Import Data
This step imports EEGLAB .SET data and saves them as .MAT files. Here, EEG trials can also be smoothed across trials using a moving window average.
This is useful to further denoise the data when one predominantly wants to look at slower dynamics across trials.
Smoothing does not dramatically alter the outcome of the group ICA. When you click on Import Data button, a figure (Figure
1)
will open to select the EEG files in a data format that EEGLAB toolbox uses. After selecting the data files,
an input dialog box will open (Figure 2) to enter the following parameters.
- 'Enter vector to correct baseline (ms)'. Enter time range in milliseconds to remove the baseline.
- Do you want to smooth the EEG trials? Selecting 'Yes' means that the trials will be smoothed using the selected moving window.
- 'Enter a number for moving point average?' Trials will be averaged using the selected window length. For now leave it as 3.
- 'How Do You Want To Sort Trials?' You have the option to sort trials based on condition and latency. For now leave it as 'No'
Note that trial sorting makes sense when (a) trial structure is different across subjects (b) sources are vastly different between conditions and or behavioral outcomes.
Sorting trials makes it easier to identify components of interest in the display GUI, it does for the most part not critically affect the accuracy of individual
component reconstruction, for illustration, in a typical oddball experiment, one can run the group ICA on unsorted trials, and then sort the resulting components
according to individual RTs, which reconstructs the RT-dependency of later components that belong to the P300 complex. EEGIFT can sort by RT and trial category, and
employs the EEG.event structure from EEGLAB for this.
- 'Enter file name (not full path) for writing data.' Data will be written in MAT file format that EEGIFT uses for analyzing EEG data.
The data will be saved in the specified output directory (Figure 3) and the data set directory name will be the same as the file name.
Figure 1: Select EEG files in .SET format.
Figure 2: EEG Parameters
Figure 3: Output directory to save the data