l - 15

orientation, t ' Íovênent and Uotor Skil ls in Divers.

H . E . R o s s .

Àbstracè.

The orientation of divers undêr water is disturbed by sevêralfactors. Visual factors inctude: optical disorti.on caused byrefraction at the facenask in vatêr; lolr visibil i ty; war.ernovement; absent or nisleading visual Iandrnarks; a;d. loss ofFeripheral vj.sion. Àuditory factors includè distortion ofangul.ar localisation and poor di.stancê 1ocalisation. Vestibu-lar factors include alternobaric vertigo, narcosis and HighPressure Nervous Syndronê. Tactife-kinaêsthêtic factors incl.u-dê! reducêd tacti lê stinutation; reduced awarèness of l ihbposition; rêducêd kno\,rledgê of achiêvêd Loconotion; d.istortedsense of body and object \^'eight; and a disruption of motors k i l 1 s .

In t roduc t ion .

The divêr operatês in a hosti le environnent which dêprives anddistorts his senses in various ways. The physiotogicàI andtechnical factors affêcting manrs survival under *ater havebeen well describêd j.n nlany nanuals. Much 1êss attention hasbeen paid to sensory and notor factors, though Êections existin sornê books (è .9 . Woods and Ly thgoe, 1971t Ross , 1974;Sh i l l i ng e t a I . , 1976; Drew e t a l . , 1 -976) , and a fe lv a rela rge ly_devoted to thê top ic (ê .g . Àdo l fson and Berghage,1 .974; Krnney , 1985) . Th is rêv iêw concênt ra tes na in ly on \ i ro rksince 19?4, though earliêr works arè nentioned wherè they areof special relevancê or ale not well known.The altêred physical and sensory ênvironnêht can give rise tonispêrceptions of bodily oriêntation and rnovernent. There arenany _ contributing factors, affecting the visual, auditory,vesÈibular and tacti le-kinaesthetic sysCêns - all ot wniàÀinteract in controlf ing posture and novênent.

V isua l fac to rs .

. l h l - i - . I h i c+^ ' l . i ^h

The unprotêcted huhan eye is extremêty long-sighted j.n watêr.The rêfractivê polrer of the cornêa is lost, beèause the densi-ty of thê cornêa is sinilar to that of rdatêr. This results ina loss o f focuss ing power o f about 45 d iop ters (K innèy 1985) .Norrnal focussing is restorêd if a facernask is worn, allowingthe eye to operate in air, Ho\.rever, thi6 introducês a distoi-t ion alue to rêfraction of l ight at the air/glass/watêr inter-facê: the rays are refractêd away frorn the norrnal when passj"ngfroin thê denser lrater into the less dense air. This causes anangular enfargement of about 4/3, and a reduction in the inaged is tancê to about 3 /4 o f thè phys ica l d is tance. I t a lso causèscurvature distorl ion, thè deviation increasing towards the

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periphery of a flat facemask in aecordancè lrith the sine Iawof refraction. Thêse distortions affêct the perceived size anddistance of objects, and their pêrceived angular oriêntationwhen not in thà centre of the field of view. Non-centralobjects appear rnore displaced towards the peri.phery than theyreal.Iy are. The result j-s that an inexperienced diver tênds tornisrêach for objects - he underreaches for all objects, andreaches too far to the left for obiects on Èhe lefÈ and toofar to thê right for objêcts on thá right. For thê sarne !êasonhe rnay perceive thê orientation of his hêad Lrith respêct to anon-central object as having too largê a deviation. This couldaffect the direction in vhich a diver swirns, if hê is attemp-ting to navigatê at an angle to a tandnark.Ref rac t ion a lso causes a d is to r t ion o f s lopê. À d ivêr sw imningalong the seabed wifl sèê a flat surface rising torn'ards hin inthe distance. This should di.stort a diverrs DèrcèDtion of thetruê horizontat - but thêre appear to be no èxperiments onthis hatter. Sinilarly a diver looking up wil l sêe thê surfaceas t i l t i ng down in the d is tancê.l ' losÈ divers show sone dêgree of inrnèdj.ate adaptation to thêsêdistortions, follóL'éd by slouêr subsequent adaptation and byopposite aftereffects on return to Iand (seê revielrs by Welchr1978; K inney , L985) . Àdapta t ion probab ly invo lves an appropr i -ate change in the galn of the vestibulo-ocular reflex (VOR),as has bêen shown for hagnifying Ienses in air (cauthier andl < O l f , I n S O n - 1 9 l 5 1 -

