very slow wave at frontal sites

[jeffreyj]

Hi,
I was wondering if anyone else has experienced problems in their recordings in which there is slow-wave noise (about .25 Hz) primarily at frontal sites. The amplitude of these waves are usually pretty large, ranging anywhere from a few hundred uV upward.
This doesn't happen for us too often, but occasionally it occurs with a few subjects in a row. We've spent a lot of time trying to figure out what it is, and we sometimes have a hunch that it's related to the subject being tired/dizzy/nervous/hot/etc.
The noise doesn't happen when we do recordings in saline, which leads us to believe that it's a subject problem instead of an equipment problem.
Any help on this would be greatly appreciated.
Thanks,
Jeff Johnson

[PB]

Hi jeffreyj,

I also see these slow waves in frontal electrodes from time to time. I am quite sure they are due to sweating. This modulates the skin conductance at the forehead and could theoretically lead to this kind of effect. I haven't done any real tests of though...

P

[Peter Praamstra]

I agree with the previous comment on this high-amplitude slow wave activity, which is likely to be due to electrodermal activity in the form of skin or sweat potentials. Probably the best source of information on this type of artefact is a paper by Picton & Hillyard (1972) Cephalic skin potentials in electroencephalography (Electroencephalography and Clinical Neurophysiology 33, 419-423). This paper also gives sensible advice on how to prevent this artefact: using a cool and relaxed environment. Electrode impedance is also important and you may be somewhat more susceptible to these artefacts with recording systems that do not require skin abrasion, such as BioSemi. In my experience (as the first and longest user of BioSemi’s active electrode systems), they rarely occur under low to normal temperatures and if they do, they usually disappear after a few minutes. They are most detrimental to recordings of slow brain potentials (readiness potential, contingent negative variation) where high-pass filtering is not an option. However, with suitable recording conditions and adequate artefact rejection there should be no problem in recording good quality slow brain potentials, as demonstrated by CNV data in papers from my lab. For instance:
- Kourtis D, Kwok, HF, Roach N, Wing AM, Praamstra P (2008) Maintaining grip: Anticipatory and reactive EEG responses to load perturbations. Journal of Neurophysiology, 99, 545-553.
- Praamstra P, Pope P. (2007) Slow brain potential and oscillatory EEG manifestations of impaired temporal preparation in Parkinson’s disease. Journal of Neurophysiology, 98, 2848-2857.
- Praamstra P, Kourtis D, Kwok HF, Oostenveld R (2006) Neurophysiology of implicit timing in serial choice reaction time performance. Journal of Neuroscience, 26, 5448-55.