Hemodynamic and metabolic responses to neuronal inhibition

Differential modulations of ipsilateral and contralateral beta (de)synchronization during unimanual force production

Unilateral movement is usually accompanied by ipsilateral activity in the primary motor cortex (M1). It is still largely unclear whether this activity reflects interhemispheric 'cross-talk' of contralateral M1 that facilitates movement, or results from processes that inhibit motor output. We investigated the role of beta power in ipsilateral M1 during unimanual force production. Significant ipsilateral beta desynchronization occurred during continuous dynamic but not during static force production. Moreover, event-related time-frequency analysis revealed bilateral desynchronization patterns, whereas post-movement synchronization was confined to the contralateral hemisphere. Our findings indicate that ipsilateral activation is not merely the result of interhemispheric cross-talk but involves additional processes. Given observations of differential blood oxygen level-dependent responses in ipsilateral and contralateral M1, and the correlation between beta desynchronization and the firing rate of pyramidal tract neurons in contralateral M1 during movement, we speculate that beta desynchronization in contra- and ipsilateral M1 arises from distinct neural activation patterns.

Transient and sustained components of the sensorimotor BOLD response in fMRI

Blood oxygenation level-dependent (BOLD) signal time courses in functional magnetic resonance imaging are estimated within the framework of general linear modeling by convolving an input function, that represents neural activity, with a canonical hemodynamic response function (HRF). Here we investigate the performance of different neural input functions and latency-optimized HRFs for modeling BOLD signals in response to vibrotactile somatosensory stimuli of variable durations (0.5, 1, 4, 7 s) in 14 young, healthy adults who were required to make button press responses at each stimulus cessation. Informed by electrophysiology and the behavioral task, three nested models with an increasing number of parameters were considered: a boxcar; boxcar and offset transient; and onset transient, boxcar and offset transient (TBT). The TBT model provided the best fit of the group-averaged BOLD time courses based on χ(2) and F statistics. Only the TBT model was capable of fitting the bimodal shape of the BOLD response to the 7-s stimulus and the relative peak amplitudes for all stimulus lengths in key somatosensory and motor areas. This suggests that the TBT model provides a more comprehensive description of brain sensorimotor responses in this experiment than provided by the simple boxcar model. Work comparing the activation maps obtained with the TBT model with magnetoencephalography data is under way.

Indication of BOLD-specific venous flow-volume changes from precisely controlled hyperoxic vs. hypercapnic calibration

Deriving cerebral metabolic rate of oxygen consumption (CMRO(2)) from blood oxygenation level-dependent (BOLD) signals involves a flow-volume parameter (α), reflecting total cerebral blood volume changes, and a calibration constant (M). Traditionally, the former is assumed a fixed value and the latter is measured under alterations in fixed inspired fractional concentrations of carbon dioxide. We recently reported on reductions in M-variability via precise control of end-tidal pressures of both hypercapnic (HC) and hyperoxic (HO) gases. In light of these findings, our aim was to apply the improved calibration alternatives to neuronal activation, making use of their distinct vasoactive natures to evaluate the α-value. Nine healthy volunteers were imaged at 3 T while simultaneously measuring BOLD and arterial spin-labeling signals during controlled, graded, HC, and HO, followed by visual (VC) and sensorimotor cortices (SMC) activation. On the basis of low M- and CMRO(2)-variability, the comparison of these calibration alternatives accurately highlighted a reduced venous flow-volume relationship (α=0.16±0.02, with α(VC)=0.12±0.04, and α(SMC)=0.20±0.02), as appropriate for BOLD modeling.

Bypassing primary sensory cortices--a direct thalamocortical pathway for transmitting salient sensory information

Detection and appropriate reaction to sudden and intense events happening in the sensory environment is crucial for survival. By combining Bayesian model selection with dynamic causal modeling of functional magnetic resonance imaging data, a novel analysis approach that allows inferring the causality between neural activities in different brain areas, we demonstrate that salient sensory information reaches the multimodal cortical areas responsible for its detection directly from the thalamus, without being first processed in primary and secondary sensory-specific areas. This direct thalamocortical transmission of multimodal salient information is parallel to the processing of finer stimulus attributes, which are transmitted in a modality-specific fashion from the thalamus to the relevant primary sensory areas. Such direct thalamocortical connections bypassing primary sensory cortices provide a fast and efficient way for transmitting information from subcortical structures to multimodal cortical areas, to allow the early detection of salient events and, thereby, trigger immediate and appropriate behavior.

Neuronal inhibition and excitation, and the dichotomic control of brain hemodynamic and oxygen responses

Brain's electrical activity correlates strongly to changes in cerebral blood flow (CBF) and the cerebral metabolic rate of oxygen (CMRO(2)). Subthreshold synaptic processes correlate better than the spike rates of principal neurons to CBF, CMRO(2) and positive BOLD signals. Stimulation-induced rises in CMRO(2) are controlled by the ATP turnover, which depends on the energy used to fuel the Na,K-ATPase to reestablish ionic gradients, while stimulation-induced CBF responses to a large extent are controlled by mechanisms that depend on Ca(2+) rises in neurons and astrocytes. This dichotomy of metabolic and vascular control explains the gap between the stimulation-induced rises in CMRO(2) and CBF, and in turn the BOLD signal. Activity-dependent rises in CBF and CMRO(2) vary within and between brain regions due to differences in ATP turnover and Ca(2+)-dependent mechanisms. Nerve cells produce and release vasodilators that evoke positive BOLD signals, while the mechanisms that control negative BOLD signals by activity-dependent vasoconstriction are less well understood. Activation of both excitatory and inhibitory neurons produces rises in CBF and positive BOLD signals, while negative BOLD signals under most conditions correlate to excitation of inhibitory interneurons, but there are important exceptions to that rule as described in this paper. Thus, variations in the balance between synaptic excitation and inhibition contribute dynamically to the control of metabolic and hemodynamic responses, and in turn the amplitude and polarity of the BOLD signal. Therefore, it is not possible based on a negative or positive BOLD signal alone to decide whether the underlying activity goes on in principal or inhibitory neurons.

