Hierarchical Brain

An explanation of the human brain

First published 1st February 2024. This is version 1.5 published 2nd March 2024.
Three pages are not yet published: sleep, memory and an index.
Copyright © 2024 Email info@hierarchicalbrain.com

Warning - the conclusions of this website may be disturbing for some people without a stable mental disposition or with a religious conviction.

Symbol schema

A symbol schema is my name for a network of neurons that, when activated, represents a concept in the brain. The total of all the symbol schemas and the connections between them in my brain is, in effect, a model of my world (including my body and my brain itself), and encompasses all my memory, knowledge and intelligence. Symbol schemas are the most important intermediate level (level four) in my seven hierarchical levels of description of the brain, and they are crucial to all higher-level functions.

Symbol schemas are created as the end result of afferent processing of incoming data (from both external and internal senses and from within the brain), and they are given meaning by the process of reinstatement which uses efferent connections back towards the origin of the data.

Contents of this page
History and other names - a brief history of proposals for networks of neurons that may represent concepts, and the names given to them.
Justification for their existence - a review of the evidence for the existence of these networks.
Definition - my definition of a symbol schema.
Further details - more details on what a symbol schema is and how it is made.
Activation and deactivation - how and when a symbol schema is activated and deactivated.
Representation and meaning - how a symbol schema represents a concept and how it gains meaning.
Creation and update - how a symbol schema is created and updated.
Numbers - some thoughts on the number of symbol schemas in a human brain.
Summary - a summary and some conclusions.
References - references and footnotes.

History and other names

Justification for their existence


Further details

Activation and deactivation

Representation and meaning

Creation and update



References For information on references, see structure of this website - references

