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Unusual visual stimulation in dynamic balance conditions: proposal for a space motion sickness test
Adv. Space Res. Vol. 14, No. 8, pp. (8)389-(8)394, 1994 Copyright © 1994 COSPAR Print~ in Great Britain. All rights reserved. 0273-1177/94 $6.00 + 0.00
Pergamon
UNUSUAL VISUAL STIMULATION IN DYNAMIC BALANCE CONDITIONS: PROPOSAL FOR A SPACE MOTION SICKNESS TEST Alexandra Stverac, Paul Bessou and Bernard Pages Laboratoire de Physiologie, U.R.A. CNRS 649, Facultd de Mddecine, Universit~ Paul Sabatier, 133 Route de Narbonne, 31062 Toulouse, France
ABSTRACT We previously demonstrated the efficiency of normal vision/unusual vestibular cues conflict to induce motion sickness. In the present study, we investigate whether, inversely, unusual visual information/normal vestibular function conflict also elicited motion sickness. The experiments were again carried out in dynamic balance conditions to increase proprioceptive input. Circular translation of the visual field with diplopia were produced by rotating Fresnel prismatic glasses. The stimulation triggered SMS-like symptoms and dynamic balance disturbance. A positive relationship was found between discomfort and balance disturbance. Unusual visual information should therefore be included in Space Motion Sickness susceptibility testing. INTRODUCTION
Space Motion Sickness (SMS) is most often attributed to a conflict between unusual information sent by certain sensorial organs and the input awaited by the CNS on the basis of the other, