plot_stat_map(z_map, bg_img=mean_img, threshold=3.0,
display_mode='z', cut_coords=3, black_bg=True,
title='Active minus Rest (Z>3)')
plt.show()
###############################################################################
# Statistical significance testing. One should worry about the
# statistical validity of the procedure: here we used an arbitrary
# threshold of 3.0 but the threshold should provide some guarantees on
# the risk of false detections (aka type-1 errors in statistics).
# One suggestion is to control the false positive rate (fpr, denoted by
# alpha) at a certain level, e.g. 0.001: this means that there is 0.1% chance
# of declaring an inactive voxel, active.
from nilearn.stats import map_threshold
_, threshold = map_threshold(z_map, alpha=.001, height_control='fpr')
print('Uncorrected p<0.001 threshold: %.3f' % threshold)
plot_stat_map(z_map, bg_img=mean_img, threshold=threshold,
display_mode='z', cut_coords=3, black_bg=True,
title='Active minus Rest (p<0.001)')
plt.show()
###############################################################################
# The problem is that with this you expect 0.001 * n_voxels to show up
# while they're not active --- tens to hundreds of voxels. A more
# conservative solution is to control the family wise error rate,
# i.e. the probability of making only one false detection, say at
# 5%. For that we use the so-called Bonferroni correction.
_, threshold = map_threshold(z_map, alpha=.05, height_control='bonferroni')
print('Bonferroni-corrected, p<0.05 threshold: %.3f' % threshold)
from nilearn.stats.second_level_model import SecondLevelModel
second_level_model = SecondLevelModel(smoothing_fwhm=2.0, mask_img=mask_img)
second_level_model.fit(gray_matter_map_filenames, design_matrix=design_matrix)
##########################################################################
# Estimating the contrast is very simple. We can just provide the column
# name of the design matrix.
z_map = second_level_model.compute_contrast(second_level_contrast=[1, 0, 0],
output_type='z_score')
###########################################################################
# We threshold the second level contrast at uncorrected p < 0.001 and plot it.
from nilearn import plotting
from nilearn.stats import map_threshold
_, threshold = map_threshold(z_map, alpha=.05, height_control='fdr')
print('The FDR=.05-corrected threshold is: %.3g' % threshold)
display = plotting.plot_stat_map(
z_map,
threshold=threshold,
colorbar=True,
display_mode='z',
cut_coords=[-4, 26],
title='age effect on grey matter density (FDR = .05)')
plotting.show()
###########################################################################
# We can also study the effect of sex by computing the contrast, thresholding it
# and plot the resulting map.
def test_all_resolution_inference():
shape = (9, 10, 11)
p = np.prod(shape)
data = norm.isf(np.linspace(1. / p, 1. - 1. / p, p)).reshape(shape)
alpha = .001
data[2:4, 5:7, 6:8] = 5.
stat_img = nib.Nifti1Image(data, np.eye(4))
mask_img = nib.Nifti1Image(np.ones(shape), np.eye(4))
# test 1: standard case
th_map = cluster_level_inference(stat_img, threshold=3, alpha=.05)
vals = th_map.get_data()
assert np.sum(vals > 0) == 8
# test 2: high threshold
th_map = cluster_level_inference(stat_img, threshold=6, alpha=.05)
vals = th_map.get_data()
assert np.sum(vals > 0) == 0
# test 3: list of thresholds
th_map = cluster_level_inference(stat_img, threshold=[3, 6], alpha=.05)
vals = th_map.get_data()
assert np.sum(vals > 0) == 8
# test 4: one single voxel
data[3, 6, 7] = 10
stat_img_ = nib.Nifti1Image(data, np.eye(4))
th_map = cluster_level_inference(stat_img_, threshold=7, alpha=.05)
vals = th_map.get_data()
assert np.sum(vals > 0) == 1
# test 5: aberrant alpha
with pytest.raises(ValueError):
cluster_level_inference(stat_img, threshold=3, alpha=2)
with pytest.raises(ValueError):
cluster_level_inference(stat_img, threshold=3, alpha=-1)
# test 6 with mask_img
th_map = cluster_level_inference(stat_img,
mask_img=mask_img,
threshold=3,
alpha=.05)
vals = th_map.get_data()
assert np.sum(vals > 0) == 8
# test 7 verbose mode
th_map = cluster_level_inference(stat_img,
threshold=3,
alpha=.05,
verbose=True)
# test 9: one-sided test
th_map, z_th = map_threshold(stat_img,
mask_img,
alpha,
height_control='fpr',
cluster_threshold=10,
two_sided=False)
assert_equal(z_th, norm.isf(.001))
# test 10: two-side fdr threshold + bonferroni
data[0:2, 0:2, 6:8] = -5.
