Controlling the contrast of the background when plotting

The dim argument controls the contrast of the background.

dim modifies the contrast of this image: dim=0 leaves the image unchanged, negative values of dim enhance it, and positive values decrease it (dim the background).

This dim argument may also be useful for the plot_roi function used to display ROIs on top of a background image.

Retrieve the data: the localizer dataset with contrast maps

from nilearn import datasets

localizer_dataset = datasets.fetch_localizer_button_task()
# Contrast map of motor task
localizer_tmap_filename = localizer_dataset.tmap
# Subject specific anatomical image
localizer_anat_filename = localizer_dataset.anat

[get_dataset_dir] Dataset created in
/home/runner/nilearn_data/brainomics_localizer
[fetch_single_file] Downloading data from https://osf.io/hwbm2/download ...
[fetch_single_file]  ...done. (3 seconds, 0 min)

[fetch_single_file] Downloading data from
https://osf.io/download/5d27ddc91c5b4a001b9ef9d0/ ...
[fetch_single_file]  ...done. (4 seconds, 0 min)

[fetch_single_file] Downloading data from
https://osf.io/download/5d27d29145253a001c3e284f/ ...
[fetch_single_file]  ...done. (2 seconds, 0 min)

[fetch_single_file] Downloading data from
https://osf.io/download/5d275ede1c5b4a001aa00c26/ ...
[fetch_single_file]  ...done. (2 seconds, 0 min)

[fetch_single_file] Downloading data from
https://osf.io/download/5d27037f45253a001c3d4563/ ...
[fetch_single_file]  ...done. (1 seconds, 0 min)

[fetch_single_file] Downloading data from
https://osf.io/download/5d7b8948fcbf44001c44e695/ ...
[fetch_single_file]  ...done. (3 seconds, 0 min)

Plotting with enhancement of background image with dim=-.5

from nilearn.plotting import plot_stat_map, show

cut_coords = (36, -27, 66)

plot_stat_map(
    localizer_tmap_filename,
    bg_img=localizer_anat_filename,
    cut_coords=cut_coords,
    threshold=3,
    title="dim=-.5",
    dim=-0.5,
)

<nilearn.plotting.displays._slicers.OrthoSlicer object at 0x7f7050033880>

Plotting with no change of contrast in background image with dim=0

plot_stat_map(
    localizer_tmap_filename,
    bg_img=localizer_anat_filename,
    cut_coords=cut_coords,
    threshold=3,
    title="dim=0",
    dim=0,
)

<nilearn.plotting.displays._slicers.OrthoSlicer object at 0x7f704fdfc100>

Plotting with decrease of contrast in background image with dim=.5

plot_stat_map(
    localizer_tmap_filename,
    bg_img=localizer_anat_filename,
    cut_coords=cut_coords,
    threshold=3,
    title="dim=.5",
    dim=0.5,
)

<nilearn.plotting.displays._slicers.OrthoSlicer object at 0x7f704f9f56a0>

Plotting with more decrease in contrast with dim=1

plot_stat_map(
    localizer_tmap_filename,
    bg_img=localizer_anat_filename,
    cut_coords=cut_coords,
    threshold=3,
    title="dim=1",
    dim=1,
)

show()

Estimated memory usage: 107 MB