Field of Science

Have you seen me before?

I have been using PCA to correct blink artifact in an EEG study that I am presenting at AMLaP in a couple weeks. Generally, I think I've gotten pretty good at detecting blinks. I do see other things that look like artifact but which I don't understand as well. For instance, look at this channel plot:
(You should be able to increase the size of the picture by opening it in a new window). So this looks a bit like a blink, but it's in the wrong place entirely. This is a 128 electrode EGI cap, with the electrodes listed sequentially (the top one is electrode 1 and the bottom is electrode 124 -- I don't use electrodes 125-128 because they tend not to have good contact with the skin).

The way EGI is laid out, the low-numbered electrodes and high-numbered electrodes are in the front, whereas the middle-numbered electrodes are in the back (check this picture), So basically what I'm seeing is being generated in the back of the head. Actually, the back left of the head, according to my PCA:
In this, the top left panel shows the localization of the signal. The top right panel shows which trials the signal occurred in. The power spectrum (bottom panel) is also quite odd. I'm going ahead and removing this component, because it's clearly artifact (the amplitude looks way too large to be true EEG), and it affects so many trials that I can't just exclude them without excluding the participant. But I'd really like to know what this is. Because maybe I *should* be excluding the participant.

So...has anyone seen something like this before?

For those wondering...

Using PCA, I was able to get rid of this artifact fairly cleanly. Here's is an imagine before removal, with the 124 electrodes stacked on one another:
You can see that strange artifact -- which looks like a blink but not quite as smooth as your typical blink -- very easily in these four trials.

Here are the same four trials after I subtracted that component, plus another component that probably is blink-related (there were two, classic-looking blinks in my data; the component above found both of those *and* those two blinks; the other component found only the two classic blinks):
You can see that the odd artifact is gone from all four trials, both otherwise, things look very similar.


Anonymous said...

I did not see what the stimulus was. Can you describe the auditory environment?

GamesWithWords said...

This is visually-presented sentences, mostly word-by-word. The room is (more or less) silent.