Coping with confounds in multivoxel pattern analysis: What should we do about reaction time differences? A comment on Todd, Nystrom & Cohen 2013

Multivoxel pattern analysis (MVPA) is a sensitive and increasingly popular method for examining differences be tween neural activation patterns that cannot be detected using classical mass-univariate analysis. Recently, Tod( et al. ("Confounds in multivariate pattern analysis: Theory and rule. representation case study", 2011 Neurolmage 77:157-165) highlighted a potential problem for these methods: high sensitivity to confounds the level of individual participants due to the use of directionless summary statistics. Unlike traditional mass univariate analyses where confounding activation differences in opposite directions tend to approximately aver age out at group level, group level MVPA results may be driven by any activation differences that can be inated in individual participants. In Todd et al.'s empirical data, factoring out differences in reaction time (RT reduced a classifier's ability to distinguish patterns of activation pertaining to two task rules. This raises two nificant questions for the field: to what extent have previous multivoxel discriminations in the literature bee driven by RT differences, and by what methods should future studies take RT and other confounds into account We build on the work of Todd eta!, and compare two different approaches to remove the effect of RT in MVP. We show that in our empirical data, in contrast to that of Todd etal., the effect of RT on rule decoding is and results were not affected by the specific details of RT modelling. We discuss the meaning of and sensitivity confounds in traditional and multivoxel approaches to fMRI analysis. We observe that the increased sensitivity MVPA comes at a price of reduced specificity, meaning that these methods in particular call for careful consider ation of what differs between our conditions of interest. We conclude that the additional complexity of the imental design, analysis and interpretation needed for MVPA is still not a reason to favour a less approach. (C) 2014 Elsevier Inc. All rights reserve