Near infrared spectroscopy (NIRS): a new tool to study hemodynamic changes during activation of brain function in human adults

Three-dimensional laser-induced photoacoustic tomography of mouse brain with the skin and skull intact

Three-dimensional laser-induced photoacoustic tomography, also referred to as optoacoustic tomography, is developed to image animal brain structures noninvasively with the skin and skull intact. This imaging modality combines the advantages of optical contrast and ultrasonic resolution. The distribution of optical absorption in a mouse brain is imaged successfully. The intrinsic optical contrast reveals not only blood vessels but also other detailed brain structures, such as the cerebellum, hippocampus, and ventriculi lateralis. The spatial resolution is primarily diffraction limited by the received photoacoustic waves. Imaged structures of the brain at different depths match the corresponding histological pictures well.

Hypoactivation of the prefrontal cortex during verbal fluency test in PTSD: a near-infrared spectroscopy study

Several studies have suggested that there is frontal dysfunction in subjects with posttraumatic stress disorder (PTSD). We investigated the relationship between alterations of the hemodynamic response of the prefrontal cortex during a cognitive task (verbal fluency task; VFT) and memory function measured using the Wechsler Memory Scale-Revised (WMS-R). The subjects were victims of the Tokyo Subway Sarin attack with (n = 8) or without (n = 26) PTSD. Hemodynamic response in the prefrontal cortex was measured using a 24-channel near-infrared spectroscopy (NIRS) system. Subjects with PTSD had a significantly smaller response of oxygenated hemoglobin and total hemoglobin during the VFT compared with those without PTSD, although there was no significant difference in performance on the VFT. Subjects with PTSD had significantly lower scores on attention and concentration in the WMS-R, which was positively correlated with the increase of total hemoglobin during the VFT. The 'frontal dysfunction' observed in subjects with PTSD may be a secondary phenomenon to reduced attentional capacity.

Brain imaging in awake infants by near-infrared optical topography

Studies of young infants are critical to understand perceptual, motor, and cognitive processing in humans. However, brain mechanisms involved are poorly understood, because the use of brain-imaging methods such as functional magnetic resonance imaging in awake infants is difficult. In the present study we show functional brain imaging of awake infants viewing visual stimuli by means of multichannel near-infrared spectroscopy, a technique that permits a measurement of cerebral hemoglobin oxygenation in response to brain activation through the intact skull without subject constraint. We found that event-related increases in oxyhemoglobin were evident in localized areas of the occipital cortex of infants aged 2-4 months in response to a brief presentation of a checkerboard pattern reversal while they maintained fixation to attention-grabbing stimuli. The dynamic change in cerebral blood oxygenation was qualitatively similar to that observed in the adult brain. This result introduces near-infrared optical topography as a method for investigating the functional development of the brain in early infancy.

Biphasic changes in tissue partial pressure of oxygen closely related to localized neural activity in guinea pig auditory cortex

An understanding of the local changes in cerebral oxygen content accompanying functional brain activation is critical for making a valid signal interpretation of hemodynamic-based functional brain imaging. However, spatiotemporal relations between changes in tissue partial pressure of oxygen (Po2) and induced neural activity remain incompletely understood. To characterize the local Po2 response to the given neural activity, the authors simultaneously measured tissue Po2 and neural activity in the identical region of guinea pig auditory cortex with an oxygen microelectrode (tip < 10 microm) and optical recording with voltage-sensitive dye (RH 795). In addition, a laser displacement gauge and a laser-Doppler flowmeter were used to monitor the spatial displacement and regional cerebral blood flow, respectively, in the Po2 measurement region. In the activated region, tissue Po2 initially decreased during the approximately 3-seconds after the onset of acoustic stimuli, and then increased during the next approximately 5 seconds. Such biphasic changes are consistently found in cortical layers I to IV. In addition, amplitude of the biphasic change was closely related to detected peak height of the optical signal changes. The results suggest that the initial decrease in tissue Po2 is coupled to the induced neural activity and depends on response time of local increase in cerebral blood flow.

Applications and limitations of perfusion-dependent functional brain mapping for neurosurgical guidance

Perfusion-dependent brain mapping modalities, such as functional magnetic resonance imaging, positron emission tomography, and optical imaging of intrinsic signals, have become increasingly popular for neurosurgical guidance because they offer a relatively rapid and noninvasive means of mapping brain function. These modalities are unique because they rely on perfusion-related signals that are coupled with neuronal activity to map the brain instead of measuring electrophysiological responses. They consequently present unique challenges to the clinician in terms of understanding the significance and limitations of the maps they produce. In particular, one must be aware of limitations of the modalities with respect to spatial specificity, sensitivity, and reliability of these maps and how the presence of intracranial lesions may further complicate these issues. The authors review the evolution, interpretation, and limitations of perfusion-based brain mapping techniques, with special attention to clinical implications of the brain maps.

Functional near-infrared optical imaging: Utility and limitations in human brain mapping

Although near-infrared spectroscopy (NIRS) was developed as a tool for clinical monitoring of tissue oxygenation, it also has potential for neuroimaging. A wide range of different NIRS instruments have been developed, and instruments for continuous intensity measurements with fixed spacing [continuous wave (CW)-type instruments], which are most readily available commercially, allow us to see dynamic changes in regional cerebral blood flow in real time. However, quantification, which is necessary for imaging of brain functions, is impossible with these CW-type instruments. Over the past 20 years, many different approaches to quantification have been tried, and several multichannel time-resolved and frequency-domain instruments are now in common use for imaging. Although there are still many problems with this technique, such as incomplete knowledge of how light propagates through the head, NIRS will not only open a window on brain physiology for subjects who have rarely been examined until now, but also provide a new direction for functional mapping studies.

Advances in optical imaging of the newborn infant brain

New methods of imaging the oxygenation, hemodynamics, and metabolism of the newborn infant brain are being developed, based on illumination of the head with near-infrared light. Techniques known as optical topography and optical tomography have the potential to provide valuable information about the function of the normal brain, and about a variety of cerebral pathology such as hypoxic-ischemia. Optical methods provide a unique means of monitoring brain oxygenation safely in an intensive care environment without interference with the normal handling of the infant. Studies on infants have focused on the assessment of steady-state regional cerebral perfusion and tissue oxygenation, as well as monitoring hemodynamic changes in response to sensory stimulation. Recent technological and methodological advances in this research field are reviewed, and the likely impact of optical imaging methods on the care of newborn infants is assessed.

