Description:
MUTimg computes the symbolic mutual information for source activity between pairs of voxels. The inputs are the MEG dataset, the SAM beamformer coefficients (weights) from SAMwts, and a parameter file. The command line usage is:
MUTimg -r <dataset_name> -m < parameter_file_name> -v
The -r flag is followed by the dataset name (with or without the .ds suffix). The -m flag is followed by the parameter file name without the .param suffix. The -v argument specifies verbose output - else MUTimg works silently.
MUTimg processes the entire dataset and does not use any markers. For those users needing mutual information specific to events or tasks, the user should design and acquire MEG data under those conditions.
MUTimg first computes the source time series for each voxel. The source time series is then converted to a symbolic time series, as follows. A vector of 4 or 5 lagged samples, with lags determined by the data lowpass frequency to avoid aliasing is extracted from the source time series. The amplitudes of these samples are ranked - yielding a rank vector (also of length 4 or 5). For a rank vector of length N there are N! possible combinations, or symbolic states. For N=4, there are 24 possible symbols. For N=5 there will be 120 symbols. The lagged sample vector is advanced one sample at a time through the source time series, generating a symbolic time series. This is saved in an N! xV matrix, where V is the number of voxels in the ROI. The symbolic mutual information for all pairs of voxels X and Y is computed from the joint and marginal probabilities of occurrence of the symbols:
If the symbolic time series Y is advanced prior to computing mutual information, then directionality of shared information is implied.
Since this is a pair-wise measure, the output is a V x V matrix. The matrix is written to the SAM subdirectory, and a NIFTI 3D+V image file is written to the directory designated by ImageDirectory (default is the SAM subdirectory). Depending upon the voxel grid spacing, this can by a huge file to view using AFNI.
The NIFTI image files will be either "ortho" (the designated ROI and grid spacing from the parameter file) or "tlrc" (Talairach space and resolution) depending upon the ImageFormat keyword used to compute the SAM weights.
The required parameter file keywords for MUTimg are:
Optional keywords include:
Examples parameters for imaging the symbolic mutual information, in Taliarach space on an 8mm grid for MUTimg:
NumMarkers | 0 | ||
XBounds | -10.0 | 10.0 | YBounds | -9.0 | 9.0 |
ZBounds | -2.0 | 15.0 | |
ImageStep | 0.5 | ||
ImageFormat | TLRC | 8.0 | |
CovBand | 50.0 | 300.0 | |
ImageBand | 50.0 | 300.0 | |
OrientBand | 14.0 | 30.0 | |
ImageMetric | MutualInfo | 6.5 | 5 |
CovType | GLOBAL | ||
MRIDirectory | /data1/mri | ||
ImageDirectory | /data2/SMI/Rest/Images |
The keyword ImageMetric directs MUTimg to compute shared information using N=5 (120 symbolic states), with Y advanced by 6.5 ms from X (the "seed" voxel)