problems of sensory prothesis

8/13/2019 Problems of Sensory Prothesis

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PRO LEMS OF SENSORY PROSTHESISN O R B E R T W I E N E R

Th is is an age in which a gre a t dea l i s be ing done to he lp the han di capped. There i s an improvement in ar t i f ic ia l l imbs for the maimed,in new sorts of spectacles , and in hearing aids. All this is returning touseful l i fe a considerable part of those who would in other generat ionshave been hope les s ly hand icapped . Even the tw o g rea te s t hand icapsf rom which humani ty suf fersbl indness and deafnessare the objec to f pow er fu l and de te rm ined a t t acks w i th the ob jec t t ha t w here thesedefec ts cannot be removed or correc ted by medica l means , these v ict ims may s t i l l be a l lowed to take , as near ly as poss ib le , a normalpart in life.

Notice that these two aff l ict ions are sensory defects , that is , defectsin the recept ion of impress ions f rom the outer universe and f romothe r hum an be ings . F ro m th e s t an dp o in t of the ou te r un ive r se , b lindness is overwhelmingly the greater of the two losses. The sense ofhea r ing , a s b rough t ou t by such inves t iga t ions a s have been m ade bythe Bel l Telephone Company in the i r s tudy of speech, and as fur therevidenced by the s ize of the speech areas in the brain, is a sensew i th vas t ly less va r ie ty th an th e sense of s ight . On th e o th er ha nd ,n o r m a l c o m m u n i c a t i o n b e t w e e n m a n a n d m a n g oe s b y m o u t h a n dear much more than by a l l o ther channels , and the socia l and emot iona l dam a ge don e by dea fness i s d i sp rop or t iona te ly g rea t w hen onecompares i t wi th the socia l loss of the b l ind . The typica l emot ionalpic ture of the deaf man i s tha t of a reserved, se l f -conta ined, neurot icpersonal i ty , whereas the typica l p ic ture of the b l ind man who hasachieved any degree whatever of equi l ibr ium is tha t of a euphor ica l lyconf ident and ra ther cocky personal i ty .

In order to make any effort to replace the lost sense, ei ther of theblind or of the deaf i t i s necessary for us to make a ra ther accura tem ea su re of w h at t h ey h av e lost . I t is th en n ece ssary for us to see ifthe re a re any re l a t ive ly unused channe l s in to the hum an ne rvous sys tem which are capable of supplying the whole or any cons iderablepar t of what i s los t . These are problems of phys io logy and comm u n i c a t i o n e n g i n e e r i n g , a n d h a v e a m o s t i m p o r t a n t m a t h e m a t i c a ls ide .

To br ing out th is mathemat ica l s ide adequate ly , I wish to refer toThe twenty-third Josiah Willard Gibbs Lecture, delivered at New York City,December 28, 1949, under the auspices of the American Mathematical Society; received by the editors April 10, 1950.

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28 NORBERT WIENER [Januarycommunication mechanisms which are more accessible to us in theirwhole than the human nervous system. A very interesting type ofmachine which is engaged in continual conversation with its surroundings is the remotely controlled power station. It is not unusualfor water power to be available under just those circumstances whichmake the site highly unsuitable for human habitation. Under theseconditions, a power station may be built without any resident engineer or operative. The place of such a resident operative is taken inpart by a visiting operative who packs in at long intervals to replaceand repair vital parts, but who is much more effectively replacedby automatic operation and long distance control.In the automatic operation of a power station, one meets much thesame problems which one finds in the operation of a railroad switching and signal system. There are certain things which must not bedone at all costs. No generator should be connected with the bus barsun til it is bro ug ht u p to speed, and is running in the proper p hase relations with the othe rs. A penstock m ust no t be thrown suddenly openin the lower end, or it will collapse under atmospheric pressure; and,on the other hand, it must not be subjected to the shock of too sudden turning on of water, or it will burst. These contingencies must beblocked in the same way as the conjunction of a closed switch and anopen signal must be blocked in railroad practice.These are relatively simple undertakings. As stated, they seemscarcely deep enough to have a mathematical form. However, behindthem is a principle which is well susceptible to mathematical treatment. The principle, namely, is that the performance of a piece ofapparatus should be fed back to it as information on which it is tooperate.What is more interesting to us mathematically at present, however, is no t these pu rely local devices which m odify the na ture of theterminal apparatus in the generating station, and may be consideredas part of the operation itself but rather the messages which go between the load dispatcher and the station. Such messages must exist.A little cloud passes over the city to which the power is sent, andthousands of lights go on in the offices, shops and homes. This immediately demands the sending of more power; and this in turnmeans the starting of new turbines, the switching in of new generators, and a thousand and one other operations. Let there be an accident in the remote generating station, and even if this accident canbe compensated for temporarily by automatic machinery, a messagemust go out to the plant engineer to send in a troubleshooter torestore the plant to maximum efficiency. In other words, the auto-


