Multimodal measurements of blood plasma and red blood cell volumes during functional brain activation

Mapping oxidative metabolism in the human brain with calibrated fMRI in health and disease

Conventional functional MRI (fMRI) with blood-oxygenation level dependent (BOLD) contrast is an important tool for mapping human brain activity non-invasively. Recent interest in quantitative fMRI has renewed the importance of oxidative neuroenergetics as reflected by cerebral metabolic rate of oxygen consumption (CMR_O2) to support brain function. Dynamic CMR_O2 mapping by calibrated fMRI require multi-modal measurements of BOLD signal along with cerebral blood flow (CBF) and/or volume (CBV). In human subjects this "calibration" is typically performed using a gas mixture containing small amounts of carbon dioxide and/or oxygen-enriched medical air, which are thought to produce changes in CBF (and CBV) and BOLD signal with minimal or no CMR_O2 changes. However non-human studies have demonstrated that the "calibration" can also be achieved without gases, revealing good agreement between CMR_O2 changes and underlying neuronal activity (e.g., multi-unit activity and local field potential). Given the simpler set-up of gas-free calibrated fMRI, there is evidence of recent clinical applications for this less intrusive direction. This up-to-date review emphasizes technological advances for such translational gas-free calibrated fMRI experiments, also covering historical progression of the calibrated fMRI field that is impacting neurological and neurodegenerative investigations of the human brain.

Auditory-visual convergence at the superior colliculus in rat using functional MRI

The superior colliculus (SC) of the midbrain has been a model structure for multisensory processing. Many neurons in the intermediate and deep SC layers respond to two or more of auditory, visual, and somatosensory stimuli as assessed by electrophysiology. In contrast, noninvasive and large field of view functional magnetic resonance imaging (fMRI) studies have focused on multisensory processing in the cortex. In this study, we applied blood oxygenation leveldependent (BOLD) fMRI on Sprague-Dawley rats receiving monaural (auditory) and binocular (visual) stimuli to study subcortical multisensory processing. Activation was observed in the left superior olivary complex, lateral lemniscus, and inferior colliculus and both hemispheres of the superior colliculus during auditory stimulation. The SC response was bilateral even though the stimulus was monaural. During visual stimulation, activation was observed in both hemispheres of the SC and lateral geniculate nucleus. In both hemispheres of the SC, the number of voxels in the activation area $( \mathrm {p}<10 -8$) and BOLD signal changes $( \mathrm {p}<0.01)$ were significantly greater during visual than auditory stimulation. These results provide functional imaging evidence that the SC is a site of auditoryvisual convergence due to its involvement in both auditory and visual processing. The auditory and visual fMRI activations likely reflect the firing of unisensory and multisensory neurons in the SC. The present study lays the groundwork for noninvasive functional imaging studies of multisensory convergence and integration in the SC.

Axial variation of deoxyhemoglobin density as a source of the low-frequency time lag structure in blood oxygenation level-dependent signals

Perfusion-related information is reportedly embedded in the low-frequency component of a blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) signal. The blood-propagation pattern through the cerebral vascular tree is detected as an interregional lag variation of spontaneous low-frequency oscillations (sLFOs). Mapping of this lag, or phase, has been implicitly treated as a projection of the vascular tree structure onto real space. While accumulating evidence supports the biological significance of this signal component, the physiological basis of the “perfusion lag structure,” a requirement for an integrative resting-state fMRI-signal model, is lacking. In this study, we conducted analyses furthering the hypothesis that the sLFO is not only largely of systemic origin, but also essentially intrinsic to blood, and hence behaves as a virtual tracer. By summing the small fluctuations of instantaneous phase differences between adjacent vascular regions, a velocity response to respiratory challenges was detected. Regarding the relationship to neurovascular coupling, the removal of the whole lag structure, which can be considered as an optimized global-signal regression, resulted in a reduction of inter-individual variance while preserving the fMRI response. Examination of the T2* and S₀, or non-BOLD, components of the fMRI signal revealed that the lag structure is deoxyhemoglobin dependent, while paradoxically presenting a signal-magnitude reduction in the venous side of the cerebral vasculature. These findings provide insight into the origin of BOLD sLFOs, suggesting that they are highly intrinsic to the circulating blood.

