Development of margination of platelet-sized particles in red blood cell suspensions flowing through Y-shaped bifurcating microchannels.

BACKGROUND: In the blood flow through microvessels, platelets exhibit enhanced concentrations in the layer free of red blood cells (cell-free layer) adjacent to the vessel wall. The motion of platelets in the cell-free layer plays an essential role in their interaction with the vessel wall, and hence it affects their functions of hemostasis and thrombosis. OBJECTIVE: We aimed to estimate the diffusivity of platelet-sized particles in the transverse direction (the direction of vorticity) across the channel width in the cell-free layer by in vitro experiments for the microchannel flow of red blood cell (RBC) suspensions containing platelet-sized particles. METHODS: Fluorescence microscope observations were performed to measure the transverse distribution of spherical particles immersed in RBC suspensions flowing through a Y-shaped bifurcating microchannel. We examined the development of the particle concentration profiles along the flow direction in the daughter channels, starting from asymmetric distributions with low concentrations on the inner side of the bifurcation at the inlet of the daughter channels. RESULTS: In daughter channels of 40 µm width, reconstruction of particle margination revealed that a symmetric concentration profile was attained in ∼30 mm from the bifurcation, independent of flow rate. CONCLUSIONS: We presented experimental evidence of particle margination developing in a bifurcating flow channel where the diffusivity of 2.9-µm diameter particles was estimated to be ∼40 µm2/s at a shear rate of 1000 s-1 and hematocrit of 0.2.

Cross-sectional distributions of normal and abnormal red blood cells in capillary tubes determined by a new technique.

BACKGROUND: In the microcirculation, red blood cells (RBCs) were observed to be confined to an axial stream surrounded by a marginal RBC depleted layer. This axial accumulation of RBCs is considered to arise from the RBC deformability. OBJECTIVE: To quantitatively evaluate the effect of RBC deformability on their axial accumulation at a flow condition comparable to that in arterioles by developing a new observation system for accurate measurements of radial RBC positions in the cross section of capillary tubes. METHODS: The cross-sectional distributions of normal and hardened RBCs as well as softened RBCs suspended in capillary tube flows were measured with high spatial resolution. A new observation system was developed in which enface views of the cross-section of the tube were obtained at small distances upstream of the outlet at various longitudinal positions in the tube. RESULTS: The radial positions of individual RBCs were detected within 1 µm accuracy. It was found that normal and softened RBCs rapidly migrated away from the wall towards the tube axis, whereas glutaraldehyde-hardened RBCs were dispersed widely over the tube cross-section, depending on the concentration of glutaraldehyde solution. CONCLUSIONS: The newly devised observation system revealed quantitatively the essential role of RBC deformability in their axial accumulation.

11.7 T MR Imaging Revealed Dilatation of Virchow-Robin Spaces within Hippocampus in Maternally Lipopolysaccharide-exposed Rats.

PURPOSE: 11.7 Tesla MRI was examined to detect Virchow-Robin spaces (VRSs) smaller than 100 µm in the rat brain. The effects of maternal exposure to lipopolysaccharide (LPS) were evaluated on basis of the number of dilated VRSs in the offspring rat brain. METHODS: T2-weighted MRI with an in-plane resolution up to 78 µm (repetition time = 5000 ms, echo time = 35 ms, slice thickness = 250 µm, imaging plane, coronal) was applied to identify VRSs. The dilated VRSs were counted in the rat brain at 5 and 10 weeks of age. The dams of half the number in each group were treated with LPS during pregnancy, and the remaining half was employed as control. LPS injection in gestation period was used to simulate maternal infections, the method of which was widely accepted as a rat model inducing neuropsychiatric disorders in the offspring. Effects of LPS exposure on the offspring rat brain were statistically investigated. RESULTS: VRSs as small as 78 µm were successfully detected by the ultra high-field MRI. All dilated VRSs were observed within lacunosum molecular layer of hippocampus, and molecular and granular layers of dentate gyrus around hippocampal fissure. In juvenile rats (5 weeks of age), the number of dilated VRSs was significantly increased in the prenatal LPS exposed rat brain (12.9 ± 2.4, n = 7) than in the control (5.3 ± 1.5, n = 7, P < 0.05), while in young adult rats (10 weeks of age), there was no significant difference in the number between the prenatal LPS exposed rat brain (3.6 ± 0.9, n = 5) and the control (2.6 ± 0.4, n = 5). CONCLUSION: The results of the present study suggested that maternal infection might cause dilatation of VRSs through neural damages especially in the dentate gyrus of the offspring rats. Thus, ultra high-field MRI can offer a promising diagnostic tool capable of determining the location of neonatal brain damage caused by maternal infections.

