Extracting high frequency oscillatory brain signals from magnetoencephalographic recordings

Oscillatory short signal bursts of cortical origin with a frequency around 600 Hz can be measured using magnetoencephalography. These so called somatosensory evoked high frequency oscillations (SE-HFO) are induced by an electrical stimulation at the wrist. Up to now only averages over several thousand stimulations yield an interpretable result. Here the spectral and temporal properties of SE-HFOs are exploited through epoch concatenation followed by temporal decorrelation to study SE-HFO single trial properties. The algorithm is a type of blind source separation and extracts an SE-HFO component, which shows a preferred phase after sorting the single trials using a wave train at 625 Hz as template. The preferred phase is not visible after sorting the raw data trials, which certainly have more noise. This indicates that the epoch concatenation temporal decorrelation is a powerful tool to study transient oscillatory signals in multichannel recordings.