Removing eye activity from EEG signals via ICA

Some background

In this previous post, I used linear regression to remove ocular artifacts from EEG signals. A popular alternative to this approach is independent component analysis (ICA). In a nutshell, ICA decomposes multi-channel EEG recordings into maximally independent components. Components that represent ocular activity can be identified and eliminated to reconstruct artifact-free EEG signals. This approach is described in more detail in Jung et al. (2000).

A comprehensive comparison between the two methods is beyond the scope of this post. Instead, I will only list some properties of both techniques.

First, the regression-based approach requires EOG channels, whereas ICA works without any reference signal.

Both methods potentially remove brain activity in addition to ocular activity. ICA can separate ocular components from brain components well if many EEG channels are available (which in turn requires a relatively large number of data samples). A cleaner separation also means that less brain activity will be removed when ocular components are eliminated. The minimum number of EEG channels required for ICA decomposition varies, but as a rule of thumb at least 20 channels seem to be necessary (the EEGLAB tutorial has more details on the amount of data required for ICA). In constrast, the regression approach works even if only a few EEG channels are available. However, the EOG reference channels always contain some amount of brain activity, which will also be removed from the data in addition to ocular activity.

The ICA method entails manual identification of ocular components, although several algorithms exist to automate this process (for example EyeCatch or ADJUST). ICA also takes much longer to compute than the regression approach (but efficient implementations are available that keep computation time to a minimum). Finally, ICA is an optimization problem that is not guaranteed to find the globally optimal solution. Depending on the initial conditions, the algorithm might find different independent components from run to run. However, this is not a huge issue in this application, because ocular components are relatively stable across decompositions.

Let’s now turn to an example to see how ICA can be used to remove ocular artifacts with MNE.

Implementation

Before we start, it is worth mentioning that ICA will generally run faster using a multi-threaded numeric library such as Intel MKL, OpenBLAS or Apple Accelerate. If you have an Anaconda environment (which I recommend in this previous post), you should already have NumPy with MKL support.

Data preprocessing

We will use the same data set that we already used with the regression approach. Specifically, we’ll use participant A01T from data set 001-2014 from the BNCI Horizon 2020 database (check out the post on using the regression approach for more details on this data set). Download this file and save it to your working directory. Note that this data set contains 22 EEG and 3 EOG channels. Although EOG channels can (and should) be used for ICA decomposition (provided that they use the same reference electrode as the EEG channels), we will only use the EEG channels here to keep things simple.

As always, we start by firing up IPython and performing the usual initial steps:

%matplotlib qt5

from scipy.io import loadmat
import mne

The data comes in a MAT file, so we use SciPy’s loadmat function to load it as a NumPy array. Note that this time we only load the fourth run containing the actual experimental data – we do not need the calibration run.

mat = loadmat("A01T.mat")
eeg = mat["data"][0, 3]["X"][0, 0] * 10e-6

We will plot ICA components as projections on the scalp surface later on. To this end, MNE needs to know the channel labels, which unfortunately are not present in the data. However, the data description contains a picture of the montage, which we can use to populate a list of channel names.

ch_names = ["Fz", "FC3", "FC1", "FCz", "FC2", "FC4", "C5", "C3", "C1", "Cz",
            "C2", "C4", "C6", "CP3", "CP1", "CPz", "CP2", "CP4", "P1", "Pz",
            "P2", "POz", "EOG1", "EOG2", "EOG3"]

We create an info object using this list, which we need to create a Raw object containing the EEG data and associated meta information (which in our case is just the sampling frequency of 250 Hz and the channel types). Finally, we assign our raw data a standard 10–20 montage, which maps the channel labels to their positions on the scalp.

info = mne.create_info(ch_names, 250, ch_types=["eeg"] * 22 + ["eog"] * 3)
raw = mne.io.RawArray(eeg.T, info)

raw.set_montage("standard_1020")

Performing ICA

ICA does not work in the presence of low-frequency drifts, so we create a copy of our raw object and apply a high-pass filter to this copy.

raw_tmp = raw.copy()
raw_tmp.filter(1, None)

We are now ready to perform ICA. First, we instantiate an ICA object and specify that we want to use the extended Infomax algorithm and a fixed random state for reproducibility. Next, we fit ica to our filtered raw data raw_tmp (note that this might take a couple of minutes depending on the speed of your computer).

ica = mne.preprocessing.ICA(method="extended-infomax", random_state=1)
ica.fit(raw_tmp)

Identifying ocular components

Our next task is to identify all ocular components. This is usually done by visual inspection, so we start by plotting all 22 independent components.

ica.plot_components(inst=raw_tmp)

Two new figures will pop up, the first 20 components are in the first figure and the last two components are in the second figure (MNE automatically creates figures with a maximum of 20 components each). Let’s focus on the first 20 components, because ocular components are generally found among these:

From these scalp projections, the component labeled as “ICA001” looks like it could represent eye movements because of its frontal location. To be sure, we can click on this component to open a new window with more details on this component (this is possible because we specified inst=raw_tmp in the method call):

Besides the scalp projection, we can now also inspect

  • the component power spectral density (which is typical for ocular activity because the characteristic EEG alpha peak is missing),
  • the epochs image, which color-codes the component activity over (virtual) epochs (which shows typical intermittent activity as blue and red stripes),
  • and the epochs variance (which in this case is not really helpful in identifying the component).

In summary, we can be pretty sure that component “ICA001” represents ocular activity. To be extra safe, let’s plot the component time courses to corroborate our assumption:

ica.plot_sources(inst=raw_tmp)

Indeed, if you scroll through the data, “ICA001” captures primarily eye movements and eye blinks.

Note that usually two ocular components can be found in the decomposition, but this is not the case in our example data (all remaining components do not seem to originate from eye activity).

Removing ocular components

In the final step, we create a list ica.exclude containing the indices of all components that should be removed when reconstructing the EEG signals. In our case, this list contains only a single element.

ica.exclude = [1]

Note that you can click on the component title (“ICA001”) in the ICA components plot created with ica.plot_components(inst=raw_tmp) to include/exclude a component (the title of an excluded component will turn gray). This will also add/remove this component from/to the underlying ica.exclude list.

Now we can apply our ICA results (without the excluded component) to a copy of the original EEG to obtain artifact-free signals:

raw_corrected = raw.copy()
ica.apply(raw_corrected)

Visualizing the results

So how did ICA perform? Let’s take a look at a segment of the original EEG containing a clear eye movment artifact:

raw.plot(n_channels=25, start=54, duration=4,
         scalings=dict(eeg=250e-6, eog=750e-6))

And here is the corrected signal:

raw_corrected.plot(n_channels=25, start=54, duration=4,
                   scalings=dict(eeg=250e-6, eog=750e-6))

Looks like ICA did a pretty decent job in removing eye artifacts.