Neuroscientists typically claim that memories are stored in synapses, but this claim makes no sense. Very long-term memories 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:
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."
Research on the lifetime of synapse proteins is 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."
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.
Well aware of some of the difficulties discussed above, neuroscientists sometimes speculate that maybe the idea of memory reactivation can explain how people can remember things for so long. It is sometimes suggested that maybe a person can remember something for decades because he keeps reactivating the memory at periodic intervals, causing the old memory to be refreshed. So, for example:
- It might be that if an old man never thought of his first boyhood sweetheart, he might be unable to remember that person. But maybe every year that man thought of his boyhood sweetheart, causing the old memory to be periodically refreshed and reactivated.
- It might be that if an old man never read again some fact he learned in high school such as the fact of Abraham Lincoln's assassination, he might have forgotten such a fact long ago. But maybe what happens is that every year the man reads some mention of Abraham Lincoln's assassination, which causes the old memory to become refreshed and reactivated.
- An old man might remember the meaning of rarely-used words he learned as a boy, because he continues to speak those words occasionally or occasionally reads the words or hears the words, causing his memory of their meaning to become refreshed or reactivated.
However, this reactivation theory does not work to explain the marvel of the lifelong preservation of memories. The fact is that even without memory reactivation, people can remember things for decades (as long as 50 or 60 years), even trivial things. I will now cite personal observations and experimental data which shows the truth of my claim in the previous sentence.
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."
The scientific paper "Extremely long-term memory and familiarity after 12 years" documents an ability of some people to remember trivial sensory experiences after many years, experiences they should have forgotten under neural theories of human memory. In 2016 the study authors rounded up 25 subjects who had been briefly exposed to some very forgettable images in a scientific experiment done between eight and fourteen years earlier: thumbnail-sized images such as a little drawing of a coffee cup and a little drawing of a hen. The subjects were tested with a set of images, half of which were the original images, and half of which were decoy images designed to be similar to the original images. The subjects were asked to guess whether or not they had seen the images before, when they were tested many years earlier. The authors expected the subjects to make guesses no more accurate than chance. But they found that the subjects were able to guess with about 55% accuracy. We read this:
"In this study we found that our group of test participants was able to recognize simple colored pictures seen for a few seconds between eight and 14 years earlier. Our best performer, who had been exposed to the pictures at most three times, was able to identify 15 pictures more than the 84 pictures expected by chance. Note that no instruction to learn the stimuli was ever given to the subjects, even at initial encoding, which makes this performance even more remarkable."
While researching tests of very long-term memory, I found a 1989 scientific paper ("On the Course of Forgetting in Very Long-Term Memory") on an interesting experiment that tested people by asking which of four titles was an actual title of a TV show. All of the actual TV shows were ones that only ran for one year. This is a good technique for testing very old memories never reactivated, because in the 1970's and 1980's in the US if a TV show ran for only one year it would almost never be shown in other years, and there would tend to be no references to it in popular culture. (This was before services such as Netflix, which might allow a program running only one year to be re-watched by many.) Here is an example of some of the questions asked to the experiment's subjects:
1974
Which of the following was a T.V. show? (a) Mandrake, (b) Shipmates (c) Private Nelson, (d) Lucas Tanner
1978
Which of the following was a T.V. show? (a) Gaslight Alley, (b) Cutting Corners (c) Black Knight, (d) Kaz
1981
Which of the following was a T.V. show? (a) Dateline Miami, (b) The Conductor (c) Discovery, (d) McClaine 's Law
In these tests the tested subjects (231 in all) scored as shown below (25% is the result expected by chance, if no recall occurred).
In my case in late August, 2025 not only did I recall that Lucas Tanner was an old TV show, but I also recalled the name of the show's star (David Hartman), and that the show was about a teacher. At the time I had not watched this 1974-1975 show or read or heard any mention of this show in the past 50 years, nor had I ever thought about the show in the past 50 years. Similarly, in November 2025 I was watching a video about unsuccessful TV shows from the year 1974. There was a mention of the TV show "Get Christie Love," which was cancelled after a single year. I recalled that the star was Teresa Graves. Graves basically had no presence in TV or movies after 1975, and in 1983 retired from show business. At the time I recalled her name (November 2025), I had not watched this 1974-1975 show or read or heard any mention of this show or Graves in the past 50 years, nor had I ever thought about the show or Graves in the past 50 years.
In a similar vein, in the year 2025 I was watching a video on the most unpopular TV shows of 1973-1974. As soon as they mentioned the show The New Perry Mason, I remembered the star of the show was Monte Markham. The show ran for only 15 episodes, and was never run in repeats after 1974, with there being virtually no mentions of it in the press since about that year. When the video mentioned the TV show Adam's Rib without showing any of its actors, I remembered that it starred Blythe Danner and a tall blonde person named Ken, who played Thomas Jefferson in the movie 1776. I remembered both of their faces well. I was right about all of those details. The male actor's name was Ken Howard. The TV show Adam's Rib ran for only 13 episodes, and never ran in repeats after 1974. When the same video mentioned the TV show Bob and Ted and Carol and Alice, I remembered that one of the stars was a short red-haired actress with the last name of Gillette. I was correct. The show ran for only 12 episodes in 1973, and never was seen after then.
