Christophe Martin _________________________________

Nature and Supplementation in Julie ou La Nouvelle Héloïse dans Rousseau between Nature and Culture : Philosophy, Literature, and Politics, éd. Anne Deneys-Tunney, Y.-C. Zarka, Berlin, Walter de Gruyter, 2016, p. 153-165 "Nature loves to hide." Heraclitus's enigmatic aphorism, as Pierre Hadot reminds us, probably signifies that what is born ("nature") tends to disappear ("to hide") or to die (Hadot 2004: 30). 1 From the beginning of the Christian era, however, Heraclitus's sentence was interpreted in a vastly different way. It was an allusion to the secrets of a nature that likes to shroud itself in mystery, and that men can only represent beneath a veil that calls for an unveiling. In the face of the secrets of nature, Hadot distinguishes two fundamental attitudes in western thought: the first, which he calls Promethean, appears in Antiquity. It encourages resorting to ruse, technology, and to a form of violence to seize nature's secrets from it and elucidate its mysteries. Encouraged by the Christian tradition, this attitude blossoms thanks to Francis Bacon and the scientific revolution of the seventeenth century, which aims to render man "the master and possessor of nature," as per Descartes's famous sentence derived from Genesis. The second attitude, which Hardot defines as Orphic, imagines, on the contrary, a state of immersion in nature leading to a kind of knowledge that is empathetic or merely contemplative, and that, respecting its veil, refrains from any violence against nature -the natural order of things being beyond our comprehension.

The eighteenth century seems to be a moment of extreme polarization between these two attitudes, with on the one side an encyclopedic movement that appears to actualize the scientific revolution initiated by Bacon and Descartes, aiming to make man "the master and possessor of nature" -consider for example d'Alembert's article "Expérimental": observation "is limited to the facts it has before its eyes, to seeing well and to detailing all the types of phenomena that the spectacle of Nature presents: it seeks on the contrary to penetrate it more deeply, […] it does not limit itself to listening to Nature, but interrogates it and pressures it" (1756: 298) -, and, on the other, an "orphic" movement whose power cannot be understated, judging particularly by the success of abbé Pluche's work, which sanctified nature and invited readers to be content with simply respecting its secrets and admiring its spectacle -"To fathom the very depths of Nature, […] is an arduous endeavor, the success of which is very uncertain, […] it is not necessary that the inner workings of the machine be open to us […]" (Pluche 1732: ix-x).

Where should we place Rousseau in this bipolarization? If he seems the most eloquent apologist, in the eighteenth century, of this orphic attitude towards nature, we will show that this orphic aspiration is far from excluding a promethean one, and that his thought bears witness to his unique effort to apply schemas of action to nature. These schemas certainly do not come under a demiurgic kind of prometheism, but seek to understand the mechanisms of nature, to fathom its mysteries, even if it means (and therein lies the paradox) exerting a type of violence onto nature. We draw from La Nouvelle Héloïse, and in particular on two of Julie's practices: her horticultural activity, and her pedagogical activity. That is, the creation of the Elysée on the one hand, and the education of her children on the other. In both cases, the idea of compensating for nature appears as a paradoxical necessity, but whose deeper logic could be that of forming a compromise between the orphic interdict forbidding any violence against nature, and the yearning to observe nature in even its smallest details, even to manipulate it according to one's needs, as long as it is by following its rules.

Fatal secrets

At first glance, Rousseau clearly incarnates, in the eighteenth century, the model for an orphic attitude regarding nature.2 A brief summary of some of the Rousseau's statements on the subject will permit us to measure the paradoxical dimensions of some of the expressions appearing in La Nouvelle Héloïse.

The third letter to Malesherbes is particularly eloquent. Describing to Malesherbes his ecstasies in Montmorency Forest, Rousseau remarks: "I believe that if I had unveiled all of the mysteries of nature, I would have felt myself to be in a less delightful situation than that stupefying ecstasy."3 Isn't that clearly saying that "the ecstasy of Being takes the place of impossible knowledge of the world," to use Starobinski's words, and that "subjective awareness of totality supplants objective discovery of nature and its laws?" (1971: 78). We should nevertheless note that the unveiling of the mysteries of nature is not for all that disqualified since it serves as a measure of the supreme pleasure that constitutes the ecstasy of being.

Such is also, we know, the principle behind the great rift between Rousseau and Diderot and the Encyclopédistes. Jacques Berchtold has fittingly noted that beyond the controversy on theatre, the attack against d'Alembert in the Letter to M. d'Alembert on Spectacles was also directed at the leader of "a cyclopean enterprise whose gaze seeks to encompass everything […], that makes science visible; the spectacle offered [by the Encyclopedia] is that of the cogs and the machinery left after analysis " (2006: 286).

And yet it is even more important to respect the veil of nature given that, for Rousseau, the violence exerted on nature in order to wrench its veil is precisely what places men outside of nature and brings about catastrophe. It is not for lack of having received, from nature, the clearest providential warnings, dissuading men from using violence in their dealings with her and from committing this fatal mistake:

The heavy veil with which she covered all her operations seemed to warn us adequately that she did not destine us for vain studies. Is there even one of her lessons from which we have known how to profit, or which we have neglected with impunity? Peoples, know once and for all that nature wanted to keep you from being harmed by sciences just as a mother wrests a dangerous weapon from her child's hands; that all secrets she hides from you are so many evils from which she protects you, and that the difficulty you find in educating yourselves is not the least of her benefits. 4

How not to deplore the "man's blindness, which, […] makes him run avidly after all the miseries of which he is susceptible, and which beneficent Nature had taken care to keep from him?"5 By what strange fatality were men not satisfied by the fruits that nature had deliberately scattered on the surface of the earth, such that they felt the need to reach into its entrails and laboriously extract the ore from them? For "it is even harder to attribute this discovery to some accidental fire, because mines are formed only in arid spots, stripped of both trees and plants; so that one would say that Nature had taken precautions to hide this deadly secret from us" (49).Much later, in the Rêveries, Rousseau returns to this mystery once more:

In itself, the mineral realm has nothing lovely or attractive. Its riches, sealed up within the bosom of the earth, seem to have been removed from the sight of man so as not to tempt his cupidity. They are there, as though in reserve, to serve one day as a supplement to the genuine riches more within his reach and for which he loses taste to the extent that he becomes corrupted. Thus he must call on industry, labor, and toil to relieve his misery. He digs in the bowels of the earth. He goes to its center, at the risk of his life and the expense of his health, to seek imaginary goods in place of the real goods it freely offered him when he knew how to enjoy them. He flees the sun and the day which he is no longer worthy to see. [START_REF] Berchtold | Le Spectacle de la nature chez Jean-Jacques Rousseau[END_REF] The "supplement" to the true riches of nature is here identified with perversion. Such is the righteous punishment that man inflicted upon himself when, instead of living in immediate proximity to Mother Nature, man sought to penetrate her entrails and take away its fatal secrets: He buries himself alive and does well, no longer deserving to live in the light of day. There, quarries, pits, forges, furnaces, an apparatus of anvils, hammers, smoke, and fire replace the gentle images of pastoral occupations. The wan faces of the wretches who languish in the foul fumes of the mines, of grimy ironsmiths, of hideous Cyclopes are the spectacle the apparatus of the mines substitutes, in the bosom of the earth, for that of greenery and flowers, of azure sky, of amorous shepherds, and of robust plowmen on its surface. (OC 1: 1066-1067 [CW 8: 62]) Substitution, a supplement for the beauties of nature, definitively points to corruption. In denying oneself the spectacle of nature by replacing it by the hideous spectacle of mines, man has not only disfigured nature, but inflicts on her the violence that, as Jacques Derrida noted, is at the very origin of society (1967: 212-213). 7 This perfectly orphic and anti-Promethean attitude is not Rousseau's last word in his work that is, as we know, entirely irreducible to any rigid form of dualism.

L'Elysée

When in the fourth part of La Nouvelle Héloïse Julie shows Saint-Preux the garden which she calls her Elysée, she evokes the process by which men disfigure nature, suggesting in turn the image of a golden age in which nature freely unveiled its charms: Moreover, nature seems to want to veil from men's eyes her true attractions, to which they are too insensible, and which they disfigure when they can get their hands on them: she flees much-frequented places; it is on the tops of mountains, deep in the forests, on desert Islands that she deploys her most stirring charms.8

Initially, Julie's discourse seems aligned with that of Rousseau's in the first Discourse. And yet a closer reading reveals a reversal in the function of the veil: what nature hides is no longer its secret "operations," as these have already been brought to light for man's misfortune, but its "true attractions," as though to put them out of reach and prevent their disfiguration. In other words the veil no longer constitutes a secret barrier. It is rather that which hones desire in lovers of nature.

In the following part of Julie's discourse, one sees more clearly the erotic and promethean undertone at the heart of the orphic discourse: "Those who love [nature] and cannot go so far to find her are reduced to doing her violence, forcing her in a way to come and live with them" (OC 2: 479-480; CW 6: 394).The expression is at the very least paradoxical when written by Rousseau: unable to "catch nature in the act" as Fontenelle put it, Julie is "reduced" (the term is rather indicative of the tension exerted in this moment upon an action that is in theory contrary to the principles of Rousseauist philosophy) to experimentally reconstruct a piece of nature. However the entire rest of the visit will aim to show that this violence is not in fact one, or rather that this violence is not contrary to nature.

It is well known what key principle presided during the elaboration of this garden and that Julie formulates within herself: "nature did it all, but under my direction, and there is nothing here that I have not designed" (Julie, 388).In this inversion of the Baconian axiom ("we cannot command Nature except by obeying her"), Julie is formulating the rules of an art of the natural, admittedly refusing any subjection to nature, but forcing her nevertheless to reside in a protected space and creating the conditions that will constrain nature to freely deploy its charms. Saint-Preux's error is believing that Julie's work is nothing other than "a passive abandon to the great productivity of nature" (Marin 122): "My goodness, it has cost you nothing but neglect […] you closed the gate […] nature alone did the rest, and you yourself could never have managed to do so well" (Julie 388).

