Variation of temporal characteristics in human cerebral hemodynamic responses to electric median nerve stimulation: a near-infrared spectroscopic study

Elevation of intracranial pressure affects the relationship between hemoglobin concentration and neuronal activation in human somatosensory cortex

During neuronal activation, a local decrease of deoxygenated hemoglobin concentration (deoxy‐Hb) occurs which is the basis of functional brain imaging with blood oxygenation level dependent functional magnetic resonance imaging (BOLD‐fMRI). Elevated intracranial pressure (eICP) has been shown to impair functional deoxy‐Hb changes. This study investigated this effect and its relation to the underlying neuronal activity in the human primary somatosensory cortex (SI). Functional near‐infrared spectroscopy (fNIRS) during somatosensory evoked potentials (SEP) monitoring was performed on 75 subjects during conditions of median nerve stimulation (MNS) and resting state, combined with normal breathing (NB) and eICP by escalating breathing maneuvers (breath holding [BH], Valsalva maneuver with 15 mmHg [V15] and 35 mmHg expiratory pressure [V35]). During NB, fNIRS revealed a typical oxygenated hemoglobin concentration (oxy‐Hb) increase with deoxy‐Hb decrease during MNS enabling SI brain mapping. Breathing maneuvers associated eICP produced a known global change of oxy‐Hb and deoxy‐Hb with and without MNS. When subtracting measurements during resting state from measurements during MNS, neither functional oxy‐Hb nor deoxy‐Hb changes could be recovered while SEPs remained unchanged. In conclusion, Valsalva‐induced eICP prevents oxy‐Hb and deoxy‐Hb changes during neuronal activation in SI. This finding raises questions on the validity of oxy‐Hb‐ and deoxy‐Hb‐based brain imaging (e.g., BOLD‐fMRI) during eICP.

Functional Spectroscopy Mapping of Pain Processing Cortical Areas During Non-painful Peripheral Electrical Stimulation of the Accessory Spinal Nerve

Peripheral electrical stimulation (PES), which encompasses several techniques with heterogeneous physiological responses, has shown in some cases remarkable outcomes for pain treatment and clinical rehabilitation. However, results are still mixed, mainly because there is a lack of understanding regarding its neural mechanisms of action. In this study, we aimed to assess its effects by measuring cortical activation as indexed by functional near infrared spectroscopy (fNIRS). fNIRS is a functional optical imaging method to evaluate hemodynamic changes in oxygenated (HbO) and de-oxygenated (HbR) blood hemoglobin concentrations in cortical capillary networks that can be related to cortical activity. We hypothesized that non-painful PES of accessory spinal nerve (ASN) can promote cortical activation of sensorimotor cortex (SMC) and dorsolateral prefrontal cortex (DLPFC) pain processing cortical areas. Fifteen healthy volunteers received both active and sham ASN electrical stimulation in a crossover study. The hemodynamic cortical response to unilateral right ASN burst electrical stimulation with 10 Hz was measured by a 40-channel fNIRS system. The effect of ASN electrical stimulation over HbO concentration in cortical areas of interest (CAI) was observed through the activation of right-DLPFC (p = 0.025) and left-SMC (p = 0.042) in the active group but not in sham group. Regarding left-DLPFC (p = 0.610) and right-SMC (p = 0.174) there was no statistical difference between groups. As in non-invasive brain stimulation (NIBS) top-down modulation, bottom-up electrical stimulation to the ASN seems to activate the same critical cortical areas on pain pathways related to sensory-discriminative and affective-motivational pain dimensions. These results provide additional mechanistic evidence to develop and optimize the use of peripheral nerve electrical stimulation as a neuromodulatory tool (NCT 03295370— www.clinicaltrials.gov).

