Welcome to the switchBoard official Blog!

SwitchBoard is an In Innovative Training Network (ITN) funded by the European Commission's Horizon 2020 programme under the Marie Curie Actions. The duration of the project is 48 months, starting on November 01, 2015.

The switchBoard consortium brings together eleven beneficiaries from eight different countries, combining the expertise of seven academic partners with excellent research and teaching records, one non-profit research organisation, and three fully integrated private sector partners. This European Training Network (ETN) is supported by six Partner Organisations as well as a management team experienced in multi-site training activities and counselled by a scientifically accomplished advisory board.

Taken together, the switchBoard training network provides an international, interdisciplinary platform to educate young scientists at the interface of neurobiology, information processing and neurotechnology.

Monday, 18 November 2019


Retinal side of optical illusions

Optical illusions are artifacts of sensory processing.  Though misperceptions, they highlight principles and objectives that govern normal neural processing. Therefore, a comprehensive theory of vision should be able to account for optical illusions. Here I will tell about retinal mechanisms hacked by such types of stimuli.
I strongly recommend arming yourself with a plan of the retina from a Figure 1.

The retina has to provide the rest of the body with sufficient information about the visual environment. On the other hand, we would like it to consume as little energy as possible. To achieve both of these goals, the retina has to be efficient and transmit only important information.  

The visual environment is an aggregate of figures and backgrounds. We are interested in their separation i.e. evaluating differences between them. A way to perform this task efficiently is to transmit information about contrast instead of an absolute light intensity. Contrast characterizes how objects differ in a reflecting light. This is an internal property of an object, thus it allows discriminating objects regardless of the absolute intensity of the incident light. Hence, there is no need to send information about it and this optimizes retinal performance.

How to estimate contrast? Figures are surrounded by other figures and background, so we estimate contrast by subtracting the signal of a given photoreceptor from the signals collected by its neighbors. This process is mediated by horizontal and amacrine cells and called “lateral inhibition”. Consequentially, bipolar cells, which collect signals from photoreceptors, and ganglion cells, which transmit it further to the brain, exhibit antagonistic center-surround receptive field arrangements. That is, these cells are excited by stimuli in their centers, and inhibited by ones in their surround. So essentially they transmit difference between center and surround.

The illusion shown in Figure 2 exploits this processing strategy. Although painted uniformly throughout, the rectangle shown appears lighter on the left and darker on the right. As we know now, the illusion occurs because instead of perceiving an objects absolute brightness we perceive how it differs from its surroundings. As the background is dimmer on the left and lighter on the right, the contrast between background and rectangle is higher leftwards. Thus, object appears brighter.

Another illusion based on this principle - perceiving not intensity, but contrast, is Hermann grid. In Figure 3, with a quick glance we can note grey blobs on the intersections of the grid.
There is a sudden increase in background luminance when we shift our gaze to an interception.
So excitation of the receptive field center remains the same, whereas inhibitory signals from the background suddenly become stronger, thereby darkening the perceived shade of the interception.
It is notable that is even simplest retina-inspired neuromorphic devices (Mahowland and Mead 1991) are susceptible to Herman grid illusion. It highlights that illusory effects arise from the very basics, almost nuts and bolts, of retinal processing. 

Light Adaptation

Although lateral inhibition removes information about absolute light intensities from the photoreceptor outputs, they still need to deal with huge variations in the level of their inputs. The reason for this is very simple: light intensity can vary over 7 orders of magnitude, but the response range of photoreceptors is quite limited. To overcome this issue photoreceptors decrease their sensitivity upon stimulation. This process underlies afterimage illusions.

Let’s consider the image in Figure 4. If you look at it for 30 seconds to 1 minute and then direct your gaze to a white object, you will perceive cyan inscription over a magenta background. This is called an “afterimage”. It is a retinal effect, so if you will move your eyes along a white object, the cyan inscription will move as well. Humans possess three types of photoreceptors for color vision: red, green and blue. When you stare at a red inscription, red-sensitive photoreceptors gradually decrease their sensitivity. That’s why white objects stimulate them to a lesser extent than it does with green and blue-sensitive photoreceptors. We perceive color as a relative activation of opponent channels: red versus green, yellow versus blue, and white versus black. So decreased activation of red channel shifts the overall color picture to its complementary color, that is - cyan. The magenta color of the background can be explained by similar logic.

