Through a glass window implanted in an opening in the rats' skulls, Moore and Boyden can shine laser light onto a few tiny blood vessels at a time. And by inserting an electrode into the brain, Moore can monitor the electrical activity of individual neurons as he turns blood flow up and down in the vessels they border. "Now I can test what happens if I turn up this faucet and turn down that one," he says. "What happens to the neurons right next to it?"

He's looking for evidence that blood flow is fine-tuning neuronal activity: "Are they firing ten spikes now when they used to fire five? Or are they firing two spikes where they used to fire five?" Either case would suggest that blood flow could play a role in neural dynamics. The work is unpublished and is in its early stages, so Moore won't disclose what he has seen. But he will say that he thinks the preliminary results provide evidence of a regulatory role for blood.

Figuring out what happens to individual neurons when blood flow is altered involves considerable technical finesse. Moore and Boyden are now building a fiber-optic system that will enable them to shine laser light into the brains of rats inside an fMRI machine, so they can observe what happens to the entire brain when they manipulate blood flow. They're also imaging individual neurons.

"Every step has been solved, but putting them together"--fiber optics, fMRI, microscopy, electrode readings--"is going to be a technical challenge," says Moore.

If Moore and Boyden's work on optically controlled smooth-muscle cells succeeds, it could have much broader implications than just illuminating the role of blood in the brain. If blood flow does contribute to epileptic seizures, it might be possible to develop an implant that uses laser light to control them.

"You never know the reality of these things when they get beyond the bench, but this is about the most promising research I've been associated with," says Moore. "This is really cool stuff. It's a fun time to be in the lab, because any day you could learn something entirely new."