A sleeping subject undergoes transcranial magnetic stimulation, in which a brief pulse of electricity is used to stimulate a small region of the brain. Using the technique, a team of University of Wisconsin-Madison scientists, led by professor of psychiatry Giulio Tononi, determined why consciousness fades when people fall into a deep sleep. When subjects are awake, the pulses elicit electrochemical signals from cells that ripple across the brain. When subjects are in deep sleep, the signals are confined to the stimulated area, showing that the brain's communication pathways are shut down, resulting in a significantly reduced state of consciousness. Image credit: courtesy Marcello Massimini/UW-Madison Department of Psychiatry Photo date: August 2005

When stimulated by brief pulses of electricity from outside the skull of a subject who is awake, cells in a specific region of the brain are able to relay those signals to critical regions that mediate perception, thought and action. This short video shows waves of electrochemical signaling rippling across the brain. Windows® Media Video: courtesy Marcello Massimini/UW-Madison Department of Psychiatry Photo date: August 2005

In a subject who is asleep, the same simulation is quickly extinguished, and the cell signaling that it initiates does not propagate far from the stimulated part of the brain. The first look at how the different regions of the brain communicate during sleep shows that faded consciousness during sleep results from a breakdown in the brain's signaling system when we fall into a deep, dreamless sleep. Windows® Media Video: courtesy Marcello Massimini/UW-Madison Department of Psychiatry Photo date: August 2005

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