Making matters worse, almost all of these implants have to be rigged to wires that snake out of the skull. They are power-hungry, shortening the life of their batteries. Surgeons have also found that the brain attacks these electrodes, covering them with a protective coat of cells that can render them useless. These problems mean that no one in 2010 can expect to carry a brain implant for life.
But none of these challenges is necessarily a showstopper. Scientists are working on new designs that can allow brain implants to shrink in size, use less power, and deliver better performance. In 2009, for example, a team of scientists at MIT succeeded in implanting a wireless electrode into a zebra finch. With the press of a button, the scientists could wirelessly transmit a signal to the song-producing region of the bird's brain. The bird instantly stopped singing.
In 2003, Boyden wondered if there might be a way to use light instead of electricity to communicate with neurons. He knew that algae have light-sensitive channels on the surface of their cells. When a photon hit the channels, they opened, up, allowing charged particles to flow in or out of the cell. Boyden imagined putting those channels on the surface of neurons. Hit a neuron with light and its new channels would open, triggering a signal.
Boyden and his colleagues pinpointed a gene for a channel in a species of algae and inserted it into viruses. They then infected neurons with the engineered viruses. Along with their own genes the viruses inserted the light-channel gene from the algae. Because the viruses were harmless, the neurons did not suffer from the infection. Instead they started using the algae gene to build channels of their own.
Boyden exposed the infected neurons to a flickering blue light. The neurons responded by crackling with spikes of voltage. "We started playing around with it and we got light-driven spikes almost on the first try," he said. "It was an idea whose time had come."
In 2005 Boyden and his colleagues published that experiment, and since then he has expanded his neuroengineer's toolkit. "We grow organisms to screen for new molecules," he told me. They went on to find new genes for receptors sensitive to different wavelengths of light. Now their engineered neurons can respond to a rainbow of signals. Boyden can get the neurons to produce a voltage spike in response to light or to go completely quiet. He can flash a particular pattern of lights to trigger signals. He can also give different types of neurons different channels by adding specific genetic handles to the channel DNA. The genes get inserted into lots of cells, but they get switched on in only one kind of neuron. Flashing different colors of light, he can switch on and shut down different groups of neurons all at once.
I visited Boyden as he was starting to see how his engineered neurons behave in real brains rather than in petri dishes. One virus he selected for his experiments, known as an adeno-associated virus, proved to be promising in human gene-therapy trials in other labs. As of 2010, it had safely delivered genes into the bodies of more than 600 people (though some of the genes have produced unintended side effects). Boyden and his colleagues successfully infected certain neurons in the brains of monkeys without causing harm to the animals. The scientists inserted an optical fiber into the monkey brains and were able to switch the neurons on with flashes, just as they do in a petri dish.
At Boyden's talk at the Singularity to Summit he unveiled a particularly stunning experiment he and his colleague Alan Horsager have run at the University of Southern California on congenitally blind mice: They infected the animals with sight.
The mice were blind thanks to mutations in the light-receptor genes in their retinas. The team wondered if they could make those neurons sensitive to light again. They loaded genes for light-sensitive channels onto viruses and injected them into the mice. The genes were targeted for the retinal neurons missing their own light receptors. Boyden gave the mice time to incorporate the genes into their eyes and, he hoped, make the channels in their neurons. Since mice can't read eye charts out loud, he and his colleagues had to use a behavioral test to see if their eyes were working. They put the mice into a little pool with barriers arranged into a maze. At one end of the pool the mice could get out of the water by climbing onto an illuminated platform. Regular mice quickly followed the light to the platform while blind mice swam around randomly. When Boyden and his colleagues put the mice infected with neuron channels in the pool, they headed for the exit almost as often as the healthy mice. As far as Boyden and his colleagues can tell, the mice can see again.