What Is the Connectome? A Map of the Brain's Wiring

The connectome is the complete map of connections in a brain: a wiring diagram showing how neurons and brain regions link up. The word borrows from genome, and the idea is similar. Just as your genome is the full set of your genes, your connectome is the full set of your neural connections. Many neuroscientists now argue that this wiring, more than the size or shape of any single region, is what makes a brain do what it does.

The scale is staggering. A human brain holds roughly 86 billion neurons, and each can form thousands of connections, adding up to something on the order of 100 trillion links. Mapping all of them at once is still beyond us, so the field works at different levels of zoom.

Three scales of the connectome

Connectomes are usually described at three scales. The microscale traces individual neurons and their synapses, the finest and most demanding level. The mesoscale maps connections between populations of neurons or small regions, often using tract-tracing in animals. The macroscale charts the large white-matter highways between major brain areas, which is what most human brain imaging captures. Each scale answers different questions, and no single method covers them all.

How a connectome gets built

At the macroscale, researchers use diffusion MRI and tractography to follow the brain's white-matter tracts in living people, no surgery required. At the meso and microscale, the classic tools are tracers, substances injected into one region that travel along neurons to reveal where they project, and, more recently, electron microscopy that reconstructs every synapse in a tiny block of tissue. The first complete connectome of any animal, the roundworm C. elegans with its 302 neurons, took researchers over a decade to finish by hand.

The primate connectivity databases

Much of what we know about mammalian wiring comes from decades of tracer studies in monkeys, painstakingly collated into shared databases. CoCoMac, for example, brought together hundreds of macaque tracing studies into a single connectivity matrix, and newer resources map the marmoset cortex in comparable detail. These primate connectivity atlases are the bridge between animal experiments and the human brain, since we cannot inject tracers into people. Assembling and referencing that literature, the neuroanatomy and connectivity of the primate cortex, is the tradition brainmeta grew out of.

Why the connectome matters

The promise of connectomics is that wiring explains function. Where regions connect shapes how information flows, and differences in connectivity are being linked to conditions from schizophrenia to autism, which is why some researchers talk about connectopathies, disorders of miswiring. It also reframes an old question about identity. The neuroscientist Sebastian Seung put it bluntly: "I am my connectome." That probably overstates the case, since a brain is also chemistry, activity, and constant change, but the wiring is a big part of the story.

The limits of the map

A connectome is a snapshot of structure, not a live recording of thought. Two brains with similar wiring can still behave differently, and connections shift with learning and age, so any map is already a little out of date the moment it is made. The field is also still arguing about how much detail you actually need. Even so, the connectome has become one of the organizing ideas of modern neuroscience, and it sits close to the personality science elsewhere on this site: your traits, like your thoughts, ultimately run on that wiring. If you are curious where personality itself comes from, see our piece on where personality comes from.