Low-frequency oscillations of cortical oxidative metabolism in waking and sleep

Origins of 1/f-like tissue oxygenation fluctuations in the murine cortex

The concentration of oxygen in the brain spontaneously fluctuates, and the distribution of power in these fluctuations has a 1/f-like spectra, where the power present at low frequencies of the power spectrum is orders of magnitude higher than at higher frequencies. Though these oscillations have been interpreted as being driven by neural activity, the origin of these 1/f-like oscillations is not well understood. Here, to gain insight of the origin of the 1/f-like oxygen fluctuations, we investigated the dynamics of tissue oxygenation and neural activity in awake behaving mice. We found that oxygen signal recorded from the cortex of mice had 1/f-like spectra. However, band-limited power in the local field potential did not show corresponding 1/f-like fluctuations. When local neural activity was suppressed, the 1/f-like fluctuations in oxygen concentration persisted. Two-photon measurements of erythrocyte spacing fluctuations and mathematical modeling show that stochastic fluctuations in erythrocyte flow could underlie 1/f-like dynamics in oxygenation. These results suggest that the discrete nature of erythrocytes and their irregular flow, rather than fluctuations in neural activity, could drive 1/f-like fluctuations in tissue oxygenation.

Imaging the response to deep brain stimulation in rodent using functional ultrasound

In this study, we explored the feasibility of using functional ultrasound (fUS) imaging to visualize cerebral activation associated with thalamic deep brain stimulation (DBS), in rodents. The ventrolateral (VL) thalamus was stimulated using electrical pulses of low and high frequencies of 10 and 100 Hz, respectively, and multiple voltages (1-7 V) and pulse widths (50-1500 μs). The fUS imaging demonstrated DBS-evoked activation of cerebral cortex based on changes of cerebral blood volume, specifically at the primary motor cortex (PMC). Low frequency stimulation (LFS) demonstrated significantly higher PMC activation compared to higher frequency stimulation (HFS), at intensities (5-7 V). Whereas, at lower intensities (1-3 V), only HFS demonstrated visible PMC activation. Further, LFS-evoked cerebral activation was was primarily located at the PMC. Our data presents the functionality and feasibility of fUS imaging as an investigational tool to identify brain areas associated with DBS. This preliminary study is an important stepping stone towards conducting real-time functional ultrasound imaging of DBS in awake and behaving animal models, which is of significant interest to the community for studying motor-related disorders.

Effects of the LHPP gene polymorphism on the functional and structural changes of gray matter in major depressive disorder

Background

A single-nucleotide polymorphism (SNP) of the LHPP gene (rs35936514) has been reported to be associated with major depressive disorder (MDD) in genome-wide association studies. However, the systems-level neural effects of rs35936514 that mediate the association are unknown. We hypothesized that variations in rs35936514 would be associated with structural and functional changes in gray matter (GM) at rest in MDD patients.

Methods

A total of 50 MDD patients and 113 healthy controls (HCs) were studied. Functional connectivity (FC) was analyzed by defining the bilateral hippocampus as the seed region. Voxel-based morphometry (VBM) was performed to assess the patterns of GM volume. The subjects were further divided into two groups: a CC homozygous group (CC; 24 MDD and 56 HC) and a risk T-allele carrier group (CT/TT genotypes; 26 MDD and 57 HC). A 2×2 analysis of variance (ANOVA: diagnosis × genotype) was used to determine the interaction effects and main effect (P<0.05).

Results

Significant diagnosis × genotype interaction effects on brain morphology and FC were noted. Compared to other subgroups, the MDD patients with the T allele showed an increased hippocampal FC in the bilateral calcarine cortex and cuneus and a decreased hippocampal FC in the right dorsolateral prefrontal cortex (DLPFC), bilateral anterior cingulate cortex (ACC), and medial prefrontal cortex (MPFC), in addition to reduced GM volume in the right DLPFC, bilateral temporal cortex, and posterior cingulate cortex (PCC).

Conclusions

LHPP gene polymorphisms may affect functional and structural changes in the GM at rest and may play an important role in the pathophysiological mechanisms of MDD.

Mitochondrial calcium homeostasis: Implications for neurovascular and neurometabolic coupling

Mitochondrial function is critical to maintain high rates of oxidative metabolism supporting energy demands of both spontaneous and evoked neuronal activity in the brain. Mitochondria not only regulate energy metabolism, but also influence neuronal signaling. Regulation of "energy metabolism" and "neuronal signaling" (i.e. neurometabolic coupling), which are coupled rather than independent can be understood through mitochondria's integrative functions of calcium ion (Ca²⁺) uptake and cycling. While mitochondrial Ca²⁺ do not affect hemodynamics directly, neuronal activity changes are mechanistically linked to functional hyperemic responses (i.e. neurovascular coupling). Early in vitro studies lay the foundation of mitochondrial Ca²⁺ homeostasis and its functional roles within cells. However, recent in vivo approaches indicate mitochondrial Ca²⁺ homeostasis as maintained by the role of mitochondrial Ca²⁺ uniporter (mCU) influences system-level brain activity as measured by a variety of techniques. Based on earlier evidence of subcellular cytoplasmic Ca²⁺ microdomains and cellular bioenergetic states, a mechanistic model of Ca²⁺ mobilization is presented to understand systems-level neurovascular and neurometabolic coupling. This integrated view from molecular and cellular to the systems level, where mCU plays a major role in mitochondrial and cellular Ca²⁺ homeostasis, may explain the wide range of activation-induced coupling across neuronal activity, hemodynamic, and metabolic responses.

Arteriovenous oscillations of the redox potential: Is the redox state influencing blood flow?

OBJECTIVE

Studies on the regulation of human blood flow revealed several modes of oscillations with frequencies ranging from 0.005 to 1 Hz. Several mechanisms were proposed that might influence these oscillations, such as the activity of vascular endothelium, the neurogenic activity of vessel wall, the intrinsic activity of vascular smooth muscle, respiration, and heartbeat. These studies relied typically on non-invasive techniques, for example, laser Doppler flowmetry. Oscillations of biochemical markers were rarely coupled to blood flow.

METHODS

The redox potential difference between the artery and the vein was measured by platinum electrodes placed in the parallel homonymous femoral artery and the femoral vein of ventilated anesthetized pigs.

RESULTS

Continuous measurement at 5 Hz sampling rate using a digital nanovoltmeter revealed fluctuating signals with three basic modes of oscillations: ∼ 1, ∼ 0.1 and ∼ 0.01 Hz. These signals clearly overlap with reported modes of oscillations in blood flow, suggesting coupling of the redox potential and blood flow.

DISCUSSION

The amplitude of the oscillations associated with heart action was significantly smaller than for the other two modes, despite the fact that heart action has the greatest influence on blood flow. This finding suggests that redox potential in blood might be not a derivative but either a mediator or an effector of the blood flow control system.

Modern Methods for Interrogating the Human Connectome

OBJECTIVES

Connectionist theories of brain function took hold with the seminal contributions of Norman Geschwind a half century ago. Modern neuroimaging techniques have expanded the scientific interest in the study of brain connectivity to include the intact as well as disordered brain.

