A Generalized Form of Context-Dependent Psychophysiological Interactions (gPPI)


University of Wisconsin – Madison
Massachusetts General Hospital
Harvard Medical School
March 24, 2011

Description

Functional MRI allows one to study task-related regional responses and taskdependent connectivity analysis using psychophysiological interaction (PPI) methods. The latter affords the additional opportunity to understand how brain regions interact in a task-related manner. In other words, a PPI analysis assesses how the activity within brain networks is modulated by varying psychological states within an fMRI task. The current implementation of PPI in Statistical Parametric Mapping (SPM8) assesses connectivity differences between two task conditions, when in practice fMRI tasks frequently employ more than two conditions. Here we propose a generalized form of context-dependent PPI (gPPI), which has the flexibility to accommodate more than two task conditions in the same PPI model. We demonstrate this using an empirical dataset. Furthermore, we compare the model fit (at the single subject level) of our new gPPI approach to standard PPI methods that have been published. Using the Akaike Information Criterion, we found the new gPPI approach improved model fit compared to standard PPI. Also, statistical comparison of results of the new gPPI versus standard PPI methods showed several regions of greater task-dependent connectivity using gPPI methods. This result taken with evidence of improved model fit using gPPI suggests that this method may have greater sensitivity than standard methods. However, this interpretation is tempered by the lack of a true gold standard to which we can compare both PPI methods. In sum, the generalized form of context-dependent PPI approach increases flexibility of statistical modeling, and our demonstration of gPPI methods using an empirical dataset indicates that gPPI may improve single-subject model-fit and sensitivity to true positive findings.

Usage

An example wrapper (looping through subjects and regions) and example input structure (P) can be found at: http://martinos.org/~mclaren/ftp/Utilities_DGM/example_structure_inputs/

The automated toolbox can do the following:

a1) produce identical results to the current implementation in SPM

a2) use the current implementation of PPI in SPM but using the regional mean instead of the eigenvariate

a3) uses a generalized form that allows a PPI for each task to be in the same model using either the regional mean of eigenvariate

b) creates the model using the output of one of the (a) options and the first level design

c) estimates the model (/results directory)

d) computes the contrasts specified

To use the toolbox, you will need to change the following things in the wrapper and input structure:

In the wrapper ppi_wrapper.m (in example_structure_inputs):

Change the following lines:

addpath(‘PPPIdirectory’)

—> replace PPPIdirectory with the location of the PPPI directory that was downloaded
addpath('spm8directory')
—> replace spm8directory with the location of spm8 (can be found by typing which spm)
Subjects={'subject1' 'subject2'};
—> put your subjects in ’’ inside the {}.
regionfile={'region1.nii'... 'region2.nii'};
—> these are the VOIs files (.nii, .img, .mat) with full paths to use for PPI, they are added to the input structure file by the wrapper.
region={'region1'... 'region2'};

—> these are the VOIs names, they are added to the input structure file by the wrapper.

load('ppi_master_template.mat');

—> change if you’re calling the master template something else.

save(['directory' region{regionnumber} '.mat'],'P');

—> change directory to a location to save the inputstructure that has the region and VOI information added to it.

Directory=['subjectdirectory'];

—> change to location of 1st level statistics. Can include variables (e.g. [‘/Data/’ Subjects{i} ‘/model/’])

load(['directory' region{regionnumber} '.mat']);

—> should match the save statement above.

save([Subjects{i} '_analysis_' region{regionnumber} '.mat'],'P');
PPPI([Subjects{i} '_analysis_' region{regionnumber} '.mat']);

—> analysis should be changed to something more identifiable (e.g. workingmemory_myname)

Now your set to run the wrapper. However, you need to modify the master_template first for the options that you want to use. You should also change the name in case others are also using it. In the case that you change the name, it must also be changed in the wrapper above.

