In the human brain, cells talk to one another through the routineexchange of electrical signals. But when people fall into a deep sleep,the higher regions of the brain - regions that during waking hours area bustling grid of neural dialogue - apparently lose their ability tocommunicate effectively, causing consciousness to fade.
Writing Friday, Sept. 30, in the journal Science, a team ofresearchers led by UW-Madison professor of psychiatry Giulio Tononireports that the fading of consciousness during dreamless sleep seemsto occur as the different regions of the cerebral cortex that mediateperception, thought and action become functionally disconnected.
Tononi and his team observed the disconnect when brief, magneticallygenerated pulses of electricity were directed to specific regions ofthe brain. The pulses stimulated an electrochemical response from thetargeted cells, which, when the subject was awake, rippled across thebrain, traveling along networks of nerve fibers to different cerebraldestinations. But when the subject was in deep sleep, the same responsewas quickly extinguished and did not travel beyond the stimulated cells.
When consciousness fades, according to Tononi, "the brain breaks down into little islands that can't talk to one another."
The new findings are important because they provide the first directclues about how the brain alters the state of consciousness duringsleep. Consciousness is a scientifically murky realm as little researchhas been conducted on how the brain sustains and alters the variousstates of mind. Tononi, one of the few scientists exploring thefrontiers of consciousness, has theorized that conscious thoughtdepends on the brain's ability to integrate information.
"Sleep is the most familiar alteration of consciousness," he says."It happens every night to all of us. Every night, when you fall intodeep sleep, your consciousness usually fades."
Indeed, research subjects woken early in the night frequently reportlittle or no conscious experience. Later in the night, and especiallyin the morning hours, subjects report vivid dreams, indicating that thelater stages of sleep can be associated with conscious experience,Tononi says.
But why does consciousness fade during deep sleep early in thenight? "You cannot say that consciousness fades because the brain shutsoff. That's not the case. Scientists have long known that the brainremains active while we sleep," Tononi says. "So what could beresponsible?"
To explore the breakdown of consciousness during sleep, Tononi andhis colleagues capitalized on a new technology - transcranial magneticstimulation - that permits precise, non-invasive activation of smallregions of the brain. Subjects are also equipped with a cap ofelectrodes to monitor the brain's electrical activity so that the cellsignals elicited by the quick bursts of electricity can be tracked.
In subjects who are awake, the pulses elicited a significantresponse: "The brain reacts in a strong and specific manner," Tononiexplains. "There is a very interesting set of activations that occurover great distances in the cortex.
"During deep sleep early in the night," he adds, "the response isshort-lived and doesn't propagate at all. Somehow, it doesn't travelanywhere."
The experiments conducted by the Wisconsin team are the first oftheir kind. The results lend support to the idea that consciousnessdepends on the ability of the brain to integrate information. In otherwords, consciousness rests on the ability of the various regions of thebrain to talk to one another.
In the brain, messages are relayed along networks of nerve fibers.Cells transmit information along those fibers electrochemically.Anatomically, the fibers are analogous to the cables that computers useto share information. But the network of nerve fibers, like a tangle ofcomputer cables, is not transparent and may not always be in use.
"What we needed to do was stimulate an area of the brain and see ifit talks to another part. We have a tool to do that now," says Tononi,referencing one of only a handful of machines in the world capable ofstimulating precise regions of the brain from outside the skull whilerecording the resulting electrical responses.
The paddle-like device is placed over the head of a subject andgenerates a magnetic field. The magnetic field, in turn, producespulses of electricity lasting less than a millisecond and that arecapable of penetrating the skull to stimulate brain cells.
"Essentially, we activate an area," Tononi says. "We can do thisanywhere in the brain. Once an area is activated, it responds bysending signals, waves that travel through the axons (nerve fibers) toother regions of the brain. At the same time, we can record how therest of the brain is responding."
The new technique promises science a way to see how the different areas of the brain communicate, Tononi says.
Beyond helping illuminate the secrets of consciousness, the newstudy, which was directed in part by Marcello Massimini, a researchassociate in the UW Psychiatric Institute and Clinics, may aid in thedevelopment of diagnostic and therapeutic tools for neurological andpsychiatric disorders that affect consciousness, such as schizophrenia.That work is now being pursued by the Wisconsin group.
In addition to Tononi and Massimini, co-authors of the Science paperinclude Fabio Ferrarelli, Reto Huber, Steve K. Esser and HarpreetSingh, all of UW-Madison.
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