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

Time-gated optical system for depth-resolved functional brain imaging

We present a time-domain optical system for functional imaging of the adult head. We first describe the instrument, which is based on a Ti:Sapphire pulsed laser (wavelength 750-850 nm) and an intensified CCD camera enabling parallel detection of multiple fibers. We characterize the system in terms of sensitivity and signal-to-noise ratio, instrument response function, cross-talk, stability, and reproducibility. We then describe two applications of the instrument: the characterization of baseline optical properties of homogeneous scattering media, and functional brain imaging. For the second application, we developed a two-part probe consisting in two squares of 4 x 4 sources and 3 x 3 detectors. The laser source is time-multiplexed to define 4 states of 8 sources that can be turned on during the same camera frame while minimizing cross-talk. On the detection side, we use for each detector 7 fibers of different lengths creating an optical delay, and enabling simultaneous detection in 7 windows (by steps of 500 ps) for each detector. This multiple window detection allows depth sensitivity. The imaging probe was tested on dynamic phantoms and a preliminary result on an adult performing a motor task shows discrimination between superficial and cortical responses to the stimulus on both hemispheres.

Resting hypofrontality in schizophrenia: A study using near-infrared time-resolved spectroscopy

Hypofrontality has been a major finding obtained from functional neuroimaging studies on schizophrenia, although there have also been contradictory results that have questioned the reality of hypofrontality. In our previous study, we confirmed the existence of activation hypofrontality by using a 2-channel continuous-wave-type (CW-type) near-infrared spectroscopy (NIRS) instrument. In this study, we employed a single-channel time-resolved spectroscopy (TRS) instrument, which can quantify hemoglobin (Hb) concentrations based on the photon diffusion theory, to investigate resting hypofrontality. A pair of incident and detecting light guides was placed on either side of the forehead at approximately Fp2-F8 or Fp1-F7 alternately in 14 male schizophrenic patients and 16 age-matched male control subjects to measure Hb concentrations at rest. The patients were also measured with a 2-channel CW-type NIRS instrument during the performance of a random number generation (RNG) task. A reduced total hemoglobin concentration (t-Hb) less than 60 microM (the mean value of the control subjects-1.5 SD) was observed bilaterally in 4 patients and only in the left side in 3 patients. Activation hypofrontality was more manifest in these patients than in the remaining 7 patients despite the same task performance. This decreased t-Hb was related to the duration of illness, and it was not observed in patients whose duration of illness was less than 10 years. These results indicate that resting hypofrontality is a chronically developed feature of schizophrenia. This does not necessarily represent frontal dysfunction, but may reflect anatomical and/or functional changes in frontal microcirculation.

Prefrontal hemodynamic activity predicts false memory--a near-infrared spectroscopy study

Evidence from lesion studies suggests an important role of the prefrontal cortex (PFC) in the reconstructive processes of episodic memory or memory distortion. Results from functional imaging studies imply PFC involvement during the illusionary recollection of non-experienced events. Here, we used a two-channel near-infrared spectroscopy (NIRS) system and conducted real-time monitoring of PFC hemodynamics, while subjects studied word lists and subsequently recognized unstudied items (false recognition). Bilateral increases in the oxygenated hemoglobin concentration ([oxy-Hb]) were observed during false recognition compared to true recognition, and a left PFC dominant increase of [oxy-Hb] was observed during encoding phases where subjects later claimed that they recognized unstudied words. Traces of semantic processing, reflected primarily in the left PFC activity, could eventually predict whether subjects falsely recognize non-experienced events.

A temporal comparison of BOLD, ASL, and NIRS hemodynamic responses to motor stimuli in adult humans

In this study, we have preformed simultaneous near-infrared spectroscopy (NIRS) along with BOLD (blood oxygen level dependent) and ASL (arterial spin labeling)-based fMRI during an event-related motor activity in human subjects in order to compare the temporal dynamics of the hemodynamic responses recorded in each method. These measurements have allowed us to examine the validity of the biophysical models underlying each modality and, as a result, gain greater insight into the hemodynamic responses to neuronal activation. Although prior studies have examined the relationships between these two methodologies through similar experiments, they have produced conflicting results in the literature for a variety of reasons. Here, by employing a short-duration, event-related motor task, we have been able to emphasize the subtle temporal differences between the hemodynamic parameters with a high contrast-to-noise ratio. As a result of this improved experimental design, we are able to report that the fMRI measured BOLD response is more correlated with the NIRS measure of deoxy-hemoglobin (R = 0.98; P < 10(-20)) than with oxy-hemoglobin (R = 0.71), or total hemoglobin (R = 0.53). This result was predicted from the theoretical grounds of the BOLD response and is in agreement with several previous works [Toronov, V.A.W., Choi, J.H., Wolf, M., Michalos, A., Gratton, E., Hueber, D., 2001. "Investigation of human brain hemodynamics by simultaneous near-infrared spectroscopy and functional magnetic resonance imaging." Med. Phys. 28 (4) 521-527.; MacIntosh, B.J., Klassen, L.M., Menon, R.S., 2003. "Transient hemodynamics during a breath hold challenge in a two part functional imaging study with simultaneous near-infrared spectroscopy in adult humans". NeuroImage 20 1246-1252.; Toronov, V.A.W., Walker, S., Gupta, R., Choi, J.H., Gratton, E., Hueber, D., Webb, A., 2003. "The roles of changes in deoxyhemoglobin concentration and regional cerebral blood volume in the fMRI BOLD signal" Neuroimage 19 (4) 1521-1531]. These data have also allowed us to examine more detailed measurement models of the fMRI signal and comment on the roles of the oxygen saturation and blood volume contributions to the BOLD response. In addition, we found high correlation between the NIRS measured total hemoglobin and ASL measured cerebral blood flow (R = 0.91; P < 10(-10)) and oxy-hemoglobin with flow (R = 0.83; P < 10(-05)) as predicted by the biophysical models. Finally, we note a significant amount of cross-modality, correlated, inter-subject variability in amplitude change and time-to-peak of the hemodynamic response. The observed co-variance in these parameters between subjects is in agreement with hemodynamic models and provides further support that fMRI and NIRS have similar vascular sensitivity.

Registering fNIR data to brain surface image using MRI templates

Functional near-infrared spectroscopy (fNIR) measures changes in the relative levels of oxygenated and deoxygenated hemoglobin and has increasingly been used to assess neural functioning in the brain. In addition to the ongoing technological developments, investigators have also been conducting studies on functional mapping and refinement of data analytic strategies in order to better understand the relationship between the fNIR signal and brain activity. However, since fNIR is a relatively new functional brain imaging modality as compared to positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), it still lacks brain-mapping tools designed to allow researchers and clinicians to easily interact with their data. The aim of this study is to develop a registration technique for the fNIR measurements using anatomical landmarks and structural magnetic resonance imaging (MRI) templates in order to visualize the brain activation when and where it happens. The proposed registration technique utilizes chain-code algorithm and depicts activations over respective locations based on sensor geometry. Furthermore, registered data locations have been used to create spatiotemporal visualization of fNIR measurements.

