from 50 years ago. Some people have tried to suggest that perhaps human memory doesn't work for such a long time, and that remembering very old memories can be explained by the idea of what is called “rehearsal.” The idea is that perhaps a 60-year-old remembering is really just remembering previous recollections that he had at an earlier age. So perhaps, this idea goes, when you are 60 you are just remembering what you remembered from your childhood at 50, and that at 50 you were just remembered what you remembered from your childhood at age 40, and so forth.

But such an idea has been disproved by experiments. A scientific study by Harry Bahrick was entitled “Semantic memory content in permastore: Fifty years of memory for Spanish learned in school.” It showed that “large portions of the originally acquired information remain accessible for over 50 years in spite of the fact the information is not used or rehearsed.” The same researcher tested a large number of subjects to find out how well they could recall the faces of high school classmates, and found very substantial recall even with a group that had graduated 47 years ago. Bahrick reported the following:

Subjects are able to identify about 90% of the names and faces of the names of their classes at graduation. The visual information is retained virtually unimpaired for at least 35 years...Free-recall probability does not diminish over 50 yr for names of classmates assigned to one or more of the Relationship Categories A through F.

I know for a fact that memories can persist for 50 years, without rehearsal. Recently I was trying to recall all kinds of details from my childhood, and recalled the names of persons I hadn't thought about for decades, as well as a Christmas incident I hadn't thought of for 50 years (I confirmed my recollection by asking my older brother about it). Digging through my memories, I was able to recall the colors (gold and purple) of a gym uniform I wore, something I haven't thought about (nor seen in a photograph) for some 47 years. Upon looking through a list of old children shows from the 1960's, I saw the title “Lippy the Lion and Hardy Har Har,” which ran from 1962 to 1963 (and was not syndicated in repeats, to the best of my knowledge). I then immediately sung part of the melody of the very catchy theme song, which I hadn't heard in 53 years. I then looked up a clip on a youtube.com, and verified that my recall was exactly correct. I also recently recalled "The Patty Duke Show" from the 1960's, a show I haven't seen in 50 years, and recalled that in the opening title sequence we saw Patty walking down some stairs. I looked up the title sequence on www.youtube.com, and verified that my 50-year-old memory was correct. This proves that a 53-year-old memory can be instantly recalled.

So in trying to explain human memory, we need to have a theory that can explain human memories that persist for 50 years. Very confusingly, scientists use the term “long-term memory” for any memory lasting longer than an hour, which is very unfortunate because almost every thing you will find on the internet (seaching for “long term memory”) does not actually explain very long-term memory such as memories lasting for 50 years.

Why LTP and Synapse Plasticity Cannot Explain Very Long-Term Memory

Now let's look at neuroscientists' theories of memories. Quora.com is a “expert answer” website which claims to give “the best answer to any question.” One of its web pages asks the question, “How are memories stored and retrieved in the human brain?” The top answer (the one with most upvotes) is by Paul King, a computational neuroscientist. King very dogmatically gives us the following answer:

At the most basic level, memories are stored as microscopic chemical changes at the connection points between neurons in the brain..As information flows through the neural circuits and networks of the brain, the activity of the neurons causes the connection points to become stronger or weaker in response. The strengthening and weakening of the synapses (synaptic plasticity) is how the brain stores information. This mechanism behind this is called "long-term potentiation" or "LTP."

But there is actually no proof that any information is being stored when synapses are strengthened. From the mere fact that synapses may be strengthened when learning occurs, we are not entitled to deduce that information is being stored in synapses, for we also see blood vessels in the leg strengthen after repeated exercise, and that does not involve information storage. In order to actually prove that a synapse is storing information, you would need to do an experiment such as having one scientist store a symbol in an animal's brain (by training), and then have another scientist (unaware of what symbol had been stored) read that symbol from some synapses in the animal's brain, correctly identifying the symbol. No such experiment has ever been done.

The idea that a memory forms after repeating strengthening of synapses is inconsistent with the fact people very commonly remember things they experienced only one time. Almost every time someone tells you about a TV show they saw last night, or a conversation they recently had with a friend, they are remembering something they only were saw or heard one time, not through repeated experiences.

The evidence does not even clearly indicate that LTP correlates with memory, as one scientist's summary of experimental results indicates (a summary utterly inconsistent with the claim LTP is a general mechanism to explain memory).

