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"""
=========================================================================
Non-parametric between conditions cluster statistic on single trial power
=========================================================================
This script shows how to compare clusters in time-frequency
power estimates between conditions. It uses a non-parametric
statistical procedure based on permutations and cluster
level statistics.
The procedure consists in:
- extracting epochs for 2 conditions
- compute single trial power estimates
- baseline line correct the power estimates (power ratios)
- compute stats to see if the power estimates are significantly different
between conditions.
"""
# Authors: Alexandre Gramfort <alexandre.gramfort@telecom-paristech.fr>
#
# License: BSD (3-clause)
import numpy as np
import matplotlib.pyplot as plt
import mne
from mne.time_frequency import tfr_morlet
from mne.stats import permutation_cluster_test
from mne.datasets import sample
print(__doc__)
###############################################################################
# Set parameters
data_path = sample.data_path()
raw_fname = data_path + '/MEG/sample/sample_audvis_raw.fif'
event_fname = data_path + '/MEG/sample/sample_audvis_raw-eve.fif'
tmin, tmax = -0.2, 0.5
# Setup for reading the raw data
raw = mne.io.read_raw_fif(raw_fname)
events = mne.read_events(event_fname)
include = []
raw.info['bads'] += ['MEG 2443', 'EEG 053'] # bads + 2 more
# picks MEG gradiometers
picks = mne.pick_types(raw.info, meg='grad', eeg=False, eog=True,
stim=False, include=include, exclude='bads')
ch_name = 'MEG 1332' # restrict example to one channel
# Load condition 1
reject = dict(grad=4000e-13, eog=150e-6)
event_id = 1
epochs_condition_1 = mne.Epochs(raw, events, event_id, tmin, tmax,
picks=picks, baseline=(None, 0),
reject=reject, preload=True)
epochs_condition_1.pick_channels([ch_name])
# Load condition 2
event_id = 2
epochs_condition_2 = mne.Epochs(raw, events, event_id, tmin, tmax,
picks=picks, baseline=(None, 0),
reject=reject, preload=True)
epochs_condition_2.pick_channels([ch_name])
###############################################################################
# Factor to downsample the temporal dimension of the TFR computed by
# tfr_morlet. Decimation occurs after frequency decomposition and can
# be used to reduce memory usage (and possibly comptuational time of downstream
# operations such as nonparametric statistics) if you don't need high
# spectrotemporal resolution.
decim = 2
freqs = np.arange(7, 30, 3) # define frequencies of interest
n_cycles = 1.5
tfr_epochs_1 = tfr_morlet(epochs_condition_1, freqs,
n_cycles=n_cycles, decim=decim,
return_itc=False, average=False)
tfr_epochs_2 = tfr_morlet(epochs_condition_2, freqs,
n_cycles=n_cycles, decim=decim,
return_itc=False, average=False)
tfr_epochs_1.apply_baseline(mode='ratio', baseline=(None, 0))
tfr_epochs_2.apply_baseline(mode='ratio', baseline=(None, 0))
epochs_power_1 = tfr_epochs_1.data[:, 0, :, :] # only 1 channel as 3D matrix
epochs_power_2 = tfr_epochs_2.data[:, 0, :, :] # only 1 channel as 3D matrix
###############################################################################
# Compute statistic
# -----------------
threshold = 6.0
T_obs, clusters, cluster_p_values, H0 = \
permutation_cluster_test([epochs_power_1, epochs_power_2],
n_permutations=100, threshold=threshold, tail=0)
###############################################################################
# View time-frequency plots
# -------------------------
times = 1e3 * epochs_condition_1.times # change unit to ms
evoked_condition_1 = epochs_condition_1.average()
evoked_condition_2 = epochs_condition_2.average()
plt.figure()
plt.subplots_adjust(0.12, 0.08, 0.96, 0.94, 0.2, 0.43)
plt.subplot(2, 1, 1)
# Create new stats image with only significant clusters
T_obs_plot = np.nan * np.ones_like(T_obs)
for c, p_val in zip(clusters, cluster_p_values):
if p_val <= 0.05:
T_obs_plot[c] = T_obs[c]
plt.imshow(T_obs,
extent=[times[0], times[-1], freqs[0], freqs[-1]],
aspect='auto', origin='lower', cmap='gray')
plt.imshow(T_obs_plot,
extent=[times[0], times[-1], freqs[0], freqs[-1]],
aspect='auto', origin='lower', cmap='RdBu_r')
plt.xlabel('Time (ms)')
plt.ylabel('Frequency (Hz)')
plt.title('Induced power (%s)' % ch_name)
ax2 = plt.subplot(2, 1, 2)
evoked_contrast = mne.combine_evoked([evoked_condition_1, evoked_condition_2],
weights=[1, -1])
evoked_contrast.plot(axes=ax2, time_unit='s')
plt.show()
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