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Vision Research 39 (1999) 2577–2587

A neural interface for a cortical vision

Richard A. Normann *, Edwin M. Maynard, Patrick J. Rousche, David J. Warren

Department of Bioengineering, Uni6ersity of Utah, 2840 Merrill Engineering Building, Salt Lake City, UT 84112, USA

Received 11 August 1998; received in revised form 7 December 1998

Abstract

The development of a cortically based vision prosthesis has been hampered by a lack of basic experiments on phosphene psychophysics. This basic research has been hampered by the lack of a means to safely stimulate large numbers of cortical neurons. Recently, a number of laboratories have developed arrays of silicon microelectrodes that could enable such basic studies on phosphene psychophysics. This paper describes one such array, the Utah electrode array, and summarizes neurosurgical, physiological and histological experiments that suggest that such an array could be implanted safely in . We also summarize a series of chronic behavioral experiments that show that modest levels of electrical currents passed into cortex via this array can evoke sensory percepts. Pending the successful outcome of biocompatibility studies using such arrays, high count arrays of penetrating microelectrodes similar to this design could provide a useful tool for studies of the psychophysics of phosphene perception in human volunteers. Such studies could provide a proof-of-concept for cortically based artificial vision. © 1999 Published by Elsevier Science Ltd. All rights reserved.

Keywords: ; Artificial vision; Electrode arrays; Multielectrode recordings; Biocompatibility

1. Introduction with an array of electrodes implanted subdurally over its surface. These studies showed that currents in the The concept of cortically based artificial vision had milliampere range were required to evoke individual its origins in studies of the functional architecture of phosphenes, and that currents passed through groups the . Wilder Penfield observed the behav- of electrodes that were spaced too close to neighboring ioral consequences of electrically stimulating various electrodes produced highly non-linear interactions be- regions of cerebral cortex and noted that there was a tween the evoked phosphenes. It became clear from rational map of visual space onto the primary visual these experiments that stimulating visual cortex via an cortex (Penfield & Rasmussen, 1950). He observed that array of surface electrodes would not be an effective electrical stimulation of the surface of the visual cortex means to produce a useful visual sense in individuals generally evoked the perception of points of light with total blindness. (called phosphenes) at specific regions in space. These Recent experiments by Schmidt et al. (1996) have observations have led a number of investigators to caused renewed interest in cortically based visual pros- propose that electrical stimulation of visual cortex via thetics. They stimulated visual cortex of a profoundly arrays of electrodes might provide the profoundly blind blind human volunteer with groups of microelectrodes with a limited form of functional vision. Subsequent that were designed to penetrate the cortex to the level experiments in the late sixties and early seventies by of its normal thalamic input. Neural stimulation via Brindley (Brindley & Lewin, 1968), Dobelle (Dobelle & Mladejovsky, 1974), Pollen (Pollen, 1975), and their penetrating electrodes has been long known to occur co-workers demonstrated that a field of individual pho- with electrical currents that are much smaller than sphenes could be evoked by stimulating visual cortex those used to excite neurons via surface stimulation. Schmidt and his coworkers demonstrated that pho- sphenes could be evoked with currents that were orders * Corresponding author. Tel.: +1-801-5817645; fax: +1-801- 58189661. of magnitude lower than those used with surface stimu- E-mail address: [email protected] (R.A. Normann) lation, and that simple patterned perceptions could be

