MOTION PERCEPTION AND SICKNESS, EYE MOVEVEMENTS AND
HUMAN PERFORMANCE
JelteBos.info
last
updated: 2017-05-11
This text is an adaptation of my inaugural
lecture at the VU University, Amsterdam, the Netherlands, presenting a
personal view, though largely based on what I have learnt from others.
INTRODUCTION
Standing and walking erect on two legs is an Art
requiring a control system with adequate sensors, ample redundancy, and smart
adaptive integration of available information. This is something humans
normally do pretty well. Normally, because there are conditions in which this
fails. After prolonged rotation we generally feel dizzy and may even
fall, which may also happen after the intake of alcohol. Also we may get
sick of motion, even by only looking at motion pictures while sitting still, do
we abort simulator trainings due to simulator sickness, and have complete naval
battles been lost due to seasickness. Furthermore we may lose sharp sight
due to vibrations, do we fall more often after the age of 70, and do some of us
suffer from vestibular diseases such as Ménière's
disease [1].
Lastly, pilots may get disoriented ultimately resulting in a controlled flight
into terrain.
To understand self-motion perception, some knowledge
on kinematics (which is about motion per se) and kinetics (which is
about the causes of motion, typically forces) may be helpful.
SOME BASIC SCIENCE
Kinematics
Object motion, i.e., that of an infinitesimal small
point, can generally be described by position (x, defined in three
dimensions (x,y,z) relative to
some point in space, it's velocity (v = dx/dt), and/or it's acceleration (a = dv/dt = d2x/dt2).
Higher order derivatives may also be defined, such as jerk (j = da/dt = d3x/dt3),
snap (s = dj/dt
= d4x/dt4), cracle
(c = ds/dt = d5x/dt5)
and pop (p = dc/dt = d6x/dt6).
The latter three are also onomatopoeic mascots used by Kellogg's, referring to the sound of chewing cornflakes.
To define the state of an object in time, either all it's time derivatives
should be known at one particular moment, or one of them should be known over a
certain interval.
A body, moreover, generally consists of several
(infinitesimal small) points. If these points are rigidly connected, the
motion of the body as a whole can be described by the linear motion of its centre
of mass, and the way all the points rotate about that centre. The state
of a (rigid) body in the 3D world is then given by six degrees of freedom:
three (linear) translations (along, e.g., the x-, y-, and z-axis),
and three angular motions about these axes. Note that, although all
points within the body may have different linear motions, the angular motion of
all points within a rigid body are equal!
Kinetics
Motion is furthermore largely determined by the laws of
physics, in particular Newton's laws of motion. These roughly state that
1. An object moves with a constant velocity (v,
characterized by direction and magnitude, the latter possibly being zero),
unless perturbed by an external force (F). This law was, in fact, already
formulated by Galilei before Newton.
2. If perturbed, the change of velocity, i.e., its acceleration (a
= dv/dt) of the body is directly
proportional to (and in the same direction as), that external force (F=ma,
with m the mass of the object).
3. If one object exerts a force (F1) on a second
object, the latter will exert a reaction force (F2) equal in
magnitude and opposite in direction to that of the first (F2=
-F1).
In addition, Newton's law of universal gravitation states
that gravity is the phenomenon that two masses attract each other by a force (Fg)
directly proportional to both masses (m1
and m2) and inversely
proportional to the square of their mutual distance (r) according Fg = k×m1m2/r2×r (with r
here denoting a unit vector along the line connecting the two masses). This also gives weight to masses according Newton's second law, i.e., Fg = mg, with g = 9.81 m/s2 the average free-fall or gravitational
acceleration at the Earth's surface.
The relationship between gravity and inertia will be detailed below.
MOTION PERCEPTION
Our central nervous system (CNS), however, does not
always seem to be aware of all these time derivatives and laws, why the laws of
perception may differ from the laws of physics. For example, perceived
velocity not necessarily equals the time derivative of (perceived) position,
and perceived acceleration does not need to equal the time derivative of
(perceived) velocity, nor does it need to be proportional to (perceived)
acceleration as given by Newton's second law. Moreover, in our 3D world
we have to deal with six degrees of freedom, three translations and three
rotations. On Earth, we have to deal with gravity in addition, which by
Newton's second and third laws gives rise to an acceleration, the free-fall
acceleration (g=-Fg/m), theoretically adding three more degrees of freedom.
