Massimini and Tononi found that their electrical echoes could be used to distinguish different levels of consciousness. In unconscious states, like dreamless sleep and general anesthesia, these echoes are very simple. There is a strong initial response in the part of the brain that was zapped, but this response dies away quickly, like the ripples caused by throwing a stone into still water. But during conscious states, the response is very different: a typical echo ranges widely over the cortical surface, disappearing and reappearing in complex patterns. The complexity of these patterns,
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