Dear Gus,
I've also come across this type of artifacts. I am also not sure about
their source. I am skeptical of including those channels in the data and
have treated them as bad channels during maxfilter processing. I then
used SSS -st 4 and that was all.
Good luck,
Marina
Gustavo Sudre wrote:
> Jason,
>
> Thanks for your reply. I did try Maxfilter with -st 4 and -st 10, but
> the jumps were still there. I'm using the default -corr value, though.
> What parameters did you find to get rid of the jumps?
>
> Best,
>
> Gus
>
> On Feb 27, 2009, at 10:21 AM, Jason Taylor wrote:
>
>> Hi Gustavo,
>>
>> I've seen those 'jumps' in raw data as well, though I'll leave it to
>> the experts to explain their origin. Have you tried using the
>> MaxFilter -st option? Whereas standard SSS seems to smear sensor jumps
>> over several channels, I've found that SSS-st nicely removes this type
>> of sensor noise.
>>
>> Cheers,
>> - Jason.
>>
>>
>> On Fri, Feb 27, 2009 at 2:59 PM, Gustavo Sudre <[log in to unmask]> wrote:
>> Sorry... here's the figure.
>>
>> Thanks,
>>
>> Gus
>>
>>
>>
>>
>> Begin forwarded message:
>>
>> From: Gustavo Sudre <[log in to unmask]>
>> Date: February 27, 2009 9:57:18 AM EST
>> To: [log in to unmask]
>> Subject: discontinuity in raw data
>>
>> Hello,
>>
>> For many repetitions in my experiments I have observed a discontinuity
>> in the raw MEG data. This happens in some of my individual trials, and
>> it is not always across several channels simultaneously. The attached
>> picture shows an example of this type of noise, for a particular trial
>> and channel. When I notice such pattern during recordings, reheating
>> the sensor usually works. However, I don't catch them all the time, so
>> it's often the case that I see this pattern in recorded data. These
>> are my questions:
>>
>> 1) What causes this noise? Can the channel be "trusted" even if it
>> shows the pattern a few times?
>>
>> 2) If I run my data through SSS, this pattern seems to appear in many
>> more channels. That means that I need to discard many more repetitions
>> after running SSS. Is this something intrinsic to the SSS algorithm
>> (i.e. to multiply this noise)? What is the reason for it to appear
>> across more channels after SSS?
>>
>> 3) If I clean up my raw data prior to SSS (e.g. discard the
>> repetitions with such pattern), and create a new FIF raw file from
>> this new data, my data won't be continuous in time anymore. I don't
>> think SSS will have a problem with it (unless I use the temporal
>> extension). Is that correct?
>>
>> 4) Would you suggest a more accurate way to deal with these
>> discontinuities (eg. wavelets?), instead of discarding (sometimes
>> precious) trials?
>>
>> 5) Assuming SSS has no problems with this new "clean" raw file, can
>> the SSS algorithm create such discontinuities by itself? If so, why?
>>
>> Thanks,
>>
>> Gus
>>
>>
>>
>>
|