Scientific Method / Science & Exploration

Sight for sore eyes: optical prosthetic mimics retina functionality

Researchers use retina's own programming language to improve sight restoration.

A retinal implant that uses electrodes to stimulate the retinal neurons, receiving images from a camera mounted on glasses.

The concept of retinal implants evokes fantastical concepts such as Deus Ex-like heads-up displays. Before we get there, though, we’d like to be able to give sight to those who are blind or losing their vision. Existing technology provides only limited improvements to vision, such as allowing people to see spots of light or edges with high contrast. To put that in numbers, we can improve someone’s vision to about 20/1,400; the legal definition for blindness is 20/200.

Retinal implants work by directly stimulating cells in the back of the eye with electrical signals, which get transmitted as visual information to the brain. This works because in many cases of blindness, diseases damage the photoreceptor cells but leave the ganglion cells, the neurons that transmit signals to the brain, intact.

Recent research efforts focused on improving the resolution of the device stimulating the retinal cells. However, higher resolution images can only help so much if the signal doesn’t match the patterns that the retinal neurons expect.

In other words, it's a bit like we've been connecting component video cables to an HDMI port, and wondering why the Xbox signal won’t show up properly on our TV.

In order to solve this problem, Sheila Nirenberg and Chethan Pandarinath of Cornell University developed a new optical prosthetic system that consists of two parts that mimic the functionality of an actual retina. An encoder takes visual information and transforms it into the form needed by the ganglion cells. This functions much like a compiler, converting human-readable source code into machine code needed by the computer. In a healthy retina, this transformation (“visual phototransduction”) is performed by the photoreceptor cells—the rods and cones. The second component is the transducer, which takes the signal from the encoder—now in the proper pattern—and stimulates the ganglion cells, in effect executing the code.

To actually create this optical implant, the authors created a system with three components. An encoder took images as input, converted them into coded electrical pulses which were fed into a minidigital light projector (mini-DLP). This, in turn, sent light pulses to the back of the retina. To make sure there's something there to detect the light, they expressed a light-sensitive protein, channelrhodopsin–2 (ChR2), that activated the ganglion cells in response to light. In effect, they created a new psuedoretina inside the eye to replace the damaged retina.

How did they know that their system was producing the correct retinal code? By hooking up electrodes to the ganglion cells in a mouse retina and playing movies with scenes such as landscapes, faces, and people walking. (Sounds pleasant, no?)

The authors used this setup to compare the activity between a normal retina, a blind retina with the new optical implant, and a blind retina with a standard implant (meaning no encoder). While the new device doesn’t perfectly replicate the activity of a normal retina, the improvement over a traditional implant is noticeable.

In another test, they tracked the eye movement of blind mice by sending signals straight to the transducer. Without any visual stimulus, the eyes of blind mice (like humans) drift around. When stimulated using uncoded signals (effectively standard implants work), the same drifting occurred. When the visual signals were coded, the eyes tracked the stimulus.

This development has the potential to transform optical implants into useful, sight-restoring technology. Obviously, further research is needed, particularly in the area of delivering the ChR2 protein to ganglion cells in human retina, but groups (including the current team) have already done this safely using viral vectors. The new component of this system, the encoder and mini-DLP device, could be placed on a pair of glasses and connected to existing stimulators.

PNAS, 2012. DOI: 10.1073/pnas.1207035109 (About DOIs)

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