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Brain energy use proposed as key to understanding consciousness

June 17, 2009
Courtesy Yale University
and World Science staff

It takes high lev­els of brain en­er­gy to main­tain con­scious­ness. That sug­gests a new way to un­der­stand the prop­er­ties of this still mys­te­ri­ous state of be­ing, a group of Yale Un­ivers­ity re­search­ers is re­port­ing.

At its sim­plest, some sci­en­tists sug­gest, con­scious­ness is the abil­ity to re­spond mean­ing­fully to ex­ter­nal stim­u­li. Most stud­ies of con­scious­ness have used im­ag­ing tech­nol­o­gy to try to pin­point ar­eas of brain ac­ti­vity dur­ing tasks such as memor­iz­a­tion or prob­lem solv­ing.

There are two prob­lems with such an ap­proach, said Yale’s Rob­ert G. Shul­man, lead au­thor of a pa­per on the new study, pub­lished this week in the on­line edi­tion of the jour­nal Pro­ceed­ings of the Na­tional Acad­e­my of Sci­ences. 

First, brain im­ag­ing has shown that many ar­eas of the brain, not just one or two, are re­cruited dur­ing tasks such as mem­o­ry tests and are of scant help in stu­dy­ing the state of be­ing con­scious. Sec­ond, the amount of en­er­gy used in such tasks is min­ute, about one per­cent of base­line en­er­gy avail­a­ble to the brain.

Brain im­ag­ing “has been look­ing at the tip of the ice­berg,” Shul­man said. “We looked at the rest of the ice­berg.” What is the oth­er 99 per­cent of en­er­gy con­sump­tion do­ing? Shul­man and col­leagues have pro­posed that it’s needed to main­tain con­scious­ness. 

Heavily an­es­the­tized peo­ple are known to show ap­prox­i­mately 50 per­cent re­duc­tions in brain en­er­gy con­sump­tion, Shul­man said. When the paws of lightly an­es­the­tized rats were stroked, im­ag­ing sig­nals were re­ceived in a wide ar­ray of brain ar­eas, but in heavily an­es­the­tized rats the sig­nals stopped at a sensa­t­ion-receiving zone known as the sen­so­ry cor­tex. Both the to­tal en­er­gy and the brain im­ag­ing sig­nals changed when the per­son or an­i­mal lost con­scious­ness, Shul­man not­ed.

“What we pro­pose is that a con­scious per­son re­quires a high lev­el of brain en­er­gy,” Shul­man said. The find­ing has pro­found im­plica­t­ions for our un­der­standing of the con­nec­tion be­tween the brain and con­scious­ness, he added. “You can think of con­scious­ness not as a prop­er­ty of the brain, but of the per­son.” 

Anes­the­si­ol­ogists con­sid­er a per­son to be in a be­hav­ior­al state of con­scious­ness when he or she can re­spond to sim­ple stim­u­li. Prop­er­ties of this state, such as the high en­er­gy and the de­lo­cal­ized brain im­ag­ing sig­nals, al­low the per­son to per­form the in­ter­con­nected ac­ti­vi­ties that make up our eve­ry­day lives. Shul­man sug­gests that these more en­er­get­ic prop­er­ties of the brain sup­port hu­man be­hav­ior and should be con­sid­ered when in­ter­pret­ing the much weaker sig­nals typ­ic­ally recorded dur­ing brain im­ag­ing stud­ies.

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It takes high levels of brain energy to maintain consciousness. That suggests a new way to understand the properties of this still mysterious state of being, a group of Yale University researchers are reporting. At its simplest, some scientists suggest, consciousness can be defined as the ability to respond meaningfully to external stimuli. Most studies of consciousness have used imaging technology to try to pinpoint areas of brain activity during tasks such as memorization or problem solving. There are two problems with such an approach, said Yale’s Robert G. Shulman, lead author of the paper, published this week in the online edition of the journal Proceedings of the National Academy of Sciences. First, brain imaging has shown that many areas of the brain, not just one or two, are recruited during tasks such as memory tests and are of scant help in studying the state of being conscious. Second, the amount of energy used in such tasks is minute, about one percent of baseline energy available to the brain. Brain imaging “has been looking at the tip of the iceberg,” Shulman said. “We looked at the rest of the iceberg.” What is the other 99 percent of energy consumption doing? Shulman and colleagues have proposed that it’s needed to maintain consciousness. Heavily anesthetized people are known to show approximately 50 percent reductions in cerebral energy consumption, Shulman said. When the paws of lightly anesthetized rats were stroked, imaging signals were received in a wide array of brain areas, but in heavily anesthetized rats the signals stopped at a sensation-receiving zone known as the sensory cortex. Both the total energy and the brain imaging signals changed when the person or animal lost consciousness, Shulman noted. “What we propose is that a conscious person requires a high level of brain energy,” Shulman said. The finding has profound implications for our understanding of the connection between the brain and consciousness, he added. “You can think of consciousness not as a property of the brain, but of the person.” Anesthesiologists consider a person to be in a behavioral state of consciousness when he or she can respond to simple stimuli. Properties of this state, such as the high energy and the delocalized brain imaging signals, allow the person to perform the interconnected activities that make up our everyday lives. Shulman suggests that these more energetic properties of the brain support human behavior and should be considered when interpreting the much weaker signals typically recorded during brain imaging studies.