[Eeglablist] Problems with a CHEPS thermode and EEG reading

Tarik S Bel-Bahar tarikbelbahar at gmail.com
Wed Nov 22 14:17:50 PST 2017

​Hello Malte,

To be extra cautious, as you intend, it is your best bet is to contact 5 to
10 authors who have worked with CHEPs previously and check in with them,
and preferably some who work with the same CHEPs machine.

It would be good to make sure that you're not getting unexpected
"electrical disturbances" from the connection between the EEG and CHEP
station., by trying things out in various combinations.

Overall this might be something usual with CHEPs setup but you can't know
for sure till you do appropriate survey and testing.

Best wishes.

If you get to some clarity about this situation, please share with the list
so that other users can benefit from your experiences.

On Mon, Nov 20, 2017 at 2:30 AM, Malte Anders <malteanders at gmail.com> wrote:

> Dear EEG Lab list,
> we are using a CHEPS thermode on a Medoc Main Station (
> https://medoc-web.com/products/pathway-model-cheps/) and a 64 Channel
> g.HIamp from gtec with active electrodes (basically electrodes with an
> amplifier in it). I am currently measuring heat evoked potentials, so
> nothing really fancy.
> When doing the eeg recordings, I am experiencing a strange behaviour I
> can't make heads and tails of. I am usually looking at an unreferenced EEG
> signal while recoding as we are doing and average referecing in the
> post-processing. Here is a screenshot of how the eeg looks _every_ time a
> trigger signal (CHEPS) of 51°C is applied to a subject (non-filtered, non
> referenced)
> https://imgur.com/ANxIPQb
> Filtering the data doesn't make it better (notch, bandpass), just
> smoother, re-referencing (either average or earlobe, doesn't matter)
> completly removes the very significant pattern and results in a what I
> would call "normal" EEG.
> I am especially worried about the very "flat sinus wave" after the trigger
> (trigger = peak temperature of 51 °C). When I measure an eeg without a
> subject and start the CHEPS electrode, no such pattern is seen. What I
> haven't tried is measuring and EEG on a subject, start the CHEPS
> measurement without the electrode applied to the subject to see if this
> pattern reoccurs, but I will try that. The only physical connection between
> EEG and CHEPS station is a +5V trigger cable which hopefully doesn't impose
> any electrical disturbances.
> I just want to know if I need to be worried about the flat sinus wave
> after the trigger. I know that it is gone after the re-referencing, but I
> am not sure if there is any electrical interference I am currently not
> paying attention to. Also the pattern before the trigger always looks
> exactly the same (up, down, up, down and then flat sinus wave)!!! As I am
> looking for potentials in a 5-10 microvolt range I just want to be extra
> cautious, and the unreferenced signal produces this pattern in the
> millivolt-range, that's why I am a bit worried.
> I am hoping this is the "normal" physical response of a subject in a heat
> evoked potential, but I just wanted to double check with somebody who has
> done evoked potentials before to rule out any interference here.
> Thank you so much!
> Malte
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