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W i l PROBLEMS OF SENSORY PROSTHESIS 29matic substation must listen, and it must speak. Since, however, itsears and its mouth have been made by engineers for these specificpurposes, and not installed by a complicated embryological process,we know far better what to expect of them than we do of humanmouths and ears.It may be remarked in passing that it is quite common practiceto transmit these messages to and from the power station either overordinary telephone lines and similar facilities, or over a carrier sentover the power line itself. In either case, they are generally codedmessages like those of teletypewriters. If we wish to go to the troubleto make them vocal, all of the resources of the telephone and phonograph are at our disposal, but there is generally no particular point inthis rather precious imitation of human function.What the casual listener may miss in this account is the fact thatthe operation of such a system involves several stages of linguistictranslation. The message from the dispatcher to the station is orshould be committed to the line in the form best suitable for linetransmission. On the other hand, it must appear in the station, notin the form of a coded sequence of dots and dashes, but in the formof a series of openings of switches, turnings on of valves, measurements of the speeds of motors or generators, and so on. The languageof command must be translated into the language of action.To understand this, we must realize what information is, and howit can be measured . Wh en I send a series of dots and d ashes, the prob lem of the information that it carries cannot be answered in terms ofthe message I send, taken by itself apart from all other messages. Asingle click ma y be ab le to do a large num ber of things, such as opening a sluice, connecting a generator to the bus bars, or replacing abu rnt out electronic valv e; but it cann ot do these things differentiallyIt is only when I know the number of separate alternatives whichmust be transmitted and combined that I can determine how manydifferent messages I must be able to send. It then appears that thesignificance of a signal is determined not only by that signal itselfbut by the whole set of signals I might have sent in place of the actual one. In other words, significance is a property belonging to ensembles.Another property belonging to larger and smaller ensembles is thatof probability, or what is its equivalent, their Lebesgue measure. Ido no t intend to go into t he w hole question of the theoretical basis ofprobability, but shall only state that, under certain rather wide conditions, a smaller set of signals has a probability when taken to bepart of a larger set, and that this probability is a number between 0


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3 NORBERT WIENER [Januaryand 1. Now, the probability that two independent events shouldhappen simultaneously is the product of their individual probabilities.On the other hand, if information is to have a significance even remotely resembling its ordinary meaning, the amount of informationgiven by two independent signals should be the sum of the amountof information which they convey separately. Under these conditions,since multiplication of probabilities corresponds to addition of information, the amount of information given by a signal can only bea constant multiple of the logarithm of its probability. Moreover,since the information is positive when the prob ability is less than one,the ratio between the logarithm of the probability and the amountof information must be negative.The notion of the logarithms of probability is a familiar one forstatistica l m echanics. The logarithm of a proba bility is calledentropyIn an ordinary statistical system, the entropy can increase spontaneously, but can never so decrease. In the transmission of information,we may lose information, but can never gain it, except from a newsource. Thus in the translation which occurs between the messagesent into the automatic station, and the operation of that station,there m ay be a loss of information, bu t there ca nno t be a gain. Und erideal circumstances, we attempt a coding which will correspond asnearly as possible to the actual operation of the power station, sothat we are not sending in unused material, as we should for exampleif we were to transmit our orders by voice. Nevertheless, it is hardlyto be expected th a t ou r code is so perfect t h a t we lose no information.Let us go from this problem to the problem of transmission of amessage by voice or by eye from person to person. There is in thefirst instance a limitation of the rate at which information can betransmitted. It depends on the system of repeatable and recognizablesignals which can be made with our vocal organs or our pen, or whatever other means we use of active communication, and which will beaccepted by the air, or whatever other channel of transmission weuse. I say recognizable, but I have already introduced an ambiguitythereby. Recognizable how, and by what? If I allow the recipient thefull choice of any detecting and analyzing instrument which he maychoose to dem and , I get a certain r ate of transmission of information,which is characteristic of the sender and of the means of transmissionb ut which is no t cha racteristic of the reception of this message by theordinary listener or viewer. It is information of this type with whichthe communication engineer has been chiefly concerned in the past,as it is by all means the simplest to handle. However, when thisinformation penetrates through the ear or the eye into those parts


