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Experimental Studies of the Eliciting Mechanism of Motion Sickness ARNE SJOBERG University of Uppsala, Sweden SUMMARY After an analysis of ship movements and calculation of the maximal acceleration values for a normal-sized passenger vessel on high seas, it is concluded that angular acceleration is unimportant in the elicitation of seasickness. The vertical up-and-down motions of a ship are the most important, and experimental motion sickness can easily be provoked in human beings and dogs exposed to vertical movements in rapid elevators or hoisting cranes. Deaf-mutes with reactionless labyrinths and dogs after labyrinthectomy have no symptoms of motion sickness after exposure to these rapid elevator movements. On board a ship on a heavy sea, eye movements similar to nystagmus have been recorded by electronystagmography (ENG). Simultaneously, on the same direct-writing paper, the pitching and rolling motions of the ship have also been recorded with accelerometers. Optical and proprioceptive impulses are not necessary for the appearance of symptoms of motion sickness. It is known from experience, however, that these impulses together facilitate and promote the appearance of the symptoms. The optical impulses facilitate the symptoms to a greater extent than do the proprioceptive ones. Results of hydromechanical studies and experiments on labyrinth models and temporal bones have shown that, when a person is exposed to linearly accelerated horizontal and vertical movements or to the movements of a ship on a heavy sea, pressure variations with accompanying displacements and flows in both the perilymph and the endolymph must occur at every point in the contents of the labyrinth. These pressure variations affect both labyrinths at the same time, but the momentary pres- sure in the corresponding points of the two labyrinths will seldom be exactly the same during these varying motions. It seems very probable that these pressure variations in the fluids of the labyrinth are of such a magnitude that the transmitted excitatory effect will create manifest symptoms of motion sickness. It thus seems justifiable to assume that the symptom complex of motion sickness arises from the two receptor systems of the labyrinth: the otoliths and the ampullar cristae. The intermittent headache and some of the psychic symptoms accompanying motion sickness may be largely due to the intracranial pressure variations caused by the linear acceleration movements. The results of some new hydromechanical experiments in the absence of gravity can be applied to the problems concerning weightlessness in prolonged space flights. The unfavorable vestibular reactions of the motion-sickness type after sharp movements of the head in Russian cosmonauts may probably have been due to the fact that, in the absence of gravity, as soon as a sudden horizontal ac- celeration takes place, the fluids of the labyrinth will be forced "outward" or flung toward the mem- branous and osseous walls of the labyrinth. The liquid in a vessel is forced toward the side away from the direction of the acceleration. The result will be a very rapid adequate excitation of the "deaffer- ented" weightless receptor systems. These unexpected afferent impulses will suddenly produce symptoms of vertigo, motion sickness, and perhaps sensory illusions. The hydromechanical hypotheses and experimental results may also plausibly explain the spatial illusion in weightlessness of the body being in an upside-down position.
THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION INTRODUCTION When I began my research 40 years ago on the fascinating problem of the mechanism eliciting seasickness, I hardly anticipated that this sub- ject would be of such great topical interest in the world of today, in the space age with its studies of vestibular problems in manned space flights of long duration. It is of primary impor- tance at the present time to prevent vestibular disturbances in weightlessness and perhaps to solve problems concerning artificial gravity. Because of the increase in sea and air travel by both civilian and military, the production of effective anti-motion-sickness drugs is equally important. For centuries, intensive studies have been made of the symptom complex which has been generally called seasickness, better known today as motion sickness, vestibular sickness, or space sickness. It was at the end of the 19th century that motion sickness was first seriously related to the inner ear, and at an early date its resemblances to Meniere's disease were pointed out. Important support for this view was obtained by the observation that deaf-mutes seldom or never became seasick (refs. 1 to 3). In 1901, Butler Savory (ref. 4) considered that seasickness was provoked reflectively from the semicircular canals, and Corning in 1904 (ref. 5) pointed out the similarity between rotatory vertigo and seasickness. In 1903, Kreidl suc- ceeded in inducing seasickness experimentally in animals subjected to artificial ship movements. Remarkably enough he does not seem to have published his results, but he is said to have shown "qu'apres section bilaterale du nerf auditif, les animaux sont insensibles aux mouve- ments artificiels" (ref. 6). Barany (ref. 7), Bruns (ref. 8), Byrne (ref. 9), and many others con- sidered that it was the angular accelerations which provoked the symptoms and they therefore tested subjects in rotating chair experiments. It was Wojatschek (ref. 10) and Quix (ref. 6) who first suggested that it was probably the otoliths and not the semicircular canals which were stimulated by ship movements. In spite of intensive studies, however, no unequivocal explanation was given for the way in which the excitatory process was primarily induced in the labyrinth. My original aim was to study seasickness experimentally on dogs subjected to artificial ship movements. ANALYSIS OF SHIP MOVEMENTS Since ship movements on rough seas are extremely complex mathematically, an analysis of them was considered necessary (fig. 1). Three principal types of oscillations are involved: (1) Plungingâa purely vertical motion; (2) Pitching âa motion around the transverse axis of the ship: and (3) Rolling âa motion around the longitudinal axis of the ship. Motions around the vertical axis of the ship â yawing âare due only to unsteady steering of the ship and probably have no actual influence on the induction of motion sickness. Added to these is a continuous motion forward. The accelerations can vary here in either a positive or a negative direction, according to the size of the ship in relation to the dimensions of the waves. In figure 1 we see that, from a mechanical point of view, a passenger undergoes the same Plunging Pitching Rolling FlcURE 1.âThree types of oscillations of a ship's movements on rough seas.
ELICITING MECHANISM OF MOTION SICKNESS movements on pitching and rolling. It may be said that these two movements are composed of a harmonic oscillatory motion and a rotatory motion. Figure 2 corresponds to the longitudinal section of a vessel with regard to pitching and to the transverse section on rolling. It is imagined that the passenger (P) is placed at a supposed point A on the vessel. Both on pitch and on roll, P describes, with point A as the median position, a to-and-fro movement from C to B. If the angle is small enough (especially if the radius, as on a ship, is large), then the path of point P can be regarded as a straight line, and the projections of P on the vertical and horizontal planes, respectively, then describe approxi- mately harmonic pendulous motions in the paths C\ and B\ and Ca and 8-i, with points P] and P-i, respectively, as median positions. If the passenger is located right in the front of the prow of the ship, then on pitching and also on plunging of the ship he is subjected mainly to a vertical harmonic pendulous motion. If. Horizontal plane on pitching, P is at the mast top or on the naviga- tion bridge, he describes instead a harmonic pendulous motion in the horizontal plane, but now forward and backward. â¢ On rolling he de- scribes instead a harmonic pendulous motion from side to side in the horizontal plane. If our passenger changes his location on the ship, then naturally the amplitude of the harmonic pendulous motion on pitch and roll varies. The amplitude becomes progressively smaller as he comes closer to the axis of rotation 0, and pro- gressively greater the further his distance from O (fig. 2). It is a well-known fact that a hori- zontal body position close to the center of gravity of the ship lessens the symptoms produced by rough seas, but it should be noted that the value of the angle is the same whatever the location of P. It is thus seen that the path of P consists ap- proximately of harmonic pendulous motions in both the vertical and the horizontal plane and, in addition, of an angular motion. The general motion to which a passenger is subjected takes place under varying axes and comprises a superposition of three pure sine curves with different amplitudes and numbers of oscillations. (See fig. 3 which illustrates condi- tionally selected sine curves.) To this supposed motion is then added the continuous accelerated motion. This general motion consists ofâ (1) A space motion (composed of plunging plus the harmonic pendulous motion in both the verti- cal and the horizontal plane on pitch and roll): (2) A combined rotatory motion which is the sum of the rotatory motions on pitch and on roll; (3) A continuous, possibly irregular, accelerated motion in the horizontal plane, which would seem to have special importance in small vessels. Since the time period and also the angle of pitch and of roll are specific for every vessel and a - Estimated angle of roll reap, pitching P -- Passenger O Axis of rotation (Transverse resp. longitud axis of the vessel) FIGURE 2.âSection of a vessel. The passenger P is placed at an imaginary point A on the vessel. The figure represents both pitch and roll. FIGURE 3.âSuperposition of three pure sine curves represent- ing motion axes to which a ship's passenger is subjected.
