  Large-scale functional networks have been extensively studied using resting
state functional magnetic resonance imaging. However, the pattern,
organization, and function of fine-scale network activity remain largely
unknown. Here we characterized the spontaneously emerging visual cortical
activity by applying independent component analysis to resting state fMRI
signals exclusively within the visual cortex. In this sub-system scale, we
observed about 50 spatially independent components that were reproducible
within and across subjects, and analyzed their spatial patterns and temporal
relationships to reveal the intrinsic parcellation and organization of the
visual cortex. We found that the visual cortical parcels were aligned with the
steepest gradient of cortical myelination, and organized into functional
modules segregated along the dorsal/ventral pathways and foveal/peripheral
early visual areas. In contrast, cortical retinotopy, folding, and
cytoarchitecture impose limited constraints to the organization of resting
state activity. From these findings, we conclude that spontaneous activity
patterns in the visual cortex are primarily organized by visual streams, likely
reflecting feedback network interactions.