Dynamic models of BOLD contrast

This personal recollection looks at the evolution of ideas about the dynamics of the blood oxygenation level dependent (BOLD) signal, with an emphasis on the balloon model. From the first detection of the BOLD response it has been clear that the signal exhibits interesting dynamics, such as a pronounced and long-lasting post-stimulus undershoot. The BOLD response, reflecting a change in local deoxyhemoglobin, is a combination of a hemodynamic response, related to changes in blood flow and venous blood volume, and a metabolic response related to oxygen metabolism. Modeling is potentially a way to understand the complex path from changes in neural activity to the BOLD signal. In the early days of fMRI it was hoped that the hemodynamic/metabolic response could be modeled in a unitary way, with blood flow, oxygen metabolism, and venous blood volume-the physiological factors that affect local deoxyhemoglobin-all tightly linked. The balloon model was an attempt to do this, based on the physiological ideas of limited oxygen delivery at baseline and a slow recovery of venous blood volume after the stimulus (the balloon effect), and this simple model of the physiology worked well to simulate the BOLD response. However, subsequent experiments suggest a more complicated picture of the underlying physiology, with blood flow and oxygen metabolism driven in parallel, possibly by different aspects of neural activity. In addition, it is still not clear whether the post-stimulus undershoot is a hemodynamic or a metabolic phenomenon, although the original venous balloon effect is unlikely to be the full explanation, and a flow undershoot is likely to be important. Although our understanding of the physics of the BOLD response is now reasonably solid, our understanding of the underlying physiological relationships is still relatively poor, and this is the primary hurdle for future models of BOLD dynamics.

Quantitative functional MRI: concepts, issues and future challenges

Since its inception 20 years ago, functional magnetic resonance imaging (fMRI) of the human brain based on the blood oxygenation level dependent (BOLD) contrast phenomenon has proliferated and matured. Today it is the predominant functional brain imaging modality with the majority of applications being in basic cognitive neuroscience where it has primarily been used as a tool to localize brain activity. While the magnitude of the BOLD response is often used in these studies as a surrogate for the level of neuronal activity, the link between the two is, in fact, quite indirect. The BOLD response is dependent upon hemodynamic (blood flow and volume) and metabolic (oxygen consumption) responses as well as acquisition details. Furthermore, the relationship between neuronal activity and the hemodynamic response, termed neurovascular coupling, is itself complex and incompletely understood. Quantitative fMRI techniques have therefore been developed to measure the hemodynamic and metabolic responses to modulations in brain activity. These methods have not only helped clarify the behaviour and origins of the BOLD signal under normal physiological conditions but they have also provided a potentially valuable set of tools for exploring pathophysiological conditions. Such quantitative methods will be critical to realize the potential of fMRI in a clinical context, where simple BOLD measurements cannot be uniquely interpreted, and to enhance the power of fMRI in basic neuroscience research. In this article, recent advances in human quantitative fMRI methods are reviewed, outstanding issues discussed and future challenges and opportunities highlighted.

Where fMRI and electrophysiology agree to disagree: corticothalamic and striatal activity patterns in the WAG/Rij rat

The relationship between neuronal activity and hemodynamic changes plays a central role in functional neuroimaging. Under normal conditions and in neurological disorders such as epilepsy, it is commonly assumed that increased functional magnetic resonance imaging (fMRI) signals reflect increased neuronal activity and that fMRI decreases represent neuronal activity decreases. Recent work suggests that these assumptions usually hold true in the cerebral cortex. However, less is known about the basis of fMRI signals from subcortical structures such as the thalamus and basal ganglia. We used WAG/Rij rats (Wistar albino Glaxo rats of Rijswijk), an established animal model of human absence epilepsy, to perform fMRI studies with blood oxygen level-dependent and cerebral blood volume (CBV) contrasts at 9.4 tesla, as well as laser Doppler cerebral blood flow (CBF), local field potential (LFP), and multiunit activity (MUA) recordings. We found that, during spike-wave discharges, the somatosensory cortex and thalamus showed increased fMRI, CBV, CBF, LFP, and MUA signals. However, the caudate-putamen showed fMRI, CBV, and CBF decreases despite increases in LFP and MUA signals. Similarly, during normal whisker stimulation, the cortex and thalamus showed increases in CBF and MUA, whereas the caudate-putamen showed decreased CBF with increased MUA. These findings suggest that neuroimaging-related signals and electrophysiology tend to agree in the cortex and thalamus but disagree in the caudate-putamen. These opposite changes in vascular and electrical activity indicate that caution should be applied when interpreting fMRI signals in both health and disease from the caudate-putamen, as well as possibly from other subcortical structures.

Limbic response to psychosocial stress in schizotypy: a functional magnetic resonance imaging study

Psychological stress causes dopamine release in the striatum and is thought to play a role in susceptibility to psychotic illness. Previous work suggests that an elevated dopaminergic response to stress may index vulnerability to psychosis in certain individuals. With functional magnetic resonance imaging, we measured stress-induced changes in brain activity in healthy individuals at elevated risk of developing psychosis. Participants were 15 controls and 25 psychometric schizotypes: 12 with positive symptom schizotypy (perceptual aberrations) and 13 with negative symptom schizotypy (physical anhedonia), as determined by questionnaires (Chapman et al., 1976; Chapman and Chapman, 1978). In the scanner, participants performed the Montreal Imaging Stress Task and a matched sensory-motor control task. Measures of self-reported stress and salivary cortisol levels were taken throughout the experiment. All three groups showed significant increases in self-reported stress and significant fMRI signal change in the striatal, limbic and cortical regions. However, the Physical Anhedonia group showed greater stress-induced striatal and limbic deactivation than the other two groups. Deactivation in the striatum was significantly correlated with Physical Anhedonia score across all subjects. Our findings suggest the presence of abnormalities in striatal response to stress in negative symptom schizotypy.

Sustained negative BOLD response in human fMRI finger tapping task

In this work, we investigated the sustained negative blood oxygen level-dependent (BOLD) response (sNBR) using functional magnetic resonance imaging during a finger tapping task. We observed that the sNBR for this task was more extensive than has previously been reported. The cortical regions involved in sNBR are divided into the following three groups: frontal, somatosensory and occipital. By investigating the spatial structure, area, amplitude, and dynamics of the sNBR in comparison with those of its positive BOLD response (PBR) counterpart, we made the following observations. First, among the three groups, the somatosensory group contained the greatest number of activated voxels and the fewest deactivated voxels. In addition, the amplitude of the sNBR in this group was the smallest among the three groups. Second, the onset and peak time of the sNBR are both larger than those of the PBR, whereas the falling edge time of the sNBR is less than that of the PBR. Third, the long distance between most sNBR foci and their corresponding PBR foci makes it unlikely that they share the same blood supply artery. Fourth, the couplings between the sNBR and its PBR counterpart are distinct among different regions and thus should be investigated separately. These findings imply that the origin of most sNBR foci in the finger-tapping task is much more likely to be neuronal activity suppression rather than "blood steal."

