Vishnu Dreaming by C. Shana Greger

How the Mind Works: Revelations

By Israel Rosenfield and Edward Ziff
The New York Review of Books, June 26, 2008

Edited by Andy Ross

The Physiology of Truth: Neuroscience and Human Knowledge
by Jean-Pierre Changeux, translated from the French by M.B. DeBevoise
Belknap Press/Harvard University Press, 324 pages

Nicotinic Acetylcholine Receptors: From Molecular Biology to Cognition
by Jean-Pierre Changeux and Stuart J. Edelstein
Odile Jacob, 284 pages

Conversations on Mind, Matter, and Mathematics
by Jean-Pierre Changeux and Alain Connes, translated from the French by M.B. DeBevoise
Princeton University Press, 260 pages

What Makes Us Think?
A Neuroscientist and a Philosopher Argue about Ethics, Human Nature, and the Brain
by Jean-Pierre Changeux and Paul Ricoeur, translated from the French by M.B. DeBevoise
Princeton University Press, 335 pages

Phantoms in the Brain: Probing the Mysteries of the Human Mind
by V.S. Ramachandran and Sandra Blakeslee, with a foreword by Oliver Sacks
Quill, 328 pages

Mirrors in the Brain: How Our Minds Share Actions and and Emotions
by Giacomo Rizzolatti and Corrado Sinigaglia, translated from the Italian by Frances Anderson
Oxford University Press, 242 pages

A Universe of Consciousness: How Matter Becomes Imagination
by Gerald M. Edelman and Giulio Tononi
Basic Books, 274 pages

Jean-Pierre Changeux is France's most famous neuroscientist. Born in 1936, Changeux began his studies with the advent both of the DNA age and of high-resolution images of the brain. Since that time, he has written a number of books exploring the functions of the brain.

The brain is a bundle of some hundred billion neurons, or nerve cells, each sharing as many as ten thousand connections with other neurons. A large crown of little dendrites extends above the body of the neuron and receives signals from other neurons, while a long axon, which conducts neural messages, projects below, occasionally shooting off to connect with other neurons. The technical term for the growth of dendrites is arborization.

Neurons use electricity to send signals through the body. But most neurons leave a gap between the terminus of the neuron, which transmits signals, and the receptor of those signals in the adjacent neuron. How signals from neurons manage to cross this synaptic cleft became the major neurophysiological question of the early twentieth century.

Most leading biologists at that time assumed that neurons would use the electricity in the nervous system to send signals across the cleft. The theory that electrical pulses would cause a chemical signal to move across the cleft seemed to rely on far too slow a mechanism. But evidence slowly accumulated in support of the chemical theory. Experiments began to suggest that the human brain makes do with components inherited from simpler organisms that have survived over the course of biological evolution.

Changeux began his work when the basic methods for neuron communication had been determined but the detailed chemical mechanisms were just opening up to research. Biologists discovered that the transmitting end of the neuron, called the nerve terminal, comes packed with tiny vesicles, each containing around five thousand molecules of the neurotransmitter. When an electrical signal moves down the neuron, it triggers the vesicles and floods the synaptic cleft with neurotransmitter molecules. These neurotransmitters then attach to the proteins called receptors on the surface of the neuron that is located just across the synaptic cleft, opening a pore and allowing ions to flow into the neuron. Thus, the chemical signal is converted back into an electrical signal, and the message is passed down the line.

In 1965, the young Changeux, working with Jacques Monod and Jeffries Wyman, attempted to explain how the structure of an enzyme could stabilize when another molecule attached to it. Changeux later saw a parallel with the nervous system. When a chemical neurotransmitter binds to a receptor it holds the ion pore open, ensuring its continuing function.

Changeux then turned to the ways that larger structures in the brain might change these basic interactions. Donald Hebb had proposed that neurons could increase the strength of their connection through repeated signals: "neurons that fire together, wire together." But researchers found that certain regulatory networks could achieve far more widespread effects by distributing specialized neurotransmitters, such as dopamine and acetylcholine, throughout entire sections of the brain.

Changeux focused on these specialized distribution networks. It was long known that nicotine acts on the same receptor as the neurotransmitter acetylcholine. Changeux recognized that this could explain both nicotine's obvious benefits as well as the drug's more puzzling long-term effects. For instance, while cigarettes are dangerous to health, some studies show that smokers tend to suffer at significantly lower rates from Alzheimer's disease and Parkinson's disease. Changeux found that nicotine, by attaching to the same receptors as acetylcholine, reproduces some of the benefits of acetylcholine by reinforcing neuronal connections throughout the brain.

Changeux has since attempted to explain how such regulatory systems can produce the experience we call consciousness. According to Changeux, the infant brain is not a blank slate, nor is it preprogrammed. Rather, the brain produces, by means of genetic action, "mental objects of a particular type that might be called prerepresentations — preliminary sketches, schemas, models."

According to this theory, spontaneous electronic activity in the brain, "acting as a Darwinian-style generator of neuronal diversity," creates dynamic, highly variable networks of nerve cells, whose variation is comparable with the variation in DNA. Those networks then give rise to the reflex movements of the newborn infant. Over time the infant's movements become better coordinated. Neural networks associated with more successful movements are reinforced as their synaptic junctions become strengthened. Darwinian competition strengthens some of these transient networks.

