Wakefulness and loss of awareness: brain and brainstem interaction in the vegetative state

The ascending reticular activating system (ARAS) modulates circadian wakefulness, which is preserved in a persistent vegetative state (PVS). Its metabolism is preserved. Impairment of metabolism in the polymodal associative cortices (i.e., precuneus) is characteristic of PVS where awareness is abolished. Because the interaction of these 2 structures allows conscious sensory perception, our hypothesis was that an impaired functional connectivity between them participates in the loss of conscious perception.

(15)O-radiolabeled water PET measurement of regional cerebral blood flow (rCBF) was performed at rest and during a proprioceptive stimulation. Ten patients in PVS and 10 controls were compared in a cross-sectional study. The functional connectivity from the primary sensorimotor cortex (S1M1) and the ARAS in both groups was also investigated.

Compared with controls, patients showed significantly less rCBF in posterior medial cortices (precuneus) and higher rCBF in ARAS at rest. During stimulation, bilateral Brodmann area 40 was less activated and not functionally correlated to S1M1 in PVS as it was in controls. Precuneus showed a lesser degree of deactivation in patients. Finally, ARAS whose activity was functionally correlated to that of the precuneus in controls was not in PVS.

Global neuronal workspace theory predicts that damage to long-distance white matter tracts should impair access to conscious perception. During persistent vegetative state, we identified a hypermetabolism in the ascending reticular activating system (ARAS) and impaired functional connectivity between the ARAS and the precuneus. This result emphasizes the functional link between cortices and brainstem in the genesis of perceptual awareness and strengthens the hypothesis that consciousness is based on a widespread neural network.