Dress & Color Constancy

Back in a day, the dress in figure 5 caused a lot of dispute about its color on the internet.
Despite the true color of the dress being blue and black, roughly 40% of people perceive it as white and gold or blue and gold (Lafer-Sousa et al. 2015). This photo highlights stunning and bizarre individual differences in color perception.

Spectral composition of a light reflected by an object, also known as color, is one of its internal properties. Thus, it can help with object identification. Hence, we would like to minimize the influence of illumination on its perception. Indeed, even though evening illumination is quite red compared to midday light, the perceived colors of your car or of tree leaves remain unchanged. This phenomenon is called color constancy, and it is this mechanism that is confused by the dress.
To have the colors of an object constant under various illuminations we simply need to discard illumination. According to experimental data, that is where differences in the dress color perception arises. First of all (Aston and Hurlbert 2017), people who see the dress as blue and black have a tendency to estimate illumination as yellowish, and those who perceive the dress as white and gold think that illumination is blue. Moreover, when the cues to the illumination are enhanced (Lafer-Souza et al. 2015) ambiguity in a dress perception disappears.
Although color constancy engages various cortical mechanisms, essentially it is a figure-background segregation task. Hence, it roots lay in a lateral inhibition, which initiates within the retina and is repeated at the various levels in the visual cortex.

Perceiving the Present

In contrast to our eukaryotic brothers, we animals are able to move and movement vastly increases the pace of life. This is why the nervous system emerged: to guide, coordinate and navigate us.  As Pavlov brilliantly demonstrated, the nervous system anticipates rather than reacts. In an ever-changing world, being ready for today means being late because preparation takes time.
Vision is there to create a map of an environment. Signal processing and transmission take time. That leads to a delay of roughly 100ms between stimulus onset and elicited perception.
This is mainly due to the slow transduction processes of photoreceptors that transduce photons into electrical signals. The visual system needs to compensate for this delay somehow if we really want to perceive the present.
According to Changizi et al. (Changizi et al. 2008) confusion of this mechanism leads to a vast number of optical illusions, like the classical geometrical illusions.
A combination of these illusions (Herring, Orbison and Ponzo) are shown in Figure 6. Motion leads to a displacement of the objects. In this case, compensation for the delay is critically important. To compensate, we need to estimate how the scene will look in the following 100 ms. To do so we use some cues to figure out the direction and speed of movement and then to model how it will affect an object.
The optical flow induced by motion engenders radial “smear’’, which is mimicked by the radial display shown in Figure 6.  It makes us think we move forward. What we see is not an actual picture, but how it will look like in the next moment. When you pass through the door, its opening goes sideways. That’s why the vertical lines in Herring illusion appears non-parallel. Surprisingly, we become closer to objects in a direction of motion while moving. As a result, the retinal area on which their image is projected, also known as angular size increases. This causes distortion of the squares shape in Figure 6, a phenomena referred to as an Orbison illusion.  Same reason underpins Ponzo illusion as well.

Figure 6   M. A. Changizi et al./Cognitive Science 32 (2008)