In vivo magnetic resonance imaging at 11.7 Tesla visualized the effects of neonatal transection of infraorbital nerve upon primary and secondary trigeminal pathways in rats.

Using 11.7T ultra high-field T2-weighted MRI, the present study aimed to investigate pathological changes of primary and secondary trigeminal pathways following neonatal transection of infraorbital nerve in rats. The trigeminal pathways consist of spinal trigeminal tract, trigeminal sensory nuclear complex, medial lemniscus, ventromedial portion of external medullary lamina and ventral posterior nucleus of thalamus. By selecting optimum parameters of MRI such as repetition time, echo time, and slice orientation, this study visualized the trigeminal pathways in rats without any contrast agents. Pathological changes due to the nerve transection were found at 8 weeks of age as a marked reduction of the areas of the trigeminal pathways connecting from the injured nerve. In addition, T2-weighted MR images of the trigeminal nerve trunk and the spinal trigeminal tract suggest a communication of CSF through the trigeminal nerve between the inside and outside of the brain stem. These results support the utility of ultra high-field MRI system for noninvasive assessment of effects of trigeminal nerve injury upon the trigeminal pathways.

Infrared thermal imaging of rat somatosensory cortex with whisker stimulation.

The present study aims to validate the applicability of infrared (IR) thermal imaging for the study of brain function through experiments on the rat barrel cortex. Regional changes in neural activity within the brain produce alterations in local thermal equilibrium via increases in metabolic activity and blood flow. We studied the relationship between temperature change and neural activity in anesthetized rats using IR imaging to visualize stimulus-induced changes in the somatosensory cortex of the brain. Sensory stimulation of the vibrissae (whiskers) was given for 10 s using an oscillating whisker vibrator (5-mm deflection at 10, 5, and 1 Hz). The brain temperature in the observational region continued to increase significantly with whisker stimulation. The mean peak recorded temperature changes were 0.048 ± 0.028, 0.054 ± 0.036, and 0.097 ± 0.015°C at 10, 5, and 1 Hz, respectively. We also observed that the temperature increase occurred in a focal spot, radiating to encompass a larger region within the contralateral barrel cortex region during single-whisker stimulation. Whisker stimulation also produced ipsilateral cortex temperature increases, which were localized in the same region as the pial arterioles. Temperature increase in the barrel cortex was also observed in rats treated with a calcium channel blocker (nimodipine), which acts to suppress the hemodynamic response to neural activity. Thus the location and area of temperature increase were found to change in accordance with the region of neural activation. These results indicate that IR thermal imaging is viable as a functional quantitative neuroimaging technique.

Impaired endothelial function may be due to decreased aortic tetrahydrobiopterin, assessed by a new flow-mediated vasodilation in vivo in hypercholesterolemic/atherogenic mice.

Tetrahydrobiopterin (BH4) is an important cofactor for endothelial nitric oxide synthase activity. The relationship between endothelial function in vivo and aortic BH4 level is not fully understood, however. In the present study, we aimed to clarify whether reduction of aortic BH4 levels contributes to endothelial dysfunction in vivo using spontaneously hyperlipidemic mice. To estimate endothelial function in vivo and in real-time state, we developed a flow-mediated vasodilation (FMV) method in mice, which measured changes in the diameter of the femoral artery in response to increased blood flow. C57BL/6 mice and apoE/low-density lipoprotein receptor double knock-out mice were fed a low-fat diet (LFD) or a high-fat diet (HFD) for 12 weeks from 6 weeks of age. HFD feeding impaired FMV in double knock-out mice, but not in C57BL/6 mice. Furthermore, HFD feeding reduced plasma NOx concentration and aortic BH4 level in double knock-out mice. Conversely, exogenous injection of BH4 (2 mg/kg) markedly increased aortic BH4 levels and restored endothelial function. In conclusion, we demonstrated that HFD feeding impaired nitric oxide-mediated endothelial function and reduced BH4 level in vivo, and that acute augmentation of aortic BH4 levels improved endothelial function. These findings indicate that BH4 is a critical determinant of nitric oxide-mediated endothelial function in hypercholesterolemia.

