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L'émergence de la conscience

The Sense of Self

Help Lien : The Electric Brain Lien : Binding problem Lien : The Visual Perception of Objects
Original modules
Tool Module: "Grandmother Cells", or Synchronous Discharges of Neurons? "Grandmother Cells", or Synchronous Discharges of Neurons?

“I have it at the tip of my tongue!”

Brain Rhythms: The Oscillations That Bind

A neuronal assembly is a group of neurons that maintain strengthened synaptic connections with one another, so that they are more likely to be active all together at the same time.

The neurons in these assemblies are not necessarily all physically close to one another. They can be distributed across various parts of the brain. Moreover, a single neuron can belong to several different assemblies and can be recruited into new assemblies at any time. Thus neuronal assemblies are not stable, but dynamic, and not necessarily localized, but often distributed.

Neuronal assemblies are also known to have other properties that must be accounted for in models of consciousness. For example, we know that even if only part of a neuronal assembly is activated initially, this activation, if it reaches a certain threshold, may then be propagated across the entire assembly. Thus, if just a portion of a memory is successfully activated in your brain, that may cause the entire neuronal assembly in question to become activated, thus filling in the lapses or gaps in this memory and enabling you to recover it in its entirety.

A phenomenon that is in a sense the opposite also occurs: some neuronal assemblies that initially have nothing to do with one another but that are repeatedly activated at the same time become more strongly connected. This is in fact the fundamental principle of learning at the cellular level, the mechanism by which our brains let us store the world’s regularities in our memories, which of course has a definite adaptive value .

A number of phenomena support the argument that the neuronal assembly is the smallest possible physical substrate for a meaningful mental representation.

Isolated neurons die in the brain every day, but an entire piece of knowledge suddenly disappears only when areas of the brain that contain large numbers of neurons are destroyed.

The brain’s ability to store new pieces of information in declarative or procedural memory has no defined limit. Because there are already such a tremendous number of neurons in the brain, the combination of these neurons into various assemblies that can also have elements in common makes the number of possible representations practically infinite as well.

Outil : L’apoptose, ou la mort cellulaire programmée Lien : Cell assemblies and phase-locked oscillations

How does a conscious idea come into your mind, and by what mechanisms does your consciousness move from one idea to another? Neurobiological models of consciousness offers some interesting new approaches to these age-old questions.

To begin our discussion of these models, we should note two things. First, these models fall within the philosophical framework of materialism. Second, the general hypothesis of these models is that there are “neural correlates” of consciousness—in other words, that any change in our mental states necessarily involves a change in the states of our neurons.

More specifically, most of these models agree that our sensory perceptions as well as our more abstract thoughts correspond to the activity of vast networks or assemblies of neurons, an activity that is subject to a complex set of dynamics (see sidebar).

Most of these models also recognize that at any given time, the brain is processing far more information than we are aware of. To cite only one example, regarding our sense of vision, we have the false impression that we are consciously aware of everything that lies in our field of vision, but actually, we consciously experience only a small portion of this scene.

In fact, at any given moment, we can be conscious of only one thing, even though the things of which we are conscious may follow each other in very rapid succession. What is it then that enables us to distinguish between the multitude of information that we process unconsciously and the unique content of our conscious mind at any given time?

In neuronal terms, the question can be stated as follows: which assembly of neurons will become the one whose activity is associated with a conscious thought at any given time? In a sense, the various neuronal assemblies are competing to pass through the narrow gateway into consciousness. Is it enough then to say that those neuronal assemblies that are the most active will form the contents of the conscious mind?

Actually, when we try to explain the difference between the conscious and the unconscious solely in terms of the amount of neuronal activity, a major problem arises: the brain already uses the activity level of neurons (physically expressed by the frequency of the nerve impulses that they emit) to represent the intensity of stimuli (for example, whether a light is brighter or dimmer, or a sound is louder or quieter).

The frequency of nerve impulses cannot simultaneously indicate the intensity of a stimulus and whether or not it is a conscious one. Hence there must be some other mechanism by which a piece of conscious content is selected. How then does the brain manage to distinguish a stimulus that is intense but unconscious (for instance, the words of a song that is playing very loud on a jukebox but that you’re not listening to) and a stimulus that is less intense but conscious (such as the sweet words that your lover whispers in your ear and that you are very much aware of)?

The brain must have some other mechanism that lets it assess the objective importance of a stimulus and at the same time distinguish between conscious and unconscious mental representations. This mechanism must also account for another problem that doesn’t seem like one if you don’t consider how sensory information is processed in the brain, but that becomes a real puzzle if you do.

This problem arises from the fact that the brain uses numerous specialized circuits to process the various properties of perceived objects in parallel. For example, when you are looking at a hat, differing visual areas in your brain’s occipital cortex will process differing properties of the hat. Some of these areas will be sensitive to the hat’s outline, while others will be sensitive to its colour, or its shape, or its texture, or its location in space, and so on. You can see what the problem must then be: how does your brain manage to integrate all these properties, which have been decoded at different locations, in order to give you the subjective perception of a single object: a hat?

(After Engel et al., 1999)

But things can get even more complicated. For example, what happens when you see a green suitcase next to a blue hat? Your brain’s visual areas for colour register the green and the blue, those for shape register a rectangle and a rounded form, those for position register one object to the left and another to the right, and so on. But where in the brain are the characteristics of each of these objects put together to form your conscious perception of two distinct objects, without confusing the characteristics of one with those of the other? That is the problem, referred to in neuroscience as the binding problem.

This problem of how the various properties of an object are bound into a single, conscious, coherent perception is closely linked with the process by which we select those neuronal assemblies that will leave the unconscious and enter into consciousness.

Among the various mechanisms that some scientists have proposed to address these two problems, the synchronization of neuronal oscillations is certainly one of the most controversial. Other scientists have gone even further, by positing that there is a second system of temporal synchronization in addition to the first one.

Can conscious phenomena be associated with the activity of a particular type of neuron? Obviously, things can’t be that simple. But some scientists do believe that a particular type of neuron—the large, elongated, spindle-shaped nerve cells known as VEN neuronsmay play a significant role in consciousness .

The name VEN neurons comes from the initials of the neurologist Constantin von Economo, who was the first to describe these neurons, in 1925. VEN neurons are bipolar neurons located exclusively in layer V of the anterior cingulate cortex and the insula. These neurons are found only in the great apes and in human beings. And as if by chance, it is humans who have by far the greater number of these neurons. This suggests that these neurons appeared quite late in the history of the brain’s evolution, something like 15 million years ago.

The relative recency of VEN neurons from an evolutionary standpoint, together with their location in the areas of the frontal lobe that are involved in our higher cognitive functions, have made them a focus of attention in certain neurobiological models of consciousness. The morphology and location of these neurons in fact suggest that they receive a wide range of stimuli which they may integrate and process very rapidly.

Lien : Von Economo Neurons, Intuition, and Phylogeny Lien : Brain cells and arrangements unique to human cerebral cortex Lien : Spindle Neurons and Frontotemporal Dementia Lien : DENDRITIC ARCHITECTURE OF THE VON ECONOMO NEURONS
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