To develop and describe a nonlinear signal-processing technique (recurrence quantification analysis) for EEG. Examined were evoked potentials in individual subjects exposed to a magnetic field.
Fifteen subjects were studied (seven males, eight females). There were two periods during which a magnetic field was presented and an intervening period during which no stimulus was applied (sham exposure). The role of the stimulus frequency and vector direction in determining the response were evaluated.
| Exposure | Parameters |
|---|---|
| Exposure 1: 30–60 Hz |
|
| Exposure source |
|
|---|---|
| Setup | test person sitting in an isolation chamber; field applied in the coronal or the sagittal plane |
| Sham exposure | A sham exposure was conducted. |
| Measurand | Value | Type | Method | Mass | Remarks |
|---|---|---|---|---|---|
| magnetic flux density | 200 µT | effective value | - | - | - |
The magnetosensory evoked potentials were independent of the frequency and the direction of the field, and were not detected using the traditional linear method of analysis (time averaging). When the results obtained within subjects were avaraged across subjects, the evoked potentials could not be detected, indicating how real nonlinear phenomena can be averaged away when the incorrect method of analysis is used.
Recurrence quantification analysis, but not linear analysis, permitted consistent demonstration of magnetosensory evoked potentials.
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