Modeling Neurovascular Coupling from Clustered Parameter Sets for Multimodal EEG-NIRS

Despite significant improvements in neuroimaging technologies and analysis methods, the fundamental relationship between local changes in cerebral hemodynamics and the underlying neural activity remains largely unknown. In this study, a data driven approach is proposed for modeling this neurovascular coupling relationship from simultaneously acquired electroencephalographic (EEG) and near-infrared spectroscopic (NIRS) data. The approach uses gamma transfer functions to map EEG spectral envelopes that reflect time-varying power variations in neural rhythms to hemodynamics measured with NIRS during median nerve stimulation. The approach is evaluated first with simulated EEG-NIRS data and then by applying the method to experimental EEG-NIRS data measured from 3 human subjects. Results from the experimental data indicate that the neurovascular coupling relationship can be modeled using multiple sets of gamma transfer functions. By applying cluster analysis, statistically significant parameter sets were found to predict NIRS hemodynamics from EEG spectral envelopes. All subjects were found to have significant clustered parameters (P < 0.05) for EEG-NIRS data fitted using gamma transfer functions. These results suggest that the use of gamma transfer functions followed by cluster analysis of the resulting parameter sets may provide insights into neurovascular coupling in human neuroimaging data.

Cortical hemodynamic responses to intravenous thiamine propyldisulphide administration detected by multichannel near infrared spectroscopy (NIRS) system

Intravenous injection of thiamine propyldisulphide (TPD), which induces sensation of a garlic-like odor, has been used as a representative subjective olfactory test in Japan. However, cortical loci activated by TPD still remain unclear. We recorded cerebral hemodynamic responses (changes in Oxy-Hb concentrations) induced by TPD administration using whole-head multi-channel near infrared spectroscopy (NIRS) system based on 3D-MRIs. TPD as an odorant and saline as a control were injected from the cephalic vein in the left forearm in ten male normosmic (five young and five elderly) subjects and five dysosmic elderly patients. The all normosmic, but not dysosmic, subjects felt the garlic-like odor in the all TPD trials. There was no significant difference in hemodynamic responses between the young and elderly normosmic subjects. However, TPD injection induced significantly larger hemodynamic responses in the bilateral operculums, bilateral dorsolateral prefrontal cortices (PFC) and anteromedial PFC in the normosmic subjects, compared with saline injection. Onset latencies of these hemodynamic responses were significantly correlated with onset latencies of subjective odor sensation in the normosmic subjects. Comparison of hemodynamic responses between the normosmic and dysosmic subjects indicated a significant difference in the bilateral operculums. The results demonstrated that Oxy-Hb increases in the bilateral operculums reflected olfactory sensation induced by TPD injection. Consideration of a route for intravenous TPD to reach the olfactory mucosa suggests that these hemodynamic responses might be attributed to food-related retronasal olfactory responses to TPD.

Cerebral hemodynamic responses induced by specific acupuncture sensations during needling at trigger points: a near-infrared spectroscopic study

Acupuncture stimulation at specific points, or trigger points (TPs), elicits sensations called "de-qi". De-qi sensations relate to the clinical efficacy of the treatment. However, it is neither clear whether de-qi sensations are associated with TPs, nor clear whether acupuncture effects on brain activity are associated with TPs or de-qi. We recorded cerebral hemodynamic responses during acupuncture stimulation at TPs and non-TPs by functional near-infrared spectroscopy. The acupuncture needle was inserted into both TPs and non-TPs within the right extensor muscle in the forearm. Typical acupuncture needle manipulation was conducted eight times for 15 s. The subjects pressed a button if they felt a de-qi sensation. We investigated how hemodynamic responses related to de-qi sensations induced at TPs and non-TPs. We observed that acupuncture stimulations producing de-qi sensations significantly decreased the Oxy-Hb concentration in the supplementary motor area (SMA), pre-supplementary motor area, and anterior dorsomedial prefrontal cortex regardless of the point stimulated. The hemodynamic responses were statistically analyzed using a general linear model and a boxcar function approximating the hemodynamic response. We observed that hemodynamic responses best fit the boxcar function when an onset delay was introduced into the analyses, and that the latency of de-qi sensations correlated with the onset delay of the best-fit function applied to the SMA. Our findings suggest that de-qi sensations favorably predict acupuncture effects on cerebral hemodynamics regardless of the type of site stimulated. Also, the effect of acupuncture stimulation in producing de-qi sensation was partly mediated by the central nervous system including the SMA.

