Tohru Ifukube, Dr. Eng.

Artifi cial organs: recent progress in artifi cial hearing and vision

Abstract Artifi cial sensory organs are a prosthetic means of sending visual or auditory information to the brain by electrical stimulation of the optic or auditory nerves to assist visually impaired or hearing-impaired people. However, clinical application of artifi cial sensory organs, except for cochlear implants, is still a trial-and-error process. This is because how and where the information transmitted to the brain is processed is still unknown, and also because changes in brain function (plasticity) remain unknown, even though brain plasticity plays an important role in meaning-ful interpretation of new sensory stimuli. This article dis-cusses some basic unresolved issues and potential solutions in the development of artifi cial sensory organs such as cochlear implants, brainstem implants, artifi cial vision, and artifi cial retinas.

Key words Artifi cial hearing · Cochlear implants · Artifi cial vision · Artifi cial retinas

Introduction

Artifi cial sensory organs are a prosthetic means of sending visual or auditory information to the brain by electrical stimulation of the optic or auditory nerves to assist visually impaired or hearing impaired people to lead a normal social life. In 2002, a survey by the Ministry of Health, Labour, and Welfare of Japan estimated that 180 000 people have grade 1–2 visual impairment and 90 000 people have grade

1–2 hearing/speech impairment in Japan. With the dramatic increase in the elderly population with severe visual and auditory impairment, interest in artifi cial sensory organs continues to grow.

Unlike functional electrical stimulation (FES) or the brain–computer interface (BCI) for artifi cial limb prosthe-ses, artifi cial sensory organs transmit visual/auditory infor-mation to the brain, the function of which changes by brain plasticity. Since much also remains unknown about brain plasticity, clinical application of artifi cial organs is still a trial-and-error process that requires continuous evaluation and improvement.

About 25 years ago, I spent a year at Stanford University doing collaborative research on a cochlear implant. Much of the controversy from that time still exists today. This article discusses some basic unresolved issues and potential solutions in the development of artifi cial sensory organs using modern research approaches.

Current state and outlook for artifi cial hearing

Establishment of the cochlear implant

The cochlear implant is an artifi cial hearing device that sends sound information to the brain’s auditory centers via electrical stimulation of the auditory nerve, which is still connected to the damaged hair cells. Many designs were developed over a 20-year period starting in the 1970s, and there was controversy over the advantages and disadvan-tages of each. Current devices are based on about 20 sound pitch range divisions, with implantation of a 20-channel electrode array in the cochlear duct to stimulate the audi-tory nerve (Fig. 1). The reason for using about 20 elec-trodes is that the stimulating current spreads through the electrically conductive lymph in the cochlear duct, and a further increase in the number of electrodes would not change the amount of information transmitted. In addition, the cochlea is a spiral-shaped organ, and it is diffi cult to place electrodes in the apex, where low-pitched sounds are picked up.

J Artif Organs (2009) 12:8–10 © The Japanese Society for Artificial Organs 2009DOI 10.1007/s10047-008-0442-3

Received: July 16, 2008

T. IfukubeDivision of Human Information Engineering, Research Institute for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, JapanTel. +81-3-5452-5065; Fax +81-3-5452-5031e-mail: ifukube@human.rcast.u-tokyo.ac.jp

This article is a translation of an article that fi rst appeared in Japanese in Jinkozoki 2007;36(3):198–200

REVIEW

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Previously proposed designs have included electrode insertion from the cochlear apex, projections as insulators between electrodes to prevent the spread of current, and winding of electrodes within the cochlea. To date, cochlear implantation has enabled more than 50 000 hearing-impaired people around the world to regain hearing that is suffi cient to carry on daily conversations. The cochlear implant is an example of a successful “artifi cial organ.”

Evaluation of brain function in cochlear implant patients has shown that normal speech areas are activated after 1 year of rehabilitation. The author have seen videos showing that children with cochlear implants who have learned to play the piano after a cochlear implant have also been able to learn to play the violin (Video by Dr. Kimitaka Kaga, who was a professor of the University of Tokyo until 2006. Unpublished). This has led to new areas in research on brain plasticity and pathways that allow recognition not only of speech but also of music.

Brainstem implants

One in 40 000 people has neurofi bromatosis, and the auditory nerve is often resected in those who undergo tumor resection for life-threatening complications. In these patients, a cochlear implant cannot be used, but auditory brainstem implants (ABIs) (Fig. 1), in which an electrode array is placed to stimulate remnant nerves to the central nervous system, may offer promise.

The cochlear nucleus is systematically arranged accord-ing to sound frequency, so there is some correlation between electrode position and perceived pitch. But in addition to the auditory nerve, the facial and glossopharyngeal nerves may also be stimulated. Thus, a maximum of only eight electrodes can be used.1 Newer devices with better fre-quency discrimination have been designed (e.g., the insert-

type ABI, Fig. 1), but high surgical invasiveness and poor correlation with pitch are still a problem. Since 1979, about 450 patients have received ABIs worldwide; however, infor-mation transfer is limited compared to a cochlear implant, so ABIs are used to augment lip reading.

