Scientific Understanding of Consciousness
• The thalamocortical system consists of continuously ongoing neural signals between nuclei in the thalamus and neurons in all modular areas of the cortex. In general, neurons are never idle. Neurons fire at about 5 Hz in the quiescent state and at rates as high as 500 Hz in the active state.
• Fast oscillations are coherent between thalamus and cortex. (Traub; Cortical Oscillations, 58)
• The thalamocortical system is continuously active from a time early in prenatal embryonic life until death.
• Signals in the thalamocortical system are defined by rates of synaptic bursts and by temporal and spatial burst patterns of synaptic activity.
• The dynamic core consists of a momentary subset of the thalamocortical system defined by active synapses. Positive feedback/reentrant signals circulate in the network of the dynamic core. The active synapses comprising the dynamic core continually change as the dynamic core updates recursively on the basis of about 100 ms.
• The idea that an iterative algorithm is carried out in the thalamocortical loop has received experimental confirmation in observed oscillations. (Mumford; Thalamus, 982)
• the reentrant signals circulating in the dynamic core create an oscillation of about 40 Hz
• Consciousness thoughts are mediated by patterns of active synapses comprised of reentrant, recursive pathways.
• 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)
• 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)
• 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)
• Processing in any given cortical area is intimately related to activity in the particular thalamic nucleus with which it has reciprocal and close connections. (LaBerge; Attentional Processing, 166)
• For most cortical areas, the main target of thalamocortical projections is layer 3. (LaBerge; Attentional Processing, 174)
• Almost 10 times as many visual neurons go down from the visual cortex to the thalamus as the other way, suggesting continuous and intimate looping between thalamus and cortex, controlled by nucleus reticularis thalami (nRt). (Baars; Neuronal Mechanisms of Consciousness, 273)
• Working memory and the prefrontal cortex along with other cortical areas.
• Thalamocortical system by itself cannot mediate consciousness. Must have support from non-conscious networks. Portions of non-conscious networks can be damaged and still leave consciousness.
• A major function of the thalamus is to gate and otherwise modulate the flow of information to the cortex.
• I have sketched my overall concept of the thalamocortical system in a diagram.
Specific and Nonspecific Functions of Thalamocortical System
Specific thalamocortical system would provide the content that relates to the external world, and the nonspecific system would give rise to the temporal conjunction, or the context that would together generate a single cognitive experience. (Llinás & Paré; Brain Modulated by Senses, 14)
Research study — Thalamocortical System Development
Reentry a key neural mechanism
The thalamocortical pathways are reciprocated by feedback pathways from the cortex back to the thalamus, forming a massive system of local loops between a thalamus and the entire cortex. (Mumford; Thalamus, 981)
As a result of corticothalamic iteration, the thalamic pattern of activity is sent back to the cortex as an enhanced view of the world. (Mumford; Thalamus, 983)
Dynamic reentrant interactions in the thalamocortical system. (Edelman; Wider than the Sky, 55)
Reentrant loops in the thalamocortical system (Llimas 1994) (Chalmers; Neural Correlate of Consciousness, 219)
Alpha rhythm of EEG may result from autorhythmic activity in closed loops between cortical and thalamic neurons. (Changeux; Neuronal Man, 72)
Link to — Association Cortices Connections
Awareness results from Thalamocortical Activity
Awareness results from the firing of a coordinated subset 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)
Overall picture that emerges from current theories is that the neural constituents of consciousness probably form a complex system of corticocortical and thalamocortical networks, each contributing some essential features to the of phenomenal level of organization. (Revonsuo; Inner Presence, 315)
Synchronous neural activity at high frequencies is the leading candidate among the potential neurophysiological mechanisms of consciousness. (Revonsuo; Inner Presence, 315)
Awareness of what happens around us and of ourselves is rooted in the complexity of the functional and anatomical networks of the thalamocortical system that enables the brain to rapidly integrate information. (Pietrini; Consciousness and Dementia, 214)
Extensive legions of the thalamocortical system are usually associated with a global loss of consciousness, such as that seen in comatose patients. (Pietrini; Consciousness and Dementia, 204)
Rudolfo Llinás at New York University has discovered that all areas of the cortex emit a steady level of noise, or oscillation, at a frequency of about 40 Hz. Some areas of the cortex, humming along at 40 Hz, are phase locked, meaning there oscillations are in unison; they keep the same beat. Llinás and other researchers have suggested that the neurons perform in synchrony because they follow a kind of conductor in the brain. The prime candidate for the conductor's function is the many intralamina nuclei, located deep within the thalamus. (Ratey; User's Guide to Brain, 133ff)
Research study — Consciousness and the Thalamocortical Loop — Rodolfo Llinás explores the basic assumption that large-scale, temporal coincidence of specific and nonspecific thalamic activity generates functional states characterizing thalamocortical iterative recurrent activity as the basis for consciousness
Research study — Thalamus Diagram
Research study — Thalamocortical Circuit Diagram
Reentrant Activity Leading to Recursion
Reentrant activity leading to recursion is a fundamental feature of thalamocortical activity, and indeed nearly all neural activity. Reentrant activity is not simple feedback but functions in a network as recursive multiple pathways, which update cyclically on a time scale of tens to hundreds of milliseconds, rapidly converging to the dynamic core’s synaptically connected neuronal network mediating an instantaneous thought. This cyclical neural activity generates gamma (~40-Hz) oscillations in the normal waking state.