Brain cortical mapping by simultaneous recording of functional near infrared spectroscopy and electroencephalograms from the whole brain during right median nerve stimulation

To investigate relationships between hemodynamic responses and neural activities in the somatosensory cortices, hemodynamic responses by near infrared spectroscopy (NIRS) and electroencephalograms (EEGs) were recorded simultaneously while subjects received electrical stimulation in the right median nerve. The statistical significance of the hemodynamic responses was evaluated by a general linear model (GLM) with the boxcar design matrix convoluted with Gaussian function. The resulting NIRS and EEGs data were stereotaxically superimposed on the reconstructed brain of each subject. The NIRS data indicated that changes in oxy-hemoglobin concentration increased at the contralateral primary somatosensory (SI) area; responses then spread to the more posterior and ipsilateral somatosensory areas. The EEG data indicated that positive somatosensory evoked potentials peaking at 22 ms latency (P22) were recorded from the contralateral SI area. Comparison of these two sets of data indicated that the distance between the dipoles of P22 and NIRS channels with maximum hemodynamic responses was less than 10 mm, and that the two topographical maps of hemodynamic responses and current source density of P22 were significantly correlated. Furthermore, when onset of the boxcar function was delayed 5-15 s (onset delay), hemodynamic responses in the bilateral parietal association cortices posterior to the SI were more strongly correlated to electrical stimulation. This suggests that GLM analysis with onset delay could reveal the temporal ordering of neural activation in the hierarchical somatosensory pathway, consistent with the neurophysiological data. The present results suggest that simultaneous NIRS and EEG recording is useful for correlating hemodynamic responses to neural activity.

Comparison of principal and independent component analysis in removing extracerebral interference from near-infrared spectroscopy signals

Near-infrared spectroscopy (NIRS) is a method for noninvasive estimation of cerebral hemodynamic changes. Principal component analysis (PCA) and independent component analysis (ICA) can be used for decomposing a set of signals to underlying components. Our objective is to determine whether PCA or ICA is more efficient in identifying and removing scalp blood flow interference from multichannel NIRS signals. Concentration changes of oxygenated (HbO(2)) and deoxygenated (HbR) hemoglobin are measured on the forehead with multichannel NIRS during hyper- and hypocapnia. PCA and ICA are used separately to identify and remove signal contribution from extracerebral tissue, and the resulting estimates of cerebral responses are compared to the expected cerebral responses. Both methods were able to reduce extracerebral contribution to the signals, but PCA typically performs equal to or better than ICA. The improvement in 3-cm signal quality achieved with both methods is comparable to increasing the source-detector separation from 3 to 5 cm. Especially PCA appears to be well suited for use in NIRS applications where the cerebral activation is diffuse, such as monitoring of global cerebral oxygenation and hemodynamics. Performance differences between PCA and ICA could be attributed primarily to different criteria for identifying the surface effect.

NIRS measurement of hemodynamic evoked responses in the primary sensorimotor cortex

To investigate the relationship between neuronal activity and hemodynamics, we carried out a near-infrared spectroscopy (NIRS) study to measure the regional changes of hemoglobin concentration associated with cortical activation in the human sensorimotor cortex (SMI) to both voluntary and nonvoluntary tasks. We measured the hemodynamic evoked responses to voluntary finger movement and nonvoluntary electrical stimulation applied on the fingers (thumb and ring finger, respectively). Measurements were performed on 6 healthy right-handed volunteers using block paradigms and we analyzed both the spatial/temporal features and the magnitude of the optical signal induced by cerebral activation during these protocols. We constantly observed an increase in the cerebral concentration of oxygenated hemoglobin at the cortical side contralateral to the stimulated side. Our findings are in agreement with results in positron emission tomography (PET), functional magnetic resonance imaging (fMRI) and EEG (Electroencephalogram).