Within-subject reproducibility of near-infrared spectroscopy signals in sensorimotor activation after 6 months

Near-infrared spectroscopy (NIRS) can measure the product of the optical path length and the concentration change in oxygenated hemoglobin (DeltaC'oxy), deoxygenated hemoglobin (DeltaC'deoxy), and their sum (DeltaC'total) in the human cerebral cortex, and it has been used for noninvasive investigation of human brain functions. We evaluate the within-subject reproducibility of the NIRS signals by repeated measurement of the sensorimotor cortex in healthy adults taken over a period of about 6 months using near-infrared (NIR) topography. The maximum signal amplitudes and the location of activation centers are compared between two sessions for each subject. The signal amplitudes vary between sessions and no consistent tendency in the changes is found among subjects. However, the distance between the activation centers identified in two sessions is relatively small, within 20 mm on average across subjects, which is comparable to the smallest distance between measurement positions in the NIR topography (21 mm). Moreover, within-subject comparisons of signal time courses show high correlation coefficients (>0.8) between the two sessions. This result, demonstrating a high within-subject reproducibility of the temporal information in NIRS signals, particularly contributes to the development of a new application of NIRS.

Cortical hemoglobin-concentration changes under the coil induced by single-pulse TMS in humans: a simultaneous recording with near-infrared spectroscopy

We measured cortical hemoglobin-concentration changes under the coil induced by single-pulse transcranial magnetic stimulation (TMS) using a technique of simultaneous recording with near-infrared spectroscopy (NIRS). Single-pulse TMS was delivered over the hand area of the left primary motor cortex at an intensity of 100, 120, or 140% of the active motor threshold (AMT). NIRS recordings were also made during sham stimulation. These four different stimulation sessions (TMS at three intensities and sham stimulation) were performed both when the subject slightly contracted the right first dorsal interosseous muscle and when relaxed it (active and resting conditions). Under the active condition with TMS at 100% AMT, we observed a transient increase in oxy-hemoglobin (oxy-Hb), which was significantly larger than sham stimulation. Under the resting conditions with TMS at 120 and 140% AMT, we observed significant decreases in both deoxy-hemoglobin (deoxyHb) and total-hemoglobin (total-Hb) as compared to sham stimulation. We suggest that the increase of oxy-Hb concentration at 100% AMT under the active condition reflects an add-on effect by TMS to the active baseline and that decrease of deoxy-Hb and total-Hb concentrations at 120 and 140% AMT under the resting condition are due to reduced baseline firings of the corticospinal tract neurons induced by a lasting inhibition provoked by a higher intensity TMS.

Extraction of depth-dependent signals from time-resolved reflectance in layered turbid media

We try a new approach with near-IR time-resolved spectroscopy, to separate optical signals originated in the upper layer from those in the lower layer and to selectively determine the absorption coefficient (mu(a)) of each layer in a two-layered turbid medium. The difference curve in the temporal profiles of light attenuation between a target and a reference medium is divided into segments along the time axis, and a slope of each segment is calculated to determine the depth-dependent mu(a). The depth-dependent mu(a) values are estimated under various conditions in which mu(a) and the reduced scattering coefficient (mu(s)') of each layer are changed with a Monte Carlo simulation and in phantom experiments. Temporal variation of them represents the difference in mu(a) between two layers when mu(s)' of a reference is the same as that of the upper layer of the target. The discrepancies between calculated mu(a) and the real mu(a) depend on the ratio of the real mu(a) of the upper layer to that of the lower layer, and our approach enables us to estimate the ratio of mu(a) between the two layers. These results suggest the potential that mu(a) of the lower layer can be determined by our procedure.

Motor-Related Intracortical Steal Phenomenon Detected by Multichannel Functional Near-Infrared Spectroscopy Imaging

BACKGROUND

Patients with severe cerebral ischemia may lose autoregulation to increase cerebral blood flow following neural activity. Although the steal phenomenon under conventional cerebral blood flow study has been known as a high-risk factor for stroke, the cerebral oxygen hemodynamics in ischemic patients during functional activation has not been thoroughly investigated. In this study, we present rare cases with intracortical steal phenomenon during motor tasks detected by multichannel functional near-infrared spectroscopy before and after surgery.

METHODS

The relative concentration change of oxygenated, deoxygenated and total hemoglobin in and around the primary sensorimotor cortex during contralateral hand grasping was investigated in 11 patients with severe internal carotid artery stenosis.

RESULTS

In 3 patients, the concentration of total hemoglobin around the primary sensorimotor cortex significantly decreased in response to motor stimulation and returned to baseline soon after termination of the motor task. This phenomenon partially disappeared postoperatively in all patients who underwent surgery. The remaining 8 patients showed no signs of total hemoglobin decrease in and around the sensorimotor cortex. In 9 patients, lack of decrease in deoxygenated hemoglobin in the center of the primary motor cortex during the motor task was observed and 3 of them showed significant increase in deoxygenated hemoglobin.

CONCLUSIONS

We have demonstrated that in some patients with severe ischemia, an abnormal motor-related steal phenomenon can be observed. This phenomenon can be modulated by surgical intervention and might imply the severity of ischemia.

Reevaluation of near-infrared light propagation in the adult human head: implications for functional near-infrared spectroscopy

Using both experimental and theoretical methods, we examine the contribution of different parts of the head to near-IR (NIR) signal. Time-resolved spectroscopy is employed to measure the mean optical path length (PL), and the absorption (mu(a)) and reduced scattering (mu(s)') coefficients in multiple positions of the human head. Monte Carlo simulations are performed on four-layered head models based on an individual magnetic resonance imaging (MRI) scan to determine mu(a) and mu(s)' in each layer of the head by solving inverse problems, and to estimate the partial path length in the brain (p-PL) and the spatial sensitivity to regions in the brain at the source-detector separation of 30 mm. The PL is closely related to the thickness of the scalp, but not to that of other layers of the head. The p-PL is negatively related to the PL and its contribution ratio to the PL is 5 to 22% when the differential path length factor is 6. Most of the signal attributed to the brain comes from the upper 1 to 2 mm of the cortical surface. These results indicate that the NIR signal is very sensitive to hemodynamic changes associated with functional brain activation in the case that changes in the extracerebral tissue are ignorable.

Study of photon migration with various source-detector separations in near-infrared spectroscopic brain imaging based on three-dimensional Monte Carlo modeling

We have simulated photon migration with various sourcedetector separations based on a three-dimensional Monte Carlo code. Whole brain MRI structure images are introduced in the simulation, and the brain model is more accurate than in previous studies. The brain model consists of the scalp, skull, CSF layer, gray matter, and white matter. We demonstrate dynamic propagating movies under different source-detector separations. The multiple backscattered intensity from every layer of the brain model is obtained by marking the deepest layer that every photon can reach. Also, the influences of an absorption target on the brain cortex are revealed.

