Visual Prosthesis

Visual Prosthesis — often termed a bionic eye — is an implantable bioelectronic system within clinical Neuroprosthetics that restores functional artificial visual perception in blind individuals by delivering spatially patterned electrical stimulation to viable neural relays along the retino-geniculo-cortical pathway. Grounded in the biophysical principle that focal electrical depolarization of visual neurons evokes discrete, retinotopically organized flashes of perceived light (phosphenes) even decades after photoreceptor loss or eye injury, visual prostheses serve two distinct clinical populations: retinal implants for outer-retinal degenerations where inner-retinal relays remain intact, and thalamic or intracortical primary visual cortex (V1) implants for total blindness caused by glaucoma, diabetic retinopathy, optic neuropathy, or ocular trauma in which the optic nerve can no longer convey signals to the brain.

Photoreceptor Loss Versus Surviving Inner-Retinal Relays

Two degenerative conditions cause most untreatable outer-retinal blindness worldwide: inherited retinitis pigmentosa (RP) (~1.5 million people globally via progressive rod-then-cone degeneration) and late-stage dry age-related macular degeneration (AMD) with geographic atrophy, which eliminates macular cones across the central 5°–15° reading field while sparing peripheral vision. Post-mortem morphometry and optical coherence tomography demonstrate that even when >95% of outer rod and cone photoreceptors die, 70% to 80% of inner-nuclear-layer bipolar cells and 30% to 50% of macular retinal ganglion cells (RGCs) survive for decades with intact optic-nerve projections.

Epiretinal Implants and Axonal Passing-Fiber Distortion

Epiretinal prostheses tack a flexible thin-film electrode array onto the inner vitreal surface adjacent to RGC somata and the nerve fiber layer. In Mark Humayun and Second Sight's 60-electrode Argus II system—the first CE-marked (2011) and FDA-approved (2013) retinal prosthesis for end-stage RP (Humayun et al., 2012)—eyeglass camera video and RF power crossed an inductive scleral coil and trans-scleral ribbon cable to 60 platinum-gray electrodes (200 μm diameter).

While Argus II restored doorway detection and mobility orientation, spatial acuity plateaued near 20/1,260 for a biophysical reason modeled by Michael Beyeler, Ione Fine, and colleagues (2019): unmyelinated RGC axons from peripheral retina sweep in arcuate bundles across the epiretinal surface between the electrodes and underlying RGC somata. Suprathreshold pulses excite passing distant axons alongside local somata, stretching intended punctate phosphenes into elongated arcuate streaks.

Subretinal Wireless Photovoltaic Arrays (PRIMA) and Suprachoroidal Arrays

Subretinal prostheses slide ultrathin microchips into the subretinal space vacated by lost photoreceptors, directly facing bipolar cells. Depolarizing graded-potential bipolar interneurons upstream of RGCs harnesses surviving amacrine–ganglion Synaptic Transmission for antagonistic center-surround contrast shaping and burst Action Potential generation, avoids inner-surface axon bundles to preserve punctate retinotopic percepts, and couples image refresh naturally to microsaccadic eye movements.

Eliminating trans-scleral wires that complicated early subretinal implants (Alpha IMS/AMS), Daniel Palanker, José-Alain Sahel, and colleagues engineered the PRIMA wireless subretinal photovoltaic microchip (Palanker et al., 2020, 2022). Measuring 2 × 2 mm × 30 μm, PRIMA tiles 378 hexagonal 100-μm silicon pixels, each combining series photodiodes around a central iridium-oxide active electrode encircled by a local hexagonal return ring that confines electric fields laterally to ~50–100 μm:

1. Pulsed Near-Infrared Augmented-Reality Projection: Pocket processors convert eyeglass camera video into pulsed 880-nm near-infrared (NIR) beams projected onto the macular chip, photovoltaically generating biphasic current in each illuminated pixel without implanted batteries or wires.
2. Simultaneous Prosthetic Central and Natural Peripheral Vision: Because 880-nm NIR light is invisible to human opsins, dry AMD geographic atrophy patients seamlessly integrate 20/460 to 20/438 central prosthetic letter-reading acuity with unperturbed natural peripheral color vision (Palanker et al., 2022).

