Analysis of the neural circuit related to auditory discrimination based on multi-dipole estimation with high temporal resolution.

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Although new imaging methods for measuring regional cerebral blood flow and metabolism such as PET and fMRI provide a detailed anatomical picture of the brain regions that are active during an sensory discrimination process, these methods fail to reveal the time course of the active regions with sufficiently high temporal resolution. On the other hand, the time course of brain activity patterns can be measured at the millisecond level of resolution through recordings of the neuromagnetic fields that are generated by active populations of nerve cells. Furthermore, recently developed whole-cortex MEG systems enabled us to measure all the brain areas at a time, without repositioning to record each area. The purpose of the present study was to construct the system for semi-automatically estimating multiple dipoles with sufficiently high temporal resolution. Furthermore, the temporal structure of the neural activity related to auditory (sensory) discrimination was investigated by using the analyzing system.The algorithm for multi-dipole estimation is based on the following three basic points : 1) In selection of the pair of the inward maximum and the outward maximum, the most neighboring pair in one hemisphere is to be selected as the first step for estimation. 2) Estimation errors are less than 10%, 15% and 20% for 4, 3 and 2 dipoles respectively. 3) Multi-dipole estimation is carried out based on spatio-temporal continuity and stability of the neuronal activities.By using the system, the following neuronal activities were observed related to auditory (sensory) discrimination : 1) activity in its modality-specific area. 2) the areas related to attention such as prefrontal area, cingulate gyrus, and so on. 3) the areas related to memory such as dorsolateral prefrontal cortex, hippocampus, parietal association cortex and temporal association cortex. These activities were estimated following the above order during the beginning, and in parallel after.