Large Neuron Groups Cannot Be Sequentially Traversed, So Brains Cannot Store Complex Learned Information

 Humans store information in many different ways, but there is a common element in almost every way that data is stored and retrieved: the element of sequential traversal. Sequential traversal means some proceeding from a current reading position to the next reading position. 

Sequential Traversal: A Crucial Aspect of Most Types of Information Retrieval

Let us look at some examples of how sequential traversal is going on in various types of information storage and information retrieval. A simple example is a book. A book is physically arranged in a way to allow a sequential traversal of its words, On a particular page, we see letters and words arranged in a particular sequence, so that the reader can read from the top left of a page to the bottom right of the page. The reader turns the pages to go from a page on the right to the next page, which is another type of sequential traversal.  The arrangement that allows sequential traversal is a crucial aspect allowing the book's information to be read.  A book would be unreadable if all of it words were just lying as tiny scraps of paper in a heap in a can the size of a large trash can. Under such an arrangement, there could be no sequential traversal, and reading the book would be impossible. 

A web page also depends crucially on sequential traversal.  When you go to a web page you see words arranged in a sequential order. You read the web page as someone would read a page of a book, from the top left to the bottom right. If the words on the page were in some random order, you could not read the page. 

Sequential traversal is also a key element of movies. When you see a movie in a movie theater, you are seeing a series of individual photos (called frames) which are sequentially displayed at a rate of 24 photos or frames per second. The physical design of the roll of film and the film projector guarantee that the frames of the movie are displayed in a particular sequential order.  There occurs sequential traversal at a steady rate, from the time the movie begins to the time the movie ends. 

Something very similar is going on when a video is displayed on a computer. The video actually consists of a series of photos, with the total number of photos being equal to about the length of the video in seconds multiplied by 24. You can use a utility such as ffmpeg to extract all of the individual photos that make up a video. When you press the Play button to play a video, there occurs sequential traversal from the beginning of the video to the end. You can stop the sequential traversal by pressing the Pause button. 

When you watch something on television, it is usually an affair of sequential traversal.  A typical TV show that is not "live" is made from a tape that is a series of photos collected by a TV camera photographing at about 24 times per second.  When a TV station plays a program at a scheduled date, a stored series of images and sounds is sequentially traversed and broadcast.  

When you access some TV show "on demand," the same sequential traversal is occurring. A live TV broadcast may not involve such a sequential traversal. There may be a continuous stream of images and sounds directly from some TV camera, without the data ever being first stored and then sequentially retrieved. But if such a broadcast is never taped, then it does not qualify as a retrieval of stored information. So such live TV broadcasts do not discredit my claim that almost all retrieval of stored information requires sequential traversal. 

On your computer you may retrieve particular documents and images. When that happens, it is very much a case of sequential traversal. When you retrieve from the hard drive of your computer some text document that you or someone else wrote, your computer finds some particular start position, and then sequentially traverses until the end of the document is reached. When you retrieve some image from your computer, something similar goes on. Typically the computer reads from some starting position on the hard drive, and then sequentially traverses the file, reading a stream of pixels, until the end of the file is reached. Rather than displaying that stream of pixels as a single line (something that would be too long to fit on your computer screen), you see the long series of pixels displayed using a rule such as "when you have filled up a line of 400 pixels, move down one line, and to the left edge, and then resume writing pixels from there." 

Sequential traversal also occurs in the retrieval of musical information.  The now-outdated technology of vinyl records used sequential traversal. To play a 33 rpm album from beginning to end, you would place the needle on a spot near the edge of the album, and the record player would rotate around and around in a circle. Sequential traversal would occur until the end of the album was reached. 

Sequential traversal would also occur when cassette tapes were played. The tape would be placed in a machine that would cause the tape to slowly move, and as that happened the tape would be sequentially traversed. 