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195 1 PROBLEMS OF SENSORY PROSTHESIS 31of the human nervous system which are specifically concerned withsound or sight, it undergoes a translation by a m echanism of which weknow something, but not everything. As a matter of fact, it undergoes two or more translations. The first of these represent what isnecessary to pen etrate into the hum an sense organ, and through th atsense organ to those parts of the brain which are in more or lesspermanent connection with it . The later translations interpret thissuperficial sensory representation into meaning, and give to what isreceived a more or less permanent place in memory and understanding. In the case of sound, the first translation is called phonetic andthe later translations are termed semanticThe problems of the loss of information in the two human translations, phonetic and semantic, are not at present susceptible to adirect investigation through the structure of the nervous system.They are however susceptible to an investigation which is empiricaland experimental. Work of this sort has been done by the Bell Telephone Company, with special reference to the recognizability of soundas speech. The original measures of information, which were givenmore or less crudely, referred to the bands of frequency necessary forthe formation of intelligible speech; and in other words were almostmore measures of what the outside line and the air could carry, thanof what the ear could translate. The reconstruction of intelligiblespeech from clicks and buzzes, by way of wave filters and resonatorsand oth er a pp ar atu s of the sort, shows th a t the loss of information between the ear and the part of the nervous system belonging to theear is overwhelmingly great, and that a crude imitation of speech involving less than 1/10 and perhaps even 1/100 of the informationcarried over a telephone line, is enough for adequate conversation.It was in view of this work that Dr. Wiesner of the ElectronicsLaboratory of MIT discussed with me the question as to whetheranything could be done for a totally deaf man, to replace the missingsense of hearing in its most important employment, that of the recognition of conversation. The question was put to me by Dr. Wiesner,and the two of us were independ ently led to the same answer: nam ely,that the recognizable part of speech was so scanty, that it was notbeyond our hope to replace it by the sense of touch backed up withade qua te equipm ent. In other words, it appeared to us th at it is perfectly possible to make the transition between sound in the outsideair, and the semantic recognition of speech, by an artificial phoneticstage, making use of touch, and supplemented by adequate electricaltools. Having come to this conclusion, we started the detailed designof such apparatus. We took speech through an ordinary microphone.


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32 NORBERT WIENER [Januarynot dissimilar to that used as a receiver for the usual hearing aids,an d qu i te ade qu ate to ta ke in a ll th a t is in te ll ig ib le in speech. W ethen spl i t the band rece ived in to a number of f requency bands , making use of wave f i l ters . As we intended to use the hand as a receiver ,five seemed th e na t ur a l n um be r of ba nd s to tak e , becau se we couldthen carry them to the f ive f ingers . However, we are not yet f inal lydecided th a t five i s the r ight num be r and m ay exper im ent on supp lement ing i t by addi t ional s ta t ions in the pa lm of the hand, so as tobr ing the number up to s ix or seven. The d i f ferent bands we have sofar taken to be of equal logar i thmic length , or length when measuredin te rms of the number of keys of the p iano corresponding to them.Whether th is i s the bes t mode of speech, we are not ye t sure . I t i s asubjec t for fur ther exper iment .