METHODS

In this review, we describe the most common techniques used to measure functional and structural connectivity, including resting state functional MRI, diffusion MRI, and electroencephalography and magnetoencephalography coherence. We also review the most common analytical approaches used for examining brain interconnectivity associated with these various imaging methods.

RESULTS

This review presents a critical analysis of the assumptions, as well as methodological limitations, of each imaging and analysis approach.

CONCLUSIONS

The overall goal of this review is to provide the reader with an introduction to evaluating the scientific methods underlying investigations that probe the human connectome.

Vascular coupling in resting-state fMRI: evidence from multiple modalities

Resting-state functional magnetic resonance imaging (rs-fMRI) provides a potential to understand intrinsic brain functional connectivity. However, vascular effects in rs-fMRI are still not fully understood. Through multiple modalities, we showed marked vascular signal fluctuations and high-level coupling among arterial pressure, cerebral blood flow (CBF) velocity and brain tissue oxygenation at <0.08 Hz. These similar spectral power distributions were also observed in blood oxygen level-dependent (BOLD) signals obtained from six representative regions of interest (ROIs). After applying brain global, white-matter, cerebrospinal fluid (CSF) mean signal regressions and low-pass filtering (<0.08 Hz), the spectral power of BOLD signal was reduced by 55.6% to 64.9% in all ROIs (P=0.011 to 0.001). The coherence of BOLD signal fluctuations between an ROI pair within a same brain network was reduced by 9.9% to 20.0% (P=0.004 to <0.001), but a larger reduction of 22.5% to 37.3% (P=0.032 to <0.001) for one not in a same network. Global signal regression overall had the largest impact in reducing spectral power (by 52.2% to 61.7%) and coherence, relative to the other three preprocessing steps. Collectively, these findings raise a critical question of whether a large portion of rs-fMRI signals can be attributed to the vascular effects produced from upstream changes in cerebral hemodynamics.

Functional ultrasound imaging of intrinsic connectivity in the living rat brain with high spatiotemporal resolution

Long-range coherences in spontaneous brain activity reflect functional connectivity. Here we propose a novel, highly resolved connectivity mapping approach, using ultrafast functional ultrasound (fUS), which enables imaging of cerebral microvascular haemodynamics deep in the anaesthetized rodent brain, through a large thinned-skull cranial window, with pixel dimensions of 100 μm × 100 μm in-plane. The millisecond-range temporal resolution allows unambiguous cancellation of low-frequency cardio-respiratory noise. Both seed-based and singular value decomposition analysis of spatial coherences in the low-frequency (<0.1 Hz) spontaneous fUS signal fluctuations reproducibly report, at different coronal planes, overlapping high-contrast, intrinsic functional connectivity patterns. These patterns are similar to major functional networks described in humans by resting-state fMRI, such as the lateral task-dependent network putatively anticorrelated with the midline default-mode network. These results introduce fUS as a powerful novel neuroimaging method, which could be extended to portable systems for three-dimensional functional connectivity imaging in awake and freely moving rodents.

Resting state functional connectivity: its physiological basis and application in neuropharmacology

Brain structures do not work in isolation; they work in concert to produce sensory perception, motivation and behavior. Systems-level network activity can be investigated by resting state magnetic resonance imaging (rsMRI), an emerging neuroimaging technique that assesses the synchrony of the brain's ongoing spontaneous activity. Converging evidence reveals that rsMRI is able to consistently identify distinct spatiotemporal patterns of large-scale brain networks. Dysregulation within and between these networks has been implicated in a number of neurodegenerative and neuropsychiatric disorders, including Alzheimer's disease and drug addiction. Despite wide application of this approach in systems neuroscience, the physiological basis of these fluctuations remains incompletely understood. Here we review physiological studies in electrical, metabolic and hemodynamic fluctuations that are most pertinent to the rsMRI signal. We also review recent applications to neuropharmacology - specifically drug effects on resting state fluctuations. We speculate that the mechanisms governing spontaneous fluctuations in regional oxygenation availability likely give rise to the observed rsMRI signal. We conclude by identifying several open questions surrounding this technique. This article is part of the Special Issue Section entitled 'Neuroimaging in Neuropharmacology'.

Hemodynamic and electrophysiological spontaneous low-frequency oscillations in the cortex: directional influences revealed by Granger causality

We used a combined electrophysiological/hemodynamic system to examine low-frequency oscillations (LFOs) in spontaneous neuronal activities (spike trains and local field potentials) and hemodynamic signals (cerebral blood flow) recorded from the anesthetized rat somatosensory and visual cortices. The laser Doppler flowmetry (LDF) probe was tilted slightly to approach the area in which a microelectrode array (MEA) was implanted for simultaneous recordings. Spike trains (STs) were converted into continuous-time rate functions (CRFs) using the ST instantaneous firing rates. LFOs were detected for all three of the components using the multi-taper method (MTM). The frequencies of these LFOs ranged from 0.052 to 0.167 Hz (mean±SD, 0.10±0.026 Hz) for cerebral blood flow (CBF), from 0.027 to 0.26 Hz (mean±SD, 0.12±0.041 Hz) for the CRFs of the STs and from 0.04 to 0.19 Hz (mean±SD, 0.11±0.035 Hz) for local field potentials (LFPs). We evaluated the Granger causal relationships of spontaneous LFOs among CBF, LFPs and CRFs using Granger causality (GC) analysis. Significant Granger causal relationships were observed from LFPs to CBF, from STs to CBF and from LFPs to STs at approximately 0.1 Hz. The present results indicate that spontaneous LFOs exist not only in hemodynamic components but also in neuronal activities of the rat cortex. To the best of our knowledge, the present study is the first to identify Granger causal influences among CBF, LFPs and STs and show that spontaneous LFOs carry important Granger causal influences from neural activities to hemodynamic signals.

Modeling the contribution of neuron-astrocyte cross talk to slow blood oxygenation level-dependent signal oscillations

A consistent and prominent feature of brain functional magnetic resonance imaging (fMRI) data is the presence of low-frequency (<0.1 Hz) fluctuations of the blood oxygenation level-dependent (BOLD) signal that are thought to reflect spontaneous neuronal activity. In this report we provide modeling evidence that cyclic physiological activation of astroglial cells produces similar BOLD oscillations through a mechanism mediated by intracellular Ca(2+) signaling. Specifically, neurotransmission induces pulses of Ca(2+) concentration in astrocytes, resulting in increased cerebral perfusion and neuroactive transmitter release by these cells (i.e., gliotransmission), which in turn stimulates neuronal activity. Noticeably, the level of neuron-astrocyte cross talk regulates the periodic behavior of the Ca(2+) wave-induced BOLD fluctuations. Our results suggest that the spontaneous ongoing activity of neuroglial networks is a potential source of the observed slow fMRI signal oscillations.

Improved detection of nicotinamide adenine dinucleotide phosphate oscillations within human neutrophils

Kinetic studies of nicotinamide adenine dinucleotide phosphate autofluorescence have been conducted in adherent neutrophils using an improved microscopic photometry system incorporating low noise excitation and detection systems. Dynamic autofluorescence oscillations were found with periods ranging from ∼4 min to ∼10 s. The largest portion of the population of oscillating neutrophils (32%) had periods near 2 min. The next largest group at 25% exhibited periods of 1 min or less. These oscillations could not be accounted for by instrument artifacts, cell shape changes away from the focal plane, or other factors. They disappeared when detergent was added to oscillating cells. Higher-frequency oscillations disappeared as cells changed shape, indicating a correlation between these two processes. This approach provides a reliable method to monitor this cellular property.

