Surface-based dataset first and second level analysis of a dataset#
Full step-by-step example of fitting a GLM (first and second level analysis) in a 10-subjects dataset and visualizing the results.
1. Download an fMRI BIDS dataset with two language conditions to contrast. 2. Project the data to a standard mesh, fsaverage5, aka the Freesurfer template mesh downsampled to about 10k nodes per hemisphere. 3. Run the first level model objects. 4. Fit a second level model on the fitted first level models.
Fetch example BIDS dataset#
We download a simplified BIDS dataset made available for illustrative purposes. It contains only the necessary information to run a statistical analysis using Nilearn. The raw data subject folders only contain bold.json and events.tsv files, while the derivatives folder includes the preprocessed files preproc.nii and the confounds.tsv files.
Here is the location of the dataset on disk.
Obtain automatically FirstLevelModel objects and fit arguments#
From the dataset directory we automatically obtain the FirstLevelModel objects with their subject_id filled from the BIDS dataset. Moreover, we obtain for each model a dictionary with run_imgs, events and confounder regressors since in this case a confounds.tsv file is available in the BIDS dataset. To get the first level models we only have to specify the dataset directory and the task_label as specified in the file names.
/home/remi/github/nilearn/env/lib/python3.11/site-packages/nilearn/interfaces/bids/query.py:49: UserWarning: 'StartTime' not found in file /home/remi/nilearn_data/fMRI-language-localizer-demo-dataset/derivatives/sub-01/func/sub-01_task-languagelocalizer_desc-preproc_bold.json.
We also need to get the TR information. For that we use the json sidecar file of the dataset’s functional images.
Project fMRI data to the surface: First get fsaverage5.
The projection function simply takes the fMRI data and the mesh. Note that those correspond spatially, as they are both in MNI space.
Empty lists in which we are going to store activation values.
z_scores_right =  z_scores_left =  for (fmri_img, confound, events) in zip( models_run_imgs, models_confounds, models_events): texture = surface.vol_to_surf(fmri_img, fsaverage.pial_right) n_scans = texture.shape frame_times = t_r * (np.arange(n_scans) + .5) # Create the design matrix # # We specify an hrf model containing Glover model and its time derivative. # The drift model is implicitly a cosine basis with period cutoff 128s. design_matrix = make_first_level_design_matrix( frame_times, events=events, hrf_model='glover + derivative', add_regs=confound) # Contrast specification contrast_values = (design_matrix.columns == 'language') * 1.0 -\ (design_matrix.columns == 'string') # Setup and fit GLM. # Note that the output consists in 2 variables: `labels` and `fit` # `labels` tags voxels according to noise autocorrelation. # `estimates` contains the parameter estimates. # We input them for contrast computation. labels, estimates = run_glm(texture.T, design_matrix.values) contrast = compute_contrast(labels, estimates, contrast_values, contrast_type='t') # We present the Z-transform of the t map. z_score = contrast.z_score() z_scores_right.append(z_score) # Do the left hemisphere exactly the same way. texture = surface.vol_to_surf(fmri_img, fsaverage.pial_left) labels, estimates = run_glm(texture.T, design_matrix.values) contrast = compute_contrast(labels, estimates, contrast_values, contrast_type='t') z_scores_left.append(contrast.z_score())
Individual activation maps have been accumulated in the z_score_left and az_scores_right lists respectively. We can now use them in a group study (one-sample study).
Prepare figure for concurrent plot of individual maps compute population-level maps for left and right hemisphere We directly do that on the value arrays.
What we have so far are p-values: we convert them to z-values for plotting.
Plot the resulting maps, at first on the left hemisphere.
<Figure size 470x400 with 2 Axes>
Next, on the right hemisphere.
Total running time of the script: (1 minutes 51.952 seconds)
Estimated memory usage: 456 MB