The thalamus rules

There appears to be a consensus that the thalamus is important to consciousness. But it is usually considered an equal partner or even as a lesser one to the neocortex is far as consciousness goes. I asked you to suspend disbelief for a short while, you can resume it later, and consider that the thalamus may be the seat and center of consciousness. Why might this be so?

First, the neocortex is too new to be the center of consciousness. It seems reasonable that an animal that purposefully moves must have some level of consciousness. Awareness and a model of the organism-in-its-environment seems required for successful, useful movement. An animal has to know where it is and where it is going even if that knowledge is very rudimentary. The thalamus is at least as old as the earliest vertebrates. Invertebrates may have a different path to whatever amount of consciousness they have (and I assume they have some).

Second, the thalamus is the central place where things come together. For most types of information it is the first integration of different sources of information. The thalamus supplies the neocortex with almost all its input.

Third, the thalamus is the source of control of consciousness for the neocortex. Signals from the brain stem to the thalamus and from the thalamus on to the neocortex are essential to initiate and maintain consciousness. Remove the thalamus and there is no consciousness. Remove great pieces of the neocortex and consciousness remains. Remove the whole neocortex and we don’t know what level of consciousness may exist; it might be a very rudimentary one that cannot be reported (so who knows).

Fourth, the thalamus develops before the neocortex and it controls the migration of neurons to take their places in the cortex. This results in a mapping of neurons in the neocortex with neurons in the thalamus. And this results in giving the functions that concern the thalamus areas to the matching neocortex areas. Large parts of the thalamus are effectively mirrored on the neocortex.

Fifth, the thalamus and neocortex communicate through feedback loops. Where the thalamus neuron sends an axon to a few neocortex neurons, the neocortex neurons send an axon back to the same thalamus neuron. The thalamo-cortical loop is a correlate of consciousness. The thalamus appears to control the behaviour of this massive feedback system. It drives the system and controls what parts of the cortex participate at any time. It appears that any part of the cortex only contributes its content to consciousness if it is driven into synchrony with other parts of the cortex and the thalamus via the thalamo-cortical loops.

Sixth, the thalamus also control the level of activity of parts of the cortex by loops that are not specific to a particular information source. This seems to give the thalamus control over which network/types of cognition the neocortex does at any time. It also appears to control attention and possibly the use of working memory.

Taken together this looks like the thalamus creates, drives, feeds with input and micro-manages the neo-cortex. It is as if the thalamus has an online cognition engine in the way it uses the cortex to detail and refine its inputs into a very useful model, a model that the thalamus then uses to construct a conscious experience and the very short-term memory of that experience.

The suspension of disbelieve can now be removed.

Here is an abstract from a Radolfo Llinas paper, Consciousness and the thalamocortical loop, International Congress Series (2003).

Attempting to understand how the brain might be organized seems, for the first time, to be a serious topic of inquiry. One aspect of its neuronal organization that seems particularly central to global function is the rich thalamocortical interconnectivity and most particularly the reciprocal nature of the thalamocortical neuronal loop function. Moreover, the interaction between the specific and nonspecific thalamic loops suggests that rather than a gate into the brain, the thalamus represents a hub from which any site in the cortex can communicate with any other such site or sites. The goal of this paper is to explore the basic assumption that large-scale, temporal coincidence of specific and nonspecific thalamic activity generates the functional states that characterize human cognition.

 

The question that has to been asked is exactly how the thalamocortical system works.

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