At any ra te , a f te r rece iv ing and separa t ing these bands we rec t i fythem , w hich m eans , nea r ly , t ha t w e rep lace the t im e-pa t t e rns o f tha tba nd b y i t s enve lope . The se rec ti fied messages are the n used to m odu la te an equal number of v ibra tors a t f requencies of something l ike100 cycles each. This is because the skin is very insensi t ive, both tonearby cons tant messages and to messages of excess ive ly h igh f requency. These modula ted messages are then car r ied e i ther to e lec t r i cal mechanical vibrators placed on the f ive f ingers , or to electrodesdes igned for the d i rec t s t imula t ion of the nerve endings in the skin .From the s tandpoint of l ightness of weight , i t would seem tha t adi rec t e lec t r ica l s t imula t ion i s des i rable . However , there are severa ldiff icul t ies to be overcome. The variable resis tances of the skin, andthe f ac t t h a t cu r ren t r a the r th an vo l t age is w ha t s t im u la te s tou ch ,bo th ind ica te th a t i t i s des i rable to use co ns tan t curre nt e lec t rodes ,which can be proper ly des igned. However , even wi th these , the margin between the lower threshold of feel ing and the lower threshold ofpa in i s so sm a l l t ha t our appara tus m us t w ork ove r a ve ry na r rowrange , and our present models need cons iderable modif ica t ion .The m echan ica l v ib ra to r , on the o the r hand , t ends to be undes i r a ble , because i t s current demand i s a t present so grea t as to requi rean ap p ar a t u s which is s t il l too he av y to be com for tably po r tab le .We bel ieve tha t whether we f ina l ly ar r ive a t mechanica l or e lec t r ica ls t im ula to r s , t he p rob lem of m ak ing the appara tus por t ab le , and in deed of comparable weight wi th tha t ac tua l ly used in exis t ing hear inga ids , i s not too diff icul t to be solved, and is more one of developmentthan any th ing e l se .Let me f irs t s tate how our resul ts s tood when we lef t off work earlyin the sum m er . Th i s w ork w as under t aken by D r . Leon Lev ine , w i ththe coopera t ion of Dr . Alex Bavelas of our Psychology Depar tment


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W ] PROBLEMS OF SENSORY PROSTHESIS

in mat ters which concerned techniques of teaching. I t was des i rableto see if there was in fact a sufficient basis of recognizable and dist inguishable sy l lables in speech for us to go ahead. In the beginning,w e d id no t exper im en t w i th the deaf but wi th hear ing people whoseears were f i l led with such a jumble of ar t i f icial noise that they werequi te comple te ly deaf for the purposes of the exper iment . We thengave them a sequence of sy l lables by the machine accompanied wi tha v isual pa t te rn d isp lay of these syl lables . Af ter t he y ha d learnedthese, we subjected them to a series of t r ial syl lables selected from thesequence; and we recorded the successes and fa i lures . The bes t runwe had was a series of 80 select ions among 12 syl lables with only sixer rors .I t has been our idea f rom the beginning tha t the learning processnecessary for achieving proficiency with the new method of receivingspeech m us t be som eth ing qu i t e long and a rduous , and w e have no twished to involve any deaf mutes in an exper iment which mightgive them a fa lse hope , or a t any ra te a bad learning technique . However , we did make a cer ta in number of exper iments wi th a b l ind deafmute , who came to us wi th h is brother . The two had learned tocommunica te together by speech, which was recognized by the deafm ut e when he p laced h is ha nd s on h is br oth er s la rynx. T he deaf m ut ecould h imsel f enuncia te a few words , but the enuncia t ion was verybrea thy and bad. Af ter he had been put in the appara tus , he was forth e f irs t t im e in his li fe able to com pa re his b ro th er s speech w ith hisow n, pa r t icu lar ly in th e enu ncia t ion of h is own na m e and th a t of h isb ro the r . Wi th in a m a t t e r o f m inu tes , and ce r t a in ly o f hours , t he im pro vem ent in h is enun c ia t ion w as no t m ere ly no t i ceab le , bu t ove r wh elm ing. I t was per fec t ly c lear to us th a t our ap pa ra tu s is an ade quate way for a deaf mute , even for a b l ind deaf mute , to moni tor h isown speech.I want to say a t th is point tha t soon af ter we had begun our work,we recognized very defini tely that i t was closely related to the visualspeech developed by the Bel l Telephone Company. I a l so wish tosay th a t t he pa ten t s w hich the B el l Te leph one C om pan y has basedon th is work were so phrased as to cover the work we had done , a l though our m e thods had no t been ac tua l ly em ployed exper im en ta l lyby th e B e ll w o rke r s . T h e adv an tag e w hich w e be l ieve our m e thod spossess over v isual speech, in addi t ion to the obvious one tha t theycan be used by b l ind deaf mutes , a re tha t we have been working forpor t ab i l i t y in the appara tus . I t w as our conv ic t ion be fo re w e s t a r t ed ,and i t has been very grea t ly conf i rmed by our resul t s , tha t the appara tus which we f ina l ly achieve should be comple te ly por table , and