  1. ^ The Principles of Psychology - William James 1890
    viewable here, downloadable here: Volume I and Volume II or see GoogleScholar.
    In Chapter 2, “The Functions of the Brain”, James quotes John Hughlings Jackson: “Brain and mind alike consist of simple elements, sensory and motor. 'All nervous centres,' says Dr Hughlings Jackson, 'from the lowest to the very highest (the substrata of consciousness), are made up of nothing else than nervous arrangements, representing impressions and movements... I do not see of what other materials the brain can be made.'”
    In Chapter 6, “The Mind-Stuff Theory”, bottom of page 173 to 175: “Now take the other cases alleged, and the other distinction, that namely between 'having' a mental state and knowing all 'about' it. ...A faint feeling may be looked back upon and classified and understood in its relations to what went before or after it in the stream of thought. But it, on the one hand, and the later state of mind which knows all these things about it, on the other, are surely not two conditions, one conscious and the other 'unconscious', of the same identical psychic fact. It is the destiny of thought that, on the whole, our early ideas are superseded by later ones, giving fuller accounts of the same realities. But none the less do the earlier and the later ideas preserve their own several substantive identities as so many several successive states of mind. To believe the contrary would make any definite science of psychology impossible. The only identity to be found among our successive ideas is their similarity of cognitive or representative function of the same objects.”
    In Chapter 6, “The Mind-Stuff Theory”, middle of page 179: “Every brain-cell has its own individual consciousness, which no other cell knows anything about, all individual consciousness being 'ejective' to each other. There is, however, among the cells one central or pontifical one to which our consciousness is attached. But the events of all the other cells physically influence this arch-cell; and through producing their joint effects on it, these other cells may be said to 'combine'.”
  2. ^ The Organization of Behavior - Hebb 1949
    downloadable here or see GoogleScholar.
    See particularly Chapter 4 “The First Stage of Perception: Growth of the Assembly”, starting on page 60. For example, first paragraph:
    “This chapter and the next develop a schema of neural action to show how a rapprochement can be made between (1) perceptual generalization, (2) the permanence of learning, and (3) attention, determining tendency, or the like. It is proposed first that a repeated stimulation of specific receptors will lead slowly to the formation of an 'assembly' of association-area cells which can act briefly as a closed system after stimulation has ceased; this prolongs the time during which the structural changes of learning can occur and constitutes the simplest instance of a representative process (image or idea). The way in which this cell-assembly might be established, and its characteristics, are the subject matter of the present chapter.”
  3. ^ A review of cell assemblies - Huyck and Passmore 2013
    doi: 10.1007/s00422-013-0555-5 downloadable here or see GoogleScholar.
    Page 1, start of Abstract: “Since the Cell Assembly (CA) was hypothesised, it has gained substantial support and is believed to be the neural basis of psychological concepts. A CA is a relatively small set of connected neurons, that through neural firing can sustain activation without stimulus from outside the CA, and is formed by learning. Extensive evidence from multiple single unit recording and other techniques provides support for the existence of CAs that have these properties, and that their neurons also spike with some degree of synchrony. Since the evidence is so broad and deep, the review concludes that CAs are all but certain. CAs are found in most cortical areas and in some sub-cortical areas, they are involved in psychological tasks including categorisation, short-term memory and long-term memory, and are central to other tasks including working memory.”
  4. ^ Memory engrams: Recalling the past and imagining the future - Josselyn and Tonegawa 2020
    doi: 10.1126/science.aaw4325 downloadable here or see GoogleScholar.
    Start of background: “The idea that memory is stored as enduring changes in the brain dates back at least to the time of Plato and Aristotle (circa 350 BCE), but its scientific articulation emerged in the 20th century when Richard Semon introduced the term 'engram' to describe the neural substrate for storing and recalling memories. Essentially, Semon proposed that an experience activates a population of neurons that undergo persistent chemical and/or physical changes to become an engram. Subsequent reactivation of the engram by cues available at the time of the experience induces memory retrieval. After Karl Lashley failed to find the engram in a rat brain, studies attempting to localize an engram were largely abandoned. Spurred by Donald O. Hebb’s theory that augmented synaptic strength and neuronal connectivity are critical for memory formation, many researchers showed that enhanced synaptic strength was correlated with memory. Nonetheless, the causal relationship between these enduring changes in synaptic connectivity with a specific, behaviorally identifiable memory at the level of the cell ensemble (an engram) awaited further advances in experimental technologies.”
  5. ^ Integrative Activity of the Brain: An Interdisciplinary Approach - Konorski 1967