In a live music performance such as the performance of a symphony or opera, there also occurs sequential traversal. A musical score is printed in a book or publication placed in front of musicians, and the musicians sequentially traverse such a manuscript, acting like readers of a book, and turning the pages. 

Sequential traversal was a key aspect of the videotapes that were used for decades to store television shows. A VCR tape would be wound like the thread wound around a spool.  As the tape played, the units at the center of the tape would be rotated, causing a sequential traversal of the tape in front of some reading unit. Once a tape had been played to its end, someone would need to rewind to its beginning so that the tape be watched again. So stores renting videotapes had a slogan of "be kind and rewind."

In the world of computer programming, the retrieval of data almost everywhere requires sequential traversal. For example:

(1) A basic type of data structure in computer programming is an array, which has all data stored in a contiguous series  Such a structure is sequentially traversed through operations such as "for" loops, which move sequentially from the first position in the array to the last position in the array. 

(2) Another very common data structure in computer programming is called a linked list. The data in a linked list may be discontinuous, with the data scattered around in different positions.  But each position must have a particular position.  The data in a linked list is sequentially traversed by using a programming loop. Even though the data is scattered, the whole list can be retrieved because each node in the list has the address of the next node in the list. 

In the world of biology, we know for sure of one case in which data is retrieved: the case of DNA. A DNA molecule has a linear structure that allows sequential retrieval, a structure that has been compared to a spiral staircase.  We know that when data is read from a DNA molecule, it is very much a case of sequential traversal, moving in one direction from one position in the molecule to another position further down the line in the same direction. 

Can we think of any kind of data storage and retrieval that does not involve sequential traversal?  There are a few. For example, a painter can paint a picture in a way that does not involve sequential traversal. Rather than moving in a "top to down, and left to right" manner, an artist can fill in features of the painting in a random order. And when someone looks at the picture, he can take it all in a single glance, rather than scanning the picture from top to bottom.  But cases such as these (involving data storage and retrieval without sequential traversal) are relatively rare compared to cases of data storage and retrieval using sequential traversal; and such cases involving no sequential traversal require very specific tools such as paint, a paintbrush and a canvas, tools that are not available in the brain. 

Why Large Neuron Groups Cannot Be Sequentially Traversed

So now that we have seen that almost all data storage and retrieval involves sequential traversal, we should consider the very important question: does the human brain have any physical architecture that might allow complex learned information to be stored and retrieved using sequential traversal?  The answer is: no, it does not. 

I could schematically depict a set of neurons with a visual like the one below. The little circles represent individual neurons. The diagram greatly understates the number of connections between individual neurons. It has been estimated that the average neuron has a synaptic connection to about 1000 other neurons. 

Consider the recall of sequential information. Imagine you are trying to recall a series of words. We might imagine that individual parts of the sequence are stored in individual neurons – perhaps something a little like the schematic visual below. 

But how could you recall the sequence in its correct order? Nerve cells are scattered throughout three dimensional space, with each neuron having many connections to other neurons (about 1000, according to many estimates). If information were stored in nerve cells, there would seem to be no way for a sequence to be stored in a way that would allow a sequential recall involving a long series, such as happens when an actor playing Hamlet recalls all of his many lines in the correct order. We can't imagine the brain simply going from one neuron to the “next” neuron to retrieve a sequence of information. This is because neurons don't exist in chains in which a particular neuron has a “next” neuron. Each neuron is connected to very many other neurons.

Below we see a map of Dupont Circle in Washington D.C.

 

Once your car gets on Dupont Circle, there is no “next” place to go. You've reached an interchange in which there are 10 roads feeding out of the circular interchange. Similarly, in the photos below we see neurons. Each one of the parts coming out of the nerve cell is a path that can be traversed from this nerve cell. Such an arrangement should not offer any support for storing a sequence of information such as the lines in a play or the notes in a song. There's no “next” route leading from one neuron to the next neuron. Every neuron is like Dupont Circle, except that there are even more paths leading out of the typical neuron.

