Abstract
Brain activity fluctuates continuously, even in the absence of changes in sensory input or motor output. These intrinsic activity fluctuations are correlated across brain regions and are spatially organized in macroscale networks. Variations in the strength, topography, and topology of correlated activity occur over time, and unfold upon a backbone of long-range anatomical connections. Subcortical neuromodulatory systems send widespread ascending projections to the cortex, and are thus ideally situated to shape the temporal and spatial structure of intrinsic correlations. These systems are also the targets of the pharmacological treatment of major neurological and psychiatric disorders, such as Parkinson’s disease, depression, and schizophrenia. Here, we review recent work that has investigated how neuromodulatory systems shape correlations of intrinsic fluctuations of large-scale cortical activity. We discuss studies in the human, monkey, and rodent brain, with a focus on non-invasive recordings of human brain activity. We provide a structured but selective overview of this work and distil a number of emerging principles. Future efforts to chart the effect of specific neuromodulators and, in particular, specific receptors, on intrinsic correlations may help identify shared or antagonistic principles between different neuromodulatory systems. Such principles can inform models of healthy brain function and may provide an important reference for understanding altered cortical dynamics that are evident in neurological and psychiatric disorders, potentially paving the way for mechanistically inspired biomarkers and individualized treatments of these disorders.
Highlights
Neural population activity in the cerebral cortex fluctuates continuously, even in the absence of changes in sensory input or motor output
We review studies that either used whole-brain “dual regression,” or correlation between a seed and the entire cortex
This study reported reduced modularity, increased participation coefficient of frontal and midline regions, and reduced rich-club coefficient
Summary
Neural population activity in the cerebral cortex fluctuates continuously, even in the absence of changes in sensory input or motor output. Input from any neuromodulatory nucleus to the cortex, changing activity state, dynamic state, or both, might translate into spatially structured correlations of neural population signals in the cortex For this reason, it is critical to consider the potential impact of neuromodulatory brainstem systems when making inferences about physiological cortico–cortical interactions (and “cortical networks”) from the correlation of intrinsic cortical activity alone. The cortical distribution of CBV responses correlated spatially with reductions in burst rate and delta-band power as measured electrophysiologically Combined, these two studies (Turchi et al, 2018; Grandjean et al, 2019) provide the strongest evidence to date of a mediation of (a part of) intrinsic activity correlations through rapid, common input from neuromodulatory brainstem nuclei that are consistent with correlated changes in activity state. Beliveau et al (2015) found relatively confined partial correlations between the cortex and anatomically and [11C]DASB PET-binding constrained delineations of the dorsal and median raphe nuclei
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