Low v is ib i l i t y and D is tance Pêrcèpt ion .visibil i ty is huch poorer in Írater than in air, êven in thêclearest lrater. Thê finê particles in the water scatter anélabsorb the Iight, the effect varying vith the r^'avelength. BfueIight is scattered and red absorbed, giving c]êar nrater a bluecast. Àdditionally there rnay bê yêllowish vegetable nattêr andother natêrial in rivers and coastal- water, producing a grêenappearance; or red rnaterial in a peat loch, giving a darkreddish brown appearance. These effects systenatically alterthe perceived colours of objêcts, dêpending on their depth andon their hori.zontal and vêrtical distance fron thê viêwêr.Changes in colour do not in themsêIves affecl a dj.verts sensêof orientation. Hovrever, thê narkêd loss in colour andbrightnêss contrast vhen viewing objêcts horizontafl-y tnakesthen appear too far away in the distancê. This is sinilar toviewing objects in a fog on land - an effect known as aerialperspec t ivê (see Ross 1975) . The increase in pêrce ivèd d is tan-ce j.s the opposite of thè êffèct caused by thê distortion ofthe fàcenask. Thê result of these cotnbinêd distorl ions is thatobjêcts appêar too near at short distancês and too far at lonqidistances, the changeover point j-ncreasing rfith thê clarity ofthe water. ft can vary fron about onê nêtrê in rnurky vater toabout 20 n i.n the clearest uater.It might bê thought thàt stereoscopic inforrnation would over-ride aeriaf perspective and ensure that objects are seen atthe i r op t ica l d is tance. Howevêr , s re reops is i s f requen l lyover r idden by conf l i c t ing in fo rna t ion . s te reops is i s par t i cu-larly poor in !,/ater, being reducêd by a factor of thrêe ornore cornpared to air. I,íhi1e facenask nagnification should

1 1 7

ihprove sterêoacuity, in practice it is dêgraded by suchfactors as low luninance contrast, the ihpoverished visualscênê, Ioss of pêripheral vision, and perhaps inappropriateaqcommodation (see Kinnêy, 1985) . other cuês that nornallyassi.st distance perception in air are tèxturê gradiênts andnotion para1lax. Têxture gradienls arê absent or tnuch reducedin host underwatêr scenês. It is clainêd that motion parallaxcontributes l itt le under water, perhaps because rêfractiondistorts the nornaf rêlation bêt$rêen hêad or êye novenents andnovêrnent o f the rê t j .na1 inagê (Fêr r is , 19?4) ,

Spêed Pêrcêptj.on,Distortions of percêivêd sizê and disCancè contributê todistortions of the pêrceivêd speed of objêcts. percêivedvelocity is pèrtly determinêd by the perceived distance tra-vêlfèd over perceived tirnê. civen consistent t i l l lê êsÈirnates.objècts travêll ing across the l ine of sight should appear totravêl further in the sane tihê, and thus faster than in air.Ob jêc ts t rave l l ing a t c lose d is tances a fong the l inê o f s igh t ,to!,rards or away frorn the divèr, should appêar to travèl astrorter distancê and thus nore slovrly than in air. Sbeedd is to re ions in the pred ic têd d i rec t iàns were found b 'y noss andRejn lan (19?2) , À t g rea ter d j .s tancês , where percê ived d is tancêsincrease due to aerial pêrspêctive, an increase in perceivêdspeeal vould bê predictêd. However, this has not beên testêd.subjects partially adapt to thesè distortions after severalninutes of vearing a facenask in vater, and shov nêgativea f te rê f fec ts ( reversed er ro rs ) in a i r .

Factors Àffecting tunbient Vision.Whên the d iver descends su f f i c ien t ly deêp h is v isua l acu i tysuffèrs due to the rêduction in the l iqht 1evel throuqh ab-sorp t ion by the par t i c les in the water l In c lear oceaÁ waterin good day l igh t , photop ic v is ion ( i l l un ina t ion grea têr thanabout 3cd/n, ) rnay continuê to dêpths of abouÈ 1O0-3OO m;rnesopic and then scoÈopic conditions (i l lunination less than10-3 cd , /n ' ) ho ld a t g rea ter depths , sone usefu l v is ion cont i -nuing to about 5OO-l-OOon. Thê thèoretical depth Iinit is hardto catculaÈe, as it depends on the attenuating properties ofthe water at diffêrent wavelengths and on thê crit ical wave-lenqths for the scotopic êye (Kinnêy, 1985) . In turbid wateror overcast conditions thê chahge to scotopic vision is ofcourse reached at nuch shallower depths. Low il luhrinationaffects visual acuity, the lurninancê contrast nêeded forobject dêtêction rising froh about 2? in photopic conditionsto 10? in nesop ic cond iL iohs and 3Og in sco top ic cond i t ions .Ho!,revêr, thê reducêd il lunination has less êffect on divêrorientation, sincê the anbiênt orienting systen operatessat is fac to r i l y a t Inesop ic leveLs .

