Prosthetic VisionWorkers resume the quest for a seeing-eye device

The cochlear implant provideshearing for deaf people by direct-ly stimulating the auditory nerve.

Scientists feel a similar prosthesis mayone day enable blind individuals to see.Teams of researchers across the coun-try are investigating the prospect of de-veloping implants that could deliverelectrical signals to neurons at variouspoints along the visual pathway or evento the visual cortex itself. Assisted bysuch instruments, enthusiasts hope, ablind person could acquire limited,though useful, vision.

The idea has a long, controversial his-tory. In the 1960s Giles S. Brindley ofthe University of Cambridge conduct-ed a series of highly publicized experi-ments. Brindley placed 80 electrodeson the surface of a sightless volunteerÕsbrain. He wired the electrodes to 80miniature radio receivers and sewedthe entire apparatus under the patientÕsscalp. When he transmitted signals tothe device, the subject reported perceiv-ing points of light, known as phos-phenes. Similar trials were conductedelsewhere until mounting skepticismconcerning safety and usefulness end-ed them.

After a lull of more than 20 years, theexperiments have resumed. ÒTimes havedeÞnitely changed,Ó says David J. Edell,who works with Joseph F. Rizzo on an

artiÞcial-vision project at the Massa-chusetts Institute of Technology andHarvard University. ÒPeople are a littlemore cautious about stepping oÝ intothe unknown with someone elseÕs bodythan Brindley was.Ó Edell reasons thatscientists have a much better under-standing of the nervous system now,an awareness that instills respect forhow diÛcult the task of creating a vi-sual prosthesis is. ÒThe idea is concep-tually very simple,Ó Edell notes. ÒBut todo it in a way thatÕs compatible withthe biological system is inordinatelydiÛcult.Ó

Nevertheless, careful research hasled to some real, if modest, success. In1992 a blind volunteer, who had elec-trodes implanted in her visual cortexby surgeons at the National Institute ofNeurological Disorders and Stroke, rec-ognized phosphene letters. Furthermore,the NIH team observed that mere micro-amps of current could evoke phos-phenes. ArtiÞcial-vision researchers givecredit for their achievements to newmaterials, developed primarily for con-sumer and military electronics. Modernelectrodes, such as those made fromsilicon, are less than one hundredththe size of those used by Brindley, theworkers say. Because they are so small,they can penetrate the visual cortexand excite a highly localized popula-tion of neurons.

Unfortunately, our visual world doesnot consist of discrete points of light.ÒIt has yet to be shown that patternedstimulation via a large number of elec-trodes will evoke a complex perception

rather than just a diÝuse blob of light,Ósays Richard A. Normann, who headsthe visual prosthetics project at the Uni-versity of Utah. To answer that question,NormannÕs group has built a three-di-mensional array of 100 electrodes in-tended to stimulate the visual cortex.To date, only subunits of 10 electrodeshave been tested at a time.

If patterned stimulation does indeedgenerate adequate images, then whatcould the blind expect to see? KennethW. Horch of the Utah team has shownthat a small number of electrodes mightproduce a diminished, but valuable, vi-sual sense. In simulation experiments,Horch blocked volunteer studentsÕ vi-sion using perforated masks. The par-ticipants saw 625 points distributedover a 1.7-degree angle, a span roughlythe size of your thumbnail at armÕslength. Under these conditions, the stu-dents were able to read text from acomputer screen at two thirds the ratethey normally could.

Teams at M.I.T. and Harvard and atDuke University are designing artiÞcial-vision systems to stimulate the retina.Such an aid could capitalize on the nat-ural image-forming processes of theeye. Still, the resolution delivered froma retinal implant would be far fromideal, and this intervention would workonly for those patients who had not sus-tained severe damage to the retina orto the optic nerve. All the same, somepatients could avoid the risk of brainsurgery.

The designs for retinal implants, liketheir cortical cousins, are challenged bya host of problems, among them the is-sue of how to attach an instrument tothe retina itself. ÒAnything thatÕs placedagainst the eye will tend to ßail aroundwhen the eye moves,Ó Normann notes.Because the eye moves very quickly, anobject of any real mass could cause serious damage. ÒThe retina is like wettissue paper,Ó Edell explains. ÒTheslightest tear can cause a retinal de-tachment.Ó To avoid such an outcome,the M.I.T.-Harvard team hopes to de-vise a silicon implant no more than 20microns thick. The researchers wouldcommunicate with the device via a laserdiode mounted on an eyeglass frameattached to a video camera.

ÒIÕm not willing to say weÕre home-free with artiÞcial vision at all,Ó Nor-mann says. ÒBut the technologies wehave developed recently will begin toallow us to ask whether it could be-come practical or not.Ó Normann sug-gests that clinical testing of cortical im-plants may be possible within the nextÞve years and that commercial artiÞcialvision systems could be a reality in the21st century. ÑKristin Leutwyler

108 SCIENTIFIC AMERICAN March 1994

SILICON ELECTRODE ARRAY, built at the University of Utah, may one day be usedto stimulate neurons in the visual cortex. The three-dimensional array contains 100electrodes, each 1.5 millimeters long and 0.08 millimeter wide at the base.

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Copyright 1994 Scientific American, Inc.