Neural Darwinism

Neural Darwinism, a large scale theory of brain function, Gerald Edelman, was initially published in 1978, MIT Press, in a book called The Mindful Brain. It was extended and published in the book Neural Darwinism - The Theory of Neuronal Group Selection, 1989.

Edelman won the Nobel Prize in 1972 for his work in immunology showing how the population of lymphocytes capable of binding to a foreign antigen is increased by differential clonal multiplication following antigen discovery. Essentially, this proved that the human body is capable of creating complex adaptive systems as a result of local events with feedback. Edelman’s interest in selective systems expanded into the fields of neurobiology and neurophysiology, and in Neural Darwinism, Edelman puts forth a theory called "neuronal group selection". It contains three major parts:

1. Anatomical connectivity in the brain occurs via selective mechanochemical events that take place epigenetically during development. This creates a diverse primary repertoire by differential reproduction.
2. Once structural diversity is established anatomically, a second selective process occurs during postnatal behavioral experience through epigenetic modifications in the strength of synaptic connections between neuronal groups. This creates a diverse secondary repertoire by differential amplification.
3. Reentrant signaling between neuronal groups allow for spatiotemporal continuity in response to real-world interactions.

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Degeneracy

With neuronal heterogeneity (called degeneracy), it is possible to test the many circuits (on the order of 30 billion neurons with an estimated one million billion connections between them in the human brain) with a diverse set of input, see which neuronal groups respond "appropriately" statistically, and wire up the brain based on the results.

Edelman goes into some detail around how the brain creates such variety, discussing how cell adhesion molecules (CAMs) and substrate adhesion molecules (SAMs) on the cell surface allow cells to dynamically control their intercellular binding properties. This surface modulation allows cell collectives to effective "signal" as the group aggregates, which helps govern morphogenesis. So morphology depends on CAM and SAM function. And CAM and SAM function also depend on developing morphology.

It is theorized that cell proliferation, cell migration, cell death, arbor distribution and neurite branching are also governed by selective processes similar to that of aforementioned cell aggregation.

Synaptic modification

Once this variegated anatomical structure in the brain is laid down during early development, it is more or less fixed. But given this numerous and diverse collection of circuitry, there is bound to be functionally equivalent albeit anatomically non-isomorphic neuronal groups capable of responding to certain sensory input. This creates a competitive environment where circuit groups proficient in their responses to certain input can be "chosen" over others by altering synaptic efficacies of those portions of the network. This leads to an increased probability of their response to similar or identical signals at a future time. This is done by altering synaptic strengths neuron-to-neuron biochemically. And adjustments to neurotransmitters allow for neural plasticity along a much quicker timetable than by anatomical changes.

Reentry

The last part of the theory attempts to explain how we experience spatiotemporal consistency in our interaction with environmental stimuli. Edelman proposes a model of reentrant signaling whereby a disjunctive, multimodal sampling of the same stimulus event correlated in time leads to self-organizing intelligence. Put another way, multiple neuronal groups can be used to sample a given stimuli set in parallel and communicate between these disjunctive groups with incurred latency.

Support for the theory

It has been suggested that Friedrich Hayek had earlier proposed a similar idea in his book The Sensory Order: An Inquiry into the Foundations of Theoretical Psychology, published in 1952.

Other leading proponents include Daniel Dennett, William H. Calvin, and Linda B. Smith.

References

• Edelman, Gerald Neural Darwinism. The Theory of Neuronal Group Selection (Basic Books, New York 1987). ISBN 0-465-04934-6
• Peter S. Eriksson et al. (1998). Neurogenesis in the Adult Human Hippocampus. Nature Medicine (4): 1313-1317.
• Hayek, F. A. The Sensory Order: An Inquiry into the Foundations of Theoretical Psychology xxii, 210 p. 1952 Paper ISBN 0-226-32094-4
• Huttenlocher, P.R. (1990) Morphometric study of human cerebral cortical development. Neuropsychologia, 28, 517-527.

See also

• Darwinism
• Evolutionary psychology
• Meme
• Psychological nativism
• Neurodevelopment
• Society of mind theory
• Complex adaptive system
• Long-term potentiation
• Genetic programming