modalities.fmri.glm

Module: modalities.fmri.glm

Inheritance diagram for nipy.modalities.fmri.glm:

Inheritance diagram of nipy.modalities.fmri.glm

This module presents an interface to use the glm implemented in nipy.algorithms.statistics.models.regression.

It contains the GLM and contrast classes that are meant to be the main objects of fMRI data analyses.

It is important to note that the GLM is meant as a one-session General Linear Model. But inference can be performed on multiple sessions by computing fixed effects on contrasts

Examples

>>> import numpy as np
>>> from nipy.modalities.fmri.glm import GeneralLinearModel
>>> n, p, q = 100, 80, 10
>>> X, Y = np.random.randn(p, q), np.random.randn(p, n)
>>> cval = np.hstack((1, np.zeros(9)))
>>> model = GeneralLinearModel(X)
>>> model.fit(Y)
>>> z_vals = model.contrast(cval).z_score() # z-transformed statistics

Example of fixed effects statistics across two contrasts

>>> cval_ = cval.copy()
>>> np.random.shuffle(cval_)
>>> z_ffx = (model.contrast(cval) + model.contrast(cval_)).z_score()

Classes

Contrast

class nipy.modalities.fmri.glm.Contrast(effect, variance, dof=10000000000.0, contrast_type='t', tiny=1e-50, dofmax=10000000000.0)

Bases: object

The contrast class handles the estimation of statistical contrasts on a given model: student (t), Fisher (F), conjunction (tmin-conjunction). The important feature is that it supports addition, thus opening the possibility of fixed-effects models.

The current implementation is meant to be simple, and could be enhanced in the future on the computational side (high-dimensional F constrasts may lead to memory breakage).

Notes

The ‘tmin-conjunction’ test is the valid conjunction test discussed in: Nichols T, Brett M, Andersson J, Wager T, Poline JB. Valid conjunction inference with the minimum statistic. Neuroimage. 2005 Apr 15;25(3):653-60. This test gives the p-value of the z-values under the conjunction null, i.e. the union of the null hypotheses for all terms.

__init__(effect, variance, dof=10000000000.0, contrast_type='t', tiny=1e-50, dofmax=10000000000.0)
Parameters:

effect: array of shape (contrast_dim, n_voxels)

the effects related to the contrast

variance: array of shape (contrast_dim, contrast_dim, n_voxels)

the associated variance estimate

dof: scalar, the degrees of freedom

contrast_type: string to be chosen among ‘t’ and ‘F’

p_value(baseline=0.0)

Return a parametric estimate of the p-value associated with the null hypothesis: (H0) ‘contrast equals baseline’

Parameters:

baseline: float, optional

Baseline value for the test statistic

Notes

The value of 0.5 is used where the stat is not defined

stat(baseline=0.0)

Return the decision statistic associated with the test of the null hypothesis: (H0) ‘contrast equals baseline’

Parameters:

baseline: float, optional,

Baseline value for the test statistic

z_score(baseline=0.0)

Return a parametric estimation of the z-score associated with the null hypothesis: (H0) ‘contrast equals baseline’

Parameters:

baseline: float, optional

Baseline value for the test statistic

Notes

The value of 0 is used where the stat is not defined

FMRILinearModel

class nipy.modalities.fmri.glm.FMRILinearModel(fmri_data, design_matrices, mask='compute', m=0.2, M=0.9, threshold=0.5)

Bases: object

This class is meant to handle GLMs from a higher-level perspective i.e. by taking images as input and output

__init__(fmri_data, design_matrices, mask='compute', m=0.2, M=0.9, threshold=0.5)

Load the data

Parameters:

fmri_data : Image or str or sequence of Images / str

fmri images / paths of the (4D) fmri images

design_matrices : arrays or str or sequence of arrays / str

design matrix arrays / paths of .npz files

mask : str or Image or None, optional

string can be ‘compute’ or a path to an image image is an input (assumed binary) mask image(s), if ‘compute’, the mask is computed if None, no masking will be applied

m, M, threshold: float, optional

parameters of the masking procedure. Should be within [0, 1]

Notes

The only computation done here is mask computation (if required)

