Synapse Strengthening is Way, Way Too Slow to Explain Instant Learning

The Kavli Foundation is a foundation founded by millions of dollars in grants from the late Fred Kavli. The foundation issues science prizes and science grants. One of its semi-annual prizes is in neuroscience. An earlier post on this blog described the bunk and misleading information that occurred when the 1 million dollar Kavli Prize in neuroscience was announced in 2024.  Recently the Kavli Prize was awarded to neuroscientists involved in work on protein synthesis in dendrites: Oswald Steward, Christine Holt, Kelsey Martin and Erin Schuman. The announcement of the prize award makes no mention of learning or memory, merely stating this: "THE KAVLI PRIZE IN NEUROSCIENCE IS AWARDED TO: Christine Holt, Kelsey Martin, Erin Schuman and Oswald Steward for the discovery of local protein translation in neurons and establishing its importance for brain development and plasticity."  

The same page has a video announcement, where someone very incorrectly makes the utterly groundless claim at the 16:32 mark that this research "came to transform our understanding of how the brain develops, adapts, and stores information." Neuroscientists have no real understanding of how a brain can store learned information, and the vague hand-waving simple-slogan "theory" of synapse strengthening is no such understanding. The same authority makes the groundless boast at the 18:15 mark that the work of Kelsey Martin "helps explain how learning and memory are stored in the brain," a boast that has no basis in truth. Neuroscientists do not have any understanding of how any learning or memory could be stored in the brain, and microscopic examination of brain tissue has never produced the slightest trace of anything learned or experienced or memorized. 

A Simons Foundation page incorrectly claims this was a reward for memory research, stating, "This year’s Kavli Prize in Neuroscience celebrates research on how neurons form and modify neural connections to enable processes such as learning and memory." That goes beyond any claim made in the written prize announcement statement, and we certainly do not know that "neural connections...enable processes such as learning and memory," which is a mere groundless dogma of neuroscientists. 

A press release by the University of California announcing this prize gives us bunk and misleading information related to this topic. We have a press release with this paragraph, preceded by the bogus header of "Transformative discoveries." The paragraph veers into falsehood at its end:

"For decades, scientists believed that proteins needed by neurons were produced primarily in the cell body. Steward’s groundbreaking electron microscopy studies revealed protein-producing machinery located near synapses, which is where cells make connections with each other, demonstrating that neurons can manufacture proteins locally where they’re needed. His subsequent research defined the mechanisms of messenger RNA transport from the nucleus and selective localization at active synapses, creating a new understanding of brain plasticity, learning and memory." 

The described research produced no actual progress in understanding learning or memory. Neuroscientists claim that protein synthesis is required for learning and memory, but their claims about this do not hold up to scrutiny, because of two reasons:

(1) All brain proteins have short lifetimes, typically less than two weeks. The proteins in the brain and its synapses are constantly being replaced. There is no credibility in attempts to explain memory formation by referring to "synapse strengthening" occurring by protein synthesis. Old humans can remember well things that happened 50 years ago, and the span of 50 years is a length of time 1000 times greater than the average lifetime of the proteins in synapses. Individual synapses cannot last for years, partially because they are connected to dendritic spines that do not last for years, and typically last for less than a few months. 

(2) Humans can learn things instantly, much faster than the time required for synapse strengthening by protein synthesis, which is at least several minutes. When someone is informed of the death of their child or parent, that person instantly forms a new memory that lasts for the rest of his life. 

It seems, therefore, that mere research into protein synthesis can never correctly be described as research that helps understand how memories form. A press release at the University of Cambridge has a paragraph that describes the research awarded the Kavli prize in neuroscience, and attempts to create some impression that a little progress has occurred related to understanding memory. But the narrative it tells is a false one, and the paragraph starts out with a misleading first sentence. The paragraph is below:

"Scientists long struggled to explain how the human brain can be so efficient – we can ultimately learn things in mere minutes. The proteins needed to enable the process in brain cells simply take too long to travel from the body of the brain cell, the neuron, to where the synapses – tiny junctions between neurons that allow them to communicate with each other or other cells – actually happen. But research spanning decades by this year’s laureates – Oswald Steward, Erin Schuman, Kelsey Martin and Christine Holt – has solved the mystery. Rather than the proteins being created in the cell body, as was previously thought, they can be produced directly on site close to where the all-important synapses happen; in the branches of the neurons appendages, called dendrites and axons. The discovery has led to a new understanding of how the brain works – and offers insights into how this process goes wrong in a range of brain disorders."

The narrative is bogus. It begins with the very misleading insinuation that human memory creation requires minutes. To the contrary, humans can form new memories instantly. So there was never a "problem of explaining how memories can be created in mere minutes." The problem was a much more difficult one: the problem of explaining how humans can create complex new memories instantly. 

The real problem (the problem of how humans can create complex new memories instantly) is not at all solved (or even appreciably reduced) by postulating that proteins are synthesized in the dendrites of cells, and are then used to bulk up synapses. The diagram below may help you understand why:

What difference would it make (under the theory that memories are stored in synapses) if some proteins are synthesized in dendrites  (the rather finger-like projections you see in the diagram above) rather than in the cell body of a neuron (surrounding the largest yellow circle in the diagram above)? Very little difference indeed. There might be a very slight decrease in the amount of time it would take newly synthesized proteins to travel to a synapse. But the difference would be small. 

A particular type of protein molecule is created by this process:

(1) Somehow the right position is found in human DNA, allowing a reading to occur from a small fraction of the DNA (called a gene), with the information being transferred into a messenger RNA molecule. How that messenger RNA is ever able to find the right gene is a mystery. The gene stores symbolic information describing the amino acid sequence of some particular protein molecule. This reading is called transcription, and occurs at a rate between 10 and 50 nucleotides per second. Since an average protein requires between 1200 and 1500 nucleotides to be read from a gene for the transcription required by the protein to occur, the transcription of a protein requires somewhere between 25 seconds and several minutes.  

(2) Somehow the messenger RNA molecule is translated into a chain of amino acids. This process is called translation, and is thought to occur at a rate of about 5 amino acids per second. Because the average protein used in synapses has about 450 amino acids, this translation must take an average of roughly 90 seconds. 

(3) Somehow (in a way that is not understood at all) that amino acid sequence quickly converts into a three-dimensional protein molecule that is folded.  This process is called protein folding. How it occurs is a mystery called the protein folding problem, which has not yet been solved. A year 2026 paper states, "The protein folding problem remains unsolved."

The Google Gemini infographic below illustrates this process:

To calculate the time required for a new protein to be created and then travel to a synapse as part of some synapse strengthening imagined to be part of a theoretical storage of memory in synapses, assuming protein synthesis in the main body of a neuron, you would need to calculate all of these different factors:

(1) The time needed for some sensory signal to travel to some neuron so that protein synthesis is somehow triggered (no one has any understanding of how sensory information could have any relation to when or where protein synthesis occurs). 