0042-6989/99/$ - see front matter © 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S0042-6989(99)00040-1 2578 R.A. Normann et al. / Vision Research 39 (1999) 2577–2587 evoked by current stimulation via small groups of these 2. A high electrode count, penetrating electrode array: microelectrodes. Unfortunately, the electrode arrays design considerations used by Schmidt et al. were too sparse to allow them to answer the key question upon which a cortical ap- The cornerstone of a visual neuroprosthetic is the proach to artificial vision must be based: does patterned interface between the functioning neurons in the visual electrical stimulation via a high electrode count elec- pathways, and implanted devices that can excite these trode array evoke discriminable patterned percepts, or neurons. This system must individually stimulate a very nondiscriminable blobs of light? If this psychophysical large number of neurons that have retained function experiment can be performed, and the former result even though more distal neurons have been irreparably maintains, cortically based artificial vision could be- damaged by the etiology of the blindness. This interface come a reality. bypasses the malfunctioning distal components in the In order to answer this critical question (and many visual pathway and directly excites neural pathways others associated with phosphene psychophysics), re- that are proximal to the implant site. Recent work at searchers need a new class of tools: arrays of microelec- the University of Utah (Jones et al., 1992), at the trodes that can be safely implanted into the visual University of Michigan by Wise et al. (Hoogerwerf & pathways, and that will allow periodic injections of Wise, 1994), and Stanford University (Kewley et al., electrical currents at many closely spaced sites. Such 1997) has focused on the use of silicon as an electrode arrays could eventually also form the cornerstone of material from which high electrode count arrays can be visual neuroprosthetic systems. It therefore is clear that fabricated. Silicon is highly biocompatible (Stensaas & progress in cortically based artificial vision systems is Stensaas, 1978; Yuen et al., 1987; Schmidt et al., 1993), directly linked to progress in the development of high can be micromachined using standard microfabrication electrode count microelectrode arrays. technologies, and can incorporate integrated electron- Over the past few years, the research efforts of an ics. The Michigan and Stanford electrode arrays have increasing number of laboratories around the world been built to take advantage of the planar photolitho- have been committed to the development of such ar- graphic manufacturing techniques used in the semicon- rays. Work has been focused mainly on arrays that will ductor industry, while the Utah arrays were designed interface to neurons of the (Humayun et al., from the ground up to meet the needs of a neural 1995; Wyatt & Rizzo, 1996; Chow & Chow, 1997; interface. As such, new manufacturing techniques had Eckmiller 1997; Zrenner et al., 1997) or to neurons of to be developed in order to build this device. These the visual cortex (Wise & Najafi, 1991; Jones et al., techniques have been described elsewhere (Jones et al., 1992; Schmidt et al., 1996), although a recent study has 1992). described phosphene generation via stimula- The Utah electrode array (UEA), shown in Fig. 1, tion (Veraart et al., 1998a,b). Because of the impor- provides a multichannel interface to the visual cortex. It tance of the neural interface in a , has a large number of 1.5 mm long electrodes (typically much of this paper will focus on one example of such a 100 in a 10×10 square grid) that project out from a penetrating cortical electrode array: the Utah electrode very thin (0.2 mm) substrate and that are separated array. We will discuss considerations that were used in from each other by 0.4 mm. The tips of the electrodes its design, how the array can be implanted in the visual are metalized with platinum to facilitate electronic to cortex, and its biocompatibility as revealed by histolog- ionic transduction. As the array’s substrate must rest ical and electrophysiological experiments. The immedi- on the cortical surface, minimizing its thickness was an ate motivation behind much of what we have developed important design consideration: if it was too thin, it at the University of Utah has not been to create vision might break upon insertion, if it was too thick, the dura neuroprosthetic systems (not enough is known about and the skull would produce a constant downward the human visual pathways to optimally design such a force on the array, tending to push it into the cortex. system today). Rather, we have been trying to create The large number of penetrating electrodes in the experimental systems that will allow researchers to bet- UEA presents a very large surface area to the cortex ter understand the vertebrate though elec- and the implanted array tends to self anchor to the trophysiological and behavioral experiments. When the cortical tissues. Such an array has the strong advantage long term biocompatibility of these experimental im- that it floats in the cortical tissues. As the cortex moves plant systems has been demonstrated, these systems due to respiration and blood pumping, or to skeletal could eventually be used in human experimentation to displacements, the array moves with it, thereby produc- study the psychophysics of phosphene perception, and ing little or no relative motion between the electrode to establish the proof-of-concept that cortical stimula- tips and the neurons near its active tips. This design tion may provide a limited, but useful visual sense to feature should therefore, produce an extremely stable those with profound blindness. interface with the surrounding neurons. R.A. Normann et al. / Vision Research 39 (1999) 2577–2587 2579

Fig. 1. A scanning electron micrograph of the 100 microelectrode, Utah electrode array (UEA).