Activation of the prefrontal cortex to trauma-related stimuli measured by near-infrared spectroscopy in posttraumatic stress disorder due to terrorism

To develop a noninvasive method for psychophysiological assessment of posttraumatic stress disorder (PTSD), 34 victims of the Tokyo Subway Sarin Attack in 1995 including 8 diagnosed as PTSD and 12 controls were examined by a multichannel near-infrared spectroscopy (NIRS) system. Hemodynamic response in the prefrontal cortex was monitored during the presentation of trauma-related and control stimuli by video images. Skin conductance response (SCR) was also examined. Oxygenated hemoglobin significantly increased during the trauma-related image in the victims with or without PTSD. Deoxygenated hemoglobin significantly decreased only in victims with PTSD. No significant alteration was found in controls. Significantly enhanced SCR was also observed in the victims with PTSD during trauma-related stimuli. The findings suggest that measurement of cerebral hemodynamic response by NIRS is useful for psychophysiological assessment of PTSD.

Fast cerebral functional signal in the 100-ms range detected in the visual cortex by frequency-domain near-infrared spectrophotometry

Brain activity is associated with physiological changes, which alter the optical properties of the tissue in the near-infrared part of the spectrum. Two major types of optical signals following functional brain activation can be distinguished: a slow signal due to hemodynamic changes and a fast signal, which is directly related to neuronal activity. The fast signal is small and therefore difficult to detect. We used a specially noise-optimized frequency-domain near-infrared spectrometer with a pi-sensor, which was expected to be particularly sensitive to deeper tissue layers, to investigate the human visual cortex during visual stimulation generated by a checkerboard. We were able to detect significant fast signals in single light bundles, but not in pi-signals. The fast signals were mostly collocated with strong slow hemodynamic signals, but showed a higher degree of localization than the latter. The latencies of 40 +/- 16 ms of the fast signals were similar between locations. Our results also indicate that the brain responds differently to a single and double (forth and back) reversal of the checkerboard, with a stronger reaction upon the double reversal.

Frontal activation during a verbal-fluency task as measured by near-infrared spectroscopy

Near-infrared spectroscopy (NIRS) is an optical method which allows non-invasive in vivo measurements of concentration changes of oxygenated hemoglobin (O(2)Hb) and deoxygenated hemoglobin (HHb) in brain tissue. The present study investigated the blood oxygenation changes that were associated with the execution of the Verbal-Fluency Test (VFT) in left and right prefrontal brain areas of 14 healthy subjects by means of NIRS. During the VFT, subjects were required to pronounce as many nouns as possible beginning with the letters "A," "F," and "S." Compared to a baseline resting condition, we found a significant increase of O2Hb (left hemisphere: baseline, 1.2+/-1.1microm; VFT(letterS), 3.56+/-2.02microm; right hemisphere: baseline, 1.26+/-1.08microm; VFT(letterS), 3.67+/-2.03microm) and a significant decrease of HHb (left hemisphere: baseline, -0.21+/-0.59microm; VFT(letterS), -0.67+/-0.60microm; right hemisphere: baseline, -0.29+/-0.53microm; VFT(letterS), -0.68+/-0.44microm) during the execution of the VFT over both hemispheres. No lateralization effects were observed. The results confirm that NIRS is suitable for the measurement of blood oxygenation changes in frontal brain areas that are associated with cognitive tasks.

Acute effects of alcohol on hemodynamic changes during visual stimulation assessed using 24-channel near-infrared spectroscopy

The purpose of this study was to evaluate the effects of alcohol on hemodynamic changes induced by visual stimulation. Ten healthy human subjects were examined using Optical Topography((R)) (Hitachi Medical Corporation: ETG-100). Each subject gradually drank 0.4 ml/kg alcohol over 10 min. Changes in oxy-hemoglobin (Hb), deoxy-Hb and total-Hb concentration were measured five times: 20 min before alcohol intake, immediately after alcohol intake, and at 20, 40 and 60 min after alcohol intake. A questionnaire was used to assess subjective feelings of alcohol. Blood-alcohol concentration (BAC) was estimated from ethanol concentration in expired air four times: immediately after alcohol intake and at 20, 40 and 60 min after alcohol intake. The visual stimulation tool was a checkerboard. It showed alternations of black and red patterns at a frequency of 8 Hz. The stimulus was displayed for 10 s after a rest of 30 s. The stimulus was repeated 10 times. Oxy-Hb concentration increased and deoxy-Hb concentration decreased during visual stimulation before and after alcohol intake, despite changes in the score of subjective feelings of alcohol and BAC. Alcohol intake does not significantly affect hemodynamic changes caused by visual stimulation in the visual cortex.

Neuronal activity-induced changes of local cerebral microvascular blood oxygenation in the rat: effect of systemic hyperoxia or hypoxia

To investigate the effect of resting blood oxygen concentration on the hemodynamic response to functional brain activation, we compared activation-induced changes in hemoglobin oxygenation during normoxia with systemic hyperoxia or mild hypoxia. Hemoglobin oxygenation changes were measured by microfiber optical spectroscopy (500-590 nm) in response to physiological whisker barrel cortex activation by whole whisker pad deflection (4 s, 4 Hz) in alpha-chloralose/urethane anesthetised male Wistar rats. During systemic hyperoxia (n=6), the stimulation-induced hyperoxygenation response was decreased and prolonged, whereas during mild systemic hypoxia (n=7) the peak response was significantly increased followed by a faster return to baseline. During normoxia, a poststimulation under- (oxy-hemoglobin) and overshoot (deoxy-hemoglobin) was observed, which disappeared during systemic hyperoxia and was pronounced during systemic hypoxia. Although averaging out below statistical significance when combining all animals, during mild systemic hypoxia a very small early increase of deoxy-Hb at the beginning of the stimulation period was conspicuous more often than during normoxic or even hyperoxic conditions. This small early increase of deoxy-Hb never preceded the onset of the oxy-Hb response, and was not accompanied by a concomitant decrease in oxy-Hb. Hyperoxia or hypoxia did not affect the induced neuronal responses. Our findings support the concept that the hemodynamic response is regulated according to the metabolic demand of oxygen within the activated tissue.

Time-series estimation of biological factors in optical diffusion tomography

We apply state space estimation techniques to the time-varying reconstruction problem in optical tomography. We develop a stochastic model for describing the evolution of quasi-sinusoidal medical signals such as the heartbeat, assuming these are represented as a known frequency with randomly varying amplitude and phase. We use the extended Kalman filter in combination with spatial regularization techniques to reconstruct images from highly under-determined time-series data. This system also naturally segments activity belonging to different biological processes. We present reconstructions of simulated data and of real data recorded from the human motor cortex (Franceschini et al 2000 Optics Express 6 49-57). It is argued that the application of these time-series techniques improves both the fidelity and temporal resolution of reconstruction in optical tomography.