Gd3+-functionalized near-infrared quantum dots for in vivo dual modal (fluorescence/magnetic resonance) imaging.

Gd(3+)-functionalized near-infrared emitting quantum dots were synthesized as a dual modal contrast agent for in vivo fluorescence imaging and magnetic resonance imaging.

Preparation and characterization of highly fluorescent, glutathione-coated near infrared quantum dots for in vivo fluorescence imaging.

Fluorescent probes that emit in the near-infrared (NIR, 700-1,300 nm) region are suitable as optical contrast agents for in vivo fluorescence imaging because of low scattering and absorption of the NIR light in tissues. Recently, NIR quantum dots (QDs) have become a new class of fluorescent materials that can be used for in vivo imaging. Compared with traditional organic fluorescent dyes, QDs have several unique advantages such as size- and composition-tunable emission, high brightness, narrow emission bands, large Stokes shifts, and high resistance to photobleaching. In this paper, we report a facile method for the preparation of highly fluorescent, water-soluble glutathione (GSH)-coated NIR QDs for in vivo imaging. GSH-coated NIR QDs (GSH-QDs) were prepared by surface modification of hydrophobic CdSeTe/CdS (core/shell) QDs. The hydrophobic surface of the CdSeTe/CdS QDs was exchanged with GSH in tetrahydrofuran-water. The resulting GSH-QDs were monodisperse particles and stable in PBS (phosphate buffered saline, pH = 7.4). The GSH-QDs (800 nm emission) were highly fluorescent in aqueous solutions (quantum yield = 22% in PBS buffer), and their hydrodynamic diameter was less than 10 nm, which is comparable to the size of proteins. The cellular uptake and viability for the GSH-QDs were examined using HeLa and HEK 293 cells. When the cells were incubated with aqueous solutions of the GSH-QDs (10 nM), the QDs were taken into the cells and distributed in the perinuclear region of both cells. After 12 hrs incubation of 4 nM of GSH-QDs, the viabilities of HeLa and HEK 293 cells were ca. 80 and 50%, respectively. As a biomedical utility of the GSH-QDs, in vivo NIR-fluorescence imaging of a lymph node in a mouse is presented.

Long-term effects of oral vitamin C supplementation on the endothelial dysfunction in the iris microvessels of diabetic rats.

The current study was aimed to investigate effects of long-term supplementation of vitamin C on the iris microcirculation in streptozotocin (STZ)-induced diabetic rats. Diabetes was induced in male Wistar-Furth rats by intravenous injection of STZ (55 mg/kg b.w.). The rats were divided into three groups: control rats (CON), STZ-induced diabetic rats (STZ), and STZ rats supplemented with vitamin C (STZ-vitC). For supplementation of vitamin C, ascorbic acid (1 g/l) was added into the drinking water. The experiments were performed at different periods (8, 12, 24 and 36 weeks) after injection of STZ. Blood glucose, tissue lipid peroxidation and plasma vitamin C were measured. To examine the endothelial function, leukocyte adhesion to the venular endothelium was evaluated in the iris post-capillaries by means of counting the number of leukocytes labeled with rhodamine 6G. Blood flow perfusion in the iris was monitored using a laser Doppler flow meter. In the STZ rats, hyperglycemia was induced with an increase in HbA(1c) and lipid peroxidation but with a decrease in the plasma vitamin C level which improved by vitamin C supplementation. The number of adherent leukocytes increased significantly, associated with reduction in the iris blood flow perfusion, at 8, 12, 24 and 36 weeks after injection of STZ. In the STZ-vitC rats, the iris blood flow perfusion was significantly increased in comparison with the STZ rats, while the leukocyte adhesion was decreased at 24 and 36 weeks. The statistical analysis shows that the leukocyte adhesion decreased with increase in the iris blood flow perfusion in STZ and STZ-vitC rats. In conclusion, vitamin supplementation suppressed leukocyte adhesion and thus endothelial dysfunction, associated with increase in iris blood flow perfusion in diabetes. The antioxidant vitamin C may be a therapeutic agent for preventing diabetic retinopathy.

Ischemic vasoconstriction and tissue energy metabolism during global cerebral ischemia in gerbils.

