A Signature of Cortical Information Processing Underlies Shifts in Global States of Consciousness
Post by Soumilee Chaudhuri
The takeaway
Altered global states of consciousness are based on a top-down information processing signature in the cortex and influenced by a) spontaneous brain activity as well as b) regional brain organization. So, consciousness is determined by a hierarchical brain-region and brain-activity dependent signature.
What's the science?
Classically, consciousness has been understood as a neural manifestation of subjective experiences and linked to several dynamic neural processes in the brain. We know that breakdowns in consciousness (during sleep, sedation, etc.) elicit complex changes in regional brain coordination and neural processing. However, we do not understand the exact relationship between shifts in global states of consciousness and brain activity in certain regions of the brain. This week in Nature Communications, Dr. Ang Li and colleagues unravel the complexity of shifting states of global consciousness by combining behavioral, neuroimaging, electrophysiological, and transcriptomic experiments.
How did they do it?
The authors hypothesized that the shift in states of global consciousness might result from differential step-by-step processing of brain activity in different regions of the cortex (the brain’s gray matter-containing outer layer). They combined several functional Magnetic Resonance imaging (fMRI) approaches to capture altered consciousness — from deep sleep to full wakefulness — in recruited volunteers. In the first step, the authors captured the change in cortical activity over space and time in three distinct conditions: a) medication induced sedation, b) normal sleep, and c) awake, resting quietly. After this, they compared the cortical fluctuations between these conditions minute by minute. The authors also performed the exact same protocol for volunteers a) on caffeine or after fasting, b) administered a psychedelic drug and c) with neuropsychiatric disorders. They also used fMRI data from the Human Connectome Project to validate the spatiotemporal signatures obtained from the experiments. Electrocorticography (ECoG) recordings from Macaque monkeys were also used to compare to obtained hierarchical signatures. Additionally, the authors used spatial transcriptomic analyses from the Allen Brain Atlas to comment on specific regional contributions to wakefulness in subjects.
What did they find?
After looking at all of the evidence across different conditions, species, and timescales, the authors found that shifts in global state of consciousness can be attributed to changes in cortical neural variability, over time. This means that the global state of consciousness hierarchically associates with the disparity in neural responses across an experiment. Additionally, these complex shifts in consciousness can be translated to a simplified low-dimensional signature, enabling understanding of changes in consciousness in individual people. The authors also noted significant elevations of this hierarchical signature in abnormal states of consciousness (such as on psychedelics, in neuropsychiatric disorders, etc.). This signature also corresponded with the complex patterns of coordination that happen during wakefulness. The authors also found that the heterogeneity in the obtained hierarchical cortical neural variability across different conditions and species was modulated by a) spontaneous waves of cortical activities and b) the histaminergic system, a system that mediates inflammation.
What's the impact?
This study is the first to show that global states of consciousness rely on top-down hierarchical information processing in the cortex. The results also provide critical preliminary evidence of the association between the histaminergic system and hierarchical cortical processing. Most importantly, the authors find that at a global level, consciousness maps to top-down information processing by the cortex and that this may not be dependent on a specific neuroanatomical location in the brain. These findings provide a holistic understanding of the neural mechanisms of different conscious states such as sleeping, caffeinated or on psychedelics. This information may help to guide therapeutic and behavioral interventions targeting disorders of consciousness, such as sleep disorders, addictions to psychedelics or psychiatric disorders.