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lionr nrarrer
E L S E V IE R
PrI 50361-9230(98)00t
l5-4
Motionsickness:
Onlyone provocative
conflict?
Willem Bles,*Jelte E. Bos, Bernd de Graaf,Eric Groenand AlexanderH. Wertheim
TNO Human Factors Researchlnstitute,Soesferberg,The Netherlands
[Received6 January1998;Accepted24 Aprit1998]
ABSTRACT:
ln rcviewingthe variousforms ot motion sickness,
th€ classic sensoryreananggmentthsory,.hasbeen redefined
by demon3tratingthat onlvone tvps of
-confl9i,"_1.::-"-":!-."9
sufficient to erplain all ditferent kinds
of motion sickness'A
mathematicaldescription is provided from the summarizing
,,Att
sratementthat
situationswhich i.i""ff" iii.t., liJililji
are charactsrisedby a conditionin whjch the senseaverticaias
determinedon the basis ol integrated intomation from the
eyes,the vestibularsystemand the nonv€stibularproprioceptors is at vadancewith the subiectiveverticalas expectedtrom
previouserped€nce," @1999ElsevierSci€nceInc.
KEYWORDS:Motion sickness,Subi€ctivovertical,visual-vestibular interactiona,Modeling.
INTRODUCTION
The so-calledsensoryrearrangementtheory on motion sickness
developedby Reasonand Brand [35] is widely accepredtoday.
The basic idea is that all situationsthat provoke motion sickness
arecharacterized
by a conditionof sensoryrearrangement
in which
the motion signalstransmittedby the eyes,the vestibularsystem
and the nonvestibularproprioceptorsare at varianceeither with
one anotheror with what is expectedfrom previous experience.
Reason and Brand specify two main categoriesof motion cue
mismatchesaccording to the sensory systems involved: an intersensoryconflict dealing with a visual-vestibularmismatchand
an intrasensoryconflict for a canal-otolithmismatch.They further
classify these mismatchesinto two types of conflict: a type I
conflict, when both systemssignal simultaneouslycontradictory
motion information,and a type2 conflict,when one systemsignals
motion in the absenceof a correspondingsignal fiom the other
sensor.Cuedry [22] extendsthe two mismatchcategorieswith a
third categorydealing with a vestibular-proprioceptor
mismatch.
Despitethe many examplesgiven by Reasonand Brand [35] or
by Grittin [20] of fbrms of motion sicknessbelonging to cerrain
types and categories,it remainsdifficult in practiceto classify a
particular provocative motion condition into only one of these
conflict categories.The heuristic motion sicknessmodel as described by Oman [33] provides a mathernaticalbasis fbr the
above-described
componentsof the sensoryrearrangement
theory
on motionsickness.
In view of the topic of this article,the point of
interestin Oman's rnodelis that motion sicknessis relatedto the
vector dif-ference
betweena vector representing
all the available
aft-ererrtsensory infbrmation and a vector representingthe expectedsensoryinti'rrntation.
When this differencevector grows.
the chanceof rnotionsicknessand the severityof the motion
sickness
will increase.
With his choiceofthe conflictvector,Oman
pinpoinrsthe conflicrto a discrepancyberweenthe sensedand
:*;;;;
informationof all individuaisensors.This is in agree:11".'::l
men^t.
with the descriptionas given by Benson[l]. InteGensory
conflictsasdescribedby ReasonandBranddo not contributeto a
conflictsodefined.The sameis truefor therulesprovidedby Stott
t401.
Althoughwe agreethat mostof the examplesof the different
conflictsas describedby Guedry[22] may lead to disorientation
and motionillusions,it is our experience
that motionsicknessis
primarilyprovokedin thosesituationswherethe determinatioo
of
the subjectivevertical,the intertal representation
of gravity, is
challenged.
This canbe illustratedby two examples.
First, afier long-durationcentrifugation,only head movements
that changethe orientationof the headrelativeto the gravity vector
provoke motion sickness.[n an upright sitting subject,roll and
pitch movementsof the head provoke motion sickness,whereas
yaw movementselicite motion illusions but no motion sickness.
With the subject in supine position, yaw and pitch head movements are provocative,but roll motion is not [7]. This illustrates
that sensory mismatchesmay induce motion illusions but only
provoke motion sicknesswhen the determinationof the subjective
vertical is at stake.