The latter, however, are dependent on the other degrees of freedom by the laws
of physics, i.e., when we tilt our body, for example, gravity rotates with
respect to our body unambiguously prescribed by that body-tilt. Here
again, our CNS not always to reckons this relationship. The ambiguities left by
the CNS may all together already explain why we are facing illusions, postural
instabilities and motion sickness.
But, there is more, as may already be exemplified by
the well-known, but ever striking visual illusion [2]
shown by the turning face right. And here again the CNS plays a crucial
role. Yet, some knowledge on our senses themselves is essential too, most
importantly the organs of balance and the eyes.
ORGANS OF BALANCE
Just behind our ears, embedded by the skull, there is a cavity (also
called the vestibule), containing two sensor: the cochlea by which
we hear, and the organs of balance by which we feel motion. The organs of
balance, in turn, consist of two parts: a set of three semi-circular
canals by which we feel angular motion, and two sets of otoliths by which
we feel linear motion.
Semicircular canals
The semi-circular canals contain fluid (endolymphe) which lags the head due to
inertia when the head moves. Within each canal, a little valve (the cupula),
connected to the head by hair cells, moves with the endolymphe, thus signalling
angular motion. If the head rotates with a constant velocity, the
endolymphe will move with the head because that cupula restricts the flow of
endolymphe and because of friction between the endolymphe and the wall of the
canals. Due to elastic properties, the cupula will then return to its
resting position, thus signalling no motion anymore. You may experience this
yourself while being rotated carefully by a friend on a desk chair, with your
eyes closed, of course. If you would then stop abruptly, the endolymphe
will keep on moving, due to inertia again, thus giving you a sense
of rotation in the opposite direction. This sensation will die out for
the same reasons as mentioned above. This illusory motion is also called
the somatogyral illusion. People in whom the organs of balance are not
functioning, this somatogyral illusion is absent. This is often the case
in totally deaf people, because the endolymphe of the cochlea is shared by the
organs of balance, and dysfunction of the one is also shared by a dysfunction
of the other. You may find a nice demonstration in the (Dutch)
educative children's programme klokhuis.
To only sense angular and not linear motion (see the otoliths below) the
cupula should be as heavy as the endolymphe (i.e., having the same specific
mass), which is the case indeed. Perhaps nice to know is that alcohol has
a lower specific mass than endolymphe. If there is sufficient alcohol in
your blood, the blood vessel containing cupula of the most vertically
oriented canal will then float upwards, thus giving a spinning sensation you
may recognize.
Otoliths
Within the sacs connecting the canals, each ear also contains two layers of
hair cells with calcium-carbonate crystals on top. These layers are also
called otoliths, and are sensitive to linear motion. The most
horizontally oriented layer (in an upright head orientation) is called the utricule (or utriculus in Latin)
and is mostly sensitive to horizontal motion. The other, most vertically
oriented layer is called the saccule (or sacculus)
and is mostly sensitive to vertical motion. Different from the utricles,
the saccules are assumed to also have an auditory function. The brushes
shown in the image above [3],
only exemplify the function of the otoliths, rather than showing an
anatomically correct image. This function, is about as follows.
In the image shown here, one of the crystals is
represented by a little blue ball, and is attached to the head, the big orange
ball, by means of a string, the gray line in
between. If then the head is accelerated upwards in space, if not
attached to the head, the crystal would remain in place if not attached to the
head. But because it is, it will be pulled upward by the string, and
because that string is flexible, that string bends. Moreover, in case of
a static acceleration, the deflection thereof is constant. The larger the
acceleration, the larger the deflection, and the cell from which the string
originates, will signal accordingly. The otoliths therefore function as
pretty good accelerometers. However, on Earth, these crystals are also
attracted by the mass of the Earth, i.e., by gravity (see
above), where the gravitational force is proportional to the masses and
inversely proportional to their mutual distance squared. Because the mass
of the Earth is huge, this force is considerable at the Earth's surface.
