, 2013a and Smith et al., 2013b). The sheer size of each dense connectome (33 GB) makes them unwieldy for many analyses that instead can benefit from compact representations of connectivity between regions defined by one or another parcellation. It is obviously preferable to use a parcellation having relatively homogeneous connectivity within the parcels rather than, say, a geographically based parcellation having demonstrably heterogeneous connectivity profiles.

Accordingly, selleck kinase inhibitor the fcMRI data provide the best available evidence on which to derive an objective brain-wide connectivity-based parcellation. There are several complementary ways to analyze fcMRI data in order to infer or identify parcels that reflect functionally distinct regions (Cohen selleckchem et al., 2008, Wig et al., 2011, Wig et al., 2013, Power et al., 2013, Blumensath et al., 2013 and Smith et al., 2013a). One powerful approach uses ICA analysis applied to group-average data (concatenated

fMRI time series) in order to identify grayordinates that share similar fMRI time courses (Smith et al., 2013a). The left half of Figure 6 shows three representative components (network “nodes”) from a 98-component ICA decomposition using data from 120 HCP subjects. These fcMRI-derived nodes are analogous to previously published resting state networks (e.g., Yeo et al., 2010) but are much smaller and reveal much finer detail. Two distinctive characteristics warrant mention. (1) Many nodes include topologically noncontiguous portions, making them unlike classically defined cortical areas. (This is particularly the case at lower ICA dimensionalities.) The noncontiguous segments usually involve symmetric portions of the two hemispheres, often involve both cerebellum and cerebral cortex, and often involve dispersed regions within a single cerebral hemisphere. This reflects the physically dispersed nature of network and subnetwork components defined by similarities in fMRI to time series. (2) In relation to topographically organized sensory and motor areas, some nodes cross multiple areal boundaries

but occupy only part of the topographic map of each area. In Figure 6A (top row), node 7 includes the face region of somatomotor cortex bilaterally and extends across areas 1, 3a, 3b, 4a, and 4b. In row 2, node 36 includes the hand region only in the left hemisphere. In rows 3 and 4, node 24 occupies central area V1, respecting the V1/V2 boundary; node 3 includes the peripheral representation of both area V1 and V2. These observations drive home the point made previously that connectivity-based parcels and architectonic parcels can differ markedly because they reflect fundamentally different aspects of cortical organization. Figure 6B shows the 98 × 98 connectivity matrix that results from correlating the time series of each ICA spatial component with one another (n = 131 subjects).