    I have not been able to get a copy of this book, so my knowledge of the contents is based on the review in reference 6 below.
  6. ^ Looking back at Jerzy Konorski’s book “Integrative Activity of the Brain”, 45 years after - Srebro 2013
    downloadable here or see GoogleScholar.
    First page, under the heading “How 'Integrative Activity of the Brain' was received”: “To some disappointment of the author, the book did not receive a worldwide attention. Indeed, only a handful of reviews of 'Integrative Activity of the Brain' appeared in scientific journals following the publication.”
    Page 455, under the heading “A brief description of the book”: “...the central concept of the proposed theory was idea of 'gnostic units' as a high level of representation of sensory stimuli by a single neuron or a small network of neurons.”
  7. ^ Genealogy of the “Grandmother Cell” - Gross 2002
    doi: 10.1177/107385802237175 downloadable here or see GoogleScholar.
    Bottom of page 512 (the first page of the paper) under the heading “Jerzy Konorski’s Gnostic Units”: “Although unknown to Lettvin, the grandmother cell idea had actually been set out in detail as a serious scientific proposal a few years earlier by the Polish neurophysiologist and neuropsychologist Jerzy Konorski in his 'Integrative Activity of the Brain' (1967), [see reference 5 above] a wide-ranging set of speculations on the neurophysiology of perception and learning.”
  8. ^ Ibid. Genealogy of the “Grandmother Cell”
    See top-right of page 512 (the first page of the paper) under the heading “Jerry Lettvin and the Birth of Mother and Grandmother Cells”: “Jerry Lettvin originated the term ‘grandmother cell’ around 1969 in his M.I.T. course titled ‘Biological Foundations for Perception and Knowledge.’”.
  9. ^ ^ Gnostic cells in the 21st century - Quiroga 2013
    downloadable here or see GoogleScholar.
    This paper is an excellent review looking back over the history of the proposal and discovery of, and various names for, networks of neurons in the human brain that represent concepts.
    Conclusion on page 469: “Concept cells are the link between perception and memory; they give an abstract and sparse representation of semantic knowledge that constitutes the building blocks for declarative memory functions. So, concept cells may then be the neural base for our thoughts, for leaving aside countless details and extracting meaning, for creating new associations and memories. They encode what is critical to retain from our experiences. ... Concept cells are not quite like the gnostic cells that Konorski once envisioned but they may be a key neural substrate for the power of human reasoning.”
  10. ^ Single Units and Sensation: A Neuron Doctrine for Perceptual Psychology? - Barlow 1972
    doi: 10.1068/p010371 downloadable here or see GoogleScholar.
    Barlow agrees with Sir Charles Sherrington that James’ ideas about single pontifical cells (see last quote of reference 1 above) cannot be correct, and that many cells in a network are needed.
    Page 390, under the heading “12.3 Pontifical cells”:
    “Thus the whole of subjective experience at any one time must correspond to a specific combination of active cells, and the 'pontifical cell' should be replaced by a number of 'cardinal cells'. Among the many cardinals only a few speak at once; each makes a complicated statement, but not, of course, as complicated as that of the pontif if he were to express the whole of perception in one utterance.”
  11. ^ ^ Godel, Escher, Bach - Douglas Hofstadter Penguin Books UK 1979
    This fascinating book, despite its title, is mostly about the functioning of the brain, although it covers many other subjects as well.
    See page 340 onwards for a discussion on symbols.
    Page 347 under the heading “Funneling into Neural Modules”:
    “One possible alternative to the grandmother cell [i.e. a single cell representation] might be a fixed set of neurons, say a few dozen, at the thin end of the 'funnel', all of which fire when Granny comes into view. And for each different recognizable object, there would be a unique network and a funneling process that would focus down onto that network. ... Such networks would be the 'symbols' in our brains.”.
    Page 348 under the heading “Modules Which Mediate Thought Processes”: “Thus we are led to the conclusion that for each concept there is a fairly well-defined module which can be triggered - a module which consists of a small group of neurons - a 'neural complex' of the type suggested earlier.”
    Page 359 under the heading “Can One Symbol Be Isolated?”: “Is it possible that one single symbol could be awakened in isolation from all other? Probably not. Just as objects in the world always exist in the context of other objects, so symbols are always connected to a constellation of other symbols.”
    Page 360 still under the same heading: “The network by which symbols can potentially trigger each other constitutes the brain’s working model of the real universe, as well as of the alternate universes which it considers (and which are every bit as important for the individual’s survival in the real world as the real world is).”
    Hofstadter also uses the term (tongue-in-cheek) “superhypercomplex cell” for a grandmother cell (page 344).
  12. ^ The brain from inside out - Gyorgy Buzsaki 2019 Oxford University Press
    doi: 10.1093/oso/9780190905385.001.0001 or see GoogleScholar.
