In a mere half-decade, the use of light to stimulate the brain has moved from basic science to the frontiers of bioengineering. By inserting into brain cells a light-sensitive protein originally found in swamp algae, engineers and scientists have begun to manipulate neurons with a dexterity that could soon vastly outstrip the capabilities of today’s electrical brain stimulation methods. This month, Patrick Degenaar reported early progress toward a non-invasive prosthetic retina that uses light to force retinal ganglion cells to fire on command, presented at the IEEE Biomedical Circuits and Systems conference.
The use of light to control neurons could enable significantly more powerful brain-machine interfaces. First, it allows biomedical engineers to activate chosen sets of neurons, not simply whatever cells happen to be near the stimulation site, as with electrodes. Light can also be used to inhibit a neuron’s firing, whereas electrodes can only stimulate. Most intriguingly of all, the engineering of light-triggered brain cells could begin to pave the way to a hybrid computer that uses an optical link to unite biological and silicon components.
But first, we’d need a light-emitting technology well suited to our neurons. One major engineering challenge is that the light source must emit 1-milliwatt-per-square-millimeter pulses to induce brain cells to fire, according to Degenaar and his colleagues at Imperial College, in London. So they set out to build a gallium nitride micro-LED array on a sapphire substrate that was capable of delivering the proper current density.
They extracted rats’ retinal ganglion cells--which ferry image information from the eye’s rods and cones to other regions of the brain--and inserted Channelrhodopsin-2 (ChRh2), one of the light-sensitive proteins. ChRh2 enables ions to flow into a cell when hit by light of a certain wavelength, causing the neuron to fire. The ChRh2-enhanced neurons were placed on an electrode array that would record their electrical activity.
The researchers' LED chip spells out the word 'optical.'
Each emitter in the 16-by-16 micro-LED array had its own current source, allowing for individual control. For the purposes of the experiment, the engineers sent the patterns to be lit from a computer to the LED chip through a USB connection. When the LEDs were centered on the cell body, the soma, they observed that each 500-millisecond pulse, delivered at a frequency of 1 Hz, indeed produced one neural spike, exactly as they’d hoped.
The next-generation LED array will incorporate on-chip microlenses to spread the light over a wider area, says Degenaar, who has since joined Newcastle University’s electrical engineering department. Eventually they hope to build a headset equipped with a camera that performs a certain amount of image processing before activating neurons optically.
Patients suffering from retinal degeneration fail to send useful signals to the visual cortex when light hits the eye’s photoreceptors. So an enormous and central challenge here will be writing the software to induce appropriate neural patterns. But with the exquisite neural control that optical stimulation allows, engineers have at least brought the goal within sight. Indeed, there’s a certain beauty in having complex, human-engineered optical systems to replace biology’s eyes.