  Functional neuroimaging studies have lead to understanding the brain as a
collection of spatially segregated functional networks. It is thought that each
of these networks is in turn composed of a set of distinct sub-regions that
together support each network's function. Considering the sub-regions to be an
essential part of the brain's functional architecture, several strategies have
been put forward that aim at identifying the functional sub-units of the brain
by means of functional parcellations. Current parcellation strategies typically
employ a bottom-up strategy, creating a parcellation by clustering smaller
units. We propose a novel top-down parcellation strategy, using time courses of
instantaneous connectivity to subdivide an initial region of interest into
sub-regions. We use split-half reproducibility to choose the optimal number of
sub-regions. We apply our Instantaneous Connectivity Parcellation (ICP)
strategy on high-quality resting-state FMRI data, and demonstrate the ability
to generate parcellations for thalamus, entorhinal cortex, motor cortex, and
subcortex including brainstem and striatum. We evaluate the subdivisions
against available cytoarchitecture maps to show that the our parcellation
strategy recovers biologically valid subdivisions that adhere to known
cytoarchitectural features.
