Returning from Oblivion
In a second experiment, by a group primarily based at the University of Turku in Finland, involving Harry Scheinin, Jaakko W. Långsjö and Michael T. Alkire, 20 volunteers were put to sleep with two different substances, dexmedetomidine and propofol (again, to make sure that the outcome does not depend on any one specific agent). After being injected with the radioactive tracer, the subjects lay down inside a PET scanner. The anesthesiologists measured the regional cerebral blood flow when the patients regained consciousness—that is, when the subjects could open their eyes again in response to a persistent command (albeit given only once every five minutes).
Subsequent statistical analysis fingered phylogenetically older regions in the brain stem (in particular, the locus coeruleus and parabrachial area that contains the noradrenergic neurons that project widely throughout the corticothalamic complex and exert broad effects on the brain). They mediate the arousal needed for behavioral responses—such as blinking the eyes—to occur. As consciousness returns, the thalamus is exuberantly active, whereas the cortex shows a much more circumscribed response, primarily in those frontal regions responsible for monitoring of the self.
The conjoined activation of both the cortex and thalamus appears at odds with the previous study, which implicated the cortex as the driver and the thalamus as the follower. Yet the two techniques (EEG versus PET imaging) measure distinct signals (voltage versus blood flow, which is 1,000 times more sluggish), compounded by the fact that the first study checked whether or not the patients were conscious every 20 seconds, whereas the second one inquired only every five minutes.
Furthermore, although consciousness waxes and wanes during anesthesia, many other processes—the overall level of brain arousal, the ability to move and to remember, the experience of pain and other sensations, and so on, each with their own neuronal signature—also vary and confound the search for the sources of consciousness. Finally, just as the sequence of operations on booting up a computer are not the same as those that occur when the machine is shut down, the brain events accompanying the return of consciousness are unlikely to be identical to those that cause consciousness to cease.
These two exemplary studies point to the difficulties, but also to the progress, of the quest to unravel the mind-body riddle.