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Researchers measure consciousness through brain activity

Aug. 14, 2013
Courtesy of Science Translational Medicine
and World Science staff

A new study seems to back up pre­vi­ous pro­pos­als that the lev­el of com­plex­ity of your brain ac­ti­vity largely de­ter­mines wheth­er you’re con­scious or not.

Sci­en­tists de­vel­oped a test of con­sciousness based on the con­cept—a test that does­n’t re­quire a pa­tient to ac­tu­ally do an­y­thing, they said.

Con­scious­ness is elu­sive, but we know it’s what van­ishes when we fall in­to a deep sleep and reap­pears when we wake up. Doc­tors typ­ic­ally de­ter­mine if a per­son is con­scious by their abil­ity to pro­cess and re­spond to ex­ter­nal com­mands, such as “open your eyes” or “squeeze my hand.” 

But these meth­ods are su­per­fi­cial, as re­search has shown in the last dec­ade that a brain to­tally dis­con­nect­ed from the out­side world may still have some aware­ness. This may hap­pen in brain-injured pa­tients who emerge from a co­ma but can’t move or un­der­stand in­struc­tions, for ex­am­ple.

One the­o­ry is that in a con­scious brain, dif­fer­ent popula­t­ions of neu­rons, or nerve cells, car­ry out their own com­puta­t­ional roles, but can’t com­mu­ni­cate with oth­er neu­ron popula­t­ions.

When the brain loses com­plex­ity, some sci­ent­ists pro­pose, neu­rons ei­ther start to be­have more si­m­i­larly al­to­geth­er (re­sult­ing in a loss of in­forma­t­ion), or their abil­ity to com­mu­ni­cate is im­paired (re­sult­ing in a loss of in­tegra­t­ion). For ex­am­ple, if you’re asleep and you hear a dog bark­ing, your brain will re­spond with ac­ti­vity in the au­di­to­ry cor­tex, the part of the brain that pro­cesses sound. But if you’re awake, the same sound might al­so in­duce thoughts of your own dog, or an­noy­ance at the loud­ness of the bark—re­sponses tied to ac­ti­vity in the brain’s mem­o­ry and emo­tion cen­ters. 

These lat­er brain pro­cesses con­tain more in­forma­t­ion. Neu­ro­sci­en­tists have been try­ing to de­vel­op ways to meas­ure con­sciousness based on this brain com­plex­ity.

In the stu­dy, Mar­cel­lo Mass­mini of the Uni­vers­ity of Mi­lan in Italy and col­leagues de­vised a tech­nique to meas­ure this brain com­plex­ity, or how much in­tegra­t­ion and in­forma­t­ion flow is hap­pening in the brain. Called the Per­turba­t­ional Com­plex­ity In­dex or PCI, their tech­nique in­volves mildly “shak­ing” the whole brain with a strong mag­net­ic stimula­t­ion and re­cord­ing the re­sponse of neu­rons. This da­ta can then be used to cal­cu­late how much in­forma­t­ion the brain is able to pro­duce as a whole. 

The re­search­ers tested the tech­nique in pa­tients with brain in­ju­ries, pa­tients un­der an­es­the­sia with dif­fer­ent drugs, and in pa­tients who were awake, in deep sleep or dream­ing. The test re­flected the par­ti­ci­pants’ lev­el of con­sciousness un­der each of these con­di­tions, they found. 

The re­sults sug­gest that dif­fer­ent lev­els of con­sciousness are tightly linked to the com­plex­ity of the brain re­sponse, they added. For ex­am­ple, find­ing a “PCI val­ue” above the sleep or an­es­the­sia lev­el in a pa­tient who is oth­erwise un­re­spon­sive would sug­gest she or he is con­scious to some ex­tent. Al­though more re­search is needed, the anal­y­sis could po­ten­tially be a use­ful tool at the hos­pi­tal bed­side for meas­ur­ing con­sciousness, they added.

The find­ings are pub­lished in the Aug. 14 is­sue of the jour­nal Sci­ence Transla­t­ional Med­i­cine.


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A new study seems to confirm previous proposals that the level of complexity of your brain activity largely determines whether you’re conscious or not. Scientists developed a test of consciousness based on the concept—a test that doesn’t require a patient to actually do anything, they said. Consciousness is elusive, but we know it’s what vanishes when we fall into a deep sleep and reappears when we wake up. Doctors typically determine if a person is conscious by their ability to process and respond to external commands, such as “open your eyes” or “squeeze my hand.” But these methods are superficial, as research has shown in the last decade that a brain totally disconnected from the outside world may still have some awareness. This may happen in brain-injured patients who emerge from a coma but are unable to move or understand commands, for example. One theory is that in a conscious brain, different populations of neurons, or nerve cells, carry out their own computational roles, but can’t communicate with other neuron populations. When the brain loses this complexity, neurons either start to behave more similarly altogether (resulting in a loss of information), or their ability to communicate is impaired (resulting in a loss of integration). For example, if you’re asleep and you hear a dog barking, your brain will respond with activity in the auditory cortex, the part of the brain that processes sound. But if you are awake, the same sound might also induce thoughts of your own dog, or annoyance at the loudness of the bark – responses tied to activity in the brain’s memory and emotion centers. The later brain processes contain more information. Neuroscientists have been trying to develop ways to measure consciousness based on this brain complexity. In the study, Marcello Massmini of the University of Milan in Italy and colleagues devised a technique to measure this brain complexity, or how much integration and information flow is happening in the brain. Called the Perturbational Complexity Index or PCI, their technique involves mildly “shaking” the whole brain with a strong magnetic stimulation and recording the response of neurons. This data can then be used to calculate how much information the brain is able to produce as a whole. The researchers tested the technique in patients with brain injuries, patients under anesthesia with different drugs, and in patients who were awake, in deep sleep or dreaming. The test reflected the participants’ level of consciousness under each of these conditions, they found. The results suggest that different levels of consciousness are tightly linked to the complexity of the brain response, they added. For example, finding a “PCI value” above the sleep or anesthesia level in a patient who is otherwise unresponsive would suggest she or he is conscious to some extent. Although more research is needed, the analysis could potentially be a useful tool at the hospital bedside for measuring consciousness, they added. The findings are published in the Aug. 14 issue of the journal Science Translational Medicine.