Gerald Edelman:

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Gerald Edelman is possibly the main contributor to the selectional theory of the immune system. When the body is attacked by a virus, it produces specially adapted protein molecules, antibodies, that attach themselves to the invaders and destroy them. Those antibodies are created by the thousands "before" the body is attacked by anything. An invasion results in a rapid increase in the rate of production of the one antibody that matches the intruder. Edelman is now applying the same concept to a selectional theory for brain development, thereby introducing population thinking to neurobiology.
Before birth the genetic instructions in each organism provide general constraints for neural development, but cannot specify the exact location and configuration of each cell. After birth innate "values", i.e. adaptive cues (such as "looking for food"), generate behavior and therefore feedback from the environment, which in turns helps "select" the neural configurations that are more suitable for survival. During this on-going process of "learning" the brain develops categories by selectively strengthening or weakening connections between neural groups. Experience "selects" one configuration of neural groups out of all the configurations that are possible.
The functioning of the brain can be explained as resulting from a morphological selection of neural groups. Neural groups "compete" to respond to environmental stimuli. Each brain is therefore different, depending on the stimuli that it encounters during its development.
Adhesion molecules determine the initial structure of neural groups, the "primary repertory". Behavior determines the secondary repertory. Repertories are organized in "maps", each map having a specific neural function. A map is a set of neurons in the brain that has a number of links to a set of receptor cells or other maps.
Maps communicate through parallel bidirectional channels, i.e. the "reentrant" signaling. Reentry is not just feedback because there can be many parallel pathways operating simultaneously. The process of reentrant signaling allows a perceptual categorization of the world, i.e. to relate independent stimuli. This ability enables higher level functions such as memory.
In Edelman's view brain processes are dynamic and stochastic.
The brain is not an "instructional" system but a "selectional" system. It evolves not by changes in a constant set of neurons but by selection of the most valuable neural groups among those that exist since birth. And the elementary unit of this process is not the single neuron, but the neural group.

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