It is a question neither of exploitation nor denaturing, but of healing, maintaining, and protecting a nature that is growing and developing: the birds are not prisoners of the garden, nor are they its guest, "it is we who are theirs" says Julie (Julie 391). While Wolmar's work in Clarens, wisely construed though it may be, comes from a rational exploitation of the wealth of nature, Julie's is removed from any relationship of domination, as Louis Marin noted: "[Julie's] direction, her order is less a commandment, the imposition of a plan or of a schema rigorously faithful to natural substance, than a connivance, a complicity of nature with Natura naturans" (Marin 1976: 122).

Rather than creating a model to serve as a norm for her actions, Julie focuses her attention on the course of things, seeking to find their coherence and profit from their evolution. In this way, the model she proposes for a scheme of action on nature shares some links with models of effective action and the "silent transformations" that François Jullien describes when he writes about Chinese wisdom. Instead of constructing an ideal form that is then projected onto things, the idea is to focus on identifying the favorable factors at the heart of their configuration (Jullien 1997: 32).9 This schema of action seems to come less from the acting subject itself than from the object. As such, it is profoundly different from the one that transforms nature into culture, as is the case for Clarens's agricultural property: the Elysée's economy is parasitic rather than exploitative. An emblem of Julie's garden are these "creeping, parasitic plants that, trained upon the tree trunks, surround their crowns in the thickest foliage and their feet in shade and coolness" that affect Saint-Preux so prodigiously (Julie 389).

In l'Elysée, nature and culture intertwine and transform each other. Julie's garden thus appears to be the art of metamorphosis. Is this project excluded from any Prometheism? Likely not, if one considers the role played in it by ruse, artifice, and illusion: Julie arranges it so that one can no longer see "anywhere the slightest trace of culture:" "all this cannot be done without a modicum of illusion" (Julie 394). And yet, as Hadot reminded us, the Greek's mēkhanē, in other words ruse, is central to the Promethean ambition: technique cunningly works with the mechanisms of nature so that it submits to the needs of men. This discrete art of transformation, under the direction of Julie, is a manipulation. "The garden is a lie because it is the height of artifice that nature is reduced to imitating: in this inverted mimesis, traces of culture have been carefully erased, grass has hidden the vestiges of work and the irregularities of nature are none other than the simulations of art." In this perspective, Julie's work is also one of fiction, "a fiction whose components are violence and illusion" (Marin 130).

Julie's negative education

And yet, this art of being natural -which represents both the ultimate height of artifice and also the fortuitous discovery of Nature as an artist who knows how to give shape to its subject -is not reserved for Julie's Elysée. The entire pedagogical doctrine of a "negative education" that is at the heart of Emile, and whose essential principles have already been developed by Julie and Wolmar in the fifth part of La Nouvelle Heloise, is also based upon the refusal to "correct nature," that is, on the refusal to exert any type of violence on her. If, in many other respects, Rousseau's debt to Locke is considerable (despite his frequent criticisms of him), the originality of his pedagogical thought here is striking. 10 Locke demands that the pedagogue keep the child's nature in highest consideration so as to avoid a frontal attack on his native propensities. He nevertheless encourages the teacher to correct them "with art [that] they may be much mended, and turned to good purpose." 11 In La Nouvelle Héloïse, Rousseau expresses through Wolmar's words (in the form of a reply to one of Saint-Preux's statements), all of the indignation that this idea of wanting to correct nature arouses in him. The draft of Wolmar's answer is even more explicit than its final version:

Correcting nature! said Wolmar interrupting me, isn't that changing it? What instruments will you use that do not belong to nature, and by what strange contradiction can you claim to make it stronger than it is itself? Are you going to combine the mind and sentiments like those disorganized bodies, or grafting men like trees to make them bear other fruits? (Julie 462) 12 The idea at play here is indeed the Lockean notion of education as a "reparation" or a "correction" of nature. The concept of negative education is not yet theorized in La Nouvelle Héloïse, but its role is already mapped out since it defines itself as the art of managing the conditions that allow nature to blossom unfettered. Furthermore, the concept of negative education includes a collaboration between nature and man, the latter taking care to release nature's force and sustain the conditions that allow it to continue its progress free of inauspicious deviations.

Hence the double need to which the principle of negative education responds. On the one hand, a need for protection: much like within the gates of l'Elysée, it is necessary to preserve the "process of nature" to be careful not to trouble -"do nothing and let nothing be done"13 or according to Wolmar's terms, "forever abstain from doing anything at all, rather than act unadvisedly" (Julie, 464). On the other hand, and this need is almost inseparable from the first, a dilatory need: one must not act prematurely, and must abstain from any inopportune intervention. "Leave nature to act for a long time before you get involved with acting in its place, lest you impede its operations" (Emile 242).

As in the case of the garden, this surrender to nature is but an appearance: like in the Elysée, nature does everything, but under the direction of the Wolmar couple. Concerning the first principle, the need for protection, Julie and Wolmar do not benefit from the same privileges as does Emile's tutor, who is guaranteed by the theoretical principle of the work a complete mastery of the child's environment. But they do benefit from an extraordinary combination of circumstances, as Julie herself remarks:

[T]he success of my ministrations depended upon a combination of circumstances that has perhaps never been seen anywhere but here. It required the understanding of an enlightened father to sort out amidst established prejudices the genuine art of governing children from their birth; it required all his patience to lend himself to its execution, without ever giving the lie to his lessons by his conduct; it required children well born in whom nature had done enough so that we could love its handiwork alone; it required having around us only intelligent and well-intentioned domestics, who never tire of entering into their masters' views; a single brutal or flattering servant would have been enough to spoil everything. In truth, when one considers how many foreign causes can set back the best designs and overturn the best-laid schemes, one has to thank fortune for all the good things one does in life, and say that proper behavior depends on a great deal on happiness. (Julie 479) Julie and Wolmar were thus able to tame fate by turning Clarens into a protected islet. It is because the community of Clarens removed itself from the rest of the world by turning in onto itself that the parents could allow their children to develop naturally without fearing that their character be depraved or constrained: "To protect them from vices that are not in them, they have

[…] a prophylactic more powerful than speeches they would not understand, or of which they would soon tire. It is the example of the morals of all those about them…" (478).

The second requirement of the inactive method demands multiplying the dilatory strategies. It is imperative to ban any corrupted social customs from inopportune intrusion in an education based on nature: "Leave nature to act for a long time before you get involved with acting in its place, lest you impede its operations" (Emile, 242).However, allowing nature to act does not mean leaving children to their own devices, any more than the work on l'Elysée consisted in closing the gates and allowing it to grow freely. Just as in the case of Julie's garden, it falls upon Saint-Preux to represent the bad reader, or perhaps Julie's foil, the one who confuses her patient work, as well as Wolmar's, by abandoning children to the forces of nature: That all would seem to me very fine, said I, if I did not see in it a disadvantage which greatly countervenes the advantages you expect from this method; it is to allow children to acquire a thousand bad habits that can only be prevented by good ones. Look at those who are left to themselves; they soon contract all the shortcomings they see exemplified, because those examples are convenient to follow, and never imitate the good, which is harder to practice. Accustomed to obtaining everything, to working in every circumstance their indiscreet will, they become rebellious, headstrong, uncontrollable… (Julie 465).

Negative education is anything but laissez-faire. 14 It is essentially the art of not perturbing nature's temporality, according to Alain Grosrichard: "If one wants to imagine the mechanisms of perversion in general terms, one can simply say it is a lack of recognition of nature's temporality. […] Man is the one who fore-warns nature and forces it to cover itself up" (1978: 52). It is an art skillfully practiced by Julie, in her garden as with her children. "Nature […] would have it that children are children before they are men. If we want to pervert that order, we will produce precocious fruits lacking both maturity and savor, which will soon rot" (Julie 461). It is particularly important not to seek to develop knowledge in the child before having sought to ensure the development of the instruments of knowledge, meaning the education of the body and of the senses: "Reason begins to take shape only after several years, and when the body has assumed a certain consistency. Nature's intention is then that the body be strengthened before the mind comes into play" (Ibid). Wolmar repeats the precept later on: "In any event, let the body mature, until reason begins to bud: Then is the moment to cultivate it" (Ibid 465).

This art that relies on the perfect effacement of the educator as well as his omnipresence also consists in selecting the most efficient stimuli. This art is so essential for Rousseau that it is the only passage in the letter on education of La Nouvelle Héloïse that he copies almost word for word from Emile:

All that surrounds him is the book in which, without thinking about it, he continually enriches his memory while waiting for his judgment to be able to profit from it. It is in the choice of these objects, it is in the care with which one constantly presents him the objects he can know, and hides from him those he ought not to know, that the true art of cultivating in him this first faculty consists; and it is in this way that one must try to form in him a storehouse of knowledge which serves his education during his youth and his conduct at all times. (Emile 248) 15 The art of educating thus consists in large part in choosing not only the right objects, but more importantly the right moment for experiences. 16 The educator manages the rate of learning. He must know how to recognize propitious moments and the ideal frequency of the experiences. It is crucial to be able to seize the moment, the occasion (Rousseau often underscores the irreversibility of certain processes). This art should not however be confused with Aristotelian Kairos. Rousseau sketches out an entirely different notion of "occasion:" not as the opportune moment that comes up, by lucky chance, to incite action and favor its success; but as "the most opportune moment to intervene in the course of a process in action." (Jullien 2002: 87).

If Julie and Wolmar as pedagogues are not to be taken as demiurges, they remain guides that redirect circumstances or control the setting of experiences. Hence the intertwinement of a vocabulary of non-intervention and that of labor and shaping. Thus for Julie, the child "will always [have the time] to learn, but there is not a moment to lose for shaping a good natural disposition in him" (Julie 581).Wolmar's explanations are an even clearer evidence of this interweaving: "Once again the question is not to change the character and bend the natural disposition, but on the contrary to push it as far as it can go, to cultivate it and keep it from degenerating; for it is thus that a man becomes all he can be, and that nature's work is culminated in him by education" (Julie 464).