This activity moves at the true location of the object or even along its leading edge (Berry et al. 1999). It means that the delay is already compensated for on the level of retinal output. The underlying mechanism is as follows: Ganglion cell activation reports that an object is passing by. Earlier we discussed receptive fields - areas from which neurons collect their inputs. Receptive fields are extended in space. Therefore, a moving object activates some of the ganglion cells ahead of its motion. It activates ganglion cells when just crosses the edge of the receptive field, but not completely passed by yet.  So in a way, the spatial extension of the ganglion cell receptive fields compensates for the delay.  One might note that such spatial extension alone is not enough for a proper delay cancelation. Since it also implies activity of ganglion cells when object have already passed by. This issue is overcome by using transient cells as they only activate briefly when an object crosses their receptive field, then quickly return to their non-activated state.
Intuitively and based on a personal experience with tracking objects, we might suggest that the quality of extrapolation and delay cancelation should depend upon a contrast, which is indeed the case. While interesting, this is not a very surprising result. We already know that due to lateral inhibition, the signal fed into ganglion cells is simply contrast. Weak signals i.e. weak contrast will just fail to activate ganglion cell in a proper way regardless of the speed, direction or any other parameter of motion.
There are more than 20 different types of ganglion cells. Each type senses its particular feature. However, regardless of those features all ganglion cells inevitably sense contrast and this sometimes creates a mess. For instance, let’s consider the next example, the “Plaid stimulus”. This stimulus consists of two gratings passing through each other. When asked about the direction of plaid motion we Homo sapiens show a bias towards grating with the higher contrast as this contrast causes stronger activation of the corresponding direction-selective cells. This subset of ganglion cells only activate when an object moves in a certain direction along vertical and horizontal axes.
There is also a certain type of retinal ganglion cells, which detect an object approaching.
It is contrast-sensitive as well. I suppose that’s why the left part of a rectangle in Figure 7 appears slightly closer.  Indeed, higher contrast causes stronger activation of approach-sensitive cells in the part of the retina where an image of this object is projected. So in terms of ganglion cell firing rates, it looks like the stimulus on the left is approaching us.
 As you remember, to perceive the present the visual system has to model how a scene will change in the next moment. And, as you might also note, objects which we approach usually becomes closer.  In addition, its angular size increases. That’s why the circle with the higher contrast in Figure 7 looks bigger.

Figure 7  M. A. Changizi et al./Cognitive Science 32 (2008)


Here I discussed some neuronal processes behind optical illusions. Namely light adaptation, lateral inhibition and compensation of neuronal delays.  These are all very general, basic and subliminal mechanism of visual processing. The bottom line here is that optical illusions stem from the organization of the nervous system itself, from its ultimate limitations and concerns. These incorrect-perceptions hack-strategies in most cases are consequences of optimizing neuronal performance. So perception of illusions is a downside of neuronal efficiency, which meanwhile nicely illustrates this efficiency.

Written by: Matthew Yedutenko


Aston, S., & Hurlbert, A. (2017). What #theDress reveals about the role of illumination priors in color perception and color constancy. Journal of Vision, 17(9):4, 1–18, doi:10.1167/17.9.4.

Berry, M.J., 2nd, Brivanlou, I.H., Jordan, T.A., and Meister, M. (1999). Anticipation of moving stimuli by the retina. Nature 398, 334–338.

Changizi M.A., Hsieh A., Nijhawan R., Kanai R., Shimojo S. Perceiving the Present and a Systematization of Illusions. Cognitive Science 32 (2008) 459–503 

Lafer-Sousa, R., Hermann, K. L., & Conway, B. R. (2015). Striking individual differences in color perception uncovered by ‘‘the dress’’ photograph. Current Biology, 25(13), R545–R546.

Mahowald Misha, Mead Carver “The Silicon Retina” Scientific American, May 1991

Monday, 11 March 2019


The switchBoard project - a review


More than two years are over since the first annual meeting in Tübingen in 2016 October, where we first met each other. We are 15 PhD-students from different parts of the world brought together in one Innovative Training Network, the switchBoard (http://www.etn-switchboard.eu/), supervised by leading scientists in the field. In this blog article we want to go through the timeline of our program to look back on some of the opportunities we got as being a part of this network.

October 2016: During our first meeting, we got to know each other, the projects and scientific backgrounds of the other early stage researchers (ESRs). It was really exciting to meet people with different cultural backgrounds but the same interest for the retina. During the same meeting, we also got the opportunity to visit the Science Center Experimenta, which was nearby Tübingen, to get some ideas for our museum exhibit, which was going to be the combined outreach activity for the second year of all ESRs. We had a lot of fun but also got many ideas from this daytrip.

May 2017: ESR Gemma Taverni and her colleagues from iniLabs organized a workshop on their Dynamic Vision Sensor (DVS) camera. Through this workshop the ESRs were able to gain knowledge about the DVS camera and other ongoing research in their labs.

June 2017: A month later, we had another workshop on ‘Recording retinal activity using microelectrode arrays (MEAs) organized by ESR Meng-Jung Lee and her colleagues at the NMI, Reutlingen. During this workshop we learned how to record and analyse light responses from the mouse retina using MEAs. In this context, we also got the opportunity to visit another company that is associated with the switchBoard network: MultiChannel Systems GMBH and learned some basics about the MEAs and retinal implants.

