The first patient perceived an activated U -shaped electrode pattern as an H . The second patient correctly identified a square electrode pattern as a match box . Moving the stimulating electrode centripetally from an extramacular to a macular region consistently demonstrated lower thresholds for the macular region. Electrical stimulation of normal retina resulted in the perception of a dark oval spot, whereas that of the retina ablated with krypton red laser and histologically verified limited outer retinal destruction resulted in seeing a focal light percept. Electrical stimulation of the full thickness argon green laser ablated retina with only ganglion cell axons present histologically resulted in the perception of a yellow linear streak.
Macular regions have lower electrical stimulation thresholds than extramacular regions.
From the laser ablated retinal tests, the site of electrical stimulation using epiretinal electrodes appears to be the inner nuclear layer. Each electrode will be capable of stimulating four-bit gray levels. The device will also have a bi-directional telemetry capability allowing for wireless stimulation, diagnostic, and recording capabilities. The retinal prosthesis under development is based on the concept of replacing photoreceptor function with an electronic device. In a healthy retina the photoreceptors initiate a neural signal in response to light.