Even if education must supplement nature as late as possible in order to respect its particular rhythms, there is no education, whether it be natural or not, without supplementation or supplement.17 And the educator is the indisputable presence of this supplementation. That is to say that there is in negative education, just as in l'Elysée, an art of the natural, a collaboration between nature and man in which man is in charge of releasing the power in nature and manage the conditions that allow nature to continue its progress free of unwanted deviation. 18 The concept of negative education as it is proposed by Rousseau tends towards a negative art: the educator's task "is only to let it arrange its work" (Emile 370). But in reality "one must use a great deal of art to prevent social man from being totally artificial" (Emile 485). 19 The work of the educator is to be found at the intersection of nature and culture, controlling a power that already exists, and inserting himself into a pre-existing process in order to better complete it.

It is not surprising that Rousseau carefully avoids the artisanal metaphors found in Locke's pedagogical texts. The Rousseauist governor is nothing like a craftsman who adjusts machines or winds up springs, he is not a tinkerer who creates things from nature. It is not a question of composing and recomposing matter but of forming a living being, and Rousseau's technical metaphors belong to the language register of culture and gardening: "Plants are shaped by cultivation, and men by education" (Emile 162). 20 The art of cultivation mentioned here does not of course refer to a rationalized and passive exploitation of nature's gifts, nor to artificial grafting which is only capable of producing monsters, and even less the murderous cultivation that insists on "fashion[ing man] in keeping with his fancy like a tree in his garden" (Emile add volume and ref. 161), but rather ultimately to the same art that allowed Julie to create her Elysée. 21 It is in this light that one can understand Rousseau's choice to raise Emile in the countryside, in a small village, in other words at the heart of a nature that is already cultivated, among men that culture has not yet monstrously denatured.

Rousseau's attitude, seemingly orphic, does therefore not exclude all promethean aims: you don't always find in Rousseau a reluctance to exhibit the "Opera's machines" or to want to "penetrate the depths of nature" to use abbé Pluche's words. Thus, visiting the Elysée constitutes also, and above all, and invitation to demystify Julie's persistence in unveiling the hidden side of the garden. The secrets of its creation have some similarities with the opening of Entretiens sur la pluralité des mondes, as Jean-Philippe Grosperrin noted (1995: 29). As to the pedagogical project, it is indissociable from an anthropological examination that implies both observation and experimentation -an essential convergence manifested even more openly in Julie's words than in those of Emile's author. Rather troubling indeed, is the manner in which the tender mother, Mme de Wolmar, admits to listening to her children with more attention than they know, and keeping an "exact record" of their slightest movements. Julie explains that she has engineered a pedagogical method corresponding "exactly to the two objectives she had in mind, that is, letting the children's natural disposition develop, and studying it" (Julie 477-478, my emphasis).This statement really helps to understand the dual nature of Emile's project, which is pedagogical and anthropological. For there is absolutely no doubt as to the exact nature of this study on which Julie embarks. Clarens is indeed, in its own way, a laboratory of origins providing access to Nature's best kept secrets: "so it is that the character [of children] develops daily before our eyes without restriction, and we can study the impulses of nature in their most secret principles" (Ibid, my emphasis). Enough to compel us to see negative education as a series of compromises between the prohibition against exerting violence upon Nature and the desire to observe her in even the most minute of details. 22 Thus, between the promethean yearning to manipulate nature by fostering a relationship of control and domination, and the orphic yearning that seeks to restrain itself to a contemplative approach and denies itself the act of unveiling, Rousseau seems to indicate the possibility of a third path. Between the Baconian, who manipulates nature, controls and dominates her, and he who, like abbé Pluche, contemplates without touching, Julie incarnates another way that consists in working with Nature, in other words, to live in society with her. Julie's garden, like her pedagogy, is not promethean, and neither are conceived as a subjection of nature, as the product of tyranny of knowledge and of the action exerted on a nature that "loves to hide," according to Heraclite's aphorism. But neither are exactly orphic either: they respect the veil of nature, and entice her to deploy her charms. Seduction is halfway between violence and gentleness. Julie's garden, and her pedagogy, point to the possibility of a harmonious seduction of nature that in the end offers the ideal image of a complete (and simultaneous) metamorphosis of both nature and culture.

with a laugh, merely the Gardener's servant; I weed the garden, I remove the bad seed, it is for him to cultivate the good" (Julie 478). 21 On horticultural metaphors in Emile, see Grosrichard 1985: 98-100. Generally speaking, gardening metaphors are very common in pedagogical discourses of the eighteenth century. See Simone Gougeaud-Arnaudeau (2000: 20[ff]). 22 On this, see Martin 2012.

See also[START_REF] Renaut | La nature aime se cacher[END_REF] article, "La nature aime se cacher."

It is primarily as such that he is portrayed in Pierre Hadot's book.

Rousseau, Lettres à Malherbes (OC 1 : 1141; January 26, 1762).

Rousseau, Discours sur les sciences et les arts(OC 3: 15 [CW 2: 11]).

Rousseau, Discours sur l'origine et les fondements de l'inégalité(OC 1 : 202; CW 3: 74)

Rêveries du promeneur solitaire, VII e Promenade (OC 1:1066-1067; CW 8: 62), my emphasis.

See alsoKofman's comments (1982: 140-141).

Julie ou La Nouvelle Héloïse, iv,), my emphasis.

Voir aussi Jullien, Les Transformations silencieuses (2009).

Regarding Rousseau's debt to Locke, see Schøsler.

Locke, Quelques pensées sur l'éducation (1904, § 102).

Variant of the text: "Correcting nature! said Wolmar interrupting me; that's a fine word; but before using it, you should have answered to what Julie just said"(OC 4: 564 [CW 6: 462]).

Émile ii (OC 4: 322 [CW X: 226]). Je ne peux pas répondre à cette question; je n'ai pas les CW

Émile ii (OC 4: 351 [CW X: 248]), my emphasis. See La NouvelleHéloïse, v,).

"If there is a favorable moment in life for this study, it is the one I have chosen for Emile" (Émile 399).

"Without childhood, no supplement would ever appear in nature. Yet the supplement is here both humanity's good fortune and the origin of its perversion"(Derrida 1969: 211).

The "systematic portion" of Émile is nothing other than the "course of nature" (Émile, preface, OC 4: 242).

"It is through the perfection of culture (and thus by pushing "denaturation") that harmony with nature can be regained, and this second nature, resulting from art, no longer defines itself as an obscure and distinct equilibrium: it is illuminated by reason and supported by moral sentiment, unknown to the primitve brute."(Starobinski 1969: 25).

"A vicious phrase in their mouth is a foreign plant whose seed the wind brought in; if I cut it off with a reprimand, it will soon sprout again: instead I quietly look for its root, and take care to pull it. I am, she said to me


Léo Pio-Lopez

Romanos Poulkouras

Damien Depannemaecker

Visual cortical prosthesis : an electrical perspective

Keywords: visual cortical prosthesis, brain-machine interface, electrical stimulation, prosthetic vision

The electrical stimulation of the visual cortices has the potential to restore vision to blind individuals. Until now, the results of visual cortical prosthetics has been limited as no prosthesis has restored a full working vision but the field has shown a renewed interest these last years thanks to wireless and technological advances. However, several scientific and technical challenges are still open in order to achieve the therapeutic benefit expected by these new devices. One of the main challenges is the electrical stimulation of the brain itself. In this review, we analyze the results in electrode-based visual cortical prosthetics from the electrical point of view. We first briefly describe what is known about the electrode-tissue interface and safety of electrical stimulation. Then we focus on the psychophysics of prosthetic vision and the state-of-the-art on the interplay between the electrical stimulation of the visual cortex and phosphene perception. Lastly, we discuss the challenges and perspectives of visual cortex electrical stimulation and electrode array design to develop the new generation implantable cortical visual prostheses.

Introduction

In 2019, according to the World Health Organization, there were 1 billion people blind and moderately-to-severely visually impaired in the world. It is predicted that this number will increase significantly in the next decades due to the ageing of the population [START_REF]World report on vision[END_REF]. Visual deficit can therefore impose a huge social and economic burden. The causes of visual deficits are multiple. It can be due to pathologies of the retina as retinis pigmentosa, age-related macular degeneration, optic nerve, retina traumatisms due to accidents or even tumors surgery [START_REF] Nathan G Congdon | Important causes of visual impairment in the world today[END_REF]. The nature of these last pathologies and traumatisms, destroying the neurons of the retina or the fibers of the optic nerve leads to the conclusion that we can't only rely on retinal prosthetics to treat and restore vision. The visual cortex remains generally intact in these different kinds of visual deficits opening the way to the field of visual cortical prosthesis for treatment.

At the origin of the field of the visual prosthetics, there is the experimental observation reporting the perception of a flash of light called phosphenes in the visual field, induced by electrical stimulation of the visual streams. This has been demonstrated for the visual cortex by [START_REF] Foerster | The cerebral cortex in man[END_REF]. Since this seminal work, different ways of inducing these phosphenes have been developed, in particular through implanted electrodes. The core idea of visual prosthetics is to produce a pixelized vision made of multiple phosphenes induced by electrical stimulation of the visual streams. We can now categorize the main stream of visual cortical prosthetics field according to the used type of electrodes: surface electrodes (subdural electrodes) and intracortical electrodes. We can consider then that the field cortical visual prosthetics with surface electrodes began with the work of Brindley and Lewin study [START_REF] Brindley | The umber of information channels needed for efficient reading[END_REF]. They showed that a blind, 52 years old patient, could perceive phosphenes by electrical stimulation of the visual cortex. This first line of research has been then followed by the Dobelle group [START_REF] William H Dobelle | Artificial vision for the blind: electrical stimulation of visual cortex offers hope for a functional prosthesis[END_REF][START_REF] Wh Dobelle | Phosphenes produced by electrical stimulation of human occipital cortex, and their application to the development of a prosthesis for the blind[END_REF][START_REF] Wm H Dobelle | braille" reading by a blind volunteer by visual cortex stimulation[END_REF][START_REF] Wm | Artificial vision for the blind by connecting a television camera to the visual cortex[END_REF].

