Kyoko Yamazaki

Experimental prediction of the wavelength-dependent path-length factor for optical intrinsic signal analysis

Analysis of the optical intrinsic signal of an exposed cortex has been applied to measurement of functional brain activation. It is important for accurate measurement of concentration changes in oxygenated hemoglobin and deoxygenated hemoglobin to consider the wavelength dependence of the mean optical path lengths for the reflectance of cortical tissue. A method is proposed to experimentally estimate the wavelength dependence of the mean optical path length in cortical tissue from the multispectral reflectance of the exposed cortex without any additional instruments. The trend in the wavelength dependence of the mean optical path length estimated by the proposed method agrees with that estimated by the model-based prediction, whereas the magnitude of the wavelength dependence predicted by the proposed method is greater than that of the model-based prediction. The experimentally predicted mean optical path length minimizes the difference in the measured changes in the concentrations of the oxygenated hemoglobin and deoxygenated hemoglobin calculated from different wavelength pairs.

Anisotropic spatial coherence of ongoing and spontaneous activities in auditory cortex

We carried out voltage-sensitive dye imaging of the guinea pig auditory cortex to determine whether the ongoing and spontaneous activities of the cortex exhibit spatial coherence reflecting the tonotopic organization of the cortex. We used independent component analysis and a signal-plus-noise model to extract ongoing activities from the observed signals including physiological noise and stimulus-evoked activities. We analyzed the cross-correlations of background activities between all pairs of recording channels and found that ongoing and spontaneous activities in the auditory cortex exhibited anisotropic spatial coherence extending along the isofrequency bands.

Comparison between Spontaneous Low‐Frequency Oscillations in Regional Cerebral Blood Volume, and Cerebral and Plethysmographic Pulsations

The brain plays a crucial role in maintaining systemic functions. Hence, the cerebral circulation is important because the tissue in the brain cannot store energy sources within it and has to constantly obtain them with oxygen from the blood flow. The mean cerebral blood flow is kept constant over a wide range of blood pressure levels by the regulation of the cerebral circulation. However, the regional cerebral blood flow (rCBF) and volume (rCBV) exhibit strong low‐frequency oscillations (LFOs) such as arterial blood pressure and heart rate, which may reflect the interaction of the cerebral and systemic circulation. To gain an insight into the regulation of the cerebral circulation, we investigated LFOs in the rCBV, cerebral pulsation (CP) and plethysmographic pulsation (PP) particularly on their spectral properties. The rCBV and PP signals were simultaneously measured by optical topography (OT: multi‐channel near infra‐red spectroscopy) and plethysmography with the subject in a resting, seated state. The...

Influence of SNR on statistical analysis of spatial extent of brain activation measured by multi-spectral imaging

The concentration changes in oxygenated and deoxygenated haemoglobin in the exposed cortex of guinea pigs evoked by the auditory stimulation are measured by the multi-spectral imaging to investigate the relationship between spatial extent of the brain activation determined by the statistical analysis and the SNR of the concentration changes in oxygenated and deoxygenated haemoglobin. The SNR of the concentration change in oxygenated haemoglobin measured by the multi-spectral imaging is generally greater than that of deoxygenated haemoglobin. The difference in SNR tends to affect the result of the spatial extent of brain activation estimated from the changes in oxygenated and deoxygenated haemoglobin. The influence of the SNR on the spatial extent is evaluated by a numerical experiment. The results of the numerical experiment are compared with the spatial extent of the brain activation estimated from the changes in oxygenated and deoxygenated haemoglobin in the exposed cortex of guinea pigs evoked by the auditory stimulation. It is found that the spatial extent of the brain activation decreases with a decrease in SNR of the concentration change. The difference in spatial extent of the brain activation estimated from the concentration changes in oxygenated and deoxygenated haemoglobin is affected by the SNR of signal.

Measurements of temporal-spatial change in blood flow and volume in exposed cortex of Guinea pig evoked by auditory stimulation

The changes in cortical blood flow and blood volume of guinea pigs during auditory stimulation are measured by optical imaging systems. In this study, the change in blood flow distribution was measured by the laser speckle method and the change in blood volume was measured by the multi-spectral imaging system. The significant increase in blood flow and volume was observed around one side of the auditory area just after the onset of the stimulation. The decrease in blood volume around the other side of the auditory area was observed whereas the blood flow surrounding the auditory area is decreased during the post-resting period.

Spatial Coherence in Auditory Cortical Activity Fluctuations

We examined activity fluctuations as ongoing and spontaneous activities that were recorded with voltage sensitive dye imaging in the auditory cortex of guinea pigs. We investigated whether such activities demonstrated spatial coherence, which represents the cortical functional organization. We used independent component analysis to extract neural activities from observed signals and a scaled signal‐plus‐noise model to estimate ongoing activities from the neural activities including response components. We mapped the correlation between the time courses in each channel and in the others for the whole observed region. Ongoing and spontaneous activities in the auditory cortex were found to have strong spatial coherence corresponding to the tonotopy, which is one of auditory functional organization.

Energetic Interrelationship between Spontaneous Low‐Frequency Fluctuations in Regional Cerebral Blood Volume, Arterial Blood Pressure, Heart Rate, and Respiratory Rhythm

Strong spontaneous fluctuations around 0.1 and 0.3 Hz have been observed in blood‐related brain‐function measurements such as functional magnetic resonance imaging and optical topography (or functional near‐infrared spectroscopy). These fluctuations seem to reflect the interaction between the cerebral circulation system and the systemic circulation system. We took an energetic viewpoint in our analysis of the interrelationships between fluctuations in cerebral blood volume (CBV), mean arterial blood pressure (MAP), heart rate (HR), and respiratory rhythm based on multivariate autoregressive modeling. This approach involves evaluating the contribution of each fluctuation or rhythm to specific ones by performing multivariate spectral analysis. The results we obtained show MAP and HR can account slightly for the fluctuation around 0.1 Hz in CBV, while the fluctuation around 0.3 Hz is derived mainly from the respiratory rhythm. During our presentation, we will report on the effects of posture on the interrela...

Path-length correction for the haemoglobin-concentration measurement using the skull cranial window by multi-spectral imaging analysis

The concentration changes in oxygenated haemoglobin and deoxygenated haemoglobin in the brain cortex of guinea pigs associated with brain activation are measured from the multi-spectral images of the cortical tissue. The cortical tissue is observed through a thinned skull. The wavelength dependence of the optical path length is considered in the calculation of haemoglobin concentration. The results are compared with those obtained from the multi-spectral images of the exposed cortex to evaluate the influence of the thinned skull on the measurement of the concentration changes by multispectral imaging system. Although the skull thickness affected the sensitivity of the change in reflectance due to decrease in optical path in the cortical tissue, the influence of skull on the wavelength dependence of the optical path length can be ignored when the skull thickness is approximately less than 100 &mgr;m.

Experimental prediction of wavelength dependence of optical path length for optical intrinsic signal analysis

In this study, we proposed a new method to experimentally estimate the wavelength dependence of the optical path lengths from multi-spectral images of the exposed cortex. The proposed method was applied to the path length correction for the multi-spectral measurements of the concentration changes in oxy-haemoglobin (oxy-Hb) and deoxy-haemoglobin (deoxy-Hb) in the cortical tissue of guinea pigs during auditory stimulation