3.1. Extracting times series to build a functional connectome¶
A functional connectome is a set of connections representing brain interactions between regions. Here we show how to extract activation time-series to compute functional connectomes.
188.8.131.52. Brain parcellations¶
Regions used to extract the signal can be defined by a “hard”
parcellation. For instance, the
nilearn.datasets has functions to
download atlases forming reference parcellation, e.g.,
For instance to retrieve the Harvard-Oxford cortical parcellation, sampled at 2mm, and with a threshold of a probability of 0.25:
from nilearn import datasets dataset = datasets.fetch_atlas_harvard_oxford('cort-maxprob-thr25-2mm') atlas_filename = dataset.maps labels = dataset.labels
Plotting can then be done as:
from nilearn import plotting plotting.plot_roi(atlas_filename)
184.108.40.206. Extracting signals on a parcellation¶
To extract signal on the parcellation, the easiest option is to use the
NiftiLabelsMasker. As any “maskers” in
nilearn, it is a processing object that is created by specifying all
the important parameters, but not the data:
from nilearn.maskers import NiftiLabelsMasker masker = NiftiLabelsMasker(labels_img=atlas_filename, standardize=True)
time_series = masker.fit_transform(frmi_files, confounds=confounds_dataframe)
Note that confound signals can be specified in the call. Indeed, to
obtain time series that capture well the functional interactions between
regions, regressing out noise sources is very important
[Varoquaux & Craddock 2013].
For data processed by fMRIPrep,
load_confounds_strategy can help you
retrieve confound variables.
load_confounds_strategy selects confounds
based on past literature with limited parameters for customisation.
For more freedoms of confounds selection,
load_confounds groups confound variables as
sets of noise components and one can fine tune each of the parameters.
See the following example for a full file running the analysis: Extracting signals from a brain parcellation.
Exercise: computing the correlation matrix of rest fmri
Try using the information above to compute the correlation matrix of
the first subject of the brain development dataset
Inspect the ‘.keys()’ of the object returned by
load_confoundsto get a set of confounds of your choice. (Note: CompCor and ICA-AROMA related options are not applicable to the brain development dataset).
load_confounds_strategyto get a set of confounds. (Note: only
scrubbingare applicable to the brain development dataset).
nilearn.connectome.ConnectivityMeasurecan be used to compute a correlation matrix (check the shape of your matrices).
matplotlib.pyplot.imshowcan show a correlation matrix.
The example above has the solution.
220.127.116.11. Probabilistic atlases¶
The definition of regions as by a continuous probability map captures
better our imperfect knowledge of boundaries in brain images (notably
because of inter-subject registration errors). One example of such an
atlas well suited to resting-state or naturalistic-stimuli data analysis is
the MSDL atlas
Probabilistic atlases are represented as a set of continuous maps, in a
4D nifti image. Visualization the atlas thus requires to visualize each
of these maps, which requires accessing them with
nilearn.image.index_img (see the corresponding example).
18.104.22.168. Extracting signals from a probabilistic atlas¶
As with extraction of signals on a parcellation, extracting signals from
a probabilistic atlas can be done with a “masker” object: the
NiftiMapsMasker. It is created by
specifying the important parameters, in particular the atlas:
from nilearn.maskers import NiftiMapsMasker masker = NiftiMapsMasker(maps_img=atlas_filename, standardize=True)
The fit_transform method turns filenames or NiftiImage objects to time series:
time_series = masker.fit_transform(frmi_files, confounds=csv_file)
A full example of extracting signals on a probabilistic: Extracting signals of a probabilistic atlas of functional regions.
Exercise: correlation matrix of rest fMRI on probabilistic atlas
Try to compute the correlation matrix of the first subject of the
dataset downloaded with
with the MSDL atlas downloaded via
Hint: The example above has the solution.
A square matrix, such as a correlation matrix, can also be seen as a “graph”: a set of “nodes”, connected by “edges”. When these nodes are brain regions, and the edges capture interactions between them, this graph is a “functional connectome”.
We can display it with the
function that take the matrix, and coordinates of the nodes in MNI space.
In the case of the MSDL atlas
nilearn.datasets.fetch_atlas_msdl), the CSV file readily comes
with MNI coordinates for each region (see for instance example:
Extracting signals of a probabilistic atlas of functional regions).
As you can see, the correlation matrix gives a very “full” graph: every node is connected to every other one. This is because it also captures indirect connections. In the next section we will see how to focus on direct connections only.
For atlases without readily available label coordinates, center coordinates can be computed for each region on hard parcellation or probabilistic atlases.
For hard parcellation atlases (eg.
nilearn.datasets.fetch_atlas_destrieux_2009), use the
nilearn.plotting.find_parcellation_cut_coordsfunction. See example: Comparing connectomes on different reference atlases
For probabilistic atlases (eg.
nilearn.datasets.fetch_atlas_msdl), use the
nilearn.plotting.find_probabilistic_atlas_cut_coordsfunction. See example: Group Sparse inverse covariance for multi-subject connectome:>>> from nilearn import plotting >>> atlas_region_coords = plotting.find_probabilistic_atlas_cut_coords(atlas_filename)