[FieldTrip] Artifact in LCMV beamformer source localization

[Bushra Riaz Syeda]

Thank you for you reply


I am already using common filter for both conditions and I have also tried weight normalisation with 'unitnoisegain', but it didn't change results.


My protocol is bit unusual in a way that electric stimulation to the finger is time locked with 200 ms after the R wave of ECG. As such, the ECG artifact is prominent at -200 ms in my data and I am using ICA to remove it as shown in figure (https://chalmersuniversity.box.com/s/deuzucqovgcfienybk7zisc3r3wjusvu)


For lcmv I am using -0.71 to -0.21 sec of prestimulus interval window ( relatively clean baseline without ECG artifact) and 0.01 to 0.51 sec of post stimulus interval window.


Could my protocol be the reason of 'center of head artifact', I didnt think it would be the case?

Any suggestions to solve this would be highly appreciated. Thank you


Kind regards

Bushra Riaz

________________________________
From: fieldtrip-bounces at science.ru.nl <fieldtrip-bounces at science.ru.nl> on behalf of Sarang S. Dalal <sarang at cfin.au.dk>
Sent: Wednesday, October 4, 2017 9:47:19 AM
To: fieldtrip at science.ru.nl
Subject: Re: [FieldTrip] Artifact in LCMV beamformer source localization

Hi Bushra,

Are you using the common filter approach to use the same spatial filter for your contrasted conditions?
http://www.fieldtriptoolbox.org/example/common_filters_in_beamforming

(Although that example is focused on DICS and PCC, a similar approach can also be used for LCMV.)

This should reduce the likelihood of the 'middle of the head' artifact. If the filters are computed independently, however, then the noise suppression characteristics of each filter can be different, so the contrast would pull out differences in each filter's noise performance.

If you are already using this approach, then you can try weight normalization, which is one strategy for removing the noise bias (and usually prevents the 'center of the head' artifact even when no contrast is done). You can run this by adding to your cfg structure:
cfg.lcmv.weightnorm = 'nai'
- or -
cfg.lcmv.weightnorm = 'unitnoisegain';

Each of those methods should give you the same result with a contrast. The NAI is simply scaled against the theoretical noise power (i.e., the noise level = 1), so is often preferred when no contrast is done.

Cheers,
Sarang




On Wed, 2017-10-04 at 07:33 +0000, Bushra Riaz Syeda wrote:

Hello



I am using lcmv to source localize somatosensory response of Electric stimulation to the index finger of the left hand of subjects.



500 ms of post-stimulus interval is contrasted to 500 ms of pre-stimulus interval. The problem is I see strong activity in the middle of the head for couple of my subjects. It is very strong for one subject as shown ( link to figure 1: https://chalmersuniversity.box.com/s/jxmq5zp8k9m9tdfpogem7ee3r4z9q3ac )and week for other subjects ( link for figure 2: https://chalmersuniversity.box.com/s/o3yl57s6phr96f9rxh4wuthjcgllr2vv ) . I thought that noise bias towards the center of the head is usually taken care of when comparing conditions. Could somebody guide me how to solve this problem. Specially figure 1 where the noise bias is so strong that I don't see any activity in somatosensory cortex.



Thank you



Kind regards

Bushra Riaz


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