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1551 lines (1225 loc) · 62.2 KB
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# Copyright 2017 Intel Corporation
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""Intersubject correlation (ISC) analysis
Functions for computing intersubject correlation (ISC) and related
analyses (e.g., intersubject funtional correlations; ISFC), as well
as statistical tests designed specifically for ISC analyses.
The implementation is based on the work in [Hasson2004]_, [Kauppi2014]_,
[Simony2016]_, [Chen2016]_, and [Nastase2019]_.
.. [Chen2016] "Untangling the relatedness among correlations, part I:
nonparametric approaches to inter-subject correlation analysis at the
group level.", G. Chen, Y. W. Shin, P. A. Taylor, D. R. Glen, R. C.
Reynolds, R. B. Israel, R. W. Cox, 2016, NeuroImage, 142, 248-259.
https://doi.org/10.1016/j.neuroimage.2016.05.023
.. [Hasson2004] "Intersubject synchronization of cortical activity
during natural vision.", U. Hasson, Y. Nir, I. Levy, G. Fuhrmann,
R. Malach, 2004, Science, 303, 1634-1640.
https://doi.org/10.1126/science.1089506
.. [Kauppi2014] "A versatile software package for inter-subject
correlation based analyses of fMRI.", J. P. Kauppi, J. Pajula,
J. Tohka, 2014, Frontiers in Neuroinformatics, 8, 2.
https://doi.org/10.3389/fninf.2014.00002
.. [Simony2016] "Dynamic reconfiguration of the default mode network
during narrative comprehension.", E. Simony, C. J. Honey, J. Chen, O.
Lositsky, Y. Yeshurun, A. Wiesel, U. Hasson, 2016, Nature Communications,
7, 12141. https://doi.org/10.1038/ncomms12141
.. [Nastase2019] "Measuring shared responses across subjects using
intersubject correlation." S. A. Nastase, V. Gazzola, U. Hasson,
C. Keysers, 2019, Social Cognitive and Affective Neuroscience, 14,
667-685. https://doi.org/10.1093/scan/nsz037
"""
# Authors: Sam Nastase, Christopher Baldassano, Qihong Lu,
# Mai Nguyen, and Mor Regev
# Princeton University, 2018
import math
import numpy as np
import logging
from scipy.spatial.distance import squareform
from itertools import combinations, permutations, product
from brainiak.fcma.util import compute_correlation
from brainiak.utils.utils import (
array_correlation, phase_randomize, p_from_null, _check_timeseries_input)
logger = logging.getLogger(__name__)
__all__ = [
"bootstrap_isc",
"compute_summary_statistic",
"isfc",
"isc",
"permutation_isc",
"phaseshift_isc",
"squareform_isfc",
"timeshift_isc",
]
MAX_RANDOM_SEED = 2**32 - 1
def isc(data, pairwise=False, summary_statistic=None, tolerate_nans=True):
"""Intersubject correlation
For each voxel or ROI, compute the Pearson correlation between each
subject's response time series and other subjects' response time series.
If pairwise is False (default), use the leave-one-out approach, where
correlation is computed between each subject and the average of the other
subjects. If pairwise is True, compute correlations between all pairs of
subjects. If summary_statistic is None, return N ISC values for N subjects
(leave-one-out) or N(N-1)/2 ISC values for each pair of N subjects,
corresponding to the upper triangle of the pairwise correlation matrix
(see scipy.spatial.distance.squareform). Alternatively, use either
'mean' or 'median' to compute summary statistic of ISCs (Fisher Z will
be applied if using mean). Input data should be a n_TRs by n_voxels by
n_subjects array (e.g., brainiak.image.MaskedMultiSubjectData) or a list
where each item is a n_TRs by n_voxels ndarray for a given subject.
Multiple input ndarrays must be the same shape. If a 2D array is supplied,
the last dimension is assumed to correspond to subjects. If only two
subjects are supplied, simply compute Pearson correlation (precludes
averaging in leave-one-out approach, and does not apply summary statistic).
