Neural Darwinism

Neural Darwinism, a large scale theory of brain function by Gerald Edelman, was initially published in 1978, in a book called The Mindful Brain (MIT Press). It was extended and published in the 1987 book Neural Darwinism – The Theory of Neuronal Group Selection.

In 1972, Edelman was awarded the Nobel Prize in Medicine or Physiology (shared with Rodney Porter of Great Britain) 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 allows for spatiotemporal continuity in response to real-world interactions. In "The Remembered Present" (1989) and later, "Bright Air, Brilliant Fire: On the Matter of the Mind" (1992) and "A Universe of Consciousness: How Matter Becomes Imagination" (2001; coauthored with Giulio Tononi), Edelman argues that thalamocortical and corticocortical reentrant signaling are critical to generating and maintaining conscious states in mammals.

Degeneracy

With neuronal heterogeneity (by Edelman called degeneracy), it is possible to test the many circuits (on the order of 30 billion neurons with an estimated one quadrillion connections between them in the human brain) with a diverse set of inputs, to see which neuronal groups respond "appropriately" statistically. Functional "distributed" (widespread) brain circuits thus emerge as a result.

Edelman goes into some detail about how brain development depends on a variety of cell adhesion molecules (CAMs) and substrate adhesion molecules (SAMs) on cell surfaces which allow cells to dynamically control their intercellular binding properties. This surface modulation allows cell collectives to effectively "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.

Edelman theorized that cell proliferation, cell migration, cell death, neuron arbor distribution, and neurite branching are also governed by similar selective processes.

Synaptic modification

Once the basic variegated anatomical structure of the brain is laid down during early development, it is more or less fixed. But given the numerous and diverse collection of available circuitry, there are 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 inputs are "chosen" through the enhancement of the synaptic efficacies of the selected network. This leads to an increased probability that the same network will respond to similar or identical signals at a future time. This occurs through the strengthening of neuron-to-neuron synapses. And these adjustments allow for neural plasticity along a fairly quick timetable.

Reentry

Main article: Reentry (neural circuitry)

The last part of the theory attempts to explain how we experience spatiotemporal consistency in our interaction with environmental stimuli. Edelman called it "reentry" and 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 stimulus 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 (Herrmann-Pillath, 1992). Other leading proponents include Jean-Pierre Changeux, Daniel Dennett, William H. Calvin, and Linda B. Smith. However, William Calvin proposes true replication in the brain, whereas Edelman's Neural Darwinism opposes the idea that there are true replicators in the brain.

Criticism of the theory

Criticism of Neural "Darwinism" was made by Francis Crick on the basis that neuronal groups are instructed by the environment rather than undergoing blind variation. A recent review by Fernando, Szathmary and Husbands explains why Edelman's Neural Darwinism is not Darwinian because it does not contain units of evolution as defined by John Maynard Smith. It is selectionist in that it satisfies the Price equation, but there is no mechanism in Edelman's theory that explains how information can be transferred between neuronal groups. A recent theory called Evolutionary Neurodynamics being developed by Eors Szathmary and Chrisantha Fernando has proposed several means by which true replication may take place in the brain. These neuronal models have been extended in a later paper by Chrisantha Fernando. In the most recent model, three plasticity mechanisms i) multiplicative STDP, ii) LTD, and iii) Heterosynaptic competition, are responsible for copying of connectivity patterns from one part of the brain to another. Exactly the same plasticity rules can explain experimental data for how infants do causal learning in the experiments conducted by Alison Gopnik. It has also been shown that by adding Hebbian learning to neuronal replicators the power of neuronal evolutionary computation may actually be greater than natural selection in organisms.

See also

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

References

• Edelman, Gerald Neural Darwinism. The Theory of Neuronal Group Selection (Basic Books, New York 1987). ISBN 0-465-04934-6
• Edelman, Gerald "The Remembered Present: A Biological Theory of Consciousness" (Basic Books, New York 1989), ISBN 0-465-06910-X
• Edelman, Gerald "Bright Air, Brilliant Fire: On the Matter of the Mind" (Basic Books, New York 1992), ISBN 0-465-052452
• Edelman, Gerald and Tononi, Giulio "A Universe of Consciousness: How Matter Becomes Imagination" (Basic Books, New York 2001), ISBN 0-465-013775
• Hayek, F.A. The Sensory Order: An Inquiry into the Foundations of Theoretical Psychology xxii, 210 p. (Routledge & Kegan Paul, London 1952) Paper ISBN 0-226-32094-4

Further reading

• (originally published in Artificial Intelligence 39 (1989) 121-139.)

External links

• How Brains Think: Evolving Intelligence, Then and Now by William H. Calvin
• Neurogenesis in the Adult Human Brain
• Johnson, George "Evolution Between the Ears", "New York Times," April 19, 1992, accessed April 16, 2007 (a critical review of Gerald Edelman's 1992 book Brilliant Air, Brilliant Fire)
• Calvin, William "Neural Darwinism: The Theory of Neuronal Group Selection", Science, 24 June 1988, accessed April 16, 2007 (a review of Gerald Edelman's book Neural Darwinism)
• Webpage of Chrisantha Fernando, first author of "Copying and Evolution of Neuronal Topology"
• A Wikiversity course that extensively discusses neural Darwinism
• The basic Neuroscience course at Wikiversity
• The Department of Neuroscience at Wikiversity