A secondexampleconcernsthe studieson optokineticcircular
vection.Despitethe absenceof correspondingvestibularinformation, motion sicknessis only occasionallyobservedduring sudden
onsets of surround motion. This is the experience of several
Europeanresearchgroupsusing the large optokineticdrums manufacturedby Tcinnies(Freiburgin Br.). Motion sicknessincidence
in this type of experimentsis estimated to be lower than lVo
(Brandt, Probst and Bles, personalcommunication).The almost
completeabsenceof motion sicknessseemsto be in conflict with
the sensoryrearrangement
theory, becausethe optokineticstimuli
createclear dif'ferencesbetweenthe sensedand expectedsensory
informationin terms of the Oman model. We came to the conclusion that the low incidenceof motion sicknessis due to the fact
that in thesedrum experimentsthe stimuli are neutralwith respect
to gravity.Of course,during optokineticcircularmotion stimulation head tilt provokes motion sickness,the so-calledpseudoCorioliseffects[2], but then the sub.jective
verticalis at stake.
These exarrples led us to redefinethe sensoryrearrangement
thcory on nrotionsickness:"All situationswhich provokemotion
sicknessare characterised
by a condition in which the sen,retl
yerticul as deterrninedon tlre basisof intesratedinfbrmationtiom
'i AtltJrcss
l i l r c o r r c s l t o n t l e n c c : D r . W r l l c n r B l c s , T N O F l u r n l r tftr a c t o r sR c s e : t r c h
I n s t i t u t c .P . O . B o x ] 3 . - 1 7 6 9Z G S t r s t c r b e r s .l ' h c N e t h c r l a n c t sF.a x :
( . 1l ) 3 1 6 3 - 5 - l9 7 7 : [ r - r n a i l : f ] l c s ( q ) ' f M . ' l N O . N L
.lt3|
BLES ET AL.
.t82
propriocepthe eyes,the vestibularsystemand the nonvestibular
tors is at variancewith t/re :;ullcctive verticul as predictedon the
basisof previottsexperience."
..SUBJECTIVE VERTICAL CONFLICT''
SICKNESS 1VIODEL
N,IOTION
The subjectivevertical conflict motion sicknessmodel, hencefbrth calledthe SV-conflictrnodel,is shown in Fig. l. The model
is an extensionof the Oman rnodel.The principlesof the Oman
modelare shown in thin linesin Fig. l, and for a detaileddescription ref'erenceis made to Oman [33]. In short, the Oman model
statesthat,basedon the desiredpositionxo, muscleactivity(nl) is
generated,
leadingto a positionx due to the body dynamics(B).
This signal,togetherwith the externalnoise rt", is detectedby the
senses(S), resultingin sensoryinformationa. Oman also assumes
that an internal rnodel that consists of the salne components
(indicatedwith a hat) computesthe expectedsensoryinfbnnation
A. Differences(the vector c) between the vectors a and A are
calculated,of which a weighted (K") amount is fed back into the
system.In this way, an optimal estimateof the actualpositioncan
be obtained.Apart from the position.r, it is obvious that the other
motion parameterslike velocity and accelerationshould be consideredas well. According to Oman, the differencevector c is the
origin of motion sickness.
The SV-conflict model extends the Oman model with a network V that constructsthe sensedvertical, v"",r' based on the
incoming sensoryinformation. Similarly, in the internal model a
network ? is addedthat constructsthe expectedvertical,i or v.*o,
based on previous experience and expectation.The difference
vector d between v"",r. and v"r.o is used to update v.ro. The
so-updatedv.*o will henceforthbe called the subjectivevertical
v",ror.However, the measurementof the subjective vertical as an
experimentalparametermay dependon the psychophysicalprocedure. For instance,the manual settingsof an unseenbar may not
necessarilylead to the same results as the visual judgment of a
luminous line. To accountfor this, we introduceda transfermodule F, the output of which is the measuredsubjectivevertical, or
V-.t*'
In contrastto Oman, we assumethat it is not vector c that
generatesmotion sicknessbut instead vector d, i.e., the vector
differencebetween v,en" and v"'o, (Fig. l). This article concentrateson the type of conflict that may lead to motion sickness.The
detaileddescriptionof the modules behind vector d that lead to
motion sicknesswill therefbreremain beyond the scope of this
article.
