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L'émergence de la conscience

HelpLien : The NMDA receptor and general anaesthetic actionLien : Evolution of ConsciousnessLien : Étomidate
Lien : KétamineLien : Vicissitudes of Consciousness, Varieties of CorrelatesLien : Talk:Neural correlates of consciousness

One should avoid jumping to conclusions about the correlations that exist between a few properties of consciousness and the sometimes rather peculiar properties of the world of the infinitely small. Molecular and quantum theories of consciousness are often criticized for this tendency to try to turn such correlations into overall explanatory models of consciousness.

For example, the correlation between the administration of certain anaesthetics and loss of consciousness might be used to justify a model that assigns a central role to the proteins of the microtubules that are affected by these substances. Or the unpredictable, changing nature of consciousness might be linked to the principle of uncertainty and the superimpositions of states in quantum physics.

Even if a new molecular mechanism were in fact found to be essential for consciousness, there would still be little chance that this mechanism alone explained the entire complex phenomenon of human consciousness. Instead, this mechanism would have to be integrated into a theory that provided explanations at higher levels of organization (cellular, neurological, psychological, etc.) as well. An excellent analogy is the way that the molecular mechanisms involved in the body's immune defences must be combined with environmental and psychological factors to explain these defences completely.


Most of the hypotheses that try to draw connections between subjective consciousness and physical events in the brain do so at the cellular level: that of individual neurons or neuronal assemblies. This approach—the search for the “neural correlates of consciousness”—is based on the assumption that the key to conscious processes can be found in the activities of these nerve cells. And in fact, the activities of the neurons and their communications with one another are central to many models of consciousness, such as those involving thalamocortical loops, synchronous 40 Hz oscillations, or the influence of the intralaminar thalamic nuclei on neuronal synchronization.

But there are also many theories that attempt to relate the functioning of human consciousness to structures at the molecular level, and even to the very strange effects of the quantum physics of the infinitely small. As scientific methods for investigating the infinitely small become more refined, more and more mechanisms will likely be discovered below the neuronal level, and it will be no surprise if some of these new mechanisms are indeed found to have an effect on human consciousness.

One molecule that may well play a role in the mechanisms of consciousness is the NMDA receptor. This large protein molecule takes the form of a channel passing through the neuronal membrane and serves as the binding site for glutamate, an excitatory neurotransmitter released into the synapses of a great many neurons. Once released into the synaptic gap, glutamate binds to the postsynaptic neuron’s NMDA receptor, causing this channel to open and thus initiating a whole series of biochemical reactions that make this synapse more efficient.

NMDA receptors thus act as critical components in the mechanism by which neurons form lasting associations by strengthening their connections with one another, thus creating what are known as neuronal assemblies. These assemblies occupy a prominent place in many neurobiological models of consciousness, so it seems entirely reasonable to assign the NMDA receptor molecule a significant role in the conscious processes of the human brain.

And that is precisely what German neurobiologist Hans Flohr has done. Flohr suggests that those synapses that have NMDA receptors are the ones that are most readily strengthened when an organism detects regularities in its environment.

Flohr also points out that anaesthetics such as ketamine block the normal excitatory effect of glutamate on NMDA receptors and cause loss of consciousness.

Another anaesthetic, nitrous oxide (“laughing gas”) is a completely different type of molecule from ketamine. The nitrous oxide molecule acts elsewhere in the series of reactions induced by the NMDA receptors but ultimately produces similar effects.

Flohr therefore concludes that normal functioning of the NMDA receptors and their secondary messengers is necessary for consciousness.

But many objections have been raised to Flohr’s approach, which does not involve any quantum effects as such. Some of his critics have pointed out that besides the NMDA receptor, there are countless other molecular mechanisms that must work properly if normal conscious function is to be maintained. Others critics have observed that normal functioning of the NMDA synapses is also important for unconscious processes and for the forming of neuronal assemblies that are not involved in thought processes.

Another major objection to Flohr’s hypothesis is that some other anaesthetics operate rather differently from ketamine and nitrous oxide. Etomidate, for example, puts subjects to sleep by potentiating their GABA receptors. According to Flohr’s hypothesis, etomidate would have to inhibit the NMDA receptors indirectly, but we have no evidence that it does. And even if it did, the two drugs should have the same anaesthetizing effects. But that is not the case either: etomidate does not have the same analgesic effects as ketamine.

This raises an even broader problem. There are countless substances that can render us unconscious, and they do so in such different ways that the resulting state of unconsciousness must be regarded as something more than the mere lack of something called “consciousness”. Hence, though scientists can generally attempt to understand phenomena by observing the effects of their absence, when the phenomenon in question is consciousness, this approach is quite insufficient.

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