The penetrating electrodes in an implanted array bonding pad on the rear surface of the substrate. must compromise as little cortical volume as possible Conduction of signals along the length of the silicon (ideally zero). Thus, each needle must be made as needles is achieved by the use of doped silicon. The slender as possible yet retain sufficient strength to entire electrode array (with the exception of the plat- withstand the implantation procedure. Further, con- inum coated tips) is insulated with a 1 micron thick sistent with concept of blunt dissection used by neu- coat of silicon nitride. An electrical connection is rosurgeons, these penetrating structures should made to each electrode by bonding an insulated 25 displace the tissues they are inserted into rather than micron diameter wire to each bond pad, and connect- cut their way through them. Thus, the needle should ing these wires to a percutaneous connector. The rear have a cylindrical (or slightly conical) rather than a of the array (with bonded lead wires) is encapsulated planar geometry. They must also be strong enough so with a silicone elastomer. Electrode impedances (mea- that they are not deflected by the tissues they are sured with a 100 nA, 1 kHz sine wave current) are inserted into. The needle electrodes in the UEA meet typically in the 100–300 kV range. these criteria and are about 80–100 microns in di- We have used both chronic and acute arrays in our ameter at their bases. They taper to a sharpened tip experiments. The acute array has all 100 electrodes that has a radius of curvature of two to three mi- brought out to a small printed circuit board contain- crons. Fig. 2 shows an electron micrograph of the tips of these electrodes. Electrodes with these dimen- ing four, 26-pin IDC connectors. The chronic system sions have been shown to be sufficiently strong to has only 11 of the 100 potentially functional elec- withstand insertion into materials that are consider- trodes brought out to a Microtech connector via 11, ably less compliant than cortical tissue (cork, balsa 25 micron diameter, platinum–iridium lead wires. The wood, even egg shell). As will be shown later, they connector is integrated into a titanium pedestal that do not bend during the insertion process, and they is mounted with titanium bone screws to the cranium. only displace about 4% of the cortical volume into Fig. 3 shows a photograph of the latest version of which they are inserted. our chronic electrode assembly that uses a 20 micron Each electrode is electrically isolated from its neigh- thick, polyimide ribbon cable rather than 12 discrete boring electrodes with a moat of glass that surrounds lead wires to connect the array to the percutaneous the base of each electrode and each electrode has a connector. 2580 R.A. Normann et al. / Vision Research 39 (1999) 2577–2587

Fig. 2. A scanning electron micrograph of the platinum coated tips of the UEA electrodes. Scale bar=0.5 mm.

