Scientific Understanding of Consciousness
Consciousness as an Emergent Property of Thalamocortical Activity

Dynamic Core

The Dynamic Core hypothesis formulated by Gerald Edelman and Giulio Tononi is the fundamental neurobiological foundation of my concept of consciousness. The dynamic core is a constantly changing hierarchical web of sparse, reentrant neural activity (perhaps <5% of total neural activity) that mediates a mental pattern of thought over a continuously advancing interval of approximately ~100 ms. The mental pattern supported by the dynamic core can be a thought pattern during the waking state, or it can be the thought pattern during the dreaming state. The thalamocortical activity begins prenatally and continues uninterrupted until death or until an abnormal state such as brain death. The major connectivity, although not exclusively, is the cortex and the thalamic nuclei.

Core consciousness refers to the here-and-now, a continuously advancing interval of perhaps a tenth of a second.

For each successive brief interval of a few hundred milliseconds, the sparse, widely distributed neuronal assembly comprising the dynamic core interacts more strongly with itself than with the rest of the neural network.

At any moment during an animal's life, only a small fraction of neurons will be strongly activated by natural stimuli. (Foldiak; Sparse Coding, 897)

Working memory, the dynamic core, and the thalamocortical system are intimately related as the mediators of consciousness. The dynamic core’s ever changing moment-to-moment composition as a network assembly of millions of synaptically connected neurons representing a thought is likely to include the most immediate activity of working memory, and will be closely synchronized with the ever-iterative thalamocortical system.

The only conclusion that can be drawn for sure about the neural substrate of consciousness is that it includes parts of the corticothalamic system. (Tononi & Laureys; Neurology of Consciousness, 390)

Dynamic core -- ever-changing composition yet ongoing integration, is generated largely, although not exclusively, within the thalamocortical system. (Edelman; Universe of Consciousness, 139)

The coordinated activity within the thalamus, anterior cingulate cortex (ACC), the ventral lateral prefrontal cortex (VLPFC), posterior parietal cortex (PPC), and the brain stem probably regulate the content of consciousness through mechanisms of executive attention. (Vogt; Cingulate Neurobiology, 384)

Neural activity does not produce an awareness event unless the prefrontal area amplifies that activity to a particular level and sustains it there for some minimum duration. (LaBerge; Attention, the Triangular Circuit, 312)

Researchers have hypothesized that the direct projections from parietal cortex into the premotor areas are unconscious, whereas projections to the premotor areas via the prefrontal cortex are related to consciousness. (Crick & Koch; Consciousness and Neuroscience, 40)

We remain agnostic with respect to the relevance of gamma oscillations to conscious perception. (Crick & Koch; Consciousness and Neuroscience, 46)

Active memory is an active cortical network, it's neurons active above certain baseline or spontaneous level of firing. (Fuster; Memory in Cerebral Cortex, 237)

Active memory as a dynamic state of cortical representation comes closer to explaining a wide range of neural phenomena of mnemonic storage. (Fuster; Memory in Cerebral Cortex, 237)

Not all neurons and connections are used all the time. The dynamically changing functional or effective connectivity gives rise to short-lived oscillations that are perpetually created and destroyed by the brain's internal dynamics. (Buzsáki; Rhythms of the Brain, 111)

The dynamic core is characterized by reentrant signals resulting in recursive updating of the synaptic state of the dynamic core on the basis of about 100 to 500 ms. There is much interconnectedness among the reentrant pathways. The instability propensity of “positive feedback” arising from many thousands of reentrant signals is restrained by the local networks of inhibitory interneurons interconnecting dendritic trees of projection neurons.

Consciousness is an emergent property of neural activity in the dynamic core. Consciousness requires a biological neural network that has evolved from molecular beginnings over a time period of billions of years.

Consciousness is the images (and thoughts and feelings) that are represented in the activated neural networks. (Hobson; Consciousness, 141)

Awareness results from the firing of a coordinated subset (dynamic core) of thalamocortical neurons that fire in some special manner for a certain length of time, probably for at least 100 or 200 msec. This firing needs to activate some type of short-term memory by either strengthening certain synapses or maintaining an elevated firing rate or both. (Koch and Crick; Neuronal Basis, 95)

The dynamic core concept is hard to pictorialize, but Edelman has shown a simplified diagram. (Edelman; Wider than the Sky, 71)

Edelman and Giulio Tononi have authored a very significant paper on Consciousness and Complexity. However, it can seem very arcane, so I have rendered my paraphrase version which should provide the gist of their thought in a form much more readable and understandable.

