Scientists Say Primitive Brain Structures Activate First as People Awaken From Anesthesia

Posted on April 5, 2012

Scientists says primitive consciousness emerges first as people awaken from anesthesia. Using brain imaging techniques in healthy volunteers, researchers have imaged the process of returning consciousness after general anesthesia. The emergence of consciousness was found to be associated with activations of deep, primitive brain structures rather than the evolutionary younger neocortex.

This image of positron emission tomography (PET) findings show that the emergence of consciousness after anesthesia is associated with activation of deep old brain structures rather than the neocortex. The Sagittal (top) and axial (bottom) sections on the left show activation in the anterior cingulate cortex, thalamus and the brainstem locus coeruleus/parabrachial area overlaid on magnetic resonance image (MRI) slices. The cortical renderings on the right show no evident activations.

The study was part of the Research Programme on Neuroscience by the Academy of Finland. It was led by Adjunct Professor Harry Scheinin, M.D. from the University of Turku, Turku, Finland in collaboration with investigators from the University of California, Irvine, USA.

Dr. Scheinin says, "We expected to see the outer bits of brain, the cerebral cortex (often thought to be the seat of higher human consciousness), would turn back on when consciousness was restored following anesthesia. Surprisingly, that is not what the images showed us. In fact, the central core structures of the more primitive brain structures including the thalamus and parts of the limbic system appeared to become functional first, suggesting that a foundational primitive conscious state must be restored before higher order conscious activity can occur."

20 young healthy volunteers were put under anesthesia in a brain scanner using either dexmedetomidine or propofol anesthetic drugs. The subjects were then woken up while brain activity pictures were being taken. Dexmedetomidine is used as a sedative in the intensive care unit setting and propofol is used for induction and maintenance of general anesthesia. Dexmedetomidine induced unconsciousness has a close resemblance to normal physiological sleep. It can be reversed with mild physical stimulation or loud voices without requiring any change in the dosage. This unique property enabled the investigators to separate the brain activity changes associated with the changing level of consciousness from the drug related effects on the brain. The changes in brain activity were imaged with positron emission tomography (PET).

The researchers say the emergence of consciousness, as assessed with a motor response to a spoken command, was associated with the activation of a core network involving subcortical and limbic regions that became functionally coupled with parts of frontal and inferior parietal cortices upon awakening from dexmedetomidine induced unconsciousness.

The same deep brain structures - the brain stem, thalamus, hypothalamus and the anterior cingulate cortex - were also first activated upon emergence from propofol anesthesia, suggesting a common, drug independent mechanism of arousal. For both drugs, activations seen upon regaining consciousness were mostly localized in deep, phylogenetically old brain structures rather than in the neocortex.

The researchers speculate that because current anesthesia monitoring technology is based on cortical electroencephalography (EEG) measurement, their results help to explain why these devices fail in differentiating the conscious and unconscious states and why patient awareness during general anesthesia may not always be detected.

THE MIND