Although it has been appreciated since the work of Cajal that the brain is comprised of neurons connected together in elaborate circuits, there has been only scant progress in mapping these circuits in detail. Here, we describe why it has been so difficult and why we believe things are about to change. Finally, we briefly discuss what value we believe these maps will have for basic and clinical neuroscience.

Probably the principal reasons why detailed circuit maps do not already exist are both the sheer number of objects that would have to be cataloged and the miniscule size of each. Each human brain contains an estimated 100 billion neurons connected through 100 thousand miles of axons and between a hundred trillion to one quadrillion synaptic connections (Shepherd, 2003) (there are only an estimated 100–400 billion stars in the Milky Way galaxy). The largest of these neural wires, myelinated projection axons, are typically smaller than 20 ums. The finest, axonal and dendrite branches, are smaller than 0.2 ums, effectively precluding even the highest resolvable conventional light microscope from tracing and identifying such connections. The raw data for the Atlas of Human Connections would require approximately 1 trillion Gigabytes (an exabyte) and could not fit in the memory of any current computer. Indeed, all the written material in the world is a small fraction of this map. By way of comparison, the entire Human Genome Project requires only a few gigabytes. Until recently, there really was no practical way to store the information needed for even a single brain map and there were no tools to make the maps in any case.

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