Picture representation during REM dreams: a redox molecular hypothesis

A novel molecular hypothesis about visual perception and imagery has recently been proposed (Bókkon, 2009; BioSystems). Namely, external electromagnetic visible photons are converted into electrical signals in the retina and are then conveyed to V1. Next, these retinotopic electrical signals (spike-related electrical signals along classical axonal-dendritic pathways) can be converted into synchronized bioluminescent biophoton signals (inside the neurons) by neurocellular radical reactions (redox processes) in retinotopically organized V1 mitochondrial cytochrome oxidase-rich visual areas. The bioluminescent photonic signals (inside the neurons) generated by neurocellular redox/radical reactions in synchronized V1 neurons make it possible to produce computational biophysical pictures during visual perception and imagery. Our hypothesis is in line with the functional roles of reactive oxygen and nitrogen species in living cells and states that this is not a random process, but rather a strict mechanism used in signaling pathways. Here, we suggest that intrinsic biophysical pictures can also emerge during REM dreams.

Continuous estimates of dynamic cerebral autoregulation during transient hypocapnia and hypercapnia

Dynamic cerebral autoregulation (CA) is the transient response of cerebral blood flow (CBF) to rapid blood pressure changes: it improves in hypocapnia and becomes impaired during hypercapnia. Batch-processing techniques have mostly been used to measure CA, providing a single estimate for an entire recording. A new approach to increase the temporal resolution of dynamic CA parameters was applied to transient hypercapnia and hypocapnia to describe the time-varying properties of dynamic CA during these conditions. Thirty healthy subjects (mean +/- SD: 25 +/- 6 yr, 9 men) were recruited. CBF velocity was recorded in both middle cerebral arteries (MCAs) with transcranial Doppler ultrasound. Arterial blood pressure (Finapres), end-tidal CO(2) (ET(CO(2)); infrared capnograph), and a three-lead ECG were also measured at rest and during repeated breath hold and hyperventilation. A moving window autoregressive moving average model provided continuous values of the dynamic CA index [autoregulation index (ARI)] and unconstrained gain. Breath hold led to significant increase in ET(CO(2)) (+5.4 +/- 6.1 mmHg), with concomitant increase in CBF velocity in both MCAs. Continuous dynamic CA parameters showed highly significant changes (P < 0.001), with a temporal pattern reflecting a delayed dynamic response of CA to changes in arterial Pco(2) and a maximal reduction in ARI of -5.1 +/- 2.4 and -5.1 +/- 2.3 for the right and left MCA, respectively. Hyperventilation led to a marked decrease in ET(CO(2)) (-7.2 +/- 4.1 mmHg, P < 0.001). Unexpectedly, CA efficiency dropped significantly with the inception of the metronome-controlled hyperventilation, but, after approximately 30 s, the ARI increased gradually to show a maximum change of 5.7 +/- 2.9 and 5.3 +/- 3.0 for the right and left MCA, respectively (P < 0.001). These results confirm the potential of continuous estimates of dynamic CA to improve our understanding of human cerebrovascular physiology and represent a promising new approach to improve the sensitivity of clinical applications of dynamic CA modeling.

Interaction between nitric oxide synthase inhibitor induced oscillations and the activation flow coupling response

The role of nitric oxide (NO) in the activation-flow coupling (AFC) response to periodic electrical forepaw stimulation was investigated using signal averaged laser Doppler (LD) flowmetry. LD measures of calculated cerebral blood flow (CBF) were obtained both prior and after intra-peritoneal administration of the non-selective nitric oxide synthase (NOS) inhibitor, N(G)-nitro-L-arginine (L-NNA) (40 mg/kg). Characteristic baseline low frequency vasomotion oscillations (0.17 Hz) were observed after L-NNA administration. These LD(CBF) oscillations were synchronous within but not between hemispheres. L-NNA reduced the magnitude of the AFC response (p<0.05) for longer stimuli (1 min) with longer inter-stimulus intervals (2 min). In contrast, the magnitude of the AFC response for short duration stimuli (4 s) with short inter-stimulus intervals (20 s) was augmented (p<0.05) after L-NNA. An interaction occurred between L-NNA induced vasomotion oscillations and the AFC response with the greatest increase occurring at the stimulus harmonic closest to the oscillatory frequency. Nitric oxide may therefore modulate the effects of other vasodilators involved in vasomotion oscillations and the AFC response.

Resting-state functional connectivity in animal models: modulations by exsanguination

We studied the spatiotemporal characteristics of the resting state low frequency fluctuations in functional MRI (fMRI), blood oxygenation level dependent (BOLD) signal in isoflurane-anesthetized rats. fMRI-BOLD measurements at 9.4 Telsa were made during normal and exsanguinated condition previously known to alter cerebral blood flow (CBF) fluctuations in anesthetized rats. fMRI signal time series were low-pass filtered and studied by spectral analysis. During normal conditions, baseline mean arterial pressure (MAP) was 110 +/- 10 mm Hg and low-frequency fluctuations in BOLD signal were observed in the frequency range of 0.01 - 0.125 Hz. Following blood withdrawal (exsanguination), MAP decreased to 68 +/- 7 mm Hg, resulting in an increase in the amplitude of the low-frequency fluctuations in BOLD signal time series and an increase in power at several frequencies between 0.01 and 0.125 Hz. Spatially, the BOLD fluctuations were confined to the cortex and thalamus spanning both hemispheres with sparse presence in the caudate putamen and hippocampus during both normal and exsanguinated states. Spatial distribution of the low frequency fluctuations in BOLD signal, from cross correlation analysis, indicates substantial inter-hemispheric synchrony similar to that observed in the conscious human brain. The behavior of the resting state BOLD signal fluctuations similar to CBF fluctuations during exsanguination indicates a myogenic dependence. Also, a high inter-hemispheric synchrony combined with different phase characteristics of the low frequency BOLD fluctuations particularly in the hippocampus relative to the cortex emphasizes distinct functional networks.

Spontaneous low-frequency blood oxygenation level-dependent fluctuations and functional connectivity analysis of the 'resting' brain

Functional magnetic resonance imaging techniques using the blood oxygenation level-dependent (BOLD) contrast are widely used to map human brain function by relating local hemodynamic responses to neuronal stimuli compared to control conditions. There is increasing interest in spontaneous cerebral BOLD fluctuations that are prominent in the low-frequency range (<0.1 Hz) and show intriguing spatio-temporal correlations in functional networks. The nature of these signal fluctuations remains unclear, but there is accumulating evidence for a neural basis opening exciting new avenues to study human brain function and its connectivity at rest. Moreover, an increasing number of patient studies report disease-dependent variation in the amplitude and spatial coherence of low-frequency BOLD fluctuations (LFBF) that may afford greater diagnostic sensitivity and easier clinical applicability than standard fMRI. The main disadvantage of this emerging tool relates to physiological (respiratory, cardiac and vasomotion) and motion confounds that are challenging to disentangle requiring thorough preprocessing. Technical aspects of functional connectivity fMRI analysis and the neuroscientific potential of spontaneous LFBF in the default mode and other resting-state networks have been recently reviewed. This review will give an update on the current knowledge of the nature of LFBF, their relation to physiological confounds and potential for clinical diagnostic and pharmacological studies.

