Ongoing physiological processes in the cerebral cortex

Section on Cognitive Neurophysiology and Imaging, Laboratory of Neuropsychology, National Institute of Mental Health, National Institutes of Health, Department of Health and Human Services, 49 Convent Dr. 1E-21, MSC 4400, Bethesda, MD 20892, USA.
NeuroImage (Impact Factor: 6.36). 10/2011; 62(4):2190-200. DOI: 10.1016/j.neuroimage.2011.10.059
Source: PubMed


Functional magnetic resonance imaging (fMRI) has revealed that the human brain undergoes prominent, regional hemodynamic fluctuations when a subject is at rest. These ongoing fluctuations exhibit distinct patterns of spatiotemporal synchronization that have been dubbed "resting state functional connectivity", and which currently serve as a principal tool to investigate neural networks in the normal and pathological human brain. Despite the wide application of this approach in human neuroscience, the neural mechanisms that give rise to spontaneous fMRI correlations are largely unknown. Here we review results of recent electrophysiological studies in the cerebral cortex of humans and nonhuman primates that link neural activity to ongoing fMRI fluctuations. We begin by describing results obtained with simultaneous fMRI and electrophysiological measurements that allow for the identification of direct neural correlates of resting state functional connectivity. We next highlight experiments that investigate the correlational structure of spontaneous neural signals, including the spatial variation of signal coherence over the cortical surface, across cortical laminae, and between the two hemispheres. In the final section we speculate on the origins and potential consequences of ongoing signals for normal brain function, and point out inherent limitations of the fMRI correlation approach.

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Available from: Alexander Maier, Feb 24, 2014
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    • "However, rsfMRI investigations were met with initial skepticism (Buckner and Vincent, 2007;Morcom and Fletcher, 2007) due to technological challenges (Birn et al., 2006;Power et al., 2012;Van Dijk et al., 2012), whose resolution defines important milestones of the field (Power et al., 2014). Of particular note is that several researchers have highlighted the uncertain interpretation of rsfMRI because of the unstructured nature of resting-state cognition (Morcom and Fletcher, 2007), as well as the complex relationship between BOLD signal fluctuations and ongoing electrophysiological activity (Leopold and Maier, 2012). Below, we focus on the literature assessing the electrophysiological correlates of rsfMRI in human and non-human primates. "

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    • "However, the global perfusion signal will likely contain neuronal contributions (Leopold and Maier, 2012; Murphy et al., 2009), and regressing out the global signal will likely bias the perfusion time series just as it biases the BOLD time series and connectivity measures. Thus, the use of global signal regression is a controversial issue in functional connectivity calculations (Saad et al., 2013). "
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    ABSTRACT: Arterial-spin labelling (ASL) is an increasingly established magnetic resonance imaging (MRI) technique that is finding broader applications in studying the healthy and diseased brain. This review addresses the use of ASL to assess brain function in the resting state. Following a brief technical description, we discuss the use of ASL in the following main categories: (1) resting-state functional connectivity (FC) measurement: the use of ASL-based cerebral blood flow (CBF) measurements as an alternative to the blood-oxygen level dependent (BOLD) technique to assess resting-state functional connectivity; (2) the link between network CBF and FC measurements: the use of network CBF as a surrogate of the metabolic activity within corresponding networks; and (3) the study of resting-state dynamic CBF-BOLD coupling and cerebral metabolism: the use of dynamic CBF information obtained using ASL to assess dynamic CBF-BOLD coupling and oxidative metabolism in the resting state. In addition, we summarize some future challenges and interesting research directions for ASL, including slice-accelerated (multiband) imaging as well as the effects of motion and other physiological confounds on perfusion based FC measurement. In summary, this work reviews the state-of-the-art of ASL, and establishes it as an increasingly viable MRI technique with high translational value in studying resting-state brain function.
    Full-text · Article · Jun 2015 · Brain Connectivity
    • "To date, studies of the neural correlates involved with spontaneous BOLD activity have used electrophysiological recording methods such as the electroencephalography (EEG), electrocorticography (ECoG), local field potential (LFP), and multi-unit activity (MUA) (Leopold and Maier 2012). Leopold and his colleagues have demonstrated that spontaneous BOLD fluctuations correlate with slow modulation of the spiking rate, MUA power, and LFP power (gamma band and 2–15 Hz range) in the monkey visual cortex at rest (Shmuel and Leopold 2008; Scholvinck et al. 2010). "
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    ABSTRACT: The spontaneous cerebral hemodynamic fluctuations observed during the resting state have been frequently visualized using functional magnetic resonance imaging (rsfMRI). However, the neuronal populations and neuroelectric characteristics underlying the functional connectivity of cerebrohemodynamic activities are poorly understood. We investigated the characteristics of bi-hemispheric functional connectivity via electrophysiology and rsfMRI in the primary sensory cortex of rats anesthetized by α-chloralose. Unlike the evoked responses, the spontaneous electrophysiological activity was concentrated in the infragranular layers and could be classified into subtypes with distinctive current sources and sinks. Both neuroelectric and rsfMRI signals were interhemispherically correlated in a layer-specific manner, suggesting that there are independent neural inputs to infragranular and granular/supragranular layers. The majority of spontaneous electrophysiological activities were bilaterally paired with delays of up to ~50 ms between each pair. The variable interhemispheric delay implies the involvement of indirect, multi-neural pathways. Our findings demonstrated the diverse activity patterns of layer-specific electrophysiological substrates and suggest the recruitment of multiple, non-specific brain regions in construction of interhemispheric functional connectivity.
    No preview · Article · Jun 2015 · Brain Structure and Function
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