At first glance, defining unconsciousness may seem easier than defining its counterpart consciousness. Indeed, consciousness has many definitions, whereas unconsciousness is "just" the absence of consciousness. However, the study of patients with epilepsy or severe disorders of consciousness (eg, coma and vegetative states) or focal deficits of consciousness (eg, blindsight) and studies on sleep and anesthesia have shown that consciousness and unconsciousness are graded. Measuring consciousness or its gradation is essential but there is little consensus on how it should be done.
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