Creating epochs of equal length¶
This tutorial shows how to create equal length epochs and briefly demonstrates an example of their use in connectivity analysis.
First, we import necessary modules and read in a sample raw data set. This data set contains brain activity that is event-related, i.e. synchronized to the onset of auditory stimuli. However, rather than creating epochs by segmenting the data around the onset of each stimulus, we will create 30 second epochs that allow us to perform non-event-related analyses of the signal.
import os import numpy as np import matplotlib.pyplot as plt import mne from mne.preprocessing import compute_proj_ecg sample_data_folder = mne.datasets.sample.data_path() sample_data_raw_file = os.path.join(sample_data_folder, 'MEG', 'sample', 'sample_audvis_raw.fif') raw = mne.io.read_raw_fif(sample_data_raw_file)
Opening raw data file /home/circleci/mne_data/MNE-sample-data/MEG/sample/sample_audvis_raw.fif... Read a total of 3 projection items: PCA-v1 (1 x 102) idle PCA-v2 (1 x 102) idle PCA-v3 (1 x 102) idle Range : 25800 ... 192599 = 42.956 ... 320.670 secs Ready.
For this tutorial we’ll crop and resample the raw data to a manageable size for our web server to handle, ignore EEG channels, and remove the heartbeat artifact so we don’t get spurious correlations just because of that.
221 events found Event IDs: [ 1 2 3 4 5 32] 221 events found Event IDs: [ 1 2 3 4 5 32] Including 3 SSP projectors from raw file Running ECG SSP computation Using channel MEG 0511 to identify heart beats. Setting up band-pass filter from 5 - 35 Hz FIR filter parameters --------------------- Designing a two-pass forward and reverse, zero-phase, non-causal bandpass filter: - Windowed frequency-domain design (firwin2) method - Hann window - Lower passband edge: 5.00 - Lower transition bandwidth: 0.50 Hz (-12 dB cutoff frequency: 4.75 Hz) - Upper passband edge: 35.00 Hz - Upper transition bandwidth: 0.50 Hz (-12 dB cutoff frequency: 35.25 Hz) - Filter length: 1000 samples (10.000 sec) Number of ECG events detected : 165 (average pulse 66 / min.) Computing projector Filtering a subset of channels. The highpass and lowpass values in the measurement info will not be updated. Filtering raw data in 1 contiguous segment Setting up band-pass filter from 1 - 35 Hz FIR filter parameters --------------------- Designing a two-pass forward and reverse, zero-phase, non-causal bandpass filter: - Windowed frequency-domain design (firwin2) method - Hamming window - Lower passband edge: 1.00 - Lower transition bandwidth: 0.50 Hz (-12 dB cutoff frequency: 0.75 Hz) - Upper passband edge: 35.00 Hz - Upper transition bandwidth: 0.50 Hz (-12 dB cutoff frequency: 35.25 Hz) - Filter length: 1000 samples (10.000 sec) Not setting metadata Not setting metadata 165 matching events found No baseline correction applied Created an SSP operator (subspace dimension = 3) 3 projection items activated Loading data for 165 events and 61 original time points ... Rejecting epoch based on MAG : ['MEG 1711'] Rejecting epoch based on MAG : ['MEG 1711'] Rejecting epoch based on MAG : ['MEG 1411', 'MEG 1421'] 3 bad epochs dropped No EEG channels found. Forcing n_eeg to 0 Adding projection: planar--0.200-0.400-PCA-01 Adding projection: planar--0.200-0.400-PCA-02 Adding projection: axial--0.200-0.400-PCA-01 Adding projection: axial--0.200-0.400-PCA-02 Done. 7 projection items deactivated Created an SSP operator (subspace dimension = 7) 7 projection items activated SSP projectors applied...
|Measurement date||December 03, 2002 19:01:10 GMT|
|Digitized points||146 points|
|Good channels||102 magnetometer, 203 gradiometer, and 0 EEG channels|
|Bad channels||MEG 2443|
|EOG channels||Not available|
|ECG channels||Not available|
|Sampling frequency||100.00 Hz|
To create fixed length epochs, we simply call the function and provide it
with the appropriate parameters indicating the desired duration of epochs in
seconds, whether or not to preload data, whether or not to reject epochs that
overlap with raw data segments annotated as bad, whether or not to include
projectors, and finally whether or not to be verbose. Here, we choose a long
epoch duration (30 seconds). To conserve memory, we set
Not setting metadata Not setting metadata 5 matching events found No baseline correction applied Created an SSP operator (subspace dimension = 7) 7 projection items activated
Characteristics of Fixed Length Epochs¶
Fixed length epochs are generally unsuitable for event-related analyses. This can be seen in an image map of our fixed length epochs. When the epochs are averaged, as seen at the bottom of the plot, misalignment between onsets of event-related activity results in noise.
Loading data for 5 events and 3000 original time points ... 0 bad epochs dropped Not setting metadata Not setting metadata 5 matching events found No baseline correction applied 0 projection items activated 0 bad epochs dropped
For information about creating epochs for event-related analyses, please see The Epochs data structure: discontinuous data.
Example Use Case for Fixed Length Epochs: Connectivity Analysis¶
Fixed lengths epochs are suitable for many types of analysis, including frequency or time-frequency analyses, connectivity analyses, or classification analyses. Here we briefly illustrate their utility in a sensor space connectivity analysis.
The data from our epochs object has shape
(n_epochs, n_sensors, n_times)
and is therefore an appropriate basis for using MNE-Python’s envelope
correlation function to compute power-based connectivity in sensor space. The
long duration of our fixed length epochs, 30 seconds, helps us reduce edge
artifacts and achieve better frequency resolution when filtering must
be applied after epoching.
Let’s examine the alpha band. We allow default values for filter parameters (for more information on filtering, please see Filtering and resampling data).
Loading data for 5 events and 3000 original time points ... Setting up band-pass filter from 8 - 12 Hz FIR filter parameters --------------------- Designing a one-pass, zero-phase, non-causal bandpass filter: - Windowed time-domain design (firwin) method - Hamming window with 0.0194 passband ripple and 53 dB stopband attenuation - Lower passband edge: 8.00 - Lower transition bandwidth: 2.00 Hz (-6 dB cutoff frequency: 7.00 Hz) - Upper passband edge: 12.00 Hz - Upper transition bandwidth: 3.00 Hz (-6 dB cutoff frequency: 13.50 Hz) - Filter length: 165 samples (1.650 sec)
If desired, separate correlation matrices for each epoch can be obtained.
For envelope correlations, this is done by passing
combine=None to the
envelope correlations function.
Now we can plot correlation matrices. We’ll compare the first and last 30-second epochs of the recording:
first_30 = corr_matrix last_30 = corr_matrix[-1] corr_matrices = [first_30, last_30] color_lims = np.percentile(np.array(corr_matrices), [5, 95]) titles = ['First 30 Seconds', 'Last 30 Seconds'] fig, axes = plt.subplots(nrows=1, ncols=2) fig.suptitle('Correlation Matrices from First 30 Seconds and Last 30 Seconds') for ci, corr_matrix in enumerate(corr_matrices): ax = axes[ci] mpbl = ax.imshow(corr_matrix, clim=color_lims) ax.set_xlabel(titles[ci]) fig.subplots_adjust(right=0.8) cax = fig.add_axes([0.85, 0.2, 0.025, 0.6]) cbar = fig.colorbar(ax.images, cax=cax) cbar.set_label('Correlation Coefficient')
Total running time of the script: ( 0 minutes 48.469 seconds)
Estimated memory usage: 8 MB