Editing Neural Darwinism (section)

=== Organization of the TNGS theory ===

''Neural Darwinism'' organizes the explanation of TNGS into three parts – somatic selection, epigenetic mechanisms, and global functions. The first two parts are concerned with how variation emerges through the interaction of genetic and epigenetic events at the cellular level in response to events occurring at the level of the developing animal nervous system. The third part attempts to build a temporally coherent model of globally unitary cognitive function and behavior that emerges from the bottom up through the interactions of the neuronal groups in real-time.

Edelman organized key ideas of the TNGS theory into three main tenets:

*Primary repertoire – developmental formation and selection of neuronal groups;
*Secondary repertoire – behavioral and experiential selection leading to changes in the strength of connections between synaptic populations that bind together neuronal groups;
*Reentrant signaling – the synchronous entrainment of reciprocally connected neuronal groups within sensorimotor maps into ensembles of coherent global activity.

The primary repertoire is formed during the period from the beginning of [[neurulation]] to the end of apoptosis. The secondary repertoire extends over the period synaptogenesis and myelination, but will continue to demonstrate developmental plasticity throughout life, albeit in a diminished fashion compared to early development.

The two repertoires deal with the issue of the relationship between genetic and epigenetic processes in determining the overall architecture of the neuroanatomy – seeking to reconcile nature, nurture, and variability in the forming the final phenotype of any individual nervous system.
 
There is no point-to-point wiring that carries a neural code through a computational logic circuit that delivers the result to the brain because

* firstly, the evidence does not lend support to such notion in a manner that is not problematic,
* secondly, the noise in the system is too great for a neural code to be coherent,
* and third, the genes can only contribute to, and constrain, developmental processes; not determine them in all their details.

Variation is the inevitable outcome of developmental dynamics.

Reentrant signalling is an attempt to explain how "coherent temporal correlations of the responses of sensory receptor sheets, motor ensembles, and interacting neuronal groups in different brain regions occur".{{sfn|Edelman|1987b|p=5}}

==== Primary repertoire- developmental selection ====

The first tenet of TNGS concerns events that are embryonic and run up to the neonatal period. This part of the theory attempts to account for the unique anatomical diversification of the brain even between genetically identical individuals. The first tenet proposes the development of a primary repertoire of degenerate neuronal groups with diverse anatomical connections are established via the historical contingencies of the primary processes of development. It seeks to provide an explanation of how the diversity of neuronal group [[phenotypes]] emerge from the organism's [[genotype]] via [[Genetics|genetic]] and [[Epigenesis (biology)|epigenetic]] influences that manifest themselves mechano-chemically at the cell surface and determine connectivity.

Edelman list the following as vital to the formation of the primary repertoire of neuronal groups but, also contributing to their anatomical diversification and variation:

* Cell division – there are repeated rounds of cell division in the formation of neuronal populations
* Cell death – there is extensive amounts of pre-programmed cell death that occurs via apoptosis within the neuronal populations.
* Process extension and elimination – the exploratory probing of the embryonic environment by developing neurons involve process extension and elimination as the neurons detect molecular gradients on neighboring cell surface membranes and the substrate of the extracellular matrix.
* CAM & SAM action – the mechanochemistry of cell and surface adhesion molecules plays a key role in the migration and connectivity of neurons as they form neuronal groups within the overall distributed population.

Two key questions with respect to this issue that Edelman is seeking to answer "in terms of developmental genetic and epigenetic events" are:{{sfn|Edelman|1987b|p=75}}

* "How does a one-dimensional genetic code specify a three-dimensional animal?"
* "How is the answer to this question consistent with the possibility of relatively rapid morphological change in relatively short periods of evolutionary time?"

==== Secondary repertoire – experiential selection ====

The second tenet of TNGS regards postnatal events that govern the development of a secondary repertoire of synaptic connectivity between higher-order populations of neuronal groups whose formation is driven by behavioral or experiential selection acting on synaptic populations within and between neuronal groups. Edelman's notion of the secondary repertoire heavily borrows from work of [[Jean-Pierre Changeux]], and his associates [[Philippe Courrège]] and [[Antoine Danchin]] – and, their theory of selective stabilization of synapses.{{sfn|Changeux|Courrège|Danchin|1973}}

===== Synaptic modification =====

Once the basic variegated anatomical structure of the primary repertoire of neuronal groups is laid down, it is more or less fixed. But given the numerous and diverse collection of neuronal group networks, there are bound to be functionally equivalent albeit anatomically non-[[isomorphic]] neuronal groups and networks capable of responding to certain sensory input. This creates a competitive environment where neuronal groups proficient in their responses to certain inputs are "differentially amplified" through the enhancement of the synaptic [[efficacy|efficacies]] of the selected neuronal group 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. These adjustments allow for [[neural plasticity]] along a fairly quick timetable.

==== Reentry ====
{{Main|Reentry (neural circuitry)}}

The third, and final, tenet of TNGS is reentry. Reentrant signalling "is based on the existence of reciprocally connected neural maps."{{sfn|Edelman|1987b|p=5}} These topobiological maps maintain and coordinate the real-time responses of multiple responding secondary repertoire networks, both unimodal and multimodal – and their reciprocal reentrant connections allow them to "maintain and sustain the spatiotemporal continuity in response to real-world signals."{{sfn|Edelman|1987b|p=5}}

The last part of the theory attempts to explain how we experience spatiotemporal consistency in our interaction with environmental stimuli. Edelman called it "[[Reentry (neural circuitry)|reentry]]" and proposes a model of reentrant signaling whereby a disjunctive, multimodal sampling of the same stimulus event correlated in time that make possible sustained physiological entrainment of distributed neuronal groups into temporally stable global behavioral units of action or perception. 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.