But Brown and others argue that devices like this give only a rudimentary measure of what's happening in the brain. "If it's slow, we think it's okay to operate; if it's fast, we think they're waking up," says Brown. "That's all we're doing."

Brown and his colleagues are using newly developed technology that allows them to study EEG waves while a patient is simultaneously having his brain imaged with functional magnetic brain imaging, an indirect measure of brain activity that is more spatially precise than EEG. Preliminary results show that some brain areas actually become more active during the course of anesthesia. It's not surprising that a broad-acting drug, which inactivates brain areas that are normally involved in selectively inhibiting brain activity, leads other areas become more active, says Brown. "This is the type of information we really need," he says.

In corresponding experiments conducted on rodents, scientists used arrays of electrodes to directly measure activity in different parts of the brain. Researchers directed by Matt Wilson, a professor of brain and cognitive sciences MIT who collaborates with Brown, found that rodents that had been given an increasing dose of an anesthetic showed characteristic changes in the rhythm of brain activity in the cortex. But activity in the hippocampus, a brain area crucial in learning and memory, remained unchanged.

"If the signature [measured via EEG] is coming from the cortex, it's not telling us what the deeper brain structures are doing, such as the arousal system, the brain stem, the amygdala, and the hippocampus," says Brown. "If EEG cannot tell you about those structures, it's not telling you about key systems."