Basic concept and future of artifi cial vision

The success of cochlear implants has served as an impetus for research on artifi cial vision in many countries. In general, a device to electrically stimulate the optic nerve within the eye is called an artifi cial retina. The broader term, which also encompasses direct stimulation of the visual areas of the brain, is artifi cial vision. In either case, the method involves division of the image detected by a camera into a grid pattern and stimulation with a two-dimensional elec-trode matrix. However, how computer chips process the information that they provide to the retina and the signifi -cance of stimuli transmitted to the visual cortex require further investigation.

Artifi cial vision

Research on artifi cial vision dates back to 1968 with a report published by Brindley et al. entitled The sensations (phos-phenes) produced by electrical stimulation of the visual cortex2 (Fig. 2). They reported “sensations of light (phos-phenes) like stars at an unknown distance in the nighttime sky.3” Subsequent research on artifi cial vision was carried out by Dobelle. Over a 20-year period of implantable device use, 15-cm Landolt ring pattern recognition at a distance of 60 cm and no problems with infection were reported.4 Despite feasibility issues and ethical concerns, this formed the basis of research in artifi cial vision.

Transmitter Receiver

Auditory center

Cochlea

Cochlear

nucleus

Auditory nerves

Electrode array

Speech processor

Cochlear implant

electrode

Fig. 1. Electrodes for artifi cial hearing and the associated implant positions. a, Cochlear implant; b, contact-type auditory brainstem implant (ABI); c, insert-type ABI

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Current artifi cial retinas

Artifi cial retinal implants are primarily indicated in par-tially blind patients with injury to the macula/fovea (where there is a high concentration of visual receptors) due to macular degeneration or retinitis pigmentosa. There are about 30 million such patients worldwide. Artifi cial retinas show the most promise in patients with residual bipolar cells (78%) and ganglion cells (30%), which are connected to visual receptors.6 In 1994, Dr. Liu developed a 4 × 4 elec-trode matrix for use in artifi cial retinas. These were implanted in volunteers with visual impairment.

About 4 years ago, I attended an international sympo-sium on artifi cial organs,5 spent 3 days with Dr. Liu, watched videos on the testing of artifi cial retinas, and participated in discussions on future directions in the fi eld. According to Dr. Liu, when the electrode matrix (Fig. 2) was fi rst devel-oped and implanted, patients were only able to detect whether a light or object was placed in front of them. But with improvements over a 10-year period, about 90% of recipients can now see faces and large fi gures, and some can even read up to 40 words per minute. However, to be able to read a newspaper, at least a 250 × 250 matrix is required. Even if this were to be developed, optimal place-ment in the retina and electrode wiring would be compli-cated issues that have not been resolved.

Artifi cial retinas have been used in several countries since 1995. In Japan, from 2001 to 2006, the Ministry of Economy, Trade, and Industry and the Ministry of Health, Labor, and Welfare sponsored a joint project entitled Research and Development of an Artifi cial Vision System. The project was featured in this journal in 2006.6 The basic design uses suprachoroidal–transretinal stimulation (STS) (Fig. 2). Instead of placement in the superior or inferior retina, electrodes are placed intrasclerally or suprachoroi-dally. This minimizes invasiveness to the retina before and after surgery. A trial device has already been evaluated in animal studies. The goal is to achieve the visual acuity to be able to count fi ngers at a distance of 30 cm. Clinical use of the device is expected by 2010.7

Future dreams of artifi cial organs

Whether the brain’s excellent plasticity in response to cochlear implant stimulation will be reproducible with respect to vision is still unanswered. Thus, at present, we cannot predict whether artifi cial retinal implants will enjoy the same success as cochlear implants. On the other hand, even though the usefulness of the cochlear implant has been demonstrated, its use in children attending special schools is somewhat limited in Japan.

At the same time, media conversion technology, such as speech-to-text conversion, is being used to help people with visual and hearing impairment. With advances in regenera-tive medicine, information technology, education, and reha-bilitation, the role of artifi cial sensory organs will continue to gain importance. Future knowledge of brain, visual, and hearing function may lead to cutting-edge technology, such as nursing-care robots. This cutting-edge technology may act as a “fi eld of dreams” providing feedback for the devel-opment of even better artifi cial sensory organs.

References

1. Takahashi H, Nakano M, Kaga K. The auditory brainstem implant. Biomech Syst 2007;4:173–216

2. Brindley GS, Lewin WS. The sensations produced by electrical stimulation of the visual cortex. J Physiol 1968;196:479–493

3. Ifukube T. Artifi cial vision/retina. In: JSAO (ed) Artifi cial organs illustrated. Tokyo: Haru Publishing, 2007;54–57

4. Dobelle WH. Artifi cial vision for the blind by connecting a televi-sion camera to the visual cortex. ASAIO J 2000;46:3–9

5. Liu W. Intraocular retinal prosthesis: a decade of learning. In: Proceedings of the 2003 International Workshop on Nano Bioe-lectronics. September 19th, 2003, Seoul, Korea, 2003

6. Kamei M, Tano Y. Artifi cial retinas. JSAO J 2006;35:348–351 (in Japanese)

7. NIDEK. 2008 (in Japanese). http://www.nidek.co.jp/artifi cial_vision_8a.html

Optic radiation

Suprachoroid

Optic nervesLateral geniculate body

Visual area

Optic tract

IntrascleraRetina

Fig. 2. Electrodes for artifi cial vision/retinas and the associated implant positions. a, Artifi cial vision; b, contact type of artifi cial retina; c, suprachoroidal–transretinal stimulation type of artifi cial retina