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

Three Topological Networks in Brain

The brain's neural network can be considered in terms of three topological networks, closely interworking together: (1) the thalamocortical system, which mediates the dynamic core of consciousness, (2) the parallel unidirectional pathways through the basal ganglia, cerebellum, hippocampus and (3) the network of diffusely projecting modulatory neurons emanating from the brain stem.

These topological networks exist in variegated forms in all mammals.  Rudimentary versions of these networks even exist in reptiles.  Functionality such as the basal ganglia have an ancient history in evolution.

Two principles of cortical organization, (1) parallel processing pathways, and (2) hierarchical organization. (Webster; Neuroanatomy of Visual Attention, 24)

Subcortical structures and their connectivity must be included in any analysis of anatomical organization of cognitive function. (Webster; Neuroanatomy of Visual Attention, 24)

Edelman and Damasio described similar organizational concepts in different words:

At the brain’s higher levels of anatomical order, it is possible to distinguish important principles of organization. Three major topological arrangements in the brain appear to be essential to understanding the brain’s global functioning. (Edelman; Universe of Consciousness, 42)

Massively recurrent circuit arrangements, feedforward and feedback loops; some of the loops are purely chemical. (Damasio; Descart’s Error, 122)

Basal ganglia receive inputs from all parts of the cerebral cortex but send their output only to the frontal lobe through the thalamus. (Kandel; Principles of Neural Science, 331)

(1) Thalamocortical system

The thalamocortical system forms the fundamental biological mechanism of consciousness, supported by other functionality.

Thalamocortical system  -  dense meshwork of reentrant connectivity between the thalamus and the cortex and between the cortical regions. (Edelman; Universe of Consciousness, 43)

Subsets of the ongoing thalamocortical activity comprise the dynamic core of consciousness, ever changing on a millisecond-by-millisecond basis.

(2) Parallel pathways through basal ganglia, cerebellum, hippocampus

A set of parallel, unidirectional neural pathways link the cortex to a set of its appendages, each with a special structure -- the cerebellum, the basal ganglia, and the hippocampus. (Edelman; Universe of Consciousness, 45)

Traditionally, the cerebellum is considered to be concerned with the coordination and synchrony of motion, although its involvement in certain aspects of thought and language appears to be substantial. (Edelman; Universe of Consciousness, 45)

Basal ganglia consists of a set of large nuclei deep in the brain that receive connections from much of the cortex, go through a series of successive synaptic steps, and then project to the thalamus and from there back to the cortex. Basal ganglia are involved in the planning and execution of complex motor and cognitive acts and are dysfunctional in Parkinson's and Huntington's diseases. (Edelman; Universe of Consciousness, 45)

The large-scale organization of the basal ganglia can be viewed as a family of reentrant loops that are organized in parallel, each taking its origin from a particular set of functionally related cortical fields, passing through the functionally corresponding portions of the basal ganglia, and returning to parts of those same cortical fields by way of specific basal ganglia recipient zones in the dorsal thalamus. (Alexander; Basal Ganglia, 139)

Hippocampus is an elongated structure that runs along the lower edge of the temporal cortex.  Inputs from many different cortical areas are funneled into the hippocampus, which deals with these inputs in a series of synaptic steps and sends projections back to many of the same cortical areas.  The hippocampus has a major role in consolidating short-term memory into long-term memory in the cerebral cortex. (Edelman; Universe of Consciousness, 45)

Parallel organization of the functionally segregated circuits going to and from cortex to basal ganglia to thalamus and back to cortex. (Edelman; Remembered Present, 134)

Long reentrant connective loops have been identified that originate in frontal areas, course through the basal ganglia and the lateral thalamus, and return to frontal areas. (Fuster; Memory in Cerebral Cortex, 77)

 

Link to — Parallel Pathways Link Cortex To Subcortical Appendages

 

(3) Neuromodulatory systems

Fan-out meshworks of diffusely projecting neuromodulatory neurons emanate from brain stem and midbrain nuclei. Reentrant circuits of the thalamocortical system are modulated by these neurotransmitters.

Noradrenergic locus coeruleus  -  fan-like 'hairnet' of fibers all over the brain, release neuromodulator noradrenaline. (Edelman; Universe of Consciousness, 46)

Small collections of neurons can deliver a dose of dopamine, norepinephrine, serotonin or acetylcholine to widespread regions of the brain including the cerebral cortex and basal ganglia. (Damasio; Descart’s Error, 120)

Neuromodulatory neurons of the brainstem - (diagram) (Hobson; Consciousness, 47)

Modulatory systems in the brain: (1) Noradrenergic, (2) Serotonergic, (3) Dopaminergic, (4) Cholinergic  -  (diagram) (Hobson; Consciousness, 75)

Neurons of the brain stem that project to the cortex use transmitters such as serotonin, norepinephrine, and dopamine. Other neurons in the brain use acetylcholine. (Crick; Astonishing Hypothesis, 103)

 

 

Link to — Consciousness Subject Outline

Further discussion — Covington Theory of Consciousness