(2) The time required for a cell or messenger RNA molecule to find the right position in DNA  from which to read the amino acid sequence needed to make the new protein molecule of a particular type. Given that DNA stores the amino acid sequences of more than 20,000 different proteins, without any kind of indexing system or sorting system,  this is kind of a "finding a needle in a haystack" situation. The mere "finding the right location to read" part should take very significant time.

(3) The time needed for the reading to occur once the right location had been found, resulting in a messenger RNA molecule matching the gene read. This is the speed of transcription.  Transcription occurs at a rate between 10 and 50 nucleotides per second. Since an average protein requires between 1200 and 1500 nucleotides to be read from a gene for the transcription required for that protein to occur, the transcription of a protein requires somewhere between 25 seconds and several minutes.  

(4) The time required for protein translation, by which the messenger RNA molecule is converted into an amino acid sequence, a specific chain of hundreds of amino acids. Translation in humans is thought to occur at a rate of about 5 amino acids per second. Because the average protein used in synapses has about 450 amino acids, this translation must take an average of roughly 90 seconds.

(5) The time required for there to occur the mysterious process of protein folding, by which a chain of amino acids forms into the 3D shape needed for a functional protein molecule.  

(6) The time needed for a newly synthesized protein molecule to travel from the neuron to the synapse. 

(7) The time needed for this newly synthesized protein molecule to somehow be integrated into the synapse, so that the synapse ends up being strengthened.

The total length of time required for this series of events would be at least three minutes, and very probably many minutes. You only slightly reduce the total length of time required for the totality of all of these things if you assume some protein synthesis going on in dendrites. That reduces the time required for only one of the items in the list above, leaving all of the other factors being just as slow as before.  

Because synapse strengthening requires a series of events that would require a total time of at least three minutes, the synaptic theory of memory (that memories are stored by synaptic strengthening) totally fails to account for the indisputable reality that humans can form permanent new memories instantly. Very rich organizations such as the Kavli Foundation can issue all of the triumphal million-dollar prize announcements that they wish. But the fact is that scientists do not have any credible explanation for how there could occur in a brain human learning which occurs instantly. The most reasonable alternative here is to discard the dogma that memory formation is a brain process. 

Synapses have not the slightest resemblance to a memory storage device. The idea that human memory can be explained by some mere idea of synapse strengthening is one that future scientists will look back on with scorn, the way today's scientists look back scornfully at 18th-century claims that health could be improved by attaching leeches to the arm. Strengthening isn't storage. 

Exhibit C That Neuroscientists Have No Understanding of How a Memory Could Form or Last in a Brain

 In 2020 on this site I published a post entitled "Exhibit A Suggesting Scientists Don't Understand How a Brain Could Store a Memory." In 2023 I published on this site a post entitled "Exhibit B That Scientists Have No Understanding of a Physical Basis of Human Memory." Now it is time for Exhibit C on this topic. 

I recently discovered a web site called The Transmitter (www.thetransmitter.org) that mainly covers neuroscience research and neuroscience theory. When read in a critical manner, an article on the last site serves to powerfully remind us that neuroscientists lack any such thing as either a real theory of memory storage or a real theory of life-long memory persistence. When scientists speak on these topics, they offer only the flimsiest catchphrases, soundbites that have the weight of soap bubbles. 

The title of the article is "What makes memories last—dynamic ensembles or static synapses?" The reference to "static synapses" is a very misleading one. Everything we know about synapses tells us that a synapse is an unstable thing that cannot last for years.

We read a neuroscientist (Jason Shepherd) making these claims:

"The debate over how information is stored in the brain is often represented as one between two extremes. One viewpoint posits that learning induces changes in gene expression that ultimately alter the structure and function of specific synapses within the physical memory circuit, or engram. These molecular changes at the synapses can remain stable for the lifetime of the memory. The other viewpoint claims that information is represented not in a specific set of cells or synapses but rather across a loose set of cells and circuits that 'drift' over time."

The narrative of two rival theories is a false one. The situation is really "no theory at all" but merely empty, vacuous sound bites and slogans such as "synapse strengthening," which may differ from one speaker to the next. The claim above that "molecular changes at the synapses can remain stable for the lifetime of the memory" is something entirely contrary to fact. We know that human memories can persist for more than 50 years. Synapses, on the other hand, are "shifting sands" type of things that are dramatically unstable. The proteins that make up synapses have an average lifetime of less than 3 weeks.  Synapses are connected to dendritic spines, which are known to have short lifetimes, not lasting for years. Remarkably synapses are built of proteins which have an average lifetime about 1000 times shorter than the maximum length of time that humans can remember things. This discrepancy is one of very many reasons why the idea that memories are stored in synapses is one of the most nonsensical ideas that scientists have ever advanced. 

Notice well the utter emptiness of what is discussed as an alternative to the utterly-vacuous-by-itself idea that memories are formed by "synapse strengthening." The alternative is presented as the idea that " information is represented not in a specific set of cells or synapses but rather across a loose set of cells and circuits that 'drift' over time." That's an utterly vague, vacuous, empty sound bite that is as much  of an empty soap bubble as the equally empty notion of "synapse strengthening." Not the slightest bit of weight is added by the next two sentences:

"In this view, the cells that initially encoded an experience are not the same set of cells that actually store the information. Indeed, the precise set of cells do not matter in this framework—the information for a specific memory is instead decoded from the computational space of firing patterns across a set of cells."

As some type of attempt to explain stable memories that can last for 50 years, this idea is as supremely goofy as the idea that memories that last for 50 years are stored in the "shifting sands" of synapses. The "firing patterns" in the brain are ever-changing. Trying to claim that stable memories are stored in "firing patterns" is as goofy as the claim that your tax records and childhood photos are stored in the wind patterns around your house. 

Shepherd gives us some "rival cases" paragraphs. Under a heading of "The case for memory engrams," he makes some untrue statements. He states this:

" In experiments that used this approach, light-sensitive receptors were expressed only in the cells active during learning. Shining a light to activate these cells days or even weeks after training resulted in the recall of a memory without any external experience or cue. This remarkable observation set the stage for the idea that 'engram' neurons that encode learning are sufficient to store and recall a memory."

No robust research of any such type ever occurred.  Shepherd is simply repeating a groundless achievement legend of neuroscientists. When you read the papers that claim to have done such things, you will always find that they were junk-science studies guilty of multiple types of Questionable Research Practices such as the use of way-too-small study group sizes, and the use of unreliable techniques for attempting to judge recall in rodents, such as the unreliable method of trying to judge "freezing behavior."

Under the heading of "the representational drift perspective," Shepherd presents nothing in the way of any evidence. We get only the most roundabout hand-waving. 

