The brain is a fascinating object of study, because of the complexity of the cognitive functions it produces but also of its intrisic complexity. Indeed, the brain is made of a distributed network of neurons, considered as information processing units and gathered in circuits to produce cerebral functions. Cerebral functions and related behaviors emerge from the interaction of these ingredients and the study of the brain as a network is a central element in most domains of neuroscience that aim at finding links between structure and function, at various levels of description. Three kinds of connectivity are generally described : structural connectivity (at the anatomical level), functional connectivity (measuring statistical dependency between neuronal activations) and effective connectivity (measuring causal interactions for tasks and information flows). Considered jointly, together with considerations related to energy consumption, embodiment, ontogenesis and phylogenesis, they are an interesting way to understand how cerebral structures and functions generate mutual constraints to produce fascinating phenomena at all the scales.