Examples

We need the example data package for this example:

from nipy.utils import example_data
from nipy.modalities.fmri.glm import FMRILinearModel
fmri_files = [example_data.get_filename('fiac', 'fiac0', run)
    for run in ['run1.nii.gz', 'run2.nii.gz']]
design_files = [example_data.get_filename('fiac', 'fiac0', run)
    for run in ['run1_design.npz', 'run2_design.npz']]
mask = example_data.get_filename('fiac', 'fiac0', 'mask.nii.gz')
multi_session_model = FMRILinearModel(fmri_files,
                                      design_files,
                                      mask)
multi_session_model.fit()
z_image, = multi_session_model.contrast([np.eye(13)[1]] * 2)

# The number of voxels with p < 0.001 given by ...
print(np.sum(z_image.get_data() > 3.09))
contrast(contrasts, con_id='', contrast_type=None, output_z=True, output_stat=False, output_effects=False, output_variance=False)

Estimation of a contrast as fixed effects on all sessions

Parameters:

contrasts : array or list of arrays of shape (n_col) or (n_dim, n_col)

where n_col is the number of columns of the design matrix, numerical definition of the contrast (one array per run)

con_id : str, optional

name of the contrast

contrast_type : {‘t’, ‘F’, ‘tmin-conjunction’}, optional

type of the contrast

output_z : bool, optional

Return or not the corresponding z-stat image

output_stat : bool, optional

Return or not the base (t/F) stat image

output_effects : bool, optional

Return or not the corresponding effect image

output_variance : bool, optional

Return or not the corresponding variance image

Returns:

output_images : list of nibabel images

The required output images, in the following order: z image, stat(t/F) image, effects image, variance image

fit(do_scaling=True, model='ar1', steps=100)

Load the data, mask the data, scale the data, fit the GLM

Parameters:

do_scaling : bool, optional

if True, the data should be scaled as percent of voxel mean

model : string, optional,

the kind of glm (‘ols’ or ‘ar1’) you want to fit to the data

steps : int, optional

in case of an ar1, discretization of the ar1 parameter

GeneralLinearModel

class nipy.modalities.fmri.glm.GeneralLinearModel(X)

Bases: object

This class handles the so-called on General Linear Model

Most of what it does in the fit() and contrast() methods fit() performs the standard two-step (‘ols’ then ‘ar1’) GLM fitting contrast() returns a contrast instance, yileding statistics and p-values. The link between fit() and constrast is done vis the two class members:

glm_results : dictionary of nipy.algorithms.statistics.models.
regression.RegressionResults instances, describing results of a GLM fit
labels : array of shape(n_voxels),
labels that associate each voxel with a results key
__init__(X)
Parameters:

X : array of shape (n_time_points, n_regressors)

the design matrix

contrast(con_val, contrast_type=None)

Specify and estimate a linear contrast

Parameters:

con_val : numpy.ndarray of shape (p) or (q, p)

where q = number of contrast vectors and p = number of regressors

contrast_type : {None, ‘t’, ‘F’ or ‘tmin-conjunction’}, optional

type of the contrast. If None, then defaults to ‘t’ for 1D con_val and ‘F’ for 2D con_val

Returns:

con: Contrast instance

fit(Y, model='ols', steps=100)

GLM fitting of a dataset using ‘ols’ regression or the two-pass

Parameters:

Y : array of shape(n_time_points, n_samples)

the fMRI data

model : {‘ar1’, ‘ols’}, optional

the temporal variance model. Defaults to ‘ols’

steps : int, optional

Maximum number of discrete steps for the AR(1) coef histogram

get_beta(column_index=None)

Accessor for the best linear unbiased estimated of model parameters

Parameters:

column_index: int or array-like of int or None, optional

The indexed of the columns to be returned. if None (default behaviour), the whole vector is returned

Returns:

beta: array of shape (n_voxels, n_columns)

the beta

get_logL()

Accessor for the log-likelihood of the model

Returns:

logL: array of shape (n_voxels,)

the sum of square error per voxel

get_mse()

Accessor for the mean squared error of the model

Returns:

mse: array of shape (n_voxels)

the sum of square error per voxel

Function

nipy.modalities.fmri.glm.data_scaling(Y)

Scaling of the data to have percent of baseline change columnwise

Parameters:

Y: array of shape(n_time_points, n_voxels)

the input data

Returns:

Y: array of shape (n_time_points, n_voxels),

the data after mean-scaling, de-meaning and multiplication by 100

mean : array of shape (n_voxels,)

the data mean