Cortex Off, Consciousness Off
This dramatic reduction in brain activity after loss of consciousness is scarcely surprising. The link between consciousness and this organ is tight, as expressed in the adage “No brain: never mind!” Yet neuroscientists are trying to track the footprints of consciousness to its actual lair. Which region in the cortex, the thalamus or elsewhere is essential to be conscious at all? Consider the following two experiments.
Twenty-five patients with Parkinson's disease were anesthetized with propofol or sevoflurane while the electrical activity of both the cortex and thalamus was monitored by a group under François Gouin of the Timone University Hospital Center at the University of the Mediterranean in Marseille, France. Their neocortex was monitored by a conventional electroencephalographic (EEG) electrode placed on the scalp on top of the head, whereas thalamic activity was recorded by an electrode implanted deep inside the brain in the subthalamic nucleus. This electrode stimulates the brain to alleviate the shaking that is the hallmark of Parkinson's. Experimenters assessed consciousness by tapping patients on the shoulder and asking them every 20 seconds to open their eyes.
When consciousness was lost after anesthesia was initiated—that is, when the patients no longer opened their eyes following the command—the cortical EEG changed dramatically, switching from low amplitude and irregular activity into readings dominated by large and slow brain waves that occur about once every second. Such so-called delta band activity is characteristic of deep sleep. Furthermore, the complexity of the cortical EEG signal decreased significantly when patients stopped responding. None of these changes occurs in the thalamic electrode at the time that consciousness is lost.
Indeed, it is only several minutes later that the thalamic voltage signal matches that of the cortex. The data—consistent for two quite different anesthetic agents, one injected and the other one inhaled—argue that the drivers for the loss of consciousness are parts (or all) of the neocortex and that the thalamus follows.