Artefact of the TMS pulse and MEPs

[mandres]

Hi

We have recently started using on-line TMS while recording EMG activity in hand and arm muscles with the BioSemi system (flat electrodes). When TMS is applied over the hand motor cortex, a motor-evoked potential (MEP) is typically observed in hand muscles 20-30 msec after the pulse (earlier in the forearm).

In some trials, however, MEPs are difficult to identify because the recovery time following the artefact of the TMS pulse includes the time window where the MEP is expected (especially in the forearm).

I read in a post about TMS and EEG that "The usual cause of long recovery times is the CM going out of range during the pulse (blue LED off)." It is not clear from this previous post whether this can also occur when the TMS is delivered over the head and the CMS (and DRL) electrode is placed over the hand/forearm ?

The next question is : how can we reduce the TMS artefact in order to observe short-latency MEPs (should be lesser than 10-20 ms to ensure appropriate measurement of MEPs in the forearm-hand muscles).

In our setup, EMG activity was measured in the right hand (index finger) and in the right forearm (close to the elbow) and the CMS electrode was placed in-between (at the level of the wrist). TMS was delivered over the left motor cortex. I had the impression that the artefact was reduced in the forearm muscle when moving the CMS close to the electrode placed over this muscle. Is it related to the role of the CMS in driving the average potential of the subject close to the AD box reference ? Should we play with the placement of the CMS electrode with respect to the other electrodes in order to reduce the TMS artefact in the EMG recordings ?

Any other tip is welcome.

Michael

[Coen]

With all electrodes (including CMS/DRL) on the arm it is unlikely that TMS causes the Common Mode to go out of range (blue LED flickering). However, you can check this in the file (even sample by sample): the status of the blue LED is saved in the trigger/status channel, see http://www.biosemi.com/faq/file_format.htm.

Experiment with the location of CMS and DRL electrodes to reduce the artifact. In addition, experiment with the location of the AD-box and TMS equipment with respect to each other, and with respect to the subject. Try to locate the AD-box as close as possible to the subject. The AD-box is galvanically isolated form the TMS equipment, but stay capacitances between the devices may influence the artifacts.

Best regards. Coen (BioSemi)