What this means is that LTP and memory have been dissociated from each other in almost every conceivable fashion. LTP can be decreased and memory enhanced. Hippocampus-dependent memory deficits can occur with no discernable effect on LTP...There will be no direct quantitative or even qualitative relationship between LTP measured experimentally and memory measured experimentally—that is already abundantly clear from the available literature...The most damning observations probably are those examples where LTP is completely lost and there is no effect on hippocampus-dependent memory formation.

A scientific paper states this about LTP:

Based on the data reviewed here, it does not appear that the induction of LTP is a necessary or sufficient condition for the storage of new memories.

LTP is so weak an effect it is hard to even detect it. A scientific paper asks the following:

Why is it so difficult to see learning-associated synaptic changes?And does their absence in numerous experiments favor the null hypothesis?

What is misleadingly called “long-term potentiation” or LTP is a not-very-long-lasting effect by which certain types of high-frequency stimulation (such as stimulation by electrodes) produces an increase in synaptic strength. Synapses are gaps between nerve cells, gaps which neurotransmitters can jump over. The evidence that LTP even occurs when people remember things is not very strong, and in 1999 a scientist stated (after decades of research on LTP) the following:

[Scientists] have never been able to see it and actually correlate it with learning and memory. In other words, they've never been able to train an animal, look inside the brain, and see evidence that LTP occurred.

Since then a few studies have claimed to find evidence that LTP occurred during learning. But there is actually an insuperable problem in the idea that long-term potentiation could explain very long-term memories. The problem is that so-called long-term potentiation is actually a very short-term phenomenon. Speaking of long-term potentiation (LTP), and using the term “decays to baseline levels” (which means “disappears”), a scientific paper says the following:

Potentiation almost always decays to baseline levels within a week. These results suggest that while LTP is long-lasting, it does not correspond to the time course of a typical long-term memory. It is recognized that many memories do not last a life-time, but taking this point into consideration, we would then have to propose that LTP is only involved in the storage of short-term to intermediate memories. Again, we would be at a loss for a brain mechanism for the storage of a long-term memory.

A more recent scientific paper (published in 2013) says something similar, although it tells us even more strongly that so-called long-term potentiation (LTP) is really a very short-term affair. For it tells us that “in general LTP decays back to baseline within a few hours.” “Decays back to baseline” means the same as “vanishes.” 

Another 2013 paper agrees that so-called long-term potentiation is really very short-lived:

LTP always decays and usually does so rapidly. Its rate of decay is measured in hours or days (for review, see Abraham 2003). Even with extended “training,” a decay to baseline levels is observed within days to a week.

Scientists distinguish between two types of LTP: an E-LTP that can be produced by a single electrical stimulus, but only lasts one to three hours, and an L-LTP that requires multiple electrical stimulations, and can last about 8 hours or a little longer. But this fails to correspond to what we know about human memory, which is that humans can form a memory lasting decades after a single sensory experience. 

So evidently long-term potentiation cannot be any foundation or mechanism for long-term memories. This is the conclusion reached by the previous paper when it makes this conclusion about long-term potentiation (LTP):

In summary, if synaptic LTP is the mechanism of associative learning—and more generally, of memory—then it is disappointing that its properties explain neither the basic properties of associative learning nor the essential property of a memory mechanism. This dual failure contrasts instructively with the success of the hypothesis that DNA is the physical realization of the gene.

The book "Neuronal Mechanisms of Memory Formation" hints on page 451 that LTP may be a poor candidate for such a thing. It says this:

Definitive empirical support for synaptic plasticity modeled by LTP being a mechanism of memory processing is still lacking. For each piece of evidence that lends some support to the theory, there is likely to be equally strong evidence to suggest the contrary. The field of research reached a veritable stalemate some years ago when so-called cornerstones of research that supported the hypothesis were unable to be replicated...and the outcome was an increasing skepticism about whether LTP can be considered a neural substrate for learning and memory.

