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A Villringer

Publications and source records attributed to A Villringer.

At least 73 records · Page 4Linked to original sources
PubMed1997

Length of resting period between stimulation cycles modulates hemodynamic response to a motor stimulus.

The influence of different lengths of the pre-stimulation resting period on the magnitude of a hemodynamic response evoked by motor stimulation was examined in 10 subjects by means of near-infrared spectroscopy (NIRS). A motor stimulus was used which has been previously established as a model for functional activation studies with NIRS. Subjects performed a 20 s finger opposition task in the hand contralateral to NIRS probe localization over left sensorimotor area (C3', according to the 10-20 system). The duration of the pre-stimulation resting period was varied from 10s to 50s and response magnitude was assessed for each of the interstimulus intervals (10 s, 20 s, 30 s, 40 s and 50 s). Data analysis showed that response magnitude in oxygenated and deoxygenated haemoglobin concentration changed with different interstimulation intervals. Interestingly the greatest NIRS response was obtained with resting period 30 s prior to stimulation; shorter and longer resting periods resulted in smaller responses. The time course and the dependence of response magnitude on interstimulus interval differed between [oxy-Hb] and [deoxy-Hb] changes. For [oxy-Hb] the previously described fast initial increase ('overshoot') and the post-stimulation undershoot was more clearly seen with long prestimulation resting periods. Cytochromeoxidase oxygenation changes did not change significantly with different interstimulus intervals. We conclude that comparisons between different functional activation studies with techniques relying on stimulus evoked changes in cerebral hemodynamics must take into account not only the quality of the experimental paradigm and the length of the stimulation period, but also that the resting period between repetitive stimulations is important for response amplitude and its time course.

Brain↗
PubMed1997

Understanding functional neuroimaging methods based on neurovascular coupling.

Functional neuroimaging techniques are usually grouped according to the employed apparatus into functional magnetic resonance imaging techniques (fMRI), nuclear medicine approaches such as single photon emission tomography (SPET) or positron emission tomography (PET), and optical approaches (measurement of intrinsic signals, near infrared spectroscopy (NIRS)). However, the physiological parameters that are measured with these methods do not necessarily follow this technical classification. On the one hand, using different imaging modalities the same physiological parameters are measured and on the other hand, using the same imaging devices completely different physiological parameters can be assessed. The present article covers those functional neuroimaging methods which measure the vascular response to functional brain activation (PET, SPET, fMRI and NIRS). First, starting with the traditional grouping of these methods, it is outlined how the specific methods assess vascular changes associated with brain activation in order to localize brain function. Based on the understanding of the underlying physiological events, subsequently, a new classification of functional neuroimaging methods is proposed.

Animals↗
PubMed1997

Non-invasive optical spectroscopy and imaging of human brain function.

Brain activity is associated with changes in optical properties of brain tissue. Optical measurements during brain activation can assess haemoglobin oxygenation, cytochrome-c-oxidase redox state, and two types of changes in light scattering reflecting either membrane potential (fast signal) or cell swelling (slow signal), respectively. In previous studies of exposed brain tissue, optical imaging of brain activity has been achieved at high temporal and microscopical spatial resolution. Now, using near-infrared light that can penetrate biological tissue reasonably well, it has become possible to assess brain activity in human subjects through the intact skull non-invasively. After early studies employing single-site near-infrared spectroscopy, first near-infrared imaging devices are being applied successfully for low-resolution functional brain imaging. Advantages of the optical methods include biochemical specificity, a temporal resolution in the millisecond range, the potential of measuring intracellular and intravascular events simultaneously and the portability of the devices enabling bedside examinations.

Brain↗
PubMed1997

Noninvasive near infrared spectroscopy monitoring of regional cerebral blood oxygenation changes during peri-infarct depolarizations in focal cerebral ischemia in the rat.