3. Surgical issue: implanting high count electrode upon this concept that appears to circumvent the above arrays mentioned problems: a system that rapidly inserts the UEA into the cortex (Rousche & Normann, 1992). A A visual prosthesis will be a highly invasive system drawing of the pneumatically actuated insertion tool we that, eventually, must contain active integrated elec- have developed is shown in Fig. 4. Array insertion is tronic circuitry. While relatively large electronic and achieved by a transfer of momentum between an accel- mechanical systems have been implanted in the body erated piston, and an insertion mass that rests against (pacemakers, artificial joints, and the cochlear prosthe- the back side of the electrode array which is to undergo sis), miniaturized devices of the mechanical and elec- implantation. When the momentum transfer takes tronic complexity of the UEA have yet to be implanted place, the array is rapidly inserted into the cortical on a long term basis in human volunteers. Whether the tissues in about 200 ms. The insertion is so rapid that implant site is intended to be the cortex or the retina the viscoelastic properties of the cortical tissues cause (or the optic nerve (Veraart et al., 1998a,b)), safe and the cortex to experience only slight mechanical dim- effective surgical procedures that are cost-effective will pling and the insertion is generally complete. Occasion- have to be developed and validated in animal models ally implantation of the UEA through surface before they are attempted in human volunteers. vasculature is accompanied by a small amount of sub- The Utah researchers have developed new surgical pial bleeding, but this typically resolves itself, and single techniques and tools that enable the UEA to be im- unit recordings of neural activity can often be made planted in cortical tissues. Even though the individual within hours after the surgical procedures are com- electrodes of the UEA are extremely sharp, early at- pleted. The implantation of a chronic UEA in a cat tempts at implanting large numbers of them into the takes typically about 4 h, and the surgical procedure visual cortex only deformed the cortical surface and has been described elsewhere (Maynard, Fernandez & resulted in incomplete implantation. Further, the com- Normann, 1999). pression of the cortical surface produced by slow me- chanical insertion can injure blood vessels, causing intracranial hemorrhage and cortical edema. 4. Biocompatibility of the Utah array Because the is a viscoelastic material, it will behave in a much more rigid fashion if the electrodes A neuroprosthetic system must be implanted into the can be inserted into the cortex at a very high velocity. nervous system and remain fully functional for periods We have developed a unique surgical instrument based that will eventually extend to many decades. This con- R.A. Normann et al. / Vision Research 39 (1999) 2577–2587 2581

Fig. 3. A photograph of a chronic 25 electrode, UEA assembly that uses a 20 micron thick, polyimide ribbon cable to connect the array to a transdermal connector. sideration places unique constraints on the architecture, 4.2. Acute recording capability materials, and surgical techniques used in the imple- mentation of the neural interface. Inattention to these An excellent index of the biocompatibility of a corti- issues can result in chronic inflammatory responses cal implant is its ability to record single- and/or multi- around the implant site and generate a thick capsule unit activity from the neurons near the electrode tip for surrounding each electrode. Because the UEA is a prolonged periods of time. If the materials used in the unique implantable structure, the consequences of its array, or the implantation techniques are not biocom- presence in the body for extended durations must be patible, neuronal processes close to the electrode track evaluated. We have studied its biocompatibility using will degenerate and it will not be possible to record histological and electrophysiological techniques. single-unit activity. However field potential activity lo- cated far from the electrode tracks might still be 4.1. Histological findings recorded from functioning neurons, and such neurons may still be stimulated effectively in a neuroprosthetic The materials of which the UEA is built: silicon, application. silicon nitride, silicon dioxide, platinum, titanium, tung- We have recorded responses from the UEA in both sten, and silicone are known to be well tolerated by the acute and chronically implanted animals to better un- CNS (Stensaas & Stensaas, 1978; Edell et al., 1992; derstand its short and long term biocompatibility. Spec- Schmidt et al., 1993). Our 6 month histological experi- imen responses with high, medium and low ments support this notion. Fig. 5a and b show hema- signal-to-noise ratios that were evoked by a bar of light toxylin and eosin stained sections of such tissue. A thin moving across the receptive fields of visual cortical capsule (2–5 microns thick) forms around each elec- units are shown in Fig. 6. To generalize these findings, trode track, but neuronal cell bodies are typically seen we have tabulated below the recording results of our in close apposition to the electrode tracks. In fact, in past 17 acute cat implants. In this table, we classified sections where the tracks are similar in diameter to responses on each electrode using the scale of Fig. 6 blood vessels, it is often difficult to tell a track from a (Table 1). vessel. Fig. 5 is an example of a particularly benign In our best acute implantations in cat area 17, we tissue response. We also have histological samples have been able to record good single- and multi-unit showing gliosis, buildup of fibrotic tissue between the responses from 68% of the electrodes (because of the array and the meninges, array displacement through the curvature of the gyri in cat cortex, it is not possible to cortex, and bleeding in some tracks. It is clear that implant all 100 electrodes in a given and some more work must be done to ensure that histological electrodes end up in sulci). More typically, however, we findings like those shown in Fig. 5 can be achieved on are able to record good quality single- and multi-unit every implantation. These brief histological observa- responses from 36% of the electrodes in a given array. tions have been elaborated upon in a recent study by Global field potentials, non-discriminable multi-unit re- Maynard on the long term consequences of UEA im- sponses, and an absence of responses are recorded from plantation (Maynard, Fernandez & Normann, 1999). the remaining electrodes. 2582 R.A. Normann et al. / Vision Research 39 (1999) 2577–2587