All my discussion of consciousness and its neurobiological basis should be viewed in the context of the dynamic core.

The brain employs multiple maps of external space, some unimodal, some multimodal, many in the cerebral cortex, especially but not exclusively in parietal lobes, but some also in thalamus and colliculi. (Tononi & Laureys; Neurology of Consciousness, 380)

Transient Groupings of Neurons for Consciousness

Susan Greenfield has some very informative discussion on transient groupings of neurons and neuronal gestalts.

Consciousness is a property of many transient groupings of neurons. Our brains are a restless grouping and re-grouping of temporarily relevant neurons with greater and lesser connectivity. (Greenfield; Centers of Mind, 85)

Consciousness may be generated at different times by shifting populations composed of different groups of neurons. (Greenfield; Centers of Mind, 88)

Dynamic reentrant interactions in the thalamocortical system. (Edelman; Wider than the Sky, 55)

Non-specialized groups of cells are distributed throughout the brain, and these groups are constantly reforming their connections and changing their size and pattern. Such groups of cells are the appropriate physical bases for multiple potential consciousnesses, only one of which is realized at any one time. (Greenfield; Centers of Mind, 89)

Concentric description of consciousness is based on a triggering epicenter which sets in motion nonlinear, concentric associations. The more extensive or sustained the associations are, the more consciousness will be experienced at that particular time. We are not conscious of each of these associations as separate components. The components conspire together to give a single experience at a specific moment in time. (Greenfield; Centers of Mind, 97)

Consciousness is spatially multiple yet effectively single at any one time. It is an emergent property of noncommitted and emergent groups of neurons that is continuously variable with respect to and always entailing a stimulus epicenter. (Greenfield; Centers of Mind, 97)

Neuronal gestalts

Neuronal gestalt -- a highly variable aggregation of neurons which is temporarily recruited around a triggering epicenter. (Greenfield; Centers of Mind, 99)

Size of the gestalt corresponds directly and simultaneously to the degree of consciousness at a given time. (Greenfield; Centers of Mind, 99)

Arousal and gestalt formation act in concert to generate consciousness. (Greenfield; Centers of Mind, 111)

Formation of neuronal gestalts and subsequent generation of consciousness are influenced by factors in the external environment. (Greenfield; Centers of Mind, 100)

Size of an existing gestalt, the depth of our current consciousness, influences how we interpret sensory inputs as they bombard us from the outside world. (Greenfield; Centers of Mind, 100)

Arousal is a powerful factor in determining the final quality of consciousness, although it is not the same as consciousness. (Greenfield; Centers of Mind, 100)

Arousal can be described as a generalized degree of alertness: it is low when we a relaxed and high when we are frightened or angry. (Greenfield; Centers of Mind, 100)

Hebbian cell assembly - spatiotemporal pattern of neuron activity. (Calvin; Neil's Brain, 282)

Link to — Dynamic Core triggers Associated Memories

Continuity of Consciousness

Our continuity of consciousness occurs as a chain of associations devolved around an epicenter. "Ripples" on one gestalt spread out to ever more remote associations, so a new epicenter starts to recruit neurons into a gestalt. This new gestalt supplants the original, and our consciousness subtly shifts. (Greenfield; Centers of Mind, 105)

Hierarchy of Gestalts

The enormous associative capabilities of the dynamic core are ideal to link or hierarchically organize a series of preexisting unconscious routines into a particular sequence. (Edelman; Universe of Consciousness, 187)

Pattern theory says that tightly coupled cortical areas seek, via some kind of relaxation functionality, to arrive at a mutual agreement in which lower areas' specific data form a fit with known, more abstract categorizations stored in higher areas' memory of prior activity. (Mumford; Neuronal Architectures, 135)

Synchronization Binding of Gestalts

A group of neurons can contribute directly to conscious experience only if it is part of a distributed functional cluster that, through reentrant interactions in the thalamocortical system, achieves high integration in hundreds of milliseconds. (Edelman; Universe of Consciousness, 144)

Dynamic core is stabilized for hundreds of milliseconds by massive reentrant loops, defined by the functional requirement that all core members interact more strongly with each other than with the rest of the brain. (Koch; Quest for Consciousness, 310)