A third retinal approach, suprachoroidal arrays (Bionic Vision Australia), places electrodes inside a scleral–choroidal pocket outside the vitreous cavity, maximizing surgical safety and stability at the cost of wider current spread across the choroid (~250–500 μm).

Bypassing the Optic Nerve via Occipital Microstimulation

Individuals blinded by end-stage glaucoma, proliferative diabetic retinopathy, optic neuropathy, or ocular trauma lack viable RGCs; neither retinal chips, viral ChrimsonR Optogenetics, nor stem-cell Retinal Organoid grafts can bridge a degenerated optic nerve. Central visual targets remain intact in the thalamic lateral geniculate nucleus (LGN) (accessible via stereotactic leads akin to Deep Brain Stimulation) and primary visual cortex (V1, Brodmann area 17), where foveal cortical magnification expands the central 10° of visual space across ~15–20 cm² of occipital surface.

Whereas early cortical surface Electrocorticography macroelectrodes implanted by Giles Brindley (1968) and William Dobelle required 1 to 5 mA currents that risked phosphene merging and focal afterdischarges (mitigated in modern subdural grids by dynamic sequential letter-tracing steering), penetrating Intracortical Microstimulation (ICMS) lowers thresholds ~100-fold. In a 2021 human trial led by Eduardo Fernández, Richard Normann, and Pieter Roelfsema (Fernández et al., 2021), a 96-channel silicon Utah Microelectrode Array implanted into occipital V1 of a woman blind for 16 years evoked stable punctate phosphenes at 30 to 67 μA with zero seizures—enabling orientation, motion, object-boundary, and block-letter discrimination when driven by a bio-inspired retina encoder.

Why does subretinal PRIMA use 880-nm near-infrared light instead of visible light?

Dry AMD patients lose central macular cones but retain healthy peripheral rods and cones. Projecting 880-nm NIR light—well beyond the ~700-nm sensitivity limit of human photopigments—drives the subretinal silicon photodiodes inside the macular scotoma without dazzling or bleaching surrounding natural peripheral vision.

Why do modern cortical visual prostheses incorporate infrared eye-tracking cameras?

Primary visual cortex maps phosphenes in eye-centered (retinotopic) coordinates, so shifting gaze moves where a stimulated electrode's phosphene appears in space. Real-time pupil tracking updates which region of the camera scene feeds the V1 array on every saccade, stabilizing perceived objects in world coordinates.

1. Humayun, M. S., Dorn, J. D., da Cruz, L., Dagnelie, G., Sahel, J.-A., Stanga, P. E., Cideciyan, A. V., Duncan, J. L., Eliott, D., Filley, E., Ho, A. C., Santos, A., Barra, A., del Priore, L., Greenberg, R. J., & Argus II Study Group. (2012). Interim results from the international trial of Second Sight's visual prosthesis. Ophthalmology, 119(4), 779–788. https://doi.org/10.1016/j.ophtha.2011.09.028
2. Beyeler, M., Nanduri, D., Weiland, J. D., Rokem, A., Boynton, G. M., & Fine, I. (2019). A model of ganglion axon pathways accounts for percepts elicited by retinal implants. Scientific Reports, 9, 9199. https://doi.org/10.1038/s41598-019-45416-4
3. Palanker, D., Le Mer, Y., Mohand-Said, S., Muqit, M., & Sahel, J.-A. (2020). Photovoltaic restoration of central vision in atrophic age-related macular degeneration. Ophthalmology, 127(8), 1097–1104. https://doi.org/10.1016/j.ophtha.2020.02.024
4. Fernández, E., Alfaro, A., Soto-Sánchez, C., Gonzalez-Lopez, P., Lozano, A. M., Peña, S., Grima, M. D., Rodil, A., Gómez, B., Chen, X., Roelfsema, P. R., Rolston, J. D., Davis, T. S., & Normann, R. A. (2021). Visual percepts evoked with an intracortical 96-channel microelectrode array inserted in human occipital cortex. Journal of Clinical Investigation, 131(23), e151331. https://doi.org/10.1172/JCI151331
5. Palanker, D., Le Mer, Y., Mohand-Said, S., & Sahel, J.-A. (2022). Simultaneous perception of prosthetic and natural vision in AMD patients. Nature Communications, 13, 513. https://doi.org/10.1038/s41467-022-28125-x