Functional neuroimaging techniques represent a useful tool to objectively quantify the residual brain function in patients with altered states of consciousness. We will here compare the cerebral function in patients who survive a severe brain injury (i.e., coma, vegetative state, minimally conscious state and locked in syndrome) with that observed in the resting conscious state, sleep, epilepsy and general anaesthesia. The interest of this work is twofold. First, severely brain injured patients represent a problem in terms of diagnosis, prognosis, treatment and daily management. Second, these patients offer the opportunity to explore human consciousness. Indeed, they present a complete ­ nearly graded ­ range of conscious states from unconsciousness (coma) to full awareness (locked-in syndrome). In what follows, we will put a special emphasis on the vegetative state. This condition represents a unique and complete dissociation between the two main components of consciousness: wakefulness -which is preserved- and awareness -which is abolished.
Compared to the conscious resting state, global brain metabolism has been shown to be significantly reduced in the vegetative state (approximately 40 to 50% of normal values). Similar values have been observed in coma, slow wave sleep and general anaesthesia. However, the recovery of consciousness from vegetative state is not always associated with substantial changes in global metabolism. This finding led us to hypothesize that some vegetative patients are unconscious not just because of a global loss of neuronal function, but rather due to an altered activity in some critical brain regions and to the abolished functional connections between them. In the vegetative state, the most dysfunctional brain regions are bilateral frontal and parieto-temporal associative cortices. Interestingly, a similar fronto-parietal network is the most active during wakefulness and the least active in coma, sleep and general anaesthesia. Despite the metabolic impairment, external stimulation still induces a significant neuronal activation (i.e., change in blood flow) in vegetative patients as shown by both noxious and auditory stimuli. However, this activation is limited to primary cortices and dissociated from higher-order associative cortices, thought to be necessary for conscious perception. Finally, we show that vegetative patients have impaired functional connections between distant cortical areas and between the thalami and the cortex and, more importantly, that recovery of consciousness is paralleled by a restoration of this cortico-thalamo-cortical interaction. Consciousness seems to rely on the functional integrity of a critical frontal-parietal 'global workspace' network and its intra- and subcortical connections.

Selected references:
Boly M, Faymonville ME, Peigneux P, Lambermont B, Damas P, Del Fiore G, Degueldre C, Franck G, Luxen A, Lamy M, Moonen G, Maquet P, Laureys S. Auditory processing in severely brain injured patients: differences between the minimally conscious state and the persistent vegetative state. Arch Neurol. 2004 Feb;61(2):233-8
Laureys S, Faymonville ME, Peigneux P, Damas P, Lambermont B, Del Fiore G, Degueldre C, Aerts J, Luxen A, Franck G, Lamy M, Moonen G, Maquet P. Cortical processing of noxious somatosensory stimuli in the persistent vegetative state. Neuroimage. 2002 Oct;17(2):732-41.
Laureys S, Faymonville ME, Luxen A, Lamy M, Franck G, Maquet P. Restoration of thalamocortical connectivity after recovery from persistent vegetative state. Lancet. 2000 May 20;355:1790-1.