When using leave-one-out approach, NaNs are ignored when computing mean
time series of N-1 subjects (default: tolerate_nans=True). Alternatively,
you may supply a float between 0 and 1 indicating a threshold proportion
of N subjects with non-NaN values required when computing the average time
series for a given voxel. For example, if tolerate_nans=.8, ISCs will be
computed for any voxel where >= 80% of subjects have non-NaN values,
while voxels with < 80% non-NaN values will be assigned NaNs. If set to
False, NaNs are not tolerated and voxels with one or more NaNs among the
N-1 subjects will be assigned NaN. Setting tolerate_nans to True or False
will not affect the pairwise approach; however, if a threshold float is
provided, voxels that do not reach this threshold will be excluded. Note
that accommodating NaNs may be notably slower than setting tolerate_nans to
False. Output is an ndarray where the first dimension is the number of
subjects or pairs and the second dimension is the number of voxels (or
ROIs). If only two subjects are supplied or a summary statistic is invoked,
the output is a ndarray n_voxels long.
The implementation is based on the work in [Hasson2004]_.
Parameters
----------
data : list or ndarray (n_TRs x n_voxels x n_subjects)
fMRI data for which to compute ISC
pairwise : bool, default: False
Whether to use pairwise (True) or leave-one-out (False) approach
summary_statistic : None or str, default: None
Return all ISCs or collapse using 'mean' or 'median'
tolerate_nans : bool or float, default: True
Accommodate NaNs (when averaging in leave-one-out approach)
Returns
-------
iscs : subjects or pairs by voxels ndarray
ISC for each subject or pair (or summary statistic) per voxel
"""
# Check response time series input format
data, n_TRs, n_voxels, n_subjects = _check_timeseries_input(data)
# No summary statistic if only two subjects
if n_subjects == 2:
logger.info("Only two subjects! Simply computing Pearson correlation.")
summary_statistic = None
# Check tolerate_nans input and use either mean/nanmean and exclude voxels
if tolerate_nans:
mean = np.nanmean
else:
mean = np.mean
data, mask = _threshold_nans(data, tolerate_nans)
# Compute correlation for only two participants
if n_subjects == 2:
# Compute correlation for each corresponding voxel
iscs_stack = array_correlation(data[..., 0],
data[..., 1])[np.newaxis, :]
# Compute pairwise ISCs using voxel loop and corrcoef for speed
elif pairwise:
# Swap axes for np.corrcoef
data = np.swapaxes(data, 2, 0)
# Loop through voxels
voxel_iscs = []
for v in np.arange(data.shape[1]):
voxel_data = data[:, v, :]
# Correlation matrix for all pairs of subjects (triangle)
iscs = squareform(np.corrcoef(voxel_data), checks=False)
voxel_iscs.append(iscs)
iscs_stack = np.column_stack(voxel_iscs)
# Compute leave-one-out ISCs
elif not pairwise:
# Loop through left-out subjects
iscs_stack = []
for s in np.arange(n_subjects):
# Correlation between left-out subject and mean of others
iscs_stack.append(array_correlation(
data[..., s],
mean(np.delete(data, s, axis=2), axis=2)))
iscs_stack = np.array(iscs_stack)
# Get ISCs back into correct shape after masking out NaNs
iscs = np.full((iscs_stack.shape[0], n_voxels), np.nan)
iscs[:, np.where(mask)[0]] = iscs_stack
# Summarize results (if requested)
if summary_statistic:
iscs = compute_summary_statistic(iscs,
summary_statistic=summary_statistic,
axis=0)[np.newaxis, :]
# Throw away first dimension if singleton
if iscs.shape[0] == 1:
iscs = iscs[0]
return iscs
def isfc(data, targets=None, pairwise=False, summary_statistic=None,
vectorize_isfcs=True, tolerate_nans=True):
"""Intersubject functional correlation (ISFC)
For each input voxel or ROI, compute the Pearson correlation between each
subject's response time series and all input voxels or ROIs in other
subjects. If a targets array is provided, instead compute ISFCs between
each input voxel time series and each voxel time series in targets across
subjects (resulting in asymmetric ISFC values). The targets array must have
the same number TRs and subjects as the input data. If pairwise is False
(default), use the leave-one-out approach, where correlation is computed
between each subject and the average of the other subjects. If pairwise is
True, compute correlations between all pairs of subjects. If a targets
array is provided, only the leave-one-out approach is supported. If
summary_statistic is None, return N ISFC values for N subjects (leave-one-
out) or N(N-1)/2 ISFC values for each pair of N subjects, corresponding to
the triangle of the correlation matrix (scipy.spatial.distance.squareform).