From the nineties, the field see the development of the visual cortical prosthesis with intracortical electrodes. Schmidt and colleagues showed that a phosphene can be induced by electrical stimulation of the visual cortex with microelectrodes penetrating the cortex. This second line of research is now more predominantly followed [START_REF] Edward J Tehovnik | Phosphene induction and the generation of saccadic eye movements by striate cortex[END_REF][START_REF] David | Visuotopic mapping through a multichannel stimulating implant in primate v1[END_REF][START_REF] Niketeghad | Brain machine interfaces for vision restoration: the current state of cortical visual prosthetics[END_REF].

But despite these advances in phosphene induction, few visual cortical prosthetis projects have emerged during this period. However, the recent publication of reviews on cortical visual prosthetics emphasize a renewed interest in this topic [START_REF] Bhowmick | An insight into assistive technology for the visually impaired and blind people: state-of-the-art and future trends[END_REF][START_REF] Niketeghad | Brain machine interfaces for vision restoration: The current state of cortical visual prosthetics[END_REF][START_REF] Niketeghad | Brain machine interfaces for vision restoration: the current state of cortical visual prosthetics[END_REF]. Indeed, new advancements in electrodes design and wireless power opened new opportunities. Nevertheless, at present, no cortical prosthesis has restored a useful vision (navigation, reading, object and facial recognition). This therefore represents an important issue and several challenges remain to be overcome.

In this article, we adopt an electrical perspective on the field of visual cortical prosthetics. We first describe the state-of-the-art about the electrode-tissue interface and safety of electrical stimulation. Then we focus on the psychophysics of prosthetic vision and the state-of-the-art on the interplay between the electrical stimulation of the visual cortex and phosphene perception. Lastly, we discuss the challenges and perspectives of visual cortex electrical stimulation and electrode array design for the development of the new generation of visual cortical prostheses.

Principles of electrical stimulation of the brain

The electrodes produce an effect on the tissue at short time scales (from millisecond to minutes) during stimulation, depending on the electrical parameters. At longer time scales (hours-years), the electrode effect on the tissue depends on frequency of recurrent stimulation and on the electrode material creating encapsulation of electrodes due to glial reaction, tissue inflammation or neuronal loss [START_REF] Biran | Neuronal cell loss accompanies the brain tissue response to chronically implanted silicon microelectrode arrays[END_REF][START_REF] Kelsey A Potter | Stab injury and device implantation within the brain results in inversely multiphasic neuroinflammatory and neurodegenerative responses[END_REF]. The reaction of biological tissue is complex and the inflammatory response may affect properties of the interface, by keeping the same electrical parameters it may change the effects of the stimulation. The factors influencing the efficiency of electrical neural stimulation seem simple, but they are not completely understood.

The principal focus of electrical stimulation is on the interface between the electrode and the neural tissue. Electrochemistry mechanisms have been extensively studied and described [START_REF] Malmivuo | Bioelectromagnetism -Principles and Applications of Bioelectric and Biomagnetic Fields[END_REF][START_REF] Dymond | Characteristics of the metal-tissue interface of stimulation electrodes[END_REF]. Charge is carried by electrons in the implant material, and then by ions in the neural tissue. The effect of the electrode on the tissue can be capacitive, where no electrons are transferred from the electrode material to the tissue, or Faradaic, where electrons are transferred from the electrode material to the electrolyte, resulting in the oxidation or reduction of different chemical species in the neural tissue [START_REF] Ranjandish | A fully fail-safe capacitive-based charge metering method for active charge balancing in deep brain stimulation[END_REF]. Ideally, stimulation should remain within the capacitive domain at all times to minimise the risk of damage to the nervous system. Chemical species produced by Faradaic reactions can either be toxic for neurons, or damage them indirectly by decreasing their ability to compensate for toxic substances, and by increasing their sensitivity to other stress factors [START_REF] Cogan | The Safe Delivery of Electrical Currents and Neuromodulation[END_REF]. The electrolysis of water may appear with a potential of only 1.23 Volt. The corresponding chemical reactions are the following:

2H 2 O + 2e --→ H 2 + 2OH - O 2 + H 2 O + 2e --→ OH -+ O 2 H
It leads to a pH modification and may damage the tissue. The pH alteration is associated with pathological brain activities such as spreading depression or seizures [START_REF] Mutch | Brain Extracellular pH Changes During Alterations in Substrate Supply[END_REF][START_REF] Raimondo | Ion dynamics during seizures[END_REF].

Most electrical stimulation of the nervous system consists of biphasic pulses. The first (usually cathodic phase) of the biphasic pulse is used to depolarise the neural membranes in the surrounding tissue and initiate the desired action potentials. Following that, a balanced anodic phase is used to reverse any chemical reactions that might have taken place during the first phase. A brief pause between the cathodic and anodic phase allows the electrode to discharge completely and return to potential equilibrium [START_REF] Cogan | Neural stimulation and recording electrodes[END_REF].

Stimulation systems used for experimental or clinical electrophysiology are either current-or voltage-driven. In the first case, a given current value is injected by the stimulator applying the necessary electric potential, while in the second, the electric potential is set and controlled by the user while the device injects the corresponding current. In general, current-driven stimulation is more reliable as it allows a control over the current injected. However, for long-term applications, it is more power consuming [START_REF] Ranjandish | A compact size charge-mode stimulator using a low-power active charge balancing method for deep brain stimulation (dbs)[END_REF][START_REF] Lettieri | Clinical outcome of deep brain stimulation for dystonia: constant-current or constant-voltage stimulation? A non-randomized study[END_REF]. Current-driven stimulation also allows to know the range of the injected charges offering a better safety criterion.

Indeed, Shannon demonstrated that one of the essential parameters for safe stimulation is the charge injected per pulse phase [START_REF] Robert V Shannon | A model of safe levels for electrical stimulation[END_REF]. This is deduced by the current level times the duration of the pulse Q = I.t. For constant current stimulation, charge can be easily derived. However, in the case of constant voltage stimulation, the current does not remain stable, allowing only for an approximation of the injected charge per phase. These authors have demonstrated that longer pulse widths require higher charge content to elicit a response, even if their absolute current value may be lower than in the case of shorter pulses [START_REF] Robert V Shannon | A model of safe levels for electrical stimulation[END_REF]. Therefore shorter pulses with minimal amplitude should be favoured to move the injected charge per phase value as far away as from the area of parametric space that can be harmful to the tissue.

In monopolar stimulation, the current flows between the stimulating electrode and the ground, usually placed away from the electrodes. The resulting electric field is diffuse. Bipolar stimulation is between two electrodes in the array, producing a more densely-packed electric field. The choice between the two largely depends on the position of the target neurons relative to the two electrode tips for bipolar stimulation. If the target is placed in between two electrode tips, bipolar stimulation may be preferred, as the generated electrical field is more focal. The decision is largely patient specific in clinical settings, such as in deep brain stimulation [START_REF] Hancu | On the (non-) equivalency of monopolar and bipolar settings for deep brain stimulation fmri studies of parkinson's disease patients[END_REF][START_REF] Stark | Monopolar and bipolar stimulation of the brain[END_REF].

In all cases, the applied voltage values (either through constant voltage stimulation, or as the voltage required to drive set current in constant current stimulation), should remain below the potential levels where irreversible electrochemical reactions begin to occur either in the electrolyte or the electrode surface. Otherwise, this would lead to tissue damage and electrode corrosion respectively. The interested reader can refer to the review of Merrill and colleagues [START_REF] Merrill | Electrical stimulation of excitable tissue: design of efficacious and safe protocols[END_REF].

The material of the electrodes has also an impact on the parameters and safety of electrical stimulation. A major advance in this area is the introduction of PEDOT and PEDOT:PSS electrodes. This kind of electrode presents electrochemical stability, high conductivity and impedance reduction [START_REF] Cui | Poly (3,4-ethylenedioxythiophene) for chronic neural stimulation[END_REF][START_REF] Anmona | PEDOT: PSS coating on gold microelectrodes with excellent stability and high charge injection capacity for chronic neural interfaces[END_REF].

