298 | Jeff Lichtman on the Wiring Diagram of the Brain

Sean Carroll speaks with neuroscientist Jeff Lichtman about the state and implications of brain mapping—specifically, the effort to describe the brain's connectome, the complete wiring diagram of how neurons connect to one another through synapses. The conversation centers on a paradox: we can now image neural connectivity in extraordinary detail, even producing a map of a cubic millimeter of human cortex, yet obtaining a wiring diagram is not the same as understanding how the brain works. Lichtman argues that the brain may be irreducibly complex—that it is its own most concise description—because understanding requires compression, and the brain may offer no shorthand simpler than itself.

The episode ranges widely across the technical and conceptual. Lichtman explains the modern methods of connectomics—serial-section electron microscopy that slices brain tissue into 10- to 30-nanometer sections and reconstructs 3D wiring through tracing—and traces the historical lineage from Ramón y Cajal's insights using the Golgi stain to today's efforts to map whole organisms like C. elegans. He shows why a complete wiring diagram still cannot predict behavior: synapses have varying strengths, neurons respond nonlinearly, and timing matters—all invisible in a static connectome. He also addresses what makes memory and learning hard: that each person's experiences produce unique, incompressible wiring changes, and that a whole human brain connectome would amount to a zettabyte of data—roughly the entire world's annual digital output—with no clear way to use it. The conversation touches on psychiatric and developmental disorders, the role of distributed circuits in encoding concepts, and whether data-driven science offers a truer path than hypothesis-first investigation.