Animals and infants conduct this miniature version of natural selection by means of what Changeux terms cognitive games. Learning through trial and error, reward and suppression, are the kind of cognitive games that are played out constantly through the brain's interaction with the environment. As successful behaviors increase in number, they strengthen the capacity to consciously manipulate the environment.

In Changeux's view, starting in the womb, spontaneous electrical activity within neurons creates highly variable networks of nerve cells. The networks are selected and reinforced by environmental stimuli, and these reinforced networks can then be said to represent the stimuli. The environment does not directly instruct the brain. Rather, working through our senses, the environment selects certain networks and reinforces the connections between them.

Critical to this process of selection, in Changeux's view, is the brain's reward system: the pleasure response. Dopamine is part of a reward system that is important in human and animal behavior, and dopamine levels are elevated in the brain when we experience pleasure or well-being. Pleasure is associated both with the anticipation of activities essential to survival and with the activities themselves.

In general, behaviors associated with pleasure are reinforced by the release of dopamine; as a result, the synaptic junctions of the associated neuronal networks are strengthened. And as they are strengthened, the changes in brain function often become permanent.

But opiates, alcohol, cannabinoids, nicotine, and other drugs can also increase the release of dopamine and subvert the normal function of the reward system. The addict's memories may be overwhelmed by the powerful neural connections previously created by the drug. Only if memory is a matter of reconstruction of latent physical traces, Changeux argues, could these kind of drug-induced long-term compulsions occur.

Changeux sketches the outlines of a plausible interpretation of the neural bases of meaning. The neural representation of a complex meaning is mostly not located in a single, hierarchically prominent neuron. Distinct populations of neurons in sensory, motor, associative, and other territories are linked as part of a distributed network, which constitutes a neural embodiment of meaning.

Changeux's ideas are similar to Gerald Edelman's theory of neural Darwinism. For both Changeux and Edelman, Darwinian selection is an essential part of the story. But they have radically different views of what selective mechanisms in the brain imply about the nature of brain function, knowledge, memory, and consciousness. Our senses, in Edelman's view, are confronted by a chaotic, constantly changing world that has no labels. The brain must create meaning from that chaos. Edelman takes the view that memory is nonrepresentational.

Changeux believes that once a set of neuronal circuits have been selected to form a memory, they become part of a relatively stable structure that "can be conceived as a set of long-lasting global representations." Though "the precise patterns of connectivity in the network may vary from individual to individual," Changeux writes, its functional relationships remain constant. In this way a "scale model" of external reality is selected and stored in memory in the brain.

Both Changeux and Edelman propose that during memory formation, our interactions with the world cause a Darwinian selection of neural circuits, much as the body, when invaded by a virus, "selects" the most potent antibodies from the enormous repertoire of antibodies made available by the body's immune system.

For Edelman, memory is the ability to repeat a mental or physical act after some time despite a changing context. Memory is not a small scale model of external reality, but a dynamic process that enables us to repeat a mental or physical act. Our senses are confronted by a chaotic, constantly changing world that has no labels, and the brain must make sense of that chaos. It is the brain's correlations of sensory information that create the knowledge we have about our surroundings.

The problem is illustrated by the case of a patient who has lost his arm in an accident, and whose brain creates a "phantom" limb in an apparent attempt to preserve a unified sense of self. For the patient, the phantom limb is painful. The brain knows there is no limb. Pain is the consequence of the incoherence between what the brain "sees" and the brain's "feeling" the presence of a phantom. Such pain is not created by an external stimulus and cannot be eliminated by painkillers.

In a therapy devised by V.S. Ramachandran, the patient put his intact hand in one side of a box and "inserted" his phantom hand in the other side. The box had a vertical mirror that showed a reflection of his intact hand. The patient observed in the mirror the image of his real hand, and was then asked to make similar movements with both "hands," which suggested to the brain real movement from the lost hand. Though the young man was aware of the trick being played on him, the pain disappeared.

In general, every recollection refers not only to the remembered event or person or object but to the person who is remembering. The very essence of memory is subjective, not mechanical, reproduction. Essential to that subjective psychology is that every remembered image inevitably contains a basic reference to the person who is remembering.

Our conscious life is a constant flow, or integration, of an immediate past and the present — what Edelman calls the remembered present. Consciousness, in this view, is neither recalled representations nor the immediate present, but something different in kind.

The importance of body image and motor activity for perception, physical movement, and thought is suggested by the recent discovery of mirror neurons. The neurons that fire when a monkey grasps an object also fire when the monkey watches a scientist grasp the same object.

We can recognize and understand the actions of others because of the mirror neurons. As Rizzolatti says, our perceptions of the motor acts and emotive reactions of others appear to be united by a mirror mechanism that triggers the same neural structures that are responsible for our own actions and emotions.

Because of the work of scientists such as Changeux, Edelman, and Rizzolatti, we have a better grasp of the complexity of subjective experiences.
 

AR  This is a good summary. I think Edelman has more persuasive views on the harder problems of consciousness and meaning than Changeux. This may not mean much, since I've read books by Edelman but not by Changeux.