What do differences between multi-voxel and univariate analysis mean? How subject-, voxel-, and trial-level variance impact fMRI analysis.
Multi-voxel pattern analysis (MVPA) has led to major changes in how fMRI data are analyzed and interpreted. Many studies now report both MVPA results and results from standard univariate voxel-wise analysis, often with the goal of drawing different conclusions from each. Because MVPA results can be sensitive to latent multidimensional representations and processes whereas univariate voxel-wise analysis cannot, one conclusion that is often drawn when MVPA and univariate results differ is that the activation patterns underlying MVPA results contain a multidimensional code. In the current study, we conducted simulations to formally test this assumption. Our findings reveal that MVPA tests are sensitive to the magnitude of voxel-level variability in the effect of a condition within subjects, even when the same linear relationship is coded in all voxels. We also find that MVPA is insensitive to subject-level variability in mean activation across an ROI, which is the primary variance component of interest in many standard univariate tests. Together, these results illustrate that differences between MVPA and univariate tests do not afford conclusions about the nature or dimensionality of the neural code. Instead, targeted tests of the informational content and/or dimensionality of activation patterns are critical for drawing strong conclusions about the representational codes that are indicated by significant MVPA results.
Department of Psychology, Texas Tech University, USA. Electronic address: tyler.h.davis@ttu.edu.
Tyler
T
Davis
Department of Psychology, Stanford University, USA. Electronic address: klarocqu@stanford.edu.
Karen F
KF
LaRocque
Department of Psychology, University of Texas at Austin, USA; Department of Neuroscience, University of Texas at Austin, USA; Imaging Research Center, University of Texas at Austin, USA.
Jeanette A
JA
Mumford
Department of Psychology, Princeton University, USA; Princeton Neuroscience Institute, Princeton University, USA.
Kenneth A
KA
Norman
Department of Psychology, Stanford University, USA; Neurosciences Program, Stanford University, USA.
Anthony D
AD
Wagner
Department of Psychology, University of Texas at Austin, USA; Department of Neuroscience, University of Texas at Austin, USA; Imaging Research Center, University of Texas at Austin, USA.
Russell A
RA
Poldrack
D000818:Animals; D003198:Computer Simulation; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D059906:Neuroimaging / Q000379:methods*; D013045:Species Specificity
Dimensionality; Distributed representations; MVPA; Voxel-level variability; fMRI analysis