Barely visible in the overhead view of the mouse is a hair-thin optical fiber that feeds through the animal's skull into the right motor cortex of its brain. As the mouse casually sniffs around and explores its white basin, a cool blue glow appears at the fiber's point of entry. At that instant, the mouse starts circling leftward around the basin, swiftly and deliberately, as though it just received marching orders. A few moments later in the video, the blue glow disappears. The mouse suddenly stops marching and reverts to its lazy meandering, eventually sitting on its haunches.

Photo: Jessica Haye and Clark Hsiao