[Eeglablist] Problems with a CHEPS thermode and EEG reading

[Malte Anders]

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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