Detecting consciousness: a unique role for neuroimaging

How can we ever know, unequivocally, that another person is conscious and aware? Putting aside deeper philosophical considerations about the nature of consciousness itself, historically, the only reliable method for detecting awareness in others has been through a predicted behavioral response to an external prompt or command. The answer may take the form of spoken words or a nonverbal signal such as a hand movement or the blink of an eye, but it is this answer, and only this answer, that allows us to infer awareness. In recent years, rapid technological developments in the field of neuroimaging have provided new methods for revealing thoughts, actions, and intentions based solely on the pattern of activity that is observed in the brain. In specialized centers, these methods are now being employed routinely to detect consciousness in behaviorally nonresponsive patients when all existing clinical techniques have failed to provide that information. In this review, I compare those circumstances in which neuroimaging data can be used to infer consciousness in the absence of a behavioral response with those circumstances in which it cannot. This distinction is fundamental for understanding and interpreting patterns of brain activity following acute brain injury and has profound implications for clinical care, diagnosis, prognosis, and medical-legal decision-making (relating to the prolongation, or otherwise, of life after severe brain injury). It also sheds light on more basic scientific questions about the nature of consciousness and the neural representation of our own thoughts and intentions.