Neuromagnetic Study on Higher Brain Functions

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A non-invasive technique, magnetoencephalography(MEG)measures the magnetic fields originating mainly from the intracellular current flow through the apical dendrites of cortical pyramidal neurons, which is caused by excitatory synaptic inputs. In this project, somatosensory-evoked, visually-evoked and movement-related magnetic fields were analyzed.(1)Cerebral magnetic fields elicited by the electrical stimulation of the median nerve were recorded. Responses at a peak latecy of 20 ms and 60 ms were generated in the primary somatosensory cortex(SI)over the contralateral scalp. The first component implies the existence of the intracellular current flow through the apical dendrites of pyramidal neurons directed from the deep layr to the superficial layr of SI, which seems to be caused by thalamic inputs to the deep layrs of SI.The second component seems to represent the synaptic activity at the superficial layr of SI probably through the intracortical neuronal circuits.(2)Visual stimuli we … More re presented at 4 or 8 positions around the center of the visual field. Waveform and distribution of the responses greatly varied depending on the stimulus position within the visual field. The electrical current dipole responsible for the first component(peak latency, 75 ms)was estimated to be around the midline of the occipital lobe, presumably the striate cortex.(3)Cerebral magnetic fields associated with visually initiated finger movements were studied. Magnetic field change preceding the onset of the movements was recorded over the contralateral scalp of the operating finger. The estimated current dipole of this component was directed anteriorly around the central sulcus. This implies the existence of the intradendritic current flow directed from the superficial layr to the deep layr of the anterior bank of the central sulcus. Previous animal experiments provide the interpretation of this component to be the activation of the hand area of the motor cortex through the cerebello-thalamo-cortical pathway.These results indicate that neuromagnetic recordings provide an effective non-invasive approach to the studies of the sensory, motor and integrative functions of the human brain, especially when combined with appropriate animal experiments. Less