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In the biomagnetic inverse problem, it is important that the optimal method be selected, especially when estimating internal electrical sources for higher functions with poor information about their source profile. In this paper, internal sources of brain magnetic fields associated with short-term memory processes were estimated by: 1) a nonlinear parameter optimization method using a multidipole model and 2) a linear optimization method using an L1 minimum norm estimation (L1-MNE) technique. Both methods showed almost the same sources: in the inferior part of the occipital lobe, the supramarginal gyrus and the angular gyrus, and the inferior frontal gyrus. These results indicate the reliability and availability of these methods for brain magnetic data associated with higher brain functions. Index Terms--Biomagnetic inverse problem, equivalent current dipole, magnetoencephalography, minimum norm estimation.

The dependency of the N1m peak amplitude and latency on the duration and frequency of stimulus is studied. The N1m is an auditory evoked brain magnetic field with a magnitude of 100 fT order. It is shown that N1m amplitudes increase with increasing stimulus duration. Amplitudes leveled when stimulus duration reached 32 ms. N1m latencies decreased with increasing stimulus duration and leveled at 32 ms.

To investigate the characteristics of visual short-term memory in humans, brain magnetic fields evoked during a delayed paired comparison task were recorded using a whole-head neuromagnetometer. The visual stimulus consisted of a circle with different colors in each quadrant. In the memory condition, subjects reacted with the index finger, when the first stimulus (Sample) was identical in color configuration to the second stimulus (Test), and with the middle finger when they differed. For the control condition, the subjects ignored the Sample, and moved the index or middle finger alternately in response to the Test. Extremely low frequency components of brain magnetic fields were observed 500 ms after the Sample onset in the temporal and/ or the occipital region in the memory condition, but not in the control condition. Sources for the low frequency components were localized in the inferior part of the occipital lobe, in the vicinity of the supramarginal gyrus and the angular gyrus, and the inferior frontal gyrus. The results suggest that the activities in the inferior part of the occipital lobe controls the storage process of short-term visual memory. Index Terms -- delayed paired comparison task, equivalent current dipole, magnetoencephalogram, short-term visual memory.

To find out which brain regions are responsible for the mental construction and recognition of a kanji character initiated by visually presented kanji radicals, rather than by information retained in the memory, a lefthenradical and the corresponding righttsukuriradical were simultaneously presented randomly to either the left or right visual field of seven subjects. Thirty lefthenradicals and the corresponding righttsukuriradicals were prepared as stimuli; this combination formed over 500 real or pseudo kanji characters. Instead of their usual left and right positions, the lefthenradical was always presented above the righttsukuriradical. As quickly and correctly as possible, the subjects released a key when two kanji radicals constituted a single real kanji character and released another key otherwise. We recorded neuromagnetic responses as well as accuracy and reaction time. Left visual field superiority was observed as regards accuracy. This is in good agreement with previous neuropsychological results. Equivalent current dipoles were localized mainly in the left and/or right occipitotemporal regions (ventral visual pathways), the bilateral occipitoparietal regions (dorsal visual pathways) including the supramarginal region, and the areas surrounding the left superior temporal cortex. We suggest that these regions are related to reading and the mental construction of a kanji character from its radicals. [ABSTRACT FROM AUTHOR]