Human Brain connectomics: Networks, techniques, and applications

The human brain is organized into a collection of interacting networks with specialized functions to support various cognitive functions. The word connectome first burst on the scene with the work of Sporns et al. [1], who urged brain researchers to advance a comprehensive structural description of the elements and connections forming the human brain. An increasing body of evidence indicates that schizophrenia, multiple sclerosis, and autism exhibit abnormal brain connections. Changes in connectivity also appear to occur as a consequence of neuron degeneration, either from natural aging or diseases such as Alzheimers disease. A connectome is hence fundamentally important for understanding brain growth, aging, and abnormality. At the micro level, the brain elements consist of single neurons, the amount of which often treads the realm of hundreds of billions, and possible connections between them numbering in the order of$10¹⁵. At a more macro (and more manageable) level, the brain is parcellated into a number of regions, where each region accounts for the activity and coactivity of a population of neurons. The colossal task of constructing a connectome calls for powerful tools for handling the vast amount of information given by advanced imaging techniques. In this article, we provide an overview of the fundamental concepts involved, the necessary techniques, and applications to date.