10 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION FIGURE 4. -Illustrating the angle of pitch (a) and of roll (/3) of a vessel at sea. are also dependent upon the nature of the wave movements, no mean values for these factors are to be found in the literature on shipbuilding. Figure 4 illustrates rolling and pitching. Angle a in this figure is the angle on rolling; i.e., the inclination of the mast toward the vertical plane. The angle on pitching (ft) represents the inclina- tion of the vessel toward the horizontal plane. For passenger ships of normal size there are cer- tain upper and lower limit values. The maximal value for roll is probably 15Â° and for pitch 5Â°. The time period (the time for a simple oscillation, D can vary; for roll and pitch it probably varies between 5 and 40 seconds (ref. 1). The greatest angular acceleration that can occur for a passenger ship of normal size (10000 to 20000 tons) is approximately 5Â°/sec2. This corresponds on rolling to an angle of 15Â° and a time period of 5 seconds. Such heavy rolling seldom occurs and then only on a very choppy heavy sea. All other values for pitch and roll lie below 2Â°/sec2 (refs. 4 and 11). EYE MOVEMENTS ON LINEAR ACCELERATIONS Now we come to a very important problem in seasickness. The vegetative explosion which characterizes Meniere's disease comprises, among other things, vertigo and nystagmus. In seasickness, on the other hand, nystagmus in its usual sense is absent on macroscopic obser- vation. In this connection it should be recalled that, in Meniere's disease, the labyrinthine ex- citation is induced on only one side and therefore nystagmus occurs. This has long been con- sidered to support the assumption that seasick- ness is only induced from the one type of mechanoreceptors of the labyrinth, the otoliths, which react to linear acceleration. The ampullar cristae, on the other hand, are considered to respond only to angular accelerated movements. The lowest value for the angular acceleration which in normal persons can induce an ocular movement in the direction of the slowest nys- tagmus phase is called its "minimum percep- tibile." The values obtained experimentally for minimum perceptibile have varied in man from T/sec2 up to 4Â° to 5Â°/sec2 (refs. 6, 12, and 13). As mentioned previously, it is only on heavy rolling that the angular accelerations reach a value of 5Â°/sec2. The Dutch investigator Nieuwenhuijsen (ref. 14), during journeys in 1958 across the Atlantic from New York to Rotterdam, and using an angular accelerometer, obtained a figure of 4.5Â°/sec2 for angular speed as a maximal value which was maintained for only a short period. Usually the maximal angu- lar acceleration was lower than 1.5Â°/sec2. In other words, it is only in extreme cases that the angular accelerations exceed the values for minimum perceptibile. and then only negli- gibly. As I have just mentioned, it has been con- sidered, generally, that these facts support the view that it is in the otoliths and not in the semi- circular canals that the cause of seasickness is to be found. Before the introduction of electronystagmog- raphy, no nystagmus could be observed with the unaided eye. As early as in 1931, however, I was able to demonstrate, with Dohlman's photo- kymograph (ref. 12), despite all sources of error, reflectory eye movements on linear up-and-down vertical acceleration motion in rapidly moving elevators. The amplitudes were 4 meters to 6 meters and the maximal speed just over 1 m/sec.
ELICITING MECHANISM OF MOTION SICKNESS 11 The principle of Dohlman's recording method was that a beam of light was reflected in a small concave mirror fixed to the eye with a rubber arrangement. The spot of light was projected onto a light-sensitive paper to a curve, a nystagmogram. In 1922, Fleisch (ref. 15) demonstrated on rabbits vertical eye movements of the nystagmus type on horizontal accelerations. Later Jong- kees and Philipszoon (ref. 16), but especially Niven, Hixson, and Correia (ref. 17) and also Guedry (ref. 18), demonstrated eye movements of nystagmus type on linear accelerations. During recent months my collaborators, C. Angelborg and R. Aust, and I have made recordings on persons on board ships belonging to the Swedish Navy and also on board large automobile ferries on the Stockholm-Gotland line. We have used a Mingograph 34 (Elema- Schiinander, Sweden) which is a multichannel direct-writing electrocardiograph with four re- cording channels an.d.a universal amplifier for recording of biological potentials; e.g.,electronys- tagmograms (ENG). By this means it has been possible to record simultaneously both horizontal and vertical eye movements and, in addition, vertical and horizontal accelerations of the ship on one and the same recording paper. For recordings of pitching and rolling ship move- ments, we have used a small accelerometer with fine sensitivity. A thin wire of constantan is used as the detector element in these trans- mitters. The resistance of the wire is varied Rough sea Calm sea Ashore Blinking FIGURE 5. â Recording of vertical nystagmus-like eye move- ments on heavy seas. according to its length. Extension of the wire is a linear function of the acceleration. In heavy seas we have so far succeeded in recording verti- cal nystagmus-like eye movements (fig. 5). These studies have not yet been published, and these results are preliminary ones. It seems justifiable, then, to conclude that seasickness symptoms are most probably not induced by angular accelerations but by har- monic oscillatory space motions in the hori- zontal and vertical planes on pitching, rolling, and plunging. Of the different motion com- ponents, the up-and-down harmonic oscillatory movements would seem to play the principal role. MAGNITUDE OF FORCES AT DIFFERENT LOCATIONS ON SHIPS To give a better idea of the enormous farces to which our bodies are subjected on board a ship during heavy seas, we have calculated the magni- tude of those movements to which a passenger is subjected by the different principal oscillations at varying locations on a passenger ship of normal size. We assume that the ship has a displacement of 10 000 tons, with a length of 120 meters and a breadth of 16 meters. The maximum angle for pitching is assumed, as discussed above, to be 5Â° and for rolling 15Â°. In reality, the oscillatory motions of the ship naturally take place around varying diagonal axes. For the sake of simplic- ity, we may regard the principal oscillations here as isolated motions around the longitudinal and transverse axes through the center of gravity. From these calculations (ref. 11), it is evident that â (1) A person located at the prow of the ship is, on pitching, thrown up and down 10 to 11 meters, with a maximum acceleration at the turning points of approximately 2 m/sec2. (2) When at the side of the ship on a level with the center of gravity, the passenger is raised and lowered 4 meters, with a maximum acceleration of just under 1 m/sec2. (3) On plunging there are maximum vertical movements of 10 meters, and the acceleration is about 2 m/sec2.
12 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION (4) On the navigation bridge, the person is thrown instead, on pitching, forward and back- ward 3 to 4 meters, with an acceleration of 1 m/sec2. On rolling he is thrown from side to side 10 meters, with a maximum acceleration of 2 m/sec2. (5) At the mast top the forward and backward movement on pitching can be 7 meters and the acceleration approximately 1.5 m/sec2, but on rolling a person can be thrown 20 to 21 meters from side to side, with an acceleration at the turn- ing point of 4 m/sec2. In some cases a person can be exposed to the sum of these vertical and horizontal accelerations. EXPERIMENTAL MOTION SICKNESS In Humans If these acceleration values are now compared with the greatest accelerations which experi- ence has shown to occur on railways, the latter values are lower; i.e., about 1 m/sec2. This conforms also with the fact that motion sickness on train journeys is considerably milder than true seasickness on board a ship. For rapid passenger elevators, on the other hand, the accelerations on starting and retardation can vary from 1 to 3 m/sec-. Experience has shown that sensitive persons subjected to elevator movements can easily have intense symptoms of motion sick- ness. Here the accelerations are of the same order of magnitude as in ship movements. By analysis of the ship movements, we concluded that the problem of inducing experimental motion sickness could be simplified by sub- jecting human beings and suitable animals â dogs or monkeys âto vertical harmonic pendulous motions. The harmonic pendulous motions just men- tioned can most easily be produced by an appara- tus as shown in figure 6(Â«) and (6). A wheel rotating at different constant rates drives a frame up and down between two guide rails. The radius of the wheel can vary. This apparatus has been used in our hydromechanical experi- ments. The apparatus is ideal for inducing mo- tion sickness experimentally. I originally intended that it should be constructed in larger dimensions, but for reasons of cost I had to use the apparatus already available. The simplest way of testing motion sickness is in large hoisting cranes or in rapid passenger elevators that are driven up and down with amplitudes of 4 to 6 meters and accelerations of I --9 FIGURE 6. â Apparatus for producing vertical harmonic pendulous motion and inducing motion sickness experimentally, (a) Schematic of wheel shown in (b) that rotates at different constant rates and drives a frame up and down between two guide rails (b).