The computationof the sensedvertical v".,," in module V is
proposedto take place accordingto the schemeshown in Fig. 2
[5,6]. The computationof v"",, is part of the signal processingof
the sensorysystemslike the vestibular,the visual and the somatosensorysystemthat all provide intormationon spatialorientation.l
To obtainonly one uniquespatialorientation,it is assumedthat all
this sensory information is integratedto obtain basically three
signals,indicatingthe sensedrotation(r,",,.), the sensedtranslation
(t,",,) and the sensedvertical (v,",,).
The integrationof rotatorymotion infbrmationto obtain r,"., iS
rather straighttbrward,becausethe sensorysystemsprovide complernentaryinfbrmation [seetbr instance4,11). A bigger problem
for the centralvestibularsystemis to cxtractv,.,,. out of the sensed
gravitoinertialfbrce vector. In view of normal humirnntoventents
that low-passfilteringof the
and locomotion,it was hypothesized
gravitoinertialtbrce vectorcould preservegravity.This is a scn-
dd k) lhe integntion pro(css .lt well.
sible approach,providedthat the angularrntltion infirrmationr"",,.
of fast hcad tilts. Mathhelps to conrpensntefbr the consequcnces
by a transtirnnatitln
is accotttplished
cnraticitlly.this contpettsatitltr
I of the coordinatefiarnewith the otolith vectors,over the angle
of the headtilt indicatedby r..,,,. Such a manipulationkeepsthe
input to the low-pass(LP) filter unchanged,the sensedvertical
afier the headtilt being determinedinitially by the rotatory motion
Z-r as shownin Fig.
infbrmationclueto the inversetransformation
2. If the changein direction of the gravitoinertialforce vector as
sensedby the otoliths dclesnot correspondexactly to the angle as
indicatedby the signal r.",,., the LP filter ensuresthat the otoliths
finally detenninev""... This is a similar modelingapproachas used
previously[6,29]. Recently,one of us showed that the abovedescribedmodule V is rnathematicallyeven identical to the Glasauermodel [l01. The LP filter fits experimentalevidenceof a
slowly developing perceptof tilt during sustainedhorizontal acceleration[4,39]. In principle,t..,,, is now the differencebetween
the gravitoinertial force vector and v.",,,.
Visual structuresthat contain direct information about what is
horizontal or vertical also determinev".... Becausetheseeffects
this interactionis supposedto take
may be almost instantaneous,
place after the LP filter. For the argumentsused here, the location
and natureof this interactionare not immediatelyrelevant.
It is hypothesizedthat the internal model usesthe same computationrules to calculatevexpor v".,nr.As mentionedabove,the
differencevector d, which should somehow serveto adjust v,,-,b;,
is thought to trigger motion sicknessand to be correlatedto the
incidence/severityof motion sickness.Becaused is a vector, not
only its magnitudebut also its directionmay affect the severityof
motion sickness.
ONLY ONE CONFLICT?
are different among the various
Motion sicknesscharacteristics
forms of motion sickness[26]. Despite thesedifferenceswe believe that all typesof motion sicknesshave one underlyingconflict
in common, which is the conflict about the subjectivevertical. So
the main questionis whetherthe conflict categoriesas describedby
can be restrictedto only this conflict,
severalauthors120,22.33,351
or, in other words, is the SV-conflict theory sufficient to account
for the different forms of motion sickness?To answer this question, we focus on the main difference between the SV-conflict
theory and the more general sensory conflict theory first and
thereafterreview the most common types of motion sicknessfrom
the viewpoint of the SV-conflict theory.
SV-Conflict Theory VersusGeneral Sensorv
RearrungementTheort,
Motion sicknesshas often been explainedas the consequence
of passive motion becausethen the expected signals 6 will be
different fiom the sensedonesa, increasingthe vectorc. However,
passivemotion does not necessarilyinfluencethe magnitudeand
the direction of v.,nr, so d is not always increasing.This is an
importantdifferencebetweenthe two concepts,becausethis might
explainwhy peopleget sick in one situationand not in the other,
despitethe fact that c is largein both situations.This is illustrated
in the following example.What happensto the vectorsc and d
when a subjectis given a suddenpuslr'?Befbre the push it nray be
are aligned.If the push is unexpected,
assuntedthat v.",. and v..,rr.,
the internalrnodelwould not itnmediatelyfbreseeany particular
changein the incoming sensoryinfbrmationi. Therefbre.veap
ONE CONFLICT IN MOTION SICKNESS
.r83
v seng
FIG I. The subjective vertical conflicl motion sickness model as described by oman
t331. extended with lhe modules that are necessaryfor the
computation of the subjective venical (thick lines). Th€ vector d is consideredt; be the conflict vector for generating motion
sickness.
