Piero Scaruffi

These are excerpts and elaborations from my book "The Nature of Consciousness"

Neural Darwinism Drawing from Jerne’s ideas, in the 1970s the US biologist Gerald Edelman applied the "selectional" theory of the immune system to the brain. His "Neural Darwinism" is a selectional theory of brain development. Edelman was after a rational explanation for two apparently bizarre facts. First, there is no way that the human genome can specify the whole complex structure of the brain. Second, individual brains are wildly diverse. One would instinctively expect the opposite: all information about the brain should be encoded in DNA and every individual should get pretty much the same brain. Edelman was aware that, before birth, the genetic instructions in each organism provide general constraints for neural development, but they 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”. Individual experience "selects" one configuration of neural groups out of all the configurations that are possible.  Note that the unit that gets selected is not the individual neuron but a neuronal group. Edelman’s neural groups are a variation on the “cortical columns” of the cortex analyzed by the US neurologist Vernon Mountcastle in 1957 (“Modality And Topographic Properties Of Single Neurons Of Cat's Somatic Sensory Cortex”). Mountcastle proved that neurons (that perform like functions) are not only organized in horizontal layers, but also in vertical columns. He revealed the modular organization of the brain. Edelman views the functioning of the brain as resulting from a morphological selection of neural groups. Neural groups "compete" to respond to environmental stimuli. That is why each brain is different: its ultimate configuration depends on the stimuli that it encounters during its development. “Adhesion” molecules determine the initial structure of neural groups, the "primary repertory".  Experience 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 to other maps. Maps communicate through parallel bidirectional channels, i.e. through "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 feature enables higher level functions such as memory. Categorization is a process of establishing a relationship between neural maps (through that reentry mechanism).  Categories (perceptual categories, such as "red" or "tall") do not exist physically. They are not located anywhere in the brain. Categories are that (on-going) process. Basically, Edelman believes that neural groups are bound to compete and evolve in a Darwinian way, and eventually self-organize as neural maps (purposeful assemblies of neural groups). A further level of organization leads to (pre-linguistic) conceptualization.  Conceptualization consists in constructing maps of the brain's own activity, or maps of maps. This process of "global mapping" indirectly retains knowledge of past activity. A concept is not a thing. It is a process.  The meaning of something is an on-going, ever-changing process. According to Edelman’s view, brain processes are dynamic and stochastic, whereas the traditional view held the brain to be static and deterministic. Furthermore, 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 were created at birth.  And the elementary unit of this process is not the single neuron, but the neural group. This Darwinian model of the brain explains the non-linearity between the complexity of the genome and that of the brain. The brain is not a direct product of the information contained in the genome. It uses much more information than is available in the genome, i.e. information derived from experience, i.e. indirectly received from the environment.   Back to the beginning of the chapter "Inside The Brain" | Back to the index of all chapters