ELICITING MECHANISM OF MOTION SICKNESS 13 about 1 to 1.5 m/sec2. The machinery of the elevators has to be suitably cooled in order to allow the heavy stress exerted by prolonged upward and downward movements with rapid braking. Persons who knew that they became easily seasick had pronounced symptoms of motion sickness after 15 to 30 minutes in these elevators. Even elevator conductors with many years of habituation easily became ill after these upward and downward movements with rapid braking and starting. However, deaf-mutes with intact tympanic membranes and reactionless labyrinths (on syringing with iced water) had not the slightest symptoms in moving elevators. In Dogs To elicit motion sickness in dogs, there has to be a more rapid change at the turning points for the up-and-down movement than is attained in elevators. This requirement could only be fulfilled in large hoisting cranes in Stockholm Harbor. The experiments were very time con- suming. The machinery of the cranes was strained to such an extent that the resistances sometimes became red hot. The animals were placed in cages with longi- tudinal sides made of wire netting, so that they could be observed easily. The maximum accelerations were 1.5 m/sec2, and the amplitudes 3 to 4 meters. Clinical Picture To these up-and-down movements in hoisting cranes, adult dogs, varying in weight from 9 to 21 kilograms, reacted with typical symptoms of motion sickness after periods varying between 10 and 30 minutes. The symptom complexes occurred in two forms: agitated and asthenic. Most animals showed the agitated form. After a few minutes they became restless and ran about in the cage, howling and barking. Their respiration increased. A number of animals had diarrhea and pollakiuria. Their salivation was greatly increased, so that saliva ran from their noses. At first they followed the move- ments with their eyes. In the last few minutes before the first vomiting attack, the dogs instinc- tively avoided keeping their eyes open. They crept together apathetically and appeared prostrate. After the first vomiting attack, they generally recovered rapidly, but after 30 to 60 minutes there was an exacerbation with further vomiting. With the asthenic form, the animals did not reach the stage of vomiting. They showed clear symptoms, however, in the form of increased salivation, polypnea, diarrhea, and pollakiuria. They lay pressed to the bottom of the cage, avoiding all movement, and showed no reaction to external stimuli such as calling, whistling, or prodding, etc. After 3 to 4 hours' "traveling," or after re- peated trials, the animals often became habitu- ated to the oscillations, and it took a rather longer time to induce symptoms. No definite eye move- ments were observed with the unaided eye. Labyrinthectomy Bilateral labyrinthectomy was performed on dogs that had previously been subjected to the elevator movements and had reacted with typical motion sickness, and microscopic examination of serial sections after the animal had been killed was made in order to establish that the removal of the labyrinth had been complete. Before the animal was killed, however, control tests were made in the hoisting cranes about a month after the operation. Even after up to 4 hours' travel- ing, the animals never showed any symptoms of motion sickness. They moved freely in their cages or lay quietly on the floor. They showed no caloric reaction after syringing with 500 to 600 ml of ice water. These experiments demonstrated unequivo- cally that a well-functioning labyrinth must be present for symptoms of motion sickness to be elicited. OPTICAL AND PROPRIOCEPTIVE IMPULSES For control of our balance, our orientation in space, and our locomotion, we depend upon im- pulses from (1) the ocular system, (2) the vestib- ular system, and (3) from muscles, joints, viscera, and skin in the form of proprioceptive impulses. It is of extreme importance to re- member that the predominant neuro-otological
14 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION symptoms are particularly referred to this principal triad of physiological equilibrium stim- uli. We all know that it is the vestibular receptor system, the end organ of the labyrinth, that gives us true information on the linear and angular acceleration to which the head is subjected in relation to the ground. As early as in 1922, Gertz (ref. 19) stressed in his studies that the efferent impulses which arise from muscle movements induce the sensory com- ponents in the reaction of the vestibular nerve and give a "propriozeptiven Schwindel" ("tactile vertigo"). This is induced most simply if, with the head and neck motionless, a person stands on a circular, rotatable plate and makes "right turns" or "left turns." When he steps down to the floor and walks forward with closed eyes, he deviates because of the proprioceptive impulses induced by the leg movements. Most persons have probably had similar sensations when first walking on land after a rough sea voyage. In German it is often said, "und dem Seemann schwankt nicht selten der Boden unter den Fiissen." These are purely proprioceptive im- pulses which, so to speak, persist when the stimuli have ceased. In this connection I want to emphasize the intimate relations between proprioceptive (cervical) impulses and the vestibular nuclei. Cervical receptors are found not only in the cervical and occipital muscles but also in the joints of the upper three cervical vertebrae. Spinovestibular communications, therefore, take place. Afferent fibers pass to the vestibular nucleus from the upper three cervical segments (refs. 20 and 21). After incision of the posterior roots to Cl, C2, and C3 in cats, Cohen (ref. 22) demonstrated equilibrium disturbances closely resembling those seen following labyrinthectomy. In spondylosis deformans in the cervical spine, the syndrome of Barre-Lieou (ref. 23) often occurs; it is also called cervical migraine or, briefly, the cervical syndrome. The main symp- tom here is vertigo of a transient type, an oc- casional feeling of uncertainty with a tendency to propulsion or lateropulsion. Pathogenetically it is considered that, on rotations of the head, the more or less arteriosclerotically changed verte- bral artery can be compressed and ischemia in the distribution area of the vestibular nuclei can occur. But with regard to the origin of the vestibular symptoms after rotation of the head in the Barre- Lieou syndrome, consideration must also be taken of the tonic, cervical, occipital, and labyrinthine reflexes, which are probably pro- voked by the proprioceptive impulses from the receptors in the muscles of the back of the head and joints in the upper part of the cervical spine. Exostosis directed posteriolaterally can constrict the intervertebral foramina, and may compress the spinal roots and produce root symptoms in the form of pains and vertigo via the spinonuclear vestibular communications just mentioned. To determine the importance of optical and proprioceptive impulses for the elicitation of motion-sickness symptoms, we made animal experiments in which these impulses were successively eliminated. The optical impulses were excluded by an occlusive bandage and by suturing the eyelids together. The surest way of eliminating the deep sensitivity would have been to incise the posterior roots or the spinal cord below the medulla oblongata. For technical reasons we limited ourselves to merely excluding the kinesthetic impulses by enclosing the entire body of the animal, including the neck, in plaster of paris. Only the head and a window over the abdomen and around the urogenital orifices were cut out. The dogs were suspended in an upright position in the cages. First the normal "elevator time" prior to the first vomiting episode in elevators traveling up and down was determined, and then the elevator times prior to vomiting (1) without optical im- pulses, (2) with plaster bandage, and (3) with plaster bandage plus occlusive bandage were measured. To exclude some degree of habitua- tion, control experiments were then performed. The results showed longer elevator times when the optical impulses had been eliminated. The longest times were obtained with the plaster bandage combined with the occlusive one on the eyes. By excluding optical impulses only, the elevator times became longer than after plaster bandaging alone. From these animal experiments it is justifiable to conclude that optical and proprioceptive im-