Frequency-domain multiplexing system for in vivo diffuse light measurements of rapid cerebral hemodynamics

A novel frequency-domain multiplexing system has been developed for in vivo measurements of rapid cerebral hemodynamics. The instrument operates in the frequency-domain with three optical wavelengths, six source positions, and two detectors. Frequency-division multiplexing was used to modulate three wavelengths (690, 786, and 830 nm) at slightly different frequencies around 70 MHz. The three laser output beams were combined and switched into different source positions by use of fast optical switches (switch time <10 ms). Three narrowband, in-phase and in-quadrature demodulators decode the modulated signals. Our full-frame-acquisition rate is 2.5 Hz, with flexibility for acquisition rates greater than 50 Hz with smaller detection areas. We evaluate the performance of the instrument with tissue phantoms, and then employ the system to measure in vivo cerebral blood oxygenation during forepaw stimulation of a rat's brain.

Optical topography: practical problems and new applications

We will briefly review the present status of optical topography and then discuss the method of improving practicality, i.e., the signal-to-noise (S/N) ratio and the spatial resolution in observations of higher-order brain functions. The optimum wavelength pair improved the S/N ratio sixfold for deoxyhemoglobin, and new configurations of light irradiation and detection positions doubled the spatial resolution. We also report on developing application fields of optical topography. This modality will bridge the gap between natural sciences, neuroscience, and pedagogy, and show actual real-time brain activity.

An event-related optical topography study of cortical activation induced by single-pulse transcranial magnetic stimulation

To visualize cortical activations during transcranial magnetic stimulation (TMS), it is necessary to measure those activations at high spatiotemporal resolution while preventing interference with the magnetic property of a coil. One suitable method that satisfies these demands is optical topography (OT), which has been used in cortical activation studies. In the present study, single-pulse TMS was applied to the left primary motor area, and cortical responses at the stimulation site were measured simultaneously with event-related OT. When TMS was applied at 110% motor threshold (MT), we observed significant oxyhemoglobin increases that were both time-locked and correlated with the hemodynamic basis function. Moreover, when TMS was applied at 90% MT, significant oxyhemoglobin increases were detected even though there were no motor-evoked potentials. These results demonstrate that OT can directly measure cortical responses to subthreshold single-pulse TMS, independent of the afferent feedback from the peripheral neuromuscular activity.

Focus diagnosis of epilepsy using near-infrared spectroscopy

PURPOSE

We sought to establish a noninvasive method for focus diagnosis of epilepsy.

METHODS

We examined the use of multichannel near-infrared spectroscopy (NIRS). It is known as a noninvasive method of functional brain mapping. We monitored cerebral blood volume change with NIRS during long-term EEG monitoring of epilepsy in 32 cases with intractable epilepsies to diagnose the epileptogenic focus.

RESULTS

In 96% of cases, NIRS showed significant hyperperfusion in the side of seizure foci, whereas ictal SPECT showed hyperperfusion in 69% of cases.

CONCLUSIONS

Ictal NIRS is a reliable method to evaluate the focus side in epilepsy, especially when it is coupled with ictal SPECT.

Incidence and management of complications during posttraumatic early rehabilitation

Early rehabilitation after traumatic brain injury has become a worldwide accepted interface between intensive care medicine and rehabilitation to aim for a better functional outcome of the surviving patients. So each chain can only be as strong as its weakest link, and there is still need for well defined quality standards depending on the medical demands during this period of treatment. Hence we were interested in quantifying the complications occurring until discharge to further rehabilitation with special regard on severe physical handicaps and organ failure necessitating surgical or intensive care therapy. Our results demonstrate that early rehabilitation is a part of intensive care medicine with enhanced approaches to preserve rehabilitation potential of the brain and for coma stimulation.

Non-invasive neuroimaging using near-infrared light

This article reviews diffuse optical brain imaging, a technique that employs near-infrared light to non-invasively probe the brain for changes in parameters relating to brain function. We describe the general methodology, including types of measurements and instrumentation (including the tradeoffs inherent in the various instrument components), and the basic theory required to interpret the recorded data. A brief review of diffuse optical applications is included, with an emphasis on research that has been done with psychiatric populations. Finally, we discuss some practical issues and limitations that are relevant when conducting diffuse optical experiments. We find that, while diffuse optics can provide substantial advantages to the psychiatric researcher relative to the alternative brain imaging methods, the method remains substantially underutilized in this field.

Educational neuroimaging: a proposed neuropsychological application of near-infrared spectroscopy (nIRS)

OBJECTIVE

To provide a description of an emerging neuroimaging methodology, near-infrared spectroscopy (nIRS), and a potential educational application of the unique aspects of this technology.

SUMMARY

nIRS is documented for its potential as a personal, portable, brain imaging system that may prove useful for cerebral monitoring in applied settings such as home, school, and work. The basis of nIRS brain imaging is reviewed, with summary descriptions of optical and neurovascular issues as well as a brief comparison to other brain imaging methodologies. Recent developments in nIRS technology are discussed, including ongoing validation efforts and potential applications for neuropsychologists. We describe one potential application of nIRS (i.e., educational neuroimaging) as an illustration of the use of nIRS technology and the potential expansion of the neuropsychologist's role in the educational setting.

CONCLUSION

nIRS holds the potential of opening new clinical questions and opportunities for neuropsychologists, and may provide a low-cost means of repeatable, neurovascular monitoring in nonmedical settings.

Regional cerebral blood flow changes associated with emotions in children

Combining a wireless telemetry system and a portable near-infrared instrument, we developed a movable optical spectroscopy system for monitoring cerebral hemodynamic changes. The patient carries a miniaturized near-infrared spectroscopy instrument on the back, and data are sent by a wireless telemetry system to a computer, without restricting patient movement. We used this system to detect hemodynamic changes associated with being startled, anticipation, and pleasant and unpleasant emotions in the bilateral prefrontal cortices of 16 right-handed 4- to 6-year-old preschool children while they were watching a 21-minute video clip consisting of various scenes that elicited emotional responses, interpolated with neutral scenes for comparison. The children were relaxed and cooperative when they were studied. Anticipation was associated with increases in cerebral blood flow in the left prefrontal cortex of the 6-year-old children, and unpleasant emotion was associated with decreases in cerebral blood flow bilaterally compared with neutral emotion, irrespective of age. No hemodynamic changes associated with the startle response were observed. Although this study should be considered preliminary, it is suggested that the emotional response is age-dependent and that the left prefrontal cortex participates in anticipation. Our newly developed system will open a window into brain physiology in children.

