The Astonishing Hypothesis: The Scientific Search for the Soul
Shadows of the Mind: A Search for the Missing Science of Consciousness
Can we find a convincing account of how brain processes cause—or even could cause—our conscious experiences? That is the question I raised in the previous issue. It is differently addressed in all the books under review, and indeed some of the writers do not think the relation of brain to consciousness is a causal relation in the first place.
Of the neurobiological theories of consciousness I have seen, the most impressively worked out and the most profound is that of Gerald Edelman, His two books discussed here are the third and the fourth in a series that began with Topobiology and Neural Darwinism. The aim of the series is to construct a global theory of the brain that situates brain science in relation to physics and evolutionary biology. The centerpiece of the series is the theory of consciousness advanced in The Remembered Present and summarized in Bright Air, Brilliant Fire.
In order to explain Edelman’s theory of consciousness I need first to explain briefly some of the central concepts and theories which he uses, especially those he uses to develop a theory of perceptual categorization. As we saw in the last issue, Crick wants to extend an account of “the binding problem” to a general account of consciousness. (The “binding problem” poses the question of how different stimulus inputs to different parts of the brain are bound together so as to produce a single, unified experience, for example, of seeing a cat.) Similarly, Edelman wants to extend an account of the development of perceptual categories—categories ranging from shapes, color, and movement to objects such as cats and dogs—into a general account of consciousness. The first idea central to Edelman is the notion of maps. A map is a sheet of neurons in the brain where the points on the sheet are systematically related to the corresponding points on a sheet of receptor cells, such as the surface of the skin or the retina of the eye. Maps may also be related to other maps. In the human visual system there are over thirty maps in the visual cortex alone.
The second idea is his Theory of Neuronal Group Selection. According to Edelman we should not think of brain development, especially in matters such as perceptual categorization and memory, as a matter of the brain learning from the impact of the environment. Rather, the brain is genetically equipped from birth with an overabundance of neuronal groups and the brain develops by a mechanism which is like Darwinian natural selection: some neuronal groups die out, others survive and are strengthened. In some parts of the brain, as many as 70 percent of the neurons die before the brain reaches maturity. The unit which gets selected is not the individual neuron, but neuronal groups of hundreds to millions of cells. The basic point is that the brain is not an instructional mechanism, but a selectional mechanism; that is, the brain does not develop by alterations in a fixed …