With its 100 billion neurons (brain cells) and its 100 trillion synapses, the human brain is a complex multi-level system. The connections among the neurons form a hierarchy of circuits, from local microcircuitry up to the level of the whole brain. Meanwhile at a lower level, every neuron and every synapse is a complex molecular machine in its own right. It is the interactions between these levels, which give rise to human behaviour, human emotion and human cognition.
The Blue Brain Project (BBP) aims to build comprehensive digital reconstructions (computer models) of the brain, which include the brain’s different levels of organization and their interactions, and which are compatible with the available experimental data. The project’s current reconstructions reproduce the detailed cellular anatomy, connectivity, and electrical behaviour of a small part of the neocortex of young rats (about one third of a cubic millimetre of cortical tissue).
The BBP’s reconstruction strategy identifies interdependencies in the experimental data (e.g. dependencies between the size of neurons and neuron densities, dependencies between the shapes of neurons and the synapses they form, dependencies between the numbers of “boutons” on axons and synapse numbers) and uses them to “constrain” the reconstruction process. Multiple, intersecting constraints allow the project to build the most faithful reconstructions possible from the sparse available experimental data– avoiding the need to “measure everything”. In fact, the BBP’s latest reconstruction of the cortical microcircuit connectivity is based on experimental data representing less than 1% of the synaptic connections in the circuit. Lab experiments providing new data and constraints make it possible to test and progressively improve the digital reconstruction.
Data and neuron models used in our current reconstructions are available on the Blue Brain Portal – a knowledge space for simulation neuroscience.