  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.

Quantitative Methods;Neurons and Cognition