Reentry is the mechanism by which the spatiotemporal coordination of diverse functional modules is achieved. Reentry synchronizes the activity of neuronal groups in different areas of the brain, binding them to function as a coherent unit. (Edelman; Universe of Consciousness, 85)

Conscious experience is associated with the activity of populations of neurons that are widely distributed in the thalamocortical system. The distributed groups of neurons must engage in strong and rapid reentrant interactions. (Edelman; Universe of Consciousness, 62)

Music's effect on sensory integration refers to how the brain organizes and interprets (in accordance with the Gestalt laws) inputs arriving simultaneously from multiple sensory modalities, such as sight, sound, smell, taste, touch, heat, pain (nociceptive), etc. (Schneck & Berger; Music Effect, 133)

Corpus callosum - approximately 200 million nerve fibers reciprocally linking the two cerebral hemispheres. (Edelman; Universe of Consciousness, 62)

Correlation of events across various maps of the brain occurs via the dynamic process of reentry. (Edelman; Universe of Consciousness, 85)

Activation patterns, conscious awareness, organized, coherent. (Singer; Neuronal Synchronization, 125)

Bayesian Inference

Psychophysical experiments indicate that humans perform near-optimal Bayesian inference in a wide variety of tasks, ranging from cue integration to decision making to motor control. (“Bayesian Inference in Brain Functionality,” Nature Neuroscience, published online 22 October 2006)

Brain activity and function in normal daily activities

Normal daily activities can provide informative examples of brain activities. Dynamic core invokes language, talking, Wernicke speech area, Broca speech area, vision, hearing, laugh, FAPs, sensory cortex, motor cortex, prefrontal cortex, thalamocortical loops, emotion. Consciousness is an emergent property of the dynamic core.

Brain is always active

Most neurons in the brain are firing intermittently even when the brain is in a quiescent waking state or sleeping state. Interneurons tend to be firing all the time, with their inhibitory connections to the dendritic trees of projection neurons. In a resting waking state, projection neurons fire at a relatively low rate, but nevertheless continue to fire stochastically whenever they receive a quorum of spikes on their thousands of dendrites.

The brain remains continuously active from a time early in the prenatal embryo until a time of accidental trauma, disease, stroke or death when the neural assemblies available for the dynamic core decline below nature’s minimum requirement.

Dynamic core concept closely aligned with other researchers’ concepts

Other researchers have concepts similar to the dynamic core, which they refer to by different names. However, Gerald Edelman has developed more of the specifics of the hypothesis.

Christof Koch states that Gerald Edelman's global aspect of consciousness with tight interaction of very big neuronal assemblies reaching clear across the brain are ideas that Koch believes may be on the right track. (Koch, 101)

Integrated mind from parcellated activity. (Damasio; Descartes' Error, 94)

Mind integration is created from the concerted action of large-scale systems by synchronizing sets of neural activity in separate brain regions. (Descartes' Error, 95)

Neural Correlate of Consciousness (NCC) is a moving target, a shifting coalition of 'strongly interacting elements', which Gerald Edelman refers to as the 'dynamic core'; perhaps composed of many temporarily coherent 'cell assemblies' of neurons envisioned by Donald Hebb. (Zeman; Consciousness, 287)

Consciousness is a dynamic organization existing within many different areas of the brain. (Johnston; Why We Feel, 122)

Quote from Susan Greenfield, professor at Oxford University: Consciousness is spatially multiple yet effectively single at any one time. It is an emergent property of non-specialized and divergent groups of neurons (gestalts) that is continuously variable. (Johnston; Why We Feel, 124)

Edelman has formulated and extended his dynamic core hypothesis in much more definitive terms. He has related his formulations to such consciousness states as slow-wave sleep, generalized epilepsy; normal waking state or REM sleep, the formulations can serve as a guide in understanding the relationships of the neural connections and loops.

Functional cluster

Edelman introduces the term “functional cluster” to refer to a subset of strongly interacting elements that is functionally demarcated from the rest of a system. The functional cluster changes composition over time of a few milliseconds via changes in strengths and modulations of synapses. (Edelman; Universe of Consciousness, 120)

Thalamocortical System

The thalamocortical system that gives rise to the dynamic core is comprised of a intricate meshwork of cortical and thalamic areas with reentrant connections. The dynamic core is composed of a functional cluster that interacts mainly with itself through an enormous complex of signals that fluctuate in time across the reentrant meshwork. Responses triggered by the reentrant dynamic core can stimulate non-conscious responses via the basal ganglia and certain thalamic nuclei and then return to the cortex. These non-conscious signals travel along parallel, polysynaptic, one-directional pathways. (Edelman; Wider than the Sky, 70)

Edelman depicts the dynamic core in a simplified diagram.

