BioSemi.nl

Hi,

Below, you find a description of the problem and my procedure. My question is twofold:

1. Might the extreme decrease of skin conductance be a result of using the wrong gel? I got it from a pharmacy according to a receipe from a seminal work on GSR measurement (I don't have the title at hand now). Is the Signa gel better suited for the GSR measurement?

2. Is there a possibility for avoiding (eg. different seating of participant/other hand posture)/removing (eg. filtering) the steep increases in conductance?


I am currently conducting an experiment with affective stimuli. I expect an effect of arousal on the GSR measurement (conductance level). However, the signals I obtain suffer from (1) a steady decrease of conductance, (2) some very high increases of conductance even during the baseline recording, and (3) resulting from the first two problems, a neglectible change of conductance during stimulus presentation.

Screening the data visually I see a steady decrease over time (ca. 8 minutes):
http://dl.dropbox.com/u/3416248/GSRissu ... inutes.jpg

The signal seems flat, only sometimes large peaks in the range of tens or hundreds of microsiemens are visible.

Zooming in to about 20 seconds of data, small skin conductance responses in the expected range of several microsimens become visible (at 1.5, 1.7, and 1.85):
http://dl.dropbox.com/u/3416248/GSRissu ... econds.jpg

However, in the face of the steady decrease of conductance the differences due to actual skin conductance responses are too weak to yield any significances.

The extreme changes in conductance happen also during the baseline period of 1 minute (begin of block). I would assume that this is rather a movement artifact, though I instruct subjects not to move and the hand rests on the back of the hand (palm upwards) with the GSR sensors attached on the second phallange of thering and middle finger.

To test the working of the GSR electrodes, I am - before the start of each experiment - clapping with my hands one time while the participant rests with closed eyes. I observe a steep increase of conductance, which is very convincing evidence that the GSR sensors are placed correctly.

Thanks for your time until here.

In the hope for some insight,
Christian

Dear Christian,

I use Signa Gel, and I too see these high drifts. I believe this is normal for any DC coupled hardware like Biosemi (someone please correct me if I am wrong).

I dont study GSR but, I study FbEEG, and it is bit annoying to see these large drifts and puzzling to differentiate between neural and non-neural sources. To add to the mystery, even nearby (as close as 15mm) electrodes in a high density FbEEG recorded with Biosemi+Signal gel combination, exhibit very different trends. For example, if we take 2s FbEEG segment and baseline correct to first sample, at the end of 2s the electrodes differ in the potentials in the range 10-100 micro volts.
See : "cnbisrv02.epfl.ch/~ggaripe/shr/gel.pdf"

At this moment, I wonder if these drifts are arising from skin conductance variation across time or the amplifier instabilities.

I pose the following questions :
1) It would be wonderful, if these drifts are explained and suggest a way to reduce them.
2) What are the ingredients of Signa Gel? Can someone give a hint please?
3) Is skin scratching is necessary to record FbEEG with Biosemi?

Sincerely,
Gangadhar GARIPELLI
CNBI EPFL Switzerland