Subsequent to this, we had a complementary skill course in Tübingen together with ESRs of another ITN program, MyFUN. This contained topics as ethics, writing of scientific papers, basic legal awareness, lab book organisation and patents. Together with getting knowledge on the above-mentioned topics, this course also gave us the opportunity to interact and build networks with other ITN members as well.

July 2017: Seven ESRs also participated in the Young Researcher Vision Camp, which is a great opportunity for young researchers to learn presenting their research in the best way and exchange knowledge about their own work with other young researchers. The castle Wildenstein, the venue for this meeting, also played a role as a beautiful setting to do this.

September 2017: For researchers it also important to convey their research to the open public. Therefore, we got a media training workshop by Anna Ross (Grasshopper Films) (http://www.grasshopper-films.de). It was a fun week, in which we learned how to arrange a picture for our research profile, to present our research in a small movie and to enhance our communication skills.

November 2017: During this month  we had our second Annual meeting and Mid-term Review when all of us met  in Bergen, Norway. The ESRs presented their progress and got feedback from the whole consortium. During this meeting we also participated in the opening ceremony of our exhibit in VilVite, the Science Center in Bergen.  The exhibit was mainly organized by ESR Rémi Fournel together with Prof. Margaret Veruki and Prof. Espen Hartveit and supported by other ESRs and PIs in the network.

July 2018: During the second Complementary Skill course, which we had in Tübingen, we practiced to give a good presentation and got an overview of communication and conflict management. Additionally, speakers with different career paths gave us an overview about career planning and job hunting outside academia.

September 2018: We had our third Annual meeting in Zürich (http://switchboardblog.blogspot.com/2018/08/3rd-annual-meeting-in-zurich-public.html). There ESRs presented their progress and got feedback on their research from the experts in the field. We also got the opportunity to visit different labs at the University of Zürich.

February 2019: As there is also a possibility to found a start-up company based on our research, an Entrepreneurship workshop was organized to provide us with some basic knowledge on this topic. We went through the process of describing a problem, finding a solution, developing a product prototype and learned to develop different business models. Sadly, this was our last workshop in Tübingen. 

Besides these meetings, courses and workshops, we also learned to improve our  communication skills by sharing our research and knowledge with the public during their outreach activities (e.g. poster presentation at TÜFFF, Blog articles, Museum exhibit, open day of the Netherlands Institute for Neuroscience, introducing the eye and retina in pupils lab).

Other than these activities each ESR also got the opportunity to do secondments in other labs within the network where we could visit the labs of their interest and gain knowledge about the ongoing research and learn other techniques which we could also incorporate into their own research.

All in all the structured ITN program allowed us to already build a network of European retina scientists, with whom we can exchange knowledge and ideas as well as build collaborations. This also helped us a lot in shaping our future career with not just acquiring knowledge from the experts in the field but also developing various interdisciplinary skills.

We, all the ESRs, are very thankful for all the experiences we gained in the last years that enabled us to become independent scientists.

And we are looking forward to our final meeting in May 2019 in Innsbruck!!

Friday, 17 August 2018



The 3rd Annual Meeting and Summer School (17-20 September) of the switchBoard project  is approaching. This time it will be organized by the University of Zürich and ETH Zürich in close collaboration with iniLabs GmbH. Main organizers in particular are early stage researcher Gemma Taverni and her principal investigator, Tobi Delbruck

They have organized a public Retina Symposium as well, to which the REGISTRATION IS OPEN for everyone, who might be interested. Speakers are coming from all over the world to share their knowledge with us! 

Register today! You are very welcome!

Friday, 18 May 2018



Képtalálat a következőre: „arvo 2018” 
Two  ESRs,  namely Matthew Yedutenko (ESR 7) from the Netherlands Institute of Neuroscience, in Amsterdam (Principal Investigator: Maarten Kamermans) and Antonia Stefanov (ESR 10) from the Italian National Research Council, Institute of Neuroscience in Pisa, Italy (Principal Investigator: Enrica Strettoi) represented the switchBoard project at one of the world's biggest and most renowned conferences, the ARVO 2018 Annual Meeting hosted by Honolulu, Hawaii this year. 

ESR 7 presented his data at a Paper Session themed to Photosensitive cells with the title: Cones adapt to higher-order stimulus statistics. He received great feedback following his presentation and was happy to answer or discuss any questions and comments he was addressed to.