Metabolic origin of BOLD signal fluctuations in the absence of stimuli

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.

Endogenous brain fluctuations and diagnostic imaging

Much of the rising health care costs in aging populations can be attributed to congenital disease and psychiatric and neurologic disorders. Early detection of changes related to these diseases can promote the development of new therapeutic strategies and effective treatments. Changes in tissue, such as damage resulting from continued functional abnormality, often exhibit a time-delay before detection is possible. Methods for detecting functional alterations in endogenous brain fluctuations allow for an early diagnosis before tissue damage occurs, enabling early treatment and a more likely positive outcome. A literature review and comprehensive overview of the current state of knowledge about endogenous brain fluctuations is presented here. Recent findings of the association between various pathological conditions and endogenous fluctuations are discussed. A particular emphasis is placed on research showing the relationship between clinical measures and pathological findings to the dynamics of endogenous fluctuations of the brain. Recent discoveries of methods for detecting abnormal functional connectivity are discussed and future research directions explored.

Spatio-temporal characteristics of low-frequency BOLD signal fluctuations in isoflurane-anesthetized rat brain

We studied the spatio-temporal characteristics of the resting state low-frequency fluctuations in fMRI-BOLD signal in isoflurane-anesthetized rats. fMRI-BOLD measurements at 9.4 T were made during normal and exsanguinated condition previously known to alter cerebral blood flow (CBF) fluctuations in anesthetized rats. fMRI signal time series were low pass filtered and studied by spectral analysis. During normal conditions, baseline mean arterial pressure (MAP) was 110+/-10 mm Hg and low-frequency fluctuations in BOLD signal were observed in the frequency range of 0.01 to 0.125 Hz. Following blood withdrawal (exsanguination), MAP decreased to 68+/-7 mm Hg, resulting in an increase in the amplitude of the low-frequency fluctuations in BOLD signal time series and an increase in power at several frequencies between 0.01 and 0.125 Hz. Spatially, the BOLD fluctuations were confined to the cortex and thalamus spanning both hemispheres with sparse presence in the caudate putamen and hippocampus during both normal and exsanguinated states. Spatial distribution of the low-frequency fluctuations in BOLD signal, from cross-correlation analysis, indicates substantial inter-hemispheric synchrony similar to that observed in the conscious human brain. The behavior of the resting state BOLD signal fluctuations similar to CBF fluctuations during exsanguination indicates a myogenic dependence. Also, a high inter-hemispheric synchrony combined with different phase characteristics of the low-frequency BOLD fluctuations particularly in the hippocampus relative to the cortex emphasizes distinct functional networks.

Continuous estimates of dynamic cerebral autoregulation: influence of non-invasive arterial blood pressure measurements

Temporal variability of parameters which describe dynamic cerebral autoregulation (CA), usually quantified by the short-term relationship between arterial blood pressure (BP) and cerebral blood flow velocity (CBFV), could result from continuous adjustments in physiological regulatory mechanisms or could be the result of artefacts in methods of measurement, such as the use of non-invasive measurements of BP in the finger. In 27 subjects (61+/-11 years old) undergoing coronary artery angioplasty, BP was continuously recorded at rest with the Finapres device and in the ascending aorta (Millar catheter, BP(AO)), together with bilateral transcranial Doppler ultrasound in the middle cerebral artery, surface ECG and transcutaneous CO(2). Dynamic CA was expressed by the autoregulation index (ARI), ranging from 0 (absence of CA) to 9 (best CA). Time-varying, continuous estimates of ARI (ARI(t)) were obtained with an autoregressive moving-average (ARMA) model applied to a 60 s sliding data window. No significant differences were observed in the accuracy and precision of ARI(t) between estimates derived from the Finapres and BP(AO). Highly significant correlations were obtained between ARI(t) estimates from the right and left middle cerebral artery (MCA) (Finapres r=0.60+/-0.20; BP(AO) r=0.56+/-0.22) and also between the ARI(t) estimates from the Finapres and BP(AO) (right MCA r=0.70+/-0.22; left MCA r=0.74+/-0.22). Surrogate data showed that ARI(t) was highly sensitive to the presence of noise in the CBFV signal, with both the bias and dispersion of estimates increasing for lower values of ARI(t). This effect could explain the sudden drops of ARI(t) to zero as reported previously. Simulated sudden changes in ARI(t) can be detected by the Finapres, but the bias and variability of estimates also increase for lower values of ARI. In summary, the Finapres does not distort time-varying estimates of dynamic CA obtained with a sliding window combined with an ARMA model, but further research is needed to confirm these findings in healthy subjects and to assess the influence of different physiological manoeuvres.

Synchronized delta oscillations correlate with the resting-state functional MRI signal

Synchronized low-frequency spontaneous fluctuations of the functional MRI (fMRI) signal have recently been applied to investigate large-scale neuronal networks of the brain in the absence of specific task instructions. However, the underlying neural mechanisms of these fluctuations remain largely unknown. To this end, electrophysiological recordings and resting-state fMRI measurements were conducted in alpha-chloralose-anesthetized rats. Using a seed-voxel analysis strategy, region-specific, anesthetic dose-dependent fMRI resting-state functional connectivity was detected in bilateral primary somatosensory cortex (S1FL) of the resting brain. Cortical electroencephalographic signals were also recorded from bilateral S1FL; a visual cortex locus served as a control site. Results demonstrate that, unlike the evoked fMRI response that correlates with power changes in the gamma bands, the resting-state fMRI signal correlates with the power coherence in low-frequency bands, particularly the delta band. These data indicate that hemodynamic fMRI signal differentially registers specific electrical oscillatory frequency band activity, suggesting that fMRI may be able to distinguish the ongoing from the evoked activity of the brain.

Spontaneous fluctuations in brain activity observed with functional magnetic resonance imaging

The majority of functional neuroscience studies have focused on the brain's response to a task or stimulus. However, the brain is very active even in the absence of explicit input or output. In this Article we review recent studies examining spontaneous fluctuations in the blood oxygen level dependent (BOLD) signal of functional magnetic resonance imaging as a potentially important and revealing manifestation of spontaneous neuronal activity. Although several challenges remain, these studies have provided insight into the intrinsic functional architecture of the brain, variability in behaviour and potential physiological correlates of neurological and psychiatric disease.