Per iphera l v is ion cont r ibu tês to ambien t o r iên ta t ion , bu ! thef ie ld o f v iew is ser ious l -y reduced by the d ivêr 's facenask . I tj-s possible to usê a large vrap-around nask, but these are notpopu lar due to the d i f f i cu l ty o f c lear i -ng the la rge a i r spaceand to the increased optical distortion in the pèriphery. Fornos t facernasks in water the use fu l hor izon ta l f iê Id o f v iew is

1 1 8

rèduced to about 80-85 degreês , conparêd to 180-190 dêgreês ina i r w i thout a Í \ask ( I ( innêy , 1985) . Th is leads to a reduc t ionin sensitivity to pêriphêral movênent, and a deterioration instereoacuity and depth percêption.The anbient orienting systen is particularfy sênsitivê tooptic f low and to vertical and horizontal l ines, all of whichÍ1ay be nisleading under vratêr. Waves and currenis rnav Droducestrêaning of particles and pl.ants past a diver, inauciirg thesensation that he is noving in thê opposite direction (vecti-on) . The underlvater topography is usualty ]acking in goodvisual indicators of the vêrtical and horizontaL. The sèabednay slope, plants nay grow out of cliffs at an anqle to thevertical, and nan-nade objects such as shiprrrecks tend to tieat an ang1e. The visual infornation available for orientationis thus nuch irnpairêd under watêr.

Àuatitory factors.

Àuditory Loss and Dirêetional Sensi.t ivity.Àuditory information norrnally contributel to spatiat orienta-tion. Howèver, thê density of water causes ]osèes and distor-tions for the auditory systen, just as it does for the visualsystêrn. The hunan ear functions poorty in vater, probablyoperating by bone conduction rather than by the rn-iddle ear:therê is a loss o f sens i t i v i t y o f about 30 :60 dec ibê ls conpa-red to air, thê differênce increasing with the frequenqy(Ho l l iên and Fe ins te in , 1976) . There is a l ,so a losa o f d i rec-tional sensitivity, the minihurn discrirninable anql-e beinq muchgreater than in a i r a t a I I ang les . D iscr in ina t io ; i s besÉ fo rbroad-band noise and for 1or1r frequenciès, and ihproveE! vJithhead hovenents and w i th t ra in ing (Goêtêrs , t -972; HoL l ien andFê ins te in , 19?6) . Man can loca l i ze pu lsed nh i tè no ise undèrvater to an accuracy of about 9-1.0 degrees undêr the bestcónditions. Though this perfornance is poorer than in air(which has a naxirnum accuracy of about t degreel , it Is conpa-rable to that of narine marïnals.The Ioss of sensitivity is partly due to the loss of one ofthe maj.n binaural cues - the j-ntensity differênce beÈweên thetwo êars . Th is occurs bêcêuse the heaá in water i s acous t ica l -ly transparent, the dênsity of the hêad and vatèr being sini-Iar. Thê head no longer casts a sound shadon, so thaÈ theintênsity of sound at the tl^ro ears renains sinilar rêqardlessof the angle of incidence. The other main binaura] cué, thetêhporal diffêrênce betveen thê two ears, is al.so affêcted.The speed of transnission of sound in air is 335 li,/s, but thisincreases to 1437 Í t / s in f rêsh ! , ra te r and about 15Ob rn seawatêr - an inc rease by a fac to r o f 4 .0 -4 .5 . Th is leducês thêÍnteraural ternporaf difference to about a quartêr of thê airva luê , e f fec t i vê ly shr ink ing the d iver 's hêad to about thes ize o f a go l f ba l l in a i r . I t i s perhaps renarkab le tha thurnans retain any local-ization abil ity in watêr.

D is to f t ion o f Aud i to ry Loca l i za t ion .Àud i to ry loca l i za t ion is d is to r ted as wê l1 as rêduced. Thêreductlon in the interaural tênporal diffêrence causes soundsto be distorted towards the nêdian plane. À sound source at 90

1 1 9

degreês to thê right or left in vatêr should bê equivalent toone at 17 degrees to the front or back in air. This thêorêti-cally leaves a large auditory lacuna (equivaLênt to a tblindspot') to èach side of the headr conpressing the apparentauditory fièld to a cone of 34 degreês to thê front and back.Becausê of the dj,stortion, a diver in darkness or zero visibi-I ity pointing towards a sound source points too closê to thened ian p lane (Wel1s and Ross , 1980) . (Th is i s thê oppos i te o fthe effêct of the optical distortion, so such experirÍrênts nustbe perforned blindfold). À diver honing in on an invisiblêsound sourcè located to one sidê usually svirns in a wide loopbêfore rêaching thê source. Leggiere et at (1970) attrj.butedth is e f fec t to a ' randorn wa lk ' , vh i lê Wêl ls and Ross (1980)clai.ned it vas due to mêdia1 bi.as. There is sone evidênce thatdivers rnay partlally adapt to the medial distortion, showingan afterêffêct arÁray from the median plane j-mnèdiatêly onre turn ing to a i r (Wel . l s and Ross , 1980) .There is also poor distance localization, due to a reducti.onor distortion of host of thê auditory distance cues. There lsa tendency to local-j-zation lrithin the hêad, haking it vêrydiff icult to locate the source of a noise such as an explosionor a boat I s êng inè .

