While recording 64 channel EEG, we stimulated the right median nerve electrically with a frequency of 6 Hz and between stimulus interval was about 2 seconds.
I time-locked EEG and averaged 100 milliseconds after stimulation.
You can fine the wave forms in the attached file.
Can anyone explain the peaks in the 20, 40, 60 and ... time instances?what are they?
Is it normal or something is wrong with our experimental stimulation or recording?
Thanks in advance.
The distance of the peaks is 20 ms, which corresponds to the mains frequency 50 Hz. There are further peaks at a distance of 6.7 ms after the main peaks. These could be caused by another phase of the three-phase system (3-phase system). It looks like the shielding of the lab is not enough. Try to operate the devices with accumulators.
Hi Nasser,
I agree with the above mentioned comment that it could be related to the resonance noise of the frequency. However, the other possibility is that there was some jitter in the actual time of the median nerve stimulation compared to when the computer intended to stimulate. I have experienced this in the past where your intention was that the stimulation was at 6 Hz and you time-lock afterwards was at the 0 ms time point. However, the timing of the actual median nerve stimulation comes early or delayed on different trials, and therefore when you average it together, it would create artificial peaks - these peaks are smaller than the MN stimulation noise at 0 ms because that is when the stimulation most frequently occur, but still large enough if even 5 or so occur around the same time point because of the large artifact relative to the signal. To check this, go into your raw data files and time-lock then ensure that the MN artifact is always at the 0 ms and not before or after in some trials.
Kind regards,
Matt
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