Advances in Imaging Brain Metabolism

Metabolism is central to neuroimaging because it can reveal pathways by which neuronal and glial cells use nutrients to fuel their growth and function. We focus on advanced magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) methods used in brain metabolic studies. ¹⁷O-MRS and ³¹P-MRS, respectively, provide rates of oxygen use and ATP synthesis inside mitochondria, whereas ¹⁹F-MRS enables measurement of cytosolic glucose metabolism. Calibrated functional MRI (fMRI), an advanced form of fMRI that uses contrast generated by deoxyhemoglobin, provides maps of oxygen use that track neuronal firing across brain regions. ¹³C-MRS is the only noninvasive method of measuring both glutamatergic neurotransmission and cell-specific energetics with signaling and nonsignaling purposes. Novel MRI contrasts, arising from endogenous diamagnetic agents and exogenous paramagnetic agents, permit pH imaging of glioma. Overall, these magnetic resonance methods for imaging brain metabolism demonstrate translational potential to better understand brain disorders and guide diagnosis and treatment.

Changes in effective diffusivity for oxygen during neural activation and deactivation estimated from capillary diameter measured by two-photon laser microscope

The relation between cerebral blood flow (CBF) and cerebral oxygen extraction fraction (OEF) can be expressed using the effective diffusivity for oxygen in the capillary bed (D) as OEF = 1 - exp(-D/CBF). The D value is proportional to the microvessel blood volume. In this study, changes in D during neural activation and deactivation were estimated from changes in capillary and arteriole diameter measured by two-photon microscopy in awake mice. Capillary and arteriole vessel diameter in the somatosensory cortex and cerebellum were measured under neural activation (sensory stimulation) and neural deactivation [crossed cerebellar diaschisis (CCD)], respectively. Percentage changes in D during sensory stimulation and CCD were 10.3 ± 7.3 and -17.5 ± 5.3 % for capillary diameter of <6 μm, respectively. These values were closest to the percentage changes in D calculated from previously reported human positron emission tomography data. This may indicate that thinner capillaries might play the greatest role in oxygen transport from blood to brain tissue.

Effect of electrical forepaw stimulation on capillary transit-time heterogeneity (CTH)

Functional hyperemia reduces oxygen extraction efficacy unless counteracted by a reduction of capillary transit-time heterogeneity of blood. We adapted a bolus tracking approach to capillary transit-time heterogeneity estimation for two-photon microscopy and then quantified changes in plasma mean transit time and capillary transit-time heterogeneity during forepaw stimulation in anesthetized mice (C57BL/6NTac). In addition, we analyzed transit time coefficient of variance = capillary transit-time heterogeneity/mean transit time, which we expect to remain constant in passive, compliant microvascular networks. Electrical forepaw stimulation reduced, both mean transit time (11.3% ± 1.3%) and capillary transit-time heterogeneity (24.1% ± 3.3%), consistent with earlier literature and model predictions. We observed a coefficient of variance reduction (14.3% ± 3.5%) during functional activation, especially for the arteriolar-to-venular passage. Such coefficient of variance reduction during functional activation suggests homogenization of capillary flows beyond that expected as a passive response to increased blood flow by other stimuli. This finding is consistent with an active neurocapillary coupling mechanism, for example via pericyte dilation. Mean transit time and capillary transit-time heterogeneity reductions were consistent with the relative change inferred from capillary hemodynamics (cell velocity and flux). Our findings support the important role of capillary transit-time heterogeneity in flow-metabolism coupling during functional activation.

