"""
==================================================
Compute a cross-spectral density (CSD) matrix
==================================================
A cross-spectral density (CSD) matrix is similar to a covariance matrix, but in
the time-frequency domain. It is the first step towards computing
sensor-to-sensor coherence or a DICS beamformer.
This script demonstrates the three methods that MNE-Python provides to compute
the CSD:
1. Using short-term Fourier transform: :func:`mne.time_frequency.csd_fourier`
2. Using a multitaper approach: :func:`mne.time_frequency.csd_multitaper`
3. Using Morlet wavelets: :func:`mne.time_frequency.csd_morlet`
"""
# Author: Marijn van Vliet
# License: BSD (3-clause)
from matplotlib import pyplot as plt
import mne
from mne.datasets import sample
from mne.time_frequency import csd_fourier, csd_multitaper, csd_morlet
print(__doc__)
###############################################################################
# In the following example, the computation of the CSD matrices can be
# performed using multiple cores. Set ``n_jobs`` to a value >1 to select the
# number of cores to use.
n_jobs = 1
###############################################################################
# Loading the sample dataset.
data_path = sample.data_path()
fname_raw = data_path + '/MEG/sample/sample_audvis_raw.fif'
fname_event = data_path + '/MEG/sample/sample_audvis_raw-eve.fif'
raw = mne.io.read_raw_fif(fname_raw)
events = mne.read_events(fname_event)
###############################################################################
# By default, CSD matrices are computed using all MEG/EEG channels. When
# interpreting a CSD matrix with mixed sensor types, be aware that the
# measurement units, and thus the scalings, differ across sensors. In this
# example, for speed and clarity, we select a single channel type:
# gradiometers.
picks = mne.pick_types(raw.info, meg='grad')
# Make some epochs, based on events with trigger code 1
epochs = mne.Epochs(raw, events, event_id=1, tmin=0, tmax=1,
picks=picks, baseline=(None, 0),
reject=dict(grad=4000e-13), preload=True)
###############################################################################
# Computing CSD matrices using short-term Fourier transform and (adaptive)
# multitapers is straightforward:
csd_fft = csd_fourier(epochs, fmin=15, fmax=20, n_jobs=n_jobs)
csd_mt = csd_multitaper(epochs, fmin=15, fmax=20, adaptive=True, n_jobs=n_jobs)
###############################################################################
# When computing the CSD with Morlet wavelets, you specify the exact
# frequencies at which to compute it. For each frequency, a corresponding
# wavelet will be constructed and convolved with the signal, resulting in a
# time-frequency decomposition.
#
# The CSD is constructed by computing the correlation between the
# time-frequency representations between all sensor-to-sensor pairs. The
# time-frequency decomposition originally has the same sampling rate as the
# signal, in our case ~600Hz. This means the decomposition is over-specified in
# time and we may not need to use all samples during our CSD computation, just
# enough to get a reliable correlation statistic. By specifying ``decim=10``,
# we use every 10th sample, which will greatly speed up the computation and
# will have a minimal effect on the CSD.
frequencies = [16, 17, 18, 19, 20]
csd_wav = csd_morlet(epochs, frequencies, decim=10, n_jobs=n_jobs)
###############################################################################
# The resulting :class:`mne.time_frequency.CrossSpectralDensity` objects have a
# plotting function we can use to compare the results of the different methods.
# We're plotting the mean CSD across frequencies.
csd_fft.mean().plot()
plt.suptitle('short-term Fourier transform')
csd_mt.mean().plot()
plt.suptitle('adaptive multitapers')
csd_wav.mean().plot()
plt.suptitle('Morlet wavelet transform')