[FieldTrip] Tms artifact removal from eeg data

[Davide Rivolta]

Dear Lennart,

Thank you for your reply. It is really useful.

Let me play a bit with it and will let you know if I have further problems.

Thanks again,

Davide

On Mon, Jan 9, 2012 at 7:44 PM, Verhagen, L. (Lennart) <
l.verhagen at fcdonders.ru.nl> wrote:

> Dear Davide,
>
> Currently, there is no dedicated processing pipeline available in
> FieldTrip that will allow you to remove TMS artifacts from EEG data.
> However, I have been using FieldTrip to analyze TMS-EEG data for the past
> two years (with success), so I am happy to give you some advice.
>
> Please remember that the difference between "good" and "bad" TMS-EEG data
> will not be made by FieldTrip (or any other analysis package), but by your
> data acquisition.
>
> There are a number of artifacts which are dealt with in different ways:
> the pulse itself, clipping, ringing step-response of the op-amp, decay of
> the RC-circuit in the filters of the amplifier, decay of the
> scalp-electrolyte-electrode circuit, muscle twitch, TMS recharging
> artifact, auditory evoked response, somatosensory evoked response.
>
> 1. Pulse: The data during the pulse (e.g. 0.3 ms) is lost and cannot be
> recovered.
> 2. Clipping: Only the Nexstim amplifier is currently theoretically able to
> hold-and-clamp your data acquisition. In all other amplifiers the signal
> will go out-of-range ("clip") between 2-6 ms, depending on your setup. In
> this time-period, your data is lost. Some people like to interpolate their
> data over the missing values. I personally don't like that.
> 3. Following the clipping of the data your operational amplifier circuit
> will be subject to a step-response. This is a characteristic ringing
> artifact often lasting up to 10 ms. You can put a lot of effort in
> modeling this artifact (search for "Kalman filter"), but I personally have
> not seen any modeling/filtering that worked adequately. Therefore many
> people consider the first 8-10 ms following the TMS pulse as lost. Some
> interpolate, I replace the values by NaNs.
> 4. Decay step-response. You are very likely to also observe a decay
> artifact in some channels, resembling an exponential decay. This decay can
> last relatively long. In horrible cases even up to 1 second, but a decay
> lasting 50-150 ms is not uncommon. I have addressed this artifact by
> developing an constrained iterative fitting algorithm that is capable of
> modeling four additive exponential decays with different time constants.
> This worked reasonable well for my data, but has not been tested on data
> acquired in any other lab (or setup for that matter). If you are
> interested, and know what you are doing, I could share my code with you
> (ft_tms_fitexpstepresp). I plan to develop this code (and a tutorial
> TMS-EEG processing pipeline) together with other TMS-EEG experts at the
> Donders, into a tested set op mfiles, which will hopefully find their
> place in the FieldTrip distribution.
> 5. the muscle twitch is notorious and cannot be easily removed.
> 6. depending on your TMS machine, you can see a recharging artifact.
> Sometimes it can be horrendous, other times it is hardly visible or
> completely absent. The best way to deal with this artifact is to avoid it.
>
> Next to replacing values by NaNs and modeling artifacts, I have also used
> Independent Component Analysis to identify and remove artifacts. Please
> keep in mind that you can only use ICA if you have already attempted to
> clean up your data by all other means. TMS artifacts can be quite strong
> and show non-linear behavior. Therefore the independent components might
> be overloaded by the extreme variance provided by the TMS pulse alone.
>
> I hope I have helped you a bit further. I can imagine that you have more
> questions. If so, it would be great if you specify them clearly. I could
> fill a whole new paper will many little tips and tricks, but that would be
> utterly impractical. By the way, a TMS toolkit focusing on multi-modal TMS
> (strong emphasis on TMS-EEG) is likely to be organized by the Donders
> early this summer, so keep an eye out on this mailing list if you are
> interested.
>
> Cheers,
> Lennart.
>
>
> > From: Davide <drivolta81 at gmail.com>
> > Date: 9 January 2012 0:00:57 CET
> > To: "fieldtrip at donders.ru.nl" <fieldtrip at donders.ru.nl>
> > Subject: [FieldTrip] Tms artifact removal from eeg data
> > Reply-To: Email discussion list for the FieldTrip project
> <fieldtrip at donders.ru.nl>
>  >
> > Dear all,
> >
> > I just wish to ask what is a good way to remove tms artifacts from eeg
> data using fieldtrip.
> >
> > The artifact lasts for around 4-8 ms after the tms pulse.
> >
> > Thanks for your help,
> > Davide
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>



-- 
Davide Rivolta, PhD
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