Functional near-infrared spectroscopy: current status and future prospects

Near-infrared spectroscopy (NIRS), which was originally designed for clinical monitoring of tissue oxygenation, has been developing into a useful tool for neuroimaging studies (functional near-infrared spectroscopy). This technique, which is completely noninvasive, does not require strict motion restriction and can be used in a daily life environment. It is expected that NIRS will provide a new direction for cognitive neuroscience research, more so than other neuroimaging techniques, although several problems with NIRS remain to be explored. This review demonstrates the strengths and the advantages of NIRS, clarifies the problems, and identifies the limitations of NIRS measurements. Finally, its future prospects are described.

Optical brain imaging in vivo: techniques and applications from animal to man

Optical brain imaging has seen 30 years of intense development, and has grown into a rich and diverse field. In-vivo imaging using light provides unprecedented sensitivity to functional changes through intrinsic contrast, and is rapidly exploiting the growing availability of exogenous optical contrast agents. Light can be used to image microscopic structure and function in vivo in exposed animal brain, while also allowing noninvasive imaging of hemodynamics and metabolism in a clinical setting. This work presents an overview of the wide range of approaches currently being applied to in-vivo optical brain imaging, from animal to man. Techniques include multispectral optical imaging, voltage sensitive dye imaging and speckle-flow imaging of exposed cortex, in-vivo two-photon microscopy of the living brain, and the broad range of noninvasive topography and tomography approaches to near-infrared imaging of the human brain. The basic principles of each technique are described, followed by examples of current applications to cutting-edge neuroscience research. In summary, it is shown that optical brain imaging continues to grow and evolve, embracing new technologies and advancing to address ever more complex and important neuroscience questions.

Cerebral hemodynamics evaluation by near-infrared time-resolved spectroscopy: Correlation with simultaneous positron emission tomography measurements

We compared pharmacologically-perturbed hemodynamic parameters (cerebral blood volume; CBV, and flow; CBF) by acetazolamide administration in six healthy human subjects studied with positron emission tomography (PET) and near-infrared (NIR) time-resolved spectroscopy (TRS) simultaneously to investigate whether NIR-TRS could measure in vivo hemodynamics in the brain tissue quantitatively. Simultaneously with the PET measurements, TRS measurements were performed at the forehead with four different optode spacing from 2 cm to 5 cm. Total hemoglobin and oxygen saturation (SO2) measured by TRS significantly increased after administration of acetazolamide at any optode spacing in all subjects. In PET study, CBV and CBF were estimated in the following three volumes of interest (VOIs) determined on magnetic resonance images, VOI1: scalp and skull, VOI2: gray matter region, VOI3: gray and white matter regions. Acetazolamide treatment elevated CBF and CBV significantly in VOI2 and VOI3 but VOI1. TRS-derived CBV was more strongly correlated with PET-derived counterpart in VOI2 and VOI3 when the optode spacing was above 4 cm, although optical signal from cerebral tissue could be caught with any optode spacing. As to increase of the CBV, 4 cm of optode spacing correlated best with VOI2. To support the result of TRS-PET experiment, we also estimated the contribution ratios of intracerebral tissue to observed absorption change based on diffusion theory. The contribution ratios at 4 cm were estimated as follows: 761 nm: 50%, 791 nm: 72%, 836 nm: 70%. These results demonstrated that NIR-TRS with 4 cm of optode spacing could measure cerebral hemodynamic responses optimally and quantitatively.

The parietal role in the sense of self-ownership with temporal discrepancy between visual and proprioceptive feedbacks

One hypothesis on how we recognize an image of, for example, an arm as our own is through the co-occurrence of multiple sensory feedbacks, especially visual and proprioceptive feedbacks, in this process. It has been suggested that the parietal lobe is the region where proprioceptive and visual information of one's own body is integrated. This study investigated parietal cortical activity during a visual-proprioceptive synchrony judgment task in which visual feedback of the subjects' own passively moving hand was delayed. The subjects were required to judge whether or not there was a delay between the proprioceptive and visual feedbacks. Parietal cortical activity, which was measured using a 48-channel near-infrared spectroscopy (NIRS) apparatus, appeared to be modulated by the length of the delay between the visual and proprioceptive feedbacks. The bilateral superior/middle parietal areas were involved in experiencing the synchrony between the visual and proprioceptive feedbacks, whereas the right inferior parietal areas were strongly activated when discrepancy between the two feedbacks was detected. We postulate that the superior portion of the parietal lobe is essential for maintaining one's own body image, while the right inferior portion is involved in detecting movements of others.

