BrainVoyager QX v2.8

From Single Voxels to Statistical Maps

The statistical analysis steps were described in the previous sections for a single voxel's time course. The described GLM analysis is performed independently for the time course of each voxel. Since a typical fMRI data set contains about 100,000 voxels, the statistical analysis is performed 100,000 times using the same design matrix X for each voxel's data. Running a GLM results in a set of estimated beta values attached to each voxel. Likewise, a specified contrast c'bv will be performed using the same contrast vector c for each voxel v, but it will use a voxel's "own" vector of beta values bv to obtain a voxel-specific t and p value. Statistical test results for individual voxels are accumulated in a 3D data set called a statistical map. To visualize a statistical map, the obtained values, e.g. t values, can be shown at the position of each voxel in the original data, which will replace the original intensity values at each voxel (showing anatomical information). A more useful approach shows the statistical values only for those voxels exceeding a specified statistical threshold. This allows visualizing "background" (anatomical) information in large parts of the brain while statistical information is shown only in those regions exhibiting significant signal modulations. While anatomical information is normally visualized using a range of grey values, supra-threshold statistical test values are typically visualized using multiple colours, for example, a red-to-yellow color range for positive values and a green-to-blue color range for negative values. With these colours, a positive (negative) t value just passing a specified threshold would be colored in red (green), while a very high positive (negative) t value would be colored in yellow (blue).

The snapshots below show a thresholded statistical map overlayed directly on recorded functional slices (FMR space), overlayed in normalized (Talariach) space (VMR data), and overlayed in cortex space (SRF data). Maps calculated in FMR space for STC time course data are called MAPs, maps calculated in VMR space for VTC data are called VMPs, and maps calculated in cortex SRF space for MTC data are called SMPs. These map formats can be loaded and saved with corresponding file extensions from respective dialogs.

Copyright © 2014 Rainer Goebel. All rights reserved.