    In Chapter 4, entitled “Neuronal Assembly”, page 92 under the heading “Reader Mechanism-Defined Cell Assembly”: “By the 1990s, my group and other laboratories worked out methods to record sufficiently large ensembles of neurons so that we could address a critical question: What determines the precise timing of a neuron’s spikes? We hypothesized, as did Hebb, that the recorded neurons take part in different assemblies, but not all assembly members are active on each occasion. We also reasoned that members of the assembly should work together within a measurable time window. ... So we tried to determine the time window within which neurons can best predict the timing of each other’s spikes. By varying the analysis window experimentally, we found that the best prediction of spike timing of single hippocampal neurons from the activity of their peers was when the time window varied between 10 and 30ms.”
  13. ^ ^ Time-locked multiregional retroactivation: a systems-level proposal for the neural substrates of recall and recognition - Damasio 1989
    doi: 10.1016/0010-0277(89)90005-X downloadable here or see GoogleScholar.
    This paper (which is quite difficult to make sense of) describes a theory of how neurons can store information about entities and events in “convergence zones” and how they can have meaning when recalled. The paper does not mention that these could be symbols for concepts, which is odd considering the subject of the next reference below, published in the same year. It does touch on where and how these convergence zones might be created, for example page 43, last paragraph: “The key to regionalization is the detection, by populations of neurons, of coincident or sequential spatial and temporal patterns of activity in the input neuron populations.”
  14. ^ ^ Concepts in the Brain - Damasio 1989
    doi: 10.1111/j.1468-0017.1989.tb00236.x (download not available).
    Page 25, second sentence: “The neural basis for the reconstructed representations is the activation of many separate neural population ensembles, distributed in various cortical regions. They constitute a related set because the activations occur within the same time-window and are co-attended.”
    and Page 26, last sentence: “The mechanism that permits coactivation of representations depends on devices I have called convergence zones, which are ensembles of neurons that 'know about' the simultaneous occurrence of patterns of activity during the perceived or recalled experience of entities and events.”
  15. ^ Convergence and divergence in a neural architecture for recognition and memory - Meyer and Damasio 2009
    doi: 10.1016/j.tins.2009.04.002 download not available but see GoogleScholar.
    Page 377, first paragraph: “The convergent-divergent connectivity principle exists at all levels of the processing hierarchy: just as first-order CDZs inscribe records of the combinatorial arrangement of knowledge fragments in early cortices, second-order CDZs inscribe records of the combinatorial arrangement of first-order CDZs, and so forth.”
    Figure 2 on my explanation of a symbol schema and its main linkages.
    This paper uses the terms “map”, “image” and “neural representation” all meaning the same thing (see reference 16 below for a comment on this).
  16. ^ Self comes to mind: constructing the conscious brain - Antonio Damasio Pantheon Books USA 2010
    Page 56: “I used to be strict about using the term image only as a synonym of mental pattern or mental image... Throughout this book, I use the terms image, map, and neural pattern almost interchangeably.”
    In general, I find Damasio’s books very readable and informative, but I find it strange that this particular terminology is both inconsistent and misleading; an 'image' is not the same as a 'map' in either technical or non-technical language: both are forms of representation, but a map is a diagrammatic representation and an image is a visual representation. There are some places in the book where it is suggested that a map is something used for actions, whereas an image is used for visual recall, but in other places this usage is not differentiated. I also think the term 'neural pattern' is vague and not helpful - does it mean a pattern in neural connections, or in neural signals, or both, and what is the pattern?
  17. ^ The strange order of things: Life, feeling and the making of cultures - Antonio Damasio Pantheon Books USA 2018
    For example, page 61: “The presence of images meant that each organism could create internal representations based on its ongoing sensory descriptions of both external and internal events.”
    Page 89: “All the words we use... are made of mental images. This is true of the auditory images of the sounds and letters and words and inflexions and of the corresponding visual symbol/letter codings that stand for those sounds... Also present in mind are countless other images regarding any object or event that pertain and describe their constitutive properties and relationships.”
    Page 89-90: “The collection of images typically related to an object or event amounts to the 'idea' of that object or event, the 'concept' of it, the meaning of it, semantics of it.”
    Damasio also sometimes uses the term 'map' as if it is only formed when conscious access to it is required. Pages 74-75: “At some point, long after nervous systems were able to respond to many features of the objects and movements that they sensed, both outside and inside their own organism, there began the ability to map the objects and events being sensed. ... Now stretch your imagination and think of maps not just of shapes or spatial locations but also of sounds..., and also think of maps built from touch, smell or taste. Stretch the imagination a bit more and think of maps built from the 'objects' and 'events' that occur within the organism... The depictions produced by this web of nervous activity, the maps, are none other than the contents of what we experience as images in our minds.”
