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Selecting the data file and pre-processing MCE User’s Guide
6 2004-05-17 NM20600A-A
Figure 4 If too strong down-sampling is applied, the aliasing is reflected in the
preview window with the folding frequency response. Decrease the filter cutoff
frequency or decimation.
3.3 Bad channels and projections
If some of the channels are flat or bad, you must set the [Bad channels] toggle and
write the channel names to the corresponding text field. The format can be “MEG
1112 1113 ...” or “MEG 1112 MEG 1113 ...”, the channels described with
four digits for Elekta Neuromag™ or Vectorview™ data and three digits for Neu-
romag-122™ data. Use the numbers in the channel names regardless of the channel
order in the fif-file. Wildcards are not supported.
External disturbance fields can be filtered using Signal Space Projection (SSP). If
the fif-datafile has a noise projection specified, you can apply it to the data and cal-
culations by selecting the [Apply projection] toggle. The projection is updated
when you change the bad channels. Normal data files where the projection infor-
mation is included and the data itself is the original non-projected data should be
used. If projected data is used, the file must also contain the projection vectors that
were used define the removed subspace. Otherwise the results may be distorted.
When you load a new file or press the [Automatic] button, the bad channels are set
automatically. The following channels are set bad:
1. Channels that are marked bad in the fif-file
2. If baseline is used, the channels where the baseline is flat
3. If baseline is used, the channels where the baseline is very noisy
The “flat channels” are the ones where the standard deviation of the raw baseline
activity is less than 10 % of the median value of others. The “noisy channels” are
the ones where the standard deviation of the baseline activity after projections is
over three times the median value. If the data includes both gradiometer and mag-
netometer data, the values are only compared with the channels with same coil
type.