would not immediately change. After the push, many sensory
systemswill immediatelysignal motion a, which meansthat the
vectorc increasesdramatically,provoking motion sicknessaccording to the generalsensoryconflict theory. However, the direction
Linear
motion
GravitoInertial
force
Visual
frame
Rotatory
motion
t o t h c n r a i n n r o c l e ls h < l w n i n F i g . l , t h c d o f c c l l i n c i s r l r a w n t o n r a k
of thc subjcctive vertical. Sce text tirr furthcr dctails.
of gravity informationas encodedin v"",,. will be hardly affected,
if at all, becauseit is not affbctedby such high-frequencylinear
motion information(Fig. 2). So v"",,,will not differ too much from
v..,or,keeping d minimal, and the subject should not get motion
BLES ET AL.
.t8.t
to the SV-conflictthcory.We know that this is the
sick accorcling
caseindced.The srrbicctis not tlrotionsick. only illlgry rtboutthe
push.The vcctorc is certainlyinrportantto guarilnteci.lppropriate
to preventtalling.
countenlleasures
betweenthe two
This exarnpleillustratesthe ntain diff-erence
rtrcticlnsicknesstheories:Large dit'fbrencesbetweensensedand
expectcdsensoryiltrlrmatign may be presentwithout provoking
mcltionsickncssas long as the sensedverticalv..,.. basedon the
verticalv..,r,.i.
detectedntotionrenuinsalignedwith the sub.iective
lnotion sickof
types
diff'erent
the
of
analysis
In the subsequent
ness,this argumentwill otien return.
o.l'RottttittrtAbout tlte
Corioli:; Elfet'ts ruul Con.sequences
Vertic'ulA.ris
The nauseaprovokedby making head nrovementsduring yaw
motion is known as the coriolis effect, and the nausea as a
of headmovementsduring optokineticsurroundmoconsequence
tion is known as the pseudo-Corioliseff-ect.Guedry [21] explained
the Coriolis effect as the consequenceof the conflict betweenthe
head tilt indicated by the otolith and neck receptors and the
clirectionof the angular velocity vector as sensedby the canals.
Detaileddata on the incidenceand the severityof motion sickness
are available fiom many experimentson Coriolis and pseudoCoriolis effects.For instance,it is well known that tilt of the head
on the shoulder in clarknessduring constantvelocity rotation is
very provocative.However, dependenton the sensedangularvelocity vector presentjust before the head tilt, the Coriolis effect
or enhanced:The more the sensedresultant
may be sr.rppressed
angularu"l,o.ity vector after the head tilt difters in directionfrom
effect
the gravitoinertialforce vector,the strongerthe nauseogenic
t231.Similar observationshave beenreportedfor visual-vestibular
interactions[3].
interactionsI l2] and for somatosensory-vestibular
This is in perfect agreementwith the SV-conflict theory on
motion sickness.By definition,the internalmodel always expects
that v,.,n,remainsaligned with gravity, independentof whether it
assum;s"thebody as turning or stationary.This seemslogical and
can be demonstratedmathematically[4]. However, when the rotation vector r.",," is not aligned with gravitY' v."., will deviate
from gravity. According to the model as shown in Fig. 2, v".," will
so d will increase.Note that v".'. is not
then differ from v",,1,1,
takes place about the gravitational
iotation
as
long
as
affected
vertical (for a more detaileddescriptionsee [4]).