Self-modulation of primary motor cortex activity with motor and motor imagery tasks using real-time fMRI-based neurofeedback

Advances in fMRI data acquisition and processing have made it possible to analyze brain activity as rapidly as the images are acquired allowing this information to be fed back to subjects in the scanner. The ability of subjects to learn to volitionally control localized brain activity within motor cortex using such real-time fMRI-based neurofeedback (NF) is actively being investigated as it may have clinical implications for motor rehabilitation after central nervous system injury and brain-computer interfaces. We investigated the ability of fifteen healthy volunteers to use NF to modulate brain activity within the primary motor cortex (M1) during a finger tapping and tapping imagery task. The M1 hand area ROI (ROI(m)) was functionally localized during finger tapping and a visual representation of BOLD signal changes within the ROI(m) fed back to the subject in the scanner. Surface EMG was used to assess motor output during tapping and ensure no motor activity was present during motor imagery task. Subjects quickly learned to modulate brain activity within their ROI(m) during the finger-tapping task, which could be dissociated from the magnitude of the tapping, but did not show a significant increase within the ROI(m) during the hand motor imagery task at the group level despite strongly activating a network consistent with the performance of motor imagery. The inability of subjects to modulate M1 proper with motor imagery may reflect an inherent difficulty in activating synapses in this area, with or without NF, since such activation may lead to M1 neuronal output and obligatory muscle activity. Future real-time fMRI-based NF investigations involving motor cortex may benefit from focusing attention on cortical regions other than M1 for feedback training or alternative feedback strategies such as measures of functional connectivity within the motor system.

A theoretical framework for estimating cerebral oxygen metabolism changes using the calibrated-BOLD method: modeling the effects of blood volume distribution, hematocrit, oxygen extraction fraction, and tissue signal properties on the BOLD signal

Calibrated blood oxygenation level dependent (BOLD) imaging, a technique used to measure changes in cerebral O(2) metabolism, depends on an accurate model of how the BOLD signal is affected by the mismatch between changes in cerebral blood flow (CBF) and cerebral metabolic rate of O(2) (CMRO(2)). However, other factors such as the cerebral blood volume (CBV) distribution at rest and with activation also affect the BOLD signal. The Davis model originally proposed for calibrated BOLD studies (Davis et al., 1998) is widely used because of its simplicity, but it assumes CBV changes are uniformly distributed across vascular compartments, neglects intravascular signal changes, and ignores blood-tissue signal exchange effects as CBV increases and supplants tissue volume. More recent studies suggest that venous CBV changes are smaller than arterial changes, and that intravascular signal changes and CBV exchange effects can bias estimated CMRO(2). In this paper, recent experimental results for the relationship between deoxyhemoglobin and BOLD signal changes are integrated in order to simulate the BOLD signal in detail by expanding a previous model to include a tissue compartment and three blood compartments rather than only the venous blood compartment. The simulated data were then used to test the accuracy of the Davis model of calibrated BOLD, demonstrating that the errors in estimated CMRO(2) responses across the typical CBF-CMRO(2) coupling range are modest despite the simplicity of the assumptions underlying the original derivation of the model. Nevertheless, the accuracy of the model can be improved by abandoning the original physical meaning of the two parameters α and β and treating them as adjustable parameters that capture several physical effects. For a 3Tesla field and a dominant arterial volume change with activation, the accuracy of the Davis model is improved with new values of α=0.14 and β=0.91.

Depression of cortical activity in humans by mild hypercapnia

The effects of neural activity on cerebral hemodynamics underlie human brain imaging with functional magnetic resonance imaging and positron emission tomography. However, the threshold and characteristics of the converse effects, wherein the cerebral hemodynamic and metabolic milieu influence neural activity, remain unclear. We tested whether mild hypercapnia (5% CO2 ) decreases the magnetoencephalogram response to auditory pattern recognition and visual semantic tasks. Hypercapnia induced statistically significant decreases in event-related fields without affecting behavioral performance. Decreases were observed in early sensory components in both auditory and visual modalities as well as later cognitive components related to memory and language. Effects were distributed across cortical regions. Decreases were comparable in evoked versus spontaneous spectral power. Hypercapnia is commonly used with hemodynamic models to calibrate the blood oxygenation level-dependent response. Modifying model assumptions to incorporate the current findings produce a modest but measurable decrease in the estimated cerebral metabolic rate for oxygen change with activation. Because under normal conditions, low cerebral pH would arise when bloodflow is unable to keep pace with neuronal activity, the cortical depression observed here may reflect a homeostatic mechanism by which neuronal activity is adjusted to a level that can be sustained by available bloodflow. Animal studies suggest that these effects may be mediated by pH-modulating presynaptic adenosine receptors. Although the data is not clear, comparable changes in cortical pH to those induced here may occur during sleep apnea, sleep, and exercise. If so, these results suggest that such activities may in turn have generalized depressive effects on cortical activity.

Physical activity and neural correlates of aging: a combined TMS/fMRI study

Aerobic exercise has been suggested to ameliorate aging-related decline in humans. Recently, evidence has indicated chronological aging is associated with decreases in measures of interhemispheric inhibition during unimanual movements, but that such decreases may be mitigated by long-term physical fitness. The present study investigated measures of ipsilateral (right) primary motor cortex activity during right-hand movements using functional magnetic resonance imaging and transcranial magnetic stimulation (TMS). Healthy, right-handed participant groups were comprised of 12 sedentary older adults, 12 physically active older adults, and 12 young adults. Active older adults and younger adults evidenced longer ipsilateral silent periods (iSP) and less positive BOLD of ipsilateral motor cortex (iM1) as compared to sedentary older adults. Across groups, duration of iSP from TMS was inversely correlated with BOLD activity in iM1 during unimanual movement. These findings suggest that increased physical activity may have a role in decreasing aging-related losses of interhemispheric inhibition.