Arranging optical fibres for the spatial resolution improvement of topographical images

Optical topography is a method for visualization of conical activity. Ways of improving the spatial resolution of the topographical image with three arrangements of optical fibres are discussed. A distribution of sensitivity is obtained from the phantom experiment, and used to reconstruct topographical images of an activation area of the brain with the fibres in each arrangement. The correlations between the activated area and the corresponding topographical images are obtained, and the effective arrangement of the optical fibres for improved resolution is discussed.

Lateralized activation in the inferior frontal cortex during syntactic processing: event-related optical topography study

Functional imaging with near-infrared light has the potential to provide novel information that cannot be obtained with other imaging techniques. An event-related paradigm has not been fully established for studying human cognitive functions with near-infrared optical imaging. We conducted language experiments to develop an event-related paradigm with optical topography (OT). We directly compared cortical activation during syntactic and semantic decision tasks, both of which involved error detection in a sentence stimulus that consisted of a noun phrase and a verb. In the syntactic decision task, subjects judged whether the presented sentence is syntactically correct, where the syntactic knowledge about the distinction between transitive and intransitive verbs was required. In the semantic decision task, subjects judged whether the presented sentence is semantically correct, where the lexico-semantic knowledge about selectional restrictions was indispensable. We found local increases in oxyhemoglobin concentration, which were selectively associated with the syntactic decision task. Activation in the left inferior frontal gyrus was detected when syntactically anomalous sentences were presented, whereas there was no significant activation in this region when semantically anomalous sentences were presented. Moreover, identical stimuli of normal sentences elicited activation in the left inferior frontal gyrus, only when the employment of syntactic knowledge was required. This task-selective activation was not observed in any other measured regions, including the right homologous region. These results demonstrate that OT techniques, when coupled with the event-related paradigm, are useful for studying the higher cognitive functions of the human cerebral cortex.

Non-invasive assessment of language lateralization by transcranial near infrared optical topography and functional MRI

Near infrared optical topography (OT) is the simultaneous acquisition of hemoglobin absorption from an array of optical fibers on the scalp to construct maps of cortical activity. We demonstrate that OT can be used to determine lateralization of prefrontal areas to a language task that has been validated by functional MRI (fMRI). Studies were performed on six subjects using a visually presented language task. Laterality was quantified by the relative number of activated pixels in each hemisphere for fMRI, and the total hemoglobin responses in each hemisphere for OT. All subjects showed varying degrees of left hemisphere language dominance and the mean laterality indices for subjects who underwent both OT and fMRI were in good agreement. These studies demonstrate that OT gives predictions of hemispheric dominance that are consistent with fMRI. Due to the ease of use and portable nature of OT, it is anticipated that optical topography will be valuable tool for neurological examinations of cognitive function.

Simultaneous recording of event-related auditory oddball response using transcranial near infrared optical topography and surface EEG

Near infrared optical topography (OT) is the measurement of hemoglobin absorption simultaneously from an array of optical fibers on the scalp to construct maps of cortical activity. We demonstrate that OT can be used to simultaneously detect and characterize the hemodynamic responses associated with an "oddball" auditory stimulus and that corresponding electrical event related potentials can be acquired simultaneously using conventional scalp recordings. In addition to the measured electrical response, the hemodynamic localization is consistent with fMRI studies, which show significant activation in the temporal and parietal cortical regions. The event-related response of total hemoglobin showed relatively slow peak latencies (5.8 +/- 0.3 s), which were also consistent with fMRI. The current study shows the regions of peak hemodynamic activity that are in closest proximity to areas of peak electrical activity. This is the first demonstration of simultaneous ERP electrical recording and non-invasive optical mapping in human subjects, which promises to be an important tool in the characterization of both normal and abnormal brain function.

Maps of optical differential pathlength factor of human adult forehead, somatosensory motor and occipital regions at multi-wavelengths in NIR

The optical differential pathlength factor (DPF) is an important parameter for physiological measurement using near infrared spectroscopy, but for the human adult head it has been available only for the forehead. Here we report measured DPF results for the forehead, somatosensory motor and occipital regions from measurements on 11 adult volunteers using a time-resolved optical imaging system. The optode separation was about 30 mm and the wavelengths used were 759 nm, 799 nm and 834 nm. Measured DPFs were 7.25 for the central forehead and 6.25 for the temple region at 799 nm. For the central somatosensory and occipital areas (10 mm above the inion), DPFs at 799 nm are 7.5 and 8.75, respectively. Less than 10% decreases of DPF for all these regions were observed when the wavelength increased from 759 nm to 834 nm. To compare these DPF maps with the anatomical structure of the head, a Monte Carlo simulation was carried out to calculate DPF for these regions by using a two-layered semi-infinite model and assuming the thickness of the upper layer to be the sum of the thicknesses of scalp and skull, which was measured from MRI images of a subject's head. The DPF data will be useful for quantitative monitoring of the haemodynamic changes occurring in adult heads.

Monitors of cerebral oxygenation

None of the monitors of cerebral oxygenation discussed above has proven to be effective enough to have become a standard of care in any given area of medical treatment. As described above, each has specific and well-defined shortcomings that prevent its widespread use. These shortcomings may not be so much a failure of technology as an acknowledgement of the complexity of our goal: a monitor that can divide the entire brain into small, focal, and discrete areas and accurately measure the oxygen tension in each one. Because we are asking for the functional equivalent of 30 or 40 simultaneous PbtO2 probes, it is small wonder that we are not yet satisfied. Of the three monitors discussed here, the greatest potential may lie with the transcranial cerebral oximetry. The cerebral oximeter has the biggest potential for improvement because it holds the most potential for technical advancement. Although, for instance, jugular venous bulb oximetric catheters may become somewhat more accurate, the biggest drawbacks in that monitor's usefulness lie in human anatomy and intracerebral blood mixing, not catheter accuracy. PbtO2 probes, also, have little room for improvement. Although every technology can be refined, the PbtO2 probes are already accurate. The fact that they are an invasive monitor, and a regional one at that, will relegate them to a limited number of cases. Cerebral oximeters hold more potential. Their greatest limitations lie in technical aspects that can be, and hopefully will be, improved upon in terms of computer technology as well as algorithm accuracy. The fact that cerebral oximeters can be used on any patient, at any time, on almost any case, makes it, potentially, truly an ideal monitor for anesthesiologists and intensivists alike. There is no certainty that any of these limitations will be surmounted, at least to the degree necessary to achieve desired accuracy. But there is much to anticipate.