Veet ibu la r facÈors .

À d ivêr rs ves t ibu la r sys ten can, in p r inc ip lê , func t ion nor -nally as in air. Neutral buoyancy does not sirnulate zerogravity: thè otoliths continue to respond to gravity even whênthê d ivêr rs body is f ]oa t ing . However , the ves t ibu la r sys tenis suscêptible to alternobaric vêrtigo (pressure vertigo), andto sevêral other effects relatêd to breathing gases at prêssu-rê (see rev iews by Kênnêdy, 1974; Farmêr , 1982) .

À l tê rnobar ic Vèr t iqo .vèrtigo can be brought on by a failurê to êquatise the pressu-re bêtweên the middlê êar and thê outer environtnent. Overpres-surè of the rniddle êar j.s probabty nore important ttran undêr-p!êssure. High pressure air trapped in thê rniddlê êar sohêhoÍi,stirnulatês the vestibular systên, causing sensations of rota-tion, tunbling or t i l t ing. Such incidents occur nore frequent-ly on the ascent nearer thê surface, wherê prêssurè changesale nore rapid and high prêssurê air is more l j.kely to becornêtrapped j.n the niddle ear. Ingelstêdt êt al (1974) coniluctedprêssure charnber studiês which showed a connection bêtweenvesti,bular nystagmus and asyrnrnêtrical êquil ibration of niddteear pressures during ascents fron shallow depths.Ross (1976) noted that thê hrost cornrnon reports froh diverswêre o f inc idents in a hêad-upr igh t pos i t lon , invo lv ing thesensatÍon of sêlf-rotation tovards the overprêssurêd ear,accompanied by fast rotation of the visual world in thê sanêdirêction. This is consistent lr ith lhe fast phase of nystagmusbeing towàrds the overpressured ear, and thê slow phase(trAcking component) ih the opposite di"rêction. It is notknown whêther thê direction leverses !,/hen the head is invêr-!ed , as migh t be pred ic ted by ana logy w i th o ther types o fver t igo . The e f fec ts a re s i rn i la r to tha t o f ca lo r j . c nys lag-

1 2 0

rnus, vhj.ch coufd be inplicated if an outèr ear bfocked \,/ ithwax-or covered by a hood suddenly clêars, aflo\,ring access ofcold watêr to the ear dmn. Rupture of the ear drím (caused byfailurê to equalise the pressuie on descent) nas sinritarè ! !êc ts . Howêver , these 1a t te r two ca têgor ies a rê ra re , andmost incidents appêar to bê casês of alfêrnobaric vertigo.Thê sensation- o f. bodi l y rotati-on is usually i i iànq ""a".warer , p robabty because o f a 1ack o f any tàc t i ) .e óont rad ic t i_on. fhê sensat ion can be inh ib i ted or ràduced by c l ingrng to a: : : I : :_ : ! l : . f i xed ob jecr . rh is observa t ion i i -anÀtasous toz ero-gravlry. experinents which shor^, that the sensation ofDoorry rotatton caused bv a 'rrotating doneÍ (rotation of the,"'hole visual f ield) can Èe reduced uf proviaing

-i i" i ir" ".,".to the as t ronaut 's fee t (young e t a f , i see) .

-

E f fec ts o f Gasês a t p rèssure .Vestibular dêconpression sj.ckness, or rvestibular bendsr, mayo9."1 9l ascênding fron rêlative1y deep dives (deepêr thanabout 100 _n)

- v i th synptoms.o f u is tead iness , à i r ; i ; "=" ,nausea.and von i t ing . ' , I sobar ic ver t igor nay a lso occur when. " : le l s . no change in to ta l a tmosphêr ic p ressure , bu t thesub lecE brêa thês a d t f fe ren t gas or gas h ix tu re i rom thêsurrounding atrnosphere. In neither cáse is Èhe mecninismc lear ly unders tood (Sh i l l i ng e t a l , 19?6)

