Functional neuroimaging in the vegetative state

The interest of functional imaging in patients in a vegetative state is twofold. First, the vegetative state continues to represent a major clinical and ethical problem, in terms of diagnosis, prognosis, treatment, everyday management and end-of-life decisions. Second, it offers a lesional approach to the study of human consciousness and adds to the international research effort on identifying the neural correlate of consciousness. Cerebral metabolism has been shown to be massively reduced in the vegetative state. However, recovery of consciousness from vegetative state seems not always associated with substantial changes in global metabolism. Recent PET data indicate that some vegetative patients are unconscious not just because of a global loss of neuronal function, but due to an altered activity in a critical fronto-parietal cortical network and to abolished functional connections within this network and with non-specific thalamic nuclei. Recovery of consciousness was shown to be paralleled by a restoration of this cortico-thalamo-cortical interaction. Despite the metabolic impairment, external stimulation still induces neuronal activation as shown by both auditory and noxious stimuli. However, this activation is limited to primary cortices and dissociated from higher-order associative cortices, thought to be necessary for conscious perception.