Functional deactivations: multiple ipsilateral brain areas engaged in the processing of somatosensory information

Somatosensory signals modulate activity throughout a widespread network in both of the brain hemispheres: the contralateral as well as the ipsilateral side of the brain relative to the stimulated limb. To analyze the ipsilateral somatosensory brain areas that are engaged during limb stimulation, we performed functional magnetic resonance imaging (fMRI) in 12 healthy subjects during electrical median nerve stimulation using both a block- and an event-related fMRI design. Data were analyzed through the use of model-dependent (SPM) and model-independent (ICA) approaches. Beyond the well-known positive blood oxygenation level-dependent (BOLD) responses, negative deflections of the BOLD response were found consistently in several ipsilateral brain areas, including the primary somatosensory cortex, the supplementary motor area, the insula, the dorsal part of the posterior cingulate cortex, and the contralateral cerebellum. Compared to their positive counterparts, the negative hemodynamic responses showed a different time course, with an onset time delay of 2.4 s and a peak delay of 0.7 s. This characteristic delay was observed in all investigated areas and verified by a second (purely tactile) event-related paradigm, suggesting a systematic difference for brain areas involved in the processing of somatosensory information. These findings may indicate that the physiological basis of these deactivations differs from that of the positive BOLD responses. Therefore, an altered model for the negative BOLD response may be beneficial to further model-dependent fMRI analyses.

Statistical Analysis of fMRI Time-Series: A Critical Review of the GLM Approach

Functional magnetic resonance imaging (fMRI) is one of the most widely used tools to study the neural underpinnings of human cognition. Standard analysis of fMRI data relies on a general linear model (GLM) approach to separate stimulus induced signals from noise. Crucially, this approach relies on a number of assumptions about the data which, for inferences to be valid, must be met. The current paper reviews the GLM approach to analysis of fMRI time-series, focusing in particular on the degree to which such data abides by the assumptions of the GLM framework, and on the methods that have been developed to correct for any violation of those assumptions. Rather than biasing estimates of effect size, the major consequence of non-conformity to the assumptions is to introduce bias into estimates of the variance, thus affecting test statistics, power, and false positive rates. Furthermore, this bias can have pervasive effects on both individual subject and group-level statistics, potentially yielding qualitatively different results across replications, especially after the thresholding procedures commonly used for inference-making.

Controlling instabilities in manipulation requires specific cortical-striatal-cerebellar networks

Dexterous manipulation requires both strength, the ability to produce fingertip forces of a specific magnitude, and dexterity, the ability to dynamically regulate the magnitude and direction of fingertip force vectors and finger motions. Although cortical activity in fronto-parietal networks has been established for stable grip and pinch forces, the cortical regulation in the dexterous control of unstable objects remains unknown. We used functional magnetic resonance imaging (fMRI) to interrogate cortical networks engaged in the control of four objects with increasing instabilities but requiring constant strength. In addition to expected activity in fronto-parietal networks we find that dexterous manipulation of increasingly unstable objects is associated with a linear increase in the amplitude of the BOLD signal in the basal ganglia (P = 0.007 and P = 0.023 for 2 compression tasks). A computational regression (connectivity) model identified independent subsets of cortical networks whose connection strengths were mutable and associated with object instability (P < 0.001). Our results suggest that in the presence of object instability, the basal ganglia may modulate the activity of premotor areas and subsequent motor output. This work, therefore, provides new evidence for the selectable cortical representation and execution of dynamic multifinger manipulation for grasp stability.

A neuronal basis for task-negative responses in the human brain

Neuroimaging studies have revealed a number of brain regions that show a reduced blood oxygenation level–dependent (BOLD) signal during externally directed tasks compared with a resting baseline. These regions constitute a network whose operation has become known as the default mode. The source of functional magnetic resonance imaging (fMRI) signal reductions in the default mode during task performance has not been resolved, however. It may be attributable to neuronal effects (neuronal firing), physiological effects (e.g., task vs. rest differences in respiration rate), or even increases in neuronal activity with an atypical blood response. To establish the source of signal decreases in the default mode, we used the calibrated fMRI method to quantify changes in the cerebral metabolic rate of oxygen (CMRO₂) and cerebral blood flow (CBF) in those regions that typically show reductions in BOLD signal during a demanding cognitive task. CBF:CMRO₂ coupling during task-negative responses were linear, with a coupling constant similar to that in task-positive regions, indicating a neuronal source for signal reductions in multiple brain areas. We also identify, for the first time, two modes of neuronal activity in this network; one in which greater deactivation (characterized by metabolic rate reductions) is associated with more effort and one where it is associated with less effort.

When desire collides with reason: functional interactions between anteroventral prefrontal cortex and nucleus accumbens underlie the human ability to resist impulsive desires

Human decisions are guided by "desire" or "reason," which control actions oriented toward either proximal or long-term goals. Here we used functional magnetic resonance imaging to assess how the human brain mediates the balance between proximal reward desiring and long-term goals, when actions promoting a superordinate goal preclude exploitation of an immediately available reward option. Consistent with the view that the reward system interacts with prefrontal circuits during action control, we found that behavior favoring the long-term goal, but counteracting immediate reward desiring, relied on a negative functional interaction of anteroventral prefrontal cortex (avPFC) with nucleus accumbens (Nacc) and ventral tegmental area. The degree of functional interaction between avPFC and Nacc further predicted behavioral success during pursuit of the distal goal, when confronted with a proximal reward option, and scaled with interindividual differences in trait impulsivity. These findings reveal how the human brain accomplishes voluntary action control guided by "reason," suggesting that inhibitory avPFC influences Nacc activity during actions requiring a restraint of immediate "desires."