The High pressure Nervous Synalrone (gpNS) has beên studiêd inI?::..::_."i1 . - lr-may. aÍfect lubjects breal.hins ó"yn.i.t,.,. qu=rn lx ru res a t depths bê low êbout 2OO m. They náy = i f fe . . ruu .eaand-vertigo, together !, ith disturban... oi uiiun"Á-ána trruocu lo rno tor sys ten (Tórók , I982) . An inc rease i " = i " " " r r^ " "velocity of thê VoR has been oÈserved ut ".rV iriqÉ-p.;;;;;": ld . : ^ : i : .d

gêses , thoush rhe cause is unc leár JÉ-^ i r " *un . t

Brar thwa i te e t a l (19?4) c fa i rnêd tha t the d izz inêss o f HPNSwas rlot accohpanied by sustained nystagmus, and aiá not origi_nate f roh the ves t j -bu tar end organ l Fa iner ' (see iá i . " . , rsaz lsuggested that it vas due to a decrease in thê normal cerebê1._1ar inhibitory rÀodulacion of the vestibu:.u. n".-iÀil -

f"ading toequa l inc reases in the tê f t . and r ign t "e - i iuu ia . - !á tn"uyr .GauthÍer (1976) noted synnecrrcal rncreases rn the lêft andriqht VOR durinq HPNS.Nitrogên. narcosis Ílay also play a part in disorientataon. ThisÍlay. be through general depression àf the central nervoussysEeh, o r th rough. rnore spec i f i c ê f fêc ts on thè ves t ibu la rsysrern . lnc reasês in body sway havê been found in p ressurecnanDer s tudres by Ado l fson e t a I (1972) , Adof tson e t a l .I l? lo) i ld rones êr . àr (1e7e). Àdoi fson át ar l re i+ l touna::?.^:::]:"!: breathing - co$pressed air at prêssures up to 10A |A . snowêd . tnc reasêd body sway on a Romberg ra i l s ba lancet e s ! , , p a r t L c u l a r l y $ r i t h t h e i r ê y e s c l o s e d . T h e a u t h o r s f o u n dno êvrqêDcê o f ves t lbu la r changes, and assuhed the CNS wasinvo lved. Howêver , Harn i f ton e t a l (19g6) a ia À inà vàs t i lu ta rchanges in subjects who breathed a rnixture ot zià nitrousoxide - a situation norÍlally equivalènt to nitrogen narcosiswhên brea th ing a i r under p ressure . The sub jec ts áhowed anrncrease o l about 502 in the ve loc i ty o f the s lov phase o f the

L2L

vOR. This suggesÈs that thê vestibular end organs, or thecentral- pathlrays controll ing nystagïus, or both, arê affectêdby nitrous oxide. The diffêrence bêtveên this and earlierstudiês hlay ]le in thê fact that Àdolfson et al (1,9?O) hêasu-red the nunber of beats and the phase differencê bebvreenstimulus and rêsponsê, whilè tlanilton êt aI (1986) measuredthê velocity of the slow phase conponent relative to theslinulus velocity. Pêrhaps thê laÈter measure is nore sensiti-ve co narcos ls .Í, lhile thè current evidence suggêsts that gases at pressure nayaffect sone aspêcts of vestibular control, it does not suggêstthat they cause a true rotary vertigo sinutar to thaÈ ofa l te rnobar ic ver t igo .

facti 1e-Rinaêsthetic Factors antt coblined Senses.

Buoyancy and Motor skil Is.The upthrust of the water counteracts thê forcê of gravity,nakj,ng objects fighter o! rnore buoyant by an ahount thatvar ies w i th thè i r vo tumê (1 g per cnr ) . The hunan body ispositively buoyant j.n sea vater, and nornally neutral orslight buoyant in fresh vatêr. À Scuba diver adjusts hiswei.ght to be as near nêutrally buoyant as possibl-e, though hisbuoyancy ineviÈably varies vith dêpth and the amount of airIeft in his cyLinder. Nêutral buoyancy sinulates zero gravi.tyto sone exLent, though gravity continues to act on the conpo-nents of the equil j.brium systen. However, thê viscosity of thewater neans that extra êffort is rêquirêd to nove the \datêraside. Movernent in water thus entaiis different forces fromnovêmênt in air, but whether thê force is grêatêr or lêÊsvali-es with the direction of riovenênt.There is reduced tacCile stinufation due to thê nuhbinq of thesk in th rough co ld , o r to i t s p ro têc t ion n i th a th ick rubbersuit. This, and othêr factors such as lack of anchoragê, leadto reiluced naniputatory abitity. À detêrioration has beenshoÍ /n Ín nany rn isce l lanêous ' s tandard tes ts ' o f sk i l l s (Àdo l f -son and Berghage, I974t Sh i l l i ng e t a l , 1976) . A grêa têrundêrstanding of thè componênts of skil ls has bêên gainêdthrough studies with a rnore theoretical basis. One such conpo-nent is the effective reduction in weight of thê diverrs arÍrand other objects, r,rhich leads to systematic mispercêptions ofwe igh t and rnass (see Ross , 1981; cassman, 1986) . fhêre is somedegree o f in i t ia l hass cons tancy , fo l towêd by subsequentadaptation and by an aftereffêct in air lrhen objects feêL tooheavy. The abil ity to distinguish between thê wêights ofobjects also dèteriorates, partly due to maladaptatj-on andpartly to coLd and other factors. position anat force estitnatesarê also affected (see Adolfson and Bèrghage, t-974) ; subrnergedsubjectÉ tend to ailn too high, take longer to locate and prêsspush-button swi.tches, and hake incorrect forcê estinates.Othêr ho tor sk i l1s de ter io ra tê , such as the ab i t i t y to tapaccura te ly bê tweên ta rgê t a rêaÉ ( fe r r , 1973, 1978; Hancock andM i l n e r , 1 9 8 2 ) . E r r o r s i n s u c h t a s k s h a y b ê d u e p a r t l y t o t h eÍncreased fo rce requ i red to novê the arn aga ins t v iscous drag ,partly to naladaptation lo the altêred force and buoyancy ef-fec ts , and par t l y to v isua l d is to r t ion o f thê ta rge t a reas

