Near-DC magnetic fields following a periodic presentation of long-duration tonebursts

The purpose of this study was to determine the time course of low-frequency (<0.1 Hz) magnetic field components evoked by long-duration tonebursts. The following two questions were of central interest. Does the sustained field (SF) show adaptation as described before for the sustained potential (SP)? How does the field amplitude return to the pre-stimulus baseline after stimulus offset?

Neuromagnetic measurements were done with a 37-channel first-order gradiometer system. The stimulus was a 1 kHz toneburst of 10 s duration presented at fixed 20 s intervals. The averaged data (high-pass filtered, 0.03 Hz cut-off) were analyzed using the model of an equivalent current dipole with time-invariant location and orientation (fixed dipole).

In the grand average of the subjects with the best signal-to-noise ratio, the SF exhibited adaptation with a time constant of 3.6 s. After stimulus offset, the amplitude of the dipole moment dropped to a lower level within 300 ms and decayed exponentially to the baseline thereafter (time constant 2.7 s).

A two-component model is proposed: One component roughly follows the envelope of the stimulus, the other behaves like a leaky integrator. A better understanding of near-DC fields appears to be crucial for the understanding of the relationship between magnetoencephalography and other functional imaging techniques like functional magnetic resonance imaging and positron emission tomography.