Shin'ichiro Kanoh

Sequential grouping of tone sequence as reflected by the mismatch negativity

Abstract.The human sequential grouping that organizes parts of tones into a group was examined by the mismatch negativity (MMN), a component of event-related potentials that reveals the sensory memory process. The sequential grouping is accomplished by the combinations of some factors, e.g., temporal and frequency proximity principles. In this study, auditory oddball stimuli in which each of the stimuli consisted of series of tone bursts, were applied to the subjects, and the MMN elicited by the deviation of the frequency of the last tone in the stimulus was investigated. The relationship between the expected phenomena of sequential grouping of tones and observed magnitudes of MMN was evaluated. It was shown that the magnitudes of MMN changed according to the configuration (number of tones, frequency) of tone sequence to be stored. This result suggested that the sequential grouping of presented tones was achieved on the preattentive auditory sensory memory process. It was also shown that the relative change of MMN magnitudes corresponded to the conditions of sequential grouping, which had been proposed by the auditory psychophysical studies. The investigation of MMN properties could reveal the nature of auditory sequential grouping.

Constant-phase-mode operation of the light-addressable potentiometric sensor

Abstract The constant-phase-mode operation of the light-addressable potentiometric sensor (LAPS) is proposed and demonstrated. In this new operation mode, the temporal change and the spatial distribution of the analyte concentration are recorded in the form of the bias voltage applied to the LAPS sensor plate, which is servo-controlled to maintain the phase of the photocurrent at a constant value with respect to the light modulation. The constant-phase-mode LAPS is advantageous for its wider measurement range and reduction of artifacts.

Differential setup of light-addressable potentiometric sensor with an enzyme reactor in a flow channel

The light-addressable potentiometric sensor (LAPS) was combined with an enzyme reactor in a fluidic channel. The fluidic channel was mounted on the sensor plate and the enzyme reactor was connected to the fluidic channel. The enzyme reactor was filled with glass beads as enzyme carrier modified by urease which catalyzed production of ammonia depending on the concentration of urea. Double-channel LAPS measurement was performed at the both side of upper and lower stream of the enzyme reactor which realized a differential measurement and eliminated the drift component of the measurement.

Basic characteristics of hardware neuron model based on CMOS negative resistance: realization of post-inhibitory rebound firing and its application

We introduced and evaluated the hardware neuron model with CMOS configuration which showed negative resistance characteristics. It was shown that this model had similar properties to those of the biological neuron, and that the post-inhibitory rebound firing (PIR firing) could be reproduced by this model. It was also found that the neural oscillator, consisting of a pair of neurons with inhibitory mutual connections, could be constructed by this hardware neuron model with the help of its PIR firing property. These results show the possibility of designing a new type of neuro-chip inspired by biological neural systems.

VISUALIZATION OF ION DISTRIBUTION BY A CHEMICAL IMAGING SENSOR

For the visualization of chemical species in the solution, markers and labels such as fluorescent dyes are often utilized. For certain applications in biology, however, toxicity of such dyes may be a problem, especially in the case of cells and tissues in culture. In this study, a label-free method of ion imaging based on a semiconductor device is developed. Chemical sensors based on semiconductor devices are advantageous for miniaturization, integration with the peripheral circuits and fabrication of various structures on their surfaces with the help of microfabrication techniques such as photolithography. A well-known semiconductor-based chemical sensor is the ion-sensitive field-effect transistor (ISFET) [1,2], which has the field effect structure or the electrolyteinsulator-semiconductor (EIS) structure. The ISFET is put to practical use, for example, in a portable pH meter. The EIS capacitive sensor [3] and the lightaddressable potentiometric sensor (LAPS) [4] are other examples of chemical sensors based on the same EIS structure. Figure 1 schematically compares the structures of these three semiconductor-based chemical sensors. Chemical sensors based on the EIS structure detect the change of the distribution of carriers in the semiconductor layer through the field effect, which responds to the change of the ion concentration of the solution in contact with the sensing surface of the insulating layer. The ISFET detects the change of the conductance of the channel between the source and the drain electrodes, whereas the EIS capacitive sensor and the LAPS detect the change of the capacitance of the depletion layer at the semiconductor-insulator interface. In the case of p-type semiconductor, the depletion layer grows thicker and the capacitance of the depletion layer becomes smaller when the sensing surface is more positively charged. In the LAPS measurement, a photocurrent is generated to detect the change of the capacitance of the depletion layer. The back surface of the sensor plate is illuminated with a light beam modulated at a frequency of several kHz, and the amplitude of the ac photocurrent is measured as a function of the bias voltage applied to the EIS system. Figure 2(a) shows typical current-voltage characteristics of a pH-sensitive LAPS with a Ta2O5 film as the insulating layer. A positive bias (applied to the solution with respect to the semiconductor substrate) in Fig. 2(a) corresponds to the depletion and inversion states of the semiconductor layer, and a negative bias corresponds to the accumulation state. In Fig. 2(b), the bias voltage at the inflection point of the current-voltage curve is plotted as a function of the pH value, showing the pH sensitivity of 57.9 mV/pH. The measuring area of a LAPS sensor is restricted to the illuminated area, and therefore, many measuring spots can be defined on the sensing surface of a single sensor plate.