when looking into water. À systematic sêries of expêrinents isneêded to distinguish bêtweên thèse variables.

Posturê and Knowlêdge of the VerticaL.The free swirnning divêr has l itt le stimulation on thê soles ofhis feèt, and a reduced loadj.ng on the gravity-resistingjoints. This loss of information should lead to a reducaion ofawareness of l inb position, as it does under zero gravity(Watt et al, 1985). Howêvêr, thê question does not appeai tohave been investigated.There is no doubt that knowl-edge of the gravitationaL verticalis reduced under water, duê to impoverj.shed visual and sones-thetic infornation. Invêstigators have used rnany differêntnethods and havê come up with lridêly varied êstinates ofangu lar e r ro rs , rang ing f ro ro 4 -180 degrêes (See Ross , 1971 iÀdo l fson and Berghagê, 1974; Sh i l l i ng e t a I , 1975) . Manyrnvêstj.gators strappêd their btindfold subjects to chairs orti l t tablês in a pool, t i l ted thên, and thèn rêquired then topoint to thê verticat or readjust their body to the vertical.Thê Inêan error estinatês werê about 7 degrees in the uprightposition and about 3O dêgrees vhen inverted; and the errorsÍncrêased vith the length of exposurê to a given ti l t anglê.such errors are considêrably larger than fo! the êquivalànttes ts in a i r .

Ross et al (1969) appêar Èo have been thê only j-nvêsÈiqatorsto test free swirnning Scuba divêrs in the sea. Thêy asÍedthêir subjects to disorient themselvês by turning

-surnersaults,

and.then to align their lrunk, extend thêir arm ánd point withtheir f inger to the gravitational vertical. They did thiseithêr upright or invêrted, and either with vision or blind-fold. the- divers were photographed bêsj.de a plurÍb l ine by tvophotographers al 90 degrees to each other. rÈe angular dávia-tions werê neasureil fron thê photographs, and thè naxinurndeviation caLculated. The rnean errors for pointinq with thef inger r , rhên fu l l y s igh têd wêre 8 .1 dêgrees upr igh i and 24 .1deqrees j"nverÈed; and when blindfold they lrere l-6.8 and 29,6dêgrees rêspect ivêIy.The typical direction of êrror in the upright position waspitch forlrard, but the dirèction vas no-re variàtte when inver-ted . À ve l l -ba lanced inver ted d iver has lhe head s l idh+r 'dorsiftexed, in " "i^i iai posil i.on to .n lnvËiiàa ;;;;;; i ""land (Cl.énênt and Rezette, 1985), but few divers achievedthis. Thê.êrrors in aligning thê trunk were sonetirnes greaterand sonetines less than that for the finger. When uprighÈ,both trunk and fingêr nornally show a pitch forward er;or, but.when inverted they nay show differênt errors. pointing errorscould be duê to uncertainty over arn and finger position inrelation to thê trunk, in addition to uncêrtainty of head-/ t runk or ien ta t ion .Thê diversr accuracy was, as expectêd, greater \ ' j .th nomalvisj-on than whên blindfold; but it sias poorer ! ' i th 'rrrhiteoutrv is io4 Lhan whên b l ind fo ld . The la t tê r à f fec t was shovrn in asecond expêrirnenÈ in which Èhê diver !/ore a scratchèd perspexplatè over the facenask, to itnitatê 'whiteout' or tov-visiLi-l i ty conditions: the whitêout nask gave grêater errors than a

123

blackout nask, particul,arly whên thê divêr ra'as uprighÈ, Theeffect may occur because attention to a valueless source ofinformation interferes with lhe abil ity to usê othêr norereliabfê sources. ÀIternatively, it nay bê that the subjèctuses the nil visual input as evidence that he has not noved,An upright diver attêmptlng to settte in a slight pitch for-ward posi.t j-on nay thus overshoot in the absence of visualfeedback, resulting in an exaggêrated êrror. This interpreta-tion is consistent with findings that stabil izêd vision leadsto greater pitch forward êrror both on land (Vidal et al,1982) and in zero grav i ty (c l -énent e t a t , 1995) .