Spatial registration of multichannel multi-subject fNIRS data to MNI space without MRI

The registration of functional brain data to the common brain space offers great advantages for inter-modal data integration and sharing. However, this is difficult to achieve in functional near-infrared spectroscopy (fNIRS) because fNIRS data are primary obtained from the head surface and lack structural information of the measured brain. Therefore, in our previous articles, we presented a method for probabilistic registration of fNIRS data to the standard Montreal Neurological Institute (MNI) template through international 10-20 system without using the subject's magnetic resonance image (MRI). In the current study, we demonstrate our method with a new statistical model to facilitate group studies and provide information on different components of variability. We adopt an analysis similar to the single-factor one-way classification analysis of variance based on random effects model to examine the variability involved in our improvised method of probabilistic registration of fNIRS data. We tested this method by registering head surface data of twelve subjects to seventeen reference MRI data sets and found that the standard deviation in probabilistic registration thus performed for given head surface points is approximately within the range of 4.7 to 7.0 mm. This means that, if the spatial registration error is within an acceptable tolerance limit, it is possible to perform multi-subject fNIRS analysis to make inference at the population level and to provide information on positional variability in the population, even when subjects' MRIs are not available. In essence, the current method enables the multi-subject fNIRS data to be presented in the MNI space with clear description of associated positional variability. Such data presentation on a common platform, will not only strengthen the validity of the population analysis of fNIRS studies, but will also facilitate both intra- and inter-modal data sharing among the neuroimaging community.

A haemodynamic response function model in spatio-temporal diffuse optical tomography

Diffuse optical tomography (DOT) is a new and effective technique for functional brain imaging. It can detect local changes in both oxygenated and deoxygenated haemoglobin concentrations in tissue based on differential absorption at multiple wavelengths. Traditional methods in spatio-temporal analysis of haemoglobin concentrations in diffuse optical tomography first reconstruct the spatial distribution at different time instants independently, then look at the temporal dynamics on each pixel, without incorporating any temporal information as a prior in the image reconstruction. In this work, we present a temporal haemodynamic response function model described by a basis function expansion, in a joint spatio-temporal DOT reconstruction of haemoglobin concentration changes during simulated brain activation. In this joint framework, we simultaneously employ spatial regularization, spectral information and temporal assumptions. We also present an efficient algorithm for solving the associated large-scale systems. The expected improvements in spatial resolution and contrast-to-noise ratio are illustrated with simulations of human brain activation.

Imaging of cerebrospinal fluid space and movement in mice using near infrared fluorescence

We developed an optical method for imaging the cerebrospinal fluid (CSF) space and the movement of CSF in mice using a near infrared fluorescence imaging methodology. Indocyanine green bound to high-density lipoprotein (ICG-HDL) was injected into the lumber subarachnoid space of nude mice. The time course of CSF movement was followed over 48 h. The imaging system was configured for epi-fluorescence measurements at near infrared wavelengths using the illumination light and narrow band excitation filtration with central wavelength of 755 nm. Emission light was filtered with a long pass filter with a cutoff at 798 nm. After the injection of ICG-HDL, a strong fluorescence signal clearly delineated the cisterna magna, bilateral supracerebellar/cerebello-pontine angle cistern, and interhemispheric cistern. Much of the fluorescent tracer was washed out within 24h after the injection. This study has therefore demonstrated that an optical method, employing near infrared fluorescence imaging, can determine the CSF space and the movement of CSF in nude mice.

Increased and decreased cortical reactivities in novelty seeking and persistence: a multichannel near-infrared spectroscopy study in healthy subjects

BACKGROUND

Near-infrared spectroscopy (NIRS) has enabled completely noninvasive measurements of regional cerebral blood volume (rCBV) changes in cortices. In the present study, we investigated the relationships between rCBV changes assessed with NIRS and two dimensions of personality, novelty seeking and persistence.

METHODS

Thirty right-handed healthy volunteers participated in the study. Their personality traits were assessed using the Temperament and Character Inventory (TCI), and changes in oxy- and deoxy-hemoglobin concentrations were monitored during 40 s unilateral finger tapping tasks over the subjects' bilateral temporal regions using a 24-channel NIRS machine.

RESULTS

The oxy-hemoglobin concentration increases were significantly correlated positively with novelty seeking scores and negatively with persistence scores in the TCI during the initial time segment of the left-finger tapping task.

CONCLUSION

Increased and decreased brain activations demonstrated using multichannel NIRS were assumed to characterize the cortical reactivities underlying novelty seeking and persistence temperament, respectively.

Intersubject variability of near-infrared spectroscopy signals during sensorimotor cortex activation

We investigate the intersubject signal variability of near-infrared spectroscopy (NIRS), which is commonly used for noninvasive measurement of the product of the optical path length and the concentration change in oxygenated hemoglobin (DeltaC'oxy) and deoxygenated hemoglobin (DeltaC'deoxy) and their sum (DeltaC'total) related to human cortical activation. We do this by measuring sensorimotor cortex activation in 31 healthy adults using 24-measurement-position near-infrared (NIR) topography. A finger-tapping task is used to activate the sensorimotor cortex, and significant changes in the hemisphere contralateral to the tapping hand are assessed as being due to the activation. Of the possible patterns of signal changes, 90% include a positive DeltaC'oxy, 76% included a negative DeltaC'deoxy, and 73% included a positive DeltaC'total. The DeltaC'deoxy and DeltaC'total are less consistent because of a large intersubject variability in DeltaC'deoxy; in some cases there is a positive DeltaC'deoxy. In the cases with no positive DeltaC'oxy in the contralateral hemisphere, there are cases of other possible changes for either or both hemispheres and no cases of no change in any hemoglobin species in either hemisphere. These results suggest that NIR topography is useful for observing brain activity in most cases, although intersubject signal variability still needs to be resolved.

Multi-channel near-infrared spectroscopy detects specific inferior-frontal activation during incongruent Stroop trials

Near-infrared spectroscopy (NIRS) is an optical method, which allows non-invasive in vivo measurements of changes in the concentration of oxygenated (O2Hb) and deoxygenated (HHb) hemoglobin in brain tissue. In the present study we investigated 10 healthy subjects by means of multi-channel NIRS (Optical Topography; ETG-100, Hitachi Medical Co., Japan) during performance of congruent and incongruent trials of the Stroop color word task. With a similar pattern of activation for both congruent and incongruent Stroop trials in the NIRS channels located left superior-frontally, the results for O2Hb and the total amount of hemoglobin (Hb-tot) indicate specific activation for interference trials in inferior-frontal areas of the left hemisphere. This result is in line with several neuroimaging studies (fMRI, PET) that have already investigated the frontal activation related to Stroop interference, which further supports the assumption that multi-channel NIRS is sensitive enough to detect spatially specific activation during the performance of cognitive tasks.

The fast optical signal--robust or elusive when non-invasively measured in the human adult?

