Retinal implants and visual prostheses: revision 1 to 3

Revision 1

Revision 3

categories: ["cybernetics"]

tags: ["vision", "sensory restoration", "implants", "blindness", "neuroprosthetics", "phosphenes"]

summary: "Implanted devices that restore rudimentary vision to blind people by electrically stimulating surviving retinal neurons or, more invasively, the visual cortex."

summary: "Implanted devices that restore rudimentary vision to blind people by electrically stimulating surviving retinal neurons or, more invasively, the visual cortex."

updated: "2026-07-27"

updated: "2026-10-29"

humanEvidence: "Devices have been implanted in blind people for over a decade; the strongest result is a 2025 European trial in which most PRIMA recipients identified letters and read words under electronic magnification."

humanEvidence: "Devices have been implanted in blind people for over a decade; the strongest result is a 2025 European trial in which most PRIMA recipients identified letters and read words under electronic magnification."

access: "Both commercially approved devices have been withdrawn and existing recipients have lost manufacturer support; access now runs through clinical trials, chiefly of the PRIMA subretinal system, at device-plus-surgery costs among the highest in ophthalmology."

access: "Both commercially approved devices have been withdrawn and existing recipients have lost manufacturer support; access now runs through clinical trials, chiefly of the PRIMA subretinal system, at device-plus-surgery costs among the highest in ophthalmology."

reversibility: "difficult"

{ "label": "Approaches", "value": "Epiretinal, subretinal, cortical" },

{ "label": "First CE-marked device", "value": "Argus II, 2011" },

{ "label": "US authorization", "value": "2013, humanitarian exemption" },

{ "label": "Electrode counts", "value": "60–400 (retinal)" },

{ "label": "Electrode counts", "value": "60-400 (retinal)" },

{ "label": "Best demonstrated acuity", "value": "Well below 20/200" },

{ "label": "Commercial status", "value": "Two leading devices withdrawn" },

{ "label": "Readiness", "value": "TRL 6" }

**Retinal implants and visual prostheses** are devices that produce visual sensation in blind people

by stimulating the surviving parts of the visual pathway with electric current. They target

conditions in which the photoreceptors have died but the downstream retina and optic nerve remain

largely intact — chiefly retinitis pigmentosa and the geographic atrophy form of age-related macular

largely intact, chiefly retinitis pigmentosa and the geographic atrophy form of age-related macular

degeneration. Every device fielded so far produces vision built from phosphenes, discrete spots of

light with no colour and little detail, and no system has restored acuity anywhere near the legal

threshold for blindness.

A camera, usually mounted on spectacles, captures a scene. A processor reduces the image to a

low-resolution activation map and transmits it to an implanted array, which converts each pixel into

a current pulse delivered to nearby retinal neurons. Because the retina is a layered circuit rather

a current pulse delivered to nearby retinal neurons. The retina is a layered circuit rather than a

than a screen, where the electrodes sit changes what they excite.

screen, so where the electrodes sit changes what they excite.

**Epiretinal** arrays sit on the inner retinal surface and stimulate retinal ganglion cells, the

output neurons whose axons form the optic nerve. This bypasses all intervening retinal processing,

output neurons whose axons form the optic nerve. This bypasses all the intervening retinal

so the signal that reaches the brain is not the code the retina would normally send; axons of

processing, so the signal that reaches the brain is not the code the retina would normally send.

passage from distant regions are also stimulated, producing elongated or displaced phosphenes.

Axons of passage from distant regions are stimulated as well, producing elongated or displaced

Argus II was the leading epiretinal device.

phosphenes. Argus II was the leading epiretinal device.

**Subretinal** arrays sit under the retina in the space the dead photoreceptors vacated and

**Subretinal** arrays sit under the retina, in the space the dead photoreceptors vacated, and

stimulate bipolar cells, letting the remaining retinal circuitry perform some of its normal

stimulate bipolar cells, which lets the remaining retinal circuitry carry out some of its normal

processing. Photovoltaic designs go further: each pixel is a photodiode that converts projected

processing. Photovoltaic designs go further. Each pixel is a photodiode that converts projected

light into local current, so the implant needs no cables and the eye's own movements scan the image

light into local current, so the implant needs no cables, and the eye's own movements scan the image

across the array, preserving the natural link between gaze and percept.

