recording of high frequency oscillatory activity

jasnam · Post by **jasnam** » Mon Jul 21, 2008 11:09 am

hi,

i'm trying to get my dept to aquire a biosemi system. i did my phd in leipzig on high frequency oscillatory activity in visual object representation. I used a biosemi system and was delighted by the signal-to-noise ratio which gave me excellent data.

however, a lecturer who is considered to be an eeg expert in my dept says that 'Biosemi does not provide the standard EEG that would be acceptable for clinical purposes. Active electrodes are known to enhance the high-frequency oscillatory components but no one knows whether this enhancement is neurophysiologically plausible.'

I thought biosemi can be used for clinical research, but not for diagnostics. however, the 2nd argument completely baffles me. Why would the signal not be neurophysiologically plausible?

thanks in advance for your reply,
Jasna

Post by **Coen** » Mon Jul 21, 2008 11:47 am

The statement about enhancement of the high-frequency oscillatory components is nonsense. I do not think that it is justified to post such accusations without any argument or proof. I feel that the "lecturer who is considered to be an eeg expert" should reveal himself or herself. In addition, you should not spread such unjustified rumors. As your know, we are always happy to discuss technical issues about our designs. However, this is something else than asking me to react on anonymous rumors by people who clearly do not know what they are talking about.

The Biosemi active electrodes provide the same EEG data as other properly designed EEG machines. The major advantage of active electrodes with respect to passive elctrodes, is the much better rejection of interference (therefore skin preparation can be omitted), and the optimally high input impedance (which ensures a flat frequency response even with high electrode impedances).

Coen (BioSemi)

jasnam · Post by **jasnam** » Mon Jul 21, 2008 12:02 pm

hi,

i'm sorry if my email has offended you. My aim was not to spread any rumours at all, but to obtain reasonable counterarguments. I have now asked this person to come forward with a reference that supports his claim, which I obviously find contrary to my own experience and my own research published using a biosemi system. However, as I am not an engineer by training, I wanted to obtain some information on this claim I'd never heard before, this was my sole motive in posting this question.

I will send you my name and affiliation to your pm so that you can see that I'm a serious researcher who employed a biosemi system in all my published eeg-research and not some rumour-monger. thanks for your prompt reply, it helped me phrase my argument, if you don't want this topic on the forum you can delete it.

cordially,
jasna

Post by **Coen** » Mon Jul 21, 2008 12:25 pm

The BioSemi ActiveTwo system with active electrodes has a flat frequency response (DC till 1/5th of the sample rate) and a high input impedance (1 TeraOhm parallel with 11 picoFahrad). This ensures that no enhancement or decrease of any frequencies in the passband is experienced, even with high electrodes impedances. The frequency response of our system can be readily checked with simple laboratory equipment (a signal generator and two bowls of salt water is enough).

Best regards, Coen