The presence of alpha rhythm with the eyes closed is a marker for wakefulness, but its representation at the cortical surface is not required for wakefulness as it is obliterated in cases of bilateral occipital infarction. It is, of course, possible that deep nuclei are still projecting the rhythm to other parts of the cerebrum and it was assumed that this maintains wakefulness but even this precept is not clear. Although for many years it has been stated that arousal causes a desynchronization of brainwave activity (in distinction to the synchronized activity of sleep), it has become apparent that during wakefulness, there also is a widespread low-voltage fast rhythm (a gamma rhythm that has a frequency of 30 to 60 Hz). This activity, coordinated by the thalamus, has been theorized to synchronize cortical activity and to account perhaps for the unification of modular aspects of experience (color, shape, motion) that are processed in different cortical regions. In this way, the rhythm is said to “bind” various aspects of a sensory experience or memory. This fast and widespread electrographic activity is not appreciated with the usual EEG surface recordings but it can be extracted by sophisticated mathematical transformations. Using such electrophysiologic methods, Meador and colleagues have shown that the gamma rhythm can be detected over the primary somatosensory cortex after an electrical stimulus on the contralateral hand is perceived, but not if the patient fails to perceive it. The clinical relevance of the rhythm is uncertain but it has elicited interest because it may give insight into several intriguing questions about conscious experience.
In the last decade, a novel perspective on consciousness has been derived from the study of functional connectivity of brain regions as reflected by functional MRI studies. At least three such interconnected networks have been detected, a salience network that has been theorized to underlie conscious perception of stimuli, a default-mode network for internal thought, and an executive-control network for externally guided awareness. Of these, Qin and colleagues found that the salience network was most closely correlated with behavioral signs of awareness but that changes in the default network corresponded to regaining consciousness after coma. These studies are interesting but at the moment only correlative and they will certainly evolve in the next decades.