Referring to this scientific paper, another paper suggests that "LTP as a memory mechanism" may be more of a dogma than something well established by observations:

Shors and Matzel,,.concluded that LTP did not meet the criteria for providing a causal mechanism of memory. To make a long argument very short, they documented instances where changes in memory occur without LTP and where LTP occurs without changes in memory.....They report that between 1974 and 1997, more than 1300 articles occurred with “LTP” in the title. Of these, fewer than 80 described any behavioral manipulation relevant to assessing changes in memory. Furthermore, the articles that contained behavioral manipulations tended to provide evidence against the hypothesis that LTP is a memory mechanism. Thus, the claim that LTP is a molecular mechanism for learning and memory may be more of a dogma of neuroscientific memory research than a hypothesis that is being rigorously tested.  

A 2014 book stated, "Although LTP is considered to be the primary model for how learning and memory storage occur at the synapse level, the evidence supporting this claim is still inconclusive and speculative." A 1995 scientific paper found. "There is a striking negative correlation of spatial learning ability with LTP." This is the exact opposite of what we should expect if LTP was some type of memory mechanism. 

But what about syntaptic plasticity, previously mentioned in my quote from the neurologist King ? Since he claimed that LTP is the mechanism behind synaptic plasticity, and LTP cannot explain any memory lasting longer than a year, then synaptic plasticity will not work to explain very long-term memories.

To study LTP, scientists typically perform something artificial called tetanic stimulation, using a frequency of 100 hertz. A normal brain does not transmit signals at a frequency so high. Of the common brain waves (alpha, beta, and gamma), only gamma waves have a frequency of greater than 30 hertz, and such gamma waves generally do not have a frequency higher than 50 hertz.  LTP can be induced using a lower frequency, but only by prolonged stimulation such as stimulation lasting minutes. That type of sluggish low-frequency stimulation cannot explain human memories that can form instantly. 

Why Synapses Cannot Explain Very Long-Term Memory

Long-term memory cannot be stored in synapses, because synapses don't last long enough. Below is a quote from a scientific paper:

A quantitative value has been attached to the synaptic turnover rate by Stettler et al (2006), who examined the appearance and disappearance of axonal boutons in the intact visual cortex in monkeys.. and found the turnover rate to be 7% per week which would give the average synapse a lifetime of a little over 3 months.

You can read Stettler's paper here. A 2019 paper documents a 16-day examination of synapses, finding "the dataset contained n = 320 stable synapses, n = 163 eliminated synapses and n = 134 formed synapses."  That's about a 33% disappearance rate over a course of 16 days, suggesting an average synapse lifetime of less than three months.

You can google for “synaptic turnover rate” for more information. We cannot believe that synapses can store-long memories for 50 years if synapses only have an average lifetime of about 3 months. The paper here says the half-life of synapses is "from days to months."

Synapses often protrude out of bump-like structures on dendrites called dendritic spines. But those spines have lifetimes of less than 2 years.  Dendritic spines last no more than about a month in the hippocampus, and less than two years in the cortex. This study found that dendritic spines in the hippocampus last for only about 30 days. This study found that dendritic spines in the hippocampus have a turnover of about 40% each 4 days. This 2002 study found that a subgroup of dendritic spines in the cortex of mice brains (the more long-lasting subgroup) have a half-life of only 120 days. A paper on dendritic spines in the neocortex says, "Spines that appear and persist are rare." While a 2009 paper tried to insinuate a link between dendritic spines and memory, its data showed how unstable dendritic spines are.  Speaking of dendritic spines in the cortex, the paper found that "most daily formed spines have an average lifetime of ~1.5 days and a small fraction have an average lifetime of ~1–2 months," and told us that the fraction of dendritic spines lasting for more than a year was less than 1 percent. A 2018 paper has a graph showing a 5-day "survival fraction" of only about 30% for dendritic spines in the cortex.  A 2014 paper found that only 3% of new spines in the cortex persist for more than 22 days. Speaking of dendritic spines, a 2007 paper says, "Most spines that appear in adult animals are transient, and the addition of stable spines and synapses is rare." A 2016 paper found a dendritic spine turnover rate in the neocortex of 4% every 2 days. A 2018 paper found only about 30% of new and existing dendritic spines in the cortex remaining after 16 days (Figure 4 in the paper). 

Furthermore, it is known that the proteins existing between the two knobs of the synapse (the very proteins involved in synapse strengthening) are very short-lived, having average lifetimes of no more than a few days. A graduate student studying memory states it like this:

It’s long been thought that memories are maintained by the strengthening of synapses, but we know that the proteins involved in that strengthening are very unstable. They turn over on the scale of hours to, at most, a few days.

