  A prominent goal of neuroimaging studies is mapping the human brain, in order
to identify and delineate functionally-meaningful regions and elucidate their
roles in cognitive behaviors. These brain regions are typically represented by
atlases that capture general trends over large populations. Despite being
indispensable to neuroimaging experts, population-level atlases do not capture
individual differences in functional organization. In this work, we present an
interactive visualization method, PRAGMA, that allows domain experts to derive
scan-specific parcellations from established atlases. PRAGMA features a
user-driven, hierarchical clustering scheme for defining temporally correlated
parcels in varying granularity. The visualization design supports the user in
making decisions on how to perform clustering, namely when to expand, collapse,
or merge parcels. This is accomplished through a set of linked and coordinated
views for understanding the user's current hierarchy, assessing intra-cluster
variation, and relating parcellations to an established atlas. We assess the
effectiveness of PRAGMA through a user study with four neuroimaging domain
experts, where our results show that PRAGMA shows the potential to enable
exploration of individualized and state-specific brain parcellations and to
offer interesting insights into functional brain networks.