Shepherd then asks eight neuroscientists for their opinions on the topic of memory storage by a brain. Shepherd follows a senseless procedure.  A good open question to ask would be something like this:

"Do you have a good, credible theory of how a brain could store memories, and how memories could persist a lifetime? If so, describe the best evidence for such a theory, and tell us how confident you are that such a theory is true."

And a good follow-up question would be questions like this:

• "Are there any physical factors in the brain that argue against such a theory? Explain how such a theory could really allow 50-year memory storage despite all the molecular and structural turnover in the brain."
• "Trying to be precise, and avoiding vague language, can you explain exactly how a detailed memory could be stored under such a theory? For example, exactly how could a brain store a page of text that someone had memorized, so that the person could retrieve that whole page?"
• "Under such a theory, how would it be possible for someone to instantly recall lots of relevant detailed information after seeing a single face or hearing a single name? For example, how could someone ever recite a paragraph describing the life of Abraham Lincoln after merely hearing his name? How could information about Lincoln stored in a brain ever be found quickly enough to allow instant recall?"

But Shepherd asks no such challenging questions to his eight neuroscientists. Instead he asks each of them the softest of softball questions. Each neuroscientist is asked these questions:

• "Is information stored in the brain at the level of cells (or circuits) or at the level of synapses?"
• "Can we reconcile observations that show distinct engram circuits seem to store memories versus observations that show the neuronal activity of these memory engram drifts?"
• "What experimental data would be helpful to reconcile these observations to help bring these theories together?"

The first question is a classic example of a stupid "either/or" question in which someone is asked to choose between two alternatives, neither of which is credible. The question is as stupid as asking, "Are UFOs spaceships from the planet Mars or spaceships from the planet Venus?" The second question is one with a false premise embedded within it. It is not true that there are "observations that show distinct engram circuits seem to store memories." Microscopic examination of brain tissue has never shown the slightest trace of anything anyone has learned or experienced. The third question is the type of question you might ask neuroscientists when they don't have any good evidence to back up their dogmas. Rather than asking them to tell about what evidence backs up their claims, you might ask them to fantasize about what type of future observations they might make that might back up their theories. 

None of the eight questioned neuroscientists has anything of any substance to offer in response to the questions. The first question at least offers an invitation for someone to start expounding about any theory he may have of neural memory storage. We get no impressive quotes in response to such a question. We get only the wobbliest hand-waving that makes the people giving the answers sound very empty-handed. 

• Andre Fenton of New York University has nothing of any substance to say. He says "information is not stored in any single element," and "it may not be practically possible to separate the process of storage from the access," both of which suggest that he has no understanding of how a brain could store a memory. People who understand how some type of information is stored do not say such things. 
• Loren Frank of the University of California gives us no impression that he understands how a brain could store a memory. He says, "It might be that changes in gene expression lead to changes in activity levels, although at the moment we really don’t understand the scope of these changes." He offers only the vaguest hand-waving, with a mention of the hippocampus. We have an example of the vaguest and most conceptually empty hand-waving in this statement by Frank: "Focusing on memories for the events of daily life, our current conception is that the events themselves drive activity across the brain, engaging specific neurons whose activity represents the various sights, sounds, smells and feelings that are part of the experience." 
• Kari Hoffman of Vanderbilt University also offers only the vaguest handwaving, an example being this statement: "I would submit that much of the heavy lifting is done at both the synaptic and circuit/ensemble level. Which levels dominate depends on factors such as memory type, when information was acquired and how it is integrated with the existing structures, themselves reflecting changes from earlier experiences. " Another statement by her indicates she has no real understanding on this topic: "That said, we may need to be careful in using the term 'these memories' or 'these memory engrams.'  Such terms suggest that experience creates biological bins to hold discrete memories, that memories exist as entities that are created 'de novo,' and that neural modifications must reside at only one level, all of which are positions that are not or may not be true." 
• Yingxi Lin of the University of Texas says, "It is, however, too early to say that those cells and synapses are sites of stored memory per se, as they may simply function to gain access to the memory."  She also says, " It is also possible that there aren’t specific sites for memory storage; cells and synapses may be part of a brain-wide code for memory expression." She seems to have no understanding of how a brain could store a memory. 
• Cian O'Donnell of Ulster University sounds like a weak scholar of neuroscience when he states, "The field has held synaptic plasticity up as the main mechanism for information storage in the brain for several decades now, and I haven’t heard any good reasons to start doubting it yet." There are very many such reasons, such as the fact that synapses are composed of proteins with very short lifetimes, the fact that synapses bear no resemblance to any system for writing or reading information, the fact that synapses do not reliably transmit information, and that synapses are connected to dendritic spines that are unstable and do not last for years. Nothing O'Donnell says makes him sound like anyone with an understanding of how a brain could store memories. 
• Timothy O'Leary of Cambridge University (not to be confused with the late Timothy Leary of Harvard) says nothing to inspire any confidence that he has any understanding of how a brain could store a memory. All he does is to reveal that he fell "hook, line and sinker" for bad neuroscience experiments using way-too-small study group sizes and the utterly unreliable technique of trying to judge recall by judging "freezing behavior." 
• Tomas Ryan of Trinity College also says  says nothing to inspire any confidence that he has any understanding of how a brain could store a memory. He engages in the emptiest of hand-waving when he says this: "It seems to me that the plausible level for the storage of long-term memories is in the topography of the connectome. So, the information is engraved through stable changes in the brain’s microanatomical circuit." The "connectome" he refers to is the collection of all synapses. But synapses are not stable, but the opposite of stable. So his claim makes no sense. 
• The last of the eight neuroscientists is Evan Schaffer of the School of Medicine at Mount Sinai. He states this: "As a consequence, I don’t think information can be stored in cells or synapses in the hippocampus in a way that is stable over a lifetime. In other parts of the brain, this may not be the case." No, actually, there is no credible storage place for memories in the brain, either in the hippocampus or anywhere else. Not sounding like anyone who understands how a brain could store memories, Schaffer also sounds like a poor student of human mental performance. Most misleadingly, he tries to suggest that humans may not be able to remember things well for weeks. He says, "On a timescale of a few days, memories seem pretty stable. On a timescale of a few weeks, there’s less evidence for stability." To the contrary, there is abundant evidence that humans can very well remember things for decades. To give one of endless examples I could cite, every opera fan knows that various opera stars are able to perfectly remember over many years the very many notes and words that make up particular opera roles. Placido Domingo, for example, performed more than 150 opera roles, many of which required singing for hours on the stage, from memory. 

Finally in the article we have a summing up by Shepherd, who sounds just as empty-handed and theory-lacking as the eight experts he has interviewed. He says this:

"Finally, neuroscientists must do a better job of defining their terms. What is 'information,' and how is it 'represented' in the brain? What is an engram?"