Guedryand Benson[23] and the SV-conflict theory implicitely
state that one should not get sick fiom rotation about an Earthvertical axis. Most researcherswould accept that theserotations
are not very provocativeindeed.However,ersmentionedearlier,in
some experimental paradigms dealing with rotations about an
Earth-verticalaxis, motion sicknesscan be evoked.But in these
conditionsothersensoryaspectsare involvedas well' For instance,
motion sicknessincidenceis high when retrievingdigits from a
nratrix lilled with digits presentedhead stationaryin front of a
who is rotatedsinusoidallyaboutthe verticalaxis,whereas
sr,rbject
the inciclelceis srnallwlrena sirnplevisualdisplayis used[221.De
Graaf, using only a horizontalanay of digits, did not find any
motion sicknessin a (unpublished)pilot stLrdywith lnany pilot
students.Whctherlogkipg up or clownchangesthe apparentlevel
of the horizon..ir.rstas looking aside af'tectsthe straight-ahead.
tirlnly. but that woulclcertainlyinfluence
rentainsto be establishctl
t)f v.",,. as call be dcrived from Fig' 2'
the detertnination
It is rcrnurkablethat the laboratoriesworking with thc large
optokineticdrunrsthat prevetltally view at the outsidcllave stl
little prgtign sickncssincitlenccwith only optokittcticlllotl()ll'
Lalloratoriesworkittg with othcr optokineticdc:viccslind rttuch
althtttrghthc stitlltrlusduratiotl
liighcr rrrotionsickncssinciclcnccs.
should be ntttstly about l5 rlrin and nlt)tion sickllessittcidence
[]ere too il reitsoll
incrclseswith drum rotittiot'tlc.g.. 15.25.381.
could be that the subjects,irladvertedly.tend to tind a stable
fiarneoutsidethc drttm by lookin-edownward to the floor
ref'erencc
rlr upward to the ceiling, even rvithoutnrakinghead nlovenlcnts
(heatl nlovententswould provoke ntotion sicknessanyhow' bttt
that is not the point here).To preventnrotionsickness.it is also
irnperativethat the tlptokinetic drut-uuroves exactly around the
verticalaxis with eract verticalstripesand is ttot swingingat all,
becausethat would inlluencethe sensedmotion path ilnd consecertainlyapply to the Tonquentlythe vertical.Theserestrictiotts
nies drums.
is positioned:Is the
Anotherpoint of interestis how the sr,rbject
headexactly in the centerof rotationduring sinusoidalrotation'l
Otherwise centrifggal forces may be introduced,affecting v.",,",
becausethe gravitoinertialforce vector is no longer aligned with
the rotation axis. This may provoke motion sicknessaccordingto
our Sv-conflict theory.Rotatingchairsfor clinical vestibulartests
mostly do not have the head exactly in the center of rotation.
Furtherresearchis neededto clarify theseobserveddiscrepancies.
Sea Sickne.ss
The most extensiveexperimentaldata setson seasicknesshave
beengatheredby o'Hanlon and Mccauley [32], McCauleyet al.
[30] and Gritlin u9l. o'Hanlon and Mccauley performeda series
of experimentswith the ship motion simulator fiom the Naval
Biodynamics Laboratory in New Orleans. They found that the
verticalmotion was the only motion componentthat contributedto
the motion sicknessincidenceand describedthis incidencemathematicallyas a function of amplitude,frequencyand exposuretime
for pure vertical sinusoidal motion. Lawther and Griffin [28],
summarizedby Griffin [19], explored the motion sicknessinci<lencetbr real ship motion. These last authorsalso fbund that the
vertical motion of the ship correspondedbest with the motion
sicknessincidence.
It has been shown by Bos and Bles Ill] that the SV-conflict
rnodel predictsthe clataset of O'Hanlon and McCauley. Here it
shouldbe mentionedthat the low-passfilter in the V module plays
an essentialrole to accomplishthat the frequencyrangearound0.2
Hz is the most provocative.This is importantbecausethis neither
follows straightforwardfrom the Ornan model [33] nor fiom
Griffin's model [19].
Wertheimet al.l42l showedthat combinationsof heavemotion
with pitch and roll motion considerablyincreasethe rnotion sicknessincidence.They, for instance,observedseveremotion sicknessin approxirnatelyhalf of their subjectswhen they were stacking crate.s(involving a lot of head movements)in the TNo ship
tnotion simulatorrunning real ship motion profiles.The heave
motion alone of this prolile wor-rldhave resulted in a motion
sicknessincidenceof maximally l%caccordingto McCauleyet iil.