Fig. 4. A drawing of the high velocity, impact insertion tool used to implant the UEA into cortical tissues.

In many multi-unit responses, single-units can easily cortex to better monitor the stability and long term be discriminated into individual responses using criteria biocompatibility of the UEA. We have recorded multi- of response kinetics. An example of three single units and single-unit evoked activity and identified single- that were recorded from a multi-unit recording is units (based on response kinetics) on many of our shown in Fig. 7A. For clarity, only five examples of electrodes. We have been able to record single- and each unit’s action potentials are illustrated. These re- multi-unit responses in cat and monkey cortex for over sponses were evoked by a randomly modulated check- 3 years (the longest intervals studied). The presence of erboard, and the responses were used to construct single- and multi-units on many electrodes, and the receptive field plots for each unit (Fig. 7B). stability of these units over periods of months provides the most compelling evidence for the biocompatibility 4.3. Chronic recording capability of the UEA.

We have used chronic implantations of the UEA in cat visual and and monkey motor 5. Behavioral experiments

Useful function will be achieved in a visual neuro- prostheses by injection of electrical currents into the visual pathways through large numbers of electrodes. Current injections can produce short term and long term complications depending upon the levels of the currents that are injected (McCreery et al., 1994) (or see Agnew, Chapter 6 for a review (Agnew & McCreery, 1990)). In order to determine the levels of current injections via the UEA that are required to evoke sensory percepts, we have conducted a series of behav- ioral experiments in cats (Rousche & Normann, 1999). Ideally, these experiments would be performed with implants in visual cortex, but, because of the ease of delivering auditory stimuli to a behaving cat, we

Fig. 6. Specimen responses evoked by a bar of light moving across the Fig. 5. (a) Hematoxylin and eosin stained sections of visual cortex receptive fields of cortical units. The slow and fast time bases implanted for 6 months with a UEA. Spacing between electrode illustrate examples of high, medium and low signal to noise ratio tracks is 0.4 mm. (b) Higher magnification of one track. recordings. R.A. Normann et al. / Vision Research 39 (1999) 2577–2587 2583

Table 1 Recording statistics from our past 17 acute implants illustrating the mean and standard deviation of the number of electrodes with high, medium and low signal-to-noise ratios for each of the arraysa

HighMedium SNRLow SNR Inactive Useful SNR\3 3BmedB1.5 B1.5 (No discernable activity) SNR \1.5

Minimum 1% 3% 0% 16% 10% Maximum 53% 52% 35% 88% 68% Mean9S.D. 20915% 16913% 899% 56923% 36919%