Vasoconstriction is known to occur in cerebral arterioles during ischemia and considered to be distinct from vasospasm seen after subarachnoid hemorrhage. To elucidate the mechanism and functional significance underlying ischemic vasoconstriction, we investigated the relationship between arteriolar constriction and tissue energy metabolism during bilateral common carotid artery occlusion in gerbils. Using video microscopy and microspectroscopy, the arteriolar caliber, the total hemoglobin (Hb) content, and the redox state of cytochrome oxidase (cyt.aa3) were monitored in the cerebral cortex in vivo. After in situ freezing of the brain, adenine nucleotides, creatine phosphate (P-Cr), and lactate levels were analyzed using high-performance liquid chromatography in vitro. Tissue damage was also assessed immunohistochemically using antibodies against microtubule-associated proteins. There was a slight reduction of the diameter of pial arterioles during the initial 1 min of ischemia. A rapid decline of total Hb and reduction of cyt.aa3 were observed with rapid decreases of P-Cr and ATP in the cortical tissue during the initial 0.5 min, but all of them showed tendencies to return toward preischemic levels at 0.5-1 min. Beyond 1.5 min, extensive vasoconstriction occurred together with further decline of total Hb, reduction of cyt.aa3, and decreases of ATP and P-Cr. Neuronal damage developed in the cerebral cortex immunohistochemically beyond 3 min. The present investigation demonstrated two phases of vasoconstriction with the possibilities that the immediate vasoconstriction likely contributed to transient improvement of cortical oxygen/energy metabolism, and the second extensive vasoconstriction was an index of tissue energy failure and imminent neuronal damage.

Noncontact backscatter-mode near-infrared time-resolved imaging system: Preliminary study for functional brain mapping.

To improve the spatial resolution and to obtain the depth information of absorbers buried in highly scattering material, we developed a noncontact backscatter-mode near-infrared time-resolved imaging system (noncontact B-TRIS) that is intended for functional human brain mapping. It consists of mode-locked Ti-sapphire lasers as light sources and a charge-coupled device camera equipped with a time-resolved intensifier as a detector. The system was tested with a white polyacetal phantom as a light-scattering medium and black polyacetal particles as absorbers. Illumination and detection of light through an objective lens system (phi = 150 mm) enabled us to capture images from an area whose diameter is about 70 mm without coming into contact with it. The scattering and absorption coefficients of the white phantom obtained by B-TRIS were similar to those obtained by a conventional time-resolved spectroscopy. Although the imaged diameter of an absorber buried within a phantom was considerably larger than the actual diameter, the center position of the absorber coincided with the actual position with accuracy <2 mm. Furthermore, the depth information can be also detected by the noncontact B-TRIS. These results suggest a potential of noncontact B-TRIS for imaging cognitive human brain function.

Velocity profiles in the rat cerebral microvessels measured by optical coherence tomography.

In order to analyze cerebral hemodynamics and its change following neural activation, the cross-sectional profiles of blood flow velocity in the rat pial microvessels and their temporal changes were measured in vivo using Doppler OCT technique (Doppler optical coherence tomography). The OCT system used in this study has axial resolution of 11 microm and lateral resolution about 14 microm in the cortical tissue. The velocity distributions along the vertical diameter of pial microvessels in a cranial window of the rats were measured at short time intervals by scanning the OCT sampling point repeatedly. The velocity profiles obtained in the pial arterioles were parabolic at any phase, although the centerline velocity pulsated following heart beats with amplitude as large as 50% of the temporal mean velocity. It indicates that the blood flow in the pial microvessels is a quasi-steady laminar flow, which is consistent with the flow expected for the case of a small Reynolds number and a small frequency parameter. The stimulus-induced increase in velocity pulsation was much larger than the increase in the mean velocity, which places a restriction on the mechanism of regulating the regional cerebral blood flow and blood volume. The results obtained in this study showed that the Doppler OCT has a potential of measuring velocity profiles and their temporal changes with both high temporal and spatial resolutions for the pial microvessels with diameter up to 200 microm.

Microembolic flow disturbances in the cerebral microvasculature with an arcadal network: a numerical simulation.

