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, the neuromagnetic activities related to higher brain functions, especially word generation and word recognition were analyzed.(1) Cortical magnetic fields during the preformance of silent word generation task were recorded over the both cerebral hemispheres.Subjects were instructed to perform Japanese word-chain game ("shiritori") continuously without vocalization and to flex their unilateral toes as soon as every new word come up to their mind.Cortical magnetic fields over the both hemispheres were simultaneously recorded using two sets of 37-ch SQUID gradiometer systems (BTi) , and were averaged 100 times on the onset of the toe flexion.In addition to the activation at the midline which corresponds to that of the foot areas of … More the motor and sensory cortex, the activation in the lateral parts of the frontal cortex of both hemispheres was obeserved.This activity preceded the onset of the toe flexion about 400 ms and was not recognized during the simple task of the toe flexion without word generation.These observations indicate that the lateral parts of the frontal cortex were activated during word generation task.(2) Cortical magnetic fields caused by the visual stimulation with Chinese characters ("kanji") were recorded over the both cerebral hemispheres.Visual stimuli of either Chinese characters or pseudo-Chinese characters were presented at random order.Significant polarity reversal with a long latency (around 500 ms) was observed over the both temporal lobes.This response was larger in the case of Chinese characters than in the case of pseudo-Chinese characters.These observations indicate that both temporal lobes were activated after the presentation of Chinese characters, and this activity may be involved in word recognition.These results indicate that neuromagnetic recordings provide an effective and noninvasive approach to elucidate the mechanisms of the integrative brain functions in human subjects. Less