So far this discussion of brain science and clinical psychiatry has spanned nonconscious subcortical processes like the dopamine reward circuit, the complex and partially understood connections of the amygdala which mediate responses to threats, and conscious experiences like anxiety, empathy, and addiction. I have not addressed the phenomenon of consciousness itself, which is essential both to understanding psychiatric disorders and treating patients. This daunting question has baffled philosophers and scientists for centuries, and it is at the forefront of research agendas today.
I have looked at models of consciousness by psychologists, philosophers, computer scientists, and neurobiologists. Most suffer from the problem faced by the blind men, each of whom felt a different part of the elephant. In addition, all modelsreach a point that feels like a blackboard mathematician’s hand-waving—the desperate vagueness we turn to when a problem is insoluble. And sometimes I fear we are dealing with the modern equivalent of epicycles, the fanciful system of interlocking circular orbits the ancient astronomer Ptolemy devised to explain how the heavenly bodies moved around the earth.
Consciousness is probably more complex than the human mind can comprehend. I expect there will be limits to how much computational models will help, since the brain is a biological system which operates by very different rules than digital computers. I had hoped to build a four-dimensional (including time) picture grounded in anatomy and physiology, but as I suggest in my discussion reentry below, the brain operates in far more than four dimensions. At this point the best approach seems to be to accept that, like the blind men, we can only understand part of the problem and hope that our minds will be able to at least partially integrate the incomplete theories we are able to construct.
The most satisfyingly proposal I have found is by Gerald Edelman, a physician neuroscientist who received the Nobel Prize in 1972 for his work on the structure of antibodies. He went on to study cell adhesion and the organization of tissues, which led to his theory of Neural Darwinism, which he also called the Theory of Neuronal Group Selection. It comes closer to explaining consciousness from a biological point of view than anything else I have found. Edelman developed his ideas in a series of scientific papers and books between 1978 and 2003; he also wrote several books for lay readers. Even these are difficult, for they draw not only on evolutionary theory but also on mathematical descriptions of complex systems. This discussion is based mainly on his 2004 book, Wider Than the Sky: The Phenomenal Gift of Consciousness. Edelman died in 2014.
Here is a sample of Edelman’s eloquence: “close your eyes and imagine myriad neural firings in millions of pathways. Some of this neural activity would occur at certain frequencies while others would show variable frequencies. Bodily activity and signals from the environment and the brain itself would modify which of the pathways were favored over others as a result of changes in synaptic strength.”
The brain contains on the order of a hundred billion neurons, connected in a thousand times that many synapses. These are unimaginable numbers—a hundred trillion seconds is over three million years. The major tracts were identified long ago, but modern neuroimaging is just beginning to sort out the brain’s functional connections. These are far from random, but unlike some other natural systems, like crystals, they are not regularly ordered.
Edelman’s theory is built on concepts of degeneracy, developmental selection, experiential selection, and reentry. The key structural elements are the dynamic core, which is a huge array of bidirectional connections involving the thalamus and various parts of the cortex; the circuits of the basal ganglia, which modify activity in the dynamic core through their connections with the thalamus; and ascending tracts using the specialized neurotransmitters dopamine, norepinephrine, serotonin, acetylcholine, and histamine, which contribute to what Edelman calls value-category memory.
Degeneracy is Edelman’s term for the ability of different structures to carry out the same function. He notes that a structure or circuit may produce a given output at one instant, then another may take over, they may shift back and forth, and other structures may participate as well. This is one aspect of the brain’s plasticity, its ability to change in response to external or internal stimuli.
Our genes determine the brain’s basic organization, but even in early development the in-utero environment influences both gene expression and epigenetic processes which determine the results of gene activity. In addition, proteins coded by one gene affect the expression of other genes.
According to Edelman, synaptic connections between neurons are strengthened by use. If one of several redundant structural systems responds more efficiently, it will be biologically selected and strengthened. This is what he means by neuronal group selection. It is a dynamic process leading to modifications which may be long-lasting but to some extent capable of change.
Such selection takes place under genetic guidance during development, but, because of random variation and in-utero environmental influences, individual nervous systems—even those of identical twins—are unique.
Overlapping developmental selection is what Edelman calls experiential selection. Sensory input, including somatosensory input resulting from motor activity starting with fetal movements in the womb, provides information which leads some synapses and circuits to be strengthened and others to fade. This an aspect of apoptosis, the process of programmed death by which we lose about a third of our neurons during development.
Edelman’s concept of reentry refers to the operation of two-way, multi-level connections in the brain. Because it is multi-level, it is more complex than simple feedback, in which information from the output of a process is used to control the process itself. Edelman believed reentry, primarily in the dynamic core circuits of the thalamus and cortex, was the basis of consciousness. For me this gets into hand-waving: I do not fully grasp how consciousness emerges from such reciprocal interactions. He seems to be saying that ever-higher levels of distinctions in sensory processing lead to abstractions and thoughts.
Edelman describes two levels of consciousness. Primary consciousness occurs in most animals and involves reentry between regions carrying out perceptual categorization and those mediating value-category memory. This results in the simple consciousness he calls the remembered present. Higher-order consciousness occurs in animals with semantic capacity, such as chimpanzees, and involves awareness of being aware. Animals with linguistic capability such as humans also have social consciousness and awareness of past and future. No additional mind, ego or soul is needed for consciousness—it emerges from the operations of complex reentrant circuits. Consciousness is not, however, the same thing as the neural processes which generate it. Edelman uses a metaphor he attributes to William James: like the music which emerges from a harp’s strings, consciousness is the result of neural activity, but just as plucking the strings is not itself music, consciousness it is not the same as neural activity.
This is a grossly abbreviated summary of Edelman’s theory. I hope to explore the neural processes which underlie consciousness in more detail future posts. For now, the main lesson for clinical psychiatry would seem to be one of humility: in the words of Lord Kelvin, our knowledge is of a meager and unsatisfactory kind.