The problem, however, is that although gravity is physically different from
inertia, the acceleration associated with it, i.e., the gravitational
acceleration, is indistinguishable from inertial acceleration, a phenomenon
also referred to as Einstein's equivalence principle [4].
As a consequence, I am in favour of drawing the
gravitational acceleration as a vector pointing upward, despite the gravity
(force) vector pointing downward.
If our central nervous system (CNS) would not reckon
this equivalence principle, it might assume the free-fall acceleration to be
inertial, thus being caused by motion. It is then easy to calculate by
integrating the free-fall acceleration (9.81m/s2) twice over an
interval of 5 minutes, we would have travelled over a vertical distance of
approximately 440km, which is about the height at which the International Space
Station is circling around us. Though it may seem trivial, the
observation that most of us generally do not experience this, is by far
trivial. Moreover, it leads to the conclusion that our CNS, despite
Einstein's equivalence principle, apparently is capable of making the
distinction between inertial and gravitational acceleration. This
extraordinary capability seems to be based on the genetical capacity of our CNS
to learn that gravity is constant (at least in our life), while inertial
self-motion is either variable or of short duration. Although I wonder
whether Einstein has given this a thought, the first to explicitly mention and
elaborate on this idea was Mayne [5].
He assumed our CNS applies a kind of low-pass filtering of otolith signals to
estimate gravity and high-pass filtering to estimate motion. This
normally seems to work quite well, but there are circumstances in which this
mechanism fails.
Somatogravic illusion
In case of an aircraft increasing its speed on the
runway before it can take-off, the inertial acceleration lasts for several
(tens) of seconds. In that case, the vector addition of the gravitational
(pointing up) and inertial (pointing forward) accelerations tilts
forward. Due to the mentioned neuronal filter, that resultant
acceleration will be assumed to be the free-fall acceleration after some
seconds, and because we have learnt that gravity is Earth vertical, we then
"think" we are tilted instead. This is called the somatogravic
illusion, and may also be experienced in a civil aircraft accelerating on the
runway. Because vision may suppresses this illusion (see also below) it
is essential not to look outside, or just to close your eyes.
Especially in military aviation, this illusion is life
threatening. If, for example, a pilot accelerates at low altitude in bad
visual (night) conditions, intending to fly straight and level, he should not
compensate for this illusion. If yet done so, this would, and actually
does so, so now and then, result in what may be called a controlled flight into
terrain.
THE EYES
Vection
Self-motion is not only perceived by the organs of
balance, but by our eyes as well. If we move, head and eyes all together,
the image of the world will move on our retinas, and this information is added in
our CNS to that of the organs of balance to improve the estimate of
self-motion. This addition can nicely be observed when in a train at a
station looking at a train leaving from the opposite platform, often giving
rise to a strong feeling of self-motion. This illusion is called
"vection" and may also be experienced in (IMAX) theatres with big
projection screens [6,7,8].
Frame and polarity
With vection, motion is essential. In getting information about gravity, static visual cues are essential too. Although visual cues do not provide information about the magnitude of gravity, they do so about its orientation. Walls, ceilings, floors, trees, all provide information about what we have learnt to be horizontal, vertical, up and down. Whenever these structures are slanting, either by accident or deliberately, in virtual environments or in real, we may get sick and have trouble keeping balance. In Japan, for example, Earth quake proof buildings sometimes get slanted as a whole, after which its inhabitants suffer from instabilities and nausea [9,10]. For esthetical reasons or for fun, ignorant architects may also incorporate slanted structures in their designs, and that too may not always be appreciated by the general public for the same reasons. I would say: think twice or ask advice.