Interpreting oxygenation-based neuroimaging signals: the importance and the challenge of understanding brain oxygen metabolism

Functional magnetic resonance imaging is widely used to map patterns of brain activation based on blood oxygenation level dependent (BOLD) signal changes associated with changes in neural activity. However, because oxygenation changes depend on the relative changes in cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO(2)), a quantitative interpretation of BOLD signals, and also other functional neuroimaging signals related to blood or tissue oxygenation, is fundamentally limited until we better understand brain oxygen metabolism and how it is related to blood flow. However, the positive side of the complexity of oxygenation signals is that when combined with dynamic CBF measurements they potentially provide the best tool currently available for investigating the dynamics of CMRO(2). This review focuses on the problem of interpreting oxygenation-based signals, the challenges involved in measuring CMRO(2) in general, and what is needed to put oxygenation-based estimates of CMRO(2) on a firm foundation. The importance of developing a solid theoretical framework is emphasized, both as an essential tool for analyzing oxygenation-based multimodal measurements, and also potentially as a way to better understand the physiological phenomena themselves. The existing data, integrated within a simple theoretical framework of O(2) transport, suggests the hypothesis that an important functional role of the mismatch of CBF and CMRO(2) changes with neural activation is to prevent a fall of tissue pO(2). Future directions for better understanding brain oxygen metabolism are discussed.

Cerebral hemodynamic responses induced by specific acupuncture sensations during needling at trigger points: a near-infrared spectroscopic study

Acupuncture stimulation at specific points, or trigger points (TPs), elicits sensations called "de-qi". De-qi sensations relate to the clinical efficacy of the treatment. However, it is neither clear whether de-qi sensations are associated with TPs, nor clear whether acupuncture effects on brain activity are associated with TPs or de-qi. We recorded cerebral hemodynamic responses during acupuncture stimulation at TPs and non-TPs by functional near-infrared spectroscopy. The acupuncture needle was inserted into both TPs and non-TPs within the right extensor muscle in the forearm. Typical acupuncture needle manipulation was conducted eight times for 15 s. The subjects pressed a button if they felt a de-qi sensation. We investigated how hemodynamic responses related to de-qi sensations induced at TPs and non-TPs. We observed that acupuncture stimulations producing de-qi sensations significantly decreased the Oxy-Hb concentration in the supplementary motor area (SMA), pre-supplementary motor area, and anterior dorsomedial prefrontal cortex regardless of the point stimulated. The hemodynamic responses were statistically analyzed using a general linear model and a boxcar function approximating the hemodynamic response. We observed that hemodynamic responses best fit the boxcar function when an onset delay was introduced into the analyses, and that the latency of de-qi sensations correlated with the onset delay of the best-fit function applied to the SMA. Our findings suggest that de-qi sensations favorably predict acupuncture effects on cerebral hemodynamics regardless of the type of site stimulated. Also, the effect of acupuncture stimulation in producing de-qi sensation was partly mediated by the central nervous system including the SMA.

Quantification of CMRO(2) without hypercapnia using simultaneous near-infrared spectroscopy and fMRI measurements

Estimation of the cerebral metabolic rate of oxygen (CMRO(2)) and cerebral blood flow (CBF) is important to investigate the neurovascular coupling and physiological components in blood oxygenation level-dependent (BOLD) signals quantitatively. Although there are methods that can determine CMRO(2) changes using functional MRI (fMRI) or near-infrared spectroscopy (NIRS), current approaches require a separate hypercapnia calibration process and have the potential to incur bias in many assumed model parameters. In this paper, a novel method to estimate CMRO(2) without hypercapnia is described using simultaneous measurements of NIRS and fMRI. Specifically, an optimization framework is proposed that minimizes the differences between the two forms of the relative CMRO(2)-CBF coupling ratio from BOLD and NIRS biophysical models, from which hypercapnia calibration and model parameters are readily estimated. Based on the new methods, we found that group average CBF, CMRO(2), cerebral blood volume (CBV), and BOLD changes within activation of the primary motor cortex during a finger tapping task increased by 39.5 +/- 21.4%, 18.4 +/- 8.7%, 12.9 +/- 6.7%, and 0.5 +/- 0.2%, respectively. The group average estimated flow-metabolism coupling ratio was 2.38 +/- 0.65 and the hypercapnia parameter was 7.7 +/- 1.7%. These values are within the range of values reported from other literatures. Furthermore, the activation maps from CBF and CMRO(2) were well localized on the primary motor cortex, which is the main target region of the finger tapping task.

A proton magnetic resonance spectroscopic study in juvenile absence epilepsy in early stages

PURPOSE

The aim of this study to evaluate the hippocampal, frontal and thalamic lobe functions in the early stage of the juvenile absence epilepsy (JAE) by magnetic resonance proton spectroscopy (MRS).

METHOD

Fourteen patients with juvenile absence epilepsy with typical absence seizures and 10 healthy volunteer controls were included in this study. The diagnosis of the patients was in accordance with EEG findings and seizure semiology. All patients had minimum twice EEG recordings and all had typical 3-Hz generalized spike and slow-wave discharges at least on one EEG. All patients had bilateral MRS of frontal, thalamic and hippocampal regions and NAA, NAA/Cr, NAA/Cho, NAA/Cho+Cr levels were detected.

RESULTS

The mean age was 14.9+/-2.05 and 14.5+/-1.7 of the JAE patients and control subjects, respectively. Mean seizure onset duration were 2.3+/-0.9 years. In patient group the frontal, thalamic and hippocampal NAA/Cr ratios were 1.65, 1.78, 1.47 in right and 1.75, 1.90, 1.42 in left, respectively. While in the control group NAA/Cr ratios were 1.64, 2.42, 1.57 in right and 1.83, 2.44, 1.47 in left, respectively. There weren't any difference in frontal and hippocampal regions, but the bilateral thalamic NAA, NAA/Cr ratios of the patients were significantly lower than control group even in early stages of the disease.

CONCLUSION

The observed reductions in NAA levels and NAA/Cr ratios of bilateral thalamic regions are consistent with epilepsy related excitoxicity as a possible underlying mechanism even in early stage of JAE. However, we believe that to generalize the results of our study a prospective multicenter study is required.

Negative blood oxygen level dependence in the rat: a model for investigating the role of suppression in neurovascular coupling

Modern neuroimaging techniques rely on neurovascular coupling to show regions of increased brain activation. However, little is known of the neurovascular coupling relationships that exist for inhibitory signals. To address this issue directly we developed a preparation to investigate the signal sources of one of these proposed inhibitory neurovascular signals, the negative blood oxygen level-dependent (BOLD) response (NBR), in rat somatosensory cortex. We found a reliable NBR measured in rat somatosensory cortex in response to unilateral electrical whisker stimulation, which was located in deeper cortical layers relative to the positive BOLD response. Separate optical measurements (two-dimensional optical imaging spectroscopy and laser Doppler flowmetry) revealed that the NBR was a result of decreased blood volume and flow and increased levels of deoxyhemoglobin. Neural activity in the NBR region, measured by multichannel electrodes, varied considerably as a function of cortical depth. There was a decrease in neuronal activity in deep cortical laminae. After cessation of whisker stimulation there was a large increase in neural activity above baseline. Both the decrease in neuronal activity and increase above baseline after stimulation cessation correlated well with the simultaneous measurement of blood flow suggesting that the NBR is related to decreases in neural activity in deep cortical layers. Interestingly, the magnitude of the neural decrease was largest in regions showing stimulus-evoked positive BOLD responses. Since a similar type of neural suppression in surround regions was associated with a negative BOLD signal, the increased levels of suppression in positive BOLD regions could importantly moderate the size of the observed BOLD response.