Reentry and Recursion

Brain architecture is characterized by abundant reciprocal connections between cortical regions, recurrent pathways that permit feedback and reactivation of active areas, and lateral inhibition that focuses neural activity within active centers by inhibiting less active adjacent regions. (Johnston; Why We Feel, 123)

Reentry is the key neural mechanism by which integration can be achieved within the thalamocortical system. (Edelman; Universe of Consciousness, 113)

Dynamic Core hypothesis (Edelman; Universe of Consciousness, 144)

The dynamic core is an ever-changing subset of activity in thalamocortical system supported by parallel loops through the basal ganglia and perhaps other subcortical areas. Activity in the dynamic core is modulated by the neuromodulatory transmitters emitting from the brain stem, et al. Edelman calls this neuromodulatory system the “value system”. The dynamic core is indeed dynamic; the neurons contributing to the dynamic core change every few milliseconds as thoughts change. The circulating, reentrant neural activity of the dynamic core binds together modular functionality in many areas of the brain that are contributing to thought at any particular instant.

The dynamic core is indeed dynamic. The neurons comprising the dynamic core change from millisecond to millisecond as thoughts change. Each different thought is a different arrangement of the neuron connections. An individual thought does not require the precise interconnection of 100s of millions of particular neurons. The participation of a specific neuron in the dynamic core is determined by the threshold effect of its requisite quorum of synapses being excited. A particular thought can be represented by a slight variation in the individual neurons involved and in the connection arrangement of those neurons.

To sustain conscious experience, it is essential that the functional cluster be highly differentiated, as indicated by high values of complexity.

The number of different thoughts that the network can implement via the synaptic connections is astronomically large. Just consider billions of neurons, each with thousands of synapses. The number of possible network connections is unimaginably large. I visualize a Dynamic Core as comprising a few percent of the billions of neurons, with each neuron having a particular combinations of synapse strengths. This combination of synapse strengths represents the memory of use of these synapses in the recent past.

Neurons comprising the dynamic core firing at a rate of about 100 Hz (typical firing rate for active neurons). The inputs to each neuron can be via thousands of synapses on dendrites receiving signals from thousands of other neurons.

Edelman’s mathematical formulations for the Dynamic Core

Edelman describes the characteristics of the dynamic core in esoteric mathematical formulations of information theory. (Edelman; Universe of Consciousness, 120-154) To those who have some understanding of information theory, the mathematical formulation can provide far more insight than a voluminous description in words. A few highlights: Entropy of the system can be considered as a (logarithmic) function reflecting the number of possible patterns of activity that the system can take, weighted on the probability of occurrence. [One hundred billion (10¹¹) neurons in the brain with one million billion (10¹⁵) synapses provides an astronomically large sample space for mental images.] A functional cluster index is defined to reflect the relative strength of the interactions within a subset of elements compared to the interactions between that subset and the rest of the system. Neural complexity is defined in terms of average mutual information between subsets and their complements. The formulation shows that high values of complexity correspond to an optimal synthesis of functional specialization and functional integration within a system. A high degree of complexity is a necessary requirement for any neural process that sustains consciousness. (Edelman; Universe of Consciousness, 121-138)

In more mundane terms, the mathematical formulation states that the dynamic core must be highly differentiated and highly integrated. Highly differentiated means that the dynamic core subset is drawn from a huge sample space of neurons and their synaptic connections. High differentiation is reflected in the fact that humans can use 60,000 or more words in language. (Ratey; User's Guide to Brain, 257) Highly integrated means that the subset comprising the dynamic core at a given instant must be related to itself much more strongly than to the rest of the neural network. An example is the tight integration necessary for the "what" and "where" pathways:

Tight integration of the "what" and "where" pathways is essential; at any time, it is necessary that the information in the two pathways be unambiguously correlated. (Niebur & Koch; Computational Architectures for Attention, 168)

High Integration Necessary for Consciousness

Through mechanisms of entrainment that involve biasing of neural networks, music can affect processes of sensory integration. (Schneck & Berger; Music Effect, 133)

Size of the Dynamic Core?