Alternatively, use either 'mean' or 'median' to compute summary statistic
of ISFCs (Fisher Z is applied if using mean). Input should be n_TRs by
n_voxels by n_subjects array (e.g., brainiak.image.MaskedMultiSubjectData)
or a list where each item is a n_TRs by n_voxels ndarray per subject.
Multiple input ndarrays must be the same shape. If a 2D array is supplied,
the last dimension is assumed to correspond to subjects. If only two
subjects are supplied, simply compute ISFC between these two subjects
(precludes averaging in leave-one-out approach, and does not apply summary
statistic). Returns vectorized upper triangle of ISFC matrices for each
subject or pair when vectorized_isfcs=True, or full (redundant) 2D ISFC
matrices when vectorized_isfcs=False. When using leave-one-out approach,
NaNs are ignored when computing mean time series of N-1 subjects (default:
tolerate_nans=True). Alternatively, you may supply a float between 0 and
1 indicating a threshold proportion of N subjects with non-NaN values
required when computing the average time series for a given voxel. For
example, if tolerate_nans=.8, ISCs will be computed for any voxel where
>= 80% of subjects have non-NaN values, while voxels with < 80% non-NaN
values will be assigned NaNs. If set to False, NaNs are not tolerated
and voxels with one or more NaNs among the N-1 subjects will be assigned
NaN. Setting tolerate_nans to True or False will not affect the pairwise
approach; however, if a threshold float is provided, voxels that do not
reach this threshold will be excluded. Note that accommodating NaNs may
be notably slower than setting tolerate_nans to False. Output is either
a tuple comprising condensed off-diagonal ISFC values and the diagonal
ISC values if vectorize_isfcs=True, or a single ndarray with shape
n_subjects (or n_pairs) by n_voxels by n_voxels 3D array if
vectorize_isfcs=False (see brainiak.isc.squareform_isfc). If targets array
is provided (yielding asymmetric ISFCs), output ISFCs are not vectorized,
resulting in an n_subjects by n_voxels by n_targets ISFC array. If
summary_statistic is supplied, output is collapsed along first dimension.
The implementation is based on the work in [Simony2016]_.
Parameters
----------
data : list or ndarray (n_TRs x n_voxels x n_subjects)
fMRI data for which to compute ISFC
targets : list or ndarray (n_TRs x n_voxels x n_subjects), optional
fMRI data to use as targets for ISFC
pairwise : bool, default: False
Whether to use pairwise (True) or leave-one-out (False) approach
summary_statistic : None or str, default: None
Return all ISFCs or collapse using 'mean' or 'median'
vectorize_isfcs : bool, default: True
Return tuple of condensed ISFCs and ISCs (True) or square (redundant)
ISFCs (False)
tolerate_nans : bool or float, default: True
Accommodate NaNs (when averaging in leave-one-out approach)
Returns
-------
isfcs : ndarray or tuple of ndarrays
ISFCs for each subject or pair (or summary statistic) per voxel pair
"""
# Check response time series input format
data, n_TRs, n_voxels, n_subjects = _check_timeseries_input(data)
# Check for optional targets input array
targets, t_n_TRs, t_n_voxels, t_n_subejcts, symmetric = (
_check_targets_input(targets, data))
if not symmetric:
pairwise = False
# Check tolerate_nans input and use either mean/nanmean and exclude voxels
if tolerate_nans:
mean = np.nanmean
else:
mean = np.mean
data, mask = _threshold_nans(data, tolerate_nans)
targets, targets_mask = _threshold_nans(targets, tolerate_nans)
# Handle just two subjects properly (for symmetric approach)
if symmetric and n_subjects == 2:
isfcs = compute_correlation(np.ascontiguousarray(data[..., 0].T),
np.ascontiguousarray(data[..., 1].T),
return_nans=True)
isfcs = (isfcs + isfcs.T) / 2
isfcs = isfcs[..., np.newaxis]
summary_statistic = None
logger.info("Only two subjects! Computing ISFC between them.")