ESR 10 was an invited moderator of the Neuroprotection Poster Session where she gained insight into the most up to date therapeutic strategies in Retinitis Pigmentosa and other retinopathies. Furthermore, she presented her data in the Photoreceptor degeneration Poster Session, where she interacted with numerous young and senior researchers, sharing opinions and inspiring each-others' research work. Following her presentation she received several invitations for job interviews from the United States. 

The Strettoi Lab at a Poster Session:

The ARVO is well-known for organizing Keynote Lectures inviting reputed and famous researchers to share their knowledge. This year one of the Keynote Lectures was held by Professor Jennifer Doudna, discoverer and inventor of the CRISP-Cas9 gene-editing technology. This technology is revolutionizing gene-therapy, the limit of our possibilities is the starry sky. 

ESR 7 with Jennifer Doudna:

Besides attending paper and poster sessions, symposia, workshops, keynote lectures and social events, the ESRs had the opportunity to get introduced to the most up-to-date technologies currently used in Vision Research and Ophthalmology at the Exhibit Area. To date, the greatest achievement in  fundus examination is a new technology, called OCT (Optical Coherence Tomography), a very quick and effective method for visualizing the living retinal structure. The instrument can produce a cross-sectional view of this nervous tissue using only the reflectance characteristics of the light simply through our pupils, as you can see it demonstrated by the Strettoi Lab.

The ARVO Meetings are excellent opportunities for young researchers to present their data, get involved in new collaborations, receive job offers, learn about new technologies and latest experimental result and to find sponsors or funding options. Next year's ARVO 2019 Annual Meeting will be organized in Vancouver, Canada. Registration and abstract submission is now open.

Wednesday, 15 November 2017



This article is written by switchBoard ESRs Prerna Srivastava, Meng-Jung Lee and David Klindt.

The European Retina Meeting (ERM) 2017 was held from 5th to 7th October in Paris. The ERM is a biennial conference series, which was started 10 years ago. It aims at bringing together people working on different fields of vision to exchange their ideas and share their innovations in the field. This platform gave ESRs a huge opportunity to present their first exciting results in front of the best scientists in the field, which not only helped them in getting valuable inputs for their project but also opened doors for collaborations.

The first day of the meeting was opened with a brief retinal anatomy session, chaired by John Dowling, focussing on the “Human and Primate Retina”. It was followed by session on “Retinal diseases and Therapies”, illuminating progress in developing new therapeutic approaches towards retinal diseases. Many different aspects of retinal disease treatment were presented. For instance, transplantation of photoreceptors derived from induced pluripotent or embryonic stem cell was found to be beneficial in treating photoreceptor degenerated diseases. Although it remained unclear if the transplanted photoreceptors integrate into the retina permanently, it showed great potential as a promising therapeutic method. In addition, new optogenetic tools were presented towards the recovery from vision loss by expressing light-sensitive channels in retinal neurons, and the studies are now moving from rodents to primates. Alternatively, organic retinal prostheses offer a complete different aspect to treating retinal diseases. By implanting the electronic device, playing the role of photoreceptors, it stimulates the remaining retinal neurons and could partially restore vision. Resolution is now the main issue to be solved. Finally, axon regeneration, a very important topic to restore vision for glaucoma patients, demonstrates now very promising results for axon elongation and the future direction will be focusing on guiding elongated axons to the right targets.

On the next day, the meeting continued with a session on the “Retinal Impact on Eye Development and Myopia”, followed by session on “Retinal Circuits” where many interesting findings were covered. The discussed topics include how the ribbon synapse are modulated in the absence of horizontal cells and their role in encoding the visual information; functional diversity of the bipolar cells and how the colour vision from regions where S and M cones have different spatial distribution is encoded in the mouse inner retina; dynamics of the circuits for directional selective retinal ganglion cells showed that the directional preference  can be tuned by repetitive stimulation by strengthening the null direction response. There were also talks linking the retinal circuits to what the animals may see in their natural environment, for instance, in zebrafish, photoreceptors sensitive to different wavelengths distribute differently in the eye corresponding to positions in visual space. These fascinating results advanced our understanding in the field.

