Hints for ICA-based artifact correction

ICA is a great tool for correcting artifacts, especially eye blinks. Here we provide some practical hints for ICA-based artifact correction.

One of the most difficult parts for new users is knowing which independent components (ICs) reflect artifacts and should be removed.  There's a really great overview here, which describes how to use the combination of scalp distribution, time course, and frequency content to distinguish among artifacts.

A fundamental assumption of ICA is that the artifact has a perfectly consistent scalp distribution.  This is true for some artifacts, such as blinks and EKG, but it may not be true for others (e.g., EMG, eye movements).  ICA works reasonably well with EMG and eye movements under some conditions (especially if all the eye movements are along a single plane), but we recommend caution with these artifacts.

A key aspect of ICA is that the number of ICs must necessarily be equal to the number of channels. You obviously aren't changing the number of underlying brain components by changing the number of electrodes, so the fact that the number of ICs changes as you vary the number of electrodes should make it clear that ICA is an imperfect approach that will not work for every kind of artifact.  Also, no matter how many electrodes you have, the number of brain components is likely to be greater than the number of ICs.  So, ICA will inevitably blend multiple brain components into a single IC, and a single brain component may be split across multiple ICs. As a result, ICA cannot be expected to figure out all the true underlying components.  In practice, ICA works best for components that are relatively large and relatively common.  For this reason, our labs mainly use ICA for blinks, which are both very large (in all participants) and fairly common (in most participants). 

Given that the number of ICs is fixed, you don't want to "waste" ICs on huge but infrequent artifacts (especially if you have a relatively small number of channels).  For example, your participants may have periods of "crazy" EEG during breaks (as a result of stretching, movement, etc.), and these periods may eat up a lot of ICs.  You may therefore want to delete sections of "crazy data" before performing ICA.  ERPLAB Toolbox has two routines that are designed to help with this.  One can automatically delete periods of data between trial blocks.  Another can detect and delete periods of crazy data.  But don't use this approach to delete ordinary artifacts -- this should be for periods in which you are seeing huge and irregular voltage deflections.

Similarly, it's important to eliminate slow drifts prior to ICA.  We generally recommend a high-pass filter with a half-amplitude cutoff of 0.1 Hz and a slope of 12 dB/octave.  You can also apply a low-pass filter if your data have a lot of high-frequency noise.  This filtering should be done on the continuous data (prior to epoching and prior to deleting segments of crazy data) and should be consistent for all participants (e.g., don't low-pass filter for some participants but not others, or use different cutoffs for different participants).

Another important bit of practical advice is that ICA involves training a neural network, and you need enough "trials" (time points) to train the network. A general heuristic is that the # of time points must be greater than 20 x (# channels)^2.  The key is that the number of channels is squared.  So, with 64 channels, you would need 81,920 points (which would be about 5.5 minutes of data with a 250 Hz sampling rate).  However, with 128 channels, you would need 4 times as many points, and with 256 channels, you would need 16 times as many points.