[30] antt of maximally 6vc accordingto Griffin [19]. So the
if snrallerships
incidenceof seasicknessttlay be underestimated
reasonfbr the
The
are
considered.
motion
roll
pitch
antl
with more
high incidencein the studyof Wertheirnet al. is that the ratherlow
fiequency pitch and roll motic'rtrare llot dealt with adeqrratelyby
canal stinrrtlation
the vestibularsystem,leadingto cross-ct-rtrpled
ln the SV-conflicttlodel tlris wtltrldlead
cluringheadtntlvelnents.
deviatitlttsd frtttl-lv..,,,,.
t9 an inadequittev.c,r.with subsequent
sillilar to what happertsduring Coriglis cttects [-ll. The fact tltat
ttt-selt
hcad movclltcrltsplay an iurportantrole in the ctthattcclllcltt
heltd
sicknesscan also be tlcrivcd frtltll thc advisc to ttrinitttiz.c
ls ntttchas prrssibleto prevclltscit sickttcsslll' T'hc
nt()vcntcltls
sc:-tsickcxperiettccth:rtsight tlf the htlrizotrttlinittliz.cs
con.ltlt()n
ONE CONFLICT IN MOTION SICKNESS
nessis ntostprobablyclueto the fact that seeingthe horizonhelps
to keepthe sensedand subjectivevcrticalaligned.
Ef'fet'tstt.l'MiL'ro- untl Hvpergrut,itv
A rnicro- or hypergravityload per se does not provoke motion
sicknesssymptoms.Continuouslychangingthe G-load level as in
parabolicflight may be provocative,but head movementsduring
the diffc'rentG-levels are the rnost provocative[13,271.Even
during and after a centritirge run at 3G* tbr l.-5 h. subjectsare
asymptomaticas long as they rernain motionless.The provocativenessof ditferent types of head movementswas investigated
afier such long-durationcentrituge runs. It was fbund that yaw
headmotion was not provocativeat all, whereaspitch and roll head
motion provoked motion sickness symptoms when they were
sitting upright.Pitch motion was found to be the most provocative.
However, when the subject subsequentlytook a supine posture,
roll movementswere not experiencedas provocativeany longer,
whereaspitch head movementsand now also yaw motion provoked sicknesssymptoms [7]. Although it is not clear how the
adaptationprocesshas influencedthe different parametersettings
during the G-load, it is clear that only thoseheadmovementswere
provocative,which changedthe orientationof the headrelativeto
the gravitationalvertical. This observationwas the basis for the
SV-conflict theory on motion sickness.
It has been reportedthat during the first days of the mission,
astronautsorient themselvesmore on the (vertical) contours of
spacelabthan after a couple of days. Becauseatter some time in
spaceverticalor horizontalhas no specilicmeaninganymore,it is
no surprisethat Coriolis experimentsin Skylab performedafter a
coupleof days in spaceproved to be not provocativeat all [18]. In
fact, this is in agreementwith the concept that motion sickness
relatesto problemswith the determinationof the vertical onlv.
Air and Car Sickness
Although there are many movements of an aircraft that may
causemotion sickness,aerobaticsis well known as being provocative among studentpilots. without a detailedanalysisit is easily
understoodin view of the foregoing discussionthat the visualvestibularconflicts thought to causemotion sicknessaccordingto
the classicsensoryrearrangementtheory also challengethe computationof the subjectivevertical under thesecircumstances.For
passengers
in civil transportaircraft, bumpy weatheris known to
provokeair sickness.varying G-loadsare the importantvestibular
stimuli in an otherwisestablevisual surrounding.It is obviousthat
thesestimuli easily lead to discrepanciesbetweenthe sensedand
the expectedvertical and thus deviatesd from zero.
Driving uphill at night along a winding road may provoke car
sicknessin the passengersin the back seat. The continuously
changinggravitoinertialfbrce vector,togetherwith the inability of
the semicircularcanalsto appropriatelysignal the angularmotion
becauseof the stablevisual interiorof the car, will affect both v".,"
and v".,n,and subsequentlyprovoke motion sicknessaccordingto
the SV-conflict model. Linear accelerationand decelerationwithout appropriate
view of the roadaheadcausescar sicknessas well
[3'11.This is also in agreementwith laboratoryexperimentsthat
showedthat horizontallow-fiequencyoscillationslead to rnorion
sicknessin a certainfiequencyrange[171.Most probablythis is
explainedby the SV-conflictrnodelin a sirlilar way as was shown
firr vertical rnotion il l l. Intuitively,one would expect that the
SV-conflictmcxlelin thesecaseswould againpredictcliscrepancies
between v.",,. :-utdv.,,n, sinrilar to what was describeclin the
sectionon seir sickness.without analyzingthis in detail. it is
obviousthat the gravitoinertialforce vector is varying in moticln
conditionsthat leaclto car sickncss.and d will not remainzero.It
-r85
is f'easible
thatthe SV-conflicttheoryis appropriate
to explainwhy
pilssengersrvho are susceptibleto motion sicknessbenefit tiom
sitting next to the driver and look at the road ahead.They anticipateon 'uvhat
maneuvers
will contenext.This explainsas well why
driversare never motion sick.