a Minimum and maximum are the lowest and highest number of electrodes with the indicated quality of recording on each of the arrays. Some arrays had a few broken electrodes, so data is presented as percent of potentially functional electrodes. targeted the auditory cortex as our implant site and fully acknowledge that such clinical systems are still used auditory stimulation rather than visual many years away, and that much basic research must stimulation. be performed before clinical systems may become a Cats were trained over a 1–2 month period to lever reality. A more immediate motivation for the creation press as a result of auditory stimulation, and auditory of the UEA has been the need for a multichannel thresholds were measured. Trained cats that performed implant system that will expedite the conduct of this this task at 90% correct were chronically implanted basic research. with the UEA. Following implantation of the UEA, we The research performed by Brindley (Brindley & interspersed current injections via the UEA and con- Lewin, 1968) and Dobelle (Dobelle & Mladejovsky, ventional auditory stimulation. Current injections that 1974), and more recently by Schmidt et al. (Schmidt et evoked auditory percepts should have resulted in a al., 1996) have provided us with a basic understanding positive lever press in the trained animals. Results from of the psychophysics of single phosphenes: their percep- a typical frequency of hearing experiment are shown in tual aspects, the currents required to evoke them, and, the psychometric function plots of Fig. 8A where the to a lesser extent, their stability. While these researchers percentage of trials that evoked a lever press is plotted also had a goal of understanding how multiple pho- as a function of the current (charge per phase) that was sphenes interact, their studies only slightly address this passed into auditory cortex. In this example, eight important question. It is this topic that will decide the frequency of hearing curves were measured from the ultimate feasibility of this technology as a means to same electrode over a 2 week interval, and the average provide a useful visual sense to the blind. 50% threshold was about 2 nC/ph. Schmidt et al. (1996) has demonstrated that safe The chronic percutaneous connectors used had lim- levels of currents (McCreery et al., 1994) can evoke ited pin counts and permitted access to a total of only individual phosphenes when delivered via electrodes 22 of the active electrodes in these three cats. Behav- that penetrate the cortex. However, we don’t know if ioral thresholds, measured on different electrodes in patterned electrical stimulation via large numbers of these three cats as the amount of charge injected per electrodes will evoke discriminable patterned percepts, phase of stimulation, ranged from 1.5 nC/ph up to 26 nC/ph. The average threshold, measured in 71 sessions or merely generalized blobs of light. For example, can a was 8.9 nC/ph. Fig. 8B shows the stability of four of trained subject discriminate the percept evoked by stim- these threshold measurements made in one cat over a 3 ulation of a square pattern of electrodes from the month period. Over the 100 days monitoring interval, percept evoked by a triangular pattern of electrodes? If thresholds varied by no more than 50%. This provides so, is this a trivial discrimination to perform, or does it an additional index of the biocompatibility of the float- require considerable attention by the subject? What is ing array design. the relation between current intensity and perceptual brightness, and how does a bright phosphene influence the perceived brightness and location of a neighboring 6. Conclusions dim phosphene? If patterned stimulation evokes pat- terned percepts, can a subject learn to associate these In this paper we have proposed that progress in patterned percepts with objects in the physical world? If cortically based visual prosthetics is closely linked to so, how much training will be required in order to progress in electrode array development, and we have make this association? These last issues are felt to be demonstrated a novel device, the UEA that could form key to a successful visual prosthetic: the basic plasticity the cornerstone upon which a cortically based visual of the higher visual pathways may cause them to re- prosthesis could be built. While the ultimate goal of our model with continued electrical stimulation to the point work has been the development of such a system, we where the phosphene images of the user’s environment 2584 R.A. Normann et al. / Vision Research 39 (1999) 2577–2587