The title of the article was "What makes memories last—dynamic ensembles or static synapses?" I re-read all of the answers to see whether anyone addressed the issue of how memories could last in a brain long enough to persist for decades. Not one of the eight neuroscientists even addressed the issue. Not one of them advanced any theory as to how memories could persist for decades. Not one of them advanced even a hypothesis about such a topic.  The issue of how memories could last for decades was simply ignored by the eight neuroscientists, none of whom had either a real theory of memory storage to advance, nor any theory of the life-long preservation of memory.  We certainly did not get any such thing when we got this piece of fantasy by Tomas Ryan:

"It seems to me that the plausible level for the storage of long-term memories is in the topography of the connectome. So, the information is engraved through stable changes in the brain’s microanatomical circuit." 

Engraved? No such engraving occurs in the brain. Nothing in a brain bears any resemblance to a system or component for writing learned information. There is zero evidence that anything bearing the slightest resemblance to engraving occurs in the brain. We see no "engraved" neurons, no "engraved" synapses, and no "engraved" dendritic spines.  Everything that has been learned about synapses shouts that a synapse cannot have any such thing as stable changes, in the sense of changes that last permanently for decades. The proteins that make up synapses have average lifetimes of less than a few weeks. And synapses are attached to dendritic spines that are known to have short lifetimes, dendritic spines that do not last for years. 

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. The same paper refers to another paper that "reported rates of [dendritic] spine eliminations in the order of 40% over an observation period of 4 days."  A paper studying the lifetimes of dendritic spines in the cortex states, "Under our experimental conditions, most spines that appear survive for at most a few days. Spines that appear and persist are rare." The rare persistence referred to was only a persistence of a few months. 

The 2023 paper here gives the graph below showing the decay rate of the volume of dendritic spines. It is obvious from the graph that they do not last for years, and mostly do not even last for six months. 

Page 278 of the same paper says, "Two-photon imaging in the Gan and Svoboda labs revealed that spines can be stable over extended periods of time in vivo but also display genesis (generation) and elimination (pruning) at a frequency of 1–4% per week." Something vanishing at a rate of 2% per week will be gone within a year. 

Below are some quotes by scientists and doctors who spoke candidly about brains and memory storage, rather than engaging in the kind of bluffing that went on from the people mentioned above:

• "Direct evidence that synaptic plasticity is the actual cellular mechanism for human learning and memory is lacking." -- 3 scientists, "Synaptic plasticity in human cortical circuits: cellular mechanisms of learning and memory in the human brain?" 
• "The fundamental problem is that we don't really know where or how thoughts are stored in the brain. We can't read thoughts if we don't understand the neuroscience behind them." -- Juan Alvaro Gallego, neuroscientist. 
• "The search for the neuroanatomical locus of semantic memory has simultaneously led us nowhere and everywhere. There is no compelling evidence that any one brain region plays a dedicated and privileged role in the representation or retrieval of all sorts of semantic knowledge."  Psychologist Sharon L. Thompson-Schill, "Neuroimaging studies of semantic memory: inferring 'how' from 'where' ".
• "How the brain stores and retrieves memories is an important unsolved problem in neuroscience." --Achint Kumar, "A Model For Hierarchical Memory Storage in Piriform Cortex." 
• "We are still far from identifying the 'double helix' of memory—if one even exists. We do not have a clear idea of how long-term, specific information may be stored in the brain, into separate engrams that can be reactivated when relevant."  -- Two scientists, "Understanding the physical basis of memory: Molecular mechanisms of the engram."
• "There is no chain of reasonable inferences by means of which our present, albeit highly imperfect, view of the functional organization of the brain can be reconciled with the possibility of its acquiring, storing and retrieving nervous information by encoding such information in molecules of nucleic acid or protein." -- Molecular geneticist G. S. Stent, quoted in the paper here. 
• "Up to this point, we still don’t understand how we maintain memories in our brains for up to our entire lifetimes.”  --neuroscientist Sakina Palida.
• "The available evidence makes it extremely unlikely that synapses are the site of long-term memory storage for representational content (i.e., memory for 'facts'’ about quantities like space, time, and number)." --Samuel J. Gershman,  "The molecular memory code and synaptic plasticity: A synthesis."
• "Synapses are signal conductors, not symbols. They do not stand for anything. They convey information bearing signals between neurons, but they do not themselves convey information forward in time, as does, for example, a gene or a register in computer memory. No specifiable fact about the animal’s experience can be read off from the synapses that have been altered by that experience.” -- Two scientists, "Locating the engram: Should we look for plastic synapses or information- storing molecules?
• " If I wanted to transfer my memories into a machine, I would need to know what my memories are made of. But nobody knows." -- neuroscientist Guillaume Thierry (link). 
• "While a lot of studies have focused on memory processes such as memory consolidation and retrieval, very little is known about memory storage" -- scientific paper (link).
• "While LTP is assumed to be the neural correlate of learning and memory, no conclusive evidence has been produced to substantiate that when an organism learns LTP occurs in that organism’s brain or brain correlate."  -- PhD thesis of a scientist, 2007 (link). 
• "Memory retrieval is even more mysterious than storage. When I ask if you know Alex Ritchie, the answer is immediately obvious to you, and there is no good theory to explain how memory retrieval can happen so quickly." -- Neuroscientist David Eagleman.
• "How could that encoded information be retrieved and transcribed from the enduring structure into the transient signals that carry that same information to the computational machinery that acts on the information?....In the voluminous contemporary literature on the neurobiology of memory, there is no discussion of these questions."  ---  Neuroscientists C. R. Gallistel and Adam Philip King, "Memory and the Computational Brain: Why Cognitive Science Will Transform Neuroscience,"  preface. 
• "The very first thing that any computer scientist would want to know about a computer is how it writes to memory and reads from memory....Yet we do not really know how this most foundational element of computation is implemented in the brain."  -- Noam Chomsky and Robert C. Berwick, "Why Only Us? Language and Evolution," page 50. 
• "When we are looking for a mechanism that implements a read/write memory in the nervous system, looking at synaptic strength and connectivity patterns might be misleading for many reasons...Tentative evidence for the (classical) cognitive scientists' reservations toward the synapse as the locus of memory in the brain has accumulated....Changes in synaptic strength are not directly related to storage of new information in memory....The rate of synaptic turnover in absence of learning is actually so high that the newly formed connections (which supposedly encode the new memory) will have vanished in due time. It is worth noticing that these findings actually are to be expected when considering that synapses are made of proteins which are generally known to have a short lifetime...Synapses have been found to be constantly turning over in all parts of cortex that have been examined using two-photon microscopy so far...The synapse is probably an ill fit when looking for a basic memory mechanism in the nervous system." -- Scientist Patrick C. Trettenbrein, "The Demise of the Synapse As the Locus of Memory: A Looming Paradigm Shift? (link).
• "Most neuroscientists believe that memories are encoded by changing the strength of synaptic connections between neurons....Nevertheless, the question of whether memories are stored locally at synapses remains a point of contention. Some cognitive neuroscientists have argued that for the brain to work as a computational device, it must have the equivalent of a read/write memory and the synapse is far too complex to serve this purpose (Gaallistel and King, 2009; Trettenbrein, 2016). While it is conceptually simple for computers to store synaptic weights digitally using their read/write capabilities during deep learning, for biological systems no realistic biological mechanism has yet been proposed, or in my opinion could be envisioned, that would decode symbolic information in a series of molecular switches (Gaallistel and King, 2009) and then transform this information into specific synaptic weights." -- Neuroscientist Wayne S. Sossin (link).
• "We take up the question that will have been pressing on the minds of many readers ever since it became clear that we are profoundly skeptical about the hypothesis that the physical basis of memory is some form of synaptic plasticity, the only hypothesis that has ever been seriously considered by the neuroscience community. The obvious question is: Well, if it’s not synaptic plasticity, what is it? Here, we refuse to be drawn. We do not think we know what the mechanism of an addressable read/write memory is, and we have no faith in our ability to conjecture a correct answer."  -- Neuroscientists C. R. Gallistel and Adam Philip King, "Memory and the Computational Brain Why Cognitive Science Will Transform Neuroscience."  page Xvi (preface). 
• "Current theories of synaptic plasticity and network activity cannot explain learning, memory, and cognition."  -- Neuroscientist Hessameddin Akhlaghpourƚ (link). 
• "It remains unclear where and how prior knowledge is represented in the brain." -- A large team of scientists, 2025 (link). 
• "How memory is stored in the brain is unknown." -- Research proposal abstract written by scientists, 2025 (link). 
• "We don’t know how the brain stores anything, let alone words." -- Scientists David Poeppel and, William Idsardi, 2022 (link).
• "If we believe that memories are made of patterns of synaptic connections sculpted by experience, and if we know, behaviorally, that motor memories last a lifetime, then how can we explain the fact that individual synaptic spines are constantly turning over and that aggregate synaptic strengths are constantly fluctuating? How can the memories outlast their putative constitutive components?" --Neuroscientists Emilio Bizzi and Robert Ajemian (link).
• "After more than 70 years of research efforts by cognitive psychologists and neuroscientists, the question of where memory information is stored in the brain remains unresolved." -- Psychologist James Tee and engineering expert Desmond P. Taylor, "Where Is Memory Information Stored in the Brain?"
• "There is no such thing as encoding a perception...There is no such thing as a neural code...Nothing that one might find in the brain could possibly be a representation of the fact that one was told that Hastings was fought in 1066." -- M. R.  Bennett, Professor of Physiology at the University of Sydney (link).
• "No sense has been given to the idea of encoding or representing factual information in the neurons and synapses of the brain." -- M. R. Bennett, Professor of Physiology at the University of Sydney (link).
• ""Despite over a hundred years of research, the cellular/molecular mechanisms underlying learning and memory are still not completely understood. Many hypotheses have been proposed, but there is no consensus for any of these."  -- Two scientists in a 2024 paper (link). 
• "We have still not discovered the physical basis of memory, despite more than a century of efforts by many leading figures. Researchers searching for the physical basis of memory are looking for the wrong thing (the associative bond) in the wrong place (the synaptic junction), guided by an erroneous conception of what memory is and the role it plays in computation." --Neuroscientist C.R. Gallistel, "The Physical Basis of Memory," 2021.
• "To name but a few examples, the formation of memories and the basis of conscious  perception, crossing  the threshold  of  awareness, the  interplay  of  electrical  and  molecular-biochemical mechanisms of signal transduction at synapses, the role of glial cells in signal transduction and metabolism, the role of different brain states in the life-long reorganization of the synaptic structure or  the mechanism of how  cell  assemblies  generate a  concrete  cognitive  function are  all important processes that remain to be characterized." -- "The coming decade of digital brain research, a 2023 paper co-authored by more than 100 neuroscientists, one confessing scientists don't understand how a brain could store memories. 
• "The human brain isn’t really empty, of course. But it does not contain most of the things people think it does – not even simple things such as ‘memories’....We don’t create representations of visual stimuli, store them in a short-term memory buffer, and then transfer the representation into a long-term memory device. We don’t retrieve information or images or words from memory registers. Computers do all of these things, but organisms do not." -- Robert Epstein,  senior research psychologist, "The Empty Brain." 
• "Despite recent advancements in identifying engram cells, our understanding of their regulatory and functional mechanisms remains in its infancy." -- Scientists claiming erroneously in 2024 that there have been recent advancements in identifying engram cells, but confessing there is no understanding of how they work (link).
• "Study of the genetics of human memory is in its infancy though many genes have been investigated for their association to memory in humans and non-human animals."  -- Scientists in 2022 (link).
• "The neurobiology of memory is still in its infancy." -- Scientist in 2020 (link). 
• "The investigation of the neuroanatomical bases of semantic memory is in its infancy." -- 3 scientists, 2007 (link). 
• "Currently, our knowledge pertaining to the neural construct of intelligence and memory is in its infancy." -- Scientists, 2011 (link). 
•  "Very little is known about the underlying mechanisms for visual recognition memory."  -- two scientists (link). 
• "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."  -- Two scientists in 2022 (link). 
• "The mechanisms governing successful episodic memory formation, consolidation and retrieval remain elusive,"  - Bogdan Draganski, cogntive neuroscientist (link). 
• " The mechanisms underlying the formation and management of the memory traces are still poorly understood." -- Three scientists in 2023 (link). 
• "The underlying electrophysiological processes underlying memory formation and retrieval in humans remains very poorly understood." --  A scientist in 2021 (link). 
• "As for the explicit types of memory, the biological underpinning of this very long-lasting memory storage is not yet understood." -- Neuroscientist Cristina M. Alberini in a year 2025 paper (link). 

New Poll Suggests About 29% of Neuroscientists Doubt the Synaptic Memory Theory

 Although it is often claimed that memories are stored in the brain (specifically in synapses), there is no place in the brain that is a plausible storage site for human memories that can last for 50 years or longer. The proteins that make up both synapses and dendritic spines are quite short-lived, being subject to very high molecular turnover which gives them an average lifetime of only a few weeks 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).  

Both synapses and dendritic spines are a “shifting sands” substrate absolutely unsuitable for storing memories that last reliably for decades. Synapses are connected to dendritic spines, which have short lifetimes. 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). 

So it should be doubly-clear that synapses cannot store memories that can last for decades. Similarly there are two reasons why information would not last long if written on maple leaves outdoors: (1) the fact that maple leaves decay after a few months, and (2) the fact that the wind tends to blow away leaves lying outdoors. 

If humans were storing their memories in brains, there would have to be a fantastically complex translation system (almost infinitely more complicated than the ASCII code or the genetic code) by which mental concepts, words, images and episodic memories are translated into neural states. But no trace of any such system has ever been found, no one has given a credible detailed theory of how it could work, and if it existed it would be a “miracle of design” that would be naturally inexplicable.

