Scientists may have gained an important insight into the age-old mystery of why consciousness fades as we nod off to sleep. Lines of communication between various parts of cerebral cortex--which buzz with activity during wakefulness--break down during slumber, researchers report today in Science.
Early neuroscientists assumed that consciousness wanes during sleep because the cortex simply shuts off. But electroencephalography (EEG) and other modern methods have since ruled out that explanation, showing that the electrical chatter and metabolism of neurons in the cortex continues unabated during sleep. That left neuroscientists with a puzzle: If the brain is still active, why does consciousness wane?
Giulio Tononi of the University of Wisconsin, Madison, suspected that a communication breakdown might be the reason. Tononi has spent years developing a theory that equates consciousness with the integration of information. Communication between regions of the cortex might be one sign of this integration--and of consciousness, Tononi says.
To test that idea, he and his team recorded electrical activity in the brains of six sleepy volunteers using high-density EEG. Before the subjects nodded off, the researchers stimulated a small patch of right frontal cortex with transcranial magnetic stimulation (TMS), a method that uses magnetic pulses to induce an electrical current inside the head. When the subjects were awake, TMS elicited waves of neural activity that spread through neighboring areas of right frontal and parietal cortex and to corresponding regions on the left side of the brain. During non-rapid eye movement (non-REM) sleep, the same TMS stimulus only elicited neural activity at the site of stimulation.
Tononi says the findings suggest that different areas of cortex do indeed stop talking to each other during non-REM sleep--a stage in which people often report little or no conscious experience on waking. An important follow-up, he says, will be to repeat the experiments during late-night REM sleep, when people often report consciouslike experiences in the form of dreams. "We would predict a pattern which is much more similar to wakefulness," he says.
The research "definitely tells us something about sleep and may have important implications for understanding the neural correlates of consciousness," says Christof Koch, a cognitive neuroscientist at the California Institute of Technology in Pasadena.