# Compute all pairwise ISFCs (only for symmetric approach)
elif pairwise:
isfcs = []
for pair in combinations(np.arange(n_subjects), 2):
isfc_pair = compute_correlation(np.ascontiguousarray(
data[..., pair[0]].T),
np.ascontiguousarray(
targets[..., pair[1]].T),
return_nans=True)
if symmetric:
isfc_pair = (isfc_pair + isfc_pair.T) / 2
isfcs.append(isfc_pair)
isfcs = np.dstack(isfcs)
# Compute ISFCs using leave-one-out approach
else:
# Roll subject axis for loop
data = np.rollaxis(data, 2, 0)
targets = np.rollaxis(targets, 2, 0)
# Compute leave-one-out ISFCs
isfcs = [compute_correlation(np.ascontiguousarray(subject.T),
np.ascontiguousarray(mean(
np.delete(targets, s, axis=0),
axis=0).T),
return_nans=True)
for s, subject in enumerate(data)]
# Transpose and average ISFC matrices for both directions
isfcs = np.dstack([(isfc_matrix + isfc_matrix.T) / 2 if
symmetric else isfc_matrix for
isfc_matrix in isfcs])
# Get ISCs back into correct shape after masking out NaNs
isfcs_all = np.full((n_voxels, t_n_voxels, isfcs.shape[2]), np.nan)
isfcs_all[np.ix_(np.where(mask)[0], np.where(targets_mask)[0])] = isfcs
isfcs = np.moveaxis(isfcs_all, 2, 0)
# Summarize results (if requested)
if summary_statistic:
isfcs = compute_summary_statistic(isfcs,
summary_statistic=summary_statistic,
axis=0)
# Throw away first dimension if singleton
if isfcs.shape[0] == 1:
isfcs = isfcs[0]
# Optionally squareform to vectorize ISFC matrices (only if symmetric)
if vectorize_isfcs and symmetric:
isfcs, iscs = squareform_isfc(isfcs)
return isfcs, iscs
else:
return isfcs
def _check_isc_input(iscs, pairwise=False):
"""Checks ISC inputs for statistical tests
Input ISCs should be n_subjects (leave-one-out approach) or
n_pairs (pairwise approach) by n_voxels or n_ROIs array or a 1D
array (or list) of ISC values for a single voxel or ROI. This
function is only intended to be used internally by other
functions in this module (e.g., bootstrap_isc, permutation_isc).
Parameters
----------
iscs : ndarray or list
ISC values
Returns
-------
iscs : ndarray
Array of ISC values
n_subjects : int
Number of subjects
n_voxels : int
Number of voxels (or ROIs)
"""
# Standardize structure of input data
if type(iscs) == list:
iscs = np.array(iscs)[:, np.newaxis]
elif isinstance(iscs, np.ndarray):
if iscs.ndim == 1:
iscs = iscs[:, np.newaxis]
# Check if incoming pairwise matrix is vectorized triangle
if pairwise:
try:
test_square = squareform(iscs[:, 0], force='tomatrix')
n_subjects = test_square.shape[0]
except ValueError:
raise ValueError("For pairwise input, ISCs must be the "
"vectorized triangle of a square matrix.")
elif not pairwise:
n_subjects = iscs.shape[0]
# Infer subjects, voxels and print for user to check
n_voxels = iscs.shape[1]
logger.info("Assuming {0} subjects with and {1} "
"voxel(s) or ROI(s) in bootstrap ISC test.".format(n_subjects,
n_voxels))
return iscs, n_subjects, n_voxels
def _check_targets_input(targets, data):
"""Checks ISFC targets input array
For ISFC analysis, targets input array should either be a list
of n_TRs by n_targets arrays (where each array corresponds to
a subject), or an n_TRs by n_targets by n_subjects ndarray. This
function also checks the shape of the targets array against the
input data array.
Parameters
----------
data : list or ndarray (n_TRs x n_voxels x n_subjects)
fMRI data for which to compute ISFC
targets : list or ndarray (n_TRs x n_voxels x n_subjects)
fMRI data to use as targets for ISFC
Returns
-------
targets : ndarray (n_TRs x n_voxels x n_subjects)
ISFC targets with standadized structure
n_TRs : int
Number of time points (TRs) for targets array
n_voxels : int
Number of voxels (or ROIs) for targets array
n_subjects : int
Number of subjects for targets array
symmetric : bool
Indicator for symmetric vs. asymmetric
"""
if isinstance(targets, np.ndarray) or isinstance(targets, list):
targets, n_TRs, n_voxels, n_subjects = (
_check_timeseries_input(targets))
if data.shape[0] != n_TRs:
raise ValueError("Targets array must have same number of "
"TRs as input data")
if data.shape[2] != n_subjects:
raise ValueError("Targets array must have same number of "
"subjects as input data")
symmetric = False
else:
targets = data
n_TRs, n_voxels, n_subjects = data.shape
symmetric = True
return targets, n_TRs, n_voxels, n_subjects, symmetric
def compute_summary_statistic(iscs, summary_statistic='mean', axis=None):
"""Computes summary statistics for ISCs
Computes either the 'mean' or 'median' across a set of ISCs. In the
case of the mean, ISC values are first Fisher Z transformed (arctanh),
averaged, then inverse Fisher Z transformed (tanh).
