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Fixing the stimulus-as-fixed-effect fallacy in task fMRI

Jacob Westfall, Thomas E. Nichols, Tal Yarkoni
doi: https://doi.org/10.1101/077131
Jacob Westfall
1Department of Psychology, University of Texas at Austin
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Thomas E. Nichols
2Department of Statistics & WMG, University of Warwick, Coventry, CV4 7AL
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Tal Yarkoni
1Department of Psychology, University of Texas at Austin
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Abstract

Most fMRI experiments record the brain’s responses to samples of stimulus materials (e.g., faces or words). Yet the statistical modeling approaches used in fMRI research universally fail to model stimulus variability in a manner that affords population generalization--meaning that researchers’ conclusions technically apply only to the precise stimuli used in each study, and cannot be generalized to new stimuli. A direct consequence of this stimulus-as-fixed-effect fallacy is that the majority of published fMRI studies have likely overstated the strength of the statistical evidence they report. Here we develop a Bayesian mixed model (the random stimulus model; RSM) that addresses this problem, and apply it to a range of fMRI datasets. Results demonstrate considerable inflation (50 - 200% in most of the studied datasets) of test statistics obtained from standard “summary statistics”-based approaches relative to the corresponding RSM models. We demonstrate how RSMs can be used to improve parameter estimates, properly control false positive rates, and test novel research hypotheses about stimulus-level variability in human brain responses.

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The copyright holder for this preprint is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under a CC-BY 4.0 International license.
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Posted September 25, 2016.
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Fixing the stimulus-as-fixed-effect fallacy in task fMRI
Jacob Westfall, Thomas E. Nichols, Tal Yarkoni
bioRxiv 077131; doi: https://doi.org/10.1101/077131
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Fixing the stimulus-as-fixed-effect fallacy in task fMRI
Jacob Westfall, Thomas E. Nichols, Tal Yarkoni
bioRxiv 077131; doi: https://doi.org/10.1101/077131

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