In the previously attached image: red = after SSS, blue = after SSS-st. May or may not be relevant.

- J.


On Fri, Feb 27, 2009 at 4:33 PM, Jason Taylor <[log in to unmask]> wrote:
Hi Gus,

I too used -st 4 and the default -corr (something like 0.980?). Perhaps we're talking about different phenomena -- unfortunately, I don't have any good examples on hand, but I'll let you know if I dig any up. Anyway, if the artefact appears in only one sensor, I would be surprised if -st didn't get rid of it.

Cheers,
- Jason.

P.S. Sorry, I meant to send the last reply to the list as well, but it was rejected because I was using the wrong email account.



On Fri, Feb 27, 2009 at 3:28 PM, Gustavo Sudre <[log in to unmask]> 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