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We describe the set-up for an electrical muscle stimulation device based on near-infrared spectroscopy (NIRS), designed for use as a brain-computer interface (BCI). Employing multi-channel NIRS, we measured evoked cerebral blood oxygenation (CBO) responses during real motor tasks and motor-imagery tasks. When a supra-threshold increase in oxyhemoglobin concentration was detected, electrical stimulation (50 Hz) of the biceps brachii muscle was applied to the side contralateral to the hand grasping task or ipsilateral to the motor-imagery task. We observed relatively stable and reproducible CBO responses during real motor tasks with an average accuracy of 100%, and during motor imagery tasks with an average accuracy of 61.5%. Flexion movement of the arm was evoked in all volunteers in association with electrical muscle stimulation and no adverse effects were noted. These findings suggest that application of the electrical muscle stimulation system based on a NIRS-BCI is non-invasive and safe, and may be useful for the physical training of disabled patients.

The relationship between changes in cerebral blood oxygenation and neuronal activity remains to be fully established. We compared somatosensory evoked potentials (SEP) and evoked cerebral blood oxygenation (CBO) changes in the sensorimotor cortex of the rat. In rats anesthetized with urethane and alpha-chloralose, we measured SEP and CBO using visible light spectroscopy (VLS) during neuronal activity. Increase of stimulus frequency caused a decrease of SEP amplitude, but an increase in concentration changes of deoxy-Hb and oxygen satu-ration. The difference in frequency responses between SEP and CBO might be caused by activation of inhibitory neurons, which could suppress excitatory neu-rons at high stimulus frequencies; activation of inhibitory neurons could reduce SEP amplitude, and increase oxygen saturation due to an increase of evoked cere-bral blood flow.

Measurement of multichannel continuous-wave near-infrared spectroscopy (CW-NIRS) is dependent on the modified Beer-Lambert law, which includes optical pathlength (PL) as an essential parameter. PLs are known to differ across different head regions and different individuals, but the distribution of PLs for the whole head has not been evaluated so far. Thus, using time-resolved near-infrared spectroscopy (TR-NIRS), we measured the optical characteristics including PL, scattering coefficients (μ′s), and absorption coefficients (μa) at three wavelengths (760, 800, 830 nm). Then, we constructed maps of these parameters on the subjects’ head surface. While the PLs in nearby channels are similar, they differ depending on the regions of the head. The PLs in the region above the Sylvian fissure tended to be shorter than those in the other regions at all of the wavelengths. The difference in the distribution of PLs may be attributed to differences in tissue absorption and scattering properties. The current study suggests the importance of considering PL differences in interpreting functional data obtained by CW-NIRS.

We examined the usefulness of near infrared time-resolved spectroscopy (TRS) for detection of vasospasm in subarachnoid hemorrhage (SAH). We investigated seven aneurysmal SAH patients with poor clinical conditions (WFNS grade V) who underwent endovascular coil embolization. Employing TRS, we measured the oxygen saturation (SO2) and baseline hemoglobin concentrations in the cortices. Measurements of TRS and transcranial Doppler sonography (TCD) were performed repeatedly for 14 days after SAH. In four of the seven patients, the SO2 and hemoglobin concentrations measured in the brain tissue of the middle cerebral artery territory remained stable after SAH. However, in three patients, TRS revealed abrupt decreases in SO2 and total hemoglobin between 5 and 9 days after SAH. Cerebral angiography performed on the same day revealed severe vasospasms in these patients. Although TCD detected the vasospasm in two of three cases, it failed to do so in one case. TRS could detect vasospasms after SAH by evaluating the cortical blood oxygenation.

Although endovascular treatment has a high success rate, it is not clear how endovascular treatment affects cerebral perfusion and hemodynamics during the perioperative period. We evaluated changes in cerebral blood oxygenation (CBO) repeatedly after endovascular treatment employing time-resolved spectroscopy (TRS). We investigated a patient (10 months old, female) who suffered cerebral arteriovenous fistula. Cerebral angiography demonstrated a pial arteriovenous fistula with three feeders (left PICA, SCA, and AICA). TRS demonstrated a decrease of oxyhemoglobin, total hemoglobin, and oxygen saturation associated with an increase of deoxyhemoglobin in all of the regions measured just after embolization, indicating that embolization improved hyperemia caused by the AV shunt. Interestingly, progressive improvement of hyperemia was observed 3 and 8 days after embolization of the feeders. The present study demonstrated that embolization of the feeders caused progressive changes in CBO and hemodynamics during the perioperative period. TRS may be a useful tool for monitoring cerebral blood perfusion changes after endovascular surgery.

