HC 3 Flashcards
What are the steps for data-analysis ERP?
- Preprocessing
- Main signal processing
- Statistical testing
What do we do when preprocessing?
-(Re-)reference of the EEG signals
-Preprocessing reduces noise signals and formats EEG data for the main signal processing
-filtering
-correction of eye movements
artefacts
-artefact rejection
-segmentation
What are the two ways to do re-referencing (offline)?
- Mastoid reference
Cz - ((Lm+Rm)/2) - Average reference
Cz - ((Fz+F3+F4…O1+Oz+O2)/nr of electrodes)
Why do we have to take account for re-referencing?
ERPs can look very different with different references. This has to do with the distance between sensors. It affects ERP morphology, but not topography. Different references sensors are:
-Nose
-Average
-Earlobes
-Non-cephalic
-Mastoids
What is noise?
Electric signals not from the brain.
What are the common types of noise?
EMG= electromyogram for face movement
EOG= electrooculogram for eye movement. Blinking is the most occuring noise.
ECG= electrocardiogram for movement
What is noise reduction?
It is a visual inspection and artefact detection/rejection. We have to inspect the data first, is it okay to use?
We use topographic interpolation to try to reduce noise. Then we use independent component analysis (ICA) for removing eye movement artefacts and then use filtering. So, we get rid of the noise.
You can either use manual or automatic rejection.
What is the difference between automatic and manual rejection?
Manual: delete a segment from the data based on visual inspection.
Automatic: delete segment from the data if some criterion is met, for example if the amplitude from of a signal > 200uV
What is artefact?
Another word for noise.
What is the difference between liberal artifact- and conservative artifact rejection?
Liberal artifact rejection= many trials in the average, but contaminated with artifacts
Conservative artifact rejection= fewer trials in the average, though less artifacts
The more trials, the smoother the data.
What is topographic interpolation?
When one or a few electrodes are very noise throughout the experiment, replace the electrode with the mean of the surrounding electrodes.
What is ocular rejection?
If the participant keeps blinking before the stimulus, throw the data away.
What is ocular correction?
Independent component analysis gets rid of the blinks without deleting data.
ICA can be used to remove eye movement and other artefacts.
What is filtering?
It is removing high or low frequencies from the EEG signal. Why should we filter raw data? To remove non-neural voltages, such as the 50Hz AC noise(computer screen omits this noise regurarly).
What are the different types of filtering?
-High-cut/low-pass filter= remove high-frequency noise
-Low-cut/high-pass filer= remove low-frequency noise
-Band-reject/Notch filter= filters out noise between the lower and upper thresholds
Different filtering can result in different ERPs. This is important to note in the method section of a paper.