MNE quickstart and background

1. Reading data into MNE

One of the first things you might be wondering about is how to get your data into mne. Assuming that you have unprocessed data, you will probably be happy with at least one of these readers:

They all have in common to return an object. See Importing data into MNE.

2. MNE gives you objects with methods

We said above that there are MNE objects. This is of course computer science jargon. What it actually means is that you get a data structure that is more than the channels by time series and the information about channel types and locations, meta-data if you want. Indeed the structures that MNE is using provide so called methods. These are nothing but functions that are configured to take the data and the meta-data of the object as parameters. Sounds complicated, but it’s actually simplifying your life as you will see below. Whether you consider Raw objects that describe continuous data, Epochs objects describing segmented single trial data, or Evoked objects describing averaged data, all have in common that they share certain methods.

  • Try raw.plot, epochs.plot, evoked.plot and any other method that has a name that starts with plot. By using the call operators () you invoke these methods, e.g. epochs.plot(). Yes, you don’t have to pass arguments but you will get an informative visualization of your data. The method knows what to do with the object. Look up the documentation for configuration options.

  • Try raw.pick_types, epochs.pick_types evoked.pick_types and any other method that has a name that starts with pick. These methods will allow you to select channels either by name or by type. Picking is MNE jargon and stands for channel selection.

  • Some of these methods can actually change the state of the object, e.g. permanently remove or transform data. To preserve your input data can explicitly use the .copy method to manipulate a copy of your inputs. Example:

    >>> raw.copy().pick_types(meg=False, eeg=True)  
  • This examplifies another important concept, that is chaining. Most methods return the object and hence allow you to write handy pipelines. Guess what this code does:

    >>> (fig = raw.copy()  
    >>>           .pick_types(meg=False, eeg=True)  
    >>>           .resample(sfreq=100)  
    >>>           .filter(1, 30)  
    >>>           .plot())  

    Yes, it creates a figure after filtering a resampled copy of the EEG data. In fact you can also recognize methods by certain linguistic cues. Methods typically use english verbs. So raw.ch_names is not a method. It’s just an attribute that cannot be invoked like a function.

  • Last but not least, many MNE objects returned a .save method that allows you to store your data into a FIFF file.

3. A key thing for MNE objects is the measurement info

Besides .ch_names another important attribute is .info. It contains the channel information and some details about the processing history. This is especially relevant if your data cannot be read using the io functions listed above. You then need to learn how to create an info. See The Info data structure.

4. MNE is modular

Beyond methods another concept that is important to get are modules. Think of them as name spaces, another computer science term. Ok, think of street names in different cities. Sending a parcel to the Washington street in New York or San Francisco typically does not involve a conflict, as these streets are in different cities. Now you know what is the idea behind a name space. You can read a lot of resources that you will find when googling accordingly. What is important here is that our modules are organized by processing contexts. Looking for I/O operations for raw data?:

>>> from mne import io

Want to do preprocessing?:

>>> from mne import preprocessing

Want to do visualization?:

>>> from mne import viz


>>> from mne import decoding

I’m sure you got it, so explore your intuitions when searching for a certain function.

5. Inspect and script

Did you happen to notice that some of the figures returned by .plot methods allow you to interact with the data? Look at raw.plot and epochs.plot for example. They allow you to update channel selections, scalings and time ranges. However, they do not replace scripting. The MNE philosophy is to facilitate diagnostic plotting but does not support doing analysis by clicking your way. MNE is meant to be a toolbox, and its your task to combine the tools by writing scripts. This should save you time in the long run by:

  1. Enabling code reuse.
  2. Documenting what you did.

Reviewers are asking you to update your analysis that you actually finished 1 year ago? Luckily you have a script.

6. Eighty percent or Python

A related point is that MNE functions are there to make it fun to process common tasks and facilitate doing difficult things. This means that you will notice certain limits here and there, the viz functions do not exactly plot things as you want them, even when using the options provided by that function. In fact our goal is to guess which are the essential 80 percent that you need in order be happy in 80 percent of the time. Where you need more Python is there for you. You can easily access the data, e.g. raw[:10, :1000] or epochs.get_data() or and manipulate them using numpy or pass them to high-level machine learning code from scikit-learn. Each .plot method returns a matplotlib figure object. Both packages have great documentations and often writing Python code amounts to looking up the right library that allows you to tackle the problem in a few lines.