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18 changes: 9 additions & 9 deletions tutorials/preprocessing/80_opm_processing.py
Original file line number Diff line number Diff line change
Expand Up @@ -77,18 +77,18 @@
# The simplest method for reducing low frequency drift in the data is to
# use a set of reference sensors away from the scalp, which only sample the
# ambient fields in the room. An advantage of this method is that no prior
# knowldge of the locations of the sensors is required. However, it assumes
# knowledge of the locations of the sensors is required. However, it assumes
# that the reference sensors experience the same interference as scalp
# recordings.
#
# To do this in our current dataset, we require a bit of housekeeping.
# There are a set of channels beginning with the name "Flux" which do not
# contain any evironmental data, these need to be set to as bad channels.
# contain any environmental data, these need to be set to as bad channels.
# Another channel -- G2-17-TAN -- will also be set to bad.
#
# For now we are only interested in removing artefacts seen below 5 Hz, so we
# initially low-pass filter the good reference channels in this dataset prior
# to regression
# to regression.
#
# Looking at the processed data, we see there has been a large reduction in the
# low frequency drift, but there are still periods where the drift has not been
Expand Down Expand Up @@ -128,14 +128,14 @@
# ------------------------------------------------------
#
# Regression of a reference channel is a start, but in this instance assumes
# the relatiship between the references and a given sensor on the head as
# that the relationship between the references and a given sensor on the head is
# constant. However this becomes less accurate when the reference is not moving
# but the subject is. An alternative method, Homogeneous Field Correction (HFC)
# only requires that the sensors on the helmet stationary relative to each
# only requires that the sensors on the helmet are stationary relative to each
# other. Which in a well-designed rigid helmet is the case.


# include gradients by setting order to 2, set to 1 for homgenous components
# include gradients by setting order to 2, set to 1 for homogeneous components
projs = mne.preprocessing.compute_proj_hfc(raw.info, order=2)
raw.add_proj(projs).apply_proj(verbose="error")

Expand All @@ -160,7 +160,7 @@
# denoising step is to calculate the power spectrum of the dataset before and
# after processing. We will use metric called the shielding factor to summarise
# the values. Positive shielding factors indicate a reduction in power, whilst
# negative means in increase.
# negative means an increase.
#
# We see that reference regression does a good job in reducing low frequency
# drift up to ~2 Hz, with 20 dB of shielding. But rapidly drops off due to
Expand Down Expand Up @@ -202,9 +202,9 @@
# --------------------------
#
# Having regressed much of the high-amplitude, low-frequency interference, we
# can now look to filtering the remnant nuisance signals. The motivation for
# can now look at filtering the remnant nuisance signals. The motivation for
# filtering after regression (rather than before) is to minimise any filter
# artefacts generated when removing such high-amplitude interfece (compared
# artefacts generated when removing such high-amplitude interference (compared
# to the neural signals we are interested in).
#
# We are going to remove the 50 Hz mains signal with a notch filter,
Expand Down
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