What's in a pattern? Examining the Type of Signal Multivariate Analysis Uncovers At the Group Level

TL;DR

This paper compares directional and non-directional group-level multivoxel pattern analysis (MVPA) in neuroimaging, revealing they uncover different brain regions and proposing a method to quantify spatial similarity of activation patterns across subjects.

Contribution

It introduces a method to measure spatial similarity in activation patterns across subjects and highlights differences between MVPA approaches in neuroimaging.

Findings

Directional and non-directional MVPA reveal different brain regions.

Higher spatial similarity in auditory regions during an auditory task.

Partial overlap between regions identified by the two approaches.

Abstract

Multivoxel pattern analysis (MVPA) has gained enormous popularity in the neuroimaging community over the past few years. At the group level, most MVPA studies adopt an "information based" approach in which the sign of the effect of individual subjects is discarded and a non-directional summary statistic is carried over to the second level. This is in contrast to a directional "activation based" approach typical in univariate group level analysis, in which both signal magnitude and sign are taken into account. The transition from examining effects in one voxel at a time vs. several voxels (univariate vs. multivariate) has thus tacitly entailed a transition from directional to non-directional signal definition at the group level. While a directional group-level MVPA approach implies that individuals have similar multivariate spatial patterns of activity, in a non-directional approach each individual may have a distinct spatial pattern. Using an experimental dataset, we show that directional and non-directional group-level MVPA approaches uncover distinct brain regions with only partial overlap. We propose a method to quantify the degree of spatial similarity in activation patterns over subjects. Applied to an auditory task, we find higher values in auditory regions compared to control regions.