Psychophysics of prosthetic vision

This field explores the design and visual information processing required for the visual tasks necessary in daily life, like reading, navigation, or the recognition of objects or faces under prosthetic vision (for a review, see Chen [START_REF] Spencer C Chen | Simulating prosthetic vision: I. visual models of phosphenes[END_REF][START_REF] Spencer C Chen | Simulating prosthetic vision: Ii. measuring functional capacity[END_REF]). This line of research is crucial for electrode array design as its results can set constraints on the number, configuration, interspace and placement of electrodes on the visual cortex. And ultimately, the choice of these parameters has implications on electrical stimulations needed to restore a useful vision as psychophysics can shed light on the need of simultaneous electrode stimulation, time of stimulation etc... Traditionally, in the studies of prosthetic vision, a video camera is on the head of the subjects recording the visual scene and sending them the visual information on a monitor through a pixelized filter simulating prosthetic vision for different tasks [START_REF] Cha | Simulation of a phosphene-based visual field: visual acuity in a pixelized vision system[END_REF][START_REF] Spencer C Chen | Simulating prosthetic vision: I. visual models of phosphenes[END_REF][START_REF] Spencer C Chen | Simulating prosthetic vision: Ii. measuring functional capacity[END_REF]. Cha and colleagues studied the optimal number of electrodes for reading. Their experiment estimated the number of pixels to 600 to restore a vision allowing reading. This minimal number was then increased to 625 (array of electrodes of 25x25) by Cha and colleagues [START_REF] Cha | Simulation of a phosphene-based visual field: visual acuity in a pixelized vision system[END_REF]. Sommerhalder [START_REF] Sommerhalder | Simulation of artificial vision: I. eccentric reading of isolated words, and perceptual learning[END_REF] reduced this optimal number and showed that 300 electrodes in foveal vision were necessary to reach a quasi-perfect reading of isolated words (almost 90% of words read correctly). They also studied reading in semi-peripheral vision (15°of visual angle) and reported that it is also possible after a period of learning of a month during one hour a day (64-85% of correct reading) with 300 electrodes. This is important given the foveal projection of the visual field on the cortex is in the calcarine sulcus and is therefore more difficult to access [START_REF] Wandell | Visual field maps in human cortex[END_REF]. Indeed, if we can't reach it, it would be possible to use electrodes in the projection of the semi-peripheral vision on the cortex. Reading with prosthetic vision is also possible with a smaller number of electrodes. Fu et al. showed that with only a low resolution of 36 (array of 6x6) and 64 phosphenes (array 8x8), reading was also possible but it will strongly affect reading speed [START_REF] Fu | Psychophysics of reading with a limited number of pixels: towards the rehabilitation of reading ability with visual prosthesis[END_REF].

For facial recognition, Vurro et al. showed that 100 electrodes allow a facial recognition included between 70-80% of recognition rates with various configurations of electrodes (squared, hexagonal, log-polar) with an advantage however for the hexagonal after a pre-treatment of the image [START_REF] Vurro | Simulation and assessment of bioinspired visual processing system for epi-retinal prostheses[END_REF]. Other results also show that a hexagonal matrix would be more successful for the prosthetic vision [START_REF] Sc Chen | Visual acuity measurement of prosthetic vision: a virtual-reality simulation study[END_REF]. More recently, a study suggests that facial recognition requires much more phosphenes than previously reported and at least an array of 32x32 electrodes is needed [START_REF] Irons | Face identity recognition in simulated prosthetic vision is poorer than previously reported and can be improved by caricaturing[END_REF].

In terms of functional performance, a low-resolution electrodes array would allow wayfinding with a 6x10 array [START_REF] Joram J Van Rheede | Simulating prosthetic vision: Optimizing the information content of a limited visual display[END_REF]. For complex scene recognition, more electrodes are needed, Zhao and colleagues suggested at least 48×48 electrodes for complete recognition would be necessary [START_REF] Zhao | Recognition of a virtual scene via simulated prosthetic vision[END_REF].

In any cases, phosphene density is the main determinant of visual acuity and a low-resolution visual prosthesis would lead to low efficiency on most acuity mediated visual tasks such as reading, object identification, face recognition, obstacle avoidance, etc... [START_REF] Cha | Simulation of a phosphene-based visual field: visual acuity in a pixelized vision system[END_REF]. Therefore, a visual cortical prosthesis able to restore a useful general vision in everyday life will likely have hundreds of electrodes inducing phosphenes in the visual field of the patient.

Phosphene perception and electrical stimulation 4.1 Visual cortical prosthesis and surface electrodes

This first line of research has been followed by the groups of Brindley [START_REF] Giles | The sensations produced by electrical stimulation of the visual cortex[END_REF][START_REF] Dn Rushton | Short-and long-term stability of cortical electrical phosphenes[END_REF] and Dobelle [START_REF] William H Dobelle | Artificial vision for the blind: electrical stimulation of visual cortex offers hope for a functional prosthesis[END_REF][START_REF] Wh Dobelle | Phosphenes produced by electrical stimulation of human occipital cortex, and their application to the development of a prosthesis for the blind[END_REF][START_REF] Wm H Dobelle | braille" reading by a blind volunteer by visual cortex stimulation[END_REF][START_REF] Wm | Artificial vision for the blind by connecting a television camera to the visual cortex[END_REF]. The pioneering work of Brindley [START_REF] Giles | The sensations produced by electrical stimulation of the visual cortex[END_REF] showed that the common evoked phosphene is described as a flash of white light, as 'a star in the sky' or having 'the size of a grain of sago at arm's length' and that the phosphene perception change and stretch with the distance from the foveal projection on the cortex [START_REF] Giles | The sensations produced by electrical stimulation of the visual cortex[END_REF]. Most of the projects who involved a clinical development have been based on surface electrodes. The advantage of this approach is the minimal invasivity of the electrodes on the cortex.

The brain-machine interface is generally made of platinum electrodes [START_REF] Giles | The sensations produced by electrical stimulation of the visual cortex[END_REF][START_REF] William H Dobelle | Artificial vision for the blind: electrical stimulation of visual cortex offers hope for a functional prosthesis[END_REF][START_REF] Wh Dobelle | Phosphenes produced by electrical stimulation of human occipital cortex, and their application to the development of a prosthesis for the blind[END_REF][START_REF] Wm H Dobelle | braille" reading by a blind volunteer by visual cortex stimulation[END_REF][START_REF] Wm | Artificial vision for the blind by connecting a television camera to the visual cortex[END_REF], with squared [START_REF] Giles | The sensations produced by electrical stimulation of the visual cortex[END_REF], or circulars shapes [START_REF] Wh Dobelle | Phosphenes produced by electrical stimulation of human occipital cortex, and their application to the development of a prosthesis for the blind[END_REF][START_REF] Wm H Dobelle | braille" reading by a blind volunteer by visual cortex stimulation[END_REF][START_REF] Wm | Artificial vision for the blind by connecting a television camera to the visual cortex[END_REF]. The array is set on subdural configuration and the electrode arrangement of the electrodes themselves is squared [START_REF] Giles | The sensations produced by electrical stimulation of the visual cortex[END_REF], or hexagonal [START_REF] Wh Dobelle | Phosphenes produced by electrical stimulation of human occipital cortex, and their application to the development of a prosthesis for the blind[END_REF][START_REF] Wm H Dobelle | braille" reading by a blind volunteer by visual cortex stimulation[END_REF][START_REF] Wm | Artificial vision for the blind by connecting a television camera to the visual cortex[END_REF]. The minimal electrode interspace to obtain two distinct phosphenes varies according to the studies. Indeed, it is about 2.4 mm for Brindley first study [START_REF] Giles | The sensations produced by electrical stimulation of the visual cortex[END_REF], 1.5 to 3 mm for Dobelle and Mladejovsky [START_REF] William H Dobelle | Artificial vision for the blind: electrical stimulation of visual cortex offers hope for a functional prosthesis[END_REF], 1.5-2 mm for [START_REF] Da Pollen | Some perceptual effects of electrical stimulation of the visual cortex in man[END_REF] and until 8 mm for Pollen and colleagues [START_REF] Daniel A Pollen | Responses of single neurons to electrical stimulation of the surface of the visual cortex[END_REF]. The minimal interspace would vary with the localization, Pollen suggests that this surprising results compared to those of Dobelle and Brindley would be due to the placement of his electrodes in the foveal vision where a large portion of cortex is used to analyze a small region of the visual field [START_REF] Daniel A Pollen | Responses of single neurons to electrical stimulation of the surface of the visual cortex[END_REF]. The size of the electrodes would not influence the phosphene perception [START_REF] Wh Dobelle | Phosphenes produced by electrical stimulation of human occipital cortex, and their application to the development of a prosthesis for the blind[END_REF]. But Pollen reported that it was possible to evoke phosphene with electrodes having a diameter of 0.25 mm and that the evoked phosphene are perceived as smaller than those obtained with electrodes of a diameter of 0.5 mm [START_REF] Da Pollen | Some perceptual effects of electrical stimulation of the visual cortex in man[END_REF].

Winawer and colleagues reported that phosphene size increases with eccentricity and amount of charge but the area activated increases with the amount of charge but not with eccentricity [START_REF] Winawer | Linking electrical stimulation of human primary visual cortex, size of affected cortical area, neuronal responses, and subjective experience[END_REF] Table 2: Electrical parameters of the stimulation of the human visual cortex to induce phosphenes with surface electrodes. The parameters refer to the common satisfactory parameters as reported in the articles.

of the stimulation. In addition, they discovered that phosphene size saturates at a relatively low current levels (around 2 mA) [START_REF] William H Bosking | Saturation in phosphene size with increasing current levels delivered to human visual cortex[END_REF]. Various works also show a relation between the amplitude of the pulse and the luminosity of the phosphene [START_REF] William H Dobelle | Artificial vision for the blind: electrical stimulation of visual cortex offers hope for a functional prosthesis[END_REF][START_REF] Wm | Artificial vision for the blind by connecting a television camera to the visual cortex[END_REF] although this relationship was observed for a study as non-linear [START_REF] Wh Dobelle | Phosphenes produced by electrical stimulation of human occipital cortex, and their application to the development of a prosthesis for the blind[END_REF]. It also seems that a frequency of stimulation superior to 30 Hz stop the flickering of the phosphene and that the reversal of polarity of pulses (+/-, -/+) has no influence on the perception of the phosphene [START_REF] William H Dobelle | Artificial vision for the blind: electrical stimulation of visual cortex offers hope for a functional prosthesis[END_REF]. Another point is the continuation of the phosphene after the interruption of the stimulus [START_REF] Giles | The sensations produced by electrical stimulation of the visual cortex[END_REF][START_REF] William H Dobelle | Artificial vision for the blind: electrical stimulation of visual cortex offers hope for a functional prosthesis[END_REF]. According to Bak and colleagues [START_REF] Bak | Visual sensations produced by intracortical microstimulation of the human occipital cortex[END_REF], this obstinacy would be due to epileptic discharges such as reported by Pollen [START_REF] Daniel A Pollen | Responses of single neurons to electrical stimulation of the surface of the visual cortex[END_REF]. The minimum thresholds of stimulation to evoke a phosphene is between 0.2-4 mA depending on the electrodes and the most common stimulation reported in the studies is biphasic and symmetric (see Table 2). Shape and localization seem to stay globally stable over time [START_REF] Dn Rushton | Short-and long-term stability of cortical electrical phosphenes[END_REF][START_REF] Niketeghad | Phosphene perceptions and safety of chronic visual cortex stimulation in a blind subject[END_REF] and multiple induced phosphenes are coplanar [START_REF] William H Dobelle | Artificial vision for the blind: electrical stimulation of visual cortex offers hope for a functional prosthesis[END_REF].