Cerebral blood flow assessment with indocyanine green bolus transit detection by near-infrared spectroscopy in the rat

In this study we evaluated the feasibility of measuring cerebral blood flow in rats by monitoring the transit of an indocyanine green bolus through the brain with multiwavelength near-infrared spectroscopy. Different volumes of a 1 mg/ml indocyanine green solution (5, 15, 25, 50 microl) were injected intravenously in the search for an optimal dose. Clear transit curves were obtained with all doses and a blood flow index could easily be determined. The indocyanine green signal obtained with the bolus of 5 microl rapidly returned to baseline and interfered minimally with the haemoglobin and cytochrome oxidase signals. This dose was used in a second study to evaluate the reproducibility of the signal and the effect of hypercapnia. Two groups of rats received 7 repetitive boli of indocyanine green. In one group, 7% CO(2) was added to the gas mixture before the second, fourth and sixth indocyanine green injection. Hypercapnia consistently caused a significant increase in blood flow index, cerebral haemoglobin concentration and O(2)-saturation. In the control group these variables remained stable in time. We conclude that monitoring of the transit of an indocyanine green bolus with multiwavelength near-infrared spectroscopy can be used to assess cerebral blood flow qualitatively in rats in combination with continuous monitoring of brain oxygenation.

Technical aspects and utility of fMRI using BOLD and ASL

Functional magnetic resonance imaging (fMRI) is an emerging methodology which provides various approaches to visualizing regional brain activity non-invasively. Although the exact mechanisms underlying the coupling between neural function and fMRI signal changes remain unclear, fMRI studies have been successful in confirming task-specific activation in a variety of brain regions, providing converging evidence for functional localization. In particular, fMRI methods based on blood oxygenation level dependent (BOLD) contrast and arterial spin labeling (ASL) perfusion contrast have enabled imaging of changes in blood oxygenation and cerebral blood flow (CBF). While BOLD contrast has been widely used as the surrogate marker for neural activation and can provide reliable information on the neuroanatomy underlying transient sensorimotor and cognitive functions, recent evidence suggests perfusion contrast is suitable for studying relatively long term effects on CBF both at rest or during activation. New developments in combining or simultaneously measuring the electrophysiological and fMRI signals allow a new class of studies that capitalize on dynamic imaging with high spatiotemporal resolution. This article reviews the biophysical bases and methodologies of fMRI and its applications to the clinical neurosciences, with emphasis on the spatiotemporal resolution of fMRI and its coupling with neurophysiology under both normal and pathophysiological conditions.

Continuous-wave near-infrared spectroscopy using pathlength-independent hypoxia normalization

A general physiological model for the hemodynamic response during altered blood flow, oxygenation, and metabolism is presented. Calculations of oxy-, deoxy-, and total hemoglobin changes during stimulation are given. It is shown that by using a global hyperoxic or mild hypoxic challenge it is possible to normalize the activation response in terms of the fractional changes in the cerebral blood volume, tissue oxygenation index, and oxygen extraction ratio, which are independent of the optical pathlength. Using a dual wavelength spectrometer, the method is validated by measuring pathlength-independent hemodynamic responses during mild hypercarbia in a rat model. Phantom experiments showed that the changes in optical pathlength were small as the hemoglobin concentration was varied over a wide range. The determination of quantitative parameters facilitates the use of continuous-wave transcranial methods by providing a means by which to characterize activation response across subjects.

Cerebral hemodynamic response to unpleasant odors in the preterm newborn measured by near-infrared spectroscopy

Newborn infants in intensive care units are exposed to several unfamiliar smells, mostly related to the nosocomial environment. How the preterm baby perceives these olfactory stimulations remains unclear. Near-infrared spectroscopy can be performed noninvasively above the olfactory cortex to monitor changes of cerebral blood flow as an indicator of cortical activation. The aim of this study was to explore by near-infrared spectroscopy how odorous substances routinely used in the neonatal intensive care unit influence bilateral cortical hemodynamics in the olfactory region of the brains of preterm infants. Specifically, a detergent (Neomidil) and an adhesive remover (Remove) have been tested. Twenty preterm neonates of gestational age 30-37 wk (mean 33.7 +/- 2.3 SD) and postconceptional age 32-37.3 wk (mean 35.5 +/- 2.75 SD) were monitored by near-infrared spectroscopy. Two optode pairs were placed above the anterior orbitofrontal gyri, which is involved in olfactory processing, on each side of the skull. Fifteen babies were exposed to the smell of a disinfectant and five babies to that of a detergent, both applied to small cotton pads. Changes of oxygenated Hb and deoxygenated Hb were recorded before, during, and after a 10-s stimulus. In 17 out of 20 babies, there was a decrease in oxygenated Hb and total Hb after the exposure to the substances. The decrease was significantly greater in the right side than in the left side. This change was different from that observed in our previous study after exposure to colostrum and the pleasant smell of vanilla, which elicited an increase in blood oxygenation in the same region. The biologic significance of this finding is unknown. We conclude that cortical hemodynamic modifications occur in the preterm newborn after exposure to preparations commonly used in the neonatal intensive care unit. A lateralization seems to occur in processing unpleasant olfactory cues.

Wavelength dependence of the precision of noninvasive optical measurement of oxy-, deoxy-, and total-hemoglobin concentration

The precision of noninvasive optical measurement of the concentration changes in oxy-, deoxy-, and total-hemoglobin depends on wavelength. For estimating the precision, we calculated the noise level of the concentration changes as the uncertainty in measurements using several wavelength pairs of light. Seven laser diodes (664-848 nm) were used simultaneously for spectroscopic measurement of brain activity during finger motor stimulation. We also used the analysis of error propagation from the uncertainty in direct measurements of absorbance changes to estimate indirectly the uncertainty of concentration changes. The measurement of the concentration changes made using an 830/664-nm pair are two times (oxy-Hb) and six times (deoxy-Hb) more precise than those made using an 830/782-nm pair.

