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.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.
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.
- "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 AkhlaghpourÆš (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).
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.