This tangle of wiry filaments is not a bird’s nest or a root system. Instead, it’s the largest map to date of the connections between brain cells—in this case, about 200 from a mouse’s visual cortex. To map the roughly 1300 connections, or synapses, between the cells, researchers used an electron microscope to take millions of nanoscopic pictures from a speck of tissue not much bigger than a dust mite, carved into nearly 3700 slices. Then, teams of “annotators” traced the spindly projections of the synapses, digitally stitching stacked slices together to form the 3D map. The completed map reveals some interesting clues about how the mouse brain is wired: Neurons that respond to similar visual stimuli, such as vertical or horizontal bars, are more likely to be connected to one another than to neurons that carry out different functions, the scientists report online today in Nature. (In the image above, some neurons are color-coded according to their sensitivity to various line orientations.) Ultimately, by speeding up and automating the process of mapping such networks in both mouse and human brain tissue, researchers hope to learn how the brain’s structure enables us to sense, remember, think, and feel.