The implementation is based on the work in [SilverDunlap1987]_.
.. [SilverDunlap1987] "Averaging correlation coefficients: should
Fisher's z transformation be used?", N. C. Silver, W. P. Dunlap, 1987,
Journal of Applied Psychology, 72, 146-148.
https://doi.org/10.1037/0021-9010.72.1.146
Parameters
----------
iscs : list or ndarray
ISC values
summary_statistic : str, default: 'mean'
Summary statistic, 'mean' or 'median'
axis : None or int or tuple of ints, optional
Axis or axes along which the means are computed. The default is to
compute the mean of the flattened array.
Returns
-------
statistic : float or ndarray
Summary statistic of ISC values
"""
if summary_statistic not in ('mean', 'median'):
raise ValueError("Summary statistic must be 'mean' or 'median'")
# Compute summary statistic
if summary_statistic == 'mean':
statistic = np.tanh(np.nanmean(np.arctanh(iscs), axis=axis))
elif summary_statistic == 'median':
statistic = np.nanmedian(iscs, axis=axis)
return statistic
def squareform_isfc(isfcs, iscs=None):
"""Converts square ISFCs to condensed ISFCs (and ISCs), and vice-versa
If input is a 2- or 3-dimensional array of square ISFC matrices, converts
this to the condensed off-diagonal ISFC values (i.e., the vectorized
triangle) and the diagonal ISC values. In this case, input must be a
single array of shape either n_voxels x n_voxels or n_subjects (or
n_pairs) x n_voxels x n_voxels. The condensed ISFC values are vectorized
according to scipy.spatial.distance.squareform, yielding n_voxels *
(n_voxels - 1) / 2 values comprising every voxel pair. Alternatively, if
input is an array of condensed off-diagonal ISFC values and an array of
diagonal ISC values, the square (redundant) ISFC values are returned.
This function mimics scipy.spatial.distance.squareform, but is intended
to retain the diagonal ISC values.
Parameters
----------
isfcs : ndarray
Either condensed or redundant ISFC values
iscs: ndarray, optional
Diagonal ISC values, required when input is condensed
Returns
-------
isfcs : ndarray or tuple of ndarrays
If condensed ISFCs are passed, a single redundant ISFC array is
returned; if redundant ISFCs are passed, both a condensed off-
diagonal ISFC array and the diagonal ISC values are returned
"""
# Check if incoming ISFCs are square (redundant)
if not type(iscs) == np.ndarray and isfcs.shape[-2] == isfcs.shape[-1]:
if isfcs.ndim == 2:
isfcs = isfcs[np.newaxis, ...]
if isfcs.ndim == 3:
iscs = np.diagonal(isfcs, axis1=1, axis2=2)
isfcs = np.vstack([squareform(isfc, checks=False)[np.newaxis, :]
for isfc in isfcs])
else:
raise ValueError("Square (redundant) ISFCs must be square "
"with multiple subjects or pairs of subjects "
"indexed by the first dimension")
if isfcs.shape[0] == iscs.shape[0] == 1:
isfcs, iscs = isfcs[0], iscs[0]
return isfcs, iscs
# Otherwise, convert from condensed to redundant
else:
if isfcs.ndim == iscs.ndim == 1:
isfcs, iscs = isfcs[np.newaxis, :], iscs[np.newaxis, :]
isfcs_stack = []
for isfc, isc in zip(isfcs, iscs):
isfc_sq = squareform(isfc, checks=False)
np.fill_diagonal(isfc_sq, isc)
isfcs_stack.append(isfc_sq[np.newaxis, ...])
isfcs = np.vstack(isfcs_stack)
if isfcs.shape[0] == 1:
isfcs = isfcs[0]
return isfcs
def _threshold_nans(data, tolerate_nans):
"""Thresholds data based on proportion of subjects with NaNs
Takes in data and a threshold value (float between 0.0 and 1.0) determining
the permissible proportion of subjects with non-NaN values. For example, if
threshold=.8, any voxel where >= 80% of subjects have non-NaN values will
be left unchanged, while any voxel with < 80% non-NaN values will be
assigned all NaN values and included in the nan_mask output. Note that the
output data has not been masked and will be same shape as the input data,
but may have a different number of NaNs based on the threshold.
