Alpha Rhythm and Visual Event-Related Fields Measurements at Room Temperature Using Magneto-Impedance Sensor System

In order to achieve the alpha rhythm and visual event-related fields (ERFs) measurements at room temperature, we have newly proposed a high-performance magneto-impedance (MI) sensor system with peak-to-peak voltage detector by synchronized switching, which utilizes the time-differential measurement in each MI element for suppressing the low-frequency noise components and increasing the stability of the sensor system. A high field detection sensitivity with good linearity is obtained via new MI sensor system, which is approximately 1.3x higher than that of the previous MI gradiometer. The noise level we achieved here is lower than 2 pT/Hz(1/2) in an unshielded environment, for the frequency range of 1-100 Hz. Meanwhile, we have demonstrated the new MI sensor system for room temperature alpha rhythm and visual ERFs measurements. We have achieved a clear sensitivity for spontaneous brain activity with SNR of about 8. As expected, the alpha rhythm signals simultaneously measured by EEG and MI sensor are significantly attenuated when the subject opens eyes and intensified with closed eyes. The considerable enhancement of alpha rhythm measured by MI sensor when the subject had closed eyes further evidences the signals recorded are emanating from spontaneous brain activity. For visual ERFs measurements, we have achieved a clear sensitivity for P300 brain activity for all subjects and demonstrated the tendency of P300 latency to increase with the target/standard discrimination difficulty.