Coping with confounds in multivoxel pattern analysis: What should wedo about reaction time differences? A comment on Todd Nystrom Cohen2013

摘要

Multivoxel pattern analysis (MVPA) is a sensitive and increasingly popular method for examining differences between neural activation patterns that cannot be detected using classical mass-univariate analysis. Recently, Todd et al. (“Confounds in multivariate pattern analysis: Theory and rule representation case study”, 2013, NeuroImage 77: 157–165) highlighted a potential problem for these methods: high sensitivity to confounds at 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 average out at group level, group level MVPA results may be driven by any activation differences that can be discriminated 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 significant questions for the field: to what extent have previous multivoxel discriminations in the literature been 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 et al. andcompare two different approaches to remove the effect of RT in MVPA. We showthat in our empirical data, in contrast to that of Todd et al., the effect of RTon rule decoding is negligible, and results were not affected by the specificdetails of RT modelling. We discuss the meaning of and sensitivity for confoundsin traditional and multivoxel approaches to fMRI analysis. We observe that theincreased sensitivity of MVPA comes at a price of reduced specificity, meaningthat these methods in particular call for careful consideration of what differsbetween our conditions of interest. We conclude that the additional complexityof the experimental design, analysis and interpretation needed for MVPA is stillnot a reason to favour a less sensitive approach.