Spikes when using response box

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Spikes when using response box

Post by tpfs »

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).
example.png (72.78 KiB) Viewed 897 times
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 ?


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Re: Spikes when using response box

Post by Coen »

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)

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