    Damasio is suggesting here that 'maps' are only created much later in evolutionary terms than the ability to recognise and react to internal and external features or influences. But how were these early organisms able to recognise and respond to these features if not by creating (simple) types of 'maps'? Any mechanism that records and remembers a stimulation, so that it can be responded to at a later time, is surely already a 'map', or symbol schema, as I call such things? When self-awareness evolved, this did not mean that the structure of maps had to change.
  18. ^ On Intelligence or On Intelligence - Jeff Hawkins with Sandra Blakeslee St. Martin’s Press USA 2004
    For example, middle of page 69: “...the cortex creates what are called invariant representations”.
    Middle of page 153:
    “The sum of all these mechanisms allows the cortex to learn sequences, make predictions, and form constant representations or 'names', for sequences. These are the basic operations for forming invariant representations.”
  19. ^ ^ Invariant visual representation by single neurons in the human brain - Quiroga, Reddy, Kreiman, Koch and Fried 2005
    doi: 10.1038/nature03687 downloadable here or see GoogleScholar.
    Second paragraph: “The subjects were eight patients with pharmacologically intractable epilepsy who had been implanted with depth electrodes to localize the focus of seizure onset. For each patient, the placement of the depth electrodes, in combination with micro-wires, was determined exclusively by clinical criteria.”
  20. ^ Brain Cells for Grandmother - Quiroga, Fried and Koch 2013
    doi: 10.1038/scientificamerican0213-30 downloadable here or see GoogleScholar.
    Page 31, first paragraph under the heading “In brief”: “For decades neuroscientists have debated how memories are stored. That debate continues today, with competing theories - one of which suggests that single neurons hold the recollection, say, of your grandmother or of a famous movie star. The alternative theory asserts that each memory is distributed across many millions of neurons. A number of recent experiments during brain surgeries provide evidence that relatively small sets of neurons in specific regions are involved with the encoding of memories.”
    Page 33, second column, end of first paragraph: “Concept cells may sometimes fire to more than one concept, but if they do, these concepts tend to be closely related.”
  21. ^ Concept cells: the building blocks of declarative memory functions - Quiroga 2012
    doi: 10.1038/nrn3251 downloadable here or see GoogleScholar.
    From the abstract, page 587: “Intracranial recordings in subjects suffering from intractable epilepsy - made during their evaluation for an eventual surgical removal of the epileptic focus - have allowed the extraordinary opportunity to study the firing of multiple single neurons in awake and behaving human subjects. These studies have shown that neurons in the human medial temporal lobe respond in a remarkably selective and abstract manner to particular persons or objects, such as Jennifer Aniston, Luke Skywalker or the Tower of Pisa. These neurons have been named 'Jennifer Aniston neurons' or, more recently, 'concept cells'. I argue that the sparse, explicit and abstract representation of these neurons is crucial for memory functions, such as the creation of associations and the transition between related concepts that leads to episodic memories and the flow of consciousness.”
  22. ^ Concept Cells through Associative Learning of High-Level Representations - Reddy and Thorpe 2014
    doi: 10.1016/j.neuron.2014.10.004 downloadable here or see GoogleScholar.
    Page 249, first paragraph: “Concept cells are highly selective neurons that seem to represent the meaning of a given stimulus in a manner that is invariant to different representations of that stimulus. For example, a single neuron in the human hippocampus was found to selectively respond to several different pictures of the actress Halle Berry, even when she was disguised as Catwoman, the role she played in one of her movies. The same neuron also responded to the letter string 'HALLE BERRY' but not to other letter strings. Later studies showed that these 'concept cells' were also activated when stimulus information was provided in other sensory modalities, for example, hearing the name of a person spoken aloud.”
  23. ^ Cognition through the lifespan: mechanisms of change - Craik and Bialystok 2006
    doi: 10.1016/j.tics.2006.01.007 downloadable here or see GoogleScholar.
    See for example page 132, under the heading “Lifespan changes in representation”: “It is generally agreed that humans encode and store relevant aspects of the external world and that these internal systems of knowledge representation are organized hierarchically and have evolved phylogenetically.”
  24. ^ Grandmother cohorts: Multiple-scale brain compression dynamics during learning of object and sequence categories - Stephen Grossberg 2016
    doi: 10.1080/23273798.2016.1232838 (download not available, although see GoogleScholar).
    This paper is a follow up to papers published in 1976-80 that described the Adaptive Resonance Theory (ART), a neural network model of brain learning. A grandmother cohort is a set of networks that each represent different categories of an object or concept.
    From the final paragraph on page 26:
