Consciousness alterations after severe brain injury

The past fifteen years have provided an unprecedented collection of discoveries that bear upon our scientific understanding of consciousness in the human brain following severe brain damage. Highlighted among these discoveries are unique demonstrations that patients with little or no behavioral evidence of conscious awareness may retain critical cognitive capacities. These first scientific demonstrations support the possibility that some severely brain-injured patients in long-standing conditions of limited behavioral responsiveness may nonetheless retain latent capacities for awareness. Such capacities include the human functions of language and higher-level cognition that, either spontaneously or by thought-directed interventions, may reemerge even at long time intervals or can remain unrecognized. Functional neuroimaging, such as positron emission tomography and functional magnetic resonance imaging, as well as electroencephalography and evoked potential studies, have offered the possibility to objectively approach covert cognitive processes in patients who are otherwise incapable of intelligible or sustained behavioral expression. Such studies have used experimental protocols to assess brain function during resting-state conditions and after external stimulation. These technologies have further permitted the detection of nonverbal command-following and even established muscle- independent means of communication with some behaviorally unresponsive patients. Such advances are expected to shed light on the gray zones between the clinical entities of consciousness and help resolve medical and ethical controversies around the management of such challenging situations.