This image is one of the first to depict, in neuron-by-neuron detail, the tangled circuits that collect, process, and archive information in the mammal brain. It shows individu­al mouse neurons--specifically, the axons from a nerve that controls eye movements--fluorescing in different colors. Such images could help researchers better understand everything from brain development to the workings of diseases like autism and schizophrenia.

Researchers at Harvard University made the image by genetically engineering mice to carry a specialized piece of DNA that, when triggered by a particular enzyme, can randomly express any of three fluor­escent proteins--yellow, red, or cyan. A single neuron contains numerous copies of this DNA segment. Once activated, each DNA piece expresses one of the proteins, giving each cell a different blend of the three colors. As a result, neurons can exhibit any of about 100 unique hues. Giving neuroscientists such a broad palette with which to paint individual brain cells will allow them to explore neural circuits as never before. "This will be an incredibly powerful tool," says Elly Nedivi, a neuro­scientist at MIT who is not involved in the research. "It will open up huge opportunities in terms of looking at neural connectivity." Scientists will probably use the engineered mice to study disease and the neural connections and disconnections that occur as young brains develop.