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

Thalamus

 

A major function of the thalamus is to gate and otherwise modulate the flow of information to the cortex. The thalamus represents the final bottleneck of information flow before it gets to cortex. For processes of attention and other behavioral requirements, evolution has established a modification and filtering of information flow at the level of thalamus before it reaches the cortex. (S. Murray Sherman (2006), Scholarpedia, 1(9):1583.)

 

The thalamus occupies a substantial part of the middle section of the brain (diencephalon). (Greenfield; Human Brain, 48)

The thalamus is sometimes considered as a seventh layer of cortex. (Tononi & Laureys; Neurology of Consciousness, 391)

As many as 50 thalamic nuclei have been identified. (Kandel; Principles of Neural Science, 341)

The thalamus is often classified in two parts -- the dorsal thalamus, which is the main bulk of it, and the ventral thalamus. (Francis Crick; Searchlight Hypothesis, 264)

Most Thalamic Nuclei are Thalamocortical rather than Sensory Relay

Most of the thalamic nuclei are specific nuclei that connect in an orderly topological pattern,  one thalamic nucleus to one area of the cortex. (Mumford; Thalamus, 981)

In addition to specific nuclei, there are also nonspecific nuclei in the thalamus that project diffusely, often onto the entire cortex. (Mumford; Thalamus, 981)

The vast majority of the nonspecific thalamic nuclei interconnect activity between brain areas, and for this reason they have sometimes been termed "association" nuclei. (LaBerge; Attentional Processing, 160)

The vast majority of signals traversing the thalamus arise from the cortex itself and from a variety of subcortical areas, notably the superior colliculus and the basal ganglia. (LaBerge; Attentional Processing, 163)

Virtually every cortical area sends signals to and receives signals from a thalamic nucleus, and there is considerable anatomical precision in the mappings between cortex and thalamus. (LaBerge; Attentional Processing, 163)

Since all regions of the cortex are connected with the thalamus, it seems highly probable that the triangular circuit exists wherever cortical columns in one area communicate with columns in another area. (LaBerge; Attention, the Triangular Circuit, 301)

The thalamus is a hub for the neocortex that provides functional shortcuts between the vast areas of the cerebral hemispheres and reduces the synaptic pathways between various cortical areas. (Buzsáki; Rhythms of the Brain, 205)

The thalamus is no longer viewed as a gigantic array of independent relays, but as a large communication hub that assists in linking large cortical areas in a flexible manner.  The principal mechanism of the cortical-thalamic-cortical flow of activity is self-sustained oscillations. (Buzsáki; Rhythms of the Brain, 186)

Subcortical activating systems in the thalamus, with its rich and diverse connectivity to the cortex, have been considered a likely site for selective engagement of specific cortical functions. (Fischler; Attention and Language, 385)

The "sensory" thalamic nuclei clearly constitute a minor part of the thalamic volume. (LaBerge; Attentional Processing, 160)

The largest nucleus in the thalamus is the pulvinar, a Greek term meaning "pillow." (LaBerge; Attentional Processing, 160)

The dorsal thalamus and epithalamus (which contains the reticular nucleus) have been implicated in attentional processing. (LaBerge; Attentional Processing, 160)

The lateral geniculate nucleus (LGN) and the medial geniculate nucleus (MGN) contains cells that relay information from the eye and ear, respectively, to the cortex. (LaBerge; Attentional Processing, 160)

 

Reticular Complex

The reticular complex is a thin sheet of neurons, in most places only a few cells thick, which partly surrounds the dorsal thalamus. (Francis Crick; Searchlight Hypothesis, 264)

All axons from the thalamus to the cerebral cortex pass through the reticular complex, as do all of the reverse projections from the cortex to the thalamus. (Francis Crick; Searchlight Hypothesis, 264)

The intralamina nuclei of the thalamus, which project very strongly to the striatum, also send their axons through the reticular complex, as may some of the axons from the globus pallidus that project back to the thalamus. (Francis Crick; Searchlight Hypothesis, 264)

Many of the axons that pass in both directions through the reticular complex give off collaterals that make excitatory synaptic contacts. (Francis Crick; Searchlight Hypothesis, 265)

Thalamus participates in Motor Functions

Some axons in the thalamus participate in motor functions, transmitting information from the cerebellum and basal ganglia to the motor regions of the frontal lobe. (Kandel; Principles of Neural Science, 341)

Major influence of the cerebellum on movement is through its connections to the ventral nuclear group of the thalamus, which connects directly to the motor cortex. (Kandel; Principles of Neural Science, 348)

Thalamus may play a role in Cognitive Functions

Some thalamic nuclei that may play a role in attention  project diffusely to large but distinctly different regions of cortex. (Kandel; Principles of Neural Science, 341)

Through its connections with the frontal lobe, the thalamus may play a role in cognitive functions such as memory. (Kandel; Principles of Neural Science, 341)

Auditory Information passes through Medial Geniculate Nucleus of Thalamus

Medial geniculate nucleus is a component of the auditory system and conveys tonotopically organized auditory information to the superior temporal gyrus of the temporal lobe. (Kandel; Principles of Neural Science, 341)

Reciprocal Connections with Cerebral Cortex

Most nuclei of the thalamus receive a prominent return projection from the cerebral cortex. (Kandel; Principles of Neural Science, 343)

Reticular nucleus — a sheet-like structure

Reticular nucleus -- outer covering of the thalamus formed by a sheet-like structure. (Kandel; Principles of Neural Science, 343)

Most of the neurons of the reticular nucleus use the inhibitory transmitter GABA, whereas most other thalamic neurons utilize the excitatory transmitter glutamate. (Kandel; Principles of Neural Science, 344)

Reticular nucleus  modulates activity of other thalamic nuclei based on its monitoring of the entirety of the thalamocortical stream of information. (Kandel; Principles of Neural Science, 344)

 

Research study — Prefrontal–Thalamo–Hippocampal circuit for Goal-Directed Navigation

Research study — Attention Gateway in Visual Thalamus

 

 

Return to — Thalamocortical System