Linear aspects of changes in deoxygenated hemoglobin concentration and cytochrome oxidase oxidation during brain activation

We used near infrared spectroscopy (NIRS) to investigate the vascular and metabolic response to brain activation in human primary and adjacent secondary visual cortex. NIRS is able to measure concentration changes in deoxygenated hemoglobin ([deoxy-Hb]) (which mainly contribute to the blood oxygenation level-dependent (BOLD) signal in functional magnetic resonance imaging (fMRI)) as well as concentration changes of oxygenated hemoglobin ([oxy-Hb]) and corpuscular blood volume ([total-Hb] = [oxy-Hb] + [deoxy-Hb]) and changes in the redox status of the cytochrome c oxidase ([Cyt-Ox]), a putative parameter for cellular oxygenation. A sound understanding of the transfer functions between stimulus parameters, neuronal activity, and vascular/metabolic parameters is important for interpretation of data acquired with indirect neuroimaging techniques like fMRI, especially in event-related design studies. In the present study we tested whether the vascular/metabolic response to stimulation can be described as a linear and time invariant system. Since linearity is a property attributed to systems that satisfy the scaling and superposition properties, as a first simple test, superposition of the responses obtained from short duration visual stimuli was used to predict the responses of longer duration stimuli. Our results showed that the predictions of [deoxy-Hb] and [Cyt-Ox] responses to stimuli of 6- to 24-s duration were satisfactory whereas predictions of [oxy-Hb] and [total-Hb] were insufficient. In a second step, a calculated convolution function of an assumed impulse response function and the stimulus function was fitted with the measured [deoxy-Hb] and [Cyt-Ox] curves to obtain amplitude, time delay, and time constant parameters. We show that predictions of cellular and vascular oxygenation responses to visual stimulation are good for 6- to 24-s stimuli duration under the assumption of a linear transfer characteristic. This model is not valid for corpuscular volume changes which affect mainly the [oxy-Hb] response. Noninvasive NIRS is shown to be a suitable method to get more direct information about neuronal-activity-associated changes in cerebral parameters which are partly reflected in BOLD signal but are not fully understood yet.

Brain auditory activation measured by near-infrared spectroscopy (NIRS) in neonates

This study presents a new measure of the hemodynamic changes to an auditory stimulus in newborns. Nineteen newborns born at 28-41 wk and aged 1 to 49 d were studied in waking and/or sleeping state, for a median time of 4 min 40 s before, 2 min 40 s during, and 3 min 5 s after an acustic stimulus (tonal sweep of frequency increasing from 2 to 4 kHz, intensity 90 dB SPL) originating 5 cm from the external auditory meatus. The emitter and detector optodes were placed over the left or right temporal region, corresponding to T3 or T4 EEG electrodes. The concentration changes in cerebral chromophores Delta[HbO2], Delta[Hb] and Deltaoxidized-reduced cytochrome aa(3) were recorded every 5 s. Changes in cerebral blood volume were calculated from the changes in total Hb x 0.89/large vessel Hb concentration. Increased oxyhemoglobin, Delta[HbO2], total Hb, Delta[Hb (sum)], and cerebral blood volume, DeltaCBV, were found in 13/19 neonates, with the exception of a neonate who only had increased in Delta[Hb], Delta[Hb (sum)] and DeltaCBV. During the stimulation phase there was a significant increase in DeltaCBV (t test, p = 0.00006) in the responsive newborns from a mean value of 0.006 (+/-0.02) mL/100 g in the pretest phase to 0.09 (+/-0.06) mL/100 g during the auditory stimulus. After the test DeltaCBV decreased to 0.04 (+/-0.07) mL/100 g (t test, p = 0.01), so did Delta[Hb (sum)] (p = 0.02). Hemodynamic responses of the subjects who showed increases in Delta[Hb (sum)] and Delta[HbO(2)] were analyzed to study the Delta[Hb]. The responder subjects could be classified into two groups according to Delta[Hb] changes: 8/13 (61.5%) showed an increase of Delta[Hb] (pattern A), while 5/13 (38.4%) showed a decrease (pattern B) (t test, p = 0.03). These two patterns did not show differences related to Delta[HbO(2)] and Delta[Hb (sum)]. The DeltaCBV changes in nonresponders presented a decrease during the test phase (t test, p = 0.04). CBV did not return to pretest values, suggesting a fronto-temporal brain pathway for storing unusual sounds. The increase in CBV followed the local increase in oxyhemoglobin and total Hb concentrations due to a greater use of oxygen in the homolateral temporal cortex of the newborns.

Simultaneous near-infrared spectroscopy monitoring of left and right occipital areas reveals contra-lateral hemodynamic changes upon hemi-field paradigm

In this study we have shown that in humans it is possible to monitor non-invasively and simultaneously both hemispheres revealing cortical oxygenation changes in the occipital area in response to a contra-lateral hemi-field paradigm. A novel multi-channel near infrared spectroscopy approach with a high temporal resolution was used. The results confirm previous findings obtained by functional magnetic resonance imaging and positron emission tomography with the advantage to measure directly not only concentration changes in deoxyhemoglobin as measured by functional magnetic resonance imaging (MRI), but also in oxyhemoglobin with low cost instrumentation potentially useful to investigate the pathophysiology of vision.

Sentence processing in the cerebral cortex

Human language is a unique faculty of the mind. It has been the ultimate mystery throughout the history of neuroscience. Despite many aphasia and functional imaging studies, the exact correlation between cortical language areas and subcomponents of the linguistic system has not been established. One notable drawback is that most functional imaging studies have tested language tasks at the word level, such as lexical decision and word generation tasks, thereby neglecting the syntactic aspects of the language faculty. As proposed by Chomsky, the critical knowledge of language involves universal grammar (UG), which governs the syntactic structure of sentences. In this article, we will review recent advances made by functional neuroimaging studies of language, focusing especially on sentence processing in the cerebral cortex. We also present the recent results of our functional magnetic resonance imaging (fMRI) study intended to identify cortical areas specifically involved in syntactic processing. A study of sentence processing that employs a newly developed technique, optical topography (OT), is also presented. Based on these findings, we propose a modular specialization of Broca's area, Wernicke's area, and the angular gyrus/supramarginal gyrus. The current direction of research in neuroscience is beginning to establish the existence of distinct modules responsible for our knowledge of language.

New radiographic techniques to evaluate cerebrovascular disorders in children

The radiographic evaluation of the pediatric patient with cerebrovascular disease has dramatically improved during the past decade. Few new technologies have been introduced, but significant new developments in data acquisition and post-processing have resulted from refinements in both software and, to a lesser extent, hardware. This review focuses on the advantages and limitations of the different imaging modalities and their recommended role in managing the pediatric patient who presents with signs or symptoms of cerebrovascular disease.