Parameters
----------
data : ndarray (n_TRs x n_voxels x n_subjects)
fMRI time series data
tolerate_nans : bool or float (0.0 <= threshold <= 1.0)
Proportion of subjects with non-NaN values required to keep voxel
Returns
-------
data : ndarray (n_TRs x n_voxels x n_subjects)
fMRI time series data with adjusted NaNs
nan_mask : ndarray (n_voxels,)
Boolean mask array of voxels with too many NaNs based on threshold
"""
nans = np.all(np.any(np.isnan(data), axis=0), axis=1)
# Check tolerate_nans input and use either mean/nanmean and exclude voxels
if tolerate_nans is True:
logger.info("ISC computation will tolerate all NaNs when averaging")
elif type(tolerate_nans) is float:
if not 0.0 <= tolerate_nans <= 1.0:
raise ValueError("If threshold to tolerate NaNs is a float, "
"it must be between 0.0 and 1.0; got {0}".format(
tolerate_nans))
nans += ~(np.sum(~np.any(np.isnan(data), axis=0), axis=1) >=
data.shape[-1] * tolerate_nans)
logger.info("ISC computation will tolerate voxels with at least "
"{0} non-NaN values: {1} voxels do not meet "
"threshold".format(tolerate_nans,
np.sum(nans)))
else:
logger.info("ISC computation will not tolerate NaNs when averaging")
mask = ~nans
data = data[:, mask, :]
return data, mask
def bootstrap_isc(iscs, pairwise=False, summary_statistic='median',
n_bootstraps=1000, ci_percentile=95, side='right',
random_state=None):
"""One-sample group-level bootstrap hypothesis test for ISCs
For ISCs from one more voxels or ROIs, resample subjects with replacement
to construct a bootstrap distribution. Input is a list or ndarray of
ISCs for a single voxel/ROI, or an ISCs-by-voxels ndarray. ISC values
should be either N ISC values for N subjects in the leave-one-out appraoch
(pairwise=False), N(N-1)/2 ISC values for N subjects in the pairwise
approach (pairwise=True). In the pairwise approach, ISC values should
correspond to the vectorized upper triangle of a square correlation matrix
(see scipy.stats.distance.squareform). Shifts bootstrap distribution by
actual summary statistic (effectively to zero) for null hypothesis test
(Hall & Wilson, 1991). Uses subject-wise (not pair-wise) resampling in the
pairwise approach. Returns the observed ISC, the confidence interval, and
a p-value for the bootstrap hypothesis test, as well as the bootstrap
distribution of summary statistics. The p-value corresponds to either a
'two-sided', 'left'-, or 'right'-sided (default) test, as specified by
side. According to Chen et al., 2016, this is the preferred nonparametric
approach for controlling false positive rates (FPRs) for one-sample tests
in the pairwise approach. Note that the bootstrap hypothesis test may not
strictly control FPRs in the leave-one-out approach.
The implementation is based on the work in [Chen2016]_ and
[HallWilson1991]_.