ELICITING MECHANISM OF MOTION SICKNESS 15 pulses are not necessary for the elicitation of symptoms of motion sickness. But as experi- ence has shown, these impulses facilitate the induction of the symptoms. The optical im- pulses have a stronger stimulatory effect than do the proprioceptive ones. HYDROMECHANICAL STUDIES The vestibular apparatus has, as is known, mechanoreceptors built into a system of fluid- filled spaces to which defined hydromechanical laws are applicable. The basic excitatory me- dium would seem to be the pressure changes, induced by external factors, which induce flows and displacements in the perilymph and which are transmitted homogeneously to the endolymph, whence the excitatory stimulus is transmitted to the sensory cells. The labyrinthine fluids are practically noncompressible. Thus, for labyrin- thine excitation to occur, the surrounding vessel walls should not be completely closed but should allow movement of labyrinthine fluids. For the perilymphatic space, such protective arrange- ments against pressure increase include (1) the perilymphatic duct, (2) the oval window, and (3) the round window. For the endolymphatic space, the endolymphatic duct and sac serve as safety valves. There seems to be good reason to suppose that, in the perilymphatic and endolymphatic spaces, similar pressure variations occur. On the basis of this supposition, it would seem suitable to use one single canal system when making a simple labyrinth model for analysis of the hydrome- chanical conditions in linear acceleration motions. We made a simple model of a laby- rinth in the form of a water-filled glass tube bent at an angle, both ends of which were closed by thin rubber membranes or bags, which were to represent the "safety valves" of the labyrinth against pressure increase. First I should like to give an example of the pressure distribution in a fluid, water, lying in a container at rest or in linear motion with a constant velocity or acceleration. This container can represent a model of a labyrinth. Apparatus Container in Vertical Motions Open container: 1. The container at rest or in linear vertical motion at constant velocity (the acceleration thus being equal to zero). If, over the open surface (V) of the liquid, atmospheric pressure = pa is prevalent, then, according to the laws of hydromechanics, the acc*o P. 9- â¢*- f\ FIGURE 1. â A water-filled glass representing a model of a labyrinth. The pressure is linear to the depth, V = open surface of the liquid; pn = atmospheric pressure; g= acceleration due to gravity; 7. = depth beneath surface of water; A = particular point at z; p = mass of liquid per unit volume.
16 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION absolute pressure p at a particular point A at depth z beneath the surface of the water will be where p = the mass of the liquid per unit of volume = the density of the liquid, and g is the acceleration caused by the force of gravity. From figure 7 we see, therefore, that the pres- sure is linear to the depth. The pressure curve appears as p = po + pgz. 2. The container in linear vertical motion with constant downward acceleration a. The pressure p on a particular point at a depth z below the surface of the water will be From figure 8 we see that the pressure in case (b) is lower than that in case (a). A downwardly directed acceleration (toward the same side as the acceleration due to gravity) results also in a pressure reduction, and this re- duction becomes greater with increase in acceleration. 3. The container in linear vertical motion with constant upward acceleration (fig. 9). An upwardly directed acceleration (opposite to the acceleration due to gravity) results in an increased pressure, and we obtain the equation of p.p.-pgz From this relationship, it must follow that, when on vertical motion the acceleration is changed, the pressure at each point of the walls of the container is changed. 4. Container in linear vertical motion with varying acceleration. The motion of the container consists of a ver- tical harmonic pendulous oscillation, for exam- ple, between points A and B in figure 10; in other words, a motion with a variable acceleration. Above the middle point O in section OA where the acceleration is directed downward, a pres- sure reduction results. In section OB, on the other hand, a pressure increase is obtained since the acceleration is directed upward. (The direc- tion of the acceleration is the same during the rising as during the falling phase.) If the container is given a form with a right- angle bend as in our labyrinth model, this ob- L P. a) P = Po + P 9 : b) P=Po+,-Â° (9â0) : FIGURE 8. â Container in linear vertical motion with constant downward acceleration. Pressure at each point of walls of container is changed when the acceleration is changed on vertical motion. Acceleration directed downward results in pressure reduction. a) p = Po + p gz b) p = PO -Â»- /> (gâa) z c) p = Po -I- p (g + a) r FIGURE 9. âContainer in linear vertical motion with constant upward acceleration resulting in increased pressure.
ELICITING MECHANISM OF MOTION SICKNESS 17 B FIGURE 10. â Container in linear vertical motion with varying acceleration. ,r s-\- Rubber membrane â¢ Glass tube (diam. 1.8 cm) Kymographic cylinder Writing arm Rubber membrane - Writing head Rubber tubing FIGURE 11. â Model of a labyrinth inform of a water-filled gloss tube. FIGURE 12.â7Vie model (fig. II) placed on the verticallv moving platform (top). Type of pressure variations (bottom). viously has no influence on the overlying pressure variations. Closed container: If the container is closed with rubber mem- branes as on our glass tube âthe model of the labyrinth (fig. 11) âand set in a vertical harmonic pendulous motion, then qualitatively the same effect must be obtained on the pressure distri- bution within the container; quantitatively, on the other hand, the conditions are changed. Mathematically, this problem can only be solved with great difficulties. In this case, account must be taken of the elastic quality of the walls of the container, of the rubber tubings, and of the rubber membrane of the writing head (fig. 11). To show that the above theoretical reasoning is also applicable to our closed model of the laby- rinth, we have demonstrated with a graphic re- cording that such pressure variations can also occur in a closed container on vertical harmonic pendulous oscillations. The model was filled with water and connected to the writing head where the movements were transmitted to the writing arm of a kymograph. The entire instrument was placed on the ver- tically moving platform of our harmonic oscilla- tion apparatus (fig. 12). The pressure variations are of the type seen at the bottom of figure 12. The accelerations varied from 3.9 to 8.9 m/sec2. Container in Horizontal Motions Open container: 1. The container at rest or in linear horizontal motion at constant velocity (the acceleration thus being equal to zero). No alteration is seen in the position of the surface of the liquid. The same equation is valid as for the open container in vertical motion (fig. 7). 2. The container in linear horizontal motion with constant acceleration. If the container is set in linear horizontal motion with constant acceleration a (cases II and III, fig. 13), the force and direction of which are depicted by the arrow, then the surface of the liquid, according to the laws of hydromechanics, will lie at right angles to the resultant of the value of the acceleration a drawn in the opposite direc- tion and the acceleration due to the force of
