BioSemi.nl

Hi everyone,

I've noticed strange spikes in my data when using the biosemi response-box.
I have 64 channels recordings on a couple of subjects while they had to respond to a task using the Biosemi response-box (RespSwitch=1). The recordings were done in an shielded booth with the ADC box next to the subject.

At the onset and offset of the response trigger, the signal displays a big spike (here the data are not rereferenced, not filtered, just epoched and with the baseline removed).
These spikes are consistent across trials, and can be seen in EDFbrowser.
I don't see them with recordings with electrodes in a bucket with water and salt.


Does anyone had similar results or has an idea where these artifacts might be coming from ?


Best,
Thomas

The voltage switching in the response button boxes may cause very small Common Mode spikes in the EEG signal. Averaging over a lot of sweeps makes the spikes visible (all the small spikes add up, the EEG noise cancels out). Since the spikes are Common Mode, referencing reduces them with a factor of at least 100.

If the spikes are visibly in the EEG before averaging, then something else is wrong

Best regards, Coen (BioSemi)