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The Journal of Neuroscience, November 19, 2008, 28(47):12535-12538; doi:10.1523/JNEUROSCI.3022-08.2008

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Brief Communications
Impulse Propagation along Thalamocortical Fibers Can Be Detected Magnetically outside the Human Brain

Tomoaki Kimura,¹ Isamu Ozaki,² and Isao Hashimoto³

¹Tsukuba University of Technology, Tsukuba 305-8521, Japan, ²Aomori University of Health and Welfare, Aomori 030-8505, Japan, and ³Kanazawa Institute of Technology, Tokyo Office, Tokyo 102-0083, Japan

Correspondence should be addressed to Isao Hashimoto, Kanazawa Institute of Technology, Ishikawaken-Kioikaikan, Room 204, 4-8 Kojimachi, Chiyoda-ku, Tokyo 102-0083, Japan. Email: ihashi{at}ael.kanazawa-it.ac.jp

Orchestrating cortical network activity with synchronous oscillations of neurons across distant regions of the brain underlies information processing in humans (Knight, 2007) and monkeys (Saalmann et al., 2007; Womelsdorf et al., 2007). Frequencies of oscillatory activities depend, to a considerable extent, on the length and conduction velocity of the tracts connecting the neural areas that participate in oscillations (Buzsáki, 2006). However, the impulse propagation along the fiber tracts in the white matter has never been visualized in humans. Here, we show, by recording magnetoencephalogram (MEG) following median nerve stimulation, that a magnetic field component, we labeled "M15," changes dynamically within 1.6–1.8 ms before the onset of magnetic M20 response generated from the primary somatosensory cortex. This new M15 component corresponds to the intracellular depolarizing action current in the thalamocortical fibers propagating with the mean conduction velocity of 29 m/s. The findings challenge the traditional view that MEG is blind to the activity of deep subcortical structures. We argue that the MEG technique holds the promise of providing novel information in impulse transmissions along not only the thalamocortical pathway but also other fiber tracts connecting distant brain areas in humans.

Key words: human thalamocortical fibers; impulse propagation; conduction velocity; magnetoencephalography; somatosensory evoked fields; M15 component

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Received June 1, 2008; revised Sept. 19, 2008; accepted Oct. 16, 2008.

Correspondence should be addressed to Isao Hashimoto, Kanazawa Institute of Technology, Ishikawaken-Kioikaikan, Room 204, 4-8 Kojimachi, Chiyoda-ku, Tokyo 102-0083, Japan. Email: ihashi{at}ael.kanazawa-it.ac.jp

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