Resting brain activity reveals two anticorrelated cortical systems linked to conscious awareness: an extrinsic system (lateral fronto-parietal areas) associated with external awareness and an intrinsic system (medial brain areas) associated with internal awareness. In 31 healthy volunteers, external and internal awareness were significantly anticorrelated, with a mean switching frequency of 0.05 Hz, similar to BOLD fMRI slow oscillations. In 22 volunteers, fMRI showed that precuneus/posterior cingulate, anterior cingulate/mesiofrontal cortices, and parahippocampal areas (intrinsic system) correlated with internal awareness, while lateral fronto-parietal cortices (extrinsic system) correlated with external awareness.
Variability in how people perceive the same sensory stimulus from one moment to the next may stem from ongoing fluctuations in baseline brain activity. This review examines evidence that activity levels in sensory brain areas before a stimulus predict whether that stimulus will be consciously perceived. The findings are discussed in the context of recent discoveries about the structure of spontaneous BOLD signal fluctuations in the awake human brain, and possible sources of these baseline fluctuations are considered.