Significant correlation between autonomic nervous activity and cerebral hemodynamics during thermotherapy on the neck

Although local thermotherapy reduces mental stress and neck stiffness, its physiological mechanisms are still not fully understood. We speculated that local thermotherapy exerts its effect, in addition to its direct peripheral effects, through the central nervous system that is involved in controlling stress responses. In the present study, we investigated the effects of a heat- and steam-generating (HSG) sheet on cerebral hemodynamics and autonomic nervous activity using near-infrared spectroscopy (NIRS) and the electrocardiograms (ECGs). Thirteen healthy young female subjects participated in this study. HSG or simple (control) sheets were repeatedly applied to the neck for 120 s with 180 s intervals of rest between applications. During the experiment, brain hemodynamic responses (changes in Oxy-Hb, Deoxy-Hb, and Total-Hb) and autonomic nervous activity based on heart rate variability (HRV) were monitored. Subjective perception of neck stiffness and fatigue was significantly improved after application of the HSG sheet. NIRS findings indicated that the application of HSG sheets decreased Oxy-Hb concentration in the anterior-dorsal region of the medial prefrontal cortex (adMPFC), while increasing parasympathetic nervous activity and decreasing sympathetic nervous activity. Furthermore, changes in Oxy-Hb in the adMPFC were significantly and negatively correlated with those in parasympathetic nervous activity during application of the HSG sheet. These findings suggest that application of the HSG sheet to the neck region induced mental relaxation and ameliorated neck stiffness by modifying activity of the adMPFC.

Differential representation of dynamic and static power grip force in the sensorimotor network

Previous studies investigating the blood oxygen level-dependent (BOLD) signal in the human sensorimotor cortex during static force (maintained for a few seconds) and dynamic force (repetitive force pulses) resulted in contradictory findings. Therefore, we conducted a whole-brain functional magnetic resonance imaging study during a visuomotor task requiring the production of either dynamic or static power grip force. Thereby we aimed at clarifying whether the BOLD signal behaves differently with dynamic and static force in the primary motor cortex, and whether it behaves in the same way in all areas and regions involved in force production. In the static condition, participants applied visually guided, isometric grip force on a dynamometer of 20% maximal voluntary contraction (MVC) and held this force for 21 s. In the dynamic condition, self-paced force pulses of 20% MVC were produced at a rate of 0.5 Hz. Static and dynamic force production activated an overlapping network of sensorimotor cortical and subcortical regions. However, the production of a significantly higher mean static force compared with the dynamic force resulted in a significantly smaller BOLD signal in the contralateral motor cortex, confirming observations of an earlier investigation. In addition, we found that the ipsilateral anterior cerebellum behaved similar to the motor cortex, whereas in all other activated regions the activation during static and dynamic force did not significantly differ. These findings demonstrate that various regions of the sensorimotor network participate differentially in the production and control of low static and dynamic grip force, and raise important questions concerning the interpretation of the BOLD signal with respect to mechanisms of neurovascular coupling.

A case of persistent visual hallucinations of faces following LSD abuse: a functional Magnetic Resonance Imaging study

In this study, we report the case of a patient experiencing hallucinations of faces that could be reliably precipitated by looking at trees. Using functional Magnetic Resonance Imaging (fMRI), we found that face hallucinations were associated with increased and decreased neural activity in a number of cortical regions. Within the same fusiform face area, however, we found significant decreased and increased neural activity according to whether the patient was experiencing hallucinations or veridical perception of faces, respectively. These findings may indicate key differences in how hallucinatory and veridical perceptions lead to the same phenomenological experience of seeing faces.

Functional magnetic resonance imaging and c-Fos mapping in rats following a glucoprivic dose of 2-deoxy-D-glucose

The glucose analogue, 2-deoxy-D-glucose (2-DG) is an inhibitor of glycolysis and, when administered systemically or centrally, induces glucoprivation leading to counter-regulatory responses, including increased feeding behaviour. Investigations into how the brain responds to glucoprivation could have important therapeutic potential, as disruptions or defects in the defence of the brain's 'glucostatic' circuitry may be partly responsible for pathological conditions resulting from diabetes and obesity. To define the 'glucostat' brain circuitry further we have combined blood-oxygen-level-dependent pharmacological-challenge magnetic resonance imaging (phMRI) with whole-brain c-Fos functional activity mapping to characterise brain regions responsive to an orexigenic dose of 2-DG [200 mg/kg; subcutaneous (s.c.)]. For phMRI, rats were imaged using a T(2)*-weighted gradient echo in a 7T magnet for 60 min under alpha-chloralose anaesthesia, whereas animals for immunohistochemistry were unanaesthetised and freely behaving. These complementary methods demonstrated functional brain activity in a number of previously characterised glucose-sensing brain regions such as those in the hypothalamus and brainstem following administration of 2-DG compared with vehicle. As the study mapped whole-brain functional responses, it also identified the orbitofrontal cortex and striatum (nucleus accumbens and ventral pallidum) as novel 2-DG-responsive brain regions. These regions make up a corticostriatal connection with the hypothalamus, by which aspects of motivation, salience and reward can impinge on the hypothalamic control of feeding behaviour. This study, therefore, provides further evidence for a common integrated circuit involved in the induction of feeding behaviour, and illustrates the valuable potential of phMRI in investigating central pharmacological actions.