18 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION FIGURE 13. â Container in linear horizontal motion. I: At rest. II: In motion. Ill: Decreased acceleration. IV: Acceleration zero. V: Acceleration in opposite direction. VI: Increase in acceleration. VII: Acceleration zero, a = linear acceleration; p=force of gravity; V=ftat surface of the fluid; z = the depth of point A under the water's surface: Pa = atmospheric pressure above the free surface of the water. gravity. The pressure p at point A at depth 2 will vary with the value of z. In case II it will be smallest, and in case VI, greatest. Thus with this motion, at each point of the con- tainer, and particularly on the walls of the con- tainer, pressure variations will occur and result in fluid flows and fluid displacements. 3. The container in linear horizontal motion with varying acceleration. Figure 13 illustrates the pressure distribution in the fluid-filled container. In case I the con- tainer is at rest, and in case II it is set in motion. The surface of the liquid takes a sloping position and the value of z diminishes. When the ac- celeration then decreases, as in case HI, the value of 2 increases slightly, since the positional angle of the surface of the liquid to the horizontal plane is slightly smaller. When the accelera- tion has declined to zero, then the velocity is constant. The surface of the liquid again is at rest (case IV); this applies both to the state of rest and to constant velocity. If the acceleration is then again changed, either in a negative or a positive direction (cases V and VI), the position of the liquid changes correspondingly as it did in cases II and III, and the value of z is increased. In case VII, accelera- tion is again zero. Figure 13 also illustrates the pressure distri- bution in a container moving in harmonic pen- dulous oscillation in the horizontal plane, with case IV as the middle position. We assume that the container moves from II to IV to VI, and back again the same way to II. The acceleration is greatest at the turning points II and VI and is equal to zero in the middle position. Accord- ingly, it can be seen that also by varying horizon- tal accelerations, pressure variations and fluid displacements occur in a similar manner. Closed container: If this container, like the model of the labyrinth, is closed by an elastic rubber membrane, it may be assumed that qualitatively the same pressure variations with flows and displacements in the fluid will occur as were seen with the vertical motions. We also found that our closed model of the labyrinth, when subjected to linear motions in the horizontal plane with varying accelerations, reacted distinctly with deviations of the writing arm. One conclusion from our hydromechanical studies and experiments was, therefore, that, in the labyrinth model closed with an elastic rubber membrane and filled with fluid, pressure variations occur at each point of its contents and especially in its walls under harmonic oscillatory motions in both the vertical and the horizontal plane. The same applies to other horizontal and vertical motions with varying accelerations. The fluid pressure variations result in flows and dis- placements, which can be easily recorded graphically by the movements of a writing arm. Most investigators of the vestibular system probably consider today that the ampullar cristae react only to angular accelerations, while it is the otoliths that respond to linear accelerations. Fleisch (ref. 15) as early as 1922, and also Magnus in 1924 (ref. 24), showed that the semi- circular canals probably reacted to linear acceleration motions. Magnus and deKleyn used a glass model of a
ELICITING MECHANISM OF MOTION SICKNESS 19 a FIGURE 14. â The Magnus and deKleyn glass model of a labyrinth. (See text.) labyrinth (fig. 14). In the ampulla there was an elastic cupula (</). The model was closed with an elastic window (e), and the inner sac âthe "membranous semicircular canal" âcommuni- cated with the outer sac by means of a "pressure reservoir," a rubber bag (/). This model reacted promptly to linear accelerations with cupula impulses both at the beginning and at the end of the movement. The writing arm in our labyrinth model is analogous to this cupula and reacts, as we have seen above, to accelerated movements in the vertical and horizontal planes. It must then be asked whether these experimental results can be applied to living persons who have been sub- jected to accelerated linear motions that provoke motion sickness in its different forms, but es- pecially in true seasickness. It seems plausible to me that the hydro- mechanical results can, on the whole, be applied to the living labyrinth. LABYRINTHINE PRESSURE VARIATIONS We must thus consider that, in persons who are subjected to ship movements in rough seas, or at any rate to linear motions in a vertical or horizontal plane where the accelerations are of such magnitude as to correspond to those on rough seas, each point of the labyrinth and its contents is affected by pressure variations. Such pressure variations occur in the walls of the labyrinth and in the labyrinthine fluids, and must be followed by displacements and flows in both the perilymph and the endolymph. As is known, one labyrinth in the human body is located on either side of the median plane of the body, the plane of symmetry. In relation to this plane the labyrinths are mirror images of each other (fig. 15). The pressure variations on linear acceleration motions discussed above must occur in the two labyrinths simultaneously. That the momentary pressures at correspond- ing points on the labyrinths are not always the same is evident from figure 15, a labyrinth model. If the two liquid-filled containers (labyrinths), placed on either side of a plane of symmetry, are set in motion with an acceleration a (correspond- ing to the arrow in fig. 15, i.e., at right angles to the plane of symmetry), the surfaces of the liq- uids assume the positions as shown in this figure. Points pi and pi are corresponding points on the two containers. The momentary pressures at these points are not equal; z-i is greater than z\. If, on the other hand, the container is set in motion in the direction of the plane of symmetry, the momentary pressures at points p, and p-> are always equal. If the movement has a component which is at right angles to the plane of symmetry, the pressures at corresponding points of the labyrinth models will never be equal, as we have seen. A passenger on board a ship in rough seas will, at many locations on the ship, be moved h Â£ Â« I Â£ Q. FKJLRE 15. â The "labyrinths" in relation to the median plane oj the body, the plane oj symmetry. (See text.)
20 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION both up and down and thrown from side to side. With the intralabyrinthine pressure variations that will be induced, the momentary pressure at corresponding points of the two labyrinths will probably seldom be absolutely the same. The question now is whether these intra- labyrinthine pressure variations induced by elevator or ship movements are of such magni- tude that the excitation of the sensory epithe- lium would be strong enough to elicit symp- toms of motion sickness. In an attempt to answer this question to some extent, we made experi- ments on fresh autopsy human temporal bones, that had been stored in a refrigerator. Since the labyrinthine fluids are practically noncompressible, the "safety valves," the labyrinth windows, have to be displaced out- ward as pressure increases inside the labyrinth. In the same way as the writing arm in our labyrinth model illustrated the pressure varia- tions which occur on up-and-down or side-to- side harmonic oscillatory motions, the outward displacement of the stapes should be able to serve as a norm, or be an approximate expres- sion of the magnitude of the intralabyrinthine fluid displacement produced by such motions. As a theoretical explanation for the caloric reaction, Barany assumed, as is well known, that cooling gives a reduction and heating an increase in the specific weight of the endolymph. This results in endolymph flows which would seem to be the basic medium of excitation of the sensory epithelium in the cupulae. On heating of the labyrinth, the volume of the labyrinthine fluids must increase and the pres- sure rise. The coefficient of heat expansion for the fluid must be greater than for the labyrin- thine capsule, and this must lead to an outward displacement of the stapes. To make an acceptable comparison, the follow- ing measurements were made on each temporal bone preparation: (1) The maximal outward displacement of the stapes on vertical oscillatory movements (2) The outward displacement of the stapes after electrical heating of the preparation Experimental Arrangement The displacements of the stapes were meas- ured electrically (figs. 16 and 17) by attaching a very small square platinum plate to the capitulum stapedis (weight 7 to 8 mg). Attached to the FlI.l RE 16. â The temporal bone preparation placed on the plate oj the apparatus for vertical harmonic oscillations. E: ebonite plate: S and St: pole screu's; T: thermocouple; M: micrometer sCretC. FlWRE M. â The temporal bone preparation electrically heated. Mi = milliioltmeter; Am = ammeter; W = resist- ance of 2000 ohm; A i = storage battery, 18 volt; Az = storage battery, 2 volt; S = pule screu; K = cnnslantan wire; Pr= preparation; St = stapes: I'l = platinum tip of the stapes; T = thermocouple; M = micrometer screu; E = ebo- nite plate.
