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"""
===================================================
Demonstrate impact of whitening on source estimates
===================================================
This example demonstrates the relationship between the noise covariance
estimate and the MNE / dSPM source amplitudes. It computes source estimates for
the SPM faces data and compares proper regularization with insufficient
regularization based on the methods described in [1]_. The example demonstrates
that improper regularization can lead to overestimation of source amplitudes.
This example makes use of the previous, non-optimized code path that was used
before implementing the suggestions presented in [1]_.
This example does quite a bit of processing, so even on a
fast machine it can take a couple of minutes to complete.
.. warning:: Please do not copy the patterns presented here for your own
analysis, this is example is purely illustrative.
References
----------
.. [1] Engemann D. and Gramfort A. (2015) Automated model selection in
covariance estimation and spatial whitening of MEG and EEG signals,
vol. 108, 328-342, NeuroImage.
"""
# Author: Denis A. Engemann <denis.engemann@gmail.com>
#
# License: BSD (3-clause)
import numpy as np
import matplotlib.pyplot as plt
import mne
from mne import io
from mne.datasets import spm_face
from mne.minimum_norm import apply_inverse, make_inverse_operator
from mne.cov import compute_covariance
print(__doc__)
##############################################################################
# Get data
data_path = spm_face.data_path()
subjects_dir = data_path + '/subjects'
raw_fname = data_path + '/MEG/spm/SPM_CTF_MEG_example_faces%d_3D.ds'
raw = io.read_raw_ctf(raw_fname % 1) # Take first run
# To save time and memory for this demo, we'll just use the first
# 2.5 minutes (all we need to get 30 total events) and heavily
# resample 480->60 Hz (usually you wouldn't do either of these!)
raw = raw.crop(0, 150.).load_data()
picks = mne.pick_types(raw.info, meg=True, exclude='bads')
raw.filter(None, 20.)
events = mne.find_events(raw, stim_channel='UPPT001')
event_ids = {"faces": 1, "scrambled": 2}
tmin, tmax = -0.2, 0.5
baseline = (None, 0)
reject = dict(mag=3e-12)
# Make forward
trans = data_path + '/MEG/spm/SPM_CTF_MEG_example_faces1_3D_raw-trans.fif'
src = data_path + '/subjects/spm/bem/spm-oct-6-src.fif'
bem = data_path + '/subjects/spm/bem/spm-5120-5120-5120-bem-sol.fif'
forward = mne.make_forward_solution(raw.info, trans, src, bem)
del src
# inverse parameters
conditions = 'faces', 'scrambled'
snr = 3.0
lambda2 = 1.0 / snr ** 2
clim = dict(kind='value', lims=[0, 2.5, 5])
###############################################################################
# Estimate covariances
samples_epochs = 5, 15,
method = 'empirical', 'shrunk'
colors = 'steelblue', 'red'
evokeds = list()
stcs = list()
methods_ordered = list()
for n_train in samples_epochs:
# estimate covs based on a subset of samples
# make sure we have the same number of conditions.
events_ = np.concatenate([events[events[:, 2] == id_][:n_train]
for id_ in [event_ids[k] for k in conditions]])
events_ = events_[np.argsort(events_[:, 0])]
epochs_train = mne.Epochs(raw, events_, event_ids, tmin, tmax, picks=picks,
baseline=baseline, preload=True, reject=reject,
decim=8)
epochs_train.equalize_event_counts(event_ids)
assert len(epochs_train) == 2 * n_train
# We know some of these have too few samples, so suppress warning
# with verbose='error'
noise_covs = compute_covariance(
epochs_train, method=method, tmin=None, tmax=0, # baseline only
return_estimators=True, rank=None, verbose='error') # returns list
# prepare contrast
evokeds = [epochs_train[k].average() for k in conditions]
del epochs_train, events_
# do contrast
# We skip empirical rank estimation that we introduced in response to
# the findings in reference [1] to use the naive code path that
# triggered the behavior described in [1]. The expected true rank is
# 274 for this dataset. Please do not do this with your data but
# rely on the default rank estimator that helps regularizing the
# covariance.
stcs.append(list())
methods_ordered.append(list())
for cov in noise_covs:
inverse_operator = make_inverse_operator(evokeds[0].info, forward,
cov, loose=0.2, depth=0.8,
rank=274)
stc_a, stc_b = (apply_inverse(e, inverse_operator, lambda2, "dSPM",
pick_ori=None) for e in evokeds)
stc = stc_a - stc_b
methods_ordered[-1].append(cov['method'])
stcs[-1].append(stc)
del inverse_operator, evokeds, cov, noise_covs, stc, stc_a, stc_b
del raw, forward # save some memory
##############################################################################
# Show the resulting source estimates
fig, (axes1, axes2) = plt.subplots(2, 3, figsize=(9.5, 5))
for ni, (n_train, axes) in enumerate(zip(samples_epochs, (axes1, axes2))):
# compute stc based on worst and best
ax_dynamics = axes[1]
for stc, ax, method, kind, color in zip(stcs[ni],
axes[::2],
methods_ordered[ni],
['best', 'worst'],
colors):
brain = stc.plot(subjects_dir=subjects_dir, hemi='both', clim=clim,
initial_time=0.175, background='w', foreground='k')
brain.show_view('ven')
im = brain.screenshot()
brain.close()
ax.axis('off')
ax.get_xaxis().set_visible(False)
ax.get_yaxis().set_visible(False)
ax.imshow(im)
ax.set_title('{0} ({1} epochs)'.format(kind, n_train * 2))
# plot spatial mean
stc_mean = stc.data.mean(0)
ax_dynamics.plot(stc.times * 1e3, stc_mean,
label='{0} ({1})'.format(method, kind),
color=color)
# plot spatial std
stc_var = stc.data.std(0)
ax_dynamics.fill_between(stc.times * 1e3, stc_mean - stc_var,
stc_mean + stc_var, alpha=0.2, color=color)
# signal dynamics worst and best
ax_dynamics.set(title='{0} epochs'.format(n_train * 2),
xlabel='Time (ms)', ylabel='Source Activation (dSPM)',
xlim=(tmin * 1e3, tmax * 1e3), ylim=(-3, 3))
ax_dynamics.legend(loc='upper left', fontsize=10)
fig.subplots_adjust(hspace=0.2, left=0.01, right=0.99, wspace=0.03)
|