Prefrontal hypooxygenation during language processing assessed with near-infrared spectroscopy

Near-infrared spectroscopy (NIRS) is a noninvasive optical method which allows in vivo measurements of relative concentrations of oxygenated (O2Hb) and desoxygenated hemoglobin (HHb). It has been successfully applied to assess the blood oxygenation changes during cerebral ischemia in man. An interesting application of NIRS is the investigation of regional cerebral blood oxygenation during physiological brain functions. In the present study, left and right anterior prefrontal brain areas of 10 healthy subjects were investigated with 2-channel NIRS during language processing (reading aloud). Nonverbal visual perception (picture observation) was performed as a control condition. Compared to the respective baseline conditions, a significant HHb increase and an O2Hb decrease as a trend were found during language processing but not during the nonverbal perceptual task, and no hemispheric differences were found. The metabolic activation patterns differed significantly between the tasks indicating the changes due to the language-related efforts but not to visual perception. Based on previous findings of metabolic brain imaging studies the results are best explained as a complementary phenomenon consisting in relative anterior prefrontal hypoperfusion due to a blood volume redistribution which favors language-related temporal areas.

Assessment of heating effects in skin during continuous wave near infrared spectroscopy

Near infrared spectroscopy is an increasingly important tool for the investigation of human brain function, however, to date there have been few systematic evaluations of accompanying thermal effects due to absorption. We have measured the spatial distribution of temperature changes during near infrared irradiation (789 nm) as a function of laser power, in both excised tissue (chicken meat and skin) and in the forearm of an awake human volunteer. Light was applied using a 1 mm optical fiber which is characteristic of the topographic system. The temperature of excised chicken tissue increased linearly with power level as 0.097 and 0.042 degrees C/mW at depths of 0 and 1 mm, respectively. Human forearm studies yielded temperature changes of 0.101, 0.038, and 0.030 degrees C/mW at depths of 0.5, 1.0, and 1.5 mm, respectively. Due to direct irradiation of the thermocouple all measurements represent the maximum temperature increase from the laser. In all cases the estimated heating effects from continuous wave optical topography systems were small and well below levels which would endanger tissue cells. The close similarity between ex vivo and in vivo measurements suggests negligible contributions from blood flow in the skin which was further supported by measurements during cuff ischemia. Heating effects decreased sharply with both depth and lateral position; thus, for optode spacings greater than a few millimeters, fibers can be treated independently. Finite element analysis confirms that the experimental results are consistent with a simple heat conduction model.

Saccadic suppression induces focal hypooxygenation in the occipital cortex

This study investigated how a decrease in neuronal activity affects cerebral blood oxygenation employing a paradigm of acoustically triggered saccades in complete darkness. Known from behavioral evidence as saccadic suppression, electrophysiologically it has been shown in monkeys that during saccades an attenuation of activity occurs in visual cortex neurons (Duffy and Burchfiel, 1975). In study A, using blood oxygen level-dependent (BOLD) contrast functional magnetic resonance imaging (fMRI), the authors observed signal intensity decreases bilaterally at the occipital pole during the performance of saccades at 2 Hz. In study B.1, the authors directly measured changes in deoxyhemoglobin [deoxy-Hb] and oxyhemoglobin [oxy-Hb] concentration in the occipital cortex with near-infrared spectroscopy (NIRS). Whereas a rise in [deoxy-Hb] during the performance of saccades occurred, there was a drop in [oxy-Hb]. In a second NIRS study (B.2), subjects performed saccades at different rates (1.6, 2.0, and 2.3 Hz). Here the authors found the increase in deoxy-Hb and the decrease of oxy-Hb to be dependent on the frequency of the saccades. In summary, the authors observed a focal hypooxygenation in the human visual cortex dependent on the saccade-frequency in an acoustically triggered saccades paradigm. This could be interpreted as evidence that corresponding to the focal hyperoxygenation observed in functional brain activation, caused by an excessive increase in cerebral blood flow (CBF) over the increase in CMRO2 during decreased neuronal activity CBF, is more reduced than oxygen delivery.

Temporal and topographical characterization of language cortices using intraoperative optical intrinsic signals

We used intraoperative optical imaging of intrinsic signals (iOIS) and electrocortical stimulation mapping (ESM) to compare functionally active brain regions in 10 awake patients undergoing neurosurgical resection. Patients performed two to four tasks, including visual and auditory naming, word discrimination, and/or orofacial movements. All iOIS maps included areas identified by ESM mapping. However, iOIS also revealed topographical specificity dependent on language task. In Broca's area, naming paradigms activated both anterior and posterior inferior frontal gyrus (IFG), while the word discrimination paradigm activated only posterior IFG. In Wernicke's area, object naming produced activations localizing over the inferior and anterior/posterior regions, while the word discrimination task activated superior and anterior cortices. These results may suggest more posterior phonological activation and more anterior semantic activations in Broca's area, and more anterior/superior phonological activation and more posterior/inferior semantic activations in Wernicke's area. Although similar response onset was observed in Broca's and Wernicke's areas, temporal differences were revealed during block paradigm (20-s) activations. In Broca's area, block paradigms yielded a boxcar temporal activation profile (in all tasks) that resembled response profiles observed in motor cortex (with orofacial movements). In contrast, activations in Wernicke's area responded with a more dynamic profile (including early and late peaks) which varied with paradigm performance. Wernicke's area profiles were very similar to response profiles observed in sensory and visual cortex. The differing temporal patterns may therefore reflect unique processing performed by receptive (Wernicke's) and productive (Broca's) language centers. This study is consistent with task-specific semantic and phonologic regions within Broca's and Wernicke's areas and also is the first report of response profile differences dependent on cortical region and language task.

Activation of olfactory cortex in newborn infants after odor stimulation: a functional near-infrared spectroscopy study

In mammals, perception of smells during the first hours of life is an essential prerequisite for adaptation of the newborn to the new extrauterine world. Functional magnetic resonance studies have shown that olfactory impression is processed in the lateral and anterior orbito-frontal gyri of the frontal lobe. Near-infrared spectroscopy (NIRS) can detect changes in oxygenated [Hb O2], and deoxygenated [Hb H] Hb during cortical activation. The aim of this study was to assess by NIRS olfactory cortex activity in newborn infants receiving olfactory stimuli. Twelve males and 11 females were studied when awake at 6 h to 8 d after birth. NIRS monitoring was carried out using two optodes placed above the left anterior orbito-frontal gyri. Each newborn was exposed for 30 s to two different smell stimuli-mother's colostrum and vanilla-and to a negative control, distilled water. Changes in Hb concentration were measured over the orbitofrontal region. During exposure to vanilla, [Hb O2] increased significantly over the left orbito-frontal area in all babies. The magnitude of the [Hb O2] increase over the illuminated region during colostrum exposure was inversely related to postnatal age. We conclude that monitoring Hb changes by NIRS can be valuable in assessing olfactory responsiveness in infants.

