The counterclockwise-running mouse was something new - a triple fusion of animal, plant, and technology - and the students knew it was a harbinger of unprecedentedly powerful ways to alter the brain. For curing diseases, to begin with, but also for understanding how the brain interacts with the body. And ultimately for fusing human and machine.

No matter how good they get, one-way prostheses can't close the loop. In theory, two-way optogenetic traffic could lead to human-machine fusions in which the brain truly interacts with the machine, rather than only giving or only accepting orders. It could be used, for instance, to let the brain send movement commands to a prosthetic arm; in return, the arm's sensors would gather information and send it back. Blue and yellow LEDs would flash on and off inside genetically altered somatosensory regions of the cortex to give the user sensations of weight, temperature, and texture. The limb would feel like a real arm. Of course, this kind of cyborg technology is not exactly around the corner. But it has suddenly leapt from the realm of wild fantasy to concrete possibility.