Studies on Response of Human Hippocampus to Random Somatosensory Stimuli by a SQUID System in a Superconducting Magnetic Shield

An essential role of a superconducting magnetic shield is described for study of higher function of human brains. The present SQUID has a sensitivity of 0.9 fT/(Hz)^1/2 at 100 Hz in a superconducting magnetic shield. SQUID sensors are made of SNS (Superconductor/Normal metal/Superconductor) junctions. Recent theories of beamformer and MUSIC (multiple signal classification) emphasize the importance of SNR (signal-to-noise ratio) of data to locate equivalent current dipoles in brains. For instance, MUSIC uses the noise subspace of the covariant matrix practically calculated by measured magnetic fields. Dynamical responses of hippocampus to novelty or unexpectedness are measured by the SQUID system in a superconducting magnetic shield and visualized by swLORETA-an extension of sLORETA (standardized low-resolution electromagnetic tomography). The SQUID system in a magnetic shield of high-Tc superconductor is yielding new information of deep areas of human brains. The sensitivity of neuromagnetic SQUID measurements is proved to be limited by the system noise temperature of a SQUID hardware system but not so-called “brain noises”.