Flow disturbance due to microembolism in the cerebral microvasculature with an arcadal network was studied by a numerical simulation. A mathematical model for flow in the arcadal network was developed, based on in vivo data of cat cerebral microvasculature and flow velocity. The network model consisted of 45 vessel segments, and 25 branching points. To simulate microvascular responses to blood flow, the following three types of responses to wall shear stress were considered; non-reactive (solid-like), cerebral arteriole, and skeletal muscle arteriole-like responses. The numerical calculation was carried out in the condition where a feeding arteriole was occlused. Flow changes in efferent vessels were evaluated for assessment of blood supply to the local area of cerebral tissue. The present simulation has demonstrated that blood flow in efferent vessels was influenced by the topology of the vascular network and the response pattern in single vessels. The arcadal structure of arterioles might be most effective in response to flow disturbances in efferent vessels.

Optical coherence tomography reveals in vivo cortical plasticity of adult mice in response to peripheral neuropathic pain.

We examined neural plasticity in mice in vivo using optical coherence tomography (OCT) of primary somatosensory (S1) and motor (M1) cortices of mice under the influence of sciatic nerve chronic constriction injury (CCI), a model of neuropathic pain widely utilized in rats. The OCT system used in this study provided cross-sectional images of the cortical tissue of mice up to a depth of about 1mm with longitudinal resolution up to 11 microm. This is the first study to evaluate neural plasticity in vivo using OCT. CCI mice exhibited cold allodynia and spontaneous pain behaviors, which are signs of neuropathic pain, 30 days after sciatic nerve ligation, when OCT observation of S1 and M1 cortices was carried out. The scattering intensity of near-infrared light within the hind paw area of S1 and M1 regions in the contralateral hemisphere was significantly higher than in the ipsilateral hemisphere. These CCI-induced increases in scattering intensity within cortical regions associated with the hind paw probably reflect elevated neural activity associated with neuropathic pain. Synapses and mitochondria are believed to have high light scattering coefficients, since they contain remarkably high concentrations of proteins and complicated membrane structure. Number densities of mitochondria and synapses are known to increase in parallel with increases in neural activity. Our findings thus suggest that neuropathic pain gives rise to neural plasticity within the hind paw area of S1 and M1 contralateral to the ligated sciatic nerve.

Microvascular hemodynamic responses to arteriovenous shunting in rat limb.

Autologus veins have been used clinically as a bypass conduit for reconstruction of small arteries, but there are few data available for microvascular response to arteriovenous (AV) shunting. This study was aimed to evaluate microvascular hemodynamic changes induced by creating AV anastomosis in rat hindlimb. Using intravital fluorescence videomicroscopy, we measured velocities of red blood cells (RBCs) flowing in the microvascular network in the control state, in the occlusion state where the superficial femoral artery (SFA) was occluded, and in the AV shunting state where the AV anastomosis was opened after occlusion of SFA. RBC velocities were measured in 155 capillaries of 6 rats using a dual window method and a frame-by-frame technique. The mean velocity and the coefficient of variation were 0.61 mm/sec and 0.90 in the control state, 0.34 mm/sec and 1.30 in the occlusion state, 0.83 mm/sec and 1.24 in the AV shunting state, respectively. These indicated that hemodynamic heterogeneity among capillaries increased with decrease in mean velocity following the arterial occlusion, while the AV shunting augmented the heterogeneity with increase in mean velocity. Capillaries with low perfusion (<0.1 mm/sec) or high perfusion (>1.0 mm/sec) were 5.8% or 20.6%, 29.6 or 5.2%, and 22.6 or 30.3% out of all measured capillaries in the control, occlusion and AV shunting conditions, respectively. In conclusion, AV shunting increased capillary perfusion and also its spatial heterogeneity, preferentially inducing high velocity in the microvasculature.

Heterogeneity of capillary flow in the retrograde microcirculation induced in rat limb by arteriovenous shunting.

Arteriovenous (AV) fistulas have been used clinically for improving adjunctive bypass patency. Such AV shunting induces retrograde flow in the microvascular network, which may induce microvascular remodeling and angiogenesis at the chronic phase. This paper was aimed to examine heterogeneity of blood flow among capillaries in the retrograde microcirculation induced by AV shunting. An AV anastomosis was created in rat hind limb. Using a dual window method or frame-by-frame technique on the fluorescence microscopic video images, we measured velocities of red blood cells (RBCs) flowing in the capillary network in three flow conditions: control (normal flow), arterial occlusion, and AV shunting (retrograde flow). For each flow condition, RBC velocities were obtained in 155 capillaries of 6 rats. By classifying all the capillaries into four groups based on the levels of RBC velocity in the occlusion state, we evaluated the mean velocities, coefficient of variation (CV), and histograms for each group of capillaries. The mean velocity and CV in each group changed significantly from the control to AV shunting states. Especially, most significant changes appeared in capillary groups where the superficial femoral artery or its collateral arteries might have a direct influence. Though the AV shunting improved capillary perfusion in the mean level, major parts of capillaries still remained at low perfusion.

