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%matplotlib inline

Compute coherence in source space using a MNE inverse solution

This example computes the coherence between a seed in the left auditory cortex and the rest of the brain based on single-trial MNE-dSPM inverse solutions.



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# Author: Martin Luessi <mluessi@nmr.mgh.harvard.edu>
#
# License: BSD (3-clause)

import numpy as np

import mne
from mne.datasets import sample
from mne.minimum_norm import (apply_inverse, apply_inverse_epochs,
                              read_inverse_operator)
from mne.connectivity import seed_target_indices, spectral_connectivity

print(__doc__)

Read the data

First we'll read in the sample MEG data that we'll use for computing coherence between channels. We'll convert this into epochs in order to compute the event-related coherence.



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data_path = sample.data_path()
subjects_dir = data_path + '/subjects'
fname_inv = data_path + '/MEG/sample/sample_audvis-meg-oct-6-meg-inv.fif'
fname_raw = data_path + '/MEG/sample/sample_audvis_filt-0-40_raw.fif'
fname_event = data_path + '/MEG/sample/sample_audvis_filt-0-40_raw-eve.fif'
label_name_lh = 'Aud-lh'
fname_label_lh = data_path + '/MEG/sample/labels/%s.label' % label_name_lh

event_id, tmin, tmax = 1, -0.2, 0.5
method = "dSPM"  # use dSPM method (could also be MNE or sLORETA)

# Load data.
inverse_operator = read_inverse_operator(fname_inv)
label_lh = mne.read_label(fname_label_lh)
raw = mne.io.read_raw_fif(fname_raw)
events = mne.read_events(fname_event)

# Add a bad channel.
raw.info['bads'] += ['MEG 2443']

# pick MEG channels.
picks = mne.pick_types(raw.info, meg=True, eeg=False, stim=False, eog=True,
                       exclude='bads')

# Read epochs.
epochs = mne.Epochs(raw, events, event_id, tmin, tmax, picks=picks,
                    baseline=(None, 0),
                    reject=dict(mag=4e-12, grad=4000e-13, eog=150e-6))

Choose channels for coherence estimation

Next we'll calculate our channel sources. Then we'll find the most active vertex in the left auditory cortex, which we will later use as seed for the connectivity computation.



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snr = 3.0
lambda2 = 1.0 / snr ** 2
evoked = epochs.average()
stc = apply_inverse(evoked, inverse_operator, lambda2, method,
                    pick_ori="normal")

# Restrict the source estimate to the label in the left auditory cortex.
stc_label = stc.in_label(label_lh)

# Find number and index of vertex with most power.
src_pow = np.sum(stc_label.data ** 2, axis=1)
seed_vertno = stc_label.vertices[0][np.argmax(src_pow)]
seed_idx = np.searchsorted(stc.vertices[0], seed_vertno)  # index in orig stc

# Generate index parameter for seed-based connectivity analysis.
n_sources = stc.data.shape[0]
indices = seed_target_indices([seed_idx], np.arange(n_sources))

Compute the inverse solution for each epoch. By using "return_generator=True" stcs will be a generator object instead of a list. This allows us so to compute the coherence without having to keep all source estimates in memory.



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snr = 1.0  # use lower SNR for single epochs
lambda2 = 1.0 / snr ** 2
stcs = apply_inverse_epochs(epochs, inverse_operator, lambda2, method,
                            pick_ori="normal", return_generator=True)

Compute the coherence between sources

Now we are ready to compute the coherence in the alpha and beta band. fmin and fmax specify the lower and upper freq. for each band, respectively.

To speed things up, we use 2 parallel jobs and use mode='fourier', which uses a FFT with a Hanning window to compute the spectra (instead of a multitaper estimation, which has a lower variance but is slower). By using faverage=True, we directly average the coherence in the alpha and beta band, i.e., we will only get 2 frequency bins.



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fmin = (8., 13.)
fmax = (13., 30.)
sfreq = raw.info['sfreq']  # the sampling frequency

coh, freqs, times, n_epochs, n_tapers = spectral_connectivity(
    stcs, method='coh', mode='fourier', indices=indices,
    sfreq=sfreq, fmin=fmin, fmax=fmax, faverage=True, n_jobs=1)

print('Frequencies in Hz over which coherence was averaged for alpha: ')
print(freqs[0])
print('Frequencies in Hz over which coherence was averaged for beta: ')
print(freqs[1])

Generate coherence sources and plot

Finally, we'll generate a SourceEstimate with the coherence. This is simple since we used a single seed. For more than one seed we would have to choose one of the slices within coh.

Note

We use a hack to save the frequency axis as time.

Finally, we'll plot this source estimate on the brain.



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tmin = np.mean(freqs[0])
tstep = np.mean(freqs[1]) - tmin
coh_stc = mne.SourceEstimate(coh, vertices=stc.vertices, tmin=1e-3 * tmin,
                             tstep=1e-3 * tstep, subject='sample')

# Now we can visualize the coherence using the plot method.
brain = coh_stc.plot('sample', 'inflated', 'both',
                     time_label='Coherence %0.1f Hz',
                     subjects_dir=subjects_dir,
                     clim=dict(kind='value', lims=(0.25, 0.4, 0.65)))
brain.show_view('lateral')