As in previous studiês, êrrors were always greater in thêinvêrtèd posi.t ion than upright. This nay be due to inexperien-ce of thê invêrtêd position, or to diff iculties in baLancingin that posturê; or it nay bê because the vêstibular system isless e f f i c iên t in tha t rangê ( the ves t ibu la r 'b l j .nd spot ' ) .The contribution of thê otoliths to orientation is known to bêgreates t in thè head-upr igh t pos i t ion on land (e .g . Schónê,1975; Young ê t a l , 19?5) , and the cont r ibu t ion o f the v isua land sonesthetic systêns is probably greatèst when invêrtêd.Since the sonèsthetÍc input is reducêd undèr water, the di-verrs sense of orientation is particutarly inpaired wheninvêrtêd. Indeed, sone authors have found an underwater irn-paÍrnent for the subjêctive vertical only in the 'head downlpos i t ion (Schóne, 1964; I , ladê , 1"973) . p r io r adapta t ion to anoff-vêrtical t i l t affects thê subjectivê vertical, but Lêch-ner-stê inleitnêr and Schóne (1980) found no obvious differencebetv?eên wet and dry conditiohs in this respect.

Locomotj.on and Gèographical orièntation.Therè is inadequate feêdback under watêr about the effects ofoners onn locornotion. The diver has diff iculty in knowing hovrfar he has swun, lrhat dêpth hê is at, and whether he hashalntainêd a straight l ine or not. Errors can occu! evên lrhenus ing nav iga t iona l a ids . Àndersen (1968) s tud ied course anddêpth kêêping. Hê investigated navigation accuracy by lheextent to r^'hich divêrs using rnagnetic conpassês devi-ated froma coursê ovêr a range of 235 n at a dêpth of 10-12 n. He foundan avêrage dev ia t ion o f about 22 n , o r 5 .2 degrêês , in compasshead ing . Wi th p rac t ice th is was reduced to about 16 m or 3 .9dêqrees. Errors werê cohmonly duê to an êrror of arn positi--on - the faifurê to hold the cohpaÊs parallê1 to the fongitu-dinal axis (direction of nolion) of the body, Another conrnonmistake was to look down instead of sighting ahêad ovêr theconpass, causing a têÍ\porary loss of oriêntation.Àndersen also studied depth-kêêping in divers using depthgauges. Subjêcts vho were neutrally buoyant or 1 kg nêgativetênded to swirn about 0.5 Íl too high at f irst, and then to sinkto about I n too Iov. Subjects 3-4 kg negative slran slightlytoo Iow at f irst, the ertor then increasinq to about 1.5 rn toolow .w i thout v is ion or nav iga t iona l a ids the d i f f i cu l t ies a rê muchgreatêr . Ross , D ick inson and Jupp (1 ,97O) j .nves t iga ted th is!,/ ith a 'tr iangle complêtionr task, in !,7hich the subject swarnb l i n d f o l d r o u n d t w o s i d e s ó F á ! r i à r a l ê h ^ l d r h d à

guÍdê1ine, and thên attênrpted to s$riÍ! unguided back to thestarting post. The divers tendêd to swim too far, and to turnthrough too srnall and ahgle conparêd ldrith walking on Land.Both these tendencies werê confirrnèd in further èxperinents bvthese au thors .

Blindfold divêrs genêrally swin in curved paths rathêr thans t ra igh t , a tendêncy a lso found in peop le wa lk inq in à f .d óFland.

-Lur ia ( r -e?ea) had ur inàiorá dï ; ; ; ; - " ; ; ; ; ' i ' t . " . i i . r - l