Indocyanine green (ICG) emits near-infrared fluorescence when it is excited by near-infrared light. The near infrared fluorescence of ICG was applied to the imaging of cerebral vessels during neurosurgical operations such as clipping of aneurysms. In this study, ICG angiography was applied to extracranial-intracranial (EC-IC) bypass surgery to evaluate the hemodynamic changes induced by bypass in moyamoya disease (MD) and non-moyamoya ischemic diseases (non-MD). These patients underwent superficial temporal artery-middle cerebral artery (STA-MCA) anastomosis. We compared the cortical areas where the bypass supplied blood flow between MD and non-MD. ICG angiography clearly demonstrated the bypass blood flow from the anastomosed STA to the cortical vessels including arteries, capillaries, and veins in both MD and non-MD. Interestingly, the anastomosed STA supplied blood flow to a larger cortical area in MD than non-MD. The bypass supplied greater extent of blood flow to the ischemic brain in MD than in non-MD. This difference might be caused by the fact that the perfusion pressure was lower in MD than in non-MD.

To determine the alterations in optical characteristics and cerebral blood oxygenation (CBO) in the frontal lobe during language tasks, we evaluated the changes in mean optical pathlength (MOP) and CBO induced by a verbal fluency task (VFT) in the right and left frontal lobes in normal adults (n = 9, mean age = 29.6 ± 4.8 years). We employed a newly developed 8-channel time-resolved near-infrared spectroscopy (TRS) instrument. The results demonstrated differences in MOP in the fronto-temporal areas with subject and wavelength; however, there was no significant difference between the right and left sides (p > 0.05). Also, the VFT did not affect the MOP significantly as compared to that before the tasks (p > 0.05). In all of the recording regions, the VFT caused increases in concentration of oxyhemoglobin and total hemoglobin associated with a decrease in deoxyhemoglobin concentration, indicating that these cortical areas were activated by the VFT. However, the mean concentration changes of oxyhemoglobin and total hemoglobin on the left side were larger than those on the right side. In addition, functional MRI demonstrated that the inferior frontal gyrus on the left side was activated in the subjects who exhibited increases in oxyhemoglobin concentration in these areas. These results suggest that TRS may be useful to study language function and to assess hemispheric dominance for language.

The sensitivity of the near-infrared spectroscopy signal to the brain activation depends on the thickness and structure of the superficial tissues. The influence of the frontal sinus, which is void region in the skull, on the sensitivity to the brain activation is investigated by the time-resolved experiments and the theoretical modelling of the light propagation in the head. In the time-resolved experiments, the mean-time of flight for the forehead scarcely depends upon the existence of the frontal sinus when probe spacing was shorter than 30 mm. The partial optical path length in the brain, which indicates the sensitivity of the near-infrared spectroscopy signal to the brain activation, in a simplified head model is predicted by Monte Carlo simulation. The influence of the frontal sinus on the sensitivity of the signal depends on the thickness of the skull and the depth of the frontal sinus.

In order to evaluate the effect of cerebral ischemia on the flavoprotein fluorescence (FPF), we compared the changes in the FPF and somatosensory evoked potential (SEP) during transient cerebral ischemia in the rat. We measured the FPF and SEP simultaneously via a cranial window made over the right sensorimotor cortex during the left median nerve stimulation in F344 rats. We compared change in FPF and SEP during cerebral ischemia for 60 min. The rCBF were rapidly recovered after reperfusion. However, the recovery rates of the FPF were significantly faster than those of the SEP after reperfusion. These findings indicate that activity-dependent changes of the FPF do not necessarily correlate with the electrical activity after transient cerebral ischemia.