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L'émergence de la conscience
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The Sense of Self


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Help Lien : State of the Art - The Psychology of Consciousness Lien : Timing of the brain events underlying access to consciousness during the attentional blink
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Any conscious state is a global phenomenon involving the activation of numerous areas in the brain. That said, certain brain structures are known to be more involved in certain types of conscious phenomena.

For example, the two phenomena of voluntary, conscious control of movement and conscious perception of an object’s properties or qualia involve the activation of different parts of the brain. When someone does relaxation-based meditation, these two phenomena tend to become disassociated: he or she has less of a feeling of conscious motor control but a heightened awareness of sensory experience.

Brain-imaging experiments on subjects in meditative states confirm that this subjective experience has objective physical correlates in the brain, such as increased activity in the hippocampus, the anterior parietal lobe and the occipital lobe. All of these areas are recognized as being active in the processing of visual and somatosensory information.


In the 1940s, Canadian neurosurgeon Wilder Penfield performed several operations on epileptic patients in which he removed the cortical tissue responsible for their seizures while the patients were awake under local anaesthesia. Before removing any tissue, Penfield applied electrical stimuli to various locations in the cortex so that he could be sure not to remove areas involved in important functions such as speech. When he stimulated the patients’ primary motor cortex in this way, their corresponding limbs moved, but the patients reported that these movements were involuntary and not intentional. These experiments thus clearly showed that the voluntary aspect of such movements does not depend on the primary motor cortex.

The premotor area and the supplementary motor area are located just anterior to the primary motor cortex. The activation of certain groups of neurons in these areas produces more specific movements of the limbs. But here again, we are far from being able to state that it is these areas that “decide” to perform any given movement.

Chercheur : Wilder Penfield Chercheur : Wilder Penfield
CAN STATES OF CONSCIOUSNESS BE MAPPED IN THE BRAIN?

When we speak of consciousness at the level of the brain as a whole, we are implicitly taking a materialist philosophical perspective. In other words, we are embracing the idea that it is the brain—and hence, physical matter—that engenders the human mind. We are also accepting that the activity of the brain’s neurons is the source of all our mental processes, such as learning, memory, perception, and language, and hence of consciousness, which in a sense emerges from all of the brain’s other attributes and so is no exception to this rule.

Once we start talking about the brain and consciousness, we must necessarily begin talking about the unconscious as well, because the brain has many specialized circuits that are constantly decoding various aspects of our environment without our being conscious of their doing so. Likewise, the vast majority of our behaviours occur automatically, without our being conscious of having initiated them. The same goes for our mother tongue, whose grammar we use correctly without even realizing it. One last example: some people suffering from brain damage can perform certain tasks correctly without being conscious of doing so.

Ultimately, we cannot deny that the vast majority of everyone’s life is governed by unconscious circuits in the brain. Even so, the stream of consciousness remains a constant presence in our daily lives. And as we are about to see, our conscious states involve extensive areas of our brains.

Three large areas of the brain seem to be especially involved in the phenomenon of consciousness. The first is the reticular formation, whose activity level influences the states of alertness, wakefulness, and sleep. Second is the thalamus, which sorts the information from the rest of the body and routes it to other parts of the brain. And finally there is the cortex, which is of crucial importance for all forms of perception and all control of voluntary movements.

Thanks to modern brain-imaging technologies (follow the Tool Module link to the left), we can also see the steps that lead to the emergence of a conscious mental image. For example, which parts of the brain must become active first, and which ones subsequently, in order for you to have a conscious visual perception?

To answer this question, neuroscientists Claire Sergent, Sylvain Baillet, and Stanislas Dehaene successfully monitored the sequences of neural activity that occur in a subject’s brain a) when a word briefly projected on a screen is perceived consciously, and b) when it is not. Whether the subject perceives the word consciously depends on how long it is projected. If it is projected for only about a quarter of a second, it will not be perceived consciously, but if it is projected for longer—say about three-quarters of a second— it will.

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What do scientists see happening in the brain when the word is projected for the shorter interval and for the longer one? Whether or not the word is ultimately perceived consciously, what happens during the first 275 milliseconds (ms) is exactly the same: only the visual cortex is activated. (This fits quite nicely with the well known modular processing in the visual cortex.) But after that, the brain activity differs according to whether or not the subject reports having consciously seen the word.

As the animation to the left shows, when the subject does see the word consciously, the activation is broadly amplified and reverberates, first through the frontal cortex (starting after the first 275 ms), then through the prefrontal, anterior cingulate, and parietal cortexes (starting after 300, 430, and 575 ms, respectively). But when the subject does not see the word consciously, the activity remains localized in the visual cortex and gradually subsides until it ceases completely after 300 ms.

It thus seems that for consciousness to exist, there must be some communication or resonance among various parts of the brain. As we have seen, conscious phenomena do not emerge from a single location in the brain; instead, they are the product of a system involving multiple areas of the brain. That is why, for instance, when someone’s brain suffers localized damage, their consciousness may be modified, but rarely eliminated completely.

Another condition for consciousness seems to be that it can arise only when the “higher” areas of the brain, such as the frontal cortex, which is connected to the circuits for emotions and decision-making, are activated.

Forward of the frontal lobes in the human brain lie the prefrontal lobes, which receive countless connections from other parts of the brain. To cite just two examples, the ventral and dorsal visual pathways, which arise from the temporal and parietal lobes, send projections to the prefrontal lobes.

The role of the prefrontal cortex is hard to define clearly, but it seems to be involved in determining the time sequence required for a given action. For example, when people who have damage to the prefrontal cortex are asked to reproduce a series of movements, they tend to produce the right movements, but in the wrong order. Also, in tests where such people are asked to demonstrate various uses for a given object, they display a great deal of rigidity in their behaviour and tend to show only the object’s most common use repeatedly. It is as if they were having trouble in inhibiting their knowledge of this most common use so that their knowledge of other uses could emerge.

Such people with damaged prefrontal lobes, as in the famous case of Phineas Gage (see figure opposite and links below), may also respond in a stereotyped way to the sight of an object, even if the social context makes that inappropriate. For instance, at the sight of a toothbrush, they might pick it up and starting brushing their teeth, even if they were in someone else’s home and the toothbrush weren’t theirs. When it is pointed out to them that their behaviour is out of place, they become confused or simply invent a story that justifies their behaviour.

Because people who have a prefrontal-lobe deficit are thus at the mercy of the slightest environmental triggers, they have problems with making plans and carrying them out. They may thus have some trouble in retrieving memories if they would need to plan and apply a search strategy to do so. Two other traits that such people very often display are a lack of spontaneity and a fair amount of indifference toward themselves and others. But despite all this, their general intelligence remains intact, so they can answer theoretical and factual questions correctly, but will rarely initiate a conversation or ask for information.

 


On September 13, 1848, Phineas Gage, an American railroad worker, was injured in an explosion in which an iron rod passed through his brain. Against all expectations, he recovered from this accident, but his behaviour was radically altered. By studying his injuries, scientists gained a better understanding of the functions of the frontal lobe. Source: Joan M.K. Tycko


Lien : Prefrontal cortex Lien : Phineas Gage Lien : The Phineas Gage Information page Lien : The Strange Tale of Phineas Gage Lien : Le cas Phineas Gage Lien : Sur les traces de Phineas Gage
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