Abstract
fMRI localizer tasks are often used to define subject specific functional regions of interest (fROIs) that contain the relevant features for subsequent analyses. fROIs are typically small and show large interindividual differences in extent and effect size. As statistical testing procedures focus on con- trolling false positives, this may lead to an ad-hoc adjustment of thresholding in some individuals. The promising likelihood ratio (LR) testing approach for fMRI (Kang et al. 2015) provides simultaneous control of both false positives and negatives by contrasting evidence in favor of true activation against evidence in favor of the null hypothesis. The authors propose to estimate the expected alternative by a percentile (e.g. 95th) across the voxels of an effect size map. However, in the context of fROIs, pre-defined observed percentiles may induce inconsistent activation across subjects. In this study we show the potential of a maximized LR approach (Bickel, 2012) for this particular application. The maximum LR is calculated over the same interval of functionally relevant alternatives for all subjects, enabling consistent localization of the fROIs in both subjects with low levels and high levels of general activity.
Highlights
Need for thresholding procedure that adjusts to general level of baseline activation: huge interindividual differences in general level activation, which results in ad hoc threshold adjustments in each individual in order to obtain anatomically plausible activation
The active region was 0.02% of the whole brain, since this proportion is typical for localizers
We evaluated the maximized LR (mLR) method with the 5th and 50th percentile of the true underlying effect size (ES) and the Likelihood ratio (LR)
Summary
Multiple testing problem: explosion of false positives. Whole brain vs regions of interest: reduction of number of voxels => impact of multiple testing ↓ Independent localizer task before main experiment to define the ROI functionally in each individual separately. Small brain regions detected with a small number of scans. This region is analyzed in main experiment.
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