Fig. 7. (A) Example of three single-units that have been isolated from a multi-unit recording from one electrode. Stimulus was a randomly modulated checkerboard. (B) Plots of the receptive fields of the three single units of (A). The plots show 12 vertical by 16 horizontal degrees of visual angle. The gray scale intensity of the receptive field maps is in units of standard deviation of a zero mean, unit variance, normal distribution. White is +25 standard deviations and black is −15 standard deviations. begin to become the defining features of the objects in specifications that can be used to create future genera- the environment. These problems present new research tion electrode arrays that may be better suited to a opportunities and challenges for the neuroscientist. visual prosthesis. However, the first step in this new line The pursuit of these basic psychophysical questions, of investigation into the psychophysics of phosphene both in the cortex and in the retina, will provide perceptions will be the development and evaluation of insights into the functional organization of the human new types of tools that will make this investigation visual system and into mechanisms of memory, learn- possible. We feel that the Utah, Michigan and Stanford ing, and cognition. One tangible result from these stud- electrode arrays are examples of such tools. ies will be the development of greatly improved design Before these tools can be used in human experimen- tation, however, their safety and efficacy must be well documented in animal experiments. The experiments described briefly in this paper and more completely elsewhere have shown that the electrode arrays with considerable geometric complexity (such as the UEA) can be safely implanted in the cerebral cortex. Our physiological studies demonstrate that such arrays can be implanted for over 3 years with little pathological complications. However, we have yet to refine our surgical techniques to the degree that we can ensure that every UEA implantation can be achieved without Fig. 8. (A) Psychometric function curves for a behavioral response to histological complication. current injections into auditory cortex via the UEA. Curves were Another significant limitation of the existing mi- measured for a single electrode on 8 separate days over a 2 week period. (B) Stability of current thresholds for electrodes over a 100 croelectrode array technology is connector technology. day period. Because our chronic implant system uses a 12 pin R.A. Normann et al. / Vision Research 39 (1999) 2577–2587 2585

Microtech connector, electrical access to 89 potentially 6.2. Retinal 6ersus cortical approaches to a 6ision useful electrodes in a 100 electrode UEA is not pro- prosthesis vided (one pin is used for a reference connection). Higher pin count connectors are commercially avail- Over the past few years, there has been considerable able, but these devices are larger and have greater interest in developing a vision prosthesis based upon insertion and removal forces. New connector designs direct retinal stimulation with extrinsic currents. Two based on the compression of anisotropic conducting general approaches have been proposed: an epiretinal sheets between aligned contact pads offer a novel ap- approach whereby the stimulating electrodes would be proach to this problem (Bioelectric Corp., Portland, implanted on the retina’s vitreal surface (Wyatt & OR), but these systems are not yet fully commercial- Rizzo 1996; Eckmiller, 1997) and a subretinal approach ized. A better approach to the problem would be to where the electrodes would be implanted between the multiplex the signals going to and from the UEA, and retina and the pigment epithelium (Chow & Chow, such integrated circuits have been built (Ji & Wise, 1997; Zrenner et al., 1997). While these two competing 1992; Jones & Normann, 1997, 1998), but these systems retinal approaches have their adherents, there are clear fail prematurely when implanted due to their high advantages and disadvantages to a retinal versus a sensitivity to ion diffusion and to the lack of a hermetic cortical approach, and these are summarized below. seal to protect them from the ions of the extracellular 6.2.1. Ad6antages of a retinal approach space. Telemetry offers an even superior solution to this interconnection problem, but such systems that can “ Could use existing physiological optics. If the photo- support the high signal bandwidth and low power sensors can be placed in the plane of the retina, the requirements of a 100 electrode array have yet to be optics of the eye could form optical images directly built. These are challenges for the engineer. on these sensors. “ Less invasive and less severe consequences of infec- tion. While the retina is central nervous tissue, a retinal infection has less severe consequences than a 6.1. Number of pixels needed for useful 6ision cortical infection. “ Closer to photoreceptors: easier spatial mapping. The researchers developing these electrode arrays are Remapping due to nonconformal retinotopy (Nord- not expecting to restore the panoramic visual sense hausen et al., 1996) would be less of a problem the enjoyed by sighted individuals. The long range goal of closer stimulating electrodes are to the photoreceptor the Utah researchers has been to develop systems that level. will provide the profoundly blind with independent “ As the implant is lower in the visual pathways, there mobility in typical visual environments. Interestingly, is more opportunity for natural processing of the the amount of visual input required to perform basic electrically stimulated images. visually guided tasks is not as great as one might expect. In a series of psychophysical experiments con- 6.2.2. Problems with a retinal approach ducted with a portable simulator of pixelized images, we suggest that a functional visual sense might be “ Retinotopy is dependent on stimulating ganglion cell restored in those with profound blindness with arrays (GC) bodies, not optic nerve fibers. If the injected containing as few as 625 electrodes (Cha et al., electrical currents preferentially excite optic nerve 1992a,b,c). A total of 625 pixels of appropriate visual fibers over GC bodies, the field of evoked pho- information (placed in a square grid, with a center-to- sphenes will not bear a rational relation to the center spacing of 4 min of arc), can provide sufficient distribution of electrodes. visual information to allow sighted volunteers to read “ Hard to ensure chronic GC/electrode apposition. In at 2/3 the rate that they could read with normal vision. order to ensure such apposition, the electrode array More importantly, 625 pixels allow volunteers to navi- will likely have to be a structure that penetrates the gate complex visual environments with normal speed retina to the level of the ganglion cell bodies. and good levels of confidence. This simulates the visual “ Encapsulation of the array will reduce GC/electrode sense that could be provided by a 25×25 electrode apposition. If the array encapsulates, it could be array, with a 0.4 mm interelectrode spacing (under the expelled from the retina, thereby decreasing GC/elec- incorrect presumption of perfectly conformal trode apposition. retinotopy). Such 625 electrode arrays have been built “ Multiple visual representations by ganglion cells. As with the same techniques used to build the UEA. The ganglion cells encode spatial, chromatic, intensity possibility of providing some degree of functional vi- and temporal information, it is not clear what fea- sion with as few as 625 electrodes is encouraging to the tures will be evoked by current injections via a Utah researchers. retinal electrode array. 2586 R.A. Normann et al. / Vision Research 39 (1999) 2577–2587

