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2.7 Neural Darwinism

1. Introduction

There are a multitude of different theories on the mind. Many more than have been discussed in this document. However, of all the ones I have seen before, I feel that this one offers the greatest hope of coming up with a real, working understanding of the science and neurobiology of how the mind works and what consciousness really is. Its author, Gerald Edelman, is a former Nobel laureate who was instrumental in cracking the mystery of how our immune systems work. After that he turned his attention to something far more difficult, attempting to understand how the neurobiology of the brain forms the mind. The main thrust of his theory of neural Darwinism is that the brain is a somatic selection system similar to evolution, and not an instructional system. (Somatic means that is over the time scale of your body instead of being on the time scale of evolution.)

2. Instruction vs. Selection

The main difference between an instructional system and a selectional system is that the instructional system uses information from the environment to change the properties of the object in question, but a selectional system has a large and varied population of objects, and the ones that are most fit for the environment are differentially reproduced. Hopefully an example Edelman uses from immunology will help clear this up.

The theory prevailing before the present one was called the theory of instruction. Its basic assumption was that, in the immune system, a foreign molecule transferred information about its shape and structure to the combining site of the antibody molecule. It then removed itself (the way a cookie cutter would be removed from dough) leaving a crevice of complementary shape that could then bind to all foreign molecules with regions having the shape with which the impression was originally made. It is obvious why this is an instructive process: Information about the three-dimensional structure is posited to be necessary to instruct the immune system how to form an antibody protein whose polypeptide chain folds around that structure to give the complementary shape.
(Edelman, Bright Air, Brilliant Fire)2.7.1

However, it was discovered that this theory was wrong. The immune system was in fact a somatic selection system. Most laypeople do not realize that evolution is really just a search algorithm, and the most efficient one ever found in the proper problem domains. It is a specific example of a selection system.

In evolution, organisms are more or less adapted to events in the environment. This adaptation occurs even when environmental changes cannot be predicted (that is even when the changes represent novelties). The process of adaptation occurs by selection on those organismal variants that are on the average fittest, and what makes them fittest does not require prior explicit information ("instruction") about the nature of the novelties in the environment. The selective environmental changes are, in general, independent of variation in the population of organisms, although selection resulting from such changes may add to that variation. In sum, there is no explicit information transferred between the environment and organisms that cause the population to change and increase its fitness. Evolution works by selection, not by instruction. There is no final cause, no teleology, no purpose guiding the overall process, the responses of which occur ex post fact in each case.
(Edelman, Bright Air, Brilliant Fire)2.7.2

The algorithm can be run on many different kinds of things and not just biological organisms in evolution. In the immune system it is run on a population of different antibody molecules. Those that encounter foreign molecules are reproduced, and those that do not are not magnified in this way. The algorithm works in the immune system on a much shorter time scale than it does in evolution. The thing that selectional systems give us in this debate that instructional systems do not, is that selectional systems do not have to have any prior knowledge of the environment, and require no explicit information transfer from the world. Whereas with the instructional system you are left with the question of who or what decides what is important for the system to learn! This leads to the endless regression of homunculi (the small man in my head) seen in figure 1.

Figure 1. The endless regression of homunculi. The idea of instruction or information processing requires someone, or something, to read it. But a similar entity is then required to read the resulting messages, and so on, endlessly. 2.7.1

3. The Stages of Selection

Stages of Selection
Figure 2. The different stages of selection in the theory of Neural Darwinism. 2.7.2

Figure 2 shows the major stages of selection that make up the theory of Neural Darwinism. The three stages are developmental selection, experiential selection, and reentrant mapping. Each section is discussed more fully below.

4. Developmental Selection

Our genes do not specify explicit blueprints for building our bodies. They are more like general guidelines. (This is a very complex subject and I will not go into details here.) So even clones with 100% identical genes will not have the exact same neural layout in their brains. The body creates a huge variation of different neural connections during development. The thing that the genes do is specify the more large scale connections and features. For instance, your genes specify that area A of the brain connects to area B. But it does not explicitly say that Neuron A at location (X1, Y1, Z1) connects to Neuron B at location (X2, Y2, Z2) with a strength of K. The brain actually creates far more connections than you actually use. Babies lose a large proportion of their neurons and connections in the first year of life. Edelman believes that the brain initially grows out a huge population of variant neuronal groups (groups consist of between one hundred and one million neurons). And it is the sampling of the environment by the newborn that then selects which of the connections and neurons to keep and which to discard. (There is a lot of evidence provided to back this up that is being skipped here. If you are interested then please read his books in the selected reading section.) This is called the primary repertoire.

5. Experiential Selection

However, eliminative selection alone could never build something as incredible as the mind. The next phase of the theory uses the experiences of the organism to strengthen or weaken populations of synapses in the primary repertoire. As you see, hear, taste, smell, and move, it effectively "carves out" a variety of functioning circuits (with strengthened synapses) from the anatomical network by selection. This is called the secondary repertoire.

6. Reentrant Mapping

It is an established fact that the mind forms a multitude of neural maps. For example, there is a map that relates the pressure receptors for each area of your body to a specific area in your brain. The next portion of the theory is the one that connects psychology to physiology. It says that brain maps interact with each other in a process called reentry.

To carry out such functions, primary and secondary repertoires must form maps. These maps are connected by massively parallel and reciprocal connections. The visual system of the monkey, for example, has over thirty different maps, each with a certain degree of functional segregation (for orientation, color, movement, and so forth), and linked to the others by parallel and reciprocal connections. Reentrant signaling occurs along these connections. This means that, as groups of neurons are selected in a map, other groups in reentrantly connected but different maps may also be selected at the same time. Correlation and coordination of such selection events are achieved by reentrant signaling and by the strengthening of interconnections between the maps within a segment of time.
(Edelman, Bright Air, Brilliant Fire)2.7.3

7. Conclusion

I have in no way done this theory justice in this short space. But hopefully I was able to give you the general idea what is involved. If this has peaked your interest then I strongly suggest you read Edelman's books. They can be very difficult to understand, but they are well worth it. This theory is very strongly based on neurobiological evidence that has largely been collected over the last fifty years. And it is the first one I have seen that gives me any real hope that we can solve the mysteries of the mind within my lifetime.

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