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5 décembre 2012 3 05 /12 /décembre /2012 07:13

Case studies suggest that some forms of consciousness may not require an intact cerebrum

The computer, smartphone or other electronic device on which you may be reading this article, tracking the weather or checking your e-mail has a kind of rudimentary brain. It has highly organized electrical circuits that store information and behave in specific, predictable ways, just like the interconnected cells in your brain. On the most fundamental level, electrical circuits and neurons are made of the same stuff—atoms and their constituent elementary particles—but whereas the human brain is conscious of itself, man-made gadgets do not know they exist.

Consciousness, most scientists would argue, is not a shared property of all matter in the universe. Rather consciousness is restricted to a subset of animals with relatively complex brains. The more scientists study animal behavior and brain anatomy, however, the more universal consciousness seems to be. A brain as complex as a human's is definitely not necessary for consciousness. On July 7 of this year, a group of neuroscientists convening at the University of Cambridge signed a document entitled “The Cambridge Declaration on Consciousness in Non-Human Animals,” officially declaring that nonhuman animals, “including all mammals and birds, and many other creatures, including octopuses,” are conscious.

Humans are more than just conscious; they are also self-aware. Scientists differ on how they distinguish between consciousness and self-awareness, but here is one common distinction: consciousness is awareness of your body and your environment; self-awareness is recognition of that consciousness—not only understanding that you exist but further comprehending that you are aware of your existence. Another way of considering it: to be conscious is to think; to be self-aware is to realize that you are a thinking being and to think about your thoughts. Presumably human infants are conscious—they perceive and respond to people and things around them—but they are not yet self-aware. In their first years of life, children develop a sense of self, learning to recognize themselves in the mirror and to distinguish between their own point of view and the perspectives of other people.

Numerous neuroimaging studies have suggested that thinking about ourselves, recognizing images of ourselves, and reflecting on our thoughts and feelings—that is, different forms of self-awareness—all involve the cerebral cortex, the outermost, intricately wrinkled part of the brain. The fact that humans have a particularly large and wrinkly cerebral cortex relative to body size supposedly explains why we seem to be more self-aware than most other animals. But new evidence is casting doubt on this idea.

“Got a Towel?”

If this anatomical hypothesis were correct, we would expect, for example, that a man missing huge portions of his cerebral cortex would lose at least some of his self-awareness. Patient R, also known as Roger, defies that expectation. Roger is a 57-year-old man who suffered extensive brain damage in 1980 after a severe bout of herpes simplex encephalitis, an inflammation of the brain caused by herpesvirus. The disease destroyed most of Roger's insular cortex, anterior cingulate cortex and medial prefrontal cortex, regions near or at the front surface of the brain that are thought to be essential for self-awareness. About 10 percent of his insula remains and only 1 percent of his anterior cingulate cortex.

Roger cannot remember much of what happened to him between 1970 and 1980, and he has great difficulty forming new memories. He cannot taste or smell either. But he still knows who he is. He recognizes himself in the mirror and in photographs, and his behavior is relatively normal.

In a paper published earlier this year postdoctoral researcher Carissa L. Philippi of the University of Wisconsin–Madison and neuroscientist David Rudrauf of the University of Iowa and their colleagues investigated the extent of Roger's self-awareness. In a mirror-recognition task, for example, a researcher pretended to brush something off of Roger's nose with a tissue that concealed black eye shadow. Fifteen minutes later the researcher asked Roger to look at himself in the mirror. Roger immediately rubbed away the black smudge on his nose and wondered aloud how it got there.

The researchers also showed Roger pictures of himself, of people he knew and of strangers. He almost always recognized himself and never mistook another person for himself. He did sometimes have difficulty recognizing a photo of his face when it appeared by itself on a black background, without any hair or clothing.

Roger also distinguished the sensation of tickling himself from the feeling of someone else tickling him and consistently found the latter more stimulating. When one researcher asked for permission to tickle Roger's armpits, he replied, “Got a towel?” As Philippi and Rudrauf note, Roger's quick wit indicates that in addition to maintaining a sense of self, he adopts the perspective of others—a talent known as theory of mind. He anticipated that the researcher would notice his sweaty armpits and used humor to preempt any awkwardness.

In another task, Roger had to use a computer mouse to drag a blue box from the center of a computer screen toward a green box in one of the corners of the screen. In some cases, the program gave him complete control over the blue box; in other cases, the program restricted his control. Roger easily discriminated between sessions in which he had full control and times when some other force was at work. In other words, he understood when he was and was not responsible for certain actions.

Given the evidence of Roger's largely intact self-awareness, Philippi, Rudrauf and their colleagues argue that the insular cortex, anterior cingulate cortex and medial prefrontal cortex cannot by themselves account for conscious recognition of oneself as a thinking being. Instead they propose that self-awareness is a far more diffuse cognitive process, relying on many parts of the brain, including regions not located in the cerebral cortex.

Laughing without a Brain

In the new study, Philippi, Rudrauf and their co-authors point to a fascinating 1999 review of children with hydranencephaly, a rare disorder in which fluid-filled sacs replace the brain's cerebral hemispheres. Children with hydranencephaly are essentially missing every part of their brain except for the brain stem and cerebellum and a few other structures. Holding a light near such a child's head illuminates the skull like a jack-o'-lantern.

Although many children with hydranencephaly appear relatively normal at birth, they often quickly develop growth problems, seizures and impaired vision. Most die within a year; some live for years or even decades. Such children lack a cerebral cortex, but at least a few give every appearance of genuine consciousness. They respond to people and things in their environment. They smile, laugh and cry. They know the difference between familiar people and strangers. And they prefer some kinds of music to others. If some children with hydranencephaly are conscious, then the brain does not require an intact cerebral cortex to produce consciousness.

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