ELICITING MECHANISM OF MOTION SICKNESS 21 platinum plate was a small piece of tinsel thread 0.6 mm long and 0.03 mm thick. The prepara- tion was placed on the ebonite plate of our apparatus for vertical harmonic oscillations (fig. 16). The platinum plate was connected to an electric circuit. With the vertical motions the stapes plate was moved, the platinum plate came into contact with the tip of the micrometer screw, the circuit was closed, and the ammeter gave a signal. The greatest distance to contact was the measure of the maximum outward dis- placement of the stapes. On the average, this displacement was 0.01 mm. The maximum acceleration value for passenger ships of normal size and for the elevators which we used was about 3 m/sec2. We used a maximum accelera- tion of 3.95 m/sec2 for the temporal bones. The driving wheel of the apparatus rotated at 60 rpm. The time for a complete oscillation up and down was 1 second. The amplitude = the radius = 10 cm. In the heating experiments the preparations were heated electrically by winding a constantan wire around the bone (fig. 17) (resistance, 12 to 15 ohms). Heat was produced by a storage battery. The temperature was measured with thermoelements of copper-constantan placed against the promontorium. In these experi- ments also, the outward displacement of the stapes was, on the average, 0.01 mm when the bone was heated 5Â° to 12Â° (measured against the wall of the labyrinth). The up-and-down vertical oscillations thus elicited intralabyrinthine pressure variations of such magnitude that, in a living person, they would probably exert such a strong excitatory effect on the sensory epithelium of the vestibular apparatus that this could well be compared with a very strong caloric effect of the type which we see today; for example, in ultrasonic treatment of Meniere's disease. The Eliciting Mechanism We have seen from the foregoing that, on rough seas or in elevators where persons are subjected to linear acceleration movements in the vertical and horizontal planes, it can be expected that pressure variations will occur simultaneously in the two labyrinths. But the momentary pres- sures at corresponding points within the two labyrinths are probably seldom of the same magnitude. As a result of the pressure variations, flows and displacements are probably transmitted to the perilymph and endolymph. These are of such magnitude that an intensive excitation of the sensory epithelium is induced in the two re- ceptor systems of the labyrinth, the otoliths and the ampullar cristae, resulting in manifest symp- toms of motion sickness. It would seem rea- sonable to ask why vestibular nystagmus cannot be recorded even by electronystagmography (ENG). On the nystagmograms, as we have just seen, only typical nystagmus-like vertical eye movements are visible. In my opinion, this may be due theoretically to the fact that the two labyrinths are stimulated simultaneously, and inhibitory impulses prevent the induction of a manifest vestibular nystagmus. Since the pressures at corresponding points of the labyrinths are not always equal, the impulses to eye movements, arising from each side, can- not completely eliminate one another, and only the small atypical nystagmic beats can occur, but not a fully developed vestibular nystagmus. Also belonging to the clinical picture of motion sickness is intermittent headache, which may perhaps be due to intracranial pressure variations which are undoubtedly provoked by ship and elevator movements. APPLICATION TO PROBLEMS OF WEIGHTLESSNESS It now seems appropriate to ask whether these results of our experimental studies on the eliciting mechanism of motion sickness can possibly give a plausible explanation for part of the vestibular disturbances in weightlessness. Graybiel (refs. 25 and 26) said that the absence of the weight factor can probably explain the relative ease with which astronauts seem to manage their strenuous space journeys and science-fiction-like walks in space. There would be no otolith stimulation. Transient weightlessness has been studied in parabolic flights with rapid jet planes, and Soviet
22 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION experimenters (refs. 27 to 29) have described the illusion of rotation in opposite directions and postrotational nystagmus. Two subjects (not professional airmen) had a sensation of being upside down during the period of weightlessness. Such spatial illusions have been observed by Soviet cosmonauts in orbital flight. These Soviets state that it is the change in afferent impulse activity that is responsible for the pro- duction of sensory reactions in weightlessness. For studies of weightlessness of longer periods, we have to turn to prolonged orbital flights. The Soviet studies in the spaceship Voskhod are of special interest. The cosmonauts had under- gone different degrees of vestibular training. "Komarov had had several years of vestibular training. Feoktistov and Yugarov only a few months." Komarov showed very high vestibular resistance before the flight, and during the flight he developed no unfavorable vestibular reactions. The other two, on the other hand, with less train- ing and low vestibular resistance, developed "vestibuloautomatic reactions" of the motion- sickness type. During orbital flight with a period of weightless- ness lasting 24 hours, "Yugarov and Feoktistov developed illusory sensations that their bodies were upside down in space." These illusions appeared when their eyes were closed and when they were open. The sensations persisted throughout the whole period of weightlessness and remained until the onset of g-loading during the descent of the ship. The two also had un- pleasant sensations of vertigo during sharp movements of the head and, consequently, "tried to move less and when performing their work they moved smoothly." After sleep, the vestibular syndrome improved, and they were able to carry out their program. Earlier, in the spaceship Vostok II, Titov had also had similar feelings of the head being down or that he was in an inverted position. This type of illusion has been described by many cosmonauts when they were still lying on their backs, but oddly enough, not apparently by the American astronauts. During sharp movements of the head, Titov was also troubled by vertigo and manifest symptoms of motion sickness. The symptoms declined after a period of sleep and disappeared altogether on descent and the return of g-forces. New Hydromechaniral Experiments As far as I know, no acceptable explanation for these illusions has been given. Likewise, the question seems to be unanswered as to whether impulses pass to the vestibular nerve from the equilibrium apparatus in weightless- ness or not. Graybiel's view seems to be the most plausible, that the otoliths are not stimu- lated in weightlessness and no afferent impulses are thus sent out to the brain. How then can we explain that (1) in weight- lessness, as a result of rapid movements of the head, i.e., linear accelerations, a sudden sensa- tion of vertigo and symptoms of motion sickness occur?; and (2) even when the space pilot is lying on his back in the capsule, throughout the period of weightlessness he has the confusing illusion that he is upside down? In this connection, Schock's experimental studies on cats (ref. 30) may be mentioned. Normal nonoperated animals became disorien- tated and confused in weightlessness. The symp- toms increased when the eyes were covered. Animals that had undergone bilateral labyrin- thectomy were unaffected, but when their optical impulses were excluded they also became dis- orientated in the state of weightlessness. Intact vision is essential for space flights. In a state of weightlessness, when the proprioceptive impulses are also eliminated, the pilots have only their sense of vision to depend upon. I have been fascinated by this peculiar state when the sensation of weight is eliminated; therefore. I have continued my previous hydromechanical studies and experiments at the Stockholm College of Technology. In collaboration with H. Bergkvist, new model experiments are being carried out. We will return now to figure 13 of the liquid- filled container, representing a model of a laby- rinth. We will imagine that the container is sub- jected to a horizontal acceleration a. Two forces are acting on the liquid: first the force of gravity g, and second the horizontal force of inertia ma (mass times acceleration). The flat surface of the liquid will assume a position at right angles
ELICITING MECHANISM OF MOTION SICKNESS 23 Open container Closed container at rest at rest Container undergoing acceleration with gravity acting, above â no gravity, below. Container closed with elastic membrane undergoing acceleration with gravity, above - no gravity below. FIGURE 18. â The liquid-filled model of a labyrinth subjected to horizontal acceleration in a state of weightlessness. The liquid is thrown out of the container. FIGURE I9. âTotal acceleration (a tot.), with large horizontal acceleration (a), representing state of weightlessness. to the resultant of the two forces at the point in question. If a is increased or g decreased, the angle will increase and thereby the depth z at the "posterior wall" of the container (e.g., case VI). When z becomes greater than the wall of the container, the liquid will be thrown out. If the container is closed with an elastic membrane, this will be displaced outward. With this reasoning, if the vertical accelera- tion is allowed to approach zero, the state of weightlessness is obtained. In the g-free space the liquid is thus thrown out of the open con- tainer (fig. 18). If the container is closed, the liquid remains in it. If the container is closed by an elastic membrane, its contents will tend to be forced against this membrane or will be flung outward or be keeled over. FIGURE 20. â Open container in linear horizontal motion with 2-gcentripetal acceleration.
