Suppression of deep brain stimulation artifacts from the electroencephalogram by frequency-domain Hampel filtering

Abstract

Objective: Currently, electroencephalography (EEG) cannot be used to record cortical activity during clinically effective DBS due to the presence of large stimulation artifact with components that overlap the useful spectrum of the EEG. A filtering method is presented that removes these artifacts whilst preserving the spectral and temporal fidelity of the underlying EEG.

Methods: The filter is based on the Hampel identifier that treats artifacts as outliers in the frequency domain and replaces them with interpolated values. Performance of the filter was tested with a synthesized DBS signal and actual data recorded during bilateral monopolar DBS.

Results: Mean increases in signal-to-noise ratio of 7.8dB for single-frequency stimulation and 13.8dB for dual-frequency stimulation are reported. Correlation analysis between EEG with synthesized artifacts and artifact-free EEG reveals that distortion to the underlying EEG in the filtered signal is negligible (r(2)>0.99).

Conclusions: Frequency-domain Hampel filtering has been shown to remove monopolar DBS artifacts under a number of common stimulation conditions used for the treatment of Parkinson's disease.

Significance: Application of frequency-domain Hampel filtering will allow the measurement of EEG in patients during clinically effective DBS and thus may increase our understanding of the mechanisms of action of this important therapeutic intervention.

Figure 1

EEG time series (top) showing the increase in signal amplitude at 30 s when the pulse generator for DBS stimulator was turned on. The time-frequency map of this signal (bottom) reveals that the artifact comprises multiple narrowband signals spaced at regular intervals. Note the transient changes in both amplitude and frequency content of the artifact over the first ~10 s of unilateral stimulation, after which the artifact’s amplitude is constant and can be considered stationary.

Figure 2

(A) 100 s and (B) 1 s segment of synthesized (red) and actual (blue) EEG contaminated with DBS artifact showing the similarity in the time-domain characteristics between the two signals.

Figure 3

(A) A 100 s segment of EEG time series contaminated with DBS artifact (top). The spectral components of the artifact are visible in the corresponding time-frequency map. (B) Results of applying the Hampel filter to the EEG time series in Fig. 3A. The time-frequency map shows that the artifact components have been greatly attenuated. Prior to applying the Hampel filter, the data were low-pass filtered with a cut-off frequency of 100 Hz, hence the difference in background activity above this frequency compared to Fig. 3A. The time series in Fig. 3B shows a corresponding reduction in amplitude. Note the different amplitude scales for the EEG time series in (A) and (B).

Figure 4

Spectrograms and time series plots showing the effects of applying the Hampel filter to artifacts from (A) subject #1, (B) subject #3, and (C) subject #4. See Figure 3 for details. Note the different amplitude scales for the EEG time series before and after filtering and between subjects.

Figure 5

Plots showing a 0.5 s section of artifact-free EEG (Blue) from subject #2, together with the same signal with synthesized DBS artifacts (Green) and the filter response (Red). The plots show that the filtered signal closely resembles the artifact-free data with little distortion and zero phase lag.