Research 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. This implies that neurons both represent probability distributions and combine those distributions according to a close approximation to Bayes' rule.
Prefrontal regions are reciprocally connected with temporal, parietal, and occipital cortices, where they receive higher-level visual, auditory, and somatosensory information. (Miller; Human Frontal Lobes, 49)
Two kinds of nervous system organization that are important to understanding how consciousness evolved: (1) brainstem, together with the limbic (hedonic) system. (2) thalamocortical system. Edelman; Bright Air, 117)
Model of Thalamocortical System
The workings of the thalamocortical system are hard to visualize and even harder to describe.
• The thalamocortical system is constantly active with circulating, reentrant neural signals exciting neural assemblies of various sizes with generally excitatory stimulation when positive behavior is needed, but with inhibitory stimulation when more quiescent behavior is needed.
• The dynamic core of consciousness can be visualized as a small subset of this thalamocortical activity, combined with supporting activity in other subcortical areas. The dynamic core is fleetingly changing in composition as the neural networks respond to momentary aggregates of spike signals passing along efficacious-synapse pathways. These efficacious-synapse pathways have been established by prior activity and experience among the thousands of neurons on dendritic trees of neurons.
• Here is a reference to a computer simulation model by Gerald Edelman:
Research Study — Computer Simulation of Thalamocortical System — Gerald Edelman’s Large-scale Model of mammalian Thalamocortical System
Damasio expresses his view on cortex and thalamus
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)
Experts comment on thalamus
Thalamus is most highly developed in mammals and especially so in primates. (Shepherd; Synaptic Organization of the Brain, 289)
Schematic three-dimensional view of right thalamus with many of its major nuclei. (Diagram) (Shepherd; Synaptic Organization of the Brain, 290)
Strong reciprocity exists in thalamocortical connections. (Shepherd; Synaptic Organization of the Brain, 295)
The neural signals are reentrant and recursive in their function. The recursive functionality is characterized by pulse-like updates as each neuron in the network transmits a burst of spike activity and is then momentarily quiescent until the signals circulating in the network return in an aggregate large enough at the input dendrites to cause the neuron to fire again. The updates may occur within some tens of milliseconds but may be longer.
The network connectivity formed by active synapses in recursive functionality is the biological basis of a thought at any particular moment. Only a small subset of brain’s synapses is active* at any particular moment; the synapses that are momentarily active changes on a millisecond-by-millisecond basis. A portion of these active synapses forms the dynamic core of consciousness. The synapse subset comprising the dynamic core changes from moment-to-moment as thoughts change.
*(active in the sense that they are contributing to the brains information processes at the moment. Even in the quiescent state, neurons fire at perhaps 10 times per second; in the active state, they fire at a rate of perhaps 100 to 500 times per second. To become active, a neuron’s dendritic tree of synapses must momentarily have combined activity that exceeds the firing threshold at the axon hillock.)
Thalamocortical system does not contain loops so much as highly connected layered local structures with massively reentrant connections. (Edelman; Bright Air, 117)
The thalamocortical system, by its hublike organization, allows radial communication of the thalamic nuclei with all aspects of the cortex. These cortical regions include the sensory, motor, and associational areas. These areas subserve a feedforward/feedback, reverberating flow of information. (Llinás; I of the Vortex, 126)
Thalamocortical system evolved to receive signals from sensory receptor sheets and give signals to voluntary muscles. (Edelman; Bright Air, 117)
Thalamocortical system is very fast in its responses (milliseconds to seconds), although its synaptic connections undergo some changes that last a lifetime. (Edelman; Bright Air, 117)
Consciousness is a noncontinuous event determined by simultaneity of activity in the thalamocortical system. (Llinás; I of the Vortex, 124)
Thalamic input from the cortex is far larger than from the peripheral sensory systems. This suggests that thalamocortical iterative activity is a main mechanism of brain function. (Llinás; I of the Vortex, 124)
Categorization response is similar to a previous response, but at a later time the neurons and synapses contributing to that response will be different. In general, they are likely to have been altered by ongoing activity in the brain. (Edelman; Bright Air, 102)
Link to — Consciousness Subject Outline
Further discussion -- Covington Theory of Consciousness