EEG 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?