Gaze stabilisation
(VCR and VOR)
Where vision plays an important role in determining
our orientation with respect to gravity and keeping balance, or organs of balance
play an important role in orienting our eyes at objects of interest. In
that respect it is essential to understand that we can only see colours with
the centre of our retinas, and this center also has
the highest density of photoreceptors and hence the largest resolving
power. Moreover, only in this central region we can see colours.
Whenever we are moving, the image of what we are focusing at may then get out
of this high resolution region. The chicken, solves this problem by keeping
the head as still in space as possible, while moving the body under that, until
so now and then the head makes a fast movement forward, as can be seen in the
animation, the real time video and the same slow motion video
Something
comparable happens when you take up a chicken and make her roll, for example,
as shown below. A more amusing version with music can be found in a commercial by Mercedes,
referring to this as magic body control. More scientifically speaking it
is called the vestibulo-collic reflex (VCR).
We,
humans, do something comparable, but by moving the eyes in the head, rather
than moving the head on the body. The basic problem here is that if this
would be controlled by vision, (left image 1) light must first be converted
into neural signals by the retina, (2) these signals be relayed to the visual
cortex for processing (3, say, the graphical card in the back of our head),
after which (4) the appropriate signals have to be relayed back to the eyes
again, i.e., to the muscles (5) that can position the eyes such that they image
the object of interest onto that central retinal part again. This,
however, is a rather complex and hence slow process, easily resulting in a
blurred image. You may observe that yourself by keeping your head still,
while looking at your hand waving at arm-length. If, however, you would
then keep your hand still while shaking your head, the image of your hand will
most probably keep in focus.
The explanation is that just behind our ears, (right
image, 1) the organs of balance detect the motion of the head, and (2) send the
information more or less directly, i.e., in a short reflex arc, back to
the eye muscles (3). If we move the head 10 degrees left, the eyes will
more or less move ten degrees right. This, in first instance [11],
does not need complex processing by that graphical card, why this reflex is
simple and fast, resulting in a sharp image, even when the head moves
relatively fast. This vestibulo-ocular
reflex (VOR) is simple, fast and involuntary, which implies it also exists
when the eyes are closed, whether you want it or not.
As explained above, the headshaking is mainly detected
by the semi-circular canals, and these do not respond to a constant angular
motion. Moreover after abruptly stopping a constant rotation, the fluid
flow within the canals will last for some while, and this too can nicely
observed when looking at the eyes. Especially, when using lenses that give an
enlarged view on the eys, and prevent a clear vision
by the subject and thus prevent visual suppression of the VOR, this is
fun. Here, the relatively slow phases are driven by the canals, while the
fast resetting jumps act comparable to the fast forward motion of the chicken's
head. All together the lsow and fast phases are
called nystagmus. This reflex allows doctors to test the function
of the organs of balance without the need of invasive examination.
Altogether, our organs of balance thus play a major
role by controlling our eye movements for gaze stabilisation. Moreover,
this may even be an evolutionary explanation why they are so close to our eyes.
The other way round, because vision is important for keeping balance, one might
say that we keep balance by our eyes, and look with our organs of balance [12].
In the following, I will elaborate on the mechanism by which we keep balance.
A BALANCE MODEL
Servo system
To keep balance, we need a control mechanism. A
simple example, a so called servo system, is shown below. Left in this illustration,
the desire to reach or maintain a certain condition or state is
represented. Though not too important for the following, you may think of
this state as a certain position or orientation in space, a certain walking
speed or acceleration or any other kinematic descriptor. Our brain has to
convert this desire into signals controlling our muscles to realise the desired
state as indicated in the centre. In addition to what we want, there may
also be external perturbations, as caused by wind, for example. This is
also where gravity comes into play. At the right of the illustration, the
estimated state based on sensory information is shown. When this estimate
is then compared with what we originally wanted, and there would still be a
difference, the control loop will continue to readjust the state of the body
until the estimated is just what we wanted.