**Cortical** devices skip the eye entirely and stimulate primary visual cortex, which makes them the

**Cortical** devices bypass the eye entirely and stimulate primary visual cortex. That makes them

only option for people whose optic nerve is destroyed by glaucoma or trauma. They are also the most

the only option for people whose optic nerve has been destroyed by glaucoma or trauma. They are also

invasive, they require a craniotomy, and the retinotopic map they must exploit is folded into a

the most invasive and require a craniotomy, and the retinotopic map they must exploit is folded into

sulcus, so much of it is difficult to reach with a surface array.

a sulcus, so much of it is hard to reach with a surface array.

```compare

{

## Development history

Electrical stimulation of the occipital cortex was known to produce phosphenes from work in the

Mid-twentieth-century work showed that electrical stimulation of the occipital cortex produces

mid-twentieth century, and Giles Brindley and William Lewin implanted an 80-electrode array over

phosphenes. In 1968 Giles Brindley and William Lewin implanted an 80-electrode array over visual

visual cortex in a blind volunteer in 1968, demonstrating that patterned percepts were possible and

cortex in a blind volunteer, demonstrating that patterned percepts were possible and that the

that the engineering was far from ready. Retinal approaches, which require far less surgery,

engineering was far from ready. Retinal approaches, which require far less surgery, dominated from

dominated from the 1990s.

the 1990s.

```timeline

[

  { "year": "1968", "title": "First cortical visual prosthesis", "text": "Brindley and Lewin implant an array of surface electrodes over the occipital cortex of a blind volunteer, who reports discrete phosphenes at predictable locations." },

  { "year": "1968", "title": "First cortical visual prosthesis", "text": "Brindley and Lewin implant an array of surface electrodes over the occipital cortex of a blind volunteer, who reports discrete phosphenes at predictable locations." },

  { "year": "2011", "title": "Argus II CE-marked", "text": "Second Sight's 60-electrode epiretinal implant is approved in Europe for retinitis pigmentosa, followed by a US humanitarian device exemption in 2013." },

  { "year": "2011", "title": "Argus II CE-marked", "text": "Second Sight's 60-electrode epiretinal implant is approved in Europe for retinitis pigmentosa, followed by a US humanitarian device exemption in 2013." },

  { "year": "2013", "title": "Subretinal photodiode arrays", "text": "Retina Implant AG's Alpha IMS, with roughly 1,500 light-sensitive pixels, receives European approval; the company later ceases operations." },

  { "year": "2013", "title": "Subretinal photodiode arrays", "text": "Retina Implant AG's Alpha IMS, with roughly 1,500 light-sensitive pixels, receives European approval; the company later ceases operations." },

  { "year": "2019–2022", "title": "Commercial collapse", "text": "Second Sight halts Argus II production and nearly winds down; Retina Implant AG closes. Implanted patients lose software support and repair paths." },

  { "year": "2019-2022", "title": "Commercial collapse", "text": "Second Sight halts Argus II production and nearly winds down; Retina Implant AG closes. Implanted patients lose software support and repair paths." },

  { "year": "2020–2021", "title": "Cortical stimulation revisited", "text": "A Utah array in the visual cortex of a blind volunteer supports letter identification, and a 1,024-channel array in monkey V1 evokes recognizable shapes." },

  { "year": "2020-2021", "title": "Cortical stimulation revisited", "text": "A Utah array in the visual cortex of a blind volunteer supports letter identification, and a 1,024-channel array in monkey V1 evokes recognizable shapes." },

  { "year": "2024–2025", "title": "Photovoltaic results and new ownership", "text": "Pixium Vision enters receivership and its PRIMA subretinal system is acquired by Science Corporation; trial results reported in 2025 describe letter and word reading with electronic zoom in most implanted participants." }

  { "year": "2024-2025", "title": "Photovoltaic results and new ownership", "text": "Pixium Vision enters receivership and its PRIMA subretinal system is acquired by Science Corporation; trial results reported in 2025 describe letter and word reading with electronic zoom in most implanted participants." }

