[Eeglablist] How to deal with 1/f noise in

derek eder derek at stenljus.se
Wed Nov 17 02:01:52 PST 2010


If amplifier 1/f noise is suspected, one possible sanity check is to 
record the same signal on multiple amplifier-channels and then sum* them.

The noise** contribution to the resulting average should be reduced by: 
   1/sqrt(n.channels)

Compare the resulting low frequency activity with those from individual 
channels.

Depending on the input-design / input-impedance of your amplifiers, 
there are practical limits to the number of channels that one can 
parallel from the same electrodes,  but 3 or 4 should not be a problem.


~Derek


*   perhaps de-mean and scale first if your channels have offsets and/or 
gain differences.
** assuming uncorrelated noise from individual channels


On 11/12/2010 03:36 PM, Gangadhar Garipelli wrote:
> Hello Arnaldo,
>
> Thanks for the attempt! :-)
>
> Yes, it is a bit tricky to record such slow oscillations. Conventional
> or classical-EEG is usually high pass at 0.5Hz. However, FbEEG is
> becoming a standard. Please check Vanhatalo et al, 2005 [1] for an
> excellent report on FbEEG and hardware (using DC coupled amplifiers)
> other related requirements.
>
> Reference :
> [I] Vanhatalo S, Viopio J, Kaila K. Full-band EEG (FbEEG): An emerging
> standard in electroencephalography. Clin. Neurophysiol., 116(1):1-8, 2005.
>
>
> On 11/12/2010 03:04 PM, Arnaldo Batista wrote:
>> Hi
>>
>> Can´t help you, but thought being generally the EEG data high-pass filtered
>> in the acquisition step, and how can you retain signal at such low
>> frequencies?
>>
>> Thanks
>>
>> Arnaldo
>>
>>
>>
>> -----Original Message-----
>> From: eeglablist-bounces at sccn.ucsd.edu
>> [mailto:eeglablist-bounces at sccn.ucsd.edu] On Behalf Of Gangadhar Garipelli
>> Sent: 05 November 2010 11:51
>> To: eeglablist at sccn.ucsd.edu
>> Subject: [Eeglablist] How to deal with 1/f noise in the low frequency
>> oscillations for on-line experiments?
>>
>> Dear all,
>>
>> I work with low frequency oscillations of the brain while a human
>> subject is cognitively engaged in a task. From the off-line analysis
>> (using zero-phase band pass FIR filters on full-band EEG), I discovered
>> that task-related cognitive signals are located in the range of [0.2
>> 0.3]Hz and in [0.6 0.8]Hz. The fluctuations/oscillations ( formally
>> called very low frequency oscillations VLFO, or infra slow oscillations
>> ISO) below 0.2Hz are REAL devil due to 1/f nature. The noise power is
>>> 100 times higher than signal's power.
>> Now as per my experimental demands, I need to estimate on-line in
>> real-time the signals mentioned in the above range and manipulate
>> stimulus presentation. Ideally, this eventually means I need to have a
>> very sharp high pass filter with almost zero group/phase delay. Which
>> sounds impossible!
>>
>> However, I should come up with a decent trade-off between SNR and
>> phase-delay. Do you have any suggestions? All suggestions ranging from
>> signal processing/machine-learning to hardware to solve this problem are
>> most welcome!
>>
>> Thanks in advance!
>> Sincerely,


-- 
Derek Eder
tlf 0704 915 715
www.stenljus.se



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