Mapping the network structure of the human brain Due to the invasive nature of most classical anatomical methods like tract tracing, these methods cannot be applied to large samples of individual brains and they cannot be deployed in vivo, hence rendering tract tracing studies in human populations and relating structural network features to brain dynamics or behavior virtually impossible. Tract tracing has an Rapamycin important role to play for

the study of anatomical Inhibitors,research,lifescience,medical connections in animal models, particularly in non-human primates,43 and it is of vital importance for validating anatomical data derived from noninvasive imaging technology.44 To the extent that such validation has been carried out, indications are that most projections identified by noninvasive imaging have counterparts in white matter fascicles described by classical anatomy. Most studies on human brain connectomics have been

carried out by charting structural connections on the basis of data coming from diffusion MRI and tractography (Figure 4). 45-48 Diffusion MRI and traclography infer the spatial orientations and trajectories Inhibitors,research,lifescience,medical of bundles of myelinated axons traversing the brain’s white matter, on the basis of measurements of the diffusion anisotropy of water or other small molecules within biological tissue. Importantly, diffusion imaging and tractography deliver inferential and statistical models Inhibitors,research,lifescience,medical of fiber anatomy but cannot directly trace or visualize anatomical connections. Methods for signal acquisition and fiber reconstruction

are under continual development, with important recent advances in imaging complex (eg, intersecting) fiber architecture,49,50 and new algorithms for improved accuracy Inhibitors,research,lifescience,medical in inferring fiber pathways, including estimates of their uncertainty and evidence.51,52 Figure 4. From imaging structural brain connectivity to network metrics. The three plots show three different ways to represent structural connections in anatomical space. (A) A set of tractography streamlines. Red, green and blue indicate fibers running along … Another Inhibitors,research,lifescience,medical area of important methodological development concerns the biological interpretation of connection weights resulting from aggregating fiber counts or probabilities into a connection matrix.53,54 New approaches for obtaining additional measures of white matter microstructure, eg, axonal diameters and packing densities,55 will likely help to refine estimates of the weight, strength, and conduction velocity of individual long-distance projections. The node degree (the number of connections Carnitine palmitoyltransferase II attached at each node) is one of the most easily accessible graph measures and it is also highly informative, as is the distribution of node degrees across the whole network. Most, if not all, complex networks found in natural, especially biological, systems have been shown to have a broad degree distribution, with a small but important admixture of nodes that maintain considerably higher numbers of connections than most other nodes.