24 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION Since, for technical reasons, it is difficult in these model experiments to eliminate g, we chose to simulate the state of weightlessness in linear acceleration by means of a large centripetal ac- celeration. As can be seen in figure 19, with a sufficiently large horizontal acceleration the total acceleration (a tot.) will represent the state of weightlessness, where, of course, g is nonexistent, with adequate accuracy. Figures 18 to 21 from our new hydromechanical model experiments illustrate and summarize the problem. Figure 20 is an open container in linear horizontal motion with a centripetal accelera- tion of 2 g. In the next figure (fig. 21) this accel- eration is 3 g, and the container is closed. In figure 22 the container is closed by an elastic membrane and is set in linear horizontal accel- eration of 15 g: this figure, as I said, represents the state of weightlesssness with adequate ac- curacy. The fluid will be forced against the membrane with a tendency to be keeled over. In figure 23(Â«) we see a closed rubber bag in a con- tainer at rest. In figure 23(6) the linear accelera- tion is 23 g and the bag has this tendency to be keeled over. Finally, in our opinion it seems plausible that these hydromechanical model data may explain why the cosmonaut in the resting state of weight- lessness, when no afferent impulses are being sent from the otoliths, suddenly has vertigo when his head is turned rapidly; i.e., when he is sub- jected to a weak linear acceleration. The weightless endolymph is as an effect of the force of mass inertia thrown against the walls of the labyrinth with its "deafferented" weightless receptor system. These new moderate, unex- pected afferent impulses will suddenly produce a strong excitatory effect on the sensory epithelium of the vestibular apparatus, inducing manifest symptoms of vertigo and motion sickness. Spatial Illusions The extremely important question of the spatial illusion, in weightlessness, of the body being in an upside-down position can also be plausibly explained. FIGURE 21. â Closed container in linear horizontal motion with 3-gcentripetalacceleration. KlGURE 22.âContainer closed by an elastic membrane. Linear horizontal 15-g acceleration.
ELICITING MECHANISM OF MOTION SICKNESS 25 As is well known, the statoconic membrane of the macula is considered to have approxi- mately twice as high a specific weight as the FIGURE 23.â (a) Closed rubber bag in container at rest, (b) With linear 23-g, acceleration, the bag has a tendency to lean over. endolymph. In other words, the membrane can easily change its position in relation to the sensory epithelium. By deviation of the sensory hairs in the viscous, gelatinous superficial mass, a mechanical transformation takes place, and the macula functions both as a position indicator and as an accelerometer. The change in position of the otolith membrane can be caused by two types of forces: the force of inertia and an alteration of the relative direction of the force of gravity. If a person, and thus his macula, is subjected to an acceleration a, the layer subjacent to the otoconic membrane, the sensory cells, tends to move in a direction op- posite to the acceleration, because of the inertia of the mass. This force, the force of mass inertia (F = ma), acts upon the macula and is oriented in a direction opposite to the acceleration. This force thus moves the otoconic membrane out of its equilibrial position, and the sensory hairs mediate a sensation of acceleration to the sensory cells. The macula is covered by a gelatinous or viscous mass, which surrounds the sensory hairs. This viscous mass probably has a fairly high fric- tional force, which will have an inhibitory effect on the movements of the otoconic membrane. The accelerations to which the head is normally subjected are small compared with the accelera- tion g due to gravity. The change in the total acceleration, acting upon the macula, will thus be relatively small when an "external" accelera- tion occurs. In the state of weightlessness, on the other hand, the conditions are different. Since there is no effect of acceleration due to gravitation, the change in acceleration is equal to the "ex- ternal" acceleration. A very moderate accelera- tion, e.g., a movement of the head, is now per- ceived instead as a large change in movement. The result may then be, as mentioned above, the pronounced and rapidly manifested attacks of motion sickness as experienced by the Soviet cosmonauts. If, on the other hand, there is in- fluence by the force of gravity, and this has a component perpendicular to the nerve endings (e.g., when the head is bent forward), the otoconic membrane will be displaced from its position of equilibrium and the macula will function here as a position indicator. When an "external" ac-
26 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION celeration has ceased, the membrane, because of the acceleration due to gravity, will return to its position of rest. In a condition of weightlessness, on the other hand, spatial illusions can occur, since there is no restoratory force to act on the membrane. There is no signal that the acceleration has ceased. If now the macula is subjected to an arbitrary small acceleration, the otoconic mem- brane will remain in its displaced equilibrial position even when the acceleration has ceased to have any influence. The influence of the frictional force of the viscous superficial layer on both the membrane and the sensory hairs will probably accentuate this condition. The macula will thus continue to indicate the presence of an acceleration which has, in fact, ceased. After, for example, a posteriorly directed ac- celeration, the individual may well have a sensa- tion of being in the face-downward position. In figure 24, finally, we attempt to visualize the mechanism of such a vestibular macular spatial illusion. In weightlessness the membrane re- mains in position and indicates an erroneous positional change. The individual will have an illusion of being in an inverted position. NO ORAVTTV. ACCELERATION BACKW, FIGURE 2^. âVisualization of the macular spatial illusion.
ELICITING MECHANISM OF MOTION SICKNESS 27 REFERENCES 1. JAMES, J. A.: The Sense of Dizziness in Deaf Mutes. Am. J. Otol., vol. 4,1882, pp. 239-254. 2. REYNOLDS, T. T.: On the Nature and Treatment of Seasickness. Lancet, vol. 1, 1884, pp. 1161-1162. 3. MINOR, J. L.: Freedom of Deaf-Mutes from Sea-sick- ness. Its Bearing Upon the Theory of Sea-sickness and Its Treatment. Memphis J. Med. Sci., vol. 1.1889. pp. 252-254. 4. SJOBERG. A.: Experimental Studies of the Eliciting Mechanism of Seasickness. Acta Oto-Laryngol., voL 13, 1929, pp. 343-347. 5. CORNING, J. L.: The Suppression of Rotary Vertigo; Its Bearing on the Prevention and Cure of Seasickness. N.Y. Med. J., vol. 80, 1904, pp. 297-299. 6. Quix. F. H.: Le Mai de Mer, le Mai des Aviateurs. Monograph No. 8. Oto-Rhino-Laryngol. Intern., 1922, pp. 825-912. 7. BARANY. R. '(translated by I. W. Voorhees): Func- tional Testing of the Vestibular Apparatus. Ann. Otol., vol. 21,1912, pp. 71-127. 8. BRUNS, O.: Wesen und Bekampfung der Seeund Luftkrankheit. Munch. Med. Wschr., vol. 73, 1926, pp. 977-979. 9. BYRNE, J.: On the Physiology of the Semicircular Canals and Their Relation to Seasickness. J. T. Dougherty, New York. 1912, 569 pp. 10. WOJATSCHEK, W.: Einige neue Erwagungen iiber das Wesen der Seekrankheit. Beitr. Anat. Physiol. Pathol. Therap. Ohres, vol. 2, 1909, pp. 336-345. 11. SJOBERG. A.: Experimentelle Studien iiber den Aus- liisungsmechanismus der Seekrankheit. Acta Oto- Laryngol., suppl. 14, 1931, pp. 1-136. 12. DoHLMAN, G.: Physikalische und physiologische Studien zur Theorie des kalorischen Nystagmus. Acta Oto- Laryngol., suppl. 5, 1925, pp. 1-96. 13. ASCHAN, G.; BERGSTEDT, M.; AND STAHLE, J.: Nystag- mographical Observations Illustrating the Cupular Mechanism in Rabbits and Pigeons. Acta Soc. Med. Upsalienis, vol. 60, 1955, pp. 89-98. 14. NIEUWENHUUSEN. J. H.: Experimental Investigations in Seasickness. Utrecht Acd. Proefschrift, 1958. 15. FLEISCH. A.: Das Labyrinth als beschleunigungsemp- findliches Organ. Pfliigers Arch. ges. Physiol., vol. 195, 1922, pp. 499-515. 16. JONGKEES, L. B. W.; AND PHILIPSZOON, A. J.: Nystag- mus Provoked by Linear Accelerations. Acta Physiol. Pharmacol. Neerl., vol. 10, 1962, pp. 239-247. 