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Neuroscientists Hunt the Line Where Consciousness Begins—and Ends

Brain scan-based experiments mapping metabolism in the brain offer a new level of resolution.

Consciousness should be a deeply spooky thing. Or, rather, the fact that we routinely lose consciousness should be a deeply spooky thing. Here, at some murky threshold that we never quite see until we've already tumbled over, self-awareness just blinks into darkness. For this time, our bodies and senses are gone and maybe we're left to dream our way through some mutated memories, but mostly it's just darkness. And then, suddenly, the light's on and we're pitched back across the threshold complete with senses and awareness and the need to pee. Once again: consciousness.


All of this is to me pretty fucked up. For one thing, I'm very protective of my body and the idea of it just laying there without "me" in it is kind of stressful. The science of consciousness isn't especially reassuring either. As a matter of extreme subjectivity, consciousness was even mostly off-limits to scientists until the 1980s and its very definition is unsettled. Where consciousness is studied, it's more likely to be in terms of psychology and as a concept or abstract idea than as a rigorously defined physiological phenomenon.

Neuroscientists are working on it, however. A study published this week in the journal Current Biology courtesy of researchers at the University of Copenhagen and Yale University, among others, has identified a clear metabolic threshold for the emergence of consciousness—"the minimal energetic requirement for the presence of conscious awareness." Should this energy level not be reached, the lights go out/stay out.

This minimum is about 47 percent of normal cerebral glucose metabolic activity. The researchers reached this figure by looking at positron emission tomography (PET) scans of 131 patients suffering from various disorders of consciousness (DOCs) along with 28 healthy controls. The work builds on previous studies looking at PET scans taken during sleep and anesthesia.

"Overall, the cerebral metabolic rate accounted for the current level, or imminent return, of awareness in 94 percent of the patient population, suggesting a global energetic threshold effect, associated with the reemergence of consciousness after brain injury," the authors explain.

Moreover, the scans revealed lower-level variations in metabolic activity—where it deviated significantly from the global average—indicating that the brain works to preserve specific cognitive or sensory modules, such as those relating to vision or language comprehension, as it goes dark. That may not mean too much for those of us whose dealings with unconsciousness involve shutting down Netflix at 12:30 AM, but for those stuck in minimally conscious states thanks to some brain injury, this self-preservation of behavior and perceptive functioning might make all of the difference.

And defining this line between a minimally-conscious state and unresponsive wakefulness syndrome—the next step down, clinically speaking—means being able to detect the potential for if not predict the return of consciousness following brain injury.

This PET-based method is, "objective and computationally simple and provides easily interpretable results," the authors write. "The direct correlation between brain metabolism and behavior further suggests that DOCs can fundamentally be understood as pathological neuroenergetic conditions and provide a unifying physiological basis for these syndromes."

Consciousness is of course far from "solved" in any general sense, scientifically or philosophically. The definition for awareness used here corresponds to the Coma Recovery Scale-Revised (CRS-R), which is scoring system based on observations of visual, auditory, and motor functioning as well as a patient's communications ability and ability to be aroused (open their eyes, focus their attention). Which is all well and good clinically, but somehow a clean quantitative assessment doesn't lessen the deep weirdness of being conscious and, well, feeling weird about it.