.. [HallWilson1991] "Two guidelines for bootstrap hypothesis testing.",
P. Hall, S. R., Wilson, 1991, Biometrics, 757-762.
https://doi.org/10.2307/2532163
Parameters
----------
iscs : list or ndarray, ISCs by voxels array
ISC values for one or more voxels
pairwise : bool, default: False
Indicator of pairwise or leave-one-out, should match ISCs structure
summary_statistic : str, default: 'median'
Summary statistic, either 'median' (default) or 'mean'
n_bootstraps : int, default: 1000
Number of bootstrap samples (subject-level with replacement)
ci_percentile : int, default: 95
Percentile for computing confidence intervals
side : str
Perform one-sided ('left' or 'right') or 'two-sided' test
random_state = int or None, default: None
Initial random seed
Returns
-------
observed : float, median (or mean) ISC value
Summary statistic for actual ISCs
ci : tuple, bootstrap confidence intervals
Confidence intervals generated from bootstrap distribution
p : float, p-value
p-value based on bootstrap hypothesis test
distribution : ndarray, n_bootstraps by voxels
Bootstrap distribution
"""
# Standardize structure of input data
iscs, n_subjects, n_voxels = _check_isc_input(iscs, pairwise=pairwise)
# Check for valid summary statistic
if summary_statistic not in ('mean', 'median'):
raise ValueError("Summary statistic must be 'mean' or 'median'")
# Compute summary statistic for observed ISCs
observed = compute_summary_statistic(iscs,
summary_statistic=summary_statistic,
axis=0)
# Set up an empty list to build our bootstrap distribution
distribution = []
# Loop through n bootstrap iterations and populate distribution
for i in np.arange(n_bootstraps):
# Random seed to be deterministically re-randomized at each iteration
if isinstance(random_state, np.random.RandomState):
prng = random_state
else:
prng = np.random.RandomState(random_state)
# Randomly sample subject IDs with replacement
subject_sample = sorted(prng.choice(np.arange(n_subjects),
size=n_subjects))
# Squareform and shuffle rows/columns of pairwise ISC matrix to
# to retain correlation structure among ISCs, then get triangle
if pairwise:
# Loop through voxels
isc_sample = []
for voxel_iscs in iscs.T:
# Square the triangle and fill diagonal
voxel_iscs = squareform(voxel_iscs, force='tomatrix')
np.fill_diagonal(voxel_iscs, 1)
# Shuffle square correlation matrix and get triangle
voxel_sample = voxel_iscs[subject_sample, :][:, subject_sample]
voxel_sample = squareform(voxel_sample, checks=False)
# Censor off-diagonal 1s for same-subject pairs
voxel_sample[voxel_sample == 1.] = np.nan
isc_sample.append(voxel_sample)
isc_sample = np.column_stack(isc_sample)
# Get simple bootstrap sample if not pairwise
elif not pairwise:
isc_sample = iscs[subject_sample, :]
# Compute summary statistic for bootstrap ISCs per voxel
# (alternatively could construct distribution for all voxels
# then compute statistics, but larger memory footprint)
distribution.append(compute_summary_statistic(
isc_sample,
summary_statistic=summary_statistic,
axis=0))
# Update random state for next iteration
random_state = np.random.RandomState(prng.randint(0, MAX_RANDOM_SEED,
dtype=np.int64))
# Convert distribution to numpy array
distribution = np.array(distribution)
# Compute CIs of median from bootstrap distribution (default: 95%)
ci = (np.percentile(distribution, (100 - ci_percentile)/2, axis=0),
np.percentile(distribution, ci_percentile + (100 - ci_percentile)/2,
axis=0))
# Shift bootstrap distribution to 0 for hypothesis test
shifted = distribution - observed
# Get p-value for actual median from shifted distribution
p = p_from_null(observed, shifted,
side=side, exact=False,
axis=0)
return observed, ci, p, distribution
def _check_group_assignment(group_assignment, n_subjects):
if type(group_assignment) == list:
pass
elif type(group_assignment) == np.ndarray:
group_assignment = group_assignment.tolist()
else:
logger.info("No group assignment provided, "
"performing one-sample test.")
