Hideo Kawaguchi

Optical topography: practical problems and new applications

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

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

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

Source of nonlinearity of the BOLD response revealed by simultaneous fMRI and NIRS.

The nonlinearity of the blood oxygenation level-dependent (BOLD) response to stimuli of different duration, particularly those of short duration, has been well studied by functional magnetic resonance imaging (fMRI). This nonlinearity is assumed to be due to neural adaptation and the nonlinearity of the response in the oxygen extraction fraction (OEF); the latter has not been examined quantitatively in humans. To evaluate how the OEF response contributes to the nonlinearity of the BOLD response to neural activity, we used simultaneous fMRI and near-infrared spectroscopy (NIRS). The responses to visual stimuli of four different durations were measured as changes in the BOLD signal and the NIRS-derived hemoglobin concentrations. The hemodynamic response nonlinearity was quantified using an impulse response function model with saturation nonlinearity scaling in the response amplitude, assuming that the unknown neural adaptation parameters varied within a physiologically feasible range. Independent of the degree of neural adaptation, the BOLD response consistently showed saturation nonlinearity similar to that of the OEF response estimated from the NIRS measures, the nonlinearity of which was greater than that of the response in the total hemoglobin concentration representing the cerebral blood volume (CBV). We also found that the contribution of the OEF response to the BOLD response was four to seven times greater than the contribution of the CBV response. Thus, we conclude that the nonlinearity of the BOLD response to neural activity originates mainly from that of the OEF response.

Separation of signal and noise from in vivo optical recording in Guinea pigs using independent component analysis

Optical recording in vivo severely suffers from the interference of heartbeat noise. So far, heartbeat noise has been minimized by subtracting from each experimental trial an average of interlaced control recordings. This method, however, is time-consuming and increases tissue damage due to phototoxicity. Here we applied independent component analysis (ICA) to in vivo optical recordings, for separation of auditory signals and noises. Our results show that ICA can be successfully used to separate sound-evoked signals and heartbeat noises. Compared with the previous method, ICA has a comparable power of separation and does not require background recordings.

Spatio-temporal analyses of stimulus-evoked and spontaneous stochastic neural activity observed by optical imaging in guinea pig auditory cortex

Stimulus-evoked response in the cortex involves random neural activity besides the deterministic responses reproducible to the stimulus. Recently, we have developed a new bright optical system that enables us to investigate the spatio-temporal patterns of such stochastic activity in the guinea pig auditory cortex without averaging. We show that (1) the stochastic neural activity is evoked by a tone-stimulus in addition to the deterministic response, and spontaneous stochastic activity is also observed in a similar manner; (2) our statistical estimation of optical responses such as variance showed that the evoked stochastic activity was increased by the sound stimulus compared to the spontaneous activity; (3) both types of stochastic activity mainly display oscillatory behavior, in the frequency range of 5-11 Hz; (4) there are no significant differences between the stimulus-induced and spontaneous stochastic neural activity in our statistical analyses using the PSD (power-spectrum density) and the spatial correlation function; (5) the spatial area of the evoked stochastic activity is not strongly correlated with the tonotopical area of the deterministic response that is mainly localized in the caudal area of field A of the guinea pig auditory cortex. Thus, the stochastic neural activity existing in the stimulus response and the spontaneous activity in the auditory cortex are possibly generated by a common neural mechanism. These results were confirmed statistically using 27 animals.

Dendrite classification in rat hippocampal neurons according to signal propagation properties: Observation by multichannel optical recording in cultured neuronal networks

Abstract Two-dimensional neuronal networks were formed using a dissociated culture of rat hippocampal neurons on glass plates. Neural activity in response to pulse stimuli applied to the neurons by whole-cell clamp electrodes was observed with a 128-channel optical recording apparatus using a voltage-sensitive dye, RH482. Dendrites emerging from the somata of single neurons were classified according to two signal-transmission properties, those with lower conduction velocities (0.12±0.034 m/s, n=24) and those with very fast conduction velocity (faster than 1.0 m/s), by evaluating the conduction velocities of pulse responses. The distinction between these two properties seemed to be related to the morphological differences in input connectivity with the axons of neighboring neurons.

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.

Isolation of neural activities from respiratory and heartbeat noises for in vivo optical recording in guinea pigs using independent component analysis

Optical recording in vivo is severely interfered by heartbeat and respiratory noises. Here we tested if these noises can be removed from in vivo optical recordings from the primary auditory cortex of the guinea pig, using independent component analysis (ICA). We applied a fast ICA algorithm combined with principal component analysis to optical recordings of long durations (9-40 s). Our results show that ICA can be successfully used to separate sound-evoked neural activities from heartbeat and respiratory noises.

Salinity increases the toxicity of silver nanocolloids to Japanese medaka embryos

To investigate the effects of salinity on the toxicity of silver nanocolloids (SNCs, 28.4 nm in diameter) in aquatic environments (freshwater, brackish water, and seawater), we exposed 15 medaka eggs in triplicate to SNCs at 10 mg L−1 in different salinities of embryo-rearing medium (ERM) (1×, 5×, 10×, 15×, 20×, and 30×) until hatching (1× ERM and 30× ERM have osmotic pressures equivalent to freshwater and seawater, respectively). With increasing concentration of ERM, SNCs aggregated to 437.3 nm in diameter in 30× ERM solution. Simultaneously, soluble silver chloro complexes (various combinations of [AgCl]0, [AgCl2]1−, [AgCl3]2−, and [AgCl4]3−) were calculated to have been formed. The patterns of the absorption spectra of SNCs and AgNO3 (a reference compound) differed markedly in ERM at different salinities, indicating that different soluble silver complexes were present in each solution. With increasing salinity, the chorion resistance decreased, and the salinity in the medaka eggs, as indicated by the osmotic pressure, increased. Simultaneously, uptake of SNCs or other silver complexes into the embryos also increased compared with that of AgNO3 in 20× and 30× ERM. In the presence of SNCs in 20× ERM, embryo hatching rate and full body lengths of post-hatch larvae were significantly lower than those with AgNO3. The toxic effects of SNCs on the hatching rate increased significantly in media of high salinity and were greater than those of AgNO3. SNCs and related silver chloro complexes exhibited higher bioavailability and medaka embryo toxicity in saline conditions than did AgNO3. SNCs pose greater ecological risks to fish embryos in high-salinity aquatic environments than in freshwater environments.

Promotion of mouse embryonic stem cell differentiation by Rho kinase inhibitor Y-27632

Rho kinase (ROCK) is one of the major downstream mediators of Rho. Rho plays crucial regulatory roles in the cellular proliferation and differentiation. Because a ROCK inhibitor, Y-27632, is known to inhibit the dissociation-induced cell death in human embryonic stem (ES) cells, we investigated the effects of this ROCK inhibitor on the differentiation of the mouse ES cells. The ROCK inhibitor promoted the differentiation of the ES cells into neurons, particularly motor and sensory neurons. The addition of both ROCK inhibitor and nerve growth factor (NGF) strongly stimulated the differentiation of the ES cells into neurons. Moreover, the ROCK inhibitor promoted the differentiation of the ES cells into muscle cells. The ES cells primarily differentiated into neurons rather than muscle cells. We found that the ROCK inhibitor may promote the neuronal differentiation of the ES cells by activating the extracellular signal-regulated kinase (ERK) signaling pathway. These results suggest that the ROCK inhibitor has a significant potential to regulate the differentiation of the ES cells.