A remarkable milestone has been reached in the resolution of retinal implants – a whopping 1520 pixels (38×40)! Following on the heels of a recent success of Argus II retinal implants developed by the Second Sight, this implant by the German Retina Implant AG brings a 25-fold increase in resolution and several other unique features. Its subretinal placement is closer to the retinal pigment epithelium than can be achieved with epiretinal placement. This provides more selective stimulation of photoreceptors and results in further improvement in the implant’s resolution. The light sensing circuitry (silicon photodiodes) is built into the implant allowing it to move along with the eye movements. This is beneficial for more natural cortical processing of visual information, as the visual map in the visual is adjusted during each saccade. Other types of retinal implants use an external videocamera (usually mounted on the glasses) that does not adjust the video information during the eye movements. The implant is 3 x 4 mm and 50 µm thick. In addition to vision restoration, the implant provides a first-ever vision-enhancing capability – the sensitivity to near-infrared light. Extending the spectrum of perceived light can have some interesting implications, such as the ability to see a thermal shape of the object (the black body radiation) even in complete darkness. The ongoing research by Prof. Eberhart Zrenner at the University of Tuebingen aims to evaluate these implants to develop strategies for further improvements in the sensitivity and targeting of the implants. According to the paper published in the November issue of Proceedings of Royal Society B, the implants have been tested in three patients with hereditary retinal degeneration. All patients could locate bright objects on a dark table, and one patient discerned shades of grey with only 15% contrast. An important question for the retinal implant community, so far not answered by the study, is: how many pixels in the implant provide truly unique information to the retina and whether this spatial threshold has been reached with a 70-µm spacing used in the implant. The answer to this question has far-reaching implications for further technology developments: 1) whether further improvements in the density of planar arrays will translate into more focal stimulation and 2) whether the stimulating sites should be microfabricated to extend from the chip toward the retina in order to achieve the intended 70-µm spatial resolution.
A remarkable milestone has been reached in the resolution of retinal implants – a whopping 1520 pixels! In addition to vision restoration, the implant provides a first-ever vision-enhancing capability – the sensitivity to near-infrared light.