With the UCSC device, scientists can precisely control individual retinal ganglion cells, a capability that will be key in next-generation implants. One of the reasons the prostheses currently in human testing have limited resolution is that they stimulate hundreds of cells simultaneously. (The diameter of the electrodes is an order of magnitude larger than that of most cells.) The five-micrometer-diameter electrodes in Litke’s chip are on par with the size of retinal ganglion cells, allowing them to stimulate individual cells. The researchers previously showed that they could simultaneously control multiple cells with a 60-electrode version of the chip, and they are developing a version with 512 electrodes.
Now that scientists have created a technology with such a precise level of control, they are using it to study the language of the retina–a language they hope prostheses will ultimately be able to speak. While the retina is often likened to a camera, it is in reality much more complicated. Light signals are captured and processed in the retina; the sequences of electrical bursts sent to the brain by the various and distinct retinal ganglion cell types encode different aspects of the visual field, such as movement, spatial patterns, color. Current prostheses use a simplified code and thus lose information, just as Morse code loses the nuanced intonations of the spoken word and the facial expressions of the speaker. “What are the patterns that really emulate what the healthy retina would be doing?” asks Alexander Sher, an assistant researcher at UCSC who is collaborating with Litke. “If you get to the point where you can stimulate individual cells, and you know how individual cells encode information, you can simulate that exactly, or nearly exactly.”
Scientists at Second Sight say that the lessons learned from these studies will be crucial to the development of next-generation prostheses. But turning the UCSC researchers’ device into an implant fit for the human eye will be challenging. “A lot of technical considerations are preventing us from jumping to really tiny electrodes,” says McMahon. “That will require further developments in electronics and packaging and software.”