“ High rotational accelerations associated with sac- system could be mitigated by such neuroprosthetic cadic eye motions could dislodge array. systems. “ Limited to outer retinal pathologies. Successful cortical implant systems will not preclude “ Will patterned stimulation evoke discriminable pat- the continued development and eventual use of retinal terned percepts or generalized blobs of light? implant systems: both approaches are seen as being synergistic and each may have an appropriate set of 6.2.3. Ad6antages of a cortical approach pathologies best suited each implant system. The next few years will see these systems start to become applied “ The skull provides a more rugged housing for the in human experiments to provide proof of concept that implant. microelectrode arrays can provide a safe and efficacious “ Penetrating arrays have been and can be simply restoration of vision to the profoundly blind. inserted into visual cortex. “ Stable stimulation site. “ Successful long term animal studies. Acknowledgements “ Provides a solution to pathologies not only of the outer retina, but to the entire retina, optic nerve, and The authors want to thank John Donoghue at Brown . University for the data on monkey . We thank Vinh Ngo and Yanping Zhang for their help in 6.2.4. Problems with a cortical approach building these microelectrode arrays. Research sup- ported by the N.S.F. (grant cIBN 9424509), a State of “ Non-conformal retinotopic mapping may require ex- Utah Center of Excellence grant, and donations from tensive electronic remapping of visual inputs onto the W.M. Keck Foundation and John A. Moran. cortical electrode array. “ Multiple visual representations in cortex. Visual cor- References tex encodes color, intensity, orientation, ocular dom- inance, and other spatial features. It is unclear what Agnew, W. F., & McCreery, D. B. (1990). Neural prostheses; funda- kind of percepts will be evoked by stimulation of mental studies. Prentice Hall biophysics and bioengineering series. large numbers of cortical units. Englewood Cliffs, NJ: Prentice-Hall. Brindley, G., & Lewin, W. (1968). Short- and long-term stability of “ Convoluted cortical anatomy can make implantation cortical electrical phosphenes. Journal of Physiology (London), of large arrays or many small arrays difficult or 196(2), 479–493. impossible. Cha, K., Horch, K., & Normann, R. A. (1992a). 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