Similarly, while watching in 2025 one of these videos about the least popular TV shows from long ago, I saw a screen merely listing a title of It's About Time, a series that ran from 1966 to 1967, and was never syndicated after then. I correctly remembered that the show was about astronauts who traveled back in time into the Stone Age. I also remembered the melody and most of the first two lines of the show's theme song, remembering that it went like this: "It's about time, it's about space, __ __ __ __ __ __ __ place." In 2025 I had an equally good recollection of the melody and lyrics of the theme song of the 1965 TV series Hank, cancelled after a single season.
Similarly, while watching in 2025 one of these videos about the least successful TV shows of 1970, there was mention of a show about young people in the American Revolutionary War. I instantly remembered the show was called "The Young Rebels." The show was canceled after 15 episodes. I am rather sure that there was never any reactivation of any memory involving the show, the show being basically unmentioned since 1970.
In December 2025 I had a dream of Meredith MacCrae, in which she stated, "Mark, this is Meredith MacCrae." After awaking, I lay in bed trying to place the name. Soon I recalled that she had been in the 1960's TV show Petticoat Junction, one I had not watched in more than 50 years (and probably not heard mentioned substantively in more than 50 years). An internet search confirmed my recollection. I had not thought of Meredith in more than 50 years, and the press hardly mentioned her after the early 1970's.
A test an old person can try is to remember some street names in cities or towns he lived in very long ago. Trying this test I correctly remembered the street name of a school that my children last attended 16 years ago, in a small town we moved away from 16 years ago. I had no sensory experiences refreshing the memory of this street name during the past 16 years, and I cannot recall ever thinking of the address in the past 16 years. Similarly, I remembered an avenue near my senior high school in a major city, an avenue with a name I have not remembered in about 50 years.
So it simply is not true that to remember something for decades, you need periodic reactivations every few years that recharge or reactivate a memory that won't last years without being strengthened. You can learn trivial little things and remember them decades later, as much as 50 years later, even with no reactivations of the memory.
An experience I had shortly before writing this post further showed this. Someone in my family had recently got a teapot, which caused me to recall the beginning of a children's song I had heard only a few times, only in a house I have not lived in for about 25 years. I remembered the first line: "I'm a little teapot, short and stout." I tried to remember the second line, but at first I could not. Later I remembered both of the first two lines of the song I had not heard or sung or remembered in about 25 years:
I'm a little teapot, short and stout
Here is my handle, here is my spout
In 2025 (as I noted at the end of the post here) I had a recollection which proved the ability of the mind to recall very old memories that have not been recalled in 50 years. 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."
Below is a quote on the same topic from an earlier post discussing why brains cannot be the storage place of very old memories:
"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). ...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."
One day in 2025 while lying in bed there strangely popped into my mind "out of nowhere" the name Toby Tyler, which I remembered was some circus movie I had seen as a child, in a theater. Looking up the name, I found it was the title of a circus movie that came out in 1960 (the title was Toby Tyler, or 10 Weeks With a Circus). I never saw the movie in a theater or on TV after 1960, and I never heard any mention of it on TV or in anything I ever read. Here we have an example of the remembering of a trivial memory from 65 years ago, a memory that was never reactivated after 1960.
In late 2025 I was watching a Youtube.com video about the 10 least popular television shows of 1966. I performed these wonders of memory recall relating to TV shows or TV personalities I have not seen or heard mentioned in about 60 years or more:
(1) Upon hearing the name of the 1966 TV show "Captain Nice" (canceled after a single season), I recalled its star was William Daniels.
(2) Upon seeing an unlabeled photo of Judy Carne (an actress I have not seen on TV in 60 years and probably not heard mentioned in 60 years), I instantly identified her as Judy Carne.
(3) Hearing mention of an "Occasional Wife" TV show I once watched in 1966, and seeing an unidentified image of its star, I first guessed the person's name as William Calley. Later (still having heard no mention of the star's name) I thought to myself something like, "No, I think it was maybe Michael Calley." The actor (who did no famous work after 1966) was named Michael Callen, a name identical to my final guess, except for a single character.
Later I watched a YouTube video of the least popular TV shows of 1969. All of the shows mentioned were cancelled after one year or less, and none ever appeared in repeats after 1970. Hearing the name of the show "My World and Welcome To It," I remembered exactly the star was the obscure actor William Windom. I also remembered that the main character's daughter would wear a noticeable orthodontic retainer. The video soon confirmed that these 55-year-old memories were correct.
In late 2025 I read an article claiming that the 1960's song "Yummy, Yummy, Yummy" was the worst song of the 1960's. Before playing a video of the song, I tried to recall the song, which I had not heard or recalled in more than 30 years. I successfully recalled the lyrics and melody of the first line of the song ("Yummy, yummy, yummy I got love in my tummy"), and also recalled the lyrics and melody of the song's middle refrain ("Ooh, love to hold you, ooh, love to kiss you").
Collectively all of these examples in this post show it is false that remembering something for decades requires periodic reactivations of the memory. Many people can remember trivial things for 30, 40 or 50 years, without any reactivations of the memory. Such abilities cannot be explained through any credible theory of brain activity, given the very high molecular turnover and synaptic turnover in the brain.
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