One main challenge of visual cortical prosthetics is the specificity of the stimulation, e.g. for one stimulation, one phosphene. Indeed, the phosphene induction by electrical stimulation with surface electrodes has been reported sometimes as non-specific [START_REF] Giles | The sensations produced by electrical stimulation of the visual cortex[END_REF][START_REF] Dn Rushton | Short-and long-term stability of cortical electrical phosphenes[END_REF][START_REF] William H Dobelle | Artificial vision for the blind: electrical stimulation of visual cortex offers hope for a functional prosthesis[END_REF][START_REF] Wh Dobelle | Phosphenes produced by electrical stimulation of human occipital cortex, and their application to the development of a prosthesis for the blind[END_REF][START_REF] Wm H Dobelle | braille" reading by a blind volunteer by visual cortex stimulation[END_REF][START_REF] Wm | Artificial vision for the blind by connecting a television camera to the visual cortex[END_REF]]. An interaction between two adjacent phosphenes is also possible appearing with the shape of a bright cloud [START_REF] Brindley | Effects of electrical stimulation of the visual cortex[END_REF]. Beauchamp and colleagues also showed that when multiple electrodes were stimulated simultaneously, phosphenes could fuse into larger visual perceptions, not allowing independent recognition of the induced phosphenes [START_REF] Michael S Beauchamp | Dynamic stimulation of visual cortex produces form vision in sighted and blind humans[END_REF]. However, more recent studies have shown reliable specific induction of phosphenes with surface electrodes separated by 6-10 mm [START_REF] Bosking | Rules governing perception of multiple phosphenes by human observers[END_REF]. They also could stimulate 4-6 electrodes and induce up to 5 phosphenes at the same time [START_REF] Bosking | Rules governing perception of multiple phosphenes by human observers[END_REF]. Therefore, it seems that producing a specific pattern using surface electrodes for different visual tasks is feasible and has been achieved for different cases (see Table 1).

Various visual tasks were achieved with these prostheses with surface electrodes. The first studies show that the recognition of letters and simple forms is possible [START_REF] Wm H Dobelle | braille" reading by a blind volunteer by visual cortex stimulation[END_REF][START_REF] Wm | Artificial vision for the blind by connecting a television camera to the visual cortex[END_REF]. The last implantation of the team of Dobelle would have allowed the recognition of objects, wayfinding and the driving of a car on a private parking lot by a non-assisted patient [START_REF] Kotler | Vision quest a half century of artificial-sight research has succeeded. and now this blind man can see. behind the bionic-eye breakthrough[END_REF].

Nowadays, the research on the visual cortical prosthesis with surface electrodes seems only conducted by SecondSight Medical Products with the Orion device [START_REF] Niketeghad | Brain machine interfaces for vision restoration: The current state of cortical visual prosthetics[END_REF]. The main arguments advanced against the use of the electrodes of surface are the currents used in the milliampere ? Biphasic 1-100 µA (as reported in [START_REF] Fernández | Toward long-term communication with the brain in the blind by intracortical stimulation: Challenges and future prospects[END_REF]) ? ? ? ? [START_REF] Fernandez | Development of a cortical visual neuroprosthesis for the blind: preliminary results[END_REF] Table 3: Electrical parameters of the stimulation of the human visual cortex to induce phosphenes with intracortical electrodes. The parameters refer to the common satisfactory parameters as reported in the articles.

order and the low phosphene resolution obtained with this technology because of the large size and interspace of the electodes at the surface of the cortex. Nevertheless, this technology presents several advantages. Indeed, it is the less invasive approach and it is compatible with the fabrication of arrays of flexible electrodes [START_REF] Viventi | Flexible, foldable, actively multiplexed, high-density electrode array for mapping brain activity in vivo[END_REF][START_REF] Lu | Flexible neural electrode array based-on porous graphene for cortical microstimulation and sensing[END_REF]. This type of array would ease for example the surgical implantation in the calcarine fissure where we can find the most of the retinotopic maps of the foveal vision in humans [START_REF] Wandell | Visual field maps in human cortex[END_REF].

Visual cortical prosthesis and intracortical electrodes

Most of the current projects follow the intracortical electrodes approach for the development of a visual cortical prosthesis. We can cite the Intracortical Visual Prosthesis Project (ICVP) at the Illinois University of Technology [START_REF] Alexander | A power and data link for a wireless-implanted neural recording system[END_REF], the Cortical visual prosthesis for the blind CORTIVIS supported by the European Commission [START_REF] Fernández | Cortivis approach for an intracortical visual prostheses[END_REF] or the Gennaris bionic vision system developed by Monash Vision Group supported by the Australian Research Council [START_REF] Lowery | Monash vision group's gennaris cortical implant for vision restoration[END_REF]. Intracortical electrodes present several advantages: they allow to reduce the necessary current to evoke a phosphene by several orders of magnitude [START_REF] Edward | Electrical stimulation of neural tissue to evoke behavioral responses[END_REF][START_REF] Schmidt | Feasibility of a visual prosthesis for the blind based on intracortical micro stimulation of the visual cortex[END_REF] and it is possible to implant hundred of electrodes on a very small surface of the cortex (4 mm 2 with CORTIVIS for example). Schmidt and colleagues showed that such intracortical electrodes allowed the perception of phosphenes with a patient blind for 22 years after glaucoma [START_REF] Schmidt | Feasibility of a visual prosthesis for the blind based on intracortical micro stimulation of the visual cortex[END_REF]. The phosphene is described as having a round shape and of size ranging from 0.2 to 2°of visual angle, without colors or blue, yellow, red but not green [START_REF] Bak | Visual sensations produced by intracortical microstimulation of the human occipital cortex[END_REF][START_REF] Schmidt | Feasibility of a visual prosthesis for the blind based on intracortical micro stimulation of the visual cortex[END_REF]. With this kind of penetrating microelectrodes, the luminance of the phosphene increases with the amplitude of pulses but experiments also reported a reduced size [START_REF] Bak | Visual sensations produced by intracortical microstimulation of the human occipital cortex[END_REF][START_REF] Schmidt | Feasibility of a visual prosthesis for the blind based on intracortical micro stimulation of the visual cortex[END_REF]. It seems that a non-linear relation exists between the amplitude of pulse and the size of the evoked phosphene. During the stimulation, the luminosity of the phosphene can also decrease through time, maybe exhibiting a saturation effect [START_REF] Schmidt | Feasibility of a visual prosthesis for the blind based on intracortical micro stimulation of the visual cortex[END_REF]. Similarly with the results obtained with the surface electrodes, when at least 3 stimulations were simultaneously applied, phosphenes seemed coplanar for the patient and one stimulation can induce several phosphenes [START_REF] Schmidt | Feasibility of a visual prosthesis for the blind based on intracortical micro stimulation of the visual cortex[END_REF]. The minimal electrodes interspace to obtain two different phosphenes is 500 µm, but microelectrodes spaced with 250 µm typically did not elicit specific phosphenes [START_REF] Schmidt | Feasibility of a visual prosthesis for the blind based on intracortical micro stimulation of the visual cortex[END_REF]. The study of Schmidt et al. [START_REF] Schmidt | Feasibility of a visual prosthesis for the blind based on intracortical micro stimulation of the visual cortex[END_REF] on humans has been stopped prematurely because of complications [START_REF] Peter | Visual prosthesis[END_REF]. A similar array of electrodes was then implanted in the macaque [START_REF] David | Visuotopic mapping through a multichannel stimulating implant in primate v1[END_REF]. Five months later, the study has also been interrupted, the monkey became lethargic because of a reaction of the nerve tissue at the level of electrodes and thus because of problems of biocompatibility.

The team of Normann developed the Utah Electrode Array (UEA), which has been approved by the Food and Drug Administration (FDA) for long-term human studies [START_REF] Richard A Normann | A neural interface for a cortical vision prosthesis[END_REF][START_REF] Richard A Normann | Representations and dynamics of representations of simple visual stimuli by ensembles of neurons in cat visual cortex studied with a microelectrode array[END_REF]. This array is a silicon-based matrix of 10x10 penetrating microelectrodes. Electrodes are long of 1.5 mm and are spaced out of 400 µm. Their tips are covered with platinum to facilitate the injection of currents. The observations on biocompatibility associated with the presence of this type of matrix can vary from an absence of reaction to the development of a gliosis or fibrosis between the array of electrodes and the brain, movement of the array or even bleedings [START_REF] Richard A Normann | A neural interface for a cortical vision prosthesis[END_REF]. Fernandez and colleagues recently implanted the UEA in the occipital cortex of a 57-year-old person during a six-month period. They found that stimulation thresholds necessary to induce phosphenes were in the 1-100 µA range [START_REF] Fernández | Toward long-term communication with the brain in the blind by intracortical stimulation: Challenges and future prospects[END_REF]. This threshold also depends on the depth of stimulation as reported by De Yoe and colleagues in monkeys [START_REF] Edgar A Deyoe | Laminar variation in threshold for detection of electrical excitation of striate cortex by macaques[END_REF]. The lower threshold has been found in layers 2/3 and more precisely at the level of the frontier between layer 3 and 4A. Another low threshold is found in layer 5. Tehovnik and colleagues, by using a similar protocol, found the lower threshold in layer 5 at a depth of 1.6 mm under the surface of the cortex [START_REF] Edward J Tehovnik | Microstimulation of visual cortex to restore vision[END_REF]. These results are coherent with those obtained by Bartlett and Doty [START_REF] Bartlett | An exploration of the ability of macaques to detect microstimulation of striate cortex[END_REF]. The chronaxies of V1 neurons for the ocular saccades are also lower in the deep layer (0.17 ms) than in the superficial (0.23 ms) [START_REF] Edward J Tehovnik | Saccadic eye movements evoked by microstimulation of striate cortex[END_REF].