Unfortunately, this control system is too easy for
three major reasons. Firstly, the sensory system, e.g., our eyes and
organs of balance, should be perfect. As shown above, however, they are
not. The semi-circular canals, for example, are insensitive to constant
angular motion. Secondly, the control loop should be fast, while
one-fifth of a second due to neuronal delays within our CNS are no
exception. When chasing a prey, or even more so when being chased by a
predator, this may be too much. And last but not least, the estimate of
acceleration should be perfect as well. This estimate, however, is
impeded by gravity, as shown by the somatogravic illusion, for example.
Internal model
That
is why our CNS is in need of another mechanism to contol
our body motion. One mechanism that may do the job is shown below this
paragraph. Somewhat based on the servo mechanism shown above, the dark
blocks that have been added represent "knowledge" our CNS seems to
have about how our body reacts to the muscles activation and how our sensors
react again to that. This knowledge is also termed a neural store,
observer, or internal model, and the signal tapped from the muscle
activation an efference copy. If that internal model would be up
to date, its estimate of the sensory estimated state, would be equal to what
the latter actually is. Further analysis then shows that also the
estimated state of the body equals the actual state of the body. In this
way, the CNS can create the best possible estimate to be compared with what we
want, thus closing the feedback loop controlling our body motion. If,
however, the output of the internal model would not be equal to what the
estimated sensory information is, due to, e.g., an external perturbation, the
difference may be fed back to the internal model of the body. This makes
a secondary control loop driving the difference as yet to zero, and again
creating a perfect estimate of the actual state of the body.
The
elegancy of this control mechanism is that it allows for sensory imperfections,
neuronal delays, and the problem caused by Einstein's equivalence principle, as
long as all inadequacies are included in the internal model. It then not
only enables an optimal control of self-initiated body motion and to correct
for external perturbations (including gravity), but a better understanding of
our motion and attitude perception, including the way our eyes are moving [8,13,14].
Moreover, it also is capable of explaining why and how we suffer from motion
sickness.
MOTION SICKNESS
Normally, we do not get sick when moving
ourselves. We only get sick from unnatural motions forced upon us, like
by a car, aircraft or ship [15,16,17].
An important detail concerns the observation that people without functioning
organs of balance do not suffer from motion sickness [18,19].
Moreover, this observation can nicely be taken into account in the observer
model shown above [20],
assuming that the conflict between the sensed and predicted sensory state of
the body provokes our gastrointestinal system. The larger the conflict,
the sicker we get. If every disadvantage has its advantage, the advantage
of sickness is that it probably incites adaptation or habituation. In the
model shown above, the conflict (between the sensed and predicted states) plays
a major role in making and keeping the internal model up-to-date, allowing for
an ecological explanation of motion sickness.
Another pleasant incidental circumstance of assuming
the above model, is that we can calculate with it. Then, this model may
be simplified by only considering external motions, thus assuming a passive
passenger. All blocks dealing with the control of self-motion may then be
eliminated. Another limitation may be that to vertical sinusoidal motion.
This leaves a relatively simple model of which all blocks can be approximated
with known mathematical (transfer) functions, resulting in the predicted amount
of motion sickness as a function of motion amplitude and frequency [16].
The coloured 3D graph show the predicted motion
sickness incidence (MSI, on the applicate or z-axis) as a function of
the motion frequency (f, on the abscissa or x-axis) and amplitude
(A, on the ordinate or y-axis). Somewhat trivial may be the
prediction that we get sicker with more motion. What may be surprising is
the prediction that we hardly get sick from motions varying with a frequency of
over 0.6 Hz and below 0.06, but are very sensitive to motions varying with a
frequency of 0.16 Hz, i.e., with a period of about 6 seconds. This graph
may thus also explain why sailing on the (Dutch) IJsselmeer
may be more provocative with respect to seasickness than the North Sea.
At the latter the waves may be higher, but will generally also be longer and
slower, the net effect being less. Even more surprising, at least to me,
is the observation that these predictions seem to be very close to what has
been observed. To that end, the inset on the top-right of the above
picture shows the results of an experiment in which more than 500 subjects have
been exposed to vertical periodic motion in a simulator for up to 2 hours each,
or less when they had to vomit [21].
