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The term Neural Darwinism is used in two different ways. In one usage it is the theory that consciousness can be explained by Darwinian selection and evolution of neural states. In the other it describes a process in neurodevelopment where synapses which are being most used are kept while least used connections are destroyed or 'pruned' to form neural pathways.
Although it was believed until 1998 that neurodevelopment ceased at the end of adolescence, there is now strong evidence that neurodevelopment continues throughout adult life.
Given this evidence, the two usages can be regarded as complementary, because:
- persistent patterns of neural activation (which seem to correspond to short term memory) appear to influence neural growth, death, and synapse formation, which appears to be the mechanism for the laying down of long term memory.
- patterns of neural connectivity in turn provide the substrate for patterns of neural activation
Neural Darwinism in theories of consciousness
The term originated as a large scale theory of brain function by Gerald Edelman, who initially published in 1978, in a book called The Mindful Brain (MIT Press). It was extended and published in the 1989 book Neural Darwinism – The Theory of Neuronal Group Selection.
Edelman was awarded 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:
- 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.
- 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.
- Reentrant signaling between neuronal groups allows for spatiotemporal continuity in response to real-world interactions.
Other leading proponents include Daniel Dennett and William H. Calvin.
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.
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.
- 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.
Criticism of the theory
Criticism of Neural "Darwinism" was made by Francis Crick who pointed to the absence of replication in the theory, a requirement for natural selection. Recent work has proposed means by which true replication may take place in the brain. Furthermore, by adding Hebbian learning to neuronal replicators the power of neuronal evolutionary computation may actually be greater than natural selection in organisms.
Neural Darwinism in neurodevelopment
The human brain rapidly creates synaptic connections between neurons after birth. The total number of synapses peaks at around 6-8 months of age. At this point the brain has approximately twice as many synapses between than will exist when the child reaches 10 years of age. Synapses are selectively destroyed or 'pruned', with the most-used synaptic connections remaining and the least-used being removed. In other words it seems the brain becomes rapidly over-connected but we only keep the useful connections. This has been thought to be loosely analogous to Darwin's 'survival of the fittest' maxim and hence has been labelled 'neural Darwinism'.
This process allows the formation of significant and long-lasting neural pathways to be influenced by experience early in life.
- Anthropic mechanism
- Complex adaptive system
- Evolutionary psychology
- Genetic programming
- Long-term potentiation
- Society of Mind theory
- Universal Darwinism
- Fernando, Karishma & Szathmary, 2008
- Fernando, Goldstein & Szathmary, 2010
- 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.
- How Brains Think: Evolving Intelligence, Then and Now by William H. Calvin
- Neurogenesis in the Adult Human Brain
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