The electrical stimulation parameters leading to phosphene induction have not been well studied in humans. The electrical stimulation parameters with intracortical electrodes (as reported in Table 3) are symmetric and the amplitude range from 1 to 100 µA. The pulse duration when reported is between 200 and 400 µs and train duration range from 125 ms to 1s (three 200 ms pulse trains, with a inter-train interval of 200 ms).

The intracortical approach presents several advantages: current in the microampere order (see Table 3), and the possibility of increasing the spatial resolution on a small part of the visual field. The main inconvenience is probably the biocompatibility. It is the most invasive technology and it could act negatively with the vascularization of the cortex [START_REF] Cohen | Prosthetic interfaces with the visual system: biological issues[END_REF]. In addition, damage can be also induced by mechanical micromovements between the pulsating neural tissue and the static implants [START_REF] Polanco | Computational assessment of neural probe and brain tissue interface under transient motion[END_REF]. Nevertheless, the studies on cats and monkeys seem to show a good reaction of tissues through time [START_REF] Liu | Stability of the interface between neural tissue and chronically implanted intracortical microelectrodes[END_REF][START_REF] David | Functional reorganization of primary visual cortex induced by electrical stimulation in the cat[END_REF][START_REF] Troyk | A model for intracortical visual prosthesis research[END_REF]. It was shown that the encapsulation of electrodes does not prevent systematically the efficiency of the stimulation [START_REF] Warren | Electrical properties of implant encapsulation tissue[END_REF][START_REF] James | Chronic neural stimulation with thin-film, iridium oxide electrodes[END_REF]. Another disadvantage is the non-soft character of the arrays of intracortical electrodes [START_REF] Alexander | A power and data link for a wireless-implanted neural recording system[END_REF][START_REF] Pr Troyk | A multifunctional neural electrode stimulation asic using neurotalk tm interface[END_REF][START_REF] Fernández | Cortivis approach for an intracortical visual prostheses[END_REF]. As a large part of the visual field map of central vision is located in the calcarine sulcus [START_REF] Wandell | Visual field maps in human cortex[END_REF], it seems difficult to reach this zone of interest with such arrays. It could be limiting factor given the importance of the foveal vision in reading, recognition of faces and the vision of details. In addition, it is still not clear if the array of penetrating microelectrodes can induce a complex pattern of phosphenes. Unlike surface electrodes, we didn't find any studies reporting the analysis of simultaneous stimulation of several electrodes or the achievement of visual tasks with such a prosthesis.

Electrical stimulation and electrode array design: challenges, potential solutions and perspectives

The studies reporting implantation of a visual cortical prosthesis either with surface or intracortical electrodes allow us to define several important points which need further investigations. V1 accessibility A large part of the the visual field map of central vision on the cortex lies in the calcarine sulcus [START_REF] Wandell | Visual field maps in human cortex[END_REF]. So far, no project has planned to implant a prosthesis in this area [START_REF] Niketeghad | Brain machine interfaces for vision restoration: the current state of cortical visual prosthetics[END_REF]. It seems very unlikely that a non-soft electrodes array will be implanted in this sulcus given the difficulty of access. However, flexible electrodes array would be a potential solution and particularly useful in order to induce phosphene in foveal vision. Some prototypes have been already developed for intracortical electrodes [START_REF] Patrick J Rousche | Flexible polyimide-based intracortical electrode arrays with bioactive capability[END_REF] and surface electrodes [START_REF] Lu | Flexible neural electrode array based-on porous graphene for cortical microstimulation and sensing[END_REF]. Cortical arrays of electrodes fabricated on smooth, flexible and ultrathin materials like parylene provide better coverage of the target area by conforming completely to the shape of the cortex. The same advances can also improve penetrating electrodes. By significantly diminishing their diameter and fabricating them on a soft, thin substrate, those implants promise to cause minimal damage during implantation, and no chronic damage, since the mismatch between a rigid material and soft neural tissue is mitigated due to the soft nature of the implant [START_REF] Joseph W Salatino | Glial responses to implanted electrodes in the brain[END_REF][START_REF] Takashi | Brain tissue responses to neural implants impact signal sensitivity and intervention strategies[END_REF]. Nevertheless, the implantation in the calcarine sulcus could lead to other technological problems including wireless transmission of information and long-term stability of the prosthesis. High-number of electrodes for restoring a useful vision In order to restore a useful vision, it will be likely necessary to induce hundreds of phosphenes to achieve basic tasks of everyday life as psychophysics of prosthetic vision seem to indicate (see section 3). At present, the studies reported either development with surface/subdural electrodes or intracortical electrodes. No projects proposed a hybrid way. However, it presents an interesting benefit/cost ratio. Indeed, to minimize invasivity, intracortical electrodes could be implanted only in semi-peripheral vision and the foveal and peripheral vision would be supported by surface electrodes stimulation. The clinician would have the advantages of both techniques and to limit their inconveniences. Intracortical electrodes would allow increasing the spatial resolution in semi-peripheral vision due to their low size and the minimal interspace electrodes of 500 µm to obtain distinct phosphenes. This technology would allow to reach a number of electrodes high enough for acuity-mediated tasks like reading, navigation or object and face recognition. In addition, the amount of current needed to induce the phosphenes will be reduced by the use of intracortical electrodes array while having a good mapping of the rest of the visual field with surface electrodes. The other way, surface electrodes would limit the invasivity of the prosthesis.

Waveform uniformity It seems difficult to conclude on the most adequate parameters of stimulation for the evocation of phosphene in the visual field of the patients: parameters are variable or incomplete between the various studies with surface or intracortical electrodes. More particularly, the shape of the electrical wave is squared or symmetrical in the majority of the studies (or not reported, see Tables 2 and3). Squared waveforms are a common shape for electrical brain stimulation. But thresholds are similar for cells and axons with the rectangular cathodic stimulation [START_REF] Jr | Which elements are excited in electrical stimulation of mammalian central nervous system: a review[END_REF][START_REF] Jankowska | Antidromic activation of renshaw cells and their axonal projections[END_REF][START_REF] Cameron | Selective microstimulation of central nervous system neurons[END_REF]. Different studies demonstrated that asymmetric waveforms can increase the selectivity of the stimulation by modifying the degree of inactivation of voltage-dependent sodium channels [START_REF] Sahin | Non-rectangular waveforms for neural stimulation with practical electrodes[END_REF][START_REF] Warren | Stimulus waveforms for selective neural stimulation[END_REF]. Imbalanced waveform can also mitigate the dissolution of platinum electrodes [START_REF] Kumsa | Electrical neurostimulation with imbalanced waveform mitigates dissolution of platinum electrodes[END_REF]. This shows the importance of the relationship between the materials used, the electrode tissue interface and the waveform. This approach should be integrated for the development of the new generation of electrode-based visual cortical prosthesis.

Variability in frequency and duration of the pulses We can also observe a strong variability in the duration of the pulse (see Table 2 and3). This lack of selectivity of the stimulation [START_REF] William H Dobelle | Artificial vision for the blind: electrical stimulation of visual cortex offers hope for a functional prosthesis[END_REF][START_REF] Wh Dobelle | Phosphenes produced by electrical stimulation of human occipital cortex, and their application to the development of a prosthesis for the blind[END_REF] and the lack of clear data makes difficult the interpretation of the results concerning the optimal electrical parameters for evoked visual responses. The frequency and duration of the pulses can clearly be a criterion to stay in a safe stimulation domain while maximizing the perceptual effect for long term use. New approaches are also being developed in the field of deep brain stimulation like the technique of current steering. It consists of using multiple electrodes to control the flow of current through the neural tissue, thus allowing a level of fine control over the volume of neural tissue activated by the stimulation. While a novel technique, its combination with high density implants holds promise of improvement, as a higher number of electrodes allows for more options in the way the current can be steered. Such novelties in both stimulation approach and implant hardware could be combined to improve the selectivity of the stimulation for visual restoration [START_REF] Cagnan | Emerging technologies for improved deep brain stimulation[END_REF][START_REF] Bettinger | Recent advances in neural interfaces-materials chemistry to clinical translation[END_REF]. Shape of electrodes The shape of the electrode has two major influences. Firstly, the shape have an impact on the electrode performance and on energy consumption, which can be an important criterion for long term implant and neural stimulation efficiency required for clinical efficacy.

The relation of perimeter to surface area have an influence on the spatial distribution of current density [START_REF] Xuefeng | Analysis of high-perimeter planar electrodes for efficient neural stimulation[END_REF] and it has been shown for example that fractal-shaped microelectrodes superior have better charge injection capacity, despite having smaller perimeter than other designs [START_REF] Park | Electrochemical evaluations of fractal microelectrodes for energy efficient neurostimulation[END_REF].

