In [ ]:
%matplotlib inline
This tutorial describes the :class:mne.Info
data structure, which keeps track
of various recording details, and is attached to :class:~mne.io.Raw
,
:class:~mne.Epochs
, and :class:~mne.Evoked
objects.
:depth: 2
We'll begin by loading the Python modules we need, and loading the same
example data <sample-dataset>
we used in the `introductory tutorial
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import os
import mne
sample_data_folder = mne.datasets.sample.data_path()
sample_data_raw_file = os.path.join(sample_data_folder, 'MEG', 'sample',
'sample_audvis_filt-0-40_raw.fif')
raw = mne.io.read_raw_fif(sample_data_raw_file)
As seen in the introductory tutorial <tut-overview>
, when a
:class:~mne.io.Raw
object is loaded, an :class:~mne.Info
object is
created automatically, and stored in the raw.info
attribute:
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print(raw.info)
However, it is not strictly necessary to load the :class:~mne.io.Raw
object
in order to view or edit the :class:~mne.Info
object; you can extract all
the relevant information into a stand-alone :class:~mne.Info
object using
:func:mne.io.read_info
:
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info = mne.io.read_info(sample_data_raw_file)
print(info)
As you can see, the :class:~mne.Info
object keeps track of a lot of
information about:
The complete list of fields is given in :class:`the API documentation
<mne.Info>`.
Querying the Info
object
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The fields in a :class:~mne.Info
object act like Python :class:`dictionary
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print(info.keys())
print() # insert a blank line
print(info['ch_names'])
Most of the fields contain :class:int
, :class:float
, or :class:list
data, but the chs
field bears special mention: it contains a list of
dictionaries (one :class:dict
per channel) containing everything there is
to know about a channel other than the data it recorded. Normally it is not
necessary to dig into the details of the chs
field — various MNE-Python
functions can extract the information more cleanly than iterating over the
list of dicts yourself — but it can be helpful to know what is in there. Here
we show the keys for the first channel's :class:dict
:
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print(info['chs'][0].keys())
Obtaining subsets of channels ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
It is often useful to convert between channel names and the integer indices
identifying rows of the data array where those channels' measurements are
stored. The :class:~mne.Info
object is useful for this task; two
convenience functions that rely on the :class:mne.Info
object for picking
channels are :func:mne.pick_channels
and :func:mne.pick_types
.
:func:~mne.pick_channels
minimally takes a list of all channel names and a
list of channel names to include; it is also possible to provide an empty
list to include
and specify which channels to exclude
instead:
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print(mne.pick_channels(info['ch_names'], include=['MEG 0312', 'EEG 005']))
print(mne.pick_channels(info['ch_names'], include=[],
exclude=['MEG 0312', 'EEG 005']))
:func:~mne.pick_types
works differently, since channel type cannot always
be reliably determined from channel name alone. Consequently,
:func:~mne.pick_types
needs an :class:~mne.Info
object instead of just a
list of channel names, and has boolean keyword arguments for each channel
type. Default behavior is to pick only MEG channels (and MEG reference
channels if present) and exclude any channels already marked as "bad" in the
bads
field of the :class:~mne.Info
object. Therefore, to get all and
only the EEG channel indices (including the "bad" EEG channels) we must
pass meg=False
and exclude=[]
:
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print(mne.pick_types(info, meg=False, eeg=True, exclude=[]))
Note that the meg
and fnirs
parameters of :func:~mne.pick_types
accept strings as well as boolean values, to allow selecting only
magnetometer or gradiometer channels (via meg='mag'
or meg='grad'
) or
to pick only oxyhemoglobin or deoxyhemoglobin channels (via fnirs='hbo'
or fnirs='hbr'
, respectively).
A third way to pick channels from an :class:~mne.Info
object is to apply
regular expression
_ matching to the channel names using
:func:mne.pick_channels_regexp
. Here the ^
represents the beginning of
the string and .
character matches any single character, so both EEG and
EOG channels will be selected:
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print(mne.pick_channels_regexp(info['ch_names'], '^E.G'))
:func:~mne.pick_channels_regexp
can be especially useful for channels named
according to the 10-20 <ten-twenty_>
_ system (e.g., to select all channels
ending in "z" to get the midline, or all channels beginning with "O" to get
the occipital channels). Note that :func:~mne.pick_channels_regexp
uses the
Python standard module :mod:re
to perform regular expression matching; see
the documentation of the :mod:re
module for implementation details.
Both :func:`~mne.pick_channels` and :func:`~mne.pick_channels_regexp` operate on lists of channel names, so they are unaware of which channels (if any) have been marked as "bad" in ``info['bads']``. Use caution to avoid accidentally selecting bad channels.
Obtaining channel type information ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Sometimes it can be useful to know channel type based on its index in the
data array. For this case, use :func:mne.channel_type
, which takes
an :class:~mne.Info
object and a single integer channel index:
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print(mne.channel_type(info, 25))
To obtain several channel types at once, you could embed
:func:~mne.channel_type
in a :term:list comprehension
:
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print([mne.channel_type(info, x) for x in (25, 76, 77, 319)])
Alternatively, you can get the indices of all channels of all channel types
present in the data, using :func:~mne.channel_indices_by_type
,
which returns a :class:dict
with channel types as keys, and lists of
channel indices as values:
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ch_idx_by_type = mne.channel_indices_by_type(info)
print(ch_idx_by_type.keys())
print(ch_idx_by_type['eog'])
Dropping channels from an Info
object
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
If you want to modify an :class:~mne.Info
object by eliminating some of the
channels in it, you can use the :func:mne.pick_info
function to pick the
channels you want to keep and omit the rest:
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print(info['nchan'])
eeg_indices = mne.pick_types(info, meg=False, eeg=True)
print(mne.pick_info(info, eeg_indices)['nchan'])
By default, :func:~mne.pick_info
will make a copy of the original
:class:~mne.Info
object before modifying it; if you want to modify it
in-place, include the parameter copy=False
.
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