Analysis of extremely low frequency brain magnetic fields associated with short-term memory and recognition processes (abstract)

Abstract

One of the advantages of neural magnetic measurement using a dc-SQUID machine is to measure magnetic fields associated with the direct current or the extremely low frequencial components of neural electric activities. In this study, the order of four tones is memorized by the subject and compared with that of another four tones. The extremely slow magnetic fields were observed by using this paradigm. Two kinds of tones (two octaves apart: the 1046.5 Hz tone and the 261.6 Hz tone) were used. A male subject memorized the order of four tones chosen randomly. After 1.4 s rest, the other four tones were presented. The subject discriminated the order of the first tones from that of the last tones in the forward order. When another long (1 s) tone was presented 2.4 s later, the subject was to push either a true or a false button. Pushing the true button meant the order of the first four tones was the same as that of the last in the forward order. Pushing the false button meant the order was not the same. The whole-cortex 64 channels of MEG data were digitized with a sampling frequency of 125 Hz, and filtered (zero phase shift, low pass digital filter with a cutoff at 20 Hz). In comparison with the control wave, the wave during memorizing clearly includes very low frequency components. We observed that the intensities of the power spectral components at 0.5 Hz during memorizing and recognition is higher than that of the control power spectral components, and tried to make the topographies using the differences between the power spectral components at 0.5 Hz during memory and recognition and the control power spectral components. Because the memory processes affect the very low frequency components of MEG, the current dipole source models are made from the data filtered below 0.5 Hz. We observed that the memory processes affected the very low frequency components of MEG. We estimated the current dipole sources using MEG data below 0.5 Hz.