In the input structure fields of the template.mat file (in example_structure_inputs):

These are set by the wrapper:

  • subject: the subject number, can also be the second argument of PPPI (set in wrapper)
  • directory: either the first-level SPM.mat directory, or if you are only estimating a PPI model, then the first-level PPI directory (set in wrapper)
  • VOI: name of VOI file (‘.nii’, ‘.img’, ‘.mat’). If you use a .mat file, it should be 3 columns containing the ijk voxel indices OR be a VOI.mat file from SPM. (set in wrapper)
  • Region: name of output file(s), reqires two names for analysis (set in wrapper) with two VOI, regions should be separated by a space inside the ’ ’. Output directory will be Region. (if 2 regions, then the two regions will be separated by a _ in the directory name. (set in wrapper)

These should be set ahead of time.

  • contrast: contrast to adjust for. Adjustments remove the effect of the null space of the contrast. Set to 0 for no adjustment. Set to a number, if you know the contrast number. Set to a contrast name, if you know the name. The default is: ‘Omnibus F-test for PPI Analyses’. If not set, reverts to default setting.
  • analysis: specifies psychophysiological interaction (‘psy’); physiophysiological interaction (‘phys’); or psychophysiophysiological interactions (‘psyphy’).
  • extract: specifies the method of ROI extraction, eigenvariate (‘eig’) or mean (‘mean’)
  • method: specifies traditional SPM PPI (‘trad’) or generalized condition-specific PPI (‘cond’). ‘trad’ is identical to SPMs current implementation.
  • equalroi: specifies the ROIs must be the same size in all subjects. NOTE: default=1 (true); set to 0 to lift the restriction. NOTE: that if your VOI is outside of the 1L mask, it will return an error message
  • FLmask: specifies that the ROI should be restricted using the mask.img from the 1L statistics. NOTE: default=0. FLmask=1 does nothing if equalroi=1.
  • VOI2: name of 2nd VOI for physiophysiological interactions
  • Weights: for traditional PPI, you must specify weight vector for each task. Order needs to be the same as Tasks below.
  • Tasks: In the generalized condition-specific PPI, you should specify the tasks to include in the analyses, but put a 0 or 1 in front of them to specify if they must exist in all sessions. NOTE: In traditional PPI, specify the tasks that go with the weights.
  • Estimate: specifies whether or not to estimate the PPI design. 1 means to estimate the design, 2 means to estimate the design from already created regressors (must be of the OUT structure), 0 means not to estimate. Default is set to 1, so it will estimate.
  • CompContrasts: 0 not to estimate any contrasts; 1 to estimate contrasts; 2 to only use PPI txt file for 1st level (not recommended); 3 to only use PPI txt file for 1st level and estimate contrasts (not recommended); 2&3 are not recommended as they potentially do not include all tasks effects in the mode. Use at your own risk. 3 can not weight the contrasts based on the number of trials. Default is 0.
  • Contrasts: cell array of tasks to create contrasts to evaluate OR it is a structure with the following fields:
    • left: tasks on left side of equation or ‘none’
    • right: tasks on right side of equation or ‘none’
    • Weighted: from Weighted below, default is 0; automatically set from Weighted
    • STAT: ‘T’ or ‘F’
    • c: contrast vector from createVec, automatically generated
    • name: name of contrast, will be defined if left blank
    • Prefix: prefix to the task name (optional), can be used to select each run
    • Contrail: suffix after task name (e.g. parametric modulators, different basis function)
    • If left blank and CompContrasts=1, then it defines all possible T contrasts for task components and across runs.
  • Weighted: Default is not to weight tasks by number of trials (0); to change this, specify which tasks should be weighted by trials. If you want to weight trials, then specify a duration longer than your events. If you have a mixed block event related design, then you can average your events based on number of trials and the blocks won’t be averaged IF Weighted is set to be a number that is shorter than the block duration and longer than your events.
  • SPMver: SPM version used to create SPM.mat files at the first level.
  • maskdir: location to save seeds if VOI was a .mat file

To set a field, type:

load template_master.mat
P.fieldname=value
save template_master.mat P

Terms of Use

This script is intended for free use by the neuroimaging community except for commercial applications.

Scripts

Download the matlab scripts: http://www.martinos.org/~mclaren/ftp/Utilities_DGM

Last Updated: 3-24-2011