Consciousness and cerebral baseline activity fluctuations

Abstract

The origin of within-subject variability in perceptual experiments is poorly understood. We here review evidence that baseline brain activity in the areas involved in sensory perception predict subsequent variations in sensory awareness. We place these findings in light of recent findings on the architecture of spontaneous BOLD fluctuations in the awake human brain, and discuss the possible origins of the observed baseline brain activity fluctuations.

Figure 1

Two components of awareness. In healthy awake volunteers as in disorders of consciousness studies, awareness has been related to the activity of a widespread frontoparietal associative network, both on the convexity and the midline. These findings are in line with the “global workspace” theory of consciousness. Awareness can in turn be divided in two main components, external and self awareness. Similarly, the awareness related frontoparietal network can be partitioned in two nonoverlapping subsystems, involved in processing the self and external‐related components of awareness. At present, the relationships between self and external awareness remain poorly understood. Upper image adapted from [Laureys, 2005], represents statistically significant hypometabolic areas as measured by FDG‐PET in the vegetative state as compared with conscious waking. Lower images are adapted from [Boly et al., 2007], showing the default network (in blue) and the lateral frontoparietal network (in red) as identified in this study by fMRI. The color code refers to voxel‐wise T values, ranging respectively from red to yellow or from blue to white.

Figure 2

Baseline brain activity predicting conscious perception of subsequent somatosensory stimuli. (A) Increased baseline brain activity in medial thalamus (Th), dorsolateral prefrontal cortex (DLPF), intraparietal sulcus/posterior parietal cortex (IPS), and anterior cingulate cortex (ACC) 3 s before stimulus presentation predicts perception of low intensity sensory stimuli. (B) Decreased baseline activity in the default brain network encompassing posterior cingulate/precuneus (Pr) and bilateral temporo–parietal junctions (TP) exert a facilitatory effect on perception of subsequent somatosensory stimuli. Color scales refer to T‐values of individual voxels. Reproduced with permission from [Boly et al., 2007). (C and D) Graphical illustration of mean group peristimulus time histograms extracted from peak areas of significance for stimulus awareness contrasts, using a finite impulse response model approach as implemented in SPM2. Results are mean effect sizes (arbitrary units) with 95% confidence intervals. In intraparietal sulcus, baseline BOLD signal before the occurrence of the stimulus presentation (blue line) is higher for events that will be perceived (P1 scores, in green) as compared to events that will be unperceived (P0 scores, in red). An inverse response shape is observed in lateral temporoparietal junction.