The last day of the meeting started with a session on “Tools against retinal diseases” that introduced the audience to new techniques in the field. Amazing images from different mouse retinal cell types –specifically labelled by adeno-associated viral vectors – were presented by Botond Roska, who is now developing similar viral vectors for the primate retina. The meeting then closed with a session on “Light Adaptation”, showing findings that overthrew the conventional point of view for visual studies. That is to say, rods can be activated with photopic illuminance and the longer they are stimulated, the more they influence visual perception.

Each day, the lecture sessions were mixed with poster sessions, where 8 of the switchBoard ESRs presented their results, namely:

· DavidKlindt on using large population recordings to discover the computations of individual neurons and group them into functional types. (ESR 1)

· PrernaSrivastava on spontaneous oscillatory networks in the degenerated retina. (ESR 2).
· MaximeZimmermann on Zebrafish colour vision: anisotropic retinal circuits match asymmetric spectral content in the natural light. (ESR 3).
· SubhashChandra Yadav on comparing AII and A8 amacrine cells in the mouse retina. (ESR 4).
· BenjaminJames on how does multivesicular release contribute to the transmission of the visual signal. (ESR 6).
· LuciaZanetti on light induced ganglion cell responses in Cav1.4 mutant mouse retinas. (ESR 9).
· Meng-JungLee on electrically imaging retinal neurons using high-density multi-electrode arrays. (ESR 11). – see picture.
· IremKilicarslan on rewiring of bipolar cells in congenital stationary blindness type 2 mouse models. (ESR 14).

Besides these interesting lecture and poster sessions, on the first day of the meeting everyone was also invited by the Mayor of Paris to a gala buffet hosted in the Hôtel de Ville to promote scientific research in the city. The venue was rather impressive and many retinal researchers gathered to enjoy this delightful evening.

Finally, to motivate young researchers to present their work, prizes were awarded to the best poster and a short talk. Mrs. Joana Neves got the prize for the best oral presentation on the title “MANF as an immune modulatory intervention to improve retinal regenerative therapies in aging” and Mrs. Rebekah Warwick for the best poster presentation on the title “Response properties of retinal ganglion cells and their underlying circuits vary with retina location”. The awardees won the registration fee and flight tickets for the upcoming ARVO 2018 meeting in Hawaii.

Tuesday, 13 June 2017




The main purpose of science is to help people obtain a better understanding of the world and to give us a better future. Researches in neurological field make us not only get a closer look to the delicacy of neurons, more importantly, help human kind to solve neuronal diseases. Scientists have been working on developing prostheses to improve the quality of lives from patients suffering from neurodegenerative diseases. The development of retinal implants, like other prostheses, aim to restore vision for the blind. Here I briefly introduce the principles of retinal prostheses and summarize some representative projects.

Concepts of Retinal Implants

Retinae are composed by very well organized layers of neurons, the photoreceptor layer, the inner nuclear layer and ganglion cell layer (check our other article ‘RETINA: OUR RULES AND CELLS WHO VIOLATE THEM’ for more detail). Most of the retinal implants are designed to benefit patients from retinitis pigmentosa (RP) or age-related macular degeneration (AMD), whose photoreceptor layers degenerate  eventually causing irreversible vision loss. However, most of the patients, even after years of suffering from these diseases, still have the remaining inner cells and ganglion cells in well contact (Weiland et al., 2011). To recover the vision, all we need is to find a good way to compensate the loss of photoreceptors; that is, a device that can sense  light and send the vision signals to the remaining retinal neurons. Therefore, the main task of retinal prostheses is to transform the light signals into electrical signals that retinae can understand.  

Retinal implants are most commonly implemented in three approaches: subretinal, epiretinal and suprachoroidal (Zrenner, 2013).

In the subretinal approach, the implant is placed right between the pigment epithelial layer, which is the layer right next to photoreceptor layer, and the (lost) photoreceptor layer. This kind of implants are usually made by light-sensitive photodiodes that make them able to transfer the light into small currents. They play the role of photoreceptors and rely on the remaining neuronal network for the rest of signal transduction. The advantages of implants from subretinal side are therefore 1. Easy positioning 2. Directly replace the damaged photoreceptors 3. No external cameras are required. 

