To find it, the scientists analyzed 36 hospital patients that had some form of brainstem damage. Twelve were in a coma and the other 24 were conscious. After imaging their brains, the scientists zeroed in on a tiny three-dimensional area of the brainstem just 2 millimeters across. Ten out of the 12 unconscious patients had damage in this area; the 24 conscious patients did not.
That gave the scientists a clue as to where the arousal component of consciousness came from. But what about the awareness component?
Again, the scientists had some information to go on, thanks to previous research. They knew that awareness was located somewhere in the brain's outer layer of neural tissue, the cortex. But where?
For an answer, they turned to the Human Connectome project, a highly detailed map of the neural connections in a healthy human brain. The scientists started from the tiny location in the damaged brainstem and followed the connections of neurons from there out into the cortex. The trail led to two areas: an area just under the left temple and a region deep in the brain behind the center of the forehead.
"When we saw this, when we found that these these were the spots that the lesions were connected to, we got pretty excited," said Fox.
It turns out that the two spots - the one under the left temple and the one behind the center of the forehead -had been implicated in other research studies as playing a role in awareness. The both contain large, oddly shaped brain cells called von Economo neurons only seen in mammals with higher order consciousness. Until this study, no one had connected them to the brainstem, where arousal is regulated. The pieces were starting to come together.
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"We knew that this finding could be provocative," Fox said.
For that reason, the scientists wanted to conduct one last investigation to see how the networks between the brainstem and the two areas in the cortex functioned in patients with disorders of consciousness. They turned to Steven Laureys of the Coma Science Group in Belgium and, using a special type of MRI, scanned the brains of 45 people, 26 who were minimally conscious and 19 who were in a vegetative state.
The images revealed that the brain networks were disrupted. Studies conducted on rodents by other researchers had shown the same brain network disruption. All of the evidence suggests that consciousness exists thanks to the networks that extend from the tiny spot on the brainstem to the two areas in the cortex.
That information could be immensely helpful in targeting regions in the brain for stimulation. Fox and other neuroscientists like him already use deep brain stimulation, which involves surgery to attach an electrode to a specific part of the brain, to treat Parkinson's disease. And noninvasive treatments, including transcranial magnetic stimulation, which is used to treat depression.
It follows that stimulation the part of the brain responsible for consciousness might help people wake from a coma. "Doing a brain stimulation therapy requires knowing where it is that you want to stimulate," Fox said.
Thanks to this new study, neuroscientists have a better idea of the regions to target.