A scientific paper states the same thing:

Experience-dependent behavioral memories can last a lifetime, whereas even a long-lived protein or mRNA molecule has a half-life of around 24 hrs. Thus, the constituent molecules that subserve the maintenance of a memory will have completely turned over, i.e. have been broken down and resynthesized, over the course of about 1 week.

The paper cited above also states this (page 6):

The mutually opposing effects of LTP and LTD further add to the eventual disappearance of the memory maintained in the form of synaptic strengths. Successive events of LTP and LTD, occurring in diverse and unrelated contexts, counteract and overwrite each other and will, as time goes by, tend to obliterate old patterns of synaptic weights, covering them with layers of new ones. Once again, we are led to the conclusion that the pattern of synaptic strengths cannot be relied upon to preserve, for instance, childhood memories.

The latest and greatest research on the lifetime of synapse proteins is the June 2018 paper “Local and global influences on protein turnover in neurons and glia.” The paper starts out by noting that one earlier 2010 study found that the average half-life of brain proteins was about 9 days, and that a 2013 study found that the average half-life of brain proteins was about 5 days. The study then notes in Figure 3 that the average half-life of a synapse protein is only about 5 days, and that all of the main types of brain proteins (such as nucleus, mitochondrion, etc.) have half-lives of 15 days or less.  The 2018 study here precisely measured the lifetimes of more than 3000 brain proteins from all over the brain, and found not a single one with a lifetime of more than 75 days (figure 2 shows the average protein lifetime was only 11 days). 

The paper here states, "Experiments indicate in absence of activity average life times ranging from minutes for immature synapses to two months for mature ones with large weights."

When you think about synapses, visualize the edge of a seashore. Just as writing in the sand is a completely unstable way to store information, long-term information cannot be held in synapses. The proteins in between the synapses are turning over very rapidly (lasting no longer than about a week), and the entire synapse is replaced every few months.

In November 2014 UCLA professor David Glanzman and his colleagues published a scientific paper publishing research results. The authors said, “These results challenge the idea that stable synapses store long-term memories." Scientific American published an article on this research, an article entitled, “Memories May Not Live in Neuron's Synapses.” Glanzman stated, “Long-term memory is not stored at the synapse,” thereby contradicting decades of statements by neuroscientists who have dogmatically made unwarranted claims that long-term memory is stored in synapses.

Why Very Long-Term Memories Cannot Be Stored in the Cell Nucleus

His research has led Glanzman to a radical new idea: that memories are not stored in synapses, but in the nerve cell nucleus. In fact, in this TED talk Glanzman dogmatically declares this doctrine. At 15:34 in the talk, Glanzman says, “memories are stored in the cell nucleus – it is stored as changes in chromatin.” This is not at all what neurologists have been telling us for the past 20 years, and few other neuroscientists have supported such an idea.

We should be extremely suspicious and skeptical whenever scientists suddenly start giving some new answer to a fundamental answer, an answer completely different from the answer they have been dogmatically declaring for years. For example, if scientists were to suddenly start telling us that galaxies are not hold together by gravity (as they've been telling us for decades), but by, say, “dark energy pulsations,” we should be extremely skeptical that the new explanation is correct. In this case, there are very good reasons why Glanzman's recently-hatched answer to where long-term memories are stored cannot be right.

Chromatin is a term meaning DNA and surrounding histone protein molecules. Histone molecules are not suitable for storing very long-term memories because they are too short-lived. A scientific paper tells us that the half-life of histones in the brain is only about 223 days, meaning that every 223 days half of the histone molecules will be replaced.

So histone molecules are not a stable platform for storing very long-term memories. But what about DNA? The DNA molecule is stable. But there are several reasons why your DNA molecules cannot be storing your memories. The first reason is that your DNA molecules are already used for another purpose – the storing of genetic information used in making proteins. DNA molecules are like a book that already has its pages printed, not a book with empty pages that you can fill. The second reason is that DNA molecules use a bare bones “amino acid” language quite unsuitable for writing all the different types of human memories. The idea that somewhere your DNA has memory of your childhood summer vacations (expressed in an amino-acid language) is laughable.