Here are some shots from the page here (part of the site here) which allows you to rotate in 3D some neurons that had their structure mapped. We see in the first image seven dendrites branching out from the body of a neuron; and those dendrites each branch out into multiple branches, so that there is a connection between each neuron and many other neurons. 

With such an arrangement there is no possibility of sequential traversal across a long sequence of neurons. There's no “next” route leading from one neuron to the next neuron. There are very many pathways leading from each neuron to some other neuron.  A neuron will typically have 5-7 main dendrites leading out of it, but those each branch out into many other other branches, as illustrated below, with very many of those dendrites connecting to other neurons. 

As I indicated before, for a sequential traversal to occur, it is not necessary for there to be a traversal of storage positions that are contiguous in space. There are data structures such as linked lists that allow long series to be sequentially retrieved even though the data may be in scattered positions that are not physically contiguous.  But such data structures not requiring contiguous positioning absolutely require that the scattered data elements have addresses. And there are no addresses in the brain. Neurons do not have neuron numbers or position coordinates or anything like addresses. Therefore you cannot have in the brain anything like a linked list that can be sequentially traversed to retrieve information from a series of neurons that are not contiguous. Linked lists require addresses for each of the nodes in the list, but neurons have no addresses. 

We can imagine a structure of some brain of an extraterrestrial creature, one that might support sequential traversal. Such an extraterrestrial's brain might have very many chains like the one below, with the circles being something like cells or neurons. The chains might be very long. It might then be possible for stored information to be sequentially traversed, by some process by which a cursor or reading unit travels along the chain. 

This is NOT how neurons are arranged in the brain

But the human has no such structure. Instead of each neuron having a single "next neuron" as the next link in a chain pointing in one direction, every neuron has very many "next neurons" located in all different directions, as the photos above suggest. Under such an arrangement there can be no sequential traversal. 

There is another reason why large groups of neurons cannot be sequentially traversed.  Sequential traversal requires some type of physical movement action that causes a particular position to temporarily become the current position. But in a brain no such thing can happen. There is no possibility of any neuron becoming the "current neuron" in an act of sequential traversal. 

Below is a table showing some of the ways in which the current reading position changes in devices in which sequential traversal occurs:

Type of sequential traversal device	What causes the current reading position to change	Is this cause microscopic?
33 RPM vinyl record and a record player	The record rotates in a circle, causing the needle's position to change	No
Cassette audio tape and cassette player	The spools of the tape player turn around in a circle, causing the part of the tape next to the reader to change continually	No
Video tape and VCR	The spools of the tape player turn around in a circle, causing the part of the tape next to the reader to change continually	No
Book	The reader's eyes move left to right, and to the next line when the end of each line is reached. Also the reader uses his finger to turn the page when the end of a page is reached.	No
Movie and film projector	The film projector causes the film spool to turn in a circle, which causes the part of the film next to the light to continually change	No
Computer hard drive	The hard drive rotates in a circle, and also the read/write head may move to a different position	No
DNA	During DNA transcription, part of the DNA is sequentially traversed by a complex RNA polymerase enzyme, and copied into an RNA transcript molecule. As the RNA polymerase molecule moves along the linear chain of the DNA molecule, the current reading position changes.	Yes

Could anything like this be happening in the brain? Nothing like the first six things mentioned can be happening in the brain, because the brain has no moving parts except for microscopic molecular parts. Could anything like the last row be happening in the brain? No sign of any such thing can be found. 

We can start to imagine the beginnings of how such a thing might work. We can imagine that there is some "reader molecule" comparable to RNA polymerase, one that has the job of traversing neurons to read some particular learned information, such as the opening lines of Shakespeare's "To Be or Not to Be" soliloquy. We can imagine that such a molecule might gather more and more information as it traverses a group of neurons. But no such molecule has been found. If it existed, it would have been discovered about the same time that RNA polymerase was discovered, around 1959. 