Intermittent peri-infarct depolarizations (PID), which spread from the vicinity of the infarction over the cortex, have been reported in focal ischemia. These depolarizations resemble cortical spreading depression except that they damage the cortex and enlarge the infarct volume possibly because of compromised oxygen delivery. The main purpose of this study was to evaluate the noninvasive technique of near-infrared spectroscopy (NIRS) for the identification of PID and to evaluate its capability for further pathophysiological studies. We used male barbiturate-anesthetized Wistar rats (n = 10) in which middle cerebral artery occlusion had been performed with a surgical thread. Middle cerebral artery occlusion resulted in a drop in parietally measured regional cerebral blood flow (laser Doppler flowmetry) to 31 +/- 8% of baseline flow. Six +/- 4 minutes after the induction of focal ischemia, 5 +/- 2 direct current deflections were recorded during a one-hour measurement period which may be regarded as PID. Measuring regional cerebral blood oxygenation changes with a NIRO 500 revealed dynamic concentration changes in the three chromophores oxyhemoglobin [HbO2], deoxyhemoglobin [Hb], and the oxidized form of cytochrome aa3 [CytO] during PID. Typically, an initial slight decrease of [HbO2] (-6.1 +/- 1.7 arbitrary units [AU] and an increase of [Hb] (+11.5 +/- 7.7 AU) were followed by an increase of [HbO2] (+10.8 +/- 4.7 AU) and a decrease of [Hb] (-4.7 +/- 5.5 AU); [CytO] decreased during the depolarizations (-2.0 +/- 1.2 AU). We conclude that NIRS can detect typical PID-associated changes in blood oxygenation. We hypothesize that during the course of PID, unlike "normal" spreading depression, hypoxygenation precedes hyperoxygenation of the microcirculation in a given cortex volume as the depolarization wave propagates through hemodynamically compromised to intact tissue. This would accord with the known damaging effect of PID. The NIRS "fingerprint" of PID encourages the search for PID during early stroke in patients.

Animals↗
PubMed1997

Functional neuroimaging. Optical approaches.

Optical methods assess different types of light tissue interactions such as light absorption, fluorescence, phosphoresence, scattering, and Doppler shift. In this article, it is reviewed how these different types of light-tissue interactions can be measured and how these measurements can be related to brain function. Based on these considerations, a new classification scheme of functional optical methods is proposed.

Absorption↗
PubMed1997

Assessment of local brain activation. A simultaneous PET and near-infrared spectroscopy study.

In five healthy human subjects, near-infrared spectroscopy (NIRS) and positron emission tomography (PET) examinations were performed simultaneously. Changes in [oxy-Hb], [deoxy-Hb] and [total-Hb] as measured by NIRS over the left forehead were compared to measurements of cerebral blood flow by PET during rest and during performance of a calculation task and a Stroop task. When a penetration depth of near-infrared light 0.9 cm into the brain cortex was assumed, a statistically significant correlation between changes in CBF and changes in [total-Hb] was found. These data confirm the validity of NIRS measurements in human adults.

Aged↗
PubMed1997

Dural sinus and cerebral venous thrombosis.

Cerebral venous thrombosis (CVT) is less frequent than arterial thrombosis, however, it is still frequently overlooked. Pathophysiologically it is characterized by a disturbance of the equilibrium between endogenous thrombogenic and fibrinolytic factors. In addition, the time course depends on the presence or absence of efficient venous collaterals. A wide variety of clinical symptoms do occur, however, they may be grouped into at least four relatively typical syndromes: a) the pseudotumor cerebri syndrome, b) the combination of headache and focal neurologic deficit, c) the combination of focal epileptic seizure (with or without Todd's paresis) and headache, and d) deep CVT characterized by headache, nausea, bilateral long-tract symptoms and usually a rapidly progressing decline in the level of consciousness. Two diagnostic routes are generally accepted, the one consisting of cranial computed tomography plus radiographic angiography, the other one consisting of magnetic resonance imaging and magnetic resonance angiography. The mainstay of therapy is partial thromboplastin time-effective anticoagulation (PTT at least doubled, target PTT between 80 and 100 secs). Anticoagulation should be performed even if intracranial hemorrhage is present. In cases where deterioration occurs despite effective heparin treatment and in subjects presenting with stupor or coma, more aggressive therapy, e.g., local fibrinolysis during venous angiography, may be considered.

Cerebrovascular Circulation↗
PubMed1996

Non-invasive functional mapping of the human motor cortex using near-infrared spectroscopy.