SintuIutur Sic'tle,r.r
Motion sicknessis also encountered
in sirnulators.
It is calred
simulatorsicknessif an individualis motion sick in the simulator
but the originalmotiondid not provokemotionsickness.
Simulator
sicknessmight be due to insufficientmotion capabilitiesof the
simulatorto mimic the amplitudesof the real motion.The motion
characteristics
of transportaircraft are such that the moving bases
of transportaircraftsimulatorscan move sufficientlyto "convince"
the human equilibrium systemthat the visually suggestedmotion
is complementedby the appropriatephysical motion stimuli. Simulator sicknessis thereforenot a common observationin these
simulators.For highly maneuverablemilitary aircraft (as well as
fbr off-terrain vehicles),the motion characteristicsof a moving
baseare ofien insufficientto stimulatethe vestibularsystem',convincingly," which may resultin simulatorsickness.It is a common
observation that experiencedfighter pilots suf-fermore from simulator sicknessthan studentpilots. This may be due to the fact that
the experiencedpilots havea fully developedexpectationabout the
incoming sensorysignals,which are not matchedon the sensory
side. (Studentpilots on the other hand might more easily suffer
from motion sicknesswhen they make their first aerobaticflights:
Their expectationtoo does not match the incoming sensoryinformation.) In flight simulatorsand in driving simulators,f-astmaneuveringis one of the provocativefactors in inducing simulator
sickness.[n view of the schemeof Fig. 2, it is obvious that then
differenceswill arise between the vectors v""r." and v.,or, because
the simulatorcannot stimulatethe vestibularsystemappropriately
in thosesituationsdue to the limited motion rangeof the simulator.
Differencesbetweenv."., and v*ubjmay also arise when there
is insufficienttemporal concordance5etweenthe visual displays
and the physicalmotion of the simulator.Such temporalproblems
may also add to the motion sicknessencounteredin virtual realitv
applications[361.
Clinical Vertigo
The SV-conflict model is also in line with our experiencein
clinical vertigo: Patientswith an acute unilateraldeficiencyshow
largediscrepancies
betweenthe subjectiveverticaland gravity [8].
Head rotation is therefore rarely about the subjective vertical,
causingdiscrepanciesbetweenv".,," and v,,n, and thereforenausea. Consequently,they pret-erto remain mbtionlessat first to
suppressvertigo and nausea.However, compensationis faster
accomplishedusingrehabilitationprogramsthat rry to mobilize the
patient as quickly as possible,in line with the Kalman filter
requirementsto learn adequatecompensationstrategies.It is of
interestthat those programsoften start to make eye movelnents
only, which apparentlyaredisturbingby themselves
[31]. Whether
this is only due to the spontaneous
nystagmusor becauseeye
movementsevoke self--motion
sensationsshouldbe sortedout. also
in view of the discussionaboveaboutmotion sicknessanclcircular
vection.
For the clinical vestibularteststhat evaluatethe rnotionsickness susceptibility.the SV-conflict theory has consequences
as
well. More attentionshould be paid to tests that involve the
subjectivevertical.especiallyto those tests that deternrinethe
sensoryweight f:rctors.lnappropriuteweighting of the sensory
intormationcorrelatesto increasednrotionsicknesssusceptibiIity,
its shown fbr instanceby Bles ct al. [9] and hy Harm ct al. [2-t],
BLES ET AL.