In second, the shape of the electrode has a direct influence on the electrical properties of the stimulation [START_REF] Brunton | A comparison of microelectrodes for a visual cortical prosthesis using finite element analysis[END_REF]. By changing the shape of electrodes, we can change the shape of the volume of activated neural tissue. Indeed, by segmenting the electrode, or, to change the shape of the contacts can increase the localization of current delivery into specific populations of neuron [START_REF] Xuefeng | Current density distributions, field distributions and impedance analysis of segmented deep brain stimulation electrodes[END_REF][START_REF] Christopher R Butson | Role of electrode design on the volume of tissue activated during deep brain stimulation[END_REF]. A combination of these effects taking into account the spatial non-homogeneity of visual projection in the cortex could significantly improve the resolution of evoked visual perceptions. Chronic stimulation The early studies report the implantation of cortical neuroprosthesis during years and even one during 20 years [START_REF] Wm | Artificial vision for the blind by connecting a television camera to the visual cortex[END_REF]. The reported complications are headaches and would be due to an activation of the meningeal fibers [START_REF] Giles | The sensations produced by electrical stimulation of the visual cortex[END_REF][START_REF] Wh Dobelle | Phosphenes produced by electrical stimulation of human occipital cortex, and their application to the development of a prosthesis for the blind[END_REF]. A reported epileptic seizure is also reported, probably induced by an over stimulation [START_REF] Kotler | Vision quest a half century of artificial-sight research has succeeded. and now this blind man can see. behind the bionic-eye breakthrough[END_REF]. More recently, Niketeghad and colleagues implanted subdural electrodes (a NeuroPace stimulator) in a blind individual [START_REF] Niketeghad | Phosphene perceptions and safety of chronic visual cortex stimulation in a blind subject[END_REF]. They don't report any serious adverse events except mild headaches and dizziness in three instances. Nevertheless, it is difficult to conclude on the parameters of safe chronic stimulation. Indeed, in addition to not having a clear view on the parameters of electrical stimulation applied in the early studies, no implantation of a fully working neuroprosthesis has been achieved, allowing the induction of hundred of phosphenes via the simultaneous activation of a large number of stimulation electrodes. Long-term stability The stability of the prosthesis for years is a challenge. Indeed, Towle and colleagues investigated the brain from a blind human subject who had a cortical visual prosthesis implanted for 36 years by Dobelle team [START_REF] Vernon L Towle | Postmortem investigation of a human cortical visual prosthesis that was implanted for 36 years[END_REF]. They found a unexpected rotation of the electrode array ranging from 25 to 40 degrees away from the midsagittal plane. They suppose that the torque of the connecting cable could be the cause of this rotation. Wireless prosthesis is a potential solution to this problem [START_REF] Niketeghad | Phosphene perceptions and safety of chronic visual cortex stimulation in a blind subject[END_REF]. Nevertheless, both approaches in cortical visual prosthetics are not equal in terms of stability. Intracortical microelectrodes are highly invasive. By penetrating the cortex, tissue responses like glial encapsulation, neuroinflammatory reponses or neuronal cell loss can prevent their long-term stability and efficiency [START_REF] Liu | Evaluation of the stability of intracortical microelectrode arrays[END_REF][START_REF] Biran | Neuronal cell loss accompanies the brain tissue response to chronically implanted silicon microelectrode arrays[END_REF][START_REF] Kelsey A Potter | Stab injury and device implantation within the brain results in inversely multiphasic neuroinflammatory and neurodegenerative responses[END_REF]. Surface/subdural electrodes are now known to have more reliable long-term stability [START_REF] Margaret C Thompson | Demonstration of a stable chronic electrocorticography-based brain-computer interface using a deep brain stimulator[END_REF][START_REF] Daryl R Kipke | Advanced neurotechnologies for chronic neural interfaces: new horizons and clinical opportunities[END_REF]. Biocompatibility and electrode materials The biocompatibility of the materials used for the implantable part is essential for long-term implantation. In this case, the biocompatibility of a material is assessed by its ability not to induce toxicity, and to produce a minimal inflammatory reaction of the tissues for the long term use. Historically, the materials for implantable electrodes manufacturing are silicone and platinum, and have been well characterized in this context [START_REF] Edell | Factors influencing the biocompatibility of insertable silicon microshafts in cerebral cortex[END_REF]. During the last decade, the development of new materials for electrodes such as the PEDOT:PSS (mixtures of poly (3, 4-ethylenedioxythiophene) and polystyrenesulfonate) has demonstrated progress in terms of biocompatibility [START_REF] Cellot | PEDOT:PSS interfaces support the development of neuronal synaptic networks with reduced neuroglia response in vitro[END_REF]. The improved conductivity and impedance of new materials like PEDOT:PSS has a direct positive impact in any kind of brain interface. The improved conductivity allows for electrodes to deliver electrical stimulation with a minimal risk of damage to tissue, while their low impedance provides a much better recording quality, therefore improving both monitoring and evoking neurophysiological activity in patients. PEDOT:PSS has the high conductivity among organic thermoelectric materials and increase charge-transfer efficiency [START_REF] Cellot | PEDOT:PSS interfaces support the development of neuronal synaptic networks with reduced neuroglia response in vitro[END_REF]. The fact that these improved properties are intrinsic to the material paves the way for fabrication of scalable electrodes down to the micro-scale that can efficiently stimulate and record activity in a very localised and selective manner. In addition, these novel materials are compatible with flexible substrates and microfabrication techniques offers additional possibilities to improve current brain implants. Both surface and intracortical electrodes can be improved through these advances. Micro-coils Another strategy outside the traditional electrode-based cortical visual prosthetics is the use of magnetic stimulation by means of micro-coils [START_REF] Woo | Implantable microcoils for intracortical magnetic stimulation[END_REF][START_REF] Fried | Abstract# 39: Towards a microcoil-based cortical visual prosthesis[END_REF]. They are small implantable inductors that magnetically activate neurons. The brain stimulation of cortical pyramidal neurons in vitro has been reported as reliable and could be confined to spatially narrow regions (<60 µm) [START_REF] Woo | Implantable microcoils for intracortical magnetic stimulation[END_REF]. Similarly to electrode stimulation, a phosphene can be induced by magnetic stimulation [START_REF] Marg | Phosphenes induced by magnetic stimulation over the occipital brain: description and probable site of stimulation[END_REF][START_REF] Kammer | Phosphenes and transient scotomas induced by magnetic stimulation of the occipital lobe: their topographic relationship[END_REF] and therefore this strategy could lead to restoration of vision. Other strategies are possible as described in [START_REF] Farnum | New vision for visual prostheses[END_REF].

Conclusion

Cortical visual prosthetics spurred a renewed interest these last years thanks to wireless and technological advances. As we saw it, compared to the retina, the optical nerve or with the LGN, the visual cortex presents a large surface of stimulation and remain usually intact after visual impairments. In this study, we focused on the electrical stimulation of the visual cortices and related aspects. We showed that despite several developments, there is still a knowledge gap concerning the safe parameters of electrical stimulation for multi-phosphene induction. To fill this gap and reach a complete clinical efficiency of the electrode-based cortical visual prosthesis seems key for the future. The next generation of visual cortical prosthesis will come hand-to-hand with new technological development of electrodes (in terms of materials and geometry), and the extension of approaches based on temporal sequences and new waveforms of stimulation to increase the efficiency and safety of phosphene induction.

Figure 1 :

 1 Figure 1: Interaction between different research fields leading to new generation of visual cortical prosthesis.

Table 1 :

 1 which is in line with non-human primate studies with microelectrodes[START_REF] Edward J Tehovnik | Microstimulation of v1 delays the execution of visually guided saccades[END_REF][START_REF] Edward J Tehovnik | Delaying visually guided saccades by microstimulation of macaque v1: spatial properties of delay fields[END_REF][START_REF] Edward J Tehovnik | Phosphene induction and the generation of saccadic eye movements by striate cortex[END_REF]. Bosking et al. also found with human subjects that the size of the phosphene depends on the amplitude Table of the different conditions and reports for clinical implantation of visual cortical neuroprostheses.

	Ref.		[11]								[28]						[29]						[31]	[30]	[48]	[62]
	Biocompatibility		Headaches								Headaches					2 days of implantation	No reported complications					?	Implantation during 20 years	for one patient	No reported complications	Epileptic seizure	Headaches and dizziness
	Visual task		None								Recognition of simple	forms				Recognition of simple	forms and letters					Braille reading at 30	words/mn	Recognition of letters	and numbers	Visual acuity of 20/120	Navigation	Driving of cars	Recognition of objects	None
	Electrodes array	80 square electrodes of platinum of 0.63 mm 2 ,	Squared configuration,	Subdural placement,	Electrodes interspace: 2-4mm			Several implanted types	Electrodes of platinum andiridium (90-10%)				Array of 64 circular platinum electrodes	of 1 mm 2	Hexagonal configuration	Subdural placement		Array of 64 electrodes of platinum	12 of the 2 mm 2 , the rest of 1 mm 2	Hexagonal configuration	Subdural placement	Array of 64 platinum electrodes of 0.78 mm 2	Hexagonal configuration	Subdural placement	A camera on glasses sends the visual	information to a computer which translates	it into electric impulses	68-72 to 242 electrodes	NeuroPace stimulator of two four-contact subdural	electrode strips implanted over the right medial oc-	cipital cortex
	Subjects		52-year-old patient affected	by a bilateral glaucoma and then		by a detachment of the left retina					15 sighted or hemianopsic patients				A 43-year-old patient affected by	a congenital cataract and then by	a glaucoma and a detachment of	the retina	A 28-year-old patient, who lost	both eyes during the Vietnam War	33-year-old patient blind by impact of	bullet	62-year-old patient who	lost sight at 32-year-old by trauma	12 patients	Blind individual with an 8-year history	of bare light perception
	Phosphene perception	Circles,without colors	Non uniform field of phosphenes	1 stimulation ->several phosphenes	Phosphene lengthened with	the estrangement of the	foveal projection	Sometimes colored phosphenes	Non uniform field of phosphenes	Variation of the size of phosphenes	with the excentricity of the stimulation	1 stimulation ->several phosphenes	Coplanar phosphenes	Phosphenes without colors		Orange for the youngest patient	Non uniform field of phosphenes		1 stimulation ->several phosphenes	Coplanar phosphenes		Phosphenes with the shape of points	1 stimulation ->several phosphenes	Phosphenes without colors	Non uniform field of phosphenes	?	Phosphene of different brightness ac-	cording to the stimulation

Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 22 December 2020 doi:10.20944/preprints202012.0562.v1

Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 22 December 2020