To practice this tutorial you can use the 5 minute recording from an Eyes Closed Rest experiment called “TutorialSignal” which you need to download from Blackboard→Course Documents→NBT material.
Unzip and save both files in a new folder in your Documents, we suggest to call this folder NBT_test_files.
How to load
Go to the menu File|Load NBT Signal.
Find the file you want to load (i.e., HN13.S002.20130610.ECR.mat); click OK.
At this moment you only have a RawSignal in the file, later you will have other versions of your signal, e.g., CleanSignal . If you have other versions of the signal in the file, you will need to select which signal versions you want to load.
Wait for the signal to load. The signal is loaded when the info becomes visible in the NBT GUI.
The NBT GUI will look similar to this:
How to plot the raw signal
Go the the menu Visualization|Plot Signals
A window pops up. Just click “OK”
You now see a plot of the raw EEG signal from all channels and for the whole time range. The EEG signal is “raw” in the sense that it is not filtered or cleaned for artifacts yet.
By using the zoom buttons in the menu bar you can select an area you want to zoom in to or back out of.
Scroll through the signal using the scroll bars in the left bottom corner, or select channels to plot by using the Select channel(s) to plot button.
After exploring these plotting tools your window might look like the figures below.
Typically, the strongest signal in eyes-closed EEG is the alpha oscillations (8-13 Hz). Try to find a channel with a clear alpha signal. Question: What is the frequency of this subject's alpha oscillations? Did your neighbor arrive at the same value?
For your report or poster you need to make screen dumps of EEG signals. Press Windows button Print Scrn on the keyboard, and an image of your screen is now on the clipboard. Go to a program like Paint, and Ctrl V (paste) the image and crop the part you want to keep.
Alternatively, you may go to “File” in the Matlab window and export the figure in a format that can be edited in another software, like Adobe Illustrator.
A power spectrum is used to quantify the power of different frequency components of a signal. You can use it to, e.g., identify the peak frequency or peak power in the alpha band (8-13 Hz).
Go to Visualization|Plot time-frequency plot and power spectrum of one channel
A window will pop up (see figure below). In the top you see a time-frequency plot for one channel. In the bottom left you see the power spectrum for that same channel. In the bottom right you can select other channels (either from the list or the scalp map) that you want to see a time-frequency plot and a power spectrum for. Question: Can you identify the peak frequency of the alpha oscillations? Question: Is it equal to the frequency you found when plotting the signal above?
Later in the course, or if you have time now, it is recommended to play with the other plotting parameters to see the possibilities for a nice visualization of your signal and its time-frequency characteristics. For example, choosing wavelet (w) instead of multitaper and decreasing the 'frequency step' size will make a more smooth TF plot.
This is a raw signal, which means it probably still contains artifacts (i.e. signals not from the brain) that will disturb the analysis. In the next tutorial you will learn how to remove these artifacts.