EEG-TMS recordings

EEG-TMS recordings

by Pierluigi Castellone, Electronics Engineer Brain Products‘ General Manager

Contents of the presentation (1) Why combining EEG and TMS? (2) Why is this application technically challenging? (3) What do we offer for EEG and TMS recording?

Reading (UK), July 7th to 8th, 2010

Workshop on EEG & TMS and EEG & fMRI

Why combining EEG and TMS? This methodology provides very interesting applications, which can be grouped as follows:

Inductive approach: The evoked potentials generated by TMS (TEPs) are measured at the cortical level to asses cortical reactivity and connectivity.

Interactive approach: To explore where, when and how TMS affects a functional network during a cognitive task.

Rhythmic approach: To interact with oscillatory brain activity in order to understand causal relationship between cortical rhythms and perceptual, cognitive or motor processes.



Why is this application technically challenging? (1) The TMS stimulation induces a very strong electrical field that can saturate the recording amplifiers for a long duration. (2) The TMS pulse induces an artifact in the EEG data that can last for hundreds of milliseconds. (3) Mains noise (50 or 60 Hz) is normally induced in EEG leads by the TMS stimulator. (4) A recharging artifact induced by the TMS stimulator affects the EEG data several milliseconds after the stimulation.



Amplifier saturation The TMS stimulation induces a very strong electrical field that can saturate the recording amplifiers for a long duration. Until few years ago the only way to avoid this phenomenon consisted in blocking the amplifier during TMS stimulation. Using BrainAmps: if a positive TTL pulse is sent to pin 23 and pin 24 is bridged to ground (pin 1), the amplifier input is zeroed for the pulse duration.



Amplifier saturation Using BrainAmps: if a positive TTL pulse is sent to pin 23 and pin 24 is bridged to ground (pin 1), the amplifier input is zeroed for the pulse duration:

In this example the TTL pulse lasts for 200 msec.




Other amplifiers use the sample and hold circuit by means of which the signal is held for a few milliseconds immediately after the TMS.




Other amplifiers use the sample and hold circuit by means of which the signal is held for a few milliseconds immediately after the TMS stimulation. Nevertheless this methodology .

causes a loss of information for the whole time in which the amplifier is blocked



Amplifier saturation BrainAmps do not require the use of the blocking function. The large dynamic range of these amplifiers allows continuous data recording: C4

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TMS induced artifact The TMS-induced artifact is very likely due to: (1) Currents induced in the electrodes and leadwires by each magnetic pulse (also known as Eddy Currents) (2) Decay of the TMS-induced charge

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TMS induced artifact The length of the artifact depends mainly on the impedance value:

With impedances below 5 K Ohm the TMS artifact lasts approximately 5 msec.

With higher impedances the artifact can last even more than 20 msec.

D. Veniero et al. / Clinical Neurophysiology



TMS induced artifact The length of the artifact doesn’t seem to be not affected by the intensity or type of stimulation. Whereas the shape of the artifact depends on the type of stimulator (biphasic or monophasic) and intensity of the stimulation.




TMS induced artifact: Our solutions The design of the BrainAmp DC and the BrainAmp MR plus allows optimal adaptation to the TMS stimulus magnitude by the ability of adjusting amplifier sensitivity and operational range. Recommended amplifier setting for EEG and TMS co-registration: A/D resolution: 0.1 or 0.5 µV depending on the stimulus magnitude Sampling rate: 5000 Hz/channel Hardware bandwidth: DC to 1000 Hz

As the TMS pulse consists of high frequency variations, the use of low sampling rate or narrower filter can result in an increase of the artifact duration. Reading (UK), July 7th to 8th, 2010


TMS induced artifact: Our solutions Brain Products’ electrode caps designed for EEG and TMS recordings have very thin electrode holders (4 mm). This solution allows placing the TMS coil very close to the scalp and elicits the evoked response with low magnitude stimulus.



TMS induced artifact: Our solutions Phase synchronizing the TMS stimulator with the recording amplifier opens up new scenarios for the removal of the TMS-induced artifact. 5Hz sine wave has been recorded during TMS stimulation. A TMS pulse artifact is clearly visible in the data.



TMS induced artifact: Our solutions Phase synchronizing the TMS stimulator with the recording amplifier opens up new scenarios for the removal of the TMS-induced artifact. Subtraction of the TMS artifact averaged across all stimuli shows very promising results.



TMS induced artifact: Our solutions Phase synchronizing the TMS stimulator with the recording amplifier opens up new scenarios for the removal of the TMS-induced artifact. Subtraction of the TMS artifact averaged across all stimuli shows very promising results.



Mains noise induced in EEG leads Mains noise induced by the TMS stimulator in EEG leads can affect the data substantially.



Mains noise induced in EEG leads Applying a notch filter would introduce a rippling of the signal and therefore a longer artifact.



Mains noise induced in EEG leads: Our solutions The PowerMAG does not induce any mains in the EEG data by:

using a fully-closed housing made of metal for the main unit

filtering noise right inside the high-voltage supply unit



Mains noise induced in EEG leads: Our solutions A cleaner signal can be recorded by orientating the cables away from the coil or coil cable. On our caps for EEG and TMS, electrode cables can be rotated 360 degrees and always allow for the best cable geometry for any stimulation point.



Recharging artifact Most of the commercially available TMS stimulators contaminate the EEG data with a coil recharge artifact.


As reported in the article published by D. Veniero et. Al on Clinical Neurophysiology, the latency of the recharging artifact increases with the increase of the power strength of the stimulation. The shape of this artifact is very similar to physiological activity and can cause misinterpretation of TEP. Reading (UK), July 7th to 8th, 2010


Recharging artifact The PowerMAG does not induce any recharging artifact by: keeping high-frequency noise in the recharging circuit low having an additional high frequency filter at the coil output




Conclusions 1. EEG and TMS co-registration is a very interesting methodology that can open up new opportunities for understanding human brain functions. 2. EEG/TMS co-registration is technically challenging as TMS-related artifacts are induced in the EEG data. 3. The BrainAmp amplifiers and the PowerMAG TMS stimulator are designed to minimize the impact of these artifacts in the EEG data. 4. It has been proved that with BrainAmp amplifiers physiological activity is visible after 5 milliseconds from the TMS stimulation. 5. Phase synchronization between BrainAmp amplifiers and the PowerMAG opens up new scenarios even for the removal of the electrical artifact produced by the magnetic pulse. Reading (UK), July 7th to 8th, 2010