17. NIVEN, J. I.; HIXSON, W. C.; AND CORREIA, M. J.: Elicitation of Horizontal Nystagmus by Periodic Linear Acceleration. Acta Oto-Laryngol., vol. 62, 1966, pp. 429-441. 18. GUEDRY, F. E.: Influence of Linear and Angular Accel- erations on Nystagmus. Second Symposium on the Role of the Vestibular Organs in Space Exploration, NASA SP-115,1966, pp. 185-196. 19. GERTZ, H.: Action Motrice Induite. Acta Med. Scand., vol. 57,1922, p. 41. 20. KORNHUBER, H.: Physiologie und Klinik des zentral- vestibularen Systems. Hals.-Nas.-Ohrenheilk., vol. 3, 1966, pp.2150-2350. 21. GERNANDT, B. E.; AND GILMAN, S.: Descending Vestibu- lar Activity and Its Modulation by Proprioceptive, Cerebellar and Reticular Influences. Exptl. Neurol., vol. 1,1959, pp. 274-304. 22. COHEN, L. A.: Role of Eye and Neck Proprioceptive Mechanisms in Body Orientation and Motor Coordina- tion. J. Neurophysiol.. vol. 24. 1961, pp. 1-11. 23. SANDSTROM. J.: Cervical Syndrome With Vestibular Symptoms. Acta Oto-Laryngol., vol. 54, 1962, pp. 207-226. 24. MAGNUS. R.: Kiirperstellung. Monographien aus dem Gesamtgebiet der Physiologie der Pflanzen und der Tiere. Springer, Berlin, 1924, p. 740. 25. GRAYBIEL, A.: Vestibular Sickness and Some of Its Im- plications for Space Flight. Neurological Aspects of Auditory and Vestibular Disorders, W. S. Fields and B. R. Alford. eds., Charles C Thomas, 1964, pp. 248-270. 26. GRAYBIEL, A.: Vestibular Problems in Relation to Space Travel. The Vestibular System and Its Diseases, R. J. Wolfson, ed., Lniv. Pennsylvania Press, 1966, pp. 443-458. 27. KAS'YAN, I. I.; KRASOVSKL, A. S.; KOLOSOV, I. A.; LOMOVA, M. A.; LEBEDEV, V. L: AND YUROV. B. N.: Some Physiologic Reactions in Man During Short Periods of Weightlessness. Federation Proc., vol. 25, pt. II. 1966. pp. 605-611. 28. YUGANOV, E. M.; GORSHKOV, A. I.; KAS'YAN, I. I.; BRYANOV, I. L; KOLOSOV, I. A.; KOPANEV, V. I.; LEBEDEV, V. L; POPOV, N. I.; AND SOLODOVNIK, F. A.: Vestibular Reactions of Cosmonauts During Flight in the Ship "Voskhod." Federation Proc., vol. 25, pt. II, 1966, pp. 767-770. 29. YUGANOV, E. M.; GORSHKOV, A. 1.: KAS'YAN, I. 1.; BRYANOV, I. I.; KOLOSOV, I. A.: KOPANEV, V. I.; LEBEDEV, V. L; POPOV, N. I.; AND SOLODOVNIK, F. A.: Vestibular Reactions of Cosmonauts During the Flight in the "Voskhod" Spaceship. Aerospace Med., vol. 37, July 1966, pp. 691-694. 30. SCHOCK, G. J. D.: A Study of Animal Reflexes During Exposure to Subgravity and Weightlessness. Aero- space Med., vol. 32, 1961, pp. 336-340.
28 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION DISCUSSION MclNally: I should like to pay tribute to Dr. Graybiel because he is pretty important to us all. You know that in science, as in everything else, there are styles, and at the present time vestibular physiology is in style. There was a time not so long ago when it was not, and cochlear physiology was the rage. Back in the 1915's and 1920's in the days of Barany, Sherrington, Magnus, and DeKleyn, physiology of the vestibular system was very popular. Dr. Graybiel took up the study of vestibular physiology, persevered in it, trained his associates, and had them ready to respond to problems involved in the space program. We owe him a tremendous debt of gratitude for what he has been able to do for vestibular physiology. It is rather interesting that our first speaker also brought us a link with these early physiologists, especially to Barany who did his work in Uppsala and won his Nobel Prize there. It is most fitting that we had one of his successors at Uppsala, Professor Sjiiberg, talk to us today. I can well remember when I first became interested in seasickness while reading articles by Professor Sjiiberg back in the 1920's. Money: Professor Sjiiberg, do you think that the pressure changes in response to linc.it accelerations are more or less important than the direct effect of the linear acceleration on the otolithic membrane? Also, could you outline the evi- dence for the conclusion that the proprioceptors are not necessary for motion sickness? Sjoberg: I believe that the pressure variations with flows and displacements probably are transmitted to the perilymph and endolymph, there inducing in the two receptor systems of the labyrinth, the otoliths and the ampullarcristae, a strong excitation of the sensory epithelium, resulting in manifest symptoms of motion sickness. Money: I understood you to say that neither the impulses from the eyes nor from the proprioceptors was necessary. Sjbberg: In my paper 1 said that, from the animal ex- periments, it is justifiable to conclude that optical and pro- prioceptive impulses are not necessary for the elicitation of symptoms, but these impulses stimulate and facilitate the induction of the symptoms. 1 agree with you that the optical is more important. Money: More important than the proprioceptors? Sjoberg: Yes. more important; the optical impulses have a stronger stimulatory effect than do the proprioceptive ones. Money: That answers my second question. Huertas: I should like to dwell a little bit more on the vibration-conducting mechanism. The labyrinth is contained in a nonelastic bony box, so to speak, which cannot be dis- tended; therefore, any change in shape of the membranous labyrinth would be due to changes in buoyancy between the membranous membrane of the labyrinth and the fluids by which it is surrounded. Do you have any evidence of the differences in specific gravity between the three elements involved âthe perilymph, membranous labyrinth, and en- dolymphâto back up your theory of displacement by vibra- tion? It is accepted today that angular acceleration can produce motion sickness; therefore, the receptors for angular accelerations must play a role in motion sickness. You did not mention such receptors as active elements in the produc- tion of motion sickness. Is there an otolith response to ac- celerations in space during zero-g or not? Dr. Gualtierotti made a similar query at the last symposium. There are otolith fibers or linear-acceleration-responding units which are sensitive to I milli-g, to a thousandth of a g. The ballistic forces of the heart produce acceleration movements of the head much greater than that; therefore, even during weight- lessness the otolith is constantly stimulated with each heartbeat. Sjoberg: It is well known that, for the statoconic mem- brane of the macula, the specific weight is considered approxi- mately twice as high as that of the endolymph. On a very winding road, naturally the receptors for angular accelerations play a role in the elicitation of motion sickness. Lansberg: I am very much impressed by the measure- ments you have made of the distance that the stapes moves out. Do you not think that, during ultrasonic therapy for Meniere's disease, there is more involved than just the heat- ing of the labyrinth? Is it not the selective destruction of the epithelium in the horizontal canal that causes the symptoms, both in the irritative and in the paralytic phase? Sjbberg: In my subsequent paper ("Experimental and Clinical Experiences and Comments on Ultrasonic Treatment of Meniere's Disease"), I go into the question of the thermal effect. In Uppsala we have experimented on rabbits and hu- man beings during ultrasonic irradiation with the head in different positions. We have shown that the initial nystag- mus of the irritative type brought about by ultrasonic treat- ment is probably a caloric reaction provoked by the endolym- phatic flow caused by the thermal effect. The nystagmus of the destructive type does not change direction when the head position is altered from face up to face down. Waite: During Gemini 7, Astronaut Frank Borman shook his head repeatedly and reported no untoward symptoms whatsoever. Has motion-sickness symptomatology been re- ported during vertical acceleration or linear acceleration with the head rigidly fixed so that no angular accelerations can take place? Sjoberg: Symptoms of motion sickness can be elicited dur- ing up-and-down harmonic pendulous movements with the head fixed. Barber: Have you any comments to make upon the inci- dence of motion sickness in bilateral otosclerosis, where each stapes is fixed? Sjoberg: I have no experience with otosclerosis in rela- tion to motion sickness. Mc.Nally: Professor Sjiiberg represents a line of very dis- tinguished investigators as does Professor Wendt who brings to us the traditions of Parker and Maxwell and Dodge.