Near infrared spectroscopy (NIRS) can detect vascular changes in cerebral cortical tissue elicited by functional stimulation. For some 10 years, another optical signal has been reported to be accessible by NIRS. This signal has been reported to correlate to the electrophysiological response rendering NIRS an exquisite non-invasive approach to investigate neurovascular coupling in the human adult. Due to their typical latency of up to hundreds of milliseconds, these signals have been termed "fast" optical signals and have been postulated to stem from scatter changes in neuronal tissue, as a fingerprint of the electrophysiological response. Here, we utter a less optimistic view on the non-invasive detectability of these changes in the human, motivated by an upper limit signal size estimation, predicting a signal size by orders of magnitude smaller than those previously reported. Also, we discuss the influence of small stimulus correlated movement artifacts potentially mimicking a fast optical signal. Based on invasive studies, we perform an upper limit estimation for changes in intensity and mean time of flight, which can be expected assuming a scatter change in the cerebral cortex while measuring on the surface of the head of an adult subject. Since the resulting numbers are far below those previously reported, we constructed a simple system, which minimizes technical noise. The system allows us to detect rather small intensity changes (2 x 10(-3)%) when averaging over approximately 3000 stimuli. Despite this outstandingly low noise level of the system, we find a reliable change in response to a sub-motor-threshold steady state median nerve stimulation in just one single subject (8 subjects examined, 4 subjects twice). Exceeding the motor threshold leads to large stimulus related artifacts, on a similar time scale and with comparable amplitude as previously reported signals. To check for potential modality specific problems, we next performed a visual stimulation study, avoiding potential motor artifacts. For the steady state visually evoked response, no subject yielded a reliable result (11 subjects examined, 4 subjects twice). The paper discusses these findings by a review of the literature on fast optical signals and their being accessible in the adult human.

Cerebral hemodynamic changes in response to an executive function task in children with attention-deficit hyperactivity disorder measured by near-infrared spectroscopy

The aim of the present study was to evaluate the hemodynamic changes in both prefrontal regions induced by a cognitive task in children with a developmental attention-deficit disorder in comparison to normal controls using near-infrared spectroscopy (NIRS). A total of 11 boys with a mean age of 10.4 (+/-1.2) years that met the DSM-IV criteria for attention-deficit hyperactivity disorder (ADHD) participated in the study and were compared with 9 healthy age- and sex-matched controls. Using a trail-making test designed for the task of connecting numbers from 1-90 in four sets, changes in oxygenated (O2Hb) and deoxygenated (HHb) hemoglobin, tissue oxygenation index (TOI), and cerebral blood volume (CBV) were measured by near-infrared spectroscopy. During the first test set, designed as a short-attention task, the children with ADHD showed significant increases in O2Hb and CBV, whereas the controls showed no significant changes. During the 4 task cycles in which extended attention was demanded, both groups showed increases in O2Hb and CBV, but only the controls showed an additional increase in HHb in the left prefrontal region. In the ADHD group only, TOI showed an increase mainly on the left side. NIRS is a sensitive tool for measuring differences in hemodynamic changes between boys with ADHD and normal controls. Overall, the normal controls showed lateralized oxygen consumption in the left prefrontal cortex during an extended-attention task, whereas the boys with ADHD showed an imbalance between oxygenated and deoxygenated hemoglobin during the short- and extended-attention tasks.

Simulation study of magnetic resonance imaging-guided cortically constrained diffuse optical tomography of human brain function

Diffuse optical imaging can measure brain activity noninvasively in humans through the scalp and skull by measuring the light intensity modulation arising from localized-activity-induced absorption changes within the cortex. Spatial resolution and localization accuracy are currently limited by measurement geometry to approximately 3 cm in the plane parallel to the scalp. Depth resolution is a more significant challenge owing to the limited angle tomography permitted by reflectance-only measurements. We combine previously established concepts for improving image quality and demonstrate, through simulation studies, their application for improving the image quality of adult human brain function. We show in a three-dimensional human head model that localization accuracy is significantly improved by the addition of measurements that provide overlapping samples of brain tissue. However, the reconstructed absorption contrast is significantly underestimated because its depth is underestimated. We show that the absorption contrast amplitude accuracy can be significantly improved by providing a cortical spatial constraint in the image reconstruction to obtain a better depth localization. The cortical constraint makes physiological sense since the brain-activity-induced absorption changes are occurring in the cortex and not in the scalp, skull, and cerebral spinal fluid. This spatial constraint is provided by segmentation of coregistered structural magnetic resonance imaging (MRI). However, the absorption contrast deep within the cortex is reconstructed superficially, resulting in an underestimation of the absorption contrast. The synthesis of techniques described here indicates that multimodality imaging of brain function with diffuse optical imaging and MRI has the potential to provide more quantitative estimates of the total and deoxyhemoglobin response to brain activation, which is currently not provided by either method independently. However, issues of depth resolution within the cortex remain to be resolved.

Signal and image processing techniques for functional near-infrared imaging of the human brain

Near-infrared spectro-imaging (NIRSI) is a quickly developing method for the in-vivo imaging of biological tissues. In particular, it is now extensively employed for imaging the human brain. In this non-invasive technique, the information about the brain is obtained from the analysis of spatial light bundles formed by the photons traveling from light sources to detectors placed on the surface of the head. Most significant problems in the functional brain NIRSI are the separation of the brain information from the physiological noise in non-cerebral tissues, and the localization of functional signals. In this paper we describe signal and image processing techniques we developed in order to measure two types of functional cerebral signals: the hemodynamic responses, and neuronal responses.

Multichannel NIRS analysis of brain activity during semantic differential rating of drawing stimuli containing different affective polarities

We used 24-channel near-infrared spectroscopy (NIRS) to measure activity in the temporal, parietal, and frontal regions of the brain in eight Japanese women while the participants rated line drawings using semantic differential scales. Participants rated the seven line drawings on 15 bipolar semantic scales, each of which belonged to one of three semantic classes: Evaluation, Activity, or Potency. Suzuki et al. [M. Suzuki, J. Gyoba, Y. Sakuta, Multichannel near-infrared spectroscopy analysis of brain activities during semantic differential rating of drawings, Tohoku Psychologica Folia 62 (2003) 86-98.] had reported previously that the right superior temporal gyrus and the right inferior parietal lobule are associated with Activity rating, while the brain regions around the central fissure were related to Potency rating. Based on these suggestions, we investigated the brain activity in these regions during rating of stimuli containing different affective polarities. When drawings were reported as 'static' or 'calm', oxyhemoglobin concentration was higher around the right superior temporal gyrus as compared to when they were considered 'noisy' or 'excitable'. Oxyhemoglobin concentrations around the central fissure were also higher when drawings were rated as 'soft', 'smooth', or 'blunt' compared to 'hard', 'rough', or 'sharp'. Any characteristic oxyhemoglobin changes were not found during the ratings on the evaluation scales. Our results suggest that activation patterns of the temporal and parietal regions are significantly modified by semantic polarities of Activity and Potency.