stat_img = nib.Nifti1Image(data, np.eye(4))
for control in ['fdr', 'bonferroni']:
th_map, _ = map_threshold(stat_img,
mask_img,
alpha=.05,
height_control=control,
cluster_threshold=5)
vals = get_data(th_map)
assert_equal(np.sum(vals > 0), 8)
assert_equal(np.sum(vals < 0), 8)
th_map, _ = map_threshold(stat_img,
mask_img,
alpha=.05,
height_control=control,
cluster_threshold=5,
two_sided=False)
vals = get_data(th_map)
assert_equal(np.sum(vals > 0), 8)
assert_equal(np.sum(vals < 0), 0)
def test_map_threshold():
shape = (9, 10, 11)
p = np.prod(shape)
data = norm.isf(np.linspace(1. / p, 1. - 1. / p, p)).reshape(shape)
alpha = .001
data[2:4, 5:7, 6:8] = 5.
stat_img = nib.Nifti1Image(data, np.eye(4))
mask_img = nib.Nifti1Image(np.ones(shape), np.eye(4))
# test 1
th_map, _ = map_threshold(stat_img,
mask_img,
alpha,
height_control='fpr',
cluster_threshold=0)
vals = get_data(th_map)
assert np.sum(vals > 0) == 8
# test 2: excessive cluster forming threshold
th_map, _ = map_threshold(stat_img,
mask_img,
threshold=100,
height_control=None,
cluster_threshold=0)
vals = get_data(th_map)
assert np.sum(vals > 0) == 0
# test 3: excessive size threshold
th_map, z_th = map_threshold(stat_img,
mask_img,
alpha,
height_control='fpr',
cluster_threshold=10)
vals = get_data(th_map)
assert np.sum(vals > 0) == 0
assert z_th == norm.isf(.0005)
# test 4: fdr threshold + bonferroni
for control in ['fdr', 'bonferroni']:
th_map, _ = map_threshold(stat_img,
mask_img,
alpha=.05,
height_control=control,
cluster_threshold=5)
vals = get_data(th_map)
assert np.sum(vals > 0) == 8
# test 5: direct threshold
th_map, _ = map_threshold(stat_img,
mask_img,
threshold=4.0,
height_control=None,
cluster_threshold=0)
vals = get_data(th_map)
assert np.sum(vals > 0) == 8
# test 6: without mask
th_map, _ = map_threshold(stat_img,
None,
threshold=4.0,
height_control=None,
cluster_threshold=0)
vals = get_data(th_map)
assert np.sum(vals > 0) == 8
# test 7 without a map
th_map, threshold = map_threshold(None,
None,
threshold=3.0,
height_control=None,
cluster_threshold=0)
assert threshold == 3.0
assert th_map == None # noqa:E711
th_map, threshold = map_threshold(None,
None,
alpha=0.05,
height_control='fpr',
cluster_threshold=0)
assert (threshold > 1.64)
assert th_map == None # noqa:E711
with pytest.raises(ValueError):
map_threshold(None, None, alpha=0.05, height_control='fdr')
with pytest.raises(ValueError):
map_threshold(None, None, alpha=0.05, height_control='bonferroni')
# test 8 wrong procedure
with pytest.raises(ValueError):
map_threshold(None, None, alpha=0.05, height_control='plop')
# Next, we specify and estimate the model.
from nilearn.stats.second_level_model import SecondLevelModel
second_level_model = SecondLevelModel().fit(cmap_filenames,
design_matrix=design_matrix)
#########################################################################
# Compute the only possible contrast: the one-sample test. Since there
# is only one possible contrast, we don't need to specify it in detail.
z_map = second_level_model.compute_contrast(output_type='z_score')
#########################################################################
# Threshold the resulting map:
# false positive rate < .001, cluster size > 10 voxels.
from nilearn.stats import map_threshold
thresholded_map1, threshold1 = map_threshold(z_map,
alpha=.001,
height_control='fpr',
cluster_threshold=10)
#########################################################################
# Now use FDR <.05 (False Discovery Rate) and no cluster-level threshold.
thresholded_map2, threshold2 = map_threshold(z_map,
alpha=.05,
height_control='fdr')
print('The FDR=.05 threshold is %.3g' % threshold2)
#########################################################################
# Now use FWER <.05 (Family-Wise Error Rate) and no cluster-level
# threshold. As the data has not been intensively smoothed, we can
# use a simple Bonferroni correction.
thresholded_map3, threshold3 = map_threshold(z_map,
alpha=.05,