Computer Accessories User Manual

Selecting the data file and pre-processing MCE User’s Guide
4 2004-05-17 NM20600A-A
3 Selecting the data file and pre-processing
Select the MEG data file by pressing the [File] button of main window (p. 22).
Select the data file from the dialog. If the file has several data sets, a selection dia-
log pops up and you can select the correct data set.
When the you have selected the data, the pre-processing dialog (see Figure 2) will
open.
Figure 2 The pre-processing dialog
3.1 Setting the filtering and baselines
Because the MEG signals are mainly concentrated to the lower frequencies, you
can increase the signal-to-noise ratio with an low-pass filter. Select the [Lowpass
filter] toggle and set appropriate cutoff frequency and transition width of the filter.
After changing the numeric values, press Enter or the Tab key to update the value.
You can view the effect on a single channel by pressing the [Channel] toggle or
the filter response with the [Impulse response] and [Freq. response] buttons.
You can zoom into the preview window with the left mouse button and revert to
automatic scaling with the [rescale] button.
Because most of the channels usually have a non-zero DC-level, you should select
a [Baseline] toggle and select a time period where there should be no real evoked
responses, typically before the stimulus.
If you are analyzing a long period or the data otherwise includes strong artificial
slow drifts, you can select a [Detrend baseline] period after the evoked response.
This is usually a safer way of getting rid of the drifts than, for example, applying
high-pass filter.
The de-trend baseline actually fits a line to the values in the two baselines and sub-
tracts it form the data (see Figure 3).