The size of the dynamic core can vary dramatically as the intensity of thought varies. The dynamic core associated with intense activity assessing danger will be very much more extensive than a dynamic core associated with relaxed reverie.

The proportion of the brain’s neurons that contribute to the dynamic core at any particular instant is unknown. I visualize the proportion at about 10-20% in typical circumstances.

How much of the brain does a thought require? Only a subset -- although not a small subset -- of the neurons in the human brain contributes directly to consciousness experience. (Edelman; Universe of Consciousness, 140-143)

Unconscious neural activity supports the dynamic core

Of course, the dynamic core is supported by the homeostatic systems of the body, including those of the brain stem, hypothalamus, and autonomic nervous system. In addition, the dynamic core is supported in metabolic functions by the vascular system with blood supply. Without these supporting functions, consciousness ceases.

What neural activity is required for consciousness?

The “neural correlate of consciousness” (NCC) is currently unknown, but current research studies are attempting to gain some insight.

Neural Structures of Core Consciousness

Second-order neural patterns of core consciousness - ensemble playing of the superior colliculi and the cingulate under the coordination of the thalamus. Cingulate and thalamus are likely to have the major roles. (Damasio; Feeling of What Happens, 181)

Research study — Intracranial Markers of Conscious Access

Consciousness is believed to be based on neural activity in the thalamocortical loops together with associated activity in modular areas of the cortex.

Consciousness is supported by essential activity in the brain stem, hypothalamus, and other areas that support the metabolic and homeostatic functions of life. (A metaphor: If you switch off the electricity, the lights go out.)

Neural activity in thalamocortical loops is characterized by reentrant functionality. Further discussion of reentry and recursion is provided in a subsequent paragraph.

Neuron Composition of the Dynamic Core

The dynamic core is indeed dynamic. The composition of the dynamic core changes every millisecond or so as thoughts change. My current hypothesis is that a minimal dynamic core requires active neurons in portions of working memory, thalamocortical loops, and the limbic system for an interval of a millisecond or so. Other modular areas of the cortex may often form a part of the dynamic core. The modular areas contributing to the dynamic core change from millisecond-to- millisecond as thoughts change. Even when thoughts seem relatively constant, the neurons contributing to the dynamic core within a modular area will change from millisecond-to- millisecond. The synapses connecting the neurons forming the dynamic core will also changing on a millisecond-to-millisecond basis.

(hypothesis)

Which neurons frequently participate in the Dynamic Core?

Which neurons, although frequently active, are never a part of the Dynamic Core? These neurons may have a supporting role.

Memories and Consciousness (Koch; Quest for Consciousness, 187)

A varying subset of neurons and synaptic connections in working memory and perhaps a few other brain areas are always included in the dynamic core. (Koch 196,199) (Zeman 281) (Neuroscience 735) (Emotional 185, 270, 271, 278, 279, 281, 282, 285, 291, 296) (LeDoux 176, 177, 187, 190, 192, 198, 199, 226, 228, 253, 292) (Damasio 96) (Drugstore 63)

A varying subset of neurons and synaptic connections from a number of the other modular areas of the brain are likely to be included in the dynamic core at any particular moment. The network composition of these neurons and synaptic connections and/or the particular set of modular areas included is likely to change every few milliseconds as thoughts change. (Delirium 203)

Several hundred milliseconds for sensing event to generate consciousness (Koch; Quest for Consciousness, 235)

Much of the supporting activity of the brain is unconscious. (LeDoux 259) (Emotional 17, 31, 67, 68)

Functions of Consciousness (Koch; Quest for Consciousness, 232)

Self-consciousness – generate a model of itself (Koch 232) (Searle 6) (Delirium 12, 141, 142)

A general strategy for circumscribing the NCC (Quest, 97)

Attention - Focused perceptual consciousness

Maintained coalition of cells in high-level sensory areas and frontal cortex. Example: Synchronized activity between inferior temporal and prefrontal cortex. This is a state of attention, as in conversation.

Fleeting consciousness

Cortical activity associated with non-attended events. Meditative state, perhaps with eyes closed.

Not conscious

Retinal and spinal cord activity. Activity that underlies eye movements, posture adjustments. Deep stages of slow-wave sleep.