The third reason is that the DNA of humans has been exhaustively analyzed by various multi-year projects such as the Human Genome Project and the ENCODE project, as well as various companies that specialize in personal analysis of the DNA of individual humans. Despite all of this huge investigation and analysis, no one has found any trace whatsoever of any type of real human memory (long-term or short-term) being stored in DNA. If you do a Google search for “can DNA store memories,” you will see various articles (most of them loosely-worded, speculative and exaggerating) that discuss various genetic effects (such as gene expression) that are not the same as an actual storage of a human memory. Such articles are typically written by people using the word “memories” in a very loose sense, not actually referring to memories in the precise sense of a recollection.

The fourth reason is that there is no known bodily mechanism by which lots of new information can be written to the storage area inside a DNA molecule. The fifth reason is that the DNA we see in brain neurons is basically identical to the DNA we see in other parts of the body (such as the DNA from foot cells). If memories were stored in DNA, the DNA in brain neurons would be much different from that of the DNA in other body parts. 

To completely defeat the idea that your memories may be stored in your DNA, I will merely remind the reader that DNA molecules are not read by brains – they are read by cells. It takes about 1 minute for a cell to read only the small part of the DNA needed to make a single protein (and DNA has recipes for thousands of proteins). If your memories were stored in DNA, it would take you hours to remember things that you can actually recall instantly. Thinking that DNA can store memories is like thinking that your refrigerator can cook a steak.

But couldn't very-long term memories just be stored in some unknown part of a neuron? No, because the proteins that make up neurons have short lifetimes. A scientist explains the timescales:

Protein half-lives in the cell range from about 2 minutes to about 20 hours, and half-lives of proteins typically are in the 2- to 4-hour time range. Okay, you say, that's fine for proteins, but what about "stable" things like the plasma membrane and the cytoskeleton? Neuronal membrane phospholipids turn over with half-lives in the minutes-to-hours range as well. The vast majority of actin microfilaments in dendritic spines of hippocampal pyramidal neurons turn over with astonishing rapidity—the average turnover time for an actin microfilament in a dendritic spine is 44 seconds...As a first approximation, the entirety of the functional components of your whole CNS [central nervous system] have been broken down and resynthesized over a 2-month time span. This should scare you. Your apparent stability as an individual is a perceptual illusion.

It is occasionally speculated that long-term memories might be stored in microtubules in a cell. But such things do not last long enough to be a storage place for memories lasting decades. A scientific paper tells us how short-lived brain microtubules are:

Neurons possess more stable microtubules compared to other cell types (Okabe and Hirokawa, 1988; Seitz-Tutter et al., 1988; Stepanova et al., 2003). These stable microtubules have half-lives of several hours and co-exist with dynamic microtubules with half-lives of several minutes.

Why Long-Term Memories Cannot Be Stored in the DNA Methylome

DNA methylation occurs when a very simple molecule becomes attached to part of a DNA molecule. Such a simple methyl molecule can act like a kind of flag that switches part of a gene on or off. The set of all of these methyl molecules attached to DNA is known as the DNA methylome. It has been suggested by a few that maybe memories are stored in this DNA methylome. 

The DNA methylome seems like a fairly stable thing, and so you don't have the “low stability” problem of very rapid protein molecule turnover that you have with the theory that memories are stored in synapses. But there are several reasons why it is not credible to maintain that human memories are being stored in such a DNA methylome.

The first reason is that we already know the function of this DNA methylome, that it is something other than storing memories. The methyl molecules that make up the methylome serve the purpose of genetic expression, a very different task than storing memories. If you were to maintain that the DNA methylome serves both of these purposes, it would be kind of like the Saturday Night Live comedic sketch that described a product like “Miracle Whip.” It went like this:

Wife: New Shimmer is a floor wax!
Husband: No, New Shimmer is a dessert topping!
Wife: It's a floor wax!
Husband: It's a dessert topping!
Wife: It's a floor wax, I'm telling you!
Husband: It's a dessert topping, you cow!
Spokesman: [ enters quickly ] Hey, hey, hey, calm down, you two. New Shimmer is both a floor wax and a dessert topping!

The second reason for doubting that memories are stored in the DNA methylome is that the DNA methylome couldn't be read with the speed needed for memory recall that is very fast. The DNA methylome consists of methyl molecules scattered across a DNA molecule. All evidence suggests that reading DNA is relatively slow. DNA transcription occurs at a rate of about 40 to 80 nucleotides per second, and there are billions of such nucleotides. But think of how fast people can recall memories. On the TV show Jeopardy we see people recalling very obscure memories in only a few seconds. When someone talks rapidly, he is retrieving language memories (such as the memory of what a particular word means) in a fraction of a second. That couldn't happen so fast if some relatively slow process of reading DNA was being used.

