When we are awake, our brains function like a contiguous mass broken into smaller regions that easily communicate with one another—think the United States minus Alaska and Hawaii. But when we begin to drift off to sleep, the distinct areas of our brain behave as separate islands with trade embargoes; all inter-regional contact dissipates.
Giulio Tononi, a professor of psychiatry at the University of Wisconsin-Madison, says this disconnection explains why we lose consciousness in the early stages of sleep. This, he argues, accounts for our inability to remember dreams when we awake from a light slumber. For over 20 years, Tononi has hypothesized that consciousness results from communication between different parts of the brain, specifically different sections of the cortex, the second outermost layer of the brain.
In an article from the new issue of the journal Science, Tononi supports his theory with research he performed on six volunteers. He delivered pulses to the subjects' brains when they were awake, in stage-one sleep, or in non-rapid eye movement (non-REM) sleep. By testing the spread of waves through their brains, Tononi demonstrated that the interconnectivity between brain regions does, in fact, fade as we progress into sleep.
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"It's as if you're injecting current in an electronic circuit in one particular spot, " said Tononi about his experiment. "You see to which other spots on the grid it's connected to and which ones are activated."
Using a combination of electroencephalography, or EEG, which monitors electrical flow in the brain via a device that looks like a swimmers cap studded with electrodes, and transcranial magnetic stimulation (TMS), which creates a magnetic field that induces impulses in the brain, Tononi's team noticed major differences in electrical activity in the brain between waking and sleeping states.
The subjects received an imperceptible electric charge in a well-connected zone in the front half of the brain. When awake, the impulse spread out like ripples on the surface of a pond, sending signals to at least five other regions. When the subjects were in non-REM sleep, the pulse was confined—it made a larger initial spike, but resulted in little to no activity in other regions of the brain. In addition, the effect of the pulse lasted twice as long when the volunteer was awake as when he or she was asleep.
"We've shown that this network is there when you're awake, but somehow, when you are asleep, although physically it's there, it's not working," says Tononi. "And that is why consciousness fades."
David McCormick, a sleep researcher at Yale, praises Tononi's study, saying that it directly proves Tononi's hypothesis on connectivity between brain regions. "What the authors are claiming, and I think it's true, is that for consciousness to occur, the cortical network has to be in a state in which the communication between parts of the cortex is rapid and easy," says McCormick. "When you're asleep, that interaction is severely disrupted. And that's probably what leads to unconsciousness in sleep."
Tononi plans to do further work, this time observing brain activity during REM sleep. He expects that during crossover from stage four non-REM sleep into REM sleep, the final stage of sleep, consciousness returns, allowing for vivid dreaming. In addition, he believes his method of investigation, combining EEG and TSM, could be applied clinically, to monitoring brain activity in vegetative states and in mental illnesses such as schizophrenia. It also may help researchers discover how anesthesia works.
"There are some drugs, some anesthetics, that reduce consciousness or propose to eliminate it, but they don't really reduce the activity of the brain," says Tononi. "The question would be: Do they work by doing the same thing as sleep, by disconnecting the areas, which remain, nevertheless, active?
"That's another interesting question, the neural basis of consciousness, which is really our goal."