Corrupt known signal with point spread¶

The aim of this tutorial is to demonstrate how to put a known signal at a desired location(s) in a mne.SourceEstimate and then corrupt the signal with point-spread by applying a forward and inverse solution.

import os.path as op

import numpy as np
from mayavi import mlab

import mne
from mne.datasets import sample

from mne.minimum_norm import read_inverse_operator, apply_inverse
from mne.simulation import simulate_stc, simulate_evoked

First, we set some parameters.

seed = 42

# parameters for inverse method
method = 'sLORETA'
snr = 3.
lambda2 = 1.0 / snr ** 2

# signal simulation parameters
# do not add extra noise to the known signals
nave = np.inf
T = 100
times = np.linspace(0, 1, T)
dt = times[1] - times[0]

# Paths to MEG data
data_path = sample.data_path()
subjects_dir = op.join(data_path, 'subjects')
fname_fwd = op.join(data_path, 'MEG', 'sample',
                    'sample_audvis-meg-oct-6-fwd.fif')
fname_inv = op.join(data_path, 'MEG', 'sample',
                    'sample_audvis-meg-oct-6-meg-fixed-inv.fif')

fname_evoked = op.join(data_path, 'MEG', 'sample',
                       'sample_audvis-ave.fif')

Load the MEG data¶

fwd = mne.read_forward_solution(fname_fwd)
fwd = mne.convert_forward_solution(fwd, force_fixed=True, surf_ori=True,
                                   use_cps=False)
fwd['info']['bads'] = []
inv_op = read_inverse_operator(fname_inv)

raw = mne.io.RawFIF(op.join(data_path, 'MEG', 'sample',
                            'sample_audvis_raw.fif'))
events = mne.find_events(raw)
event_id = {'Auditory/Left': 1, 'Auditory/Right': 2}
epochs = mne.Epochs(raw, events, event_id, baseline=(None, 0), preload=True)
epochs.info['bads'] = []
evoked = epochs.average()

labels = mne.read_labels_from_annot('sample', subjects_dir=subjects_dir)
label_names = [l.name for l in labels]
n_labels = len(labels)

Estimate the background noise covariance from the baseline period¶

cov = mne.compute_covariance(epochs, tmin=None, tmax=0.)

Generate sinusoids in two spatially distant labels¶

# The known signal is all zero-s off of the two labels of interest
signal = np.zeros((n_labels, T))
idx = label_names.index('inferiorparietal-lh')
signal[idx, :] = 1e-7 * np.sin(5 * 2 * np.pi * times)
idx = label_names.index('rostralmiddlefrontal-rh')
signal[idx, :] = 1e-7 * np.sin(7 * 2 * np.pi * times)

Find the center vertices in source space of each label¶

We want the known signal in each label to only be active at the center. We create a mask for each label that is 1 at the center vertex and 0 at all other vertices in the label. This mask is then used when simulating source-space data.

hemi_to_ind = {'lh': 0, 'rh': 1}
for i, label in enumerate(labels):
    # The `center_of_mass` function needs labels to have values.
    labels[i].values.fill(1.)

    # Restrict the eligible vertices to be those on the surface under
    # consideration and within the label.
    surf_vertices = fwd['src'][hemi_to_ind[label.hemi]]['vertno']
    restrict_verts = np.intersect1d(surf_vertices, label.vertices)
    com = labels[i].center_of_mass(subject='sample',
                                   subjects_dir=subjects_dir,
                                   restrict_vertices=restrict_verts,
                                   surf='white')

    # Convert the center of vertex index from surface vertex list to Label's
    # vertex list.
    cent_idx = np.where(label.vertices == com)[0][0]

    # Create a mask with 1 at center vertex and zeros elsewhere.
    labels[i].values.fill(0.)
    labels[i].values[cent_idx] = 1.

Create source-space data with known signals¶

Put known signals onto surface vertices using the array of signals and the label masks (stored in labels[i].values).

stc_gen = simulate_stc(fwd['src'], labels, signal, times[0], dt,
                       value_fun=lambda x: x)

Plot original signals¶

Note that the original signals are highly concentrated (point) sources.

kwargs = dict(subjects_dir=subjects_dir, hemi='split', smoothing_steps=4,
              time_unit='s', initial_time=0.05, size=1200,
              views=['lat', 'med'])
clim = dict(kind='value', pos_lims=[1e-9, 1e-8, 1e-7])
figs = [mlab.figure(1), mlab.figure(2), mlab.figure(3), mlab.figure(4)]
brain_gen = stc_gen.plot(clim=clim, figure=figs, **kwargs)

Simulate sensor-space signals¶

Use the forward solution and add Gaussian noise to simulate sensor-space (evoked) data from the known source-space signals. The amount of noise is controlled by nave (higher values imply less noise).

evoked_gen = simulate_evoked(fwd, stc_gen, evoked.info, cov, nave,
                             random_state=seed)

# Map the simulated sensor-space data to source-space using the inverse
# operator.
stc_inv = apply_inverse(evoked_gen, inv_op, lambda2, method=method)

Plot the point-spread of corrupted signal¶

Notice that after applying the forward- and inverse-operators to the known point sources that the point sources have spread across the source-space. This spread is due to the minimum norm solution so that the signal leaks to nearby vertices with similar orientations so that signal ends up crossing the sulci and gyri.

figs = [mlab.figure(5), mlab.figure(6), mlab.figure(7), mlab.figure(8)]
brain_inv = stc_inv.plot(figure=figs, **kwargs)

Exercises¶

Change the method parameter to either dSPM or MNE to explore the effect of the inverse method.

Try setting evoked_snr to a small, finite value, e.g. 3., to see the effect of noise.

Total running time of the script: ( 0 minutes 58.022 seconds)

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