All axes in this graph are equal to that of the predicted values, thus showing
a remarkable resemblance. Though not the ultimate proof of the pudding,
this resemblance, stated carefully, at least does not speak against the
framework presented here.
Sickness and
performance
With respect to sickness, it may be worth mentioning
that this not only concerns a luxury problem. Anarchasis,
in the 6th century BC, for example, already divided humans into three
categories: the living, the dead, and the seasick. Also the reservists
shown left below [22]
suggest, if I may say so, not being capably of winning a war right after the
depicted transport. Although this picture may look like a caricature
(which it yet isn't), the observational data shown right further substantiate
this problem [23].
Moreover, from other studies, we now also know that the effect of seasickness
on crew performance can even be larger than the effect of the ship motion per
se [24].
|
|
|
Cybersickness
So far, the above considerations on sickness only
concerned physical motion, thus concerning our organs of balance. As may
be assumed from the considerations on vision, what we see may also affect
sickness. And this indeed is the case, in a positive as well as negative
sense. It is, for example, common sense among sailors that looking at the
horizon has a positive (alleviating) effect on seasickness [25,26].
The other way round, looking at motion pictures can be sickening [27],
even when being immobile oneself. The latter is then generally referred
to as visually induced motion sickness or more specifically cybersickness
in case of computer gaming, for example. With these games getting better,
computer screens getting bigger, and 3D getting more common, cybersickness may
get a serious issue (see also below). A special fact of interest may be
(and for me it certainly is) that people without functioning organs of balance
do not only suffer from (physical) motion sickness, but also not from visually
induced motion sickness [28].
Sickness, furthermore, not only is an issue with respect to gaming, with flight
and driving simulators it is an issue too. 50% drop-out due to sickness
has been shown not to be an exception [29,30].
Then, it may be essential to make an explicit distinction between motion
sickness and simulator sickness, the latter only referring to sickness in case
the simulated real condition would not give rise to sickness. The way all
different kinds of motion sickness are related is shown in the Venn diagram
below. Here, I deliberately drew space motion sickness separate from the
others, because of the rare condition of the virtual absence of gravity.
Postural
(in)stability
Another reason why research on cybersickness may be of
interest, concerns the observation that most of us are more instable after
watching motion pictures than before watching [31].
This not only has been observed in the lab, but also in a normal (3D) cinema
audience [32],
and it makes sense to assume that after several hour (or even days) of gaming
this effect is even more obvious. An intriguing thought concerns the
possibility that the more we get used to watching motion pictures with respect
to sickness (see below), the more instable we (seem to) get. Another
provoking thought may be the observation that a comparable instability as
observed after watching motion pictures is known from taking alcohol.
Although comparing apples with oranges, the following graph in combination with
the fact that in some countries postural stability is taken as a criterion for
safe driving, is telling [31].
COROLLARIES
Back
to basic it seems to make sense assuming that perception is is
not the direct effect of sensory information. Instead, it seems likely
that perception is the result of a process aiming at making an optimal estimate
of what reality is, and this process takes certain knowledge into account,
assumed predictive for the future. This knowledge may, furthermore, be
made more concrete by an internal model. Perception, stated differently,
thus seems to be the result of an interpretation made by our CNS, only using
indirect information from our senses. This will then have certain
consequences, like the effect of watching a manipulated passport photograph
shown above. There are, however, many more examples, some of which I will
touch on in the remainder of this text.
Age
Babies, for example, do not suffer from motion
sickness. They may throw up, doing that even regularly, but they do not
do that due to motion. Given the sketched frame of mind, the idea seems
to be that babies are only moved passively, no motion control mechanism being
active yet. As soon as they start moving themselves, especially when
standing up, however, they are in need of such a control mechanism, and
creating or filling up the internal model or neural store is then at
issue. That process, however, is then troubled by the fact that both the
body and the nervous system are continuously growing, i.e., liable to
changes. Until the age of about 20 years, the internal model therefore
lags reality continuously, and only after that it can really
"settle". This, accordingly, should, and according the grafph below apparently is the period of life in which most
conflicts between sensed and predicted signals are present, and we are also
most susceptible to motion sickness. For the same reason, the
preservation and further fine tuning of the internal model results in
habituation, 80 year olds being about four times less susceptible to
seasickness than 20 year olds, for example [33].
