Detecting slow narrowband modulation in EEG signals.

We observed an unusual modulatory phenomenon in the electroencephalogram (EEG) of pediatric patients with acquired brain injury. The modulation is orders of magnitude slower than the fast EEG background activity, necessitating new analysis procedures to systematically detect and quantify the phenomenon. We propose a method for analyzing spatial and temporal relationships associated with slow, narrowband modulation of EEG. We extract envelope signals from physiological frequency bands of EEG. Then, we construct a sparse representation of the spectral content of the envelope signal across sliding windows. For the latter, we use an augmented LASSO regression to incorporate spatial and temporal filtering into the solution. The method can be applied to windows of variable length, depending on the desired frequency resolution. The sparse estimates of the envelope power spectra enable the detection of narrowband modulation in the millihertz frequency range. Subsequently, we are able to assess non-stationarity in the frequency and spatial relationships across channels. The method can be paired with unsupervised anomaly detection to identify windows with significant modulation. We validated such findings by applying our method to a control set of EEGs. To our knowledge, no methods have been previously proposed to quantify second order modulation at such disparate time-scales. We provide a general EEG analysis framework capable of detecting signal content below 0.1 Hz, which is especially germane to clinical recordings that may contain multiple hours worth of continuous data. • We propose a novel method to quantify very slow (< 0.01 Hz) EEG modulation as a function of time. • Our method enables tracking of the dominant modulation frequency and spatial distribution. • Paired with unsupervised anomaly detection, the method allows for detection of significant modulation. • We successfully applied our methods to EEG from neurocritical care patients and controls. [ABSTRACT FROM AUTHOR]