Measurements of consciousness in the vegetative state.

In The Lancet, Damian Cruse and colleagues' study 1 examines 16 patients in the vegetative state. This disorder is defi ned as wakefulness without conscious awareness of self and environment. The investigators showed that three (19%) of the 16 patients could generate EEG responses to two commands involving motor imagery, although the patients were otherwise behaviourally unresponsive. But how can this discovery be understood? Researchers typically accept a distinction between the contents and levels of consciousness. 2 Contents of consciousness are defi ned as subjective experience— eg, the taste of coff ee, feeling of pain, or experience of the colour red. In the study of levels of consciousness, 2 three distinct stages of degraded consciousness have been described: coma, the vegetative state, and the minimally conscious state. Diff erentiation between the stages is based on behavioural criteria. Patients in the vegetative state diff er from those in a coma because they can be aroused, yet both groups are considered fully unconscious. Patients in the minimally conscious state are believed to have fl uctuating consciousness and are distinguished from the vegetative state when an outside observer (a doctor in most cases) thinks the patient has a minimum understanding of self or the environment (eg, a voluntary attempt to communicate). Other patients with severe brain injury, who are not in the minimally conscious state, are typically believed to be more conscious 3 than minimally conscious patients. Immediate problems occur with attempts to operationalise these defi nitions, not least with the aim to develop a bedside test for consciousness. Levels of consciousness, in the standard scale, are defi ned as perceptible signs of being in contact with the outside world. Whereas this defi nition might make intuitive sense, it is not identical to subjective experience. In cognitive science, nearly all mental events exist with and without consciousness. Complex types of behaviour, response inhibition, task switching, instruction following, confl ict monitoring, and error detection have all been described as fully functional in the absence of reportable conscious experience. 4 Similarly, complex aspects of perception, such as semantic interpretation, can occur unconsciously. 5 No patient has been discovered who could follow instructions, yet was completely unconscious. However, with the scarcity of external validation methods in the study of consciousness, to imagine how such a patient could be identifi ed is diffi cult. Yet fi ndings from some studies 6 indicate that so-called split-brain patients react …