enter image description hereEEG was taken inside an fMRI, including ECG, EXG and eye tracking system. Sampling rate was 5000 Hz. Sync box was used. After running Brain Product Analyzer gradient artifact removal (using markers from the fMRI for each volume, and after removal- down sampling to 250 Hz.)- one could still identify high frequency "ripples" (high frequency oscillations) which ruined the CB cleaning. I planned to add some low pass or some other filter to get rid of these, and before performing the filter checked the spectrum of the noise and found the high frequency noises shown below. Any idea what causes these?


  • $\begingroup$ They seem to be harmonics of 15 Hz. Can you find anything recurring at that frequency in the time based EEG? A raw time based EEG trace would help. What frequency were the stimuli given, if any? Does the scanner interfere? What's the sampling rate of the scanner? Does it occur at 15 Hz? $\endgroup$ – AliceD Mar 6 '18 at 22:30
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    $\begingroup$ Hi, thanks- I believe these are merely gradient artifact remnants- although the scanner events were marked correctly- Brain product analyzer doesn't clean them all. Thank you! $\endgroup$ – user2522941 Mar 7 '18 at 19:54

These seem to be simply gradient artifact remnants- Brain products analyzer didn't fully clean them.

Looking over the spectrogram of another EEG before the gradient artifact removal- it seems as if this noise is merely leftovers of the gradient artifact caused by the fMRI system (noticed that these spikes were greatly reduced by the brain vision analyzer gradient artifact cleaner, but in some cases- this transform left much noise in these frequencies).

  • $\begingroup$ Could you expand on your answer? We expect answers to be more than a one-liner. I deleted it at first, but I just saw that you were the question's OP, so I undeleted it, but I'd encourage you to explain what you found and how that solved your issue so that others can benefit from your findings. $\endgroup$ – AliceD Mar 7 '18 at 20:31
  • $\begingroup$ Any additional information and especially sources may get people to upvote your answer +1 from my side. Thanks $\endgroup$ – AliceD Mar 8 '18 at 8:04
  • $\begingroup$ Also please accept your answer If it's sufficient $\endgroup$ – AliceD Mar 11 '18 at 23:04

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