We applied non-invasive multisite near-infrared spectroscopy (NIRS) to assess oxygenation changes during performance of a sequential finger opposition task in five healthy human adults. Oxygenation response was localized anatomically using three-dimensional high-resolution magnetic resonance imaging (3D MRI). NIRS measurements showed a localized increase in [oxy-Hb] and a decrease in [deoxy-Hb] in all subjects. The largest response was obtained when the measurement position was over the primary motor and sensory cortex hand area. Interestingly, changes in [deoxy-Hb] seemed to be more localized than changes in [oxy-Hb]. We conclude that this simple, non-invasive and flexible optical bedside method may be used for functional brain mapping.

Adult↗
PubMed1996

Functional magnetic resonance imaging shows localized brain activation during serial transcranial stimulation in man.

Area and depth penetration of transcranial stimulation methods such as transcranial electrical stimulation (TES) are poorly defined. We investigated the feasibility of a simultaneous TES and fMRI measurement. The aim was to compare the signal intensity changes measured using BOLD fMRI during sequential finger movement with the signal response during artificial transcranial stimulation. Tes induced contralateral finger contractions and in T2* weighted images a transient signal increase was observed in the area underlying the electrodes. Compared with the signal obtained during sequential finger movements, the area activated by TES was more localized, signal amplitude, was smaller and there was no post-stimulus undershoot. These data indicate that TES induces a local blood flow increase associated with a drop in the concentration of deoxyhaemoglobin.

Brain↗
PubMed1996

Near infrared spectroscopy in the diagnosis of Alzheimer's disease.

Near infrared spectroscopy (NIRS) is a new technique that permits noninvasive monitoring of cerebral blood and tissue oxygenation. Recently, we and others have shown that NIRS measurements are sensitive enough to follow changes in cerebral hemoglobin oxygenation due to activation of brain function. Based on these findings we have assessed the influence of aging as well as the influence of neurodegeneration on cerebral hemoglobin oxygenation during mental work. The typical NIRS pattern in young healthy subjects while performing calculation tasks measured in the frontal cortex were increases in oxygenated hemoglobin [HbO2] and total hemoglobin [HbT] while reduced hemoglobin [HbR] decreased. Elderly healthy subjects showed a significant lower mean increase in [HbO2] and [HbT] levels. Regression analysis revealed an age-dependent decline in activation-induced local increase of [HbO2] as well as [HbT]. Furthermore, we monitored changes in cerebral hemoglobin oxygenation in the frontal cortex while patients with probable Alzheimer's disease (AD) performed cognitive tasks. Whereas elderly healthy subjects (as well as patients with major depression, age-associated memory impairments or vascular dementia) again showed clear increases in the local concentrations of [HbO2] and [HbT] during brain activation, AD patients showed significant decreases compared to the baseline levels in both variables that were most pronounced in the parietal cortex. To clarify whether the different patterns in cerebral hemoglobin oxygenation during cognitive activation were due to an altered functional brain organization in AD or to alterations in the cerebrovascular response to neuronal activation, we are currently performing simultaneous NIRS and (015-H20-)PET measurements during performance of a cognitive task (Stroop test). Our finding of a regional reduced oxygen supply during activation of brain function may be of relevance to the development and the time course of neurodegeneration.

Aging↗
PubMed1996

Simultaneous recording of cerebral blood oxygenation changes during human brain activation by magnetic resonance imaging and near-infrared spectroscopy.

Changes in cerebral blood oxygenation due to functional activation of the primary sensorimotor cortex during a unilateral finger opposition task were simultaneously mapped by deoxyhemoglobin-sensitive magnetic resonance imaging (MRI) and monitored by near-infrared spectroscopy (NIRS). Activation foci along the contralateral central sulcus displayed task-associated increases in MRI signal intensity, indicating a concomitant decrease of the focal concentration of deoxyhemoglobin. This interpretation was confirmed by simultaneous reductions in deoxyhemoglobin measured optically. Since observation of the latter effect required exact spatial matching of the MRI-detected activation foci and position of the fiber optic bundles ("optodes") used for transmitting and receiving light, it may be concluded that optical recordings of changes in deoxyhemoglobin during functional challenge probe only a restricted brain tissue region. While deoxyhemoglobin responses seen by NIRS were smaller for ipsi- than for contralateral finger movements, task-related increases in oxyhemoglobin were rather similar between both conditions and, thus, seem to be less specific. Furthermore, no consistent changes were obtained for total hemoglobin during task performance, possibly due to the short timing of the repetitive protocol. In general, results underline, in humans, the hitherto assumed signal physiology for functional brain mapping by oxygenation-sensitive MRI and allow assessment of both constraints and practicability of functional studies by NIRS.

