Metabolic Origin of Bold Signal Fluctuations in the Absence of Stimuli

Abstract

Blood oxygen level-dependent (BOLD) functional magnetic resonance imaging studies have shown the existence of ongoing blood flow fluctuations in the absence of stimuli. Although this so-called 'resting-state activity' appears to be correlated across brain regions with apparent functional relationship, its origin might be predominantly vascular and not directly representing neuronal signaling. To investigate this, we simultaneously measured BOLD and perfusion signals on healthy human subjects (n = 11) and used their ratio (BOLD/perfusion ratio or BPR) as an indicator of metabolic demand. BPR during rest and sleep was compared with that during a visual task (VT) and a breath-holding task (BH), which are challenges with substantial and little metabolic involvement, respectively. Within the visual cortex, BPR was 3.76±1.23 during BH, which was significantly higher than during the VT (1.76 ± 0.27) and rest (1.56 ± 0.41). Meanwhile, BPR values during VT and rest were not significantly different, suggesting a similar metabolic involvement. Eight subjects showed stage 1 and 2 sleep, during which temporally correlated BOLD and perfusion activity continued. In these subjects, there was no significant difference in BPR between the sleep and waking conditions (1.79 ± 0.54 and 1.66 ± 0.67, respectively), but both were lower than the BPR during BH. These data suggest that resting-state activity, at least in part, represents a metabolic process.

Low-frequency calcium oscillations accompany deoxyhemoglobin oscillations in rat somatosensory cortex

“…A direct measure of bulk CBF would help distinguish whether flow is changing independent of RBC velocity and whether this is occurring throughout the vascular tree. Recent work on MRI has demonstrated CBF fluctuations at frequencies similar to those of BOLD fMRI oscillations (50,51), but it is unclear that the higher frequencies reported in this study would have been detected owing to technical issues. It will be interesting to determine whether there is a higher-frequency cerebral blood flow (CBF) oscillation that correlates with the higher-frequency red cell velocity (CBFv) measured here.…”

“…5, D 0 and D 1 ). For LSI flow image reconstruction the dynamic speckle contrast was calculated based on a 5 × 5 pixel moving window (spatial resolution of 5 × 6 ∼30 μm for LSI) across the whole field of view (FOV, e.g., 5 × 6 mm 2 ) to convert flow index (proportional to flow rate) from the acquired raw photographic images (24,51).…”

Simulation study of brain blood flow regulation by intra-cortical arterioles in an anatomically accurate large human vascular network: Part I: Methodology and baseline flow

“…Thus, they might turn out to be of importance, especially in the context of high field imaging, where voxel size approaches the minimal size studied in the present work (250 μm), and in studies focused on signal fluctuations in the absence of stimuli (e.g. during sleep, Fukunaga et al, 2008).…”

Partitioning of Physiological Noise Signals in the Brain with Concurrent Near-Infrared Spectroscopy and fMRI

“…Other studies provide evidence suggesting that the respiratory and cardiac pulsation effects contribute to the LFO through aliasing (Bhattacharyya and Lowe, 2004;Birn et al, 2006Birn et al, , 2008Shmueli et al, 2007;van Buuren et al, 2009). Another contributor to the LFO may be neuronal signal from 'resting state functional connectivity' networks (Damoiseaux et al, 2006;Fox and Raichle, 2007) that are presumed to arise from spontaneous neuronal events (at frequencies below 0.1 Hz) that are synchronized in time between distant brain regions, leading to fluctuations in metabolic demands in the resting brain (Balduzzi et al, 2008;Fransson, 2005;Fukunaga et al, 2008;Suckling et al, 2008;Zuo et al, 2010). Since these fluctuations occur in the same frequency band as the LFO, the relationship to physiological noise is unclear.…”