Relationship between evolving epileptiform activity and delayed loss of mitochondrial activity after asphyxia measured by near-infrared spectroscopy in preterm fetal sheep

Early onset cerebral hypoperfusion after birth is highly correlated with neurological injury in premature infants, but the relationship with the evolution of injury remains unclear. We studied changes in cerebral oxygenation, and cytochrome oxidase (CytOx) using near-infrared spectroscopy in preterm fetal sheep (103-104 days of gestation, term is 147 days) during recovery from a profound asphyxial insult (n= 7) that we have shown produces severe subcortical injury, or sham asphyxia (n= 7). From 1 h after asphyxia there was a significant secondary fall in carotid blood flow (P < 0.001), and total cerebral blood volume, as reflected by total haemoglobin (P < 0.005), which only partially recovered after 72 h. Intracerebral oxygenation (difference between oxygenated and deoxygenated haemoglobin concentrations) fell transiently at 3 and 4 h after asphyxia (P < 0.01), followed by a substantial increase to well over sham control levels (P < 0.001). CytOx levels were normal in the first hour after occlusion, was greater than sham control values at 2-3 h (P < 0.05), but then progressively fell, and became significantly suppressed from 10 h onward (P < 0.01). In the early hours after reperfusion the fetal EEG was highly suppressed, with a superimposed mixture of fast and slow epileptiform transients; overt seizures developed from 8 +/- 0.5 h. These data strongly indicate that severe asphyxia leads to delayed, evolving loss of mitochondrial oxidative metabolism, accompanied by late seizures and relative luxury perfusion. In contrast, the combination of relative cerebral deoxygenation with evolving epileptiform transients in the early recovery phase raises the possibility that these early events accelerate or worsen the subsequent mitochondrial failure.

Intrinsic brain activity sets the stage for expression of motivated behavior

Research in many species has provided increasingly detailed information on relevant, primarily subcortical brain systems supporting the expression of basic appetites and drives. While basic appetites and drives are essential for adaptation and survival in any environment, they are naturally constrained by an organism's inherent biology and modulated as circumstances dictate. The brain mechanisms which serve to constrain and modulate them, however, remain much less well understood. We suggest that the manner in which such constraint and potential modulation is achieved likely involves processes that emerge from the coordinated behavior of multiple brain systems, and functional brain imaging techniques such as PET and fMRI are beginning to help us understand aspects of such coordination. In this review we argue that, in pursuit of this understanding, we must focus not only on changes evoked in brain systems during various behaviors, but also on the ongoing and very costly intrinsic activity within these systems, for the latter may be at least as important as the evoked activity in terms of brain function in general and the constraint and modulation of basic appetites and drives in particular. Distinguishing intrinsic from evoked activity in the context of functional brain imaging experiments is challenging, however. Here we review some evolving strategies for doing so.

Signal processing times in neutrophil activation: dependence on ligand concentration and the relative phase of metabolic oscillations

Intracellular NAD(P)H oscillations exhibited by polarized neutrophils display congruent with 20 s periods, which are halved to congruent with 10 s upon stimulation with chemotactic peptides such as FNLPNTL (N-formyl-nle-leu-phe-nle-tyr-lys). By monitoring this frequency change, we have measured accurately the time interval between stimulus and metabolic frequency changes. A microscope flow chamber was designed to allow rapid delivery of FNLPNTL to adherent cells. Using fluorescein as a marker, we found delivery to be complete and stable throughout the chamber within approximately 400 ms. Peptides were injected into the chamber at concentrations ranging from 10(-6) to 10(-9) M. Injections also varied with respect to the relative phase of a cell's NAD(P)H oscillations. The time interval between injection of 10(-6) M FNLPNTL and the acquisition of congruent with 10 s period metabolic oscillations was found to be 12.2+/-3.3 s when injections occurred at the NAD(P)H oscillation peak whereas the lag time was 22.5+/-4.8 s when coinciding with a trough. At 10(-8) M FNLPNTL, lag times were found to be 26.1+/-5.2 and 30.5+/-7.3 s for injections at NAD(P)H peaks and troughs, respectively. FNLPNTL at 10(-9) M had no effect on metabolic oscillations, consistent with previous studies. Our experiments show that the kinetics of transmembrane signal processing, in contrast to a simple transmembrane chemical reaction, can depend upon both ligand dose and its temporal relationship with intracellular metabolic oscillations.

Fluorescence spectroscopic detection of mitochondrial flavoprotein redox oscillations and transient reduction of the NADPH oxidase-associated flavoprotein in leukocytes

Steady-state and time-resolved fluorescence spectroscopy and fluorescence microscopy of leukocyte flavoproteins have been performed. Both living human peripheral blood monocytes and neutrophils have been utilized as experimental models, as the former relies much more heavily on mitochondrial metabolism for energy production than the latter. We confirm previous studies indicating that cellular flavoproteins absorb at 460 nm and emit at 530 nm, very similar to that of the FAD moiety. Furthermore, the emission properties of intracellular flavoproteins were altered by the metabolic inhibitors rotenone, antimycin A, azide, cyanide, DNP (2,4-dinitrophenol), and FCCP [carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone]. Kinetic studies revealed flavoprotein emission oscillations in both monocytes and neutrophils. The flavoprotein intensity oscillations correlated with the physiological status of the cell and the nature of membrane receptor ligation. Microscopy revealed the presence of flavoprotein fluorescence in association with the plasma membrane, intracellular granules and distributed throughout the cytoplasm, presumably within mitochondria. Metabolic inhibitors such as cyanide suggest that the plasma membrane and granular components are cyanide-insensitive and therefore are likely associated with the flavoprotein component of the NADPH oxidase, which is located in these two compartments. This interpretation was found to be consistent with structural localization of the NADPH oxidase using an antibody molecule specific for this protein. Using peripheral blood neutrophils, which display less active mitochondria, and time-resolved emission spectroscopy, we show that the NADPH oxidase-associated flavoprotein undergoes a periodic transient reduction of about 54+/-2 ms in living cells. This finding is consistent with prior studies indicating that propagating substrate (NADPH) waves periodically promote electron transport across the NADPH oxidase.

Short-term variability of cerebral blood flow velocity responses to arterial blood pressure transients

The time course of mean beat-to-beat changes in cerebral blood flow velocity changes induced by spontaneous transients in mean arterial blood pressure was studied in a group of 39 healthy subjects, ages 40 +/- 15 (SD) years. Continuous 10-min noninvasive recordings of cerebral blood flow velocity (CBFV) from both middle cerebral arteries (MCA) with Doppler ultrasound (US) and simultaneous beat-to-beat arterial blood pressure (ABP) were made. A total of 522 spontaneous positive transients of ABP and CBFV were extracted with a maximum of 15 transients for each subject. The CBFV transient amplitude was normalized by the corresponding ABP change and the area-under-the-curve (AUC) of the falling phase was used to classify the CBFV regulatory response as either weak, moderate or strong. The coherent average of ABP and CBFV of each category confirmed the consistency of this classification, reinforced by the agreement of separate averages for recordings from the right and left MCA. All 39 subjects showed at least two categories of transients, with all three categories present in 33 subjects (right MCA) and 29 subjects (left MCA), respectively. These results indicate a significant short-term variability of CBFV responses in healthy subjects whose origin remains unexplained.

Multiple sclerosis: low-frequency temporal blood oxygen level-dependent fluctuations indicate reduced functional connectivity initial results

PURPOSE

To study the correlation of low-frequency blood oxygenation level-dependent (BOLD) fluctuations on magnetic resonance (MR) images obtained of the left- and right-hemisphere primary motor regions in healthy control subjects and patients with multiple sclerosis (MS).