    “...ART category learning mechanisms can enable the learning of specific and invariant object categories, and the learning of list chunks that represent sequences of items that are temporarily stored in working memory. ...The categories that are learned by these mechanisms tend to be compact, involving small numbers of cells or cell populations, even perhaps grandmother cells as a limiting case. Balanced against this property, however, is the fact that networks of such categories are often needed to represent multiple aspects of an object or of a stored sequence of events. Such networks are called grandmother cohorts to emphasize the balance between compression and distribution that is needed to learn predictive representations of the world in space and time.”
  25. ^ Natural speech reveals the semantic maps that tile human cerebral cortex - Huth, de Heer, Griffiths, Theunissen and Gallant 2016
    doi: 10.1038/nature17637 downloadable here or see GoogleScholar.
    Supplementary Information downloadable here.
    See also accompanying video and interactive tour.
  26. ^ Categorization and Concepts - Goldstone, Kersten and Carvalho from Volume 3, Chapter 8 of Stevens’ Handbook of Experimental Psychology and Cognitive Neuroscience - Fourth Edition 2018
    doi: 10.1002/9781119170174.epcn308 downloadable here or see GoogleScholar.
    Page 276, under the heading “What are concepts?”: “... a concept is a mental representation of a class or individual and deals with what is being represented and how that information is typically used during the categorization”.
    Page 278, under the heading “What do concepts do for us?”: “... concepts function as filters. We do not have direct access to our external world. We only have access to our world as filtered through our concepts. Concepts are useful when they provide informative or diagnostic ways of structuring this world.”
    and “Concepts are cognitive elements that combine together to generatively produce an infinite variety of thoughts.”
  27. ^ ^ Sparse Representation in the Human Medial Temporal Lobe - Waydo, Kraskov, Quiroga, Fried and Koch 2006
    doi: 10.1523/JNEUROSCI.2101-06.2006 downloadable here or see GoogleScholar.
    Page 10234, second paragraph, concludes that several million neurons represent a typical stimulus, and that each neurons fires in response to 50-100 distinct representations. However, these figures are calculated using a number of assumptions that could in fact be very different from the assumed values.
  28. ^ Degeneracy and complexity in biological systems - Edelman and Gally 2001
    doi: 10.1073/pnas.231499798 downloadable here or see GoogleScholar.
    Page 13765, bottom left, last five lines: “Typically, neurons in the brain receive synaptic input from many thousands of other neurons so that in humans, for example, there are approximately one billion synapses in each cubic millimeter of brain gray matter. The pattern of connectivity created by so many synapses within such a tiny volume of tissue in one animal could not be genetically prespecified and, thus, must be unique to each individual. Indeed, the degree of degeneracy in neural connectivity probably dwarfs that of any other system discussed in this review.”
  29. ^ Hidden in plain view: degeneracy in complex systems - Mason, Dominguez, Winter and Grignolio 2015
    doi: 10.1016/j.biosystems.2014.12.003 downloadable here or see GoogleScholar.
    Abstract, page 1: “Degeneracy is a word with two meanings. The popular usage of the word denotes deviance and decay. In scientific discourse, degeneracy refers to the idea that different pathways can lead to the same output.”
    Page 4: “Degeneracy is present across multiple scales of brain organization - from neurotransmitters and synapses, through cortical and subcortical regions...”
    and: “for ... brain networks to be functionally robust, structurally different elements must be able to provide similar outputs, that is, they must be degenerate. A degenerate network is thus able to provide reliable output even when the input is neither labeled nor identical across occurrences, and when the input is presented in a noisy environment with a large number of competing stimuli, which overwhelmingly characterizes the conditions under which the brain operates.”
    See also concluding remarks on page 5.
  30. ^ Degeneracy and redundancy in cognitive anatomy - Friston and Price 2003
    doi: 10.1016/S1364-6613(03)00054-8 downloadable here or see GoogleScholar.
    This short letter explains the difference between degeneracy and redundancy. Degeneracy is necessary for redundancy, but degeneracy is more than redundancy, it is a relationship between structure and function.
  31. ^ Abstract and concrete concepts have structurally different representational frameworks - Crutch and Warrington 2005
    doi: 10.1093/brain/awh349 downloadable here or see GoogleScholar.
    Two-thirds of the way through the Summary: “...abstract concepts, but not concrete concepts, are represented in an associative neural network.”
  32. ^ Recognition-by-Components: A Theory of Human Image Understanding - Biederman 1987
    doi: 10.1037/0033-295X.94.2.115 downloadable here or see GoogleScholar.
    This psychological study concludes (Page 127, top of second column) that there are around 30,000 readily discriminable concrete objects, but this does not mean that any single individual has actually come across all of these and therefore created a symbol schema for each. The methodology uses the concept of geons and seems a reasonable estimate as far as it goes, but only includes one category of items in my list of things that must be represented by symbol schemas.

Page last uploaded Sat Mar 2 02:55:43 2024 MST