486
who firund increascdweightingof the visual intirrrnationin sufastrof'erersfroln sca sickness-attd ttrotitlttsickness-susceptible
courses.
nauts.respectively.For motion sicknessdesensitizatitln
trainingwith Corioliseff'ectswith firll view of the I'isualsurrtlund
accordingto the SV-conllicttheoryin view
shouldbe lesseft'ectil'e
of the absenceof a conflictbetweenv.cn.and v,ub.i.It would only
the subjectthat
eftect in that it detnonstrates
have a psycholo-uical
motionsicknesswill not alwaysfbllow on headmovementsduring
rotation.
DISCUSSION AND CONCLUSION
we proposea theory on motion sicknessthat is built aroundthe
of the vertical,(i.e', the
of the internalrepresentation
cletermination
theory can be seen
SV-conflict
so-called
This
subjectivevertical).
theory, in
as a simplilicationof the classicsensoryrearrangement
which motion sicknessis attributed to the presenceof sensory
conflicts.According to the SV-conflict theory, however, there is
only one conflict of interest:that betweenthe subjective(or expected;vertical and the sensedvertical.The advantageof having
only one underlying conflict is that stimuli no longer need to be
classifiedinto different types of conflicts,such as is the casewith
the sensory rearrangementtheory. In the short review of wellknown provocativestimuli, we did not encountermajor problems
in upholdingthe SV-conflict theory. Despitethe oflen qualitative
description,the examplesshowed that the theory fits the experimental data quite well, in our opinion better than the classic
sensoryrearrangementtheory.
The SV-conflicttheory was mathematicallyimplementedas an
extensionof the model of Oman [33] by adding modulesfor the
calculationof the sensedand subjectivevertical and the conflict
vector betweenthesetwo. We believe that the SV-conflict model
is straightforwardin analyzingwhethersituationsare provocative
in terms of motion sicknessor not. There is no need for applying
different weight factors for different situationsto explain why
people get sick in one and not in the other condition. Moreover,
witli respectto the clescriptivemotion sicknessincidencemodels
u9,301, th" Sv-.onflict model doesnot requirespecialfiltering of
input signalsor the restrictionto pure vertical motion'
one uncertaintyin the model is how exactly the feedbackis
organizedby which the subjectiveverticalis updatedby the sensed
vertical. This requires further researchand should also address
questionsregardingadaptationand habituation.We assumethat
for habituationto a motion environment,apart from the vector c
and the gain K. in the Oman model, the vector d and the gain Ku
are importantas well. For instance.sllpposea personis habituated
to a particularpatternof ship motion. When this personchanges
orieniationaboard the ship (e'g., from upright to supine or from
facing bow to facing starboard).the stimulationof sensorysystems
will changeas well. In termsof sensorystimulationthen,a quite
complex habituation process of the multidimensionalvector c
Still, it is common experiencethat changing
would be reqr-rired.
one's orientationaboarda ship does not causemotion sicknessto
pop up a-tain. In our opinion. this could be accountedfclr by
assurninghabituationof the sgbjectivevertical.which does not
depenclon the body orientationaboiird the ship.r Moreover, this
can be a much siu-rplerprocessbecauseit only concernsthe
to a specificship
vector d. Peoplehabitrratecl
three-dimensional
new habituation
a
require
and
again
get
sick
pattern
nlay
rnotion
processwhen tfte nrgti6n patternof the ship changcsdue t6 a
transitiott.for instancc,I'rornthe North Sea wave patternsto those
of the AtlanticOcean.
It is an interestingqucstionwhy the subjectivevertical is stl
impoftant that problernsin detenniningthe subjectivevertical
One could arguethat the verticalis critical
causemotion sickttc-ss.
fbr the organismto maintainthe uprightposturalposition.This is
by the view of someworkers in this held who have built
supportecl
an entire motion sickness theory around the control of body
orientation[37]. Anotherpossibleexplanationcould be the finding
that the carcliovascularsystem needs inforrnation to keep the
cardiacoutplrt adequatefbr each body position [431.
In conclusion,the assumptionthat all notion sickness-provoking conflictscan be pinneddown to only one conflictis justifiedin
view of the analysisof the differenttypesof motion sickness.[t has
been demonstratedthat most of the experimentaldata on motion
sicknesscan be explainedsufficientlyby the SV-conflict model. It
is also clear that some experiments in the literature (like the
different types of optokinetic stimulus apparatus)should be analyzed in more detail to solve the apparentdiscrepanciesin motion
sicknessdata.
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