JTW's Evolutionary Origins - Author: Edelman, Gerald M.
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 Gerald M. Edelman |
The Limbic-Brainstem and Thalamocortical Systems
"There are, grossly speaking, two kinds of nervous system organization that are important to understanding how consciousness evolved. These systems are very different in their organization...
The Limbic-Brainstem System - A Hedonic Value System
The first [organization is called the Limbic-Brainstem system and is] concerned with appetite, sexual and consummatory behavior, and evolved defensive behavior patterns.
It is a value system; it is extensively connected to many different body organs, the endocrine system, and the autonomic nervous system.
Together, these systems regulate heart and respiratory rate, sweating, digestive functions, and the like, as well as bodily cycles related to sleep and sex.
It will come as no surprise to learn that the circuits respond relatively slowly (in periods ranging from seconds to months), and that they do not consist of detailed maps.
They have been selected during evolution to match the body, not to match large numbers of unanticipated signals from the outside world.
These systems evolved early to take care of bodily functions; they are systems of the interior.
The Thalamocortical System - A Somatosensory System
The second major nervous system organization is quite different. It is called the thalamocortical system...
The thalamocortical system consists of the thalamus and the cortex acting together, a system that evolved to receive signals from sensory receptor sheets and to give signals to voluntary muscles.
It is very fast in its responses (taking from milliseconds to seconds), although its synaptic connections undergo some changes that last a lifetime...
[T]he cerebral cortex, is arranged in a set of maps, which receive inputs from the outside world via the thalamus.
Unlike the limbic-brain stem system, it does not contain loops so much as highly connected, layered local structures with massively reentrant connections.
In many places these are topographically arranged.
The cerebral cortex is a structure adapted to receive a dense and rapid series of signals from the world through many sensory modalities simultaneously...
It evolved later than the limbic-brainstem system to permit increasingly sophisticated motor behavior and the categorization of world events.
To handle time, as well as space, the cortical appendages - the cerebellum, basal ganglia, and hippocampus - evolved along with the cortex to deal with succession both in actual motion and in memory.
Two Linked Nervous System Architectures
The two systems, limbic-brain stem and thalamocortical, were linked during evolution.
The later-evolving cortical system served learning behavior that was adaptive to increasingly complex environments.
Because this behavior was clearly selected to serve the physiological needs and values mediated by the earlier limbic-brain stem system, the two systems had to be connected in such a way that their activities could be matched.
Indeed, such a matching is a critical part of learning.
If the cortex is concerned with the categorization of the world and the limbic-brain stem system is concerned with value (or setting it adjustments to evolutionarily selected physiological patterns), then learning may be seen as the means by which categorization occurs on a background of value to result in adaptive changes in behavior that satisfy value."
(Gerald M. Edelman, 1992, pp. 117-118)
- Edelman, Gerald M.
- Bright Air, Brilliant Fire: On the Matter of Mind
- Basic Books, Inc.
- BF 161.E34 1992
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