Just getting old therefore seems to be a fair advice against motion sickness,
at least seasickness.
Dysfunction
Another
example, though not completely different. Apart from age, there are
several reasons why the functioning of our senses may vary over time. An
inflammation of the vestibular nerve (called a vestibular neuritis by the ENT
doctor), for example, may cause an acute problem. The way the endolymphe
within behaves in our semi-circular canals may change, seems to be the cause of
the longer lasting Ménière's
disease. The nice thing about the sketched framework is that these
changes do not necessarily need to cause a permanent problem. As long as
the internal model is capable of adapting to the new condition, the conflict
between sensed and predicted signals can be kept small, thus also minimising
the associated problems like sickness and postural instabilities. The
problems will then mainly be evident in the acute phase when either the senses
are instable, or the internal model not yet up-to-date. The process of
getting this internal model up-to-date also refers to adaptation, habituation,
or vestibular compensation, and may even be the major reason for our CNS to
apply an internal model anyway.
Mal-de-debarquement
The last example I would like to mention here again
concerns motion sickness. Especially sailors are familiar with the fact
that we get used to ship motion within hours to days, also accompanied with a
reduction in sickness. After debarking, they also know about a motion
sensation like still bein on the boat, and this
motion can also be accompanied by sickness again. Normally the latter
disappears within hours to days. In some, however, it does not. This is
referred to as mal-de-debarquement, or when it
lasts for more than days, sometimes even years, mal-de-debarquement
syndrome. Intriguingly, it has
been observed that in most of these cases the organs of balance are still
functioning normally, why it seems likely that the internal model got
disorganised, not being able to get organised again. From a mathematical
point of view, one may then think of optimisation of the internal model
transfer functions by minimising the mentioned conflict as the cost function,
the process being caught in a local minimum, far off the optimal solution.
CHALLENGES
If
our CNS does indeed apply an internal model to optimise the control of body
motion, and if indeed it does so by keeping that up-to-date using a conflict
related to motion sickness, this also offers possibilities for cure of diseases
often assumed untreatable so far. To further validate the framework
presented here and then to find out how a malfunctioning internal model can
best be cured, I consider a major challenge for the coming years. The
results may then be rewarding, not only for patients suffering from vertigo,
dizziness and nausea. It may also be of benefit for the treatment of
elderly who keep afraid of falling after an unlucky fall.
As
may be clear from this text, motion sickness is not a disease. Moreover,
it seems to be inherent to a normal, i.e., healthy life, especially when
unnatural motion is at issue. To explain motion sickness in all of its
manifestations, an internal model seems to be of particular use. Hence,
here too the further elaboration of how our CNS may apply this approach will
likely also reveal solutions not thought of before.
Another
challenge is the optimising of motion simulators. With the aimed
reduction of costs, training on the job is often replaced by training in a
simulator. If motion would then be a factor of importance in real, it
should also be in the simulation. A problem with the simulation of motion
is that most often, the vehicle to be simulated is capable making much larger
motions than the simulator can. An aircraft can go all over the world, a
flight simulator should generally stay in its building. Although from a
technical point of view it then makes sense to minimise the difference(s)
between the motion parameters (kinematic and kinetic) of the real vehicle and
the simulator, from a human perspective it makes more sense to minimise the
difference between the perceptions of both. A motion perception model
would then be helpful, especially when not only taking our senses into account
but also what is sometimes referred to as mental set, or cognition.
To do so, an internal model may also be of benefit.
And, who knows, one day we may even be able to show
neuronal structures actually operating like an internal model. Up to that
moment the challenge is to make its use by our CNS not only plausible as done
in this text, but also probable. For scientific reasons it might as well
be good if it could be proven not to exist.
NOTES
1.
Not "meunière", for that
relates to the way of preparing, e.g., trout.