]

```

## Current state

As of 2026 the field is in an unusual position: its two commercially approved devices have been

As of 2026 the field is in an unusual position. Its two commercially approved devices have been

withdrawn, and its most promising results come from a system whose developer went insolvent.

The PRIMA subretinal photovoltaic chip is a wireless array of a few hundred pixels implanted under

the macula, driven by near-infrared images projected from augmented-reality glasses. Feasibility

the macula and driven by near-infrared images projected from augmented-reality glasses. Feasibility

work in geographic atrophy showed that patients could perceive prosthetic vision in the atrophic

central field while retaining their natural peripheral vision, an unusual property of the

central field while keeping their natural peripheral vision, an unusual property of the

approach.[^palanker2020] Results from the pivotal European trial, reported in 2025, described most

implanted participants regaining the ability to identify letters and read words when the projected

image was electronically magnified. This is the strongest functional result any visual prosthesis

has produced, and it should be read carefully: reading with a zoom function at a few pixels per

has produced, and it needs careful reading: reading with a zoom function at a few pixels per letter

letter is not restored sight.

is not restored sight.

Cortical work has been rebuilt on modern intracortical hardware. A 96-channel [[utah-array]]

[[cortical-visual-prosthesis|Cortical work]] has been rebuilt on modern intracortical hardware. A 96-channel [[utah-array]]

implanted in the visual cortex of a blind volunteer produced stable, reproducible phosphenes and

supported identification of simple letter shapes.[^fernandez2021] A high-channel-count array in

monkey V1 evoked percepts of shapes and letters assembled from many simultaneous

phosphenes.[^chen2020] Both establish feasibility; neither approaches a usable device.

phosphenes.[^chen2020] Both establish feasibility, and neither comes close to a usable device.

## Why resolution is the binding constraint

The intuition that a prosthesis with *n* electrodes yields an *n*-pixel image is wrong in a way that

It is tempting to assume that a prosthesis with *n* electrodes yields an *n*-pixel image. The

governs the whole field. Current spreads, so adjacent electrodes excite overlapping populations and

assumption is wrong, and the ways it fails govern the whole field. Current spreads, so adjacent

phosphenes merge. Epiretinal stimulation activates axons passing over the array from other retinal

electrodes excite overlapping populations and their phosphenes merge. Epiretinal stimulation

regions, so a phosphene may appear far from the stimulated site and be drawn out into a streak.

activates axons passing over the array from other retinal regions, so a phosphene may appear far

Percepts fade under sustained stimulation, so images must be refreshed. And the phosphene map is

from the stimulated site and be drawn out into a streak. Percepts fade under sustained stimulation,

idiosyncratic to each patient and must be characterized individually.

so images must be refreshed. The phosphene map is also idiosyncratic to each patient and has to be

characterized individually.

Simulation studies of prosthetic vision in sighted observers suggest that face recognition and

fluent reading require on the order of hundreds to thousands of well-separated, independently

controllable phosphenes. Fielded retinal devices have delivered tens. The gap is not merely

controllable phosphenes. Fielded retinal devices have delivered tens. Manufacturing alone cannot

manufacturing: packing electrodes more densely without proportionally increasing current confinement

close the gap. Packing electrodes more densely without a matching gain in current confinement does

does not increase the number of distinguishable percepts, the same saturation that limits the

not increase the number of distinguishable percepts; the same saturation limits the

[[cochlear-implant]] to roughly eight effective channels.

> [!caution] What "restored sight" means in this field

> Reported successes are usually object localization, motion detection, and letter identification

> Reported successes are usually object localization, motion detection and letter identification

> under favourable conditions. Users typically retain their cane and continue to rely on other

> under favourable conditions. Users typically keep their cane and continue to rely on other senses.

> senses. Press coverage that describes patients as "seeing again" overstates every result published

> Press coverage that describes patients as "seeing again" overstates every result published to

> to date.

> date.

## Risks and the abandoned-patient problem

Surgical risks include retinal detachment, hypotony, conjunctival erosion over the implant, and

endophthalmitis; cortical devices add the risks of craniotomy and of a percutaneous connector where

endophthalmitis. Cortical devices add the risks of craniotomy and, where one is used, of a

one is used. Long-term electrode-tissue stability is limited by the same foreign-body response that

percutaneous connector. Long-term electrode-tissue stability is limited by the same foreign-body

degrades cortical recordings in a [[brain-computer-interface]].

response that degrades cortical recordings in a [[brain-computer-interface]].