MATERIALS AND METHODS

Sixteen healthy volunteers and 20 patients with MS underwent MR imaging with a 1.5-T imager by using a protocol designed to monitor low-frequency BOLD fluctuations. Data for low-frequency BOLD fluctuations were acquired with subjects at rest and during continuous performance of a bilateral finger-tapping task. These data were low-pass filtered (<0.08 Hz), and cross correlations of all acquired pixels to a region of interest in the left precentral gyrus were calculated. Confidence levels were calculated from the cross correlations. The fraction of pixels in the right precentral gyrus above a confidence level of 95% for correlation with the left precentral gyrus was calculated for each subject.

RESULTS

A plot of the fraction of the right precentral gyrus with high correlation with the left precentral gyrus for the finger-tapping state versus the resting state showed a clear discrimination between patients with MS and control subjects. Compared with control subjects, patients with MS generally had a smaller fraction of the pixels in the right precentral gyrus above the confidence level. This finding indicates that our method results in greater than 60% sensitivity and 100% specificity for discriminating patients with MS from control subjects. No significant correlation was found between clinical measures of MS disease and correlations of low-frequency BOLD fluctuations between left and right precentral gyri.

CONCLUSION

On the basis of the connectivity measure of low-frequency BOLD fluctuations, patients with MS exhibited lower functional connectivity between right- and left-hemisphere primary motor cortices when compared with that in control subjects.

Sleep impairment by diethyldithiocarbamate in rat. Protective effects of pre-conditioning and antioxidants

Dithiocarbamates, a class of compounds widely used in medicine and agriculture, have been reported to impair sleep structure. These effects have been attributed to the decrease in norepinephrine levels induced by these drugs. However, it has also been recently demonstrated that most of the mechanisms by which dithiocarbamates damage cell function involve changes in oxidative environment. To verify the potential relevance of the latter mechanism in the sleep impairment, we examined the sleep response of adult rats to an acute administration of diethyldithiocarbamate (DDTC). At the dose of 0.6 g/kg, DDTC induced fragmentation and a decrease in slow wave sleep (SWS), and a dramatic loss of paradoxical sleep (PS). These changes occurred soon after the treatment (day 0), persisted the following day (day 1), partially recovered on day 3, and regained near basal values on day 6. No sleep anomalies were observed with a lower dose of DDTC (0.06 mg/kg). On the other hand, when the higher dose of DDTC was given in association with either one of two antioxidants, alpha-tocopherol or melatonin, the amounts of SWS and PS significantly improved even on day 1, suggesting that the DDTC effects on sleep involved an impairment of the brain oxidative balance. Likewise, administration of the lower dose of DDTC 5 days before the higher dose induced a much earlier recovery of normal sleep, presumably due to the development of a tolerance to DDTC. On the whole, the data suggest that the brain oxidative environment may play a role in the mechanisms subserving sleep regulation.

Neutrophil oscillations: temporal and spatiotemporal aspects of cell behavior

Neutrophil activation is an essential event in inflammatory responses. How cells coordinate, integrate, manage, and distribute information on physiologically-relevant timescales are not well understood. Although neutrophil oscillators have been known for many years, their biological roles have not been identified. We suggest that intracellular oscillators (such as NAD(P)H, pH, calcium, and so on) account for functional oscillations (e.g., superoxide and NO production, cytolytic marker release, pericellular proteolysis, and actin assembly). In addition to these well-known temporal oscillations, we have recently discovered self-organized traveling chemical waves in neutrophils; these waves respond to extracellular signals and have distinct origins that coincide with a cell's uropod, lamellipodium, or adherence site. The fundamental physico-chemical features of cell chemistry will have an increasing role in our understanding of leukocyte function.

Spontaneous low frequency oscillations of cerebral hemodynamics and metabolism in human adults

We investigated slow spontaneous oscillations in cerebral oxygenation in the human adult's visual cortex. The rationale was (1) to demonstrate their detectability by near infrared spectroscopy (NIRS); (2) to analyze the spectral power of as well as the phase relationship between the different NIRS parameters (oxygenated and deoxygenated hemoglobin and cytochrome-oxidase; oxy-Hb/deoxy-Hb/Cyt-ox). Also (3) influences of functional stimulation and hypercapnia on power and phase shifts were investigated. The results show that-in line with the literature-low frequency oscillations (LFO) centred around 0.1 s(-1) and even slower oscillations at about 0.04 s(-1) (very low frequency, VLFO) can be distinguished. Their respective power differs between oxy-Hb, deoxy-Hb, and Cyt-ox. Either frequency (LFO and VLFO) is altered in magnitude by functional stimulation of the cortical area examined. Also we find a change of the phase shift between the vascular parameters (oxy-Hb, tot-Hb) and the metabolic parameter (Cyt-ox) evoked by the stimulation. It is shown that hypercapnia attenuates the LFO in oxy-Hb and deoxy-Hb.

CONCLUSIONS

(1) spontaneous vascular and metabolic LFO and VLFO can be reproducibly detected by NIRS in the human adult. (2) Their spectral characteristics and their response to hypercapnia are in line with those described in exposed cortex (for review see (Hudetz et al., 1998)) and correspond to findings with transcranial doppler sonography (TCD) (Diehl et al., 1995) and fMRI (Biswal et al., 1997). (3) The magnitude of and phase relation between NIRS-parameters at the LFO may allow for a local noninvasive assessment of autoregulatory mechanisms in the adult brain.

Spectroscopic analysis of neural activity in brain: increased oxygen consumption following activation of barrel cortex

This research investigates the hemodynamic response to stimulation of the barrel cortex in anaesthetized rats using optical imaging and spectroscopy (Bonhoeffer and Grinvald, 1996; Malonek and Grinvald, 1996; Mayhew et al., 1999). A slit spectrograph was used to collect spectral image data sequences. These were analyzed using an algorithm that corrects for the wavelength dependency in the optical path lengths produced by the light scattering properties of tissue. The analysis produced the changes in the oxy- and deoxygenation of hemoglobin following stimulation. Two methods of stimulation were used. One method mechanically vibrated a single whisker, the other electrically stimulated the whisker pad. The electrical stimulation intensity varied from 0.4 to 1.6 mA. The hemodynamic responses to stimulation increased as a function of intensity. At 0.4 mA they were commensurate with those from the mechanical stimulation; however, the responses at the higher levels were greater by a factor of approximately 10. For both methods of data collection, the results of the spectroscopic analysis showed an early increase in deoxygenated hemoglobin (Hbr) with no evidence for a corresponding decrease in oxygenated hemoglobin (HbO(2)). Evidence for increased oxygen consumption (CMRO(2)) was obtained by converting the fractional changes in blood volume (Hbt) into estimates of changes in blood flow (Grubb et al., 1974) and using the resulting time course to scale the fractional changes in Hbr. The results show an early increase CMRO(2) peaking approximately 2 s after stimulation onset. Using these methods, we find evidence for increased oxygen consumption following increased neural activity even at low levels of stimulation intensity.