Decrease in prefrontal hemoglobin oxygenation during reaching tasks with delayed visual feedback: a near-infrared spectroscopy study

Visual feedback of hand movement is crucial to accurate reaching. Although previous studies have extensively examined spatial alteration of visual feedback (e.g., prism adaptation), temporal delay of visual feedback has been less explored. In the present study, we investigated the effect of delayed visual feedback of the moving hand in a reaching task. The prefrontal cortical activity was measured by near-infrared spectroscopy (NIRS). Twelve subjects performed reaching tasks under two conditions where visual feedback of their own hand was delayed by 200 ms (delay condition) or 0 ms (normal condition). Introducing the visual feedback delay significantly disrupted the reaching performance, although the subjects gradually adapted to the delay during the experiment. There was a clear tendency to overreach the target in the delay condition, even after the reaching movement had been practiced sufficiently in the normal condition. We observed marked oxy- and total-Hb decreases in the dorsal prefrontal area in the delay conditions. The decrease began shortly after task onset and diminished during the rest period, indicating that the decrease was task-induced. Furthermore, the oxy- and total-Hb decreases were significantly correlated with task performance--the degree of decrease was larger as the subject made more errors. We suggest that the decreases in oxy- and total-Hb at the dorsal prefrontal area are related with the visuomotor recalibration process.

Effect of stimulus frequency on human cerebral hemodynamic responses to electric median nerve stimulation: a near-infrared spectroscopic study

We examined the effect of stimulus frequency on optically recorded hemodynamic responses to electric median nerve stimulation. Electric stimuli were delivered to the right median nerve with an intensity of 90% of motor threshold. Four different stimulus frequencies (2, 5, 10, and 20 Hz) were administered in each subject. By means of a multi-channel near-infrared spectroscopic instrument, changes in concentration of oxygenated hemoglobin were continuously measured over the left scalp. After 20 Hz stimulation, we found two spatially and temporally distinct hemodynamic responses. One lasted beyond 60 s, and the center of this response was located over the secondary somatosensory area. The other had a transient duration starting immediately after the stimulus onset and was located in the primary somatosensory hand area. Both responses were linearly augmented as a function of the stimulus frequency. Since temporal activation patterns are different in two somatosensory areas, real-time optical monitoring is necessary in evaluation of hemodynamic responses to electric nerve stimulation.

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.

Beyond the visible--imaging the human brain with light

Optical approaches to investigate cerebral function and metabolism have long been applied in invasive studies. From the neuron cultured to the exposed cortex in the human during neurosurgical procedures, high spatial resolution can be reached and several processes such as membrane potential, cell swelling, metabolism of mitochondrial chromophores, and vascular response can be monitored, depending on the respective preparation. The authors focus on an extension of optical methods to the noninvasive application in the human. Starting with the pioneering work of Jöbsis 25 years ago, near-infrared spectroscopy (NIRS) has been used to investigate functional activation of the human cerebral cortex. Recently, several groups have started to use imaging systems that allow the generation of images of a larger area of the subject's head and, thereby, the production of maps of cortical oxygenation changes. Such images have a much lower spatial resolution compared with the invasively obtained optical images. The noninvasive NIRS images, however, can be obtained in undemanding set-ups that can be easily combined with other functional methods, in particular EEG. Moreover, NIRS is applicable to bedside use. The authors briefly review some of the abundant literature on intrinsic optical signals and the NIRS imaging studies of the past few years. The weaknesses and strengths of the approach are critically discussed. The authors conclude that NIRS imaging has two major advantages: it can address issues concerning neurovascular coupling in the human adult and can extend functional imaging approaches to the investigation of the diseased brain.