Study on Neuromagnetic Imaging of the Functional Organization of the Human Brain

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Two techniques in magnetics, magnetoencephalography (MEG) and magnetic brain stimulation, have made it possible in recent years to better understand the functional organization of the brain. The first part of this study concerns about the neuromagnetic imaging using SQUID magnetometer. We developed a highly sensitive dc-SQUID system with a first-order gradiometer. We have measured MEG activities in normal subjects during sleep and waking stages, and we have obtained more accurate information about sources of K-complexes and delta waves than obtained by electrical measurements alone. MEG is a useful method for diagnosis of functional brain diseases as well as for the study of higher brain function, such as cognition, memory and blending of brain function. We have studied the frontal midline theta (Fm rheta) activity in human subjects during mental processes such as mental calculation. The Fm rheta activity appears in the front central area of the head when a subject concentrates his/her mind during a task such as mental calculation. We have also studied the spatio-temporal properties of visual evoked responses associated with the perception of visual apparent movement. The results in the EEG recording suggest that "visual apparent movement perception" can be produced in the visual association area or higher levels in the nervous system. It is possible to estimate the source of visual apparent movement using the MEG technique.The latter part of the study concerns about the magnetic stimulation of the brain. We have developed a method of focal and vectorial magnetic stimulation of the brain. The basic idea is to concentrate induced eddy currents locally in the vicinity of a target by a pair of opposing pulsed magnetic fields. Using this method, we were able to stimulate the human cortex within a 5-mm resolution.