2. Thanks to Eric Groen and Walter van Dijk, Soesterberg.
3.
Thanks to Bernd de Graaf and Peter Schep, Soesterberg.
4.
Einstein A
(1907). On the relativity principle and the conclusions drawn from
it. Jahrbuch der Radioaktivitat und Elektronik
4:411-462.
5.
Mayne R (1974). A
systems concept of the vestibular organs. In: H.H. Kornhuber (ed),
Handbook of sensory physiology. IV-2 Vestibular system, (Springer Verlag,
Berlin, pp 493-580.
6.
It may be interesting to
know that the time it takes to experience vection is about the time it takes
for the endolymphe to stand still in the semicircular canals. When
rotating with the eyes open, the addition of visual and vestibular signals thus
gives a fair estimate of self-rotation.
9.
Kitahara M, Uno R (1967). Equilibrium and vertigo in a
tilting environment. Annals Oto-Rhino-Laryngology 76:166-177.
10. De Wit G (1973). The Flying Enterprise symptom.
Oto-Rhino-Laryngology 35:248-251.
11. This reflex, however, is affected by the so called “velocity
storage” mechanisme, resulting in a longer lasting sensation of rotation
as can be explained on the function of the semicircular
canals only (see also note 13).
12. Wit G de, Vente PEM, Bos JE, Bles W. (1999) Het snelle labyrint ten dienste van het trage oog / The fast labyrinth for the benefit of the slow eye. Nederlands Tijdschrift KNO-Heelkunde 5:7-10.
13. Merfeld DM, Young LR, Oman CM,
Shelhamer MJ (1993). A multidimensional model
of the effect of gravity on the spatial orientation of the monkey.
Journal of Vestibular Research 3:141-161.
15. Bles W, Bos JE, Graaf B
de, Groen E, Wertheim AH (1998). Motion sickness: only one provocative conflict?
Brain Research Bulletin 47:481-487.
18. Irwin JA (1881). The pathology of seasickness. The Lancet
2:907–909.
19. James W (1882). The sense of dizziness in deaf-mutes. Am. J.
Otol. 4:239–254.
20. Here I assume that the neuronal filter making the distiction between inertia and
gravity, only operates on the (afferent) signals coming from the otoliths
within the organs of balance. Because this process also takes time within
the internal model, there will always be a difference between the actually
sensed signals of people with functioning organs of balance and the prediction
thereof by the internal model. In people without functioning organs of
balance there are no afferent otolith signals, and with an accordingly adapted
internal model there can also be no conflict (see also note 16).
22. Thanks to Michael McCauley, California.
24. Bos JE, Valk PJL,
Hogervorst MA, Munnoch K, Perrault D, Colwell JL
(2008). TNO contribution to the Quest 303 trial - Human performance assessed by
a vigilance and tracking test, a multi-attribute task, and by dynamic visual
acuity. TNO report, TNO Human Factors, Soesterberg, the Netherlands
TNO-DV 2008 A267.
25. Bos JE, MacKinnon SN, Patterson A (2005). Motion sickness
symptoms in a ship motion simulator: effects of inside, outside, and no view.
Aviation Space and Environmental Medicine 76:1111–1118.
26. Bos JE Houben MMJ,
Lindenberg J. (2012). Optimising human performance by reducing motion sickness and enhancing
situation awareness with an intuitive artificial 3D Earth-fixed visual
reference. Proc. Maritime Systems and Technology Conference, Malmö,
Sweden, 11-13 September. pp. 1-10.
27. Bos JE, Bles W, Groen
EL (2008). A theory on visually induced motion sickness. Displays 29:47-57.
31. Emmerik ML van, Vries
SC de, Bos JE (2011). Internal and external fields of view affect cybersickness.
Displays 32:169–174.
32. Bos JE, Ledegang WD,
Lubeck AJA, Stins, JF (2013). Cineramasickness and postural instability.
Ergonomics 56:1430-1436.
33. Bos JE, Damala D, Lewis C, Ganguly A, Turan O (2007). Susceptibility to seasickness. Ergonomics 50:890–901.