The brains of people under anesthesia respond to stimuli as they do in the deepest part of sleep – lending credence to a developing theory of consciousness and suggesting a new method to assess loss of consciousness in conditions such as coma.
Scientists at the University of Wisconsin School of Medicine and Public Health, led by brain researcher Fabio Ferrarelli, reported their findings in this week’s edition of the Proceedings of the National Academy of Science.
The group gave the anesthetic midazolam, commonly used at lower doses in “conscious sedation” procedures such as colonoscopies, to volunteers.
Then they used transcranial magnetic stimulation (TMS), a noninvasive technique to stimulate the brain cortical neurons from the scalp, in combination with electroencephalography (EEG), which recorded the TMS-evoked brain responses. What they found is a pattern that looks much as it does when the brain is in deep, non-rapid eye movement (non-REM) sleep, another condition when consciousness fades.
Co-author and consciousness expert Giulio Tononi says that when the brain is unconscious it appears to lose the connectivity that underlies the coordinated, yet differentiated responses to electrical stimuli observed when the brain is awake or in REM sleep. The group’s earlier studies demonstrated the differences between the sleeping and awake brain.
“Based on a theory about how consciousness is generated, we expect to see a response that is both integrated and differentiated when the brain is conscious,” says Tononi, professor of psychiatry. “When there is a loss of consciousness, either due to sleep or anesthesia, the response is radically different. We see a stereotyped burst of activity that remains localized and fades quickly.”
The team believes that the response patterns observed in the awake brain, characterized by long-lasting activations moving over time to different cortical areas, reflect the connectivity of the cortical areas activated by TMS. This could be because when we are awake, the cortex is involved in many activities which require a constant communication between different cortical areas. But in the unconscious brain, this connectivity is temporarily lost, and therefore the TMS-evoked brain responses remain localized.
Ferrarelli says the results lend weight to the idea that a breakdown of cortical connectivity is a key aspect of loss of consciousness, and are consistent with the “integrated information theory of consciousness.”
Co-author Dr. Robert Pearce, chair and professor of anesthesiology at UW SMPH, said it is interesting that the cortical responses under anesthesia were so similar to changes seen during natural sleep.
“The idea that some anesthetics “hijack” the natural sleep-promoting centers was proposed recently by others,” says Pearce. “While our present findings do not directly confirm this hypothesis, they are consistent with a set of shared mechanisms. That is, that the loss of functional connectivity between brain regions is a characteristic that sleep and anesthesia share, and that we think might be causal in the loss of consciousness in both cases.” …
What is IIT?
Since the early days of computers, scholars have argued that the subjective, phenomenal states that make up the life of the mind are intimately linked to the information expressed at that time by the brain. Yet they have lacked the tools to turn this hunch into a concrete and predictive theory. Enter psychiatrist and neuroscientist Giulio Tononi of the University of Wisconsin–Madison. Tononi has developed and refined what he calls the integrated information theory (IIT) of consciousness.
An Integrated Theory
IIT is based on two axiomatic pillars.
First, conscious states are highly differentiated; they are informationally very rich. You can be conscious of an uncountable number of things: you can watch your son’s piano recital, for instance; you can see the flowers in the garden outside or the Gauguin painting on the wall. Think of all the frames from all the movies you have ever seen or that have ever been filmed or that will be filmed! Each frame, each view, is a specific conscious percept.
Second, this information is highly integrated. No matter how hard you try, you cannot force yourself to see the world in black-and-white, nor can you see only the left half of your field of view and not the right. When you’re looking at your friend’s face, you can’t fail to also notice if she is crying. Whatever information you are conscious of is wholly and completely presented to your mind; it cannot be subdivided. Underlying this unity of consciousness is a multitude of causal interactions among the relevant parts of your brain. If areas of the brain start to disconnect or become fragmented and balkanized, as occurs in deep sleep or in anesthesia, consciousness fades and might cease altogether. Consider split-brain patients, whose corpus callosum—the 200 million wires linking the two cortical hemispheres—has been cut to alleviate severe epileptic seizures. The surgery literally splits the person’s consciousness in two, with one conscious mind associated with the left hemisphere and seeing the right half of the visual field and the other mind arising from the right hemisphere and seeing the left half of the visual field. – ref