The distinctive risk is commercial. When Second Sight stopped supporting Argus II, several hundred

implanted patients were left with devices that could not be repaired or upgraded, and some lost

function when hardware failed. The episode was widely reported and has become the reference case for

function when hardware failed. The episode was widely reported. It has become the reference case for

the argument that manufacturers of implanted neurotechnology incur obligations that outlast their

the argument that manufacturers of implanted neurotechnology take on obligations that outlast their

business models.[^spectrum2022] Proposed remedies include escrowed device documentation, mandated

open-sourcing on discontinuation, and explicit disclosure of company-failure risk in consent

documents. None is required by regulators as of 2026, and the same exposure applies to every

documents. As of 2026 regulators require none of them, and the same exposure applies to every

implant discussed under [[neuroprosthetics]] and to the emerging commercial implants covered by

[[neurorights]] debates.

Cost compounds the exposure. Device, vitreoretinal surgery, and the months of rehabilitation needed

Cost adds to the exposure. The device, vitreoretinal surgery, and the months of rehabilitation

to learn to interpret phosphenes place these systems among the most expensive interventions per

needed to learn to interpret phosphenes put these systems among the most expensive interventions per

patient in ophthalmology, for a functional gain that is real but small — the trade-off examined

patient in ophthalmology, for a functional gain that is real but small. That trade-off is examined

under [[access-and-inequality]]. Parts of the blind community also reject the framing of blindness

as a defect awaiting correction, an argument developed at length under

as a defect awaiting correction. The argument is developed at length under

[[disability-rights-and-enhancement]] and less prominent here than in deafness only because

[[disability-rights-and-enhancement]]; it is less prominent here than in deafness only because

prosthetic vision has never worked well enough to force the question.

## Alternatives and outlook

Prostheses compete with biological approaches that did not exist when the field began.

Prostheses now compete with biological approaches that did not exist when the field began.

[[somatic-gene-therapy]] delivered by [[aav-vectors]] is approved for one inherited retinal

dystrophy caused by *RPE65* mutations, which slows or partially reverses loss in a small patient

dystrophy caused by *RPE65* mutations, where it slows or partially reverses loss in a small patient

population; it treats a cause rather than substituting for the sense. [[optogenetics]] offers a middle path — making surviving retinal

population; it treats a cause instead of substituting for the sense. [[optogenetics]] offers a

ganglion cells directly light-sensitive, then driving them with goggles that convert scenes into

middle path, making surviving retinal ganglion cells directly sensitive to light and then driving

patterned light. Partial visual function was reported in a single patient with retinitis pigmentosa

them with goggles that convert scenes into patterned light. Partial visual function was reported in

in 2021, and other opsin-based programmes have since been tested in early trials.[^sahel2021]

a single patient with retinitis pigmentosa in 2021, and other opsin-based programmes have since been

Photoreceptor and pigment-epithelium replacement derived from

tested in early trials.[^sahel2021] Photoreceptor and pigment-epithelium replacement derived from

[[induced-pluripotent-stem-cells]] has been attempted in small numbers of patients for macular

degeneration, and retinal organoids grown by the methods described under

degeneration. Retinal organoids grown by the methods described under [[tissue-engineering]] are a

[[tissue-engineering]] are a source of transplantable cells rather than a route to a working eye.

source of transplantable cells rather than a route to a working eye.

None of these will help someone whose retina is entirely gone or whose optic nerve is severed, which

is why cortical prostheses continue despite their difficulty. The honest near-term expectation is a

is why work on cortical prostheses continues despite their difficulty. A realistic near-term

subretinal device that gives central-field pattern vision to people with geographic atrophy, a large

expectation is a subretinal device that gives central-field pattern vision to people with geographic

and growing population, alongside continued failure to deliver anything resembling normal sight. The

atrophy, a large and growing population, alongside continued failure to deliver anything resembling

unresolved scientific question is whether the visual system can learn to interpret an artificial

normal sight. The unresolved scientific question is whether the visual system can learn to interpret

code at all, or whether prosthetic vision is permanently limited to what a naive read-out of

an artificial code at all, or whether prosthetic vision is permanently limited to what a naive

phosphenes can convey — a question that [[neural-decoding]] research on the write side of the

read-out of phosphenes can convey. [[neural-decoding]] research on the write side of the interface

interface has barely begun to address.

has barely begun to address it.

## See also