Near-DC magnetic fields following a periodic presentation of long-durationtonebursts

Objectives: The purpose of this study was to determine the time course of l ow-frequency (<0.1 Hz) magnetic field components evoked by long-duration to nebursts. The following two questions were of central interest. Does the su stained field (SF) show adaptation as described before for the sustained po tential (SP)? How does the field amplitude return to the pre-stimulus basel ine after stimulus offset? Methods: Neuromagnetic measurements were done with a 37-channel first-order gradiometer system. The stimulus was a 1 kHz toneburst of 10 s duration pr esented at fixed 20 s intervals. The averaged data (high-pass filtered, 0.0 3 Hz cut-off) were analyzed using the model of an equivalent current dipole with time-invariant location and orientation (fixed dipole). Results: 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 st imulus offset, the amplitude of the dipole moment dropped to a lower level within 300 ms and decayed exponentially to the baseline thereafter (time co nstant 2.7 s). Conclusions: A two-component model is proposed: One component roughly follo ws 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 und erstanding of the relationship between magnetoencephalography and other fun ctional imaging techniques like functional magnetic resonance imaging and p ositron emission tomography. (C) 2001 Elsevier Science Ireland Ltd. All rig hts reserved.