To create these cubes, Sebastian Seung of MIT and Daniel Berger, a postdoc in his lab, shaved 256 microscopically thin slices off the outermost layer of a mouse brain. Each slice was imaged under an electron microscope; the digital pictures were then stacked to construct a 3-D representation of the sliced region (top left). In reality the cube is only six-thousandths of a millimeter wide. Each cell in the image (both neurons and the support cells known as glia) was color-coded by hand, a process that took 150 hours.

To visualize the elements that make up neurons--axons, dendrites, and cell bodies--Seung and Berger used a computer to draw 3-D surfaces around the neural structures found in the cube. The bottom-right image depicts the outlines of neurons only, with glial cells removed; the bottom--left cube visualizes all the axons, the parts of neurons that transmit signals. The top--right cube shows only dendrites, the signal-receiving portions of neurons. Together the images give a sense of how the nervous system allocates space among glial cells, axons, and dendrites.