One theory that tries to explain consciousness argues that the essence of consciousness is the integration of information. For example, communication between different areas of the brain (e.g. cortex) might be one sign of this integration and of consciousness. To test this hypothesis researchers at the University of Wisconsin led by Dr. Guilio Tononi recorded electrical activity in the brains of six sleepy volunteers using high density EEG. Before the subjects nodded off, the researchers stimulated a small path of right frontal cortex with transcranial magnetic stimulation, a noninvasive method that uses magnetic pulses to induce electrical activity inside the head. As results were published in this week's Science: (photo of TMS machine working on subject)

The EEG recordings revealed how the neural actvity triggered by TMS spread from the site of stimulation to other parts of the brain. The team repeated the experiment once the subjects had entered non-rapid eye movement (non-REM) sleep. Noise canceling earphones ensured that subjects couldn't detect the sound of teh TMS magnet.

When the subjects were awake, TNS elicited waves of neural activity that spread through neighboring area of the right frontal and parietal cortex and to corresponding regions on the left side of the brain. During non-REM sleep, the same TMS stimulus only elicited neural activity at the site of stimulation.

The researchers say that the finding suggest that different areas of the cortex do indeed stop talking to each other during non-REM sleep- a stage of sleep on which people often report little or no conscious experience on waking. A follow up experiment is scheduled that will pulse the brain during late-night REM sleep, a time when researchers expect to see a pattern which is much more similar to wakefulness.