To test the hypothesis that a magnetic field comparable to a field generated during a MR scan produced changes in human brain electrical activity that persisted until the field was removed (called a "presence effect" to distinguish it from evoked potentials).
Individual subjects (n=22, 9 males, 13 females) served as their own controls. As a positive control a binaural 424 Hz tone was presented.
Following an acclimation period, every subject underwent three blocks of trials: the magnetic field was applied in either the first or third block, as determined randomly from subject to subject. The data from the block where the field was not applied were analyzed as a negative control (sham exposure). The auditory stimulus was applied in the middle block (positive control).
|Exposure duration||2 s on - 5 s off - for 560 s|
Using non-linear recurrence analysis, changes in brain activity lasting 1 sec (the longest interval considered) were found in 21 of 22 subjects. The "presence effect" was not detected using linear analysis and was reversible, as indicated by a return of brain activity to baseline levels in all subjects within 2 seconds of field offset.
The authors suggest that actual MRI magnetic fields produce nonlinear steady-state perturbations of brain dynamical activity. The effect may influence the picture of brain connectivity inferred in some functional MRI studies.