Renewal of the neurophysiology of language: functional neuroimaging

Functional neuroimaging methods have reached maturity. It is now possible to start to build the foundations of a physiology of language. The remarkable number of neuroimaging studies performed so far illustrates the potential of this approach, which complements the classical knowledge accumulated on aphasia. Here we attempt to characterize the impact of the functional neuroimaging revolution on our understanding of language. Although today considered as neuroimaging techniques, we refer less to electroencephalography and magnetoencephalography studies than to positron emission tomography and functional magnetic resonance imaging studies, which deal more directly with the question of localization and functional neuroanatomy. This review is structured in three parts. 1) Because of their rapid evolution, we address technical and methodological issues to provide an overview of current procedures and sketch out future perspectives. 2) We review a set of significant results acquired in normal adults (the core of functional imaging studies) to provide an overview of language mechanisms in the "standard" brain. Single-word processing is considered in relation to input modalities (visual and auditory input), output modalities (speech and written output), and the involvement of "central" semantic processes before sentence processing and nonstandard language (illiteracy, multilingualism, and sensory deficits) are addressed. 3) We address the influence of plasticity on physiological functions in relation to its main contexts of appearance, i.e., development and brain lesions, to show how functional imaging can allow fine-grained approaches to adaptation, the fundamental property of the brain. In closing, we consider future developments for language research using functional imaging.

Improved sensitivity to cerebral hemodynamics during brain activation with a time-gated optical system: analytical model and experimental validation

Time domain (TD) diffuse optical measurement systems are being applied to neuroimaging, where they can detect hemodynamics changes associated with cerebral activity. We show that TD systems can provide better depth sensitivity than the more traditional continuous wave (CW) systems by gating late photons, which carry information about deep layers of the brain, and rejecting early light, which is sensitive to the superficial physiological signal clutter. We use an analytical model to estimate the contrast due to an activated region of the brain, the instrumental noise of the systems, and the background signal resulting from superficial physiological signal clutter. We study the contrast-to-noise ratio and the contrast-to-background ratio as a function of the activation depth and of the source-detector separation. We then present experimental results obtained with a time-gated instrument on the motor cortex during finger-tapping exercises. Both the model and the experimental results show a similar contrast-to-noise ratio for CW and TD, but that estimation of the contrast is experimentally limited by background fluctuations and that a better contrast-to-background ratio is obtained in the TD case. Finally, we use the time-gated measurements to resolve in depth the brain activation during the motor stimulus.

Optical Topography with Near-Infrared Spectroscopy During a Verbal-Fluency Task

Abstract: In this study we investigated the changes in brain oxygenation in 22 subjects during a verbal fluency task (VFT) using multichannel near-infrared spectroscopy (NIRS). To assess topographical effects in more detail we measured over bilateral prefrontal and over the left frontolateral cortex in 11 subjects. We found significantly higher increases in O2HB, as a sign of cerebral activation, during the VFT compared to a verbal repetition control task over the left prefrontal and the inferior left lateral brain areas. These results underscore the value of NIRS to assess functional brain activation during cognitive tasks. Furthermore, it shows the advantage of multichannel compared to previously used single channel NIRS measurements.

Measurement of brain activity by near-infrared light

We review our most recent results on near-IR studies of human brain activity, which have been evolving in two directions: detection of neuronal signals and measurements of functional hemodynamics. We discuss results obtained so far, describing in detail the techniques we developed for detecting neuronal activity, and presenting results of a study that, as we believe, confirms the feasibility of neuronal signal detection. We review our results on near-IR measurements of cerebral hemodynamics, which are performed simultaneously with functional magnetic resonance imaging (MRI) These results confirm the cerebral origin of hemodynamic signals measured by optical techniques on the surface of the head. We also show how near-IR methods can be used to study the underlying physiology of functional MRI signals.

Eigenvector-based spatial filtering for reduction of physiological interference in diffuse optical imaging

Diffuse optical imaging is an effective technique for noninvasive functional brain imaging. However, the measurements respond to systemic hemodynamic fluctuations caused by the cardiac cycle, respiration, and blood pressure, which may obscure or overwhelm the desired stimulus-evoked response. Previous work on this problem employed temporal filtering, estimation of systemic effects from background pixels, or modeling of interference signals with predefined basis functions, with some success. However, weak signals are still lost in the interference, and other complementary methods are desirable. We use the spatial behavior of measured baseline signals to identify the interference subspaces. We then project signals components in this subspace out of the stimulation data. In doing so, we assume that systemic interference components will be more global spatially, with higher energy, than the stimulus-evoked signals of interest. Thus, the eigenvectors corresponding to the largest eigenvalues of an appropriate correlation matrix form the basis for an interference subspace. By projecting the data onto the orthogonal nullspace of these eigenvectors, we can obtain more localized response, as reflected in improved contrast-to-noise ratio and correlation coefficient maps.

Bilateral prefrontal cortex oxygenation responses to a verbal fluency task: a multichannel time-resolved near-infrared topography study

The letter-fluency task-induced response over the prefrontal cortex is investigated bilaterally on eight subjects using a recently developed compact, eight-channel, time-resolved, near-IR system. The cross-subject mean values of prefrontal cortex oxygen saturation (SO2) were 68.8+/-3.2% (right) and 71.0+/-3.6% (left), and of total hemoglobin concentration (tHb) were 69.6+/-9.6 microM (right) and 69.5+/-9.9 microM (left). The typical cortical activation response to the cognitive task [characterized by an increase in oxyhemoglobin (O2Hb) with a concurrent decrease in deoxyhemoglobin (HHb)] at each measurement point is observed in only four subjects. In this subset, the amplitude of the O2Hb increase and HHb decrease is uniform over each prefrontal cortex area and comparable between the two hemispheres. These findings agree with previous studies using continuous wave functional near-IR spectroscopy and functional magnetic resonance imaging, therefore demonstrating the potential of a time-resolved spectroscopy approach. In addition, a significant increase in SO2 levels was observed in the right (1.1+/-0.5%) compared to left side of the prefrontal cortex (0.9+/-0.5%) (P=0.005). A different pattern of cortical activation (characterized by the lack of HHb decrease or even increased HHb) was observed in the remaining subjects.

Investigation of cerebral haemodynamics by near-infrared spectroscopy in young healthy volunteers reveals posture-dependent spontaneous oscillations

Autonomic reflexes enable the cardiovascular system to respond to gravitational displacement of blood during changes in posture. Spontaneous oscillations present in the cerebral and systemic circulation of healthy subjects have demonstrated a regulatory role. This study assessed the dynamic responses of the cerebral and systemic circulation upon standing up and the posture dependence of spontaneous oscillations. In ten young healthy volunteers, blood pressure and cerebral haemodynamics were continuously monitored non-invasively using the Portapres and near-infrared spectroscopy (NIRS), respectively. Oscillatory changes in the cerebral NIRS signals and the diastolic blood pressure (DBP) signal have been identified by the fast Fourier analysis. Blood pressure increased during standing and returned to basal level when volunteers sat on a chair. The mean value of cerebral tissue oxygen index (TOI) as measured by NIRS did not demonstrate any significant changes. Oscillatory changes in DBP, oxyhaemoglobin concentration [O2Hb] and TOI showed a significant increase when subjects were standing. Investigation of the low frequency component (approximately 0.1 Hz) of these fluctuations revealed posture dependence associated with activation of autonomic reflexes. Systemic and cerebral changes appeared to preserve adequate blood flow and cerebral perfusion during standing in healthy volunteers. Oscillatory changes in [O2Hb] and TOI, which may be related to the degree of cerebral sympathetic stimulation, are posture dependent in healthy subjects.

