No significant small-study inflation of whole-brain effect sizes in 1,245 published unthresholded task-fMRI maps on NeuroVault

Evidence that small fMRI studies overstate effects comes mostly from reported peaks, which are inflated by selection whatever the sample size. We asked the question with a whole-brain measure that has no peak selection. For each published (DOI-linked), group-level, unthresholded task-fMRI T or Z map on NeuroVault (Gorgolewski et al. 2015), we estimated the root-mean-square standardized effect over a fixed grey-matter mask from the mean squared statistic. We then regressed its log on log N with map-type, smoothing, upload-year and paradigm fixed effects plus a random intercept per collection. Everything was preregistered (prereg:f7ecdaf6…) on a frozen map list. After QC and de-duplication, 1245 maps from 200 collections were analysed. The slope was β = -0.101387 (CI -0.272087 to 0.069313), so the locked decision is inconclusive. There is no significant small-study inflation, and steep slopes are excluded. The data cannot rule out a modest slope at the preregistered boundary.