Cell morphological changes in venous remodeling induced by arteriovenous grafting in rat limb.

Vascular remodeling induced in rat limb by arteriovenous (AV) shunting was investigated by evaluating changes in vascular diameter and cell morphology. In Wistar rats, a vein graft was implanted in situ in the hind limb. Flow-rate in the grafted vein was assessed by measuring flow in the common femoral artery using an ultrasonic flowmeter. Nuclei and actin filaments of the venous wall were stained with propidium iodine and phalloidine-FITC, and the samples were observed using confocal laser microscopy. The grafted veins became circular in cross-section with increase in diameter during two weeks after AV shunting. Owing to the increase in diameter, the estimated wall shear stress was not increased so much as the flow-rate. The confocal laser microscopic observation showed that endothelial cells (ECs) and smooth muscle cells (SMCs) in the grafted veins were either aligned well (2 out of 8 samples), or ECs were denudated and SMCs were disrupted (in 6 out of 8 samples). The cell density of ECs was unchanged from the control level. In conclusion, the grafted vein was remodeled with morphological changes in ECs and SMCs during 2 weeks after AV shunting.

Paramagnetic artifact and safety criteria for human brain mapping.

Biological effects of magnetic field and their safety criteria, especially effects of gradient magnetic field on the cerebral and pulmonary circulation during functional brain mapping are still unclear. Here we estimated that magnetically induced artifacts for the blood oxygenation level- and flow- based functional magnetic resonance imaging are less than 0.1%, and disturbance in the pulmonary circulation is less than 1.3% even if the field strength of magnetic resonance system is risen up to 10 tesla. These paramagnetic effects are considered to be small and harmless during human brain mapping.

In vivo imaging of the rat cerebral microvessels with optical coherence tomography.

A technique called optical coherence tomography (OCT) was applied to in vivo observation of microcirculation in the rat cerebral cortex. The OCT system used in this study provided cross-sectional images of the cerebral cortical tissue up to about 1 mm depth with longitudinal resolution up to 8 microm. It could visualize cross-sectional structure of the dura, arachnoid membrane, cortical tissue, and pial microvessels through the cranial window. Pial microvessels with diameter larger than several 10 microm could be detected to observe their cross-sectional shape, while the microvessels within the cortical tissue with smaller diameter were not discernible. The OCT observation revealed that the pial microvessels showed different spatial configurations depending on the cerebral preparations with intact dura and without dura. Stimulus responses of the somatosensory cortices were also different among the preparation methods; Delayed swelling of the cortical surface appeared in the somatosensory cortex following the electrical stimulation of the hind paw in the case of dura removal, which was restricted to a thin surface layer with less than several 10 microm. It is considered to reflect the reactive hyperemia accompanying the neuronal activation. Doppler frequency shift due to the blood flow was detected in pial arterioles. This phenomenon is promising to provide the velocity profile within microvessels and may be applicable to the functional imaging of the brain.

Circulatory basis of fMRI signals: relationship between changes in the hemodynamic parameters and BOLD signal intensity.

Blood oxygenation level-dependent functional magnetic resonance imaging (BOLD-fMRI) is widely used as a tool for functional brain mapping. During brain activation, increases in the regional blood flow lead to an increase in blood oxygenation and a decrease in paramagnetic deoxygenated hemoglobin (deoxy-Hb), causing an increase in the MR signal intensity at the site of brain activation. However, not a few studies using fMRI have failed to detect activation of areas that ought to have been activated. We assigned BOLD-positive (an increase in the signal intensity), BOLD-negative (a decrease in the signal intensity), and BOLD-silent (no change) brain activation to respective circulatory conditions through a description of fMRI signals as a function of the concentration of oxygenated Hb (oxy-Hb) and deoxy-Hb obtained with near-infrared optical imaging (NIOI). Using this model, we explain the sensory motor paradox in terms of BOLD-positive, BOLD-negative, and BOLD-silent brain activation.