The third reason for doubting that memories are stored in the DNA methylome is that the methylome does not grow in size as learning occurs. As discussed here, the DNA methylome is larger (percentage-wise) in a newborn baby than in either a young adult or an old man.

The fourth reason for doubting that memories are stored in the DNA methylome is that methylation suppression experiments do not affect memory very dramatically. Scientists have ways of suppressing DNA methylation, and they have tested the effects of such suppression on learning and memory.  A scientific paper says that “inhibiting DNA methylation alters olfactory extinction but not acquisition learning.” Another scientific paper says that when DNA methylation was inhibited, “long-term memory strength itself was not affected.” 

A scientific paper states the following:

Prior studies found no effects of zeb, a DNA methylation inhibitor (Zhou et al., 2002), on learning in A. mellifera (Lockett et al., 2010; Biergans et al., 2012). More recently, Biergans et al. (2016), conducted a meta-analysis of multiple honey bee studies with methylation inhibitors (zeb and RG108), but found no strong overall effect of inhibiting DNA methylation on honey bee learning.

These are not the type of very dramatic effects on learning and memory that one would expect from DNA methylation inhibition if memories were being stored in the methylome. Other studies claiming a stronger effect of DNA methylation inhibition are typically unreliable studies using fewer than 15 animals per study group, and in such low-statistical-power studies there is a high chance of a false alarm or false positive.

There are actually two drugs for humans that work mainly by inhibiting DNA methlyation: azacitidine and decitabine.  If DNA methlyation was a mechanism for memory storage, we would expect that the side effect lists for these drugs would make some mention of a possible effect on learning or memory. But these drugs do not produce such a side effect. It has been claimed by the few proponents of memory stored in the DNA methylation marks that such marks are a stable medium for writing information. But search for "DNA methylation turnover" and you will find contrary claims, such as a paper entitled "Rapid turnover of DNA methylation in human cells." 

When It Comes to Explaining Very Long-Term Memory, Our Neuroscientists Are in Disarray

So how can we summarize the current state of scientific thought on how long-term memory is stored? The word that comes to mind is: disarray. In this matter our scientists are flailing about, wobbling this way and that way; but they aren't getting anywhere in terms of presenting a plausible answer as to how very long-term memory can be stored in the brain. Our scientists have done nothing to plausibly solve the permanence problem – the problem that very long-term memories cannot be explained by evoking transient “shifting sands” mechanisms such as LTP which last much less than a year (or in neurons, which are rebuilt every two months due to protein turnover). On this matter our scientists have merely presented explanatory facades – theories that do not hold up to scrutiny, like some movie studio building facade that you can see is a fake when you walk around and look behind it, finding no rooms behind the front.

Another sign of this disarray is a 2013 scientific paper with the title, “"Very long-term memories may be stored in the pattern of holes in the perineuronal net." After basically explaining in its first paragraph why current theories of long-term memories do not work and are not plausible, the author goes on to suggest a wildly imaginative and absurdly ornate speculation that perhaps the brain is a kind of a giant 3D punchcard, storing information like data used to be stored on the old 2D punchcards used by IBM electronic machinery in the 1970's. The author provides no good evidence for this wacky speculation, mainly discussing imaginary experiments that would lend support to it. The very appearance of such a paper is another sign that currently scientists have no good explanation for very long-term memory. I may note that IBM punchcards only worked because they were read by IBM punchcard-reader machines. In order for the brain to work as giant 3D punchcard, we would have to imagine a brain-reader machine that is nowhere to be found in the human body. There has never existed such a thing as a punchcard that can read itself.

This scientific article quotes a neuroscientist speculating about memory storage. The article says:

Neuroscience has also been struggling to find where the brain stores its memories. “They may be ‘hiding’ in high-dimensional cavities,” Markram speculates.

High-dimensional cavities? Cavities are holes, not information storage media. I think the quote bolsters my claim that scientists do not have any plausible explanation of how brains can be storing memories for 50 years. 