- "

straight l ine, nake 90 dêgree turns to the left and right, anda 180 dêgree turn. The nêdian êrror was 12 degrêes. Errors tothê lêft and right wêre about êquaI, and individual subjectswere fairly consistent in thêir dirèction of êrror lrhetherthêy vere wêaring Scuba tanks or not, and whether they lrereswinrning blindfol.d or sightêd in low visibil i tv watêr íabout 2n) . Er ro rs were no t cor re la ted w i th hand or foàL pre ference,or with the relative strength of the têgs. Luria (1979b) alsofound that errors werê not corrècted by using svinfins ofunequal l"ength, or attaching a rudder to thê Scuba tank. Thebias is probably of central origin, perhaps due to an asynne-try of vestibular functioning or to an irnbalance in the fin-aes thê t ic sensès .we normally see stationary oblects as stàtionary, dêspitehowenent of thê rêtinal inage causêd by our own eyê, head orbody novenênts. It is somêtines argued that vj-sual stabil ityoccurs because the brain cohpensates for thê effects of suchrnovenènts. .Others argue that corrêction is unnecessary, sinceobject motion is sêen only i,rhên objêcts nove in relation toother objects. The contribution of active and passivê r,^.iwmovenènts to visual stabiliiy ""iè i"".Áiróà'i.á-tv-ná.!-iíaLennt-e (1969), by observing the apparenÈ movênênt of aftêrima-gês and of f ixed l ights vhen swirnhinq. A betaliqht ía snallglowing l ight) was fi.xed in a blackeá-out face mask, to provi-de a s t ihu lus tha t was f i xed in re la t ion to the d iver ,s i teadnovenents. In addition, an aftêrinage was obtained flon aflashgun, to providê a stinulus that was fixed in relation toeye novenênts. when !^rearing lhe nask the outside worli l cannotbe sêên, so any apparent novenênt of thê betalight or after-irnage nust be due to correction for èye rnovenenÈs or sel.frnotion. Whên a diver noves activêly (è.g. turns a for\rardsornêrsauft), he init ialty sêes the bètalight and afterrmagejerk back in the opposite direction to hj.nself (upvards inthis case). the afterinage rnoving a IoÈ further tËan thebètalight. Às he continuês to nove in the sane direction theafterÍrnage and bêtalight gradually return to centre, and bothappear to nove with hi-h. Thê init ial disptacenent of thebetalight rnay be due to an unconpensated rêf]ex eye rnovenent?but this is unlikely sincê thê aflerinaqê noves in the sarnedirêction, and rnay appea! to hove right outside the visualfield. While the init iàl effêct is hard to exptai-n, thê subse-quent steady novehènt of the l ight and afterinage shov thatthê .v isua l s tab i l i t y sys ten co l rec ts àdequatê Iy fo r ac t i vebodily novenênt,Thê apparent movêhent is seen only duringactive or intênded novêhent, Slov passivê novement - sinXing,rising and drift ing - has no effect. À diver nay atternpt tos\,rim upsrards, but actually sink: hê then sêês the betaliqht as

rising up with hin, even though lte nay know fron pressurechanges in his êars that he is sinking. This obsêrvationdernonsÈrates that rcognitivê! infornation about depth andhovement does not fêed into the visual stabil ity system. Itseêns that thê diver qorrects for his intêndêd rather than hisactual bodily novenent.

civen the reduction in sênsory feêdback unClêr watêr, one rnightexpect divers to undêrêstinate the distance they have travet-}ed. Ross, Dickinson and Jupp (1970) found that when svirnrninghorizontally along a rope on the seabêd (depth about 15n) ,inexpêrienced divers lended to slrin too far whên askêd toreproduce distances of about 5-25 n. this was true both wit-hout vlsion, and with vision in !, 'ater of low visibi. l i .ty (about5 rn ) . Ross and Frank l in (1976) asked s igh téd d ivèrs to descendtó a spêcifiéd depth (without looking up) . Àt shallov dèpths(lêss than about 11 n) they s.!.ran too dêêp, thus underestina-ting thêir truê dêpthi but at greater depths they did notdescênd deêp ênough, thus overestinating their depth. Ross,King and Snowdên (19?0) led diverÊ to various depths bêtvrêên 3and 20 n and thên asked then Èo look up to the surface andnêkè a verbal judgenent of its distance: divêrs init iallyunderestihatêd their depth, but their judgênents inprovêd withpractice. cassnan (l-985) found that \^'hen sightêd di.vers follo-wed a ropê dor,rn the sloping bed of a quarry, thêy tênded toestinate their depth as shallowe! than it was. The underesti-nation incrêased over the depth 3-15 m, and was grèater forêxperiênced ttran novicê divêrs. Thê conflict bet!íêên thêsefindings is probably due to thê naturê of thê test (whêthèr anurnerical- êstinate fron a givên position, or a requirernênt toswirn to a stated depth or distance), and to the experience ofthê subjects. Novicês are particularly prone to anxiety, andnost subjects prêfer lo err on the side of safely when askêdto swirn dêep. Ànxiety nay have Iitt le êffêct on nunericalêstinates, or on horizontal swinning at shallow dêplhs. It isclear that lnorê systernatic sludies are needed to uncover thevar iab les a f fec t ing the es t ina t ion o f d is tance t rave l led .

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