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34 NORB ERT WIENER [Januaryshould not deprive one of the concurrent use of such an importantsense as vision. I t is only by this po rtability th a t th e speak er s wordscan be continually monitored, so that he is never allowed to fall intothe faulty speech of the congenital deaf mute. We hope that, with theuse of this app ara tus , the speech -training of the deaf m ute can be begun in early childhood, and that although it will be arduous training,it may not at that period be too different from that training whichhearing children receive in learning to speak.It will be a curious study to see what these hearing aids will dofor the musical sense. They were not designed for this purpose, and Ido not expect any very great results. Still, people like Helen Kellerhav e reached a surprising degree of proficiency in recognizing musicalvibrations with their fingers, and it is scarcely to be expected thatthe results with these machines will be worse than with the barefingers. All in all, however, the social function of he aring is so nearlyexclusively a matter of the reception of speech, that a person who canfollow speech in the basis of sound carried by the air, and can do thiswith reasonable proficiency, can scarcely be considered socially deaf.It is a little unfortunate that we have to use so valuable a part ofthe body as the hand for a receiver. We intend to use the left handalone, and even as far as this is concerned, we intend to hold thereceivers in a flexible glove, so that a gross use of the grasp by thehand is not incompatible with the simultaneous reception of speech.The trouble with the other sensitive receiving parts of the body isthat the best of them lie around the mouth which must be left withou t impedimen t to participate in conversation.It is thus conceivable at least to us that deafness as a considerablesocial problem might even be eliminated, except for those not mentally able to face the stringent training needed, or for those withserious concomitant physical defects. We have no such expectationfor the replacement of vision.There is reason to believe that the total amount of information received at a given time by a normal eye is of the order of 50 to 100times what is received in the same time by a normal ear. In this, thetyp e of m easure m ent of vision as given b y th e oculist s ratio is deceptive. Vision which is 50/100 means ability to recognize details at 50feet which would ord inarily be recognized a t 100. How ever, the num ber of details varies as the square of the distance, so th a t from thispoint of view 30 /100 vision should give abo ut a qu arte r of the am oun tof possible information which normal vision gives. On this basis,if the whole auditory region of the cortex were turned over to thepurposes of vision, we should expect about 10/100 vision. This is


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95 ] PROBLEMS OF SENSORY PROSTHESIS 3 5

not blindness, but very poor vision indeed. Touch is an even worsesurrogate for vision. Under these circumstances, it scarcely seemsworth while to try to make a universal apparatus to replace vision forall purposes.There are two directions in which progress should be expected.One of these is a read ing a pp ar atu s, w hich will trans late the pages ofan ordinary book into a sound or touch pattern recognizable by theblind. The other is an aid whereby the blind man may go aroundmore freely ou t of doors and in unfamiliar room s. In the constructionof an apparatus of this latter sort, there is a limitation below as wellas a limitation above. Any apparatus must be better than what theblind m an ca n do for himself b y using such odds and ends of sensa tionas au dito ry echo , air tem pe ratu re, air pressure of the forehead, and soforth.Finally, besides the problem of the blind and thedeaf the problemof the maimed is also one with a profound sensory aspect which isoften neglected. The better an artificial limb is from the standpointof variability of performance and flexibility, the more information asto its position and as to the strains on it which the cripple needs if heis to walk securely. Similar remarks apply to artificial hands. Thereis a very con siderable future in the a rt of applying strain and pressuregauges to artificial limbs, in such a way as to furnish the cripple withbetter sensory monitoring than he can obtain with the aid of anything but the skin of his stump. What the cripple has lost is not onlymotion, but sensation as well. He is an ataxic, as well as a paralytic,and anything that can be done to furnish him with sensory feedbackchannels other than those which he normally possesses is likely tomake a great contribution to his effectiveness and to his well-being.M A S S A C H U S E T T S I N S T I T U T E O F T E C H N O L O G Y

problems of sensory prothesis

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