Often the modern neuroscientist will engage in pretentious talk which makes it sound as if there is some understanding of how very long-term memory storage can occur. But just occasionally we will get a little candor from our neuroscientists, such as when neuroscientist Sakina Palida admitted in 2015, “Up to this point, we still don’t understand how we maintain memories in our brains for up to our entire lifetimes.”

Concluding Remarks on Long-Term Memory

For the reasons given above, there is no plausible mechanism by which brains such as ours could be storing memories lasting longer than a year. There are only a few possible physical candidates for things that might store very long-term memory in our brain, and as we have seen, none of them are plausible candidates for a storage of very long-term memory.

The fact that our neurologists claim to have theories as to how very long-term memories could be stored does not mean that any such theory is tenable. Imagine if you lived on a planet in which your consciousness and long-term memory was due to a soul, and that the first time scientists dissected a brain, they found that the brain was filled with sawdust. No doubt such scientists would get busy inventing clever theories purporting to explain how sawdust can generate consciousness and long-term memories.

I may note that memories stretching back 50 years are inexplicable not merely from a neurological standpoint but also from a Darwinian standpoint. As I will argue later, from the standpoint of survival of the fittest and natural selection, there is no reason why any primate organism should ever need to remember anything for longer than about a year or two (it would work just fine to just keep remembering last year's memories). I may note that according to an article on wikipedia.com, the average life span in the Bronze Age was only 26 years old. There is no reason why natural selection (prior to the Bronze Age) would have equipped us to remember things for a length of time twice the average life span in the Bronze Age, and it is not plausible that very long-term memories are a recent evolutionary development.

In this post and in other places on this site, when I use the term “memory” I am referring to things such as episodic memories of life events, learned vocabulary, learned facts, and learned visual information that we can recall (such as identifying the name of an object, or recognizing a face). I do not refer to muscular skills such as the skill of how to ride a bike. Such skills are best referred to as “muscular skills” rather than memory.

Postscript: Richard Huganir led a study that was specifically dedicated to trying to find long-lived proteins in synapses. None were found.  The actual scientific paper found no such thing. Quite to the contrary, the paper found the following:

• Studying thousands of brain proteins, the study found that virtually all proteins in brains are very short-lived, with half-lives of less than a week.
  
• Table 2 of the paper gives specific half-life estimates for the most long-lasting brain proteins, and in this table only 10 out of thousands of brain proteins had half-lives of 10 days or longer.
  
• Of the proteins whose half-life is estimated in Table 2, only one of them has a half-life of longer than 30 days, that protein having a half-life of only 32 days.
  
• A graph in the paper indicates that none of the synapse proteins had a half-life of more than 35 days.
  

Below is a graph from the Huganir paper. It shows that the study found that virtually all proteins in synapses are very short-lived.

Below is another graph from the same paper. It shows that the study found that virtually all proteins in synapses are very short-lived.

Judging from these graphs, none of the proteins found had a half-life of longer than 35 days, and only a few had a half-life of more than 14 days.

The scientific paper (like very many scientific papers) had a misleading title that does not match its observations. The title was "Identification of long-lived synaptic proteins by proteomic analysis of synaptosome protein turnover." No such proteins were identified. All of the proteins studied had short lifetimes of less than 75 days; and almost all of the proteins studied had lifetimes of less than a week.

Post-postscript: In 2025 I had a recollection which proved the ability of the mind to recall very old memories that have not been recalled in several decades. For some reason I recalled a book I had read about 50 years ago, and never since: the science fiction book "Galaxies Like Grains of Sand" by Brian Aldiss. I remembered some lines from the book. I wrote them down on paper like this:

"The mirror of the past lies shattered. The fragments you hold in your hand."

After I wrote this recollection of something I had not read, thought of or heard quoted in fifty years, I borrowed the book on www.archive.org.  I see that the lines were these (almost exactly as I remembered them)

"The long mirror of the past is shattered...Only a few fragments are left, and these you hold in your hand." 

Post-post-postscript: The 2025 paper here states that  "the synaptic turnover rate is as high as 1% per day in the visual cortex." That is a rate of about 100% replacement every four months. If that is the typical rate at which synapes are replaced, synapses cannot be the storage place of memories lasting decades.  Quoting an even higher rate of synaptic turnover, the paper here states, "A recent imaging study revealed that the synaptic turnover rate in hippocampal CA1 cells is very high, with an estimated lifetime of 1–2 weeks (Attardo et al., 2015)."