Noninvasive dynamic imaging of seizures in epileptic patients

Epileptic seizures are due to abnormal synchronized neuronal discharges. Techniques measuring electrical changes are commonly used to analyze seizures. Neuronal activity can be also defined by concomitant hemodynamic and metabolic changes. Simultaneous electroencephalogram (EEG)-functional MRI (fMRI) measures noninvasively with a high-spatial resolution BOLD changes during seizures in the whole brain. Until now, only a static image representing the whole seizure was provided. We report in 10 focal epilepsy patients a new approach to dynamic imaging of seizures including the BOLD time course of seizures and the identification of brain structures involved in seizure onset and discharge propagation. The first activation was observed in agreement with the expected location of the focus based on clinical and EEG data (three intracranial recordings), thus providing validity to this approach. The BOLD signal preceded ictal EEG changes in two cases. EEG-fMRI may detect changes in smaller and deeper structures than scalp EEG, which can only record activity form superficial cortical areas. This method allowed us to demonstrate that seizure onset zone was limited to one structure, thus supporting the concept of epileptic focus, but that a complex neuronal network was involved during propagation. Deactivations were also found during seizures, usually appearing after the first activation in areas close or distant to the activated regions. Deactivations may correspond to actively inhibited regions or to functional disconnection from normally active regions. This new noninvasive approach should open the study of seizure generation and propagation mechanisms in the whole brain to groups of patients with focal epilepsies.

Interhemispheric neuroplasticity following limb deafferentation detected by resting-state functional connectivity magnetic resonance imaging (fcMRI) and functional magnetic resonance imaging (fMRI)

Functional connectivity magnetic resonance imaging (fcMRI) studies in rat brain show brain reorganization following peripheral nerve injury. Subacute neuroplasticity was observed 2 weeks following transection of the four major nerves of the brachial plexus. Direct stimulation of the intact radial nerve reveals a functional magnetic resonance imaging (fMRI) activation pattern in the forelimb regions of the sensory and motor cortices that is significantly different from that observed in normal rats. Results of this fMRI experiment were used to determine seed voxel regions for fcMRI analysis. Intrahemispheric connectivities in the sensorimotor forelimb representations in both hemispheres are largely unaffected by deafferentation, whereas substantial disruption of interhemispheric sensorimotor cortical connectivity occurs. In addition, significant intra- and interhemispheric changes in connectivities of thalamic nuclei were found. These are the central findings of the study. They could not have been obtained from fMRI studies alone-both fMRI and fcMRI are needed. The combination provides a general marker for brain plasticity. The rat visual system was studied in the same animals as a control. No neuroplastic changes in connectivities were found in the primary visual cortex upon forelimb deafferentation. Differences were noted in regions responsible for processing multisensory visual-motor information. This incidental discovery is considered to be significant. It may provide insight into phantom limb epiphenomena.

Negative BOLD during tongue movement: a functional magnetic resonance imaging study

The aim of this functional magnetic resonance imaging (fMRI) study was to evaluate negative blood oxygen level-dependent (BOLD) signals during voluntary tongue movement. Deactivated (Negative BOLD) regions included the posterior parietal cortex (PPC), precuneus, and middle temporal gyrus. Activated (Positive BOLD) regions included the primary somatosensory-motor area (SMI), inferior parietal lobule, medial frontal gyrus, superior temporal gyrus, insula, lentiform nucleus, and thalamus. The results were not consistent with previous studies involving unilateral hand and finger movements showing the deactivation of motor-related cortical areas including the ipsilateral MI. The areas of Negative BOLD in the PPC and precuneus might reflect specific neural networks relating to voluntary tongue movement.

Negative BOLD effect on somato-motor inhibitory processing: an fMRI study

Inhibiting inappropriate behavior and thoughts in the current context is an essential ability for humans, but the neural mechanisms for response inhibitory processing are a matter of continuous debate. The aim of this event-related functional magnetic resonance imaging (fMRI) study was to evaluate the negative blood oxygen level dependent (BOLD) effect on inhibitory processing during go/no-go paradigms. Fifteen subjects performed two different types of somatosensory go/no-go paradigm: (1) button press and (2) count. Go and no-go stimuli were presented with an even probability. We observed a common negative activation during Movement No-go and Count No-go trials in the right SFG, corresponding to BA 8. These findings suggest that the right SFG region was responsible for the negative BOLD effect on inhibitory processing, which was independent of the required response mode. We hypothesized several possible explanations for the deactivation of the SFG during no-go trials.

Effects of aging on cerebral blood flow, oxygen metabolism, and blood oxygenation level dependent responses to visual stimulation

Calibrated functional magnetic resonance imaging (fMRI) provides a noninvasive technique to assess functional metabolic changes associated with normal aging. We simultaneously measured both the magnitude of the blood oxygenation level dependent (BOLD) and cerebral blood flow (CBF) responses in the visual cortex for separate conditions of mild hypercapnia (5% CO(2)) and a simple checkerboard stimulus in healthy younger (n = 10, mean: 28-years-old) and older (n = 10, mean: 53-years-old) adults. From these data we derived baseline CBF, the BOLD scaling parameter M, the fractional change in the cerebral metabolic rate of oxygen consumption (CMRO(2)) with activation, and the coupling ratio n of the fractional changes in CBF and CMRO(2). For the functional activation paradigm, the magnitude of the BOLD response was significantly lower for the older group (0.57 +/- 0.07%) compared to the younger group (0.95 +/- 0.14%), despite the finding that the fractional CBF and CMRO(2) changes were similar for both groups. The weaker BOLD response for the older group was due to a reduction in the parameter M, which was significantly lower for older (4.6 +/- 0.4%) than younger subjects (6.5 +/- 0.8%), most likely reflecting a reduction in baseline CBF for older (41.7 +/- 4.8 mL/100 mL/min) compared to younger (59.6 +/- 9.1 mL/100 mL/min) subjects. In addition to these primary responses, for both groups the BOLD response exhibited a post-stimulus undershoot with no significant difference in this magnitude. However, the post-undershoot period of the CBF response was significantly greater for older compared to younger subjects. We conclude that when comparing two populations, the BOLD response can provide misleading reflections of underlying physiological changes. A calibrated approach provides a more quantitative reflection of underlying metabolic changes than the BOLD response alone.