And there are several reasons why such a reader molecule traversing many neurons cannot exist. The first reason involves the "no next neuron" issue discussed above. The RNA polymerase molecule can gather a longer and longer RNA transcript as it reads more and more of a part of DNA. The RNA polymerase molecule can find the next spot to move by simply moving further in the same direction, moving farther down the chain. But given the physical structure of neurons, as suggested in the photos above,  nothing similar can happen. For each neuron there is no "next neuron." Every neuron is connected to very many other neurons, supposedly something like 1000. So sequential traversal cannot occur to get a particular sequence of stored information. 

There is another "show stopper" here, the fact that synapses do not reliably transmit information. Tests have shown that synapses only transmit signals with a reliability of 50% or less. So a "reader molecule" trying to traverse a set of neurons to gather information would never be able to reliably extract information from some sequence of neurons. Whenever such a molecule reached a particular neuron, it would be like someone in the circular roundabout at  Dupont Circle (shown above) where are there ten different directions to go to. Except that it would be 100 times worse, because coming from  every neuron there would be a thousand different synaptic connections, each leading to different neurons, making it impossible to retrieve sequential information by sequential traversal. 

When I search for "average distance between neurons in micrometers," I get an answer of around 20-50 micrometers. When I search for "how far do neurotransmitters travel" I get an answer of "neurotransmitters travel a distance of tens to hundreds of micrometers." A neuroscience textbook says this:

"In contrast, the distance over which neurotransmitters act is miniscule. At many synapses, transmitters bind only to receptors on the postsynaptic cell that directly underlies the presynaptic terminal (Figure 6.2A); in such cases, the transmitter acts over distances less than a micrometer. Even when neurotransmitters diffuse locally to alter the electrical properties of multiple postsynaptic (and sometimes presynaptic) cells in the vicinity (Figure 6.2B), they act only over distances of tens to hundreds of micrometers."

It seems, therefore, that there is no complex molecule transmitted over any long sequence of neurons (such as twenty neurons), a reality that just gives another strong reason for thinking that neuron groups cannot be sequentially traversed in any way that could involve some retrieval of complex learned information. A person imagining some sequential traversal of neurons might vaguely say "the brain traverses" some sequence of neurons, but for that to be a physical reality there would need to be either a moving anatomical part that moves over such a line of neurons, or a complex molecule that moves over such a distance to gather up the sequence. The brain has no visible moving parts, and there seems to be no complex neurotransmitter molecule that ever travels across some long sequence of neurons. 

The facts force us to a shocking conclusion contrary to the unfounded dogmas of neuroscientists about what goes on in the brain. The facts of how neurons are organized in the brain force us to conclude: there cannot be any sequential traversal in the brain amounting to a retrieval of complex stored information. For example, when someone recalls the thirty word of the first sentence of the Gettysburg Address, as many Americans can do, that cannot be happening through a retrieval of information stored in a brain. We cannot imagine any sequential traversal of neurons that would allow the retrieval of a long series of words. 

The conclusion reached here is consistent with the reality that no one has ever been able to retrieve any sequence of learned information by studying brain tissue. A large amount of brain tissue has been retrieved from living subjects who underwent brain operations. Such tissue is often normal, healthy brain tissue. For example, to treat severe epileptic seizures, large fractions of the brain may be removed, as much as half of a brain. No one microscopically studying such extracted brain tissue has ever found the slightest trace of learned information. No one has ever found a single word stored in a brain, nor has anyone ever found a single image of something someone saw by studying removed brain tissue. Besides removal of brain tissue from living patients, very many people agreed to donate their brains to medical science upon death. Very many brains of people who recently died have been studied. Such study has never retrieved a single word or a single image extracted from a brain. The utter failure of the microscopic investigation of brain tissue to produce any sign of learned information is consistent with what I have discussed in this post: that the brain has a type of physical architecture that should prevent the sequential traversal of learned information from ever occurring. 