Intracerebral hemodynamics probed by near infrared spectroscopy in the transition between wakefulness and sleep

Previous imaging studies have shown that cerebral metabolism is gradually reduced at the beginning of sleep. Few studies have examined the sleep state transition periods from wakefulness to sleep and sleep to wakefulness. The current study used the Near Infrared Spectroscopy (NIRS) technique to describe the intracerebral hemodynamics at the frontal pole in the circumscribed period between wakefulness and sleep. Nine healthy young adults were studied during afternoon naps. Optical probes were placed on the forehead and EEG electrodes on the scalp. At sleep onset oxygenated hemoglobin (oxy-Hb) was reduced (P<0.01) and deoxygenated hemoglobin (deoxy-Hb) showed a near significant reduction (P<0.063). At sleep offset there were increases in oxy-Hb (P<0.005) and deoxy-Hb (P<0.05). In 18 of 26 transitions to sleep there was a coordinated fall in both NIRS parameters, we call the Switch Point, that lasted a mean of 3.6 s. In 32 of 36 transitions to wakefulness there was an analogous Switch Point that lasted a mean of 3.4 s. Before and after the Switch Point, changes were small and the relationship between oxy-Hb and deoxy-Hb was a combination of parallel and reciprocal fluctuations. A synchronized, parallel and short-lived change in oxy-Hb and deoxy-Hb is a discrete event in the transition period between wakefulness and sleep. The concentration of these light absorbing molecules is abruptly set to a new level at sleep-wake transitions and probably reflects the different perfusion demands of these states.

Visuospatial imagery is a fruitful strategy for the digit span backward task: a study with near-infrared optical tomography

Our newly developed 64-channel time-resolved optical tomographic imaging system using near-infrared light enables us to obtain a quantitative image of hemoglobin concentration changes associated with neuronal activation in the human brain ¿H. Eda, I. Oda, Y. Ito, Y. Wada, Y. Oikawa, Y. Tsunazawa, M. Takada, Y. Tsuchiya, Y. Yamashita, M. Oda, A. Sassaroll, Y. Yamada, M. Tamura, Multi-channel time-resolved optical tomographic imaging system, Rev. Sci. Instrum., 70 (1999) 3595-3602. Here, we used this optical imaging system to demonstrate that the backward digit span (DB) task activated the dorsolateral prefrontal cortex (DLPFC) of each hemisphere more than the forward digit span (DF) task in healthy adult volunteers, and higher performance of the DB task was closely related to the activation of the right DLPFC. These results suggest that visuospatial imagery is a useful strategy for the DB task. Optical tomography described here is a new modality of neuropsychological studies.

Simultaneous assessment of brain tissue oxygenation and cerebral perfusion during orthostatic stress

Simultaneous registration of cerebral tissue oxygenation parameters obtained by near infrared spectroscopy (NIRS), intracranial blood flow velocity (CBFV) measured by transcranial Doppler sonography (TCD) and basic cardiovascular parameters was carried out during a passive 80 degrees head-up tilt table test in 15 patients with a history of orthostatic syncope and 20 control subjects. In normals, the cardiovascular parameters showed a specific course after changing to a vertical position: the heart rate increased, the mean arterial blood pressure remained unchanged, and the CBFV decreased. The NIRS measurements showed an increase in deoxyhemoglobin (HHb) and a decline in oxyhemoglobin (O(2)Hb) and the regional oxygen saturation (RSAT). Patients had a significantly more prominent decline in arterial blood pressure (p < 0.001), CBFV (p < 0.001) and RSAT (p = 0.04). Five patients experienced symptoms of (pre)syncope during the experiment, which were associated with a further sudden and marked (>10%) drop of O(2)Hb. The results indicate that the combination of TCD and NIRS increases the understanding of hemodynamic and metabolic changes during orthostatic stress, which may lead to individually suited therapeutic procedures.

Cerebral oxygenation state in childhood moyamoya disease: a near-infrared spectroscopy study

Near-infrared spectroscopy was used to measure the relative concentration changes of oxygenated hemoglobin (HbO(2)), deoxygenated hemoglobin (HbR), and total hemoglobin (t-Hb) before, during, and after hyperventilation in eight patients with childhood moyamoya disease who underwent surgery. The patients were divided into the following two groups: those with and those without re-build-up phenomenon on electroencephalography. All patients except one exhibited decreased concentrations of HbO(2) and t-Hb during hyperventilation and decreased concentrations of HbO(2) and increased concentrations of HbR after hyperventilation. The difference between the hemoglobin concentration during and after hyperventilation was significantly greater in the group with the re-build-up phenomenon than the group without it (P <0.03, Mann-Whitney U test). The patients with the re-build-up phenomenon had lower HbO(2) and higher HbR concentrations after hyperventilation. The concentrations of HbO(2) and HbR reflect an alteration in the oxygenated blood supply and/or oxygen use. Thus, it is strongly suggested that regional cerebral hypoxia and metabolic oxygen disturbances play an important role in the occurrence of the re-build-up phenomenon. Near-infrared spectroscopy is a useful noninvasive method to evaluate the efficacy of surgical treatment in childhood moyamoya disease.

Acute carotid occlusion alters the activation flow coupling response to forepaw stimulation in a rat model

BACKGROUND AND PURPOSE

To determine whether the hemodynamic response to functional stimulation is sensitive to proximal arterial occlusion, we measured the activation flow coupling response in a rat model of acute reversible vascular occlusion.

METHODS

In alpha-chloralose-anesthetized rats (n=18), laser Doppler measurements were made through a thinned skull over the somatosensory cortex in response to electrical forepaw stimulation. Signal-averaged responses to 4 and 8 seconds of electrical forepaw stimulation were obtained before, during, and shortly after acute unilateral or bilateral carotid occlusion produced with the use of a surgically placed snare.

RESULTS

Baseline cerebral blood flow was significantly decreased over the forepaw region of the somatosensory cortex after both occlusion of the carotid contralateral to the stimulated forepaw and bilateral occlusion compared with preocclusion (P<0.05). Postocclusion and ipsilateral occlusion led to a nonsignificant increase in baseline cerebral blood flow compared with preocclusion. Contralateral carotid occlusion and bilateral occlusion significantly prolonged the temporal characteristics of the flow response, especially the delay to peak (P<0.05), compared with preocclusion, whereas ipsilateral carotid occlusion significantly shortened the delay to peak (P<0.05). Only contralateral carotid occlusion produced a significant reduction in the peak amplitude of the flow response compared with preocclusion (P<0.05).

CONCLUSIONS

These findings suggest that temporal characteristics of functional activation responses are sensitive to alterations in the proximal arterial supply and, conversely, that functional activation studies must be interpreted with consideration of proximal arterial disease.