Neural Darwinism

Gerald Edelman points to model of gamma globulin molecule (1972 photograph).

From Wikipedia: 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 1989 book Neural Darwinism – The Theory of Neuronal Group Selection.

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 these 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.
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 (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.

This work presents a radical new view of the function of the brain and nervous system. It suggests that the nervous system in each individual operates as a selective system resembling natural selection in evolution but operating different mechanisms. By providing a fundamental neural basis for categorization of the things of the world it unifies perception, action and learning. This theory revises our view of memory, considering it as a dynamic process of recategorization which has implications for the various psychological states from attention to dreaming. It will stimulate discussion about the mind-body problem, the origins of knowledge and the perceptual bases of language. The author won the 1972 Nobel Prize for Physiology of Medicine.