Prefrontal hemodynamic response to verbal-fluency task and hyperventilation in bipolar disorder measured by multi-channel near-infrared spectroscopy

BACKGROUND

Many neuroimaging studies of patients with bipolar disorder have demonstrated functional hypofrontality (reduced activation of the frontal cortex), although this finding is still controversial. We previously found hypoactivation of the left prefrontal region in remitted subjects with bipolar disorder measured by one-channel near-infrared spectroscopy (NIRS). The aim of the present study was to clarify whether or not this finding was due to altered cerebral lateralization or caused by reduced cerebrovascular reactivity.

METHODS

We enrolled nine remitted patients with bipolar disorder and nine normal controls. Hemodynamic responses in the prefrontal cortex during the verbal-fluency and hyperventilation tasks were monitored by 24-channel NIRS, which can measure oxygenated hemoglobin (OxyHb), deoxygenated hemoglobin, and total hemoglobin (TotalHb).

RESULTS

The increases of OxyHb and TotalHb in the bipolar group were significantly smaller than that in the controls during the verbal-fluency task. The response of TotalHb during hyperventilation in the bipolar group was weaker than that in the controls.

LIMITATIONS

The sample size was small.

CONCLUSIONS

These findings suggest that the bilateral hypofrontality to a cognitive task is seen in remitted subjects with bipolar disorder, which may be related to vascular function as measured by the response to hyperventilation.

Determination of language dominance with near-infrared spectroscopy: comparison with the intracarotid amobarbital procedure

PURPOSE

To determine the efficacy of near-infrared spectroscopy (NIRS) in identifying the hemisphere associated with language by measuring changes in bilateral lateral inferior frontal blood flow during a word generation task in epilepsy surgery patients and healthy volunteers.

METHODS

Sixteen patients who underwent the intracarotid amobarbital procedure (IAP) and eight healthy right-handed/right-footed individuals were tested with NIRS during a word generation paradigm.

RESULTS

Increases in lateral inferior frontal total hemoglobin concentrations agreed with the IAP in 11 of 16 patients, including 2 of 3 with right hemisphere (atypical) speech dominance (P = 0.13). NIRS revealed increases in lateral inferior frontal total hemoglobin concentrations congruent with language dominance predicted by handedness/footedness in 18 of 24 subjects, including 100% of healthy right-handed subjects (P = 0.02).

CONCLUSION

NIRS is a simple, non-invasive, safe modality for measuring cerebral blood flow. Further research and development of testing procedures and instrumentation are needed before routine implementation in pre-surgical testing.

Tissue spectroscope: a novel in vivo approach to real time monitoring of tissue vitality

Optical monitoring of various tissue physiological and biochemical parameters in real-time represents a significant new approach and a tool for better clinical diagnosis. The Tissue Spectroscope (TiSpec), developed and applied in experimental and clinical situations, is the first medical device that enables the real-time monitoring of three parameters representing the vitality of the tissue. Tissue vitality, which is correlated to the oxygen balance in the tissue, is defined as the ratio between O(2) supply and O(2) demand. The TiSpec enables the monitoring of microcirculatory blood flow (O(2) supply), mitochondrial NADH redox state (O(2) balance), and tissue reflectance, which correlates to blood volume. We describe in detail the theoretical basis for the monitoring of the three parameters and the technological aspects of the TiSpec. The comparison between the TiSpec and the existing single parameter monitoring instruments shows a statistically significant correlation as evaluated in vitro as well as in various in vivo animal models. The results presented originated in a pilot study performed in vivo in experimental animals. Further research is needed to apply this technology clinically. The clinical applications of the TiSpec include two situations where the knowledge of tissue vitality can improve clinical practice. The major application is the monitoring of "nonvital" organs of the body [i.e., the skin, gastrointestinal (G-I) tract, urethra] in emergency situations, such as in the operating rooms and intensive care units. Also, the monitoring of specific (vital) organs, such as the brain or the heart, during surgical procedure is of practical importance.

Attention-induced frontal brain activation measured by near-infrared spectroscopy

In eight right-handed healthy males aged 9.6-12.9 years, changes in the frontal cerebral concentrations of oxygenated, deoxygenated, and total hemoglobin, as well as in tissue oxygenation index and cytochrome oxidase aa(3) were measured by near-infrared spectroscopy. The males were evaluated using a trail-making test with four test cycles interrupted by relax breaks. During the first cycle of this task, a significant increase in deoxygenated hemoglobin was documented in the left frontal region. In the extended course of the test, a significant increase in oxygenated and total hemoglobin was observed on both sides, indicating an increase in cerebral blood volume. However, only on the left side was an increase in deoxygenated hemoglobin documented. In children, changes in cerebral oxygenation induced by a cognitive task could be measured by near-infrared spectroscopy. In a short and extended attention task, a lateralized increase in oxygen consumption was documented by an increase in deoxygenated hemoglobin. In an extended attention task, an increase in oxygenated and total hemoglobin was recorded additionally in both frontal brain regions, indicating a compensatory increase in cerebral perfusion. Further studies are required to examine the significance of this technique in testing differences in children with neurobehavioral disabilities.

Sex and age dependencies of cerebral blood volume changes during cognitive activation: a multichannel near-infrared spectroscopy study

In this study, we measured the change in cerebral hemoglobin concentrations during a cognitive task using multichannel near-infrared spectroscopy (NIRS), and investigated the relationship between regional cerebral blood volume and sex, age, and task performance. Thirty-nine healthy volunteers (24 males and 15 females; mean age, 33.0 years) participated after giving their informed consent and performed a word fluency task. The relative oxy-hemoglobin concentration ([oxy-Hb]) was measured using frontal and temporal probes with two sets of 24-channel NIRS machines. The effects of sex, age, and task performance on [oxy-Hb] changes were analyzed using analysis of covariance: with sex, age, and task performance as independent variables, and [oxy-Hb] changes as dependent variables, and years of education as covariates. The effects on [oxy-Hb] increase were significant in many channels in the frontal and temporal probes for sex, that is the most prominent effect, and in a few frontal channels for age: [oxy-Hb] increases were larger in males than in females, and in the young than in the middle-aged. The effects on [oxy-Hb] increase were not significant for task performance, but [oxy-Hb] increases in subjects with low performance tended to be larger than those in subjects with high performance. The results demonstrated that multichannel NIRS could detect cerebral activation during cognitive tasks and clarify sex- and age-dependent differences in such cerebral activation. Sex- and age-dependent differences in cerebral activation, as demonstrated in the present study, should be considered when interpreting cerebral blood volume, cerebral blood flow, and cerebral glucose metabolism data.