If human brains actually stored conceptual and experiential memories, the human brain would have to have both a write mechanism by which exact information can be precisely written, and a read mechanism by which exact information can be precisely read. The brain seems to have neither of these things. There is nothing in the brain similar to the “read-write” heads found in computers.  

If memories were to be stored in a brain, it would take you "ages" to retrieve an answer to a question, because brains are totally lacking in any of the things that make fast retrieval possible: sorting, addresses and indexes. The brain has no type of addresses or coordinates or indexes. The brain has a structure in which neurons are rooted in place like trees in a forest, and synapses are rooted in place like the roots of trees in a forest. With such a physical arrangement, sorting is impossible. 

 So if memories were stored in brains, you would have to suffer the most ridiculously long delays every time you wanted to retrieve knowledge or a memory. 

As discussed here, humans can form new memories instantly, at a speed much faster than would be possible if we were using our brains to store such memories. It is typically claimed that memories are stored by “synapse strengthening” and protein synthesis, but such things are relatively sluggish processes that do not work fast enough to explain the formation of memories that can occur instantly.

For decades microscopes have been powerful enough to detect memories in brains, if memories existed in brains. Very much brain tissue has been studied by the most powerful microscopes: both brain tissue extracting from living patients, and brain tissue extracted from someone very soon after he died. Very many thousands of brains preserved soon after death have been microscopically examined.  Microscopes now allow us to see very clearly what is in the tiniest brain structures such as dendritic spines and synapse heads. But microscopic examination of brain tissue has failed to reveal any trace whatsoever of learned information in a brain.  No one has found a single letter of the alphabet stored in a brain; no has found a single number stored in a brain; and no one has ever found even a single pixel of something someone saw a day or more before.  If memories were stored in human brains, microscopes would have revealed decisive evidence of such a thing decades ago.  But no such evidence has appeared. 

There is nothing in the brain that looks like learned information stored according to some systematic format that humans understand or do not understand. Even when scientists cannot figure out a code used to store information, they often can detect hallmarks of encoded information. For example, long before Europeans were able to decipher how hieroglyphics worked, they were able to see a repetition of symbolic tokens that persuaded them that some type of coding system was being used. Nothing like that can be seen in the brain. We see zero signs that synapses or dendritic spines are any such things as encoded information. 

We know that human memory recall can occur massively with complete accuracy. There are numerous cases of people who memorized with complete accuracy the text of books of hundreds of pages. But synapses do not reliably transmit information. Scientists have repeatedly told us that an individual synapse will transmit a nerve signal with a reliability of 50% or less. So every time a nerve signal crosses a synapse, it is a coin flip as to whether that signal will be successfully carried across the synapse gap. So how could memories could ever be reliably retrieved from synapses? That would require a gigantic number of traversals across synaptic gaps, with a 50% chance of failure during each such traversal. You could never get perfect recall of large bodies of text from such a state of affairs, or even recall that was 10% accurate. 

The theory that human memories that can last for 50 years and can be instantly recalled are stored in synapses is a theory that contradicts pretty much everything we know about systems that can permanently store and instantly retrieve information, and contradicts everything we know about synapses, and contradicts everything we observe about the best examples of human memory performance. You could reasonably compare such a theory to the theory that certain clouds in the sky are nuclear missile bases set up by the Swiss to threaten your nation. That would pretty much contradict everything we know about the Swiss, everything we know about clouds, and everything we know about nuclear missile bases. 

But, you may say, "We should trust the theory of synaptic memory because all the neuroscientists believe in it." Do they really? We don't know that at all. To determine what percentage of neuroscientists believe the dogmas typically stated by neuroscientists, you need well-designed secret ballot opinion polls; and such polls are almost never done.  A recent study attempted to poll neuroscientists on their beliefs about memory storage.  The study (which you can read here) is entitled "What are memories made of? A survey of neuroscientists on the structural basis of long-term memory." 

Unfortunately the study fails to follow some of the main principles that should be followed by any study attempting to do an opinion poll of scientists. Specifically:

(1) The survey was not a secret ballot survey. Scientists were sent emails encouraging them to participate in the survey. The survey form promised anonymity, but it seemed like an arrangement where the participants had to trust that those running the survey would not reveal how individual scientists had voted. Anyone responding might have suspected that there might be some way in which his responses could be identified publicly as coming from him, if there were some future  breach in the promised confidentiality. 

Imagine you are a scientist getting an email like this:

Hello, Professor Waterskein. We have here a survey we would like you to fill out, asking you about all kinds of controversial questions. Please send it back to us. Don't worry, we won't ever publicly reveal which answers you gave. We'll only publish the collected results from the entire group of respondents. 

Are you going to feel it is safe to speak your mind? Or, are you going to still fear that somehow your answers might get you into trouble if they are too candid? I think you might tend to "play it safe" by assuming the person who sent you this email (who you know nothing about) cannot be trusted. 

Devising a true secret-ballot opinion poll (as opposed to a "promised anonymity" opinion poll) requires some cleverness and ingenuity, which did not go on in this case. So we can't know how much the responses were affected by scientists thinking "I had better answer as they expect me to answer."

(2) Contrary to all good standards of properly doing opinion polls, the badly fumbling survey organizers wrote a survey form in which the survey questions are preceded by statements strongly tending to bias  respondents towards a particular type of answer.  The survey form is found here. On page 4 of the survey form, before any questions are asked, we have the statement, "Memories are not standalone entities but are embedded within the complex network structure of the brain." Such a statement precedes questions about the nature of memory creation. But the statement "stacks the deck" in favor of a particular type of answer that could be given to one or more of the questions later asked. 

The clumsiness here is very big. It is a cardinal rule of serious polling that you should not precede questions with statements tending to yield particular types of responses to that questions. So, for example, if you are doing an objective poll about Presidential Candidate William Tygersoll, you absolutely should not precede your questions about this person with a statement such as "Here are some questions about that great American hero and patriot William Tygersoll."  Or, to give another example, if you polling people about some scandal involving this candidate, you absolutely should not be preceding your questions by some statement such as "Many are deeply upset about the scandal involving William Tygersoll. We would like to ask you your opinion."

Equally bad is that the survey conductors have stated on page 4 of their survey form this untrue claim: "Some studies have already demonstrated the ability to decode simple information such as visual field from brain maps (e.g. Scholl et. al. 2020." The reference is to the "no real evidence at all" study here involving a way-too-small sample size of only three monkeys. The quoted claim is an untrue one. The paper mentioned did not "demonstrate the ability to decode simple information such as visual field from brain maps."