However, they currently  face the problem of power supply, meaning they either need  huge amount of light in order to generate sufficient current, usually a lot higher than the light from nature environment. Patients today  need to carry an external power source which provides sufficient voltage for stimulation.
On the other hand, epiretinal implants are placed directly on the retinal ganglion cell layer. As retinal ganglion cells act as the output of the visual signal to the brain, implants no longer rely on the remaining neural network; instead, the implant itself directly transfers the images into electrical pulses to the optic nerve. For that reason, epiretinal implants are accompanied by external cameras. The electrical stimuli, compared to subretinal approach, act  directly onto the ganglion cells or their axons and could also help patients even there are barely no remaining healthy cells. Disadvantages are that they are harder to fix on retina since only one side of the implant is attached to the retina, they need an extra force to stabilize the position. More importantly, this approach will need the full understanding of the activities from dozens types of retinal ganglion cells without activating axons of passage.  
In the suprachoroidal approach  the implant is placed between choroid and sclera, and is similar to subretinal approach; however stimulating from a larger distance and therefore requiring larger electrodes. (Luo and da Cruz, 2014).

Few Representative Examples
Argus® II

The Argus® II prosthesis implement a 60 electrodes micro electrode arrays (MEAs) in an epiretinal fashion. Images are captured by a video processing unit adapted to eyeglasses, later on sent to the implant in a wireless way. This implant  has  received a CE mark for medical devices (for Europe) and FDA approval :Currently  more than 100 patients have received these implants.
This implant help patients with bare or no light perception to increase their abilities of recognize and discriminate forms, localize targets, detect motion, and navigate. The best visual acuity is reported by 20/1262.
Each of the Alpha-IMS implants comprises  1500 photosensitive pixels and is implanted in the subretinal side of the fovea, the area with the highest visual acuity. The photodiodes capture light and transfer it into stimulation currents, which activate downstream inner neurons; The external power supply is magnetically attached to a subdermal internal coil (Stingl et al., 2013). This implant has also been commercially available in Europe and is going through human clinical trial. Patients with Alpha IMS implants restore partially the ability of recognition of objects and help them avoid dangerous obstacles on the road. The best reported visual acuity is 20/546.

These are the two most advanced examples of retinal implants. Other ongoing consortia like, Pixium,  Boston Retinal Implant or TSIC, Taiwan Sub-retinal Implant Consortium  are developing own implants.

(Cheng et al., 2017)

Challenges and Future

Despite of the few successful cases and  advances  made over the last decade, there are still challenges for retinal implants.. How are the effects to the chronic stimulation to both the function of the implants and to the remaining retinal neurons remain unclear. The resolution that implants can provided is another important issue. Eeven the best implant so far can only allow patients see objects vaguely. To increase the visual acuity, there are still a lot of engineering challenges to overcome. Other issues like the significant remodeling of neural network after photoreceptor degeneration or the lack of understanding of the interface between retina and the implants are the open questions to be answered.

Although there are difficulties to conquer, simple light sensitivity already help blind people greatly improve their quality of lives. More studies to the fundamental retinal neurosciences are going to help us explore the possibility and to break through the boundaries of technology. In the future, implants with better spatial and temporal resolution can be expected.

Current Status of Projects

(Cheng et al., 2017)


Cheng, D.L., Greenberg, P.B., and Borton, D.A. (2017). Advances in Retinal Prosthetic Research: A Systematic Review of Engineering and Clinical Characteristics of Current Prosthetic Initiatives. Curr Eye Res 42, 334-347.
Luo, Y.H., and da Cruz, L. (2014). A review and update on the current status of retinal prostheses (bionic eye). Br Med Bull 109, 31-44.
Stingl, K., Bartz-Schmidt, K.U., Besch, D., Braun, A., Bruckmann, A., Gekeler, F., Greppmaier, U., Hipp, S., Hoertdoerfer, G., Kernstock, C., et al. (2013). Artificial vision with wirelessly powered subretinal electronic implant alpha-IMS. Proceedings of the Royal Society B-Biological Sciences 280, 20130077.
Weiland, J.D., Cho, A.K., and Humayun, M.S. (2011). Retinal Prostheses: Current Clinical Results and Future Needs. Ophthalmology 118, 2227-2237.
Zrenner, E. (2013). Fighting blindness with microelectronics. Science translational medicine 5, 210ps216-210ps216.