if group_assignment and len(group_assignment) != n_subjects:
raise ValueError("Group assignments ({0}) "
"do not match number of subjects ({1})!".format(
len(group_assignment), n_subjects))
return group_assignment
def _get_group_parameters(group_assignment, n_subjects, pairwise=False):
# Set up dictionary to contain group info
group_parameters = {'group_assignment': group_assignment,
'n_subjects': n_subjects,
'group_labels': None, 'groups': None,
'sorter': None, 'unsorter': None,
'group_matrix': None, 'group_selector': None}
# Set up group selectors for two-group scenario
if group_assignment and len(np.unique(group_assignment)) == 2:
group_parameters['n_groups'] = 2
# Get group labels and counts
group_labels = np.unique(group_assignment)
groups = {group_labels[0]: group_assignment.count(group_labels[0]),
group_labels[1]: group_assignment.count(group_labels[1])}
# For two-sample pairwise approach set up selector from matrix
if pairwise:
# Sort the group_assignment variable if it came in shuffled
# so it's easier to build group assignment matrix
sorter = np.array(group_assignment).argsort()
unsorter = np.array(group_assignment).argsort().argsort()
# Populate a matrix with group assignments
upper_left = np.full((groups[group_labels[0]],
groups[group_labels[0]]),
group_labels[0])
upper_right = np.full((groups[group_labels[0]],
groups[group_labels[1]]),
np.nan)
lower_left = np.full((groups[group_labels[1]],
groups[group_labels[0]]),
np.nan)
lower_right = np.full((groups[group_labels[1]],
groups[group_labels[1]]),
group_labels[1])
group_matrix = np.vstack((np.hstack((upper_left, upper_right)),
np.hstack((lower_left, lower_right))))
np.fill_diagonal(group_matrix, np.nan)
group_parameters['group_matrix'] = group_matrix
# Unsort matrix and squareform to create selector
group_parameters['group_selector'] = squareform(
group_matrix[unsorter, :][:, unsorter],
checks=False)
group_parameters['sorter'] = sorter
group_parameters['unsorter'] = unsorter
# If leave-one-out approach, just user group assignment as selector
else:
group_parameters['group_selector'] = group_assignment
# Save these parameters for later
group_parameters['groups'] = groups
group_parameters['group_labels'] = group_labels
# Manage one-sample and incorrect group assignments
elif not group_assignment or len(np.unique(group_assignment)) == 1:
group_parameters['n_groups'] = 1
# If pairwise initialize matrix of ones for sign-flipping
if pairwise:
group_parameters['group_matrix'] = np.ones((
group_parameters['n_subjects'],
group_parameters['n_subjects']))
elif len(np.unique(group_assignment)) > 2:
raise ValueError("This test is not valid for more than "
"2 groups! (got {0})".format(
len(np.unique(group_assignment))))
else:
raise ValueError("Invalid group assignments!")
return group_parameters
def _permute_one_sample_iscs(iscs, group_parameters, i, pairwise=False,
summary_statistic='median', group_matrix=None,
exact_permutations=None, prng=None):
"""Applies one-sample permutations to ISC data
Input ISCs should be n_subjects (leave-one-out approach) or
n_pairs (pairwise approach) by n_voxels or n_ROIs array.
This function is only intended to be used internally by the
permutation_isc function in this module.
Parameters
----------
iscs : ndarray or list
ISC values
group_parameters : dict
Dictionary of group parameters
i : int
Permutation iteration
pairwise : bool, default: False
Indicator of pairwise or leave-one-out, should match ISCs variable
summary_statistic : str, default: 'median'
Summary statistic, either 'median' (default) or 'mean'
exact_permutations : list
List of permutations
prng = None or np.random.RandomState, default: None
Initial random seed
Returns
-------
isc_sample : ndarray
Array of permuted ISC values
"""
# Randomized sign-flips
if exact_permutations:
sign_flipper = np.array(exact_permutations[i])
else:
sign_flipper = prng.choice([-1, 1],
size=group_parameters['n_subjects'],
replace=True)
# If pairwise, apply sign-flips by rows and columns
if pairwise:
matrix_flipped = (group_parameters['group_matrix'] * sign_flipper
* sign_flipper[
:, np.newaxis])
sign_flipper = squareform(matrix_flipped, checks=False)
# Apply flips along ISC axis (same across voxels)
isc_flipped = iscs * sign_flipper[:, np.newaxis]
# Get summary statistics on sign-flipped ISCs
isc_sample = compute_summary_statistic(
isc_flipped,
summary_statistic=summary_statistic,
axis=0)
return isc_sample
def _permute_two_sample_iscs(iscs, group_parameters, i, pairwise=False,
summary_statistic='median',
exact_permutations=None, prng=None):
"""Applies two-sample permutations to ISC data
Input ISCs should be n_subjects (leave-one-out approach) or
n_pairs (pairwise approach) by n_voxels or n_ROIs array.
This function is only intended to be used internally by the
permutation_isc function in this module.
Parameters
----------
iscs : ndarray or list
ISC values
group_parameters : dict
Dictionary of group parameters
i : int
Permutation iteration
pairwise : bool, default: False
Indicator of pairwise or leave-one-out, should match ISCs variable
summary_statistic : str, default: 'median'
Summary statistic, either 'median' (default) or 'mean'
exact_permutations : list
List of permutations
prng = None or np.random.RandomState, default: None
Initial random seed