Adult↗
PubMed1996

Systemic nitric oxide synthase inhibition does not affect brain oxygenation during cortical spreading depression in rats: a noninvasive near-infrared spectroscopy and laser-Doppler flowmetry study.

Cortical spreading depression (CSD) has been implicated in the migraine aura and in stroke. This study demonstrates near-infrared spectroscopy (NIRS) for the first time as capable of noninvasive on-line detection of CSD in the pentobarbital-anesthetized rat. CSD was accompanied by a brief and rapid increase of regional CBF (by laser-Doppler flowmetry) to 200-400% baseline. NIRS demonstrates that this hyperperfusion is associated with concentration increases of oxyhemoglobin, while deoxyhemoglobin decreases. Simultaneously, oxygen partial pressure, measured on the brain surface with a solid-state polarographic probe, was shown to be raised by at least 14 mm Hg during CSD. Oxygen-dependent phosphorescence life-time quenching measurements confirmed this finding. NIRS data on cytochrome aa3, however, showed a CSD-related shift toward a more reduced state, despite raised blood oxygenation. This may suggest either limited O2 transport from the blood to mitochondria or decreased oxygen utilization during CSD as supposed by theories about compartmentalization of energy metabolism favoring glycolytic rather than aerobic energy supply during CSD. However, the data on cytochrome aa3 warrant caution and are discussed critically. Nitric oxide synthase inhibition by systemic application of N'-nitro-L-arginine had no significant effect on the perfusion response or the tissue PO2 during CSD. During most CSD episodes, a brief decrease in MABP by 4-8 mm Hg was noted that might be caused by functional decortication during CSD.

Animals↗
PubMed1996

Role of nitric oxide synthase inhibition in leukocyte-endothelium interaction in the rat pial microvasculature.

We investigated the role of nitric oxide (NO) in leukocyte-endothelium interaction, blood-brain barrier (BBB) function and oxygen free-radical production in the rat pial microcirculation. In a closed cranial window preparation (dura removed) over the parietal cortex of pentobarbital-anesthetized Wistar rats, NO synthase (NOS) was inhibited by systemic and/or topical application of N omega-nitro-L-arginine (L-NNA) under physiological conditions and during leukotriene B4 (LTB4) activation. Circulating leukocytes were labeled by intravenous injection of rhodamine 6G. We used a confocal laser scanning microscope (CLSM) and studied leukocyte rolling and sticking in pial veins and arteries before and after NOS inhibition. At the end of the experiments, sodium-fluorescein was injected intravenously to test BBB integrity. Brain cortex oxygen free-radical production was investigated in the cranial window preparation using lucigenin-enhanced chemiluminescence (CL). L-NNA application did not lead to significant changes in leukocyte-endothelium interaction, BBB function, and oxygen free-radical production under physiological conditions [leukocyte-endothelium interaction: control (n = 5), L-NNA systemically (n = 5), L-NNA topically (n = 5): at baseline rollers/100 microns: 0.76 +/- 0.55, 0.64 +/- 0.94, 0.44 +/- 0.55 and stickers/100 microns: 0.90 +/- 0.28, 0.76 +/- 0.24, 0.84 +/- 0.42; at 60 min rollers/100 microns: 1.49 +/- 0.66, 1.21 +/- 0.99, 0.67 +/- 0.66 and stickers/100 microns: 1.04 +/- 0.20, 1.19 +/- 0.23, 1.21 +/- 0.54; oxygen free-radical production (n = 4): CL count before L-NNA application 35 +/- 17 cps, after 1 h of topical superfusion of L-NNA 38 +/- 14 cps; p < 0.05]. In contrast to the results achieved under physiological conditions, a significant further increase of rolling leukocytes and BBB permeability occurred due to NOS inhibition under LTB4-activated conditions [76 +/- 47% significant (p < or = 0.01, n = 7) further increase of rollers/100 microns due to 60 min L-NNA application following the activation period of 120 min LTB4 superfusion]. Our results support a modulatory role for NO in leukocyte-endothelium interaction and BBB permeability in the pial microcirculation when this interaction is increased.

Animals↗