Biosemi for recording motor-evoked potentials

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Erman
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Joined: Tue Dec 04, 2012 4:51 pm

Biosemi for recording motor-evoked potentials

Post by Erman »

Hi,

I have been using a Biosemi system to record motor-evoked potentials (MEPs) using TMS. Active electrodes on the FDI muscle of the right hand record the MEPs. I have been recording at 2048Hz sampling rate (speed mode 4).

Could I ask what the hardware filter settings are set at with this kind of sampling - particularly the high end of the bandpass?

The MEPs currently recorded look a little strange. If you look at the attachment of what a normal MEP should look like (normal_MEP.jpg) and compare it to the MEPs I have been getting (myMEP.jpg), there are some differences. For example, the waves that should be present just after the MEP are entirely absent in my MEP. This seems like it might be an issue with filtering.

Finally, after some TMS pulses in the raw data, the signal also struggles to return to baseline following the TMS pulse artefact. I wonder if this is also likely to be a product of amplifier settings, or some kind of interaction between the TMS pulse and the amplifier?

Any help would be enormously appreciated.

Thank you,
Erman
Attachments
My MEP
My MEP
myMEP.jpg (14.34 KiB) Viewed 18691 times
Normal MEP
Normal MEP
normal_MEP.jpg (23.2 KiB) Viewed 18691 times

Coen
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Re: Biosemi for recording motor-evoked potentials

Post by Coen »

ActiceTwo does not show the filter ringing problem seen in the "normal" MEP. For filter info, see viewtopic.php?f=1&t=351

ActiveTwo is a DC system, return to baseline after overload takes 5 samples (due to FIR filter in ADC). Increased baseline return time after (several) TMS pulses is almost surely caused by electrode polarization (provided Common Mode remains in range during the pulse, blue LED should remain ON).

Best regards, Coen (BioSemi)

ctischler
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Joined: Thu Jul 27, 2017 6:57 pm

Re: Biosemi for recording motor-evoked potentials

Post by ctischler »

As a follow-up to the question regarding recovery time, if it takes 5 samples to recover, does that mean increasing the sampling rate will allow for faster return to baseline? I am trying to figure out how to measure the peak-to-peak MEP amplitude while I deliver TMS over M1 and record from EEG.

Thank you in advance for your help,
Claudia

Coen
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Re: Biosemi for recording motor-evoked potentials

Post by Coen »

A higher sample rate does indeed result in faster pulse response. This will mean faster return to baseline in practical measurements provided that the amplifier response is the limiting factor. This is usually not the case with TMS, where the return to baseline speed is usually determined by electrode polarization effects (see TMS threads). So changing the sample rate won't have an effect.

Best regards, Coen (BioSemi)

LMarreel
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Joined: Wed May 04, 2022 7:11 pm

Re: Biosemi for recording motor-evoked potentials

Post by LMarreel »

Hi,

I would like to follow up on the suggested issue of ' electrode polarization' and the TMS threads mentioned.

I am not entirely sure I understand what 'electrode polarization' is, how it comes to be or can be avoided (if at all possible). The only explanation I can find in the TMS threads relates to the 'CM going out of range' issue. Which I assume is a different issue ["...surely caused by electrode polarization (provided Common Mode remains in range ..." ].

Thank you,
Lena

Coen
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Re: Biosemi for recording motor-evoked potentials

Post by Coen »

TMS may cause electrical currents through the electrodes. The skin-gel-electrode interface behaves like a battery or electrolytic capacitor and may become charged (polarized) due to these currents. This results in high, slowly decaying, offset voltages.

Best regards, Coen (BioSemi)

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