Improved specific loss power on cancer cells by hyperthermia and MRI contrast of hydrophilic Fex Co1-x Fe2 O4 nanoensembles

Ferrite-based ferri/superparamagnetic nanoparticles can be rapidly heated by an external alternating magnetic field (AMF) to induce tissue necrosis of the adjacent microenvironment, but in addition provide magnetic resonance imaging (MRI) contrast utilizing enhanced water relaxivity. Here we characterized nanoensembles of Fe-Co mixed spinel ferrites (i.e. Fe_x Co_1-x Fe₂ O₄ , where x ranges from 0.2 to 0.8) synthesized by chemical co-precipitation. With nanoensembles of increasing Co content the saturation magnetization improved, while lattice parameter remained relatively constant. MRI water (transverse) relaxivity at 11.7 T was also boosted with increasing Co content. Efficiency of AMF-induced heating was quite comparable for the nanoensembles with either chitosan or polyethylene glycol (PEG) coating except for PEG-coated Fe_0.2 Co_0.8 Fe₂ O₄ , which was twice as less efficient as others. While toxicity of the nanoensembles with either coating examined on 9L tumor cell cultures showed no significant differences, upon AMF exposure (i.e. heat-induced necrosis) Fe_x Co_1-x Fe₂ O₄ composition with different values of x showed quite dramatic effects on cell death of tumor cells with both coatings. This study lays the ground work for further characterization of other mixed spinel ferrites, and in addition we expect that chitosan and PEG coated Fe_x Co_1-x Fe₂ O₄ of all the compositions will have good potential for preclinical applications in vivo. Copyright © 2016 John Wiley & Sons, Ltd.

Optical Coherence Tomography angiography reveals laminar microvascular hemodynamics in the rat somatosensory cortex during activation

The BOLD (blood-oxygen-level dependent) fMRI (functional Magnetic Resonance Imaging) signal is shaped, in part, by changes in red blood cell (RBC) content and flow across vascular compartments over time. These complex dynamics have been challenging to characterize directly due to a lack of appropriate imaging modalities. In this study, making use of infrared light scattering from RBCs, depth-resolved Optical Coherence Tomography (OCT) angiography was applied to image laminar functional hyperemia in the rat somatosensory cortex. After defining and validating depth-specific metrics for changes in RBC content and speed, laminar hemodynamic responses in microvasculature up to cortical depths of >1mm were measured during a forepaw stimulus. The results provide a comprehensive picture of when and where changes in RBC content and speed occur during and immediately following cortical activation. In summary, the earliest and largest microvascular RBC content changes occurred in the middle cortical layers, while post-stimulus undershoots were most prominent superficially. These laminar variations in positive and negative responses paralleled known distributions of excitatory and inhibitory synapses, suggesting neuronal underpinnings. Additionally, the RBC speed response consistently returned to baseline more promptly than RBC content after the stimulus across cortical layers, supporting a "flow-volume mismatch" of hemodynamic origin.

Quantitative basis for neuroimaging of cortical laminae with calibrated functional MRI

Layer-specific neurophysiologic, hemodynamic, and metabolic measurements are needed to interpret high-resolution functional magnetic resonance imaging (fMRI) data in the cerebral cortex. We examined how neurovascular and neurometabolic couplings vary vertically in the rat's somatosensory cortex. During sensory stimulation we measured dynamic layer-specific responses of local field potential (LFP) and multiunit activity (MUA) as well as blood oxygenation level-dependent (BOLD) signal and cerebral blood volume (CBV) and blood flow (CBF), which in turn were used to calculate changes in oxidative metabolism (CMR(O2)) with calibrated fMRI. Both BOLD signal and CBV decreased from superficial to deep laminae, but these responses were not well correlated with either layer-specific LFP or MUA. However, CBF changes were quite stable across laminae, similar to LFP. However, changes in CMR(O2) and MUA varied across cortex in a correlated manner and both were reduced in superficial lamina. These results lay the framework for quantitative neuroimaging across cortical laminae with calibrated fMRI methods.