Simultaneous event-related potential and near-infrared spectroscopic studies of semantic processing

Near-infrared optical topography (NIROT) signals and event-related potentials (ERPs) were measured simultaneously during a semantic processing task to evaluate the ability of these techniques to detect hemodynamic and electrophysiologic responses generated by semantic anomalies and to compare these results to earlier independent functional magnetic resonance imaging (fMRI) and ERP measurements. Candidate brain regions were first identified from activations detected by NIROT during a simple block-design task (in this case reading sentences vs. passive viewing), and defined regions (corresponding to Broca's and Wernicke's areas) were used for guiding the localization of optodes and electrodes for recording during tasks involving semantic anomalies. In five of six subjects, ERP measurements showed the characteristic N400 wave, whereas event-related NIROT showed results that agreed with previous fMRI studies. There were transient hemodynamic signals recorded in specific optodes that corresponded to activation in Broca's area, but slightly anterior to the region activated during the simple reading task, and in Wernicke's area, but slightly inferior to that for the simple reading task. A between-subject correlation of the ERP and NIRS data was also employed to identify areas of activation. The highest correlations were obtained in Broca's area, centered more anterior than for the reading task and in Wernicke's area, slightly inferior to that for the reading task. This study confirms that event-related studies are feasible using NIROT and produce results similar to those obtained with fMRI. Even though the spatial resolution is lower in NIROT than in fMRI, small differences in the locations of activation centers could be detected with NIROT. This, together with the feasibility of simultaneous ERP recording, makes NIROT attractive as a new approach to studying language function in healthy subjects as well as in those with functional abnormalities.

Noninvasive optical imaging in the visual cortex in young infants

During the developmental stage, the brain undergoes anatomic, functional, and metabolic changes necessary to support the complex adaptive behavior of a mature individual. Estimation of developmental changes occurring in different regions of the brain would provide a means of relating various behavioral phenomena to maturation-specific brain structures, thereby providing useful information on structure-function relationships in both normal and disease states. We used multichannel near-infrared spectroscopy (MNIRS), a new noninvasive imaging technique for revealing the course of neural activity in selected brain regions, to monitor the activities of the visual cortex as mirrored by hemodynamic responses in infants subjected to photostimulation during natural sleep. In the infants, oxyhemoglobin and total hemoglobin decreased and deoxyhemoglobin increased in the visual cortex with photostimulation. This pattern of responses was different from the response pattern in adults reported previously. The different patterns of responses to photostimulation in the visual cortices of infants and adults might reflect developmental and behavioral differences. It may reflect a different functional organization of the visual cortex in infants or ongoing retinal development. Our results demonstrated that regional hemodynamic change could be detected in a small area around the visual cortex. MNIRS offers considerable potential for research and noninvasive clinical applications.

Improving the diffuse optical imaging spatial resolution of the cerebral hemodynamic response to brain activation in humans

We compare two geometries of sources and detectors for optimizing the diffuse optical imaging resolution of brain activation in humans. Because of limitations in the instruments' dynamic range, most diffuse optical brain activation images have used only nonoverlapping measurements. We demonstrate theoretically and with a human experiment that a simple geometry of sources and detectors can provide overlapping measurements within the limitation of instrumentation dynamic range and produce an image resolution and localization accuracy that is twofold better.

The physiology and metabolism of neuronal activation: in vivo studies by NMR and other methods

In this article, a review is made of the current knowledge concerning the physiology and metabolism of neuronal activity, as provided by the application of NMR approaches in vivo. The evidence furnished by other functional spectroscopic and imaging techniques, such as PET and optical methods, are also discussed. In spite of considerable amounts of studies presented in the literature, several controversies concerning the mechanisms underlying brain function still remain, mainly due to the difficult assessment of the single vascular and metabolic dynamics which generally influence the functional signals. In this framework, methodological and technical improvements are required to provide new and reliable experimental elements, which can support or eventually modify the current models of activation.

Practicality of wavelength selection to improve signal-to-noise ratio in near-infrared spectroscopy

Near-infrared spectroscopy (NIRS), which can be used to detect changes in the concentration of oxygenated hemoglobin (oxy-Hb) and deoxygenated hemoglobin (deoxy-Hb) in tissue by using illumination at two different wavelengths, is often applied to noninvasive measurements of human brain functions. It is common to use two wavelengths that are on opposite sides of the point where the optical absorptions of oxy- and deoxy-Hb are equal (about 800 nm) but an optimal wavelength pair has not yet been determined. In this study, we conducted simultaneous recordings at five wavelengths (678, 692, 750, 782, and 830 nm) to determine the best wavelength for pairing with 830 nm. A theory suggests that pairing a shorter wavelength with 830 nm can provide more sensitivity because of the larger difference in absorption coefficients of hemoglobin between two wavelengths. The changes measured in four cortical areas (frontal, occipital, parietal, and temporal) showed that the noise level when the 678-, 692-, and 750-nm wavelengths were paired with 830 nm was usually lower than when the 782-nm wavelength was paired with 830 nm, which is consistent with theoretical prediction. Moreover, the signal-to-noise ratios (S/Ns) and wavelength dependencies of the power detected in all areas and subjects together suggest that the 692-nm pairing had the highest S/N. This suggests that the optimal wavelengths depend on not only the difference in the absorption coefficients of hemoglobin but also on the optical properties in the measurement area, which affect the strength of the attenuation data. The 692-nm wavelength is thus a more optimal choice than wavelengths around 780 nm for pairing with 830 nm to measure Hb changes induced by cortical activation. The improved S/N enables more sensitive statistical analysis, which is essential to functional mapping with NIRS.

Prefrontal activation through task requirements of emotional induction measured with NIRS

The medial prefrontal cortex is believed to be involved in emotional experiences, but also in situations in which attention and self-monitoring is required. Therefore, it might be that the medial prefrontal cortex is not only activated by the emotional state per se, but rather through the task requirements that were used to induce emotions in the laboratory. The present study investigated the change of oxygenation in the left and right prefrontal cortex measured with near-infrared spectroscopy (NIRS) from 14 subjects during two kinds of emotional induction, which differed in the self-monitoring requirements. The task with the higher self-monitoring requirements resulted in an increased concentration of oxygenated hemoglobin (O(2)HB). This activation pattern was not observed during the emotional induction with the fewer self-monitoring requirements, although the subjective ratings indicated that both tasks induced comparable emotional states. The results indicate that task requirements should be taken into account when investigating the neural substrate of emotions.

