Part 2: Mainly on eye tracking and pupillometry data.
Controlling for eye-related artifacts in EEG is a standard protocol, so I wanted to compile a list of the sources of electrical activity that result in distortion of EEG signal.
we may not need to remove artifacts
I outlined in my last post a couple of examples using deep learning models to either “learn noise” in order to remove it, or utilize noise with the end goal of assisting classification. There are similarities in how researchers use the word “noise” to how words like “artifacts” or “distortions” are also used, in my opinion.
Including distortions in EEG may be essential to some research. A quick example of this is Tevis Gher's project mentioned in the last post, where correction of blinking and other eye tracking related events would remove useful information that could aid in classification.
I just mean to note that aggressive filtering or thresholding when finding independent components can lead to cases where we remove brain activity that affects the system of interest (e.g. EEG) and another that produces “artifacts” (e.g. blinking) to the detriment of the goal of a study (e.g. decision-making in a visual task where activity in certain regions of the brain affect blinking rates). In the parenthesized example of blink rate, it may be important to not remove blink artifacts when studying attention in children with ADHD, since Caplan et al 1 showed it may be a critical discriminatory measure between ADHD and non-ADHD populations.
eye related artifacts
With that out of the way, I wanted to summarize the types of corrections that seem to be most common in EEG processing, related to eye tracking data. This is based on (and the quotes below derive from) a very nice, controlled study by Plöchl, Ossandón and König in 2012 2.
|artifact||what is it||what causes it (quotes from Plöchl et al)|
|corneo-retinal dipole movement||large ocular movements||
|blinks (eyelid-induced)||spontaneous or willful blinking||
|eyelid-saccades||saccade-accompanying ballistic eyelid movements||
|saccadic spike potential||potential appearing right before saccade onset||depends on the type of eye movements. Characteristically, it's
|microsaccades||involuntary, small eye movements that occur during fixations||
correction of eye artifacts
This will be covered in the next post.