Noninvasive fMRI investigation of interaural level difference processing in the rat auditory subcortex

Objective

Interaural level difference (ILD) is the difference in sound pressure level (SPL) between the two ears and is one of the key physical cues used by the auditory system in sound localization. Our current understanding of ILD encoding has come primarily from invasive studies of individual structures, which have implicated subcortical structures such as the cochlear nucleus (CN), superior olivary complex (SOC), lateral lemniscus (LL), and inferior colliculus (IC). Noninvasive brain imaging enables studying ILD processing in multiple structures simultaneously.

Methods

In this study, blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) is used for the first time to measure changes in the hemodynamic responses in the adult Sprague-Dawley rat subcortex during binaural stimulation with different ILDs.

Results and Significance

Consistent responses are observed in the CN, SOC, LL, and IC in both hemispheres. Voxel-by-voxel analysis of the change of the response amplitude with ILD indicates statistically significant ILD dependence in dorsal LL, IC, and a region containing parts of the SOC and LL. For all three regions, the larger amplitude response is located in the hemisphere contralateral from the higher SPL stimulus. These findings are supported by region of interest analysis. fMRI shows that ILD dependence occurs in both hemispheres and multiple subcortical levels of the auditory system. This study is the first step towards future studies examining subcortical binaural processing and sound localization in animal models of hearing.

Cerebral blood volume changes during brain activation

Cerebral blood volume (CBV) changes significantly with brain activation, whether measured using positron emission tomography, functional magnetic resonance imaging (fMRI), or optical microscopy. If cerebral vessels are considered to be impermeable, the contents of the skull incompressible, and the skull itself inextensible, task- and hypercapnia-related changes of CBV could produce intolerable changes of intracranial pressure. Because it is becoming clear that CBV may be useful as a well-localized marker of neural activity changes, a resolution of this apparent paradox is needed. We have explored the idea that much of the change in CBV is facilitated by exchange of water between capillaries and surrounding tissue. To this end, we developed a novel hemodynamic boundary-value model and found approximate solutions using a numerical algorithm. We also constructed a macroscopic experimental model of a single capillary to provide biophysical insight. Both experiment and theory model capillary membranes as elastic and permeable. For a realistic change of input pressure, a relative pipe volume change of 21±5% was observed when using the experimental setup, compared with the value of approximately 17±1% when this quantity was calculated from the mathematical model. Volume, axial flow, and pressure changes are in the expected range.

Arteries dominate volume changes during brief functional hyperemia: evidence from mathematical modelling

Variations in local neural activity are accompanied by rapid, focal changes in cerebral blood flow and volume. While a range of observations have shown that dilation occurs in cerebral arteries, there is conflicting evidence about the significance of volume changes in post-arteriole vessels. Here, we reconcile the competing observations using a new mathematical model of the hemodynamic response. First, we followed a 'top down' approach, without constraining the model, but using experimental observations at progressively more detailed scales to ensure physiological behaviour. Then, we blocked dilation of post-arteriole vessels, and predicted observations at progressively more aggregated scales (a 'bottom up' approach). Predictions of blood flow, volume, velocity, and vessel diameter changes were consistent with experimental observations. Interestingly, the model predicted small, slow increases in capillary and venous diameter in agreement with recent in vivo data. Blocking dilation in these vessels led to erroneous volume predictions. The results are further evidence that arteries make up the majority of blood volume increases during brief functional activation. However, dilation of capillaries and veins appears to be increasingly significant during extended stimulation. These are important considerations when interpreting results from different neurovascular imaging modalities.

IRON fMRI measurements of CBV and implications for BOLD signal

Changes in cerebral blood volume (CBV) and blood magnetization each induce changes in the transverse relaxation rate of MRI signal that are associated with changes in cerebral activity. BOLD signal, the preeminent method for non-invasive localization of task-induced brain function in human subjects, reflects a combination of changes in CBV and blood magnetization. Intravenous injection of paramagnetic contrast media, usually iron oxide particles surrounded by larger macromolecules, can overwhelm the BOLD response and sensitize signal to blood plasma volume, a method we have deemed "IRON" fMRI. The practical advantage of this technique is the ability to optimize blood magnetization at any echo time, enabling high detection power and the use of short echo times; for these reasons, IRON fMRI has become a valuable imaging tool in animal models. The temporal response of blood plasma volume is quite different from blood flow and BOLD signal; thus, CBV has been identified as a prominent source of transient features of the BOLD response. This article reviews the methodological advantages of the IRON method and how CBV measurements have informed our understanding of the BOLD response.

