[Eeglablist] Problems with a CHEPS thermode and EEG reading

Malte Anders malteanders at gmail.com
Wed Nov 22 23:44:25 PST 2017

Dear EEG Lab list,

I dug a bit deeper and my nightmares have come true... at least now I know
what is going wrong. See this screenshot:

The EEG is unreferenced, thats why its in the 1 mV range. The blue line
marks the spot where the subject was handed the CHEPS electrode. Before
that, somebody else was holding the electrode. As you can see, the signal
was perfectly clean before the subject took the thermode in her hand. She
_only_ touched the outer metal case of the thermode. Filters can be seen in
the top row (bandpass < 1Hz, > 60Hz, notch 48-52 Hz). We're in Europe so
notch is between 48-52, those filters were just applied for visualization

After the thick blue line, there is some random noise in the line which
can't be 50 Hz as i have notch filtered the EEG! I am currently out of
ideas but still pretty happy as I found the culprit for my messed up
data... The signal is gone after re-referencing e.g. to earlobes - which
makes sense - but a noise like that should not be in an EEG whatsoever so I
need to solve this problem.

The red line (Trigger 9) is a 51 °C CHEPS stimulus and this is the pattern
I get every time I am applying a stimulus. Before the trigger: up down up
down and then after the trigger: 0,5 seconds of basically nothing (flat
line which, if zoomed in, shows a sinus wave as seen in the first post). As
this is the crucial part for me (obviously) this can't be a good sign...

The thermode looks like this:

The CHEPS machine looks like this:

The red circle marks the _only_ connection to the EEG which is a 5V trigger
signal. Even if this is disconnected I get a really weird EEG reading as
seen in the first screenshot so this can't be a problem. There are _no_
other physical connections besides the subject who is holding the thermode.

The EEG ist +5 V and draws 6A Maximum. Both are connected to a power outlet
(230 Volts, 50 Hz) in the same room. I still have no idea if this makes a
difference though.

I am happy for any input here. Thank you so much!

2017-11-20 11:30 GMT+01:00 Malte Anders <malteanders at gmail.com>:

> 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

Mit freundlichen Grüßen,

Malte Anders
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