People remember things, but there has never been any evidence that brains store the things that people remember. Everywhere the facts of neuroscience and the facts of human memory performance contradict the claim that the brain stores memories. One of these facts is the fact that humans can instantly acquire permanent new memories, as a person does when he learns of the death of one of his parents or sons or daughters. Such instant learning would be impossible if memory was a brain phenomenon, as all theories of brain learning appeal vaguely to processes such as synapse strengthening that occur only very slowly. Another example of those facts involves the short lifetimes of proteins in the brain. The average lifetime of proteins in the brain and its synapses is less than two weeks. Because of all the very high molecular turnover in the brain, you do not have the same synapses or the same dendritic spines that you had five years ago. But humans can reliably remember detailed memories acquired fifty or sixty years ago. A third example of such facts is the simple fact that humans can instantly recall many facts about a person as soon as they hear the name of such a person. Such instant recall should be impossible in a brain utterly lacking in addresses, indexes and sorting, the things that make instant recall possible using devices that humans build. 

So how did we ever get a community of experts claiming that the brain stores memories?  That is easily explained on sociological and psychological grounds. Around the year 1800, when very little was known about the brain, there arose in universities various types of belief communities populated by people passionately opposed to the idea of a human soul.  Such people founded neuroscience departments where "brains make minds" and "brains store memories" were required belief tenets.  During the two centuries since then, innumerable observations have contradicted such dogmas. But when a passionate belief community arises, it can continue to teach its dogmas as a kind of sacred creed that it is taboo to challenge. So for generation after generation the "brains make minds" and "brains store memories" tenets have been passed down from one generation to the next. 

When each new generation of neuroscience professor arises, it is not a case of the new generation independently reaching conclusions matching those of their professor predecessors. It is instead a case of a belief tradition being passed from one generation to the next, in an authoritarian "do not challenge the teachings of your teachers" fashion, similar to what goes on in organized religions when old belief traditions (often outdated ones) are passed on from one generation to the next. 

What happens is that neuroscientists are constantly making dogmatic claims that memories are stored in brains; but a search for ignorance confessions by neuroscientists on this topic will find many quotes that would never be made if such dogmatic claims were true. Here are some of these quotes by doctors and scientists, with links to the sources of such quotes:

• "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). 
• "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, cognitive neuroscientist (link). 

I mentioned above the trivial case of being able to remember the first sentence of the Gettysburg Address. It is a fact that humans can display sequential recall abilities hundreds of times greater than that, which makes the arguments stated above all the more weighty. For example:

• The mathematician Leonhard Euler could recite the entire Aeneid from beginning to end, a work of 9896 lines.  Another mathematician (Alexander Aitken) also memorized the whole Aeneid, and could recite the first 1000 digits of pi. 

• Between age 59 and age 67 a person memorized all 10,565 lines of Milton's Paradise Lost, recalling the entire work over a three-day period.

• The famous conductor Toscanini was able to keep conducting despite bad eyesight, because he had memorized the musical scores of a very large number of symphonies and operas. According to the 1920 newspaper article here, he had so well-memorized 150 opera scores that he "never even glances at a score when conducting."

• A scientific paper says, "Rajan S. Mahadevan ...was listed in the Guinness Book of World Records (McWhirter, 1983) for reciting pi to 31,811 places."  

• Salvatore Baccaloni memorized 168 opera roles. George Vogan de Arrezo memorized the entire text of Virgil's Aeneid (consisting of 9,896 lines). Aitken and JB  performed similar feats when they memorized epic poems of about 10,000 lines. Leste May Williams memorized 12,000 verses of the Bible, including the entire New Testament. The New Testament has about 180,000 words, so the feat of Leste May Williams would seem to be far more impressive than the memorization of Virgil's Aeneid, which has only 63,719 words. The same feat of memorizing the New Testament was achieved by a male minister (Henry M. Halley).

• It is well known that many Muslim scholars have memorized the entire text of their holy book, a book with 6236 verses.