Combining independent component analysis and correlation analysis to probe interregional connectivity in fMRI task activation datasets

A new approach in studying interregional functional connectivity using functional magnetic resonance imaging (fMRI) is presented. Functional connectivity may be detected by means of cross correlating time course data from functionally related brain regions. These data exhibit high temporal coherence of low frequency fluctuations due to synchronized blood flow changes. In the past, this fMRI technique for studying functional connectivity has been applied to subjects that performed no prescribed task ("resting" state). This paper presents the results of applying the same method to task-related activation datasets. Functional connectivity analysis is first performed in areas not involved with the task. Then a method is devised to remove the effects of activation from the data using independent component analysis (ICA) and functional connectivity analysis is repeated. Functional connectivity, which is demonstrated in the "resting brain," is not affected by tasks which activate unrelated brain regions. In addition, ICA effectively removes activation from the data and may allow us to study functional connectivity even in the activated regions.

Spontaneous fluctuations in cerebral blood flow: insights from extended-duration recordings in humans

To determine the dependence of cerebral blood flow (CBF) on arterial pressure over prolonged time periods, we measured beat-to-beat changes in mean CBF velocity in the middle cerebral artery (transcranial Doppler) and mean arterial pressure (Finapres) continuously for 2 h in six healthy subjects (5 men and 1 woman, 18-40 yr old) during supine rest. Fluctuations in velocity and pressure were quantified by the range [(peak - trough)/mean] and coefficients of variation (SD/mean) in the time domain and by spectral analysis in the frequency domain. Mean velocity and pressure over the 2-h recordings were 60 +/- 7 cm/s and 83 +/- 8 mmHg, associated with ranges of 77 +/- 8 and 89 +/- 10% and coefficients of variation of 9.3 +/- 2.2 and 7.9 +/- 2.3%, respectively. Spectral power of the velocity and pressure was predominantly distributed in the frequency range of 0.00014-0.1 Hz and increased inversely with frequency, indicating characteristics of an inverse power law (1/f(alpha)). However, linear regression on a log-log scale revealed that the slope of spectral power of pressure and velocity was steeper in the high-frequency (0.02-0.5 Hz) than in the low-frequency range (0.002-0.02 Hz), suggesting different regulatory mechanisms in these two frequency ranges. Furthermore, the spectral slope of pressure was significantly steeper than that of velocity in the low-frequency range, consistent with the low transfer function gain and low coherence estimated at these frequencies. We conclude that 1) long-term fluctuations in CBF velocity are prominent and similar to those observed in arterial pressure, 2) spectral power of CBF velocity reveals characteristics of 1/f(alpha), and 3) cerebral attenuation of oscillations in CBF velocity in response to changes in pressure may be more effective at low than that at high frequencies, emphasizing the frequency dependence of cerebral autoregulation.

Spectroscopic analysis of changes in remitted illumination: the response to increased neural activity in brain

Imaging of neural activation has been used to produce maps of functional architecture and metabolic activity. There is some uncertainty associated with the sources underlying the intrinsic signals. It has been reported that following increased neural activity there was little increased oxygen consumption ( approximately 5%), although glucose consumption increased by approximately 50%. The research we describe uses a modification of the Beer-Lambert Law called path-length scaling analysis (PLSA) to analyze the spectra of the hemodynamic and metabolic responses to vibrissal stimulation in rat somatosensory cortex. The results of the PLSA algorithm were compared with those obtained using a linear spectrographic analysis method (we refer to this as LMCA). There are differences in the results of the analysis depending on which of the two algorithms (PLSA or LMCA) is used. Using the LMCA algorithm, we obtain results showing an increase in the volume of Hbr at approximately 2 s, following onset of stimulation but no complementary decrease in oxygenated haemoglobin (HbO(2)). These results are similar to a previous report. In contrast, after using the PLSA algorithm, the time series of the chromophore changes shows no evidence for an increase in the volume of deoxygenated haemoglobin (Hbr). However, after further analysis of the time series from the PLSA using general linear models (GLM) to remove contributions from low frequency baseline oscillations, both the HbO(2) and Hbr times series of the response to stimulation were found to be biphasic with an early decrease in saturation peaking approximately 1 s after onset of stimulation followed by a larger increase in saturation peaking at approximately 3 s. Finally, following the PLSA-then-GLM analysis procedure, we do not find convincing evidence for an increase in cytochrome oxidation following stimulation, though we demonstrate the PLSA algorithm to be capable of disassociating changes in cytochrome oxidation state from changes in hemoglobin oxygenation.

Slow oscillations of cytochrome oxidase redox state and blood volume in unanesthetized cat and rabbit cortex. Interhemispheric synchrony

The purpose of this study was to determine the frequency characteristics and the degree of interhemispheric synchrony of slow (< 0.5 Hz), spontaneous oscillations of the regional cortical cytochrome oxidase redox state (CYT) and blood volume (CBV) in unanesthetized animals. We implanted bilateral cortical windows and electrodes for polysomnography in 7 cats and 3 rabbits. The animals were atraumatically restrained during multiple 3-6 hour sessions for up to 8 weeks, and relative changes in the cortical CYT and CBV were monitored by dual wavelength reflectance spectrophotometry at 603 nm and 590 nm. Continuous oscillations of CYT and CBV, unrelated to pulse or respiration, were always observed in each animal. Frequency (FFT) analysis over time revealed a nonstationary distribution of frequencies below 0.4 Hz, with most of the spectral power being contained in the 0-0.25 Hz band during both waking and sleep. Although the time-frequency plots of the CYT and CBV signals were similar, an occasional dissociation between the CYT and CBV oscillations was found. Analysis of simultaneous bilateral cortical optical recordings revealed a significant and sustained interhemispheric cross-correlation over time between the CYT as well as the CBV oscillations during stable recordings as long as 60 min. We conclude that: 1) CYT and CBV levels normally oscillate at < 0.4 Hz in the unanesthetized cat and rabbit cortex; 2) these complex oscillations, whose frequencies are non-stationary over time, nevertheless show sustained interhemispheric synchrony between 50 mm2 homotopic cortical regions; and 3) these oscillations may in part represent fluctuations of the metabolic rate.

Spontaneous fluctuations in cerebral oxygen supply. An introduction

Spontaneous, low frequency (4-12 cpm) fluctuations, independent of the cardiac and respiratory cycles, in human and animal brains were first recorded with the O2 polarographic technique in the late 1950s. They were seen in NADH and cytochrome oxidase and associated with spontaneous vasomotion pial and large cerebral arteries. Renewed interest in spontaneous fluctuations was generated by studies with laser-Doppler flowmetry (LDF), reflectance oximetry and functional MRI. Spontaneous fluctuations were consistently produced when cerebral perfusion was challenged by systemic or local manipulations; the fluctuation amplitude reached 30-40% of the mean. The most potent stimuli are hypotension, hyperventilation, cerebral artery occlusion and cerebral vasoconstriction elicited, for example, by a nitric oxide synthase inhibitor but not by indomethacin. The fluctuations are suspended by CO2 and halothane at concentrations that produce hyperemia. Recently, spontaneous fluctuations were recorded by LDF microprobes in areas as small as 130 microns and by video-microscopy in single capillaries. The fluctuations were absent in severe, focally ischemic brain territories. The dependence of spontaneous fluctuations on intravascular pressure argues for the importance of a myogenic mechanism, however, neuronal modulation may also play a role. Coherence of small vessel vasomotion may be required for the emergence of regional flow fluctuations. There is a need to elucidate the spatial and frequency domains in which fluctuations are present under normal physiological conditions and those in which they may reflect brain injury and pathologies of diagnostic or prognostic value.