The salient characteristics of the central effects of acupuncture needling: limbic-paralimbic-neocortical network modulation

Human and animal studies suggest that acupuncture produces many beneficial effects through the central nervous system. However, the neural substrates of acupuncture actions are not completely clear to date. fMRI studies at Hegu (LI4) and Zusanli (ST36) indicated that the limbic system may play an important role for acupuncture effects. To test if this finding applies to other major classical acupoints, fMRI was performed on 10 healthy adults during manual acupuncture at Taichong (LV3), Xingjian (LV2), Neiting (ST44), and a sham point on the dorsum of the left foot. Although certain differences could be observed between real and sham points, the hemodynamic response (BOLD signal changes) and psychophysical response (sensory experience) to acupuncture were generally similar for all four points. Acupuncture produced extensive deactivation of the limbic-paralimbic-neocortical system. Clusters of deactivated regions were seen in the medial prefrontal cortex (frontal pole, pregenual cingulate), the temporal lobe (amygdala, hippocampus, and parahippocampus) and the posterior medial cortex (precuneus, posterior cingulate). The sensorimotor cortices (somatosensory cortices, supplementary motor cortex), thalamus and occasional paralimbic structures such as the insula and anterior middle cingulate cortex showed activation. Our results provide additional evidence in support of previous reports that acupuncture modulates the limbic-paralimbic-neocortical network. We hypothesize that acupuncture may mediate its antipain, antianxiety, and other therapeutic effects via this intrinsic neural circuit that plays a central role in the affective and cognitive dimensions of pain as well as in the regulation and integration of emotion, memory processing, autonomic, endocrine, immunological, and sensorimotor functions.

Comparison of hypercapnia-based calibration techniques for measurement of cerebral oxygen metabolism with MRI

MRI may be used to measure fractional changes in cerebral oxygen metabolism via a metabolic model. One step commonly used in this measurement is calibration with image data acquired during hypercapnia, which is a state of increased CO2 content of the blood. In this study some commonly used hypercapnia-inducing stimuli were compared to assess their suitability for the calibration step. The following stimuli were investigated: (a) inspiration of a mixture of 4% CO2, 21% O2 and balance N2; (b) 30-s breath holding; and (c) inspiration of a mixture of 4% CO2 and 96% O2 (i.e., carbogen). Measurements of BOLD and cerebral blood flow made on nine subjects during the different hypercapnia-inducing stimuli showed that each stimulus leads to a different calibration of the model. We argue that of the aforementioned stimuli, inspiration of 4% CO2, 21% O2 and balance N2 should be preferred for the calibration as the other stimuli produce responses that violate assumptions of the metabolic model.

Spatial and effector processing in the human parietofrontal network for reaches and saccades

It is generally accepted that interactions between parietal and frontal cortices subserve the visuomotor processing for eye and hand movements. Here, we used a sequential-instruction paradigm in 3-T functional MRI to test the processing of effector and spatial signals, as well as their interaction, as a movement is composed and executed in different stages. Subjects prepared either a saccade or a reach following two successive visual instruction cues, presented in either order. One cue instructed which effector to use (eyes, right hand); the other signaled the spatial goal (leftward vs. rightward target location) of the movement. During the first phase of the prepared movement, after cueing of either goal or effector information, we found significant spatial goal selectivity but no effector specificity along the parietofrontal network. During the second phase of the prepared movement, when both goal and effector information were available, we found a large overlap in the neural circuitry involved in the planning of eye and hand movements. Gradually distributed along this network, we observed clear spatial goal selectivity and limited, but significant, effector specificity. Regions in the intraparietal sulcus and the dorsal premotor cortex were selective to both goal location and motor effector. Taken together, our results suggest that the relative weight of spatial goal and effector selectivity changes along the parietofrontal network, depending on the status of the movement plan.

Metabolic and hemodynamic events after changes in neuronal activity: current hypotheses, theoretical predictions and in vivo NMR experimental findings

Unraveling the energy metabolism and the hemodynamic outcomes of excitatory and inhibitory neuronal activity is critical not only for our basic understanding of overall brain function, but also for the understanding of many brain disorders. Methodologies of magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) are powerful tools for the noninvasive investigation of brain metabolism and physiology. However, the temporal and spatial resolution of in vivo MRS and MRI is not suitable to provide direct evidence for hypotheses that involve metabolic compartmentalization between different cell types, or to untangle the complex neuronal microcircuitry, which results in changes of electrical activity. This review aims at describing how the current models of brain metabolism, mainly built on the basis of in vitro evidence, relate to experimental findings recently obtained in vivo by (1)H MRS, (13)C MRS, and MRI. The hypotheses related to the role of different metabolic substrates, the metabolic neuron-glia interactions, along with the available theoretical predictions of the energy budget of neurotransmission will be discussed. In addition, the cellular and network mechanisms that characterize different types of increased and suppressed neuronal activity will be considered within the sensitivity-constraints of MRS and MRI.

Cortical imaging of event-related (de)synchronization during online control of brain-computer interface using minimum-norm estimates in frequency domain

It is of wide interest to study the brain activity that correlates to the control of brain-computer interface (BCI). In the present study, we have developed an approach to image the cortical rhythmic modulation associated with motor imagery using minimum-norm estimates in the frequency domain (MNEFD). The distribution of cortical sources of mu activity during online control of BCI was obtained with the MNEFD. Contralateral decrease (event-related desynchronization) and ipsilateral increase (event-related synchronization) are localized in the sensorimotor cortex during online control of BCI in a group of human subjects. Statistical source analysis revealed that maximum correlation with movement imagination is localized in sensorimotor cortex.

Quantitative multifocal fMRI shows active suppression in human V1

Multifocal functional magnetic resonance imaging has recently been introduced as an alternative method for retinotopic mapping, and it enables effective functional localization of multiple regions-of-interest in the visual cortex. In this study we characterized interactions in V1 with spatially and temporally identical stimuli presented alone, or as a part of a nine-region multifocal stimulus. We compared stimuli at different contrasts, collinear and orthogonal orientations and spatial frequencies one octave apart. Results show clear attenuation of BOLD signal from the central region in the multifocal condition. The observed modulation in BOLD signal could be produced either by neural suppression resulting from stimulation of adjacent regions of visual field, or alternatively by hemodynamic saturation or stealing effects in V1. However, we find that attenuation of the central response persists through a range of contrasts, and that its strength varies with relative orientation and spatial frequency of the central and surrounding stimulus regions, indicating active suppression mechanisms of neural origin. Our results also demonstrate that the extent of the signal spreading is commensurate with the extent of the horizontal connections in primate V1.