Multichannel near-infrared spectroscopy in depression and schizophrenia: cognitive brain activation study

BACKGROUND

Recent developments in near-infrared spectroscopy (NIRS) have enabled the noninvasive clarification of brain functions in psychiatric disorders with measurement of hemoglobin concentrations as cerebral blood volume.

METHODS

Ten patients with depression, 13 patients with schizophrenia, and 16 age- and gender-matched healthy control subjects participated in the study after giving consent. The relative concentrations of oxyhemoglobin [oxyHb] were measured with frontal and temporal probes every.1 sec during word fluency and unilateral finger tapping tasks, with two 24-channel NIRS machines.

RESULTS

The [oxyHb] increase patterns during the word fluency task varied among the three groups, although their task performances were similar: the depression group was characterized by a smaller [oxyHb] increase during the first half of the task period and the schizophrenic group by a small trough of [oxyHb] at the start of the task period and [oxyHb] re-increase in the posttask period. [OxyHb] increases during the finger-tapping task were rather larger in the patient groups than in the control group.

CONCLUSIONS

The characteristic time courses of [oxyHb] changes in the frontal lobe were elucidated for depression and schizophrenia. Near-infrared spectroscopy, with its noninvasiveness and high time resolution, can be a useful tool for research and clinical purposes in psychiatry.

Bilateral near-infrared monitoring of the cerebral concentration and oxygen-saturation of hemoglobin during right unilateral electro-convulsive therapy

Reductions in right prefrontal cerebral blood flow have been correlated with symptomatic improvement in depressed individuals receiving electroconvulsive therapy (ECT). Non-invasive near infrared spectroscopy has previously been shown to reliably measure changes in cerebral hemoglobin concentrations and oxygen saturation. In this study, we measured the concentration and oxygen saturation of hemoglobin on the right and left frontal brain regions of nine patients during right unilateral ECT. In all patients, we have found that the electrically induced seizure causes a stronger cerebral deoxygenation on the side ipsilateral to the electrical current (-21+/-5%) with respect to the contralateral side (-6+/-4%). On the brain side ipsilateral to the ECT electrical discharge, we have consistently observed a discharge-induced decrease in the total hemoglobin concentration, i.e. in the cerebral blood volume, by -7+/-3 microM, as opposed to an average increase by 6+/-3 microM on the contralateral side. The ipsilateral decrease in blood volume is assigned to a vascular constriction associated with the electrical discharge, as indicated by the observed decrease in cerebral oxy-hemoglobin concentration and minimal change in deoxy-hemoglobin concentration during the electrical discharge on the side of the discharge. These findings provide indications about the cerebral hemodynamic/metabolic mechanisms associated with ECT, and may lead to useful parameters to predict the individual clinical outcome of ECT.

Spatiotemporal characteristics of hemodynamic changes in the human lateral prefrontal cortex during working memory tasks

The prefrontal cortex (PFC) is widely believed to subserve mental manipulation and monitoring processes ascribed to the central executive (CE) of working memory (WM). We attempted to examine and localize the CE by functional imaging of the frontal cortex during tasks designed to require the CE. Using near-infrared spectroscopy, we studied the spatiotemporal dynamics of oxygenated hemoglobin (oxy-Hb), an indicator of changes in regional cerebral blood flow, in both sides of lateral PFC during WM intensive tasks. In most participants, increases in oxy-Hb were localized within one subdivison during performance of the n-back task, whereas oxy-Hb increased more diffusely during the random number generation (RNG) task. Activation of the ventrolateral PFC (VLPFC) was prominent in the n-back task; both sustained and transient dynamics were observed. Transient dynamics means that oxy-Hb first increases but then decreases to less than 50% of the peak value or below the baseline level before the end of the task. For the RNG task sustained activity was also observed in the dorsolateral PFC (DLPFC), especially in the right hemisphere. However, details of patterns of activation varied across participants: subdivisions commonly activated during performance of the two tasks were the bilateral VLPFCs, either side of the VLPFC, and either side of the DLPFC in 4, 2, and 4 of the 12 participants, respectively. The remaining 2 of the 12 participants had no regions commonly activated by these tasks. These results suggest that although the PFC is implicated in the CE, there is no stereotyped anatomical PFC substrate for the CE.

Noninvasive measurement of neuronal activity with near-infrared optical imaging

Diffuse optical imaging (DOI) alone offers the possibility of simultaneously and noninvasively measuring neuronal and vascular signals in the brain with temporal resolution of up to 1 ms. However, while optical measurement of hemodynamic signals is well established, optical measurement of neuronal activation (the so-called fast signal) is just emerging and requires further optimization and validation. In this work, we present preliminary studies in which we measured the fast signal in 10 healthy volunteers during finger-tapping, tactile stimulation, and electrical median nerve stimulation. We used an instrument (CW4) with 8 source (690 and 830 nm) and 16 detector positions-more optodes than the instruments in previously reported studies. This allowed us to record the ipsilateral and contralateral sensorimotor cortex simultaneously, while at the same time measuring the evoked hemodynamic response. We used an acquisition time of 25 ms per image; after averaging approximately 1000 events, the signal-to-noise ratio was approximately 10(4). Since the expected relative intensity changes due to the fast signal (approximately 10(-3)) are smaller than the relative intensity changes due to physiological effects (approximately 10(-1)), we enhanced the suppression of competing signals such as the heartbeat-associated intensity changes, and established five criteria with which to assess the robustness of the fast signal. We detected the fast signal in 43% of the measurements during finger-tapping, 60% of those during tactile stimulation, and 23% of those during electrical median nerve stimulation. The relative changes in intensity associated with the fast signal were approximately 0.07% and the latency of the signal was approximately 100 ms.

Differences in the hemodynamic response to event-related motor and visual paradigms as measured by near-infrared spectroscopy

Several current brain imaging techniques rest on the assumption of a tight coupling between neural activity and hemodynamic response. The nature of this neurovascular coupling, however, is not completely understood. There is some evidence for a decoupling of these processes at the onset of neural activity, which manifests itself as a momentary increase in the relative concentration of deoxyhemoglobin (HbR). The existence of this early component of the hemodynamic response function, however, is controversial, as it is inconsistently found. Near infrared spectroscopy (NIRS) allows quantification of levels of oxyhemoglobin (HbO(2)) and HbR during task performance in humans. We acquired NIRS data during performance of simple motor and visual tasks, using rapid-presentation event-related paradigms. Our results demonstrate that rapid, event-related NIRS can provide robust estimates of the hemodynamic response without artifacts due to low-frequency signal components, unlike data from blocked designs. In both the motor and visual data the onset of the increase in HbO(2) occurs before HbR decreases, and there is a poststimulus undershoot. Our results also show that total blood volume (HbT) drops before HbO(2) and undershoots baseline, raising a new issue for neurovascular models. We did not find early deoxygenation in the motor data using physiologically plausible values for the differential pathlength factor, but did find one in the visual data. We suggest that this difference, which is consistent with functional magnetic resonance imaging (fMRI) data, may be attributable to different capillary transit times in these cortices.