The relationship of oxygen delivery to absolute haemoglobin oxygenation and mitochondrial cytochrome oxidase redox state in the adult brain: a near-infrared spectroscopy study

Near-infrared spectroscopy was used to determine the effect of changes in the rate of oxygen delivery to the adult rat brain on the absolute concentrations of oxyhaemoglobin, deoxyhaemoglobin and the redox state of the CuA centre in mitochondrial cytochrome oxidase. The cytochrome oxidase detection algorithm was determined to be robust to large changes in haemoglobin oxygenation and concentration. By assuming complete haemoglobin deoxygenation and CuA reduction following mechanical ventilation on 100% N2O, the absolute concentration of oxyhaemoglobin (35 microM), deoxyhaemoglobin (27 microM) and the redox state of CuA (82% oxidized) were calculated in the normal adult brain. The mean arterial blood pressure was decreased by exsanguination. When the pressure reached 100 mmHg, haemoglobin oxygenation started to fall, but the total haemoglobin concentration and oxidized CuA levels only fell when cerebral blood volume autoregulation mechanisms failed at 50 mmHg. Haemoglobin oxygenation fell linearly with decreases in the rate of oxygen delivery to the brain, but the oxidized CuA concentration did not start to fall until this rate was 50% of normal. The results suggest that the brain maintains more than adequate oxygen delivery to mitochondria and that near-infrared spectroscopy may be a good measure of oxygen insufficiency in vivo.

Interhemispheric synchrony of slow oscillations of cortical blood volume and cytochrome aa3 redox state in unanesthetized rabbits

In order to study spontaneous, slow oscillations of regional oxidative metabolism and blood flow in the normal, unanesthetized cortex, adult rabbits were implanted with bilateral cortical windows and electrodes for polysomnography. Relative changes in the cortical intramitochondrial redox state of cytochrome aa3 (CYT) and blood volume (CBV) were monitored by dual-wavelength reflectance spectrophotometry. Continuous, non-stationary oscillations (< 0.5 Hz) of both CYT and CBV were observed during waking and non-REM sleep. Cross-correlation analysis revealed a predominant interhemispheric synchrony of these oscillations which were unrelated to the heart rate, breathing, or electrocorticogram pattern. These findings suggest a dynamic linkage of slowly varying metabolic and vascular processes between unanesthetized cortical regions of 50 mm2 surface area.

Simultaneous assessment of flow and BOLD signals in resting-state functional connectivity maps

We have recently demonstrated using functional magnetic resonance imaging the presence of synchronous low-frequency fluctuations of signal intensities from the resting human brain that have a high degree of temporal correlation (p < 0.0001) both within and across the sensorimotor cortex. A statistically significant overlap between the resting-state functional connectivity map and the task-activation map due to bilateral finger tapping was obtained. Similar results have been obtained in the auditory and visual cortex. Because the pulse sequence used for collecting data was sensitive to blood flow and blood oxygenation, these low-frequency fluctuations of signal intensity may have arisen from variations of both. The objective of this study was simultaneously to determine the contribution of the blood oxygenation level signal and the flow signal to physiological fluctuations in the resting brain using the flow-sensitive alternating inversion recovery pulse sequence. In all subjects, the functional connectivity maps obtained from BOLD had a greater coincidence with task-activation maps than the corresponding functional connectivity maps obtained from blood-flow signals at the same level of statistical significance. Results of this study suggest that while variations in blood flow might contribute to functional connectivity maps, BOLD signals play a dominant role in the mechanism that gives rise to functional connectivity in the resting human brain.

Hypercapnia reversibly suppresses low-frequency fluctuations in the human motor cortex during rest using echo-planar MRI

Using magnetic resonance (MR) echo-planar imaging (EPI), we recently demonstrated the presence of low-frequency fluctuations (< 0.1 Hz) in MR signal intensity from the resting human brain that have a high degree of temporal correlation (p < 10(-3)) within and across associated regions of the sensorimotor cortex. These fluctuations in MR signal intensity are believed to arise from fluctuations in capillary blood flow and oxygenation. A substantial overlap between the activation map generated by bilateral finger tapping and temporally-correlated voxels from the sensorimotor cortex obtained during rest was observed. In the work reported here, we investigated whether respiratory hypercapnia, which is known to suspend spontaneous oscillations in regional cerebral blood flow, influences these low-frequency fluctuations. The magnitude of low-frequency fluctuations was reversibly diminished during hypercapnia, resulting in a substantial decrease of the temporal correlation both within and across contralateral hemispheres of the sensorimotor cortex. After the breathing mixture was returned to ambient air, the magnitude and spatial extent of the temporal correlation of low-frequency fluctuations returned to normal. Results of this study support the hypothesis that low-frequency physiological fluctuations observed by MR in the human cortex and spontaneous flow oscillations observed in early studies by laser-Doppler flowmetry (LDF) in the cortex of the rat are identical and are secondary to fluctuations in neuronal activity.

Slow rhythmic oscillations of blood pressure, intracranial pressure, microcirculation, and cerebral oxygenation. Dynamic interrelation and time course in humans

BACKGROUND AND PURPOSE

Various biological signals show nonpulsatile, slow rhythmic oscillations. These include arterial blood pressure (aBP), blood flow velocity in cerebral arteries, intracranial pressure (ICP), cerebral microflow, and cerebral tissue PO2. Generation and interrelations between these rhythmic fluctuations remained unclear. The aim of this study was to analyze whether stable dynamic interrelations in the low-frequency range exist between these different variables, and if they do, to analyze their exact time delay.

METHODS

In a clinical study, 16 comatose patients with either higher-grade subarachnoid hemorrhage or severe traumatic brain injury were examined. A multimodal digital data acquisition system was used to simultaneously monitor aBP, flow velocity in the middle cerebral artery (FVMCA), ICP, cerebral microflow, and oxygen saturation in the jugular bulb (SjO2). Cross-correlation as a means to analyze time delay and correlation between two periodic signals was applied to a time series of 30 minutes' duration divided into four segments of 2048 data points (approximately 436 seconds) each. This resulted in four cross-correlations for each 30-minute time series. If the four cross-correlations were consistent and reproducible, averaging of the original cross-correlations was performed, resulting in a representative time delay and correlation for the complete 30-minute interval.

RESULTS

Reproducible cross-correlations and stable dynamic interrelations were found between aBP, FVMCA, ICP, and SjO2. The mean time delay between aBP and ICP was 6.89 +/- 1.90 seconds, with a negative correlation in 81%. A mean time delay of 1.50 +/- 1.29 seconds (median, 0.85 seconds) was found between FVMCA and ICP, with a positive correlation in 94%. The mean delay between ICP and SjO2 was 9.47 +/- 2.21 seconds, with a positive correlation in 77%. Mean values of aBP and ICP did not influence the time delay and dynamic interrelation between the different parameters.

CONCLUSIONS

These results strongly support Rosner's theory that ICP B-waves are the autoregulatory response of spontaneous fluctuations of cerebral perfusion pressure. There is casuistic evidence that failure of autoregulation significantly modifies time delay and the correlation between aBP and ICP.