Once again the scientists conducting the survey have violated the first rule of conducting a survey, which is "do not do anything to pre-sell a particular answer."  Here is Question 11 that appears in the survey form:

"11.Some neuroscientists have suggested that while molecular and subcellular details play a role, the majority of information for long-term memories is likely physically stored in the brain at the level of neuronal connectivity patterns and ensembles of synaptic strengths (e.g. Poo et al., 2016).

To what extent do you agree with the following statement: ' The structural basis of long-term memories primarily consists of lasting changes in neuronal connectivity and ensembles of synaptic strengths, rather than in molecular or subcellular details.'  "

This is a blunder from any standpoint of trying to objectively survey the opinion of scientists. We have a survey question that is preceded by a sentence pre-selling a particular answer to the survey question, as if the survey conductors were interested in pushing a particular response.  

Here are the responses the survey got from the question above asking neuroscientists about agreement with the following statement: "The structural basis of long-term memories primarily consists of lasting changes in neuronal connectivity and ensembles of synaptic strengths, rather than in molecular or subcellular details."

Strongly Agree:  51 respondents

Agree:  111 respondents

Not Sure: 37 respondents

Disagree:  26 respondents

Strongly Disagree: 5 respondents

So 68 out of 230 respondents (or about 29%) refused to endorse the synaptic theory of memory, even though the survey was strongly pre-selling such a theory. 13% flatly said they disagreed or strongly disagreed with such a theory.  We can only wonder how much higher that 29% figure would be if a true secret ballot had been used, and if the survey had followed proper standards of opinion surveying, such as not trying to pre-sell some answer to the questions it was asking. 

Clearly there is no consensus of neuroscientists about the theory that memories are stored in synapses. We are being misinformed when people try to suggest that such a consensus exists. A bad example of that type of misleading statement occurred in the paper "What is memory? The present state of the engram," a paper with many misstatements and many references to junk neuroscience studies failing to qualify as robust research. In that paper Mu-ming Poo stated, "There is now general consensus that persistent modification of the synaptic strength via LTP and LTD of pre-existing connections represents a primary mechanism for the formation of memory engrams." The new poll discussed above shows there is no such consensus. 

When someone tries to make a theory sound more popular than it is, they have done one of the bad deceits of science theory pitchmen. It's a deceit as old as the hills. It works by people trying to make some not-yet-triumphant theory gain more popularity by insinuating that almost everyone already believes in it. For a discussion of the trickery and equivocation and deceit that so often occurs in such cases, see my post "So Much Misleading Talk Occurs in Claims of a Scientific Consensus."

A huge problem with the question asking about long-term memory is that is worded as a choice between two different materialist ideas of memory, both of which assume that memories are stored in brains. 

A better-designed poll might have asked a question such as this:

"Which reflects your thinking:

• 'Human memories are stored mainly in synapses.'
• 'Human memories are stored by some other brain mechanism, perhaps something involving neurons or brain chemistry.'
• 'Most memories are not stored in brains, and human memory is mostly a spiritual, psychic or metaphysical phenomenon, or some other subtle reality different from information storage in brains.'
• 'I don't know/no answer.'

Given a question such as this, and also a secret ballot not requiring respondents to trust the confidentiality of those doing a survey,  I doubt whether even 60% of neuroscientists would choose the first answer. 

The poll discussed above also shows us that most of those professing belief in the synaptic theory of memory lack a strong confidence in it. When given a set of poll choices allowing you to choose "agree" or "strongly agree," a mere 22% of the respondents chose to say that they "strongly agree" with the theory of synaptic memory storage. 

I have been generously referring above to a synaptic theory of memory, although it is probably more accurate to say that such a thing is not even a theory, but merely a small group of vague, vacuous jargon phrases repeated by scientists who have yet to develop a real theory on this topic. It's really a "there's no there there" situation. 

Given the huge diversity of the types of things that human can remember, an actual theory of neural memory encoding would require maybe 1,000,000 times more effort than involved in the production of the diagram above. 

Below are some relevant quotes, all statements by scientists:

• "Direct evidence that synaptic plasticity is the actual cellular mechanism for human learning and memory is lacking." -- 3 scientists, "Synaptic plasticity in human cortical circuits: cellular mechanisms of learning and memory in the human brain?" 
• "How the brain stores and retrieves memories is an important unsolved problem in neuroscience." --Achint Kumar, "A Model For Hierarchical Memory Storage in Piriform Cortex." 
• "We are still far from identifying the 'double helix' of memory—if one even exists. We do not have a clear idea of how long-term, specific information may be stored in the brain, into separate engrams that can be reactivated when relevant."  -- Two scientists, "Understanding the physical basis of memory: Molecular mechanisms of the engram."
• "There is no chain of reasonable inferences by means of which our present, albeit highly imperfect, view of the functional organization of the brain can be reconciled with the possibility of its acquiring, storing and retrieving nervous information by encoding such information in molecules of nucleic acid or protein." -- Molecular geneticist G. S. Stent, quoted in the paper here. 
• "Up to this point, we still don’t understand how we maintain memories in our brains for up to our entire lifetimes.”  --neuroscientist Sakina Palida.
• "The available evidence makes it extremely unlikely that synapses are the site of long-term memory storage for representational content (i.e., memory for 'facts'’ about quantities like space, time, and number)." --Samuel J. Gershman,  "The molecular memory code and synaptic plasticity: A synthesis."
• "Synapses are signal conductors, not symbols. They do not stand for anything. They convey information bearing signals between neurons, but they do not themselves convey information forward in time, as does, for example, a gene or a register in computer memory. No specifiable fact about the animal’s experience can be read off from the synapses that have been altered by that experience.” -- Two scientists, "Locating the engram: Should we look for plastic synapses or information- storing molecules?
• " If I wanted to transfer my memories into a machine, I would need to know what my memories are made of. But nobody knows." -- neuroscientist Guillaume Thierry (link). 
• "While a lot of studies have focused on memory processes such as memory consolidation and retrieval, very little is known about memory storage" -- scientific paper (link).
• "While LTP is assumed to be the neural correlate of learning and memory, no conclusive evidence has been produced to substantiate that when an organism learns LTP occurs in that organism’s brain or brain correlate."  -- PhD thesis of a scientist, 2007 (link). 
• "Memory retrieval is even more mysterious than storage. When I ask if you know Alex Ritchie, the answer is immediately obvious to you, and there is no good theory to explain how memory retrieval can happen so quickly." -- Neuroscientist David Eagleman.
• "How could that encoded information be retrieved and transcribed from the enduring structure into the transient signals that carry that same information to the computational machinery that acts on the information?....In the voluminous contemporary literature on the neurobiology of memory, there is no discussion of these questions."  ---  Neuroscientists C. R. Gallistel and Adam Philip King, "Memory and the Computational Brain: Why Cognitive Science Will Transform Neuroscience,"  preface. 
• "The very first thing that any computer scientist would want to know about a computer is how it writes to memory and reads from memory....Yet we do not really know how this most foundational element of computation is implemented in the brain."  -- Noam Chomsky and Robert C. Berwick, "Why Only Us? Language and Evolution," page 50. 
• "When we are looking for a mechanism that implements a read/write memory in the nervous system, looking at synaptic strength and connectivity patterns might be misleading for many reasons...Tentative evidence for the (classical) cognitive scientists' reservations toward the synapse as the locus of memory in the brain has accumulated....Changes in synaptic strength are not directly related to storage of new information in memory....The rate of synaptic turnover in absence of learning is actually so high that the newly formed connections (which supposedly encode the new memory) will have vanished in due time. It is worth noticing that these findings actually are to be expected when considering that synapses are made of proteins which are generally known to have a short lifetime...Synapses have been found to be constantly turning over in all parts of cortex that have been examined using two-photon microscopy so far...The synapse is probably an ill fit when looking for a basic memory mechanism in the nervous system." -- Scientist Patrick C. Trettenbrein, "The Demise of the Synapse As the Locus of Memory: A Looming Paradigm Shift? (link).
• "Most neuroscientists believe that memories are encoded by changing the strength of synaptic connections between neurons....Nevertheless, the question of whether memories are stored locally at synapses remains a point of contention. Some cognitive neuroscientists have argued that for the brain to work as a computational device, it must have the equivalent of a read/write memory and the synapse is far too complex to serve this purpose (Gaallistel and King, 2009; Trettenbrein, 2016). While it is conceptually simple for computers to store synaptic weights digitally using their read/write capabilities during deep learning, for biological systems no realistic biological mechanism has yet been proposed, or in my opinion could be envisioned, that would decode symbolic information in a series of molecular switches (Gaallistel and King, 2009) and then transform this information into specific synaptic weights." -- Neuroscientist Wayne S. Sossin (link).
• "We take up the question that will have been pressing on the minds of many readers ever since it became clear that we are profoundly skeptical about the hypothesis that the physical basis of memory is some form of synaptic plasticity, the only hypothesis that has ever been seriously considered by the neuroscience community. The obvious question is: Well, if it’s not synaptic plasticity, what is it? Here, we refuse to be drawn. We do not think we know what the mechanism of an addressable read/write memory is, and we have no faith in our ability to conjecture a correct answer."  -- Neuroscientists C. R. Gallistel and Adam Philip King, "Memory and the Computational Brain Why Cognitive Science Will Transform Neuroscience."  page Xvi (preface). 
• "Current theories of synaptic plasticity and network activity cannot explain learning, memory, and cognition."  -- Neuroscientist Hessameddin Akhlaghpourƚ (link). 
• "How memory is stored in the brain is unknown." -- Research proposal abstract written by scientists, 2025 (link). 
• "We don’t know how the brain stores anything, let alone words." -- Scientists David Poeppel and, William Idsardi, 2022 (link).
• "If we believe that memories are made of patterns of synaptic connections sculpted by experience, and if we know, behaviorally, that motor memories last a lifetime, then how can we explain the fact that individual synaptic spines are constantly turning over and that aggregate synaptic strengths are constantly fluctuating? How can the memories outlast their putative constitutive components?" --Neuroscientists Emilio Bizzi and Robert Ajemian (link).
• "After more than 70 years of research efforts by cognitive psychologists and neuroscientists, the question of where memory information is stored in the brain remains unresolved." -- Psychologist James Tee and engineering expert Desmond P. Taylor, "Where Is Memory Information Stored in the Brain?"
• "There is no such thing as encoding a perception...There is no such thing as a neural code...Nothing that one might find in the brain could possibly be a representation of the fact that one was told that Hastings was fought in 1066." -- M. R.  Bennett, Professor of Physiology at the University of Sydney (link).
• "No sense has been given to the idea of encoding or representing factual information in the neurons and synapses of the brain." -- M. R. Bennett, Professor of Physiology at the University of Sydney (link).
• ""Despite over a hundred years of research, the cellular/molecular mechanisms underlying learning and memory are still not completely understood. Many hypotheses have been proposed, but there is no consensus for any of these."  -- Two scientists in a 2024 paper (link). 
• "We have still not discovered the physical basis of memory, despite more than a century of efforts by many leading figures. Researchers searching for the physical basis of memory are looking for the wrong thing (the associative bond) in the wrong place (the synaptic junction), guided by an erroneous conception of what memory is and the role it plays in computation." --Neuroscientist C.R. Gallistel, "The Physical Basis of Memory," 2021.
• "To name but a few examples, the formation of memories and the basis of conscious  perception, crossing  the threshold  of  awareness, the  interplay  of  electrical  and  molecular-biochemical mechanisms of signal transduction at synapses, the role of glial cells in signal transduction and metabolism, the role of different brain states in the life-long reorganization of the synaptic structure or  the mechanism of how  cell  assemblies  generate a  concrete  cognitive  function are  all important processes that remain to be characterized." -- "The coming decade of digital brain research, a 2023 paper co-authored by more than 100 neuroscientists, one confessing scientists don't understand how a brain could store memories. 
• "The human brain isn’t really empty, of course. But it does not contain most of the things people think it does – not even simple things such as ‘memories’....We don’t create representations of visual stimuli, store them in a short-term memory buffer, and then transfer the representation into a long-term memory device. We don’t retrieve information or images or words from memory registers. Computers do all of these things, but organisms do not." -- Robert Epstein,  senior research psychologist, "The Empty Brain." 
• "Despite recent advancements in identifying engram cells, our understanding of their regulatory and functional mechanisms remains in its infancy." -- Scientists claiming erroneously in 2024 that there have been recent advancements in identifying engram cells, but confessing there is no understanding of how they work (link).
• "Study of the genetics of human memory is in its infancy though many genes have been investigated for their association to memory in humans and non-human animals."  -- Scientists in 2022 (link).
• "The neurobiology of memory is still in its infancy." -- Scientist in 2020 (link). 
• "The investigation of the neuroanatomical bases of semantic memory is in its infancy." -- 3 scientists, 2007 (link). 
• "Currently, our knowledge pertaining to the neural construct of intelligence and memory is in its infancy." -- Scientists, 2011 (link). 
•  "Very little is known about the underlying mechanisms for visual recognition memory."  -- two scientists (link). 
• "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."  -- Two scientists in 2022 (link). 
• "The mechanisms governing successful episodic memory formation, consolidation and retrieval remain elusive,"  - Bogdan Draganski, cogntive neuroscientist (link). 
• " The mechanisms underlying the formation and management of the memory traces are still poorly understood." -- Three scientists in 2023 (link). 
• "The underlying electrophysiological processes underlying memory formation and retrieval in humans remains very poorly understood." --  A scientist in 2021 (link). 
• "As for the explicit types of memory, the biological underpinning of this very long-lasting memory storage is not yet understood." -- Neuroscientist Cristina M. Alberini in a year 2025 paper (link).