Functional micro-ultrasound imaging of rodent cerebral hemodynamics

Healthy cerebral microcirculation is crucial to neuronal functioning. We present a new method to investigate microvascular hemodynamics in living rodent brain through a focal cranial window based on high-frequency ultrasound imaging. The method has a temporal resolution of 40ms, and a 100μm in-plane and 600μm through-plane spatial resolution. We use a commercially available high-frequency ultrasound imaging system to quantify changes in the relative cerebral blood volume (CBV) by measuring the scattered signal intensity from an ultrasound contrast agent circulating in the vasculature. Generalized linear model analysis is then used to produce effect size and significance maps of changes in cerebral blood volume upon electrical stimulation of the forepaw. We observe larger CBV increases in the forelimb representation of the primary somatosensory cortex than in the deep gray matter with stimuli as short as 2s (5.1 ± 1.3% vs. 3.3 ± 0.6%). We also investigate the temporal evolution of the blood volume changes in cortical and subcortical gray matter, pial vessels and subcortical major vessels, and show shorter response onset times in the parenchymal regions than in the neighboring large vessels (1.6 ± 1.0s vs. 2.6 ± 1.3s in the cortex for a 10 second stimulus protocol). This method, which we termed functional micro-ultrasound imaging or fMUS, is a novel, highly accessible, and cost-effective way of imaging rodent brain microvascular topology and hemodynamics in vivo at 100micron resolution over a 1-by-1cm field of view with 10s-100s frames per second that opens up a new set of questions regarding brain function in preclinical models of health and disease.

Longitudinal functional magnetic resonance imaging in animal models

Functional magnetic resonance imaging (fMRI) has had an essential role in furthering our understanding of brain physiology and function. fMRI techniques are nowadays widely applied in neuroscience research, as well as in translational and clinical studies. The use of animal models in fMRI studies has been fundamental in helping elucidate the mechanisms of cerebral blood-flow regulation, and in the exploration of basic neuroscience questions, such as the mechanisms of perception, behavior, and cognition. Because animals are inherently non-compliant, most fMRI performed to date have required the use of anesthesia, which interferes with brain function and compromises interpretability and applicability of results to our understanding of human brain function. An alternative approach that eliminates the need for anesthesia involves training the animal to tolerate physical restraint during the data acquisition. In the present chapter, we review these two different approaches to obtaining fMRI data from animal models, with a specific focus on the acquisition of longitudinal data from the same subjects.

Neurovascular and Neurometabolic Couplings in Dynamic Calibrated fMRI: Transient Oxidative Neuroenergetics for Block-Design and Event-Related Paradigms

Functional magnetic resonance imaging (fMRI) with blood-oxygenation level dependent (BOLD) contrast is an important tool for mapping brain activity. Interest in quantitative fMRI has renewed awareness in importance of oxidative neuroenergetics, as reflected by cerebral metabolic rate of oxygen consumption(CMRO2), for supporting brain function. Relationships between BOLD signal and the underlying neurophysiological parameters have been elucidated to allow determination of dynamic changes inCMRO2 by "calibrated fMRI," which require multi-modal measurements of BOLD signal along with cerebral blood flow (CBF) and volume (CBV). But how doCMRO2 changes, steady-state or transient, derived from calibrated fMRI compare with neural activity recordings of local field potential (LFP) and/or multi-unit activity (MUA)? Here we discuss recent findings primarily from animal studies which allow high magnetic fields studies for superior BOLD sensitivity as well as multi-modal CBV and CBF measurements in conjunction with LFP and MUA recordings from activated sites. A key observation is that while relationships between neural activity and sensory stimulus features range from linear to non-linear, associations between hyperemic components (BOLD, CBF, CBV) and neural activity (LFP, MUA) are almost always linear. More importantly, the results demonstrate good agreement between the changes inCMRO2 and independent measures of LFP or MUA. The tight neurovascular and neurometabolic couplings, observed from steady-state conditions to events separated by <200 ms, suggest rapid oxygen equilibration between blood and tissue pools and thus calibrated fMRI at high magnetic fields can provide high spatiotemporal mapping ofCMRO2 changes.

A dynamic model of neurovascular coupling: implications for blood vessel dilation and constriction

Neurovascular coupling in response to stimulation of the rat barrel cortex was investigated using concurrent multichannel electrophysiology and laser Doppler flowmetry. The data were used to build a linear dynamic model relating neural activity to blood flow. Local field potential time series were subject to current source density analysis, and the time series of a layer IV sink of the barrel cortex was used as the input to the model. The model output was the time series of the changes in regional cerebral blood flow (CBF). We show that this model can provide excellent fit of the CBF responses for stimulus durations of up to 16 s. The structure of the model consisted of two coupled components representing vascular dilation and constriction. The complex temporal characteristics of the CBF time series were reproduced by the relatively simple balance of these two components. We show that the impulse response obtained under the 16-s duration stimulation condition generalised to provide a good prediction to the data from the shorter duration stimulation conditions. Furthermore, by optimising three out of the total of nine model parameters, the variability in the data can be well accounted for over a wide range of stimulus conditions. By establishing linearity, classic system analysis methods can be used to generate and explore a range of equivalent model structures (e.g., feed-forward or feedback) to guide the experimental investigation of the control of vascular dilation and constriction following stimulation.

Cerebral oxygen demand for short-lived and steady-state events

Because of the importance of oxidative energetics for cerebral function, extraction of oxygen consumption (CMR(O2)) from blood oxygenation level-dependent (BOLD) signal using multi-modal measurements of blood flow (CBF) and volume (CBV) has become an accepted functional magnetic resonance imaging (fMRI) technique. This approach, termed calibrated fMRI, is based on a biophysical model which describes tissue oxygen extraction at steady-state. A problem encountered for calculating dynamic CMR(O2) relates to concerns whether the conventional BOLD model can be applied transiently. In particular, it is unclear whether calculation of CMR(O2) differs between short and long stimuli. Linearity was experimentally demonstrated between BOLD-related components and neural activity, thereby making it possible to use calibrated fMRI in a dynamic manner. We used multi-modal fMRI and electrophysiology, in alpha-chloralose anesthetized rats during forepaw stimulation to show that respective transfer functions (of BOLD, CBV, CBF) generated by deconvolution with neural activity are time invariant, for events in the millisecond to minute range. These results allowed extraction of a significant component of the BOLD signal that can be ascribed to CMR(O2) transients. We discuss the importance of minimizing residual signal, represented by the difference between modeled and raw signals, in convolution analysis of multi-modal signals.

A time-invariant visco-elastic windkessel model relating blood flow and blood volume

The difference between the rate of change of cerebral blood volume (CBV) and cerebral blood flow (CBF) following stimulation is thought to be due to circumferential stress relaxation in veins (Mandeville, J.B., Marota, J.J.A., Ayata, C., Zaharchuk, G., Moskowitz, M.A., Rosen, B.R., Weisskoff, R.M., 1999. Evidence of a cerebrovascular postarteriole windkessel with delayed compliance. J. Cereb. Blood Flow Metab. 19, 679-689). In this paper we explore the visco-elastic properties of blood vessels, and present a dynamic model relating changes in CBF to changes in CBV. We refer to this model as the visco-elastic windkessel (VW) model. A novel feature of this model is that the parameter characterising the pressure-volume relationship of blood vessels is treated as a state variable dependent on the rate of change of CBV, producing hysteresis in the pressure-volume space during vessel dilation and contraction. The VW model is nonlinear time-invariant, and is able to predict the observed differences between the time series of CBV and that of CBF measurements following changes in neural activity. Like the windkessel model derived by Mandeville, J.B., Marota, J.J.A., Ayata, C., Zaharchuk, G., Moskowitz, M.A., Rosen, B.R., Weisskoff, R.M., 1999. Evidence of a cerebrovascular postarteriole windkessel with delayed compliance. J. Cereb. Blood Flow Metab. 19, 679-689, the VW model is primarily a model of haemodynamic changes in the venous compartment. The VW model is demonstrated to have the following characteristics typical of visco-elastic materials: (1) hysteresis, (2) creep, and (3) stress relaxation, hence it provides a unified model of the visco-elastic properties of the vasculature. The model will not only contribute to the interpretation of the Blood Oxygen Level Dependent (BOLD) signals from functional Magnetic Resonance Imaging (fMRI) experiments, but also find applications in the study and modelling of the brain vasculature and the haemodynamics of circulatory and cardiovascular systems.

Oxidative neuroenergetics in event-related paradigms

Energetic basis of neural activity provides a solid foundation for noninvasive neuroimaging with calibrated functional magnetic resonance imaging (fMRI). Calculating dynamic changes in cerebral oxidative energy utilization (CMR(O(2))) is limited by uncertainties about whether or not the conventional blood oxygenation level-dependent (BOLD) model can be applied transiently using multimodal measurements of blood flow (CBF) and volume (CBV) that affect the BOLD signal. A prerequisite for dynamic calibrated fMRI is testing the linearity of multimodal signals within a temporal regimen, as assessed by signal strength (i.e., both intensity and width). If each hyperemic component (BOLD, CBV, CBF) is demonstrated to be linear with neural activity under various experimental conditions, then the respective transfer functions generated by deconvolution with neural activity should be time invariant and thus could potentially be used for calculating CMR(O(2)) transients. Hyperemic components were investigated at 11.7 T in alpha-chloralose-anesthetized rats and combined with electrophysiological recordings of local field potential (LFP) and multiunit activity (MUA) from the cortex during forepaw stimulation, in which stimulus number and frequency were varied. Although relationships between neural activity and stimulus features ranged from linear to nonlinear, associations between hyperemic components and neural activity were linear. Specific to each hyperemic component, a universal transfer function (with LFP or MUA) yielded predictions in agreement with experimental measurements. The results identified a component of the BOLD signal that can be attributed to significant changes in CMR(O(2)), even for temporal events separated by <200 ms.

Remote effects of focal hippocampal seizures on the rat neocortex

Seizures have both local and remote effects on nervous system function. Whereas propagated seizures are known to disrupt cerebral activity, little work has been done on remote network effects of seizures that do not propagate. Human focal temporal lobe seizures demonstrate remote changes including slow waves on electroencephalography (EEG) and decreased cerebral blood flow (CBF) in the neocortex. Ictal neocortical slow waves have been interpreted as seizure propagation; however, we hypothesize that they reflect a depressed cortical state resembling sleep or coma. To investigate this hypothesis, we performed multimodal studies of partial and secondarily generalized limbic seizures in rats. Video/EEG monitoring of spontaneous seizures revealed slow waves in the frontal cortex during behaviorally mild partial seizures, contrasted with fast polyspike activity during convulsive generalized seizures. Seizures induced by hippocampal stimulation produced a similar pattern, and were used to perform functional magnetic resonance imaging weighted for blood oxygenation and blood volume, demonstrating increased signals in hippocampus, thalamus and septum, but decreases in orbitofrontal, cingulate, and retrosplenial cortex during partial seizures, and increases in all of these regions during propagated seizures. Combining these results with neuronal recordings and CBF measurements, we related neocortical slow waves to reduced neuronal activity and cerebral metabolism during partial seizures, but found increased neuronal activity and metabolism during propagated seizures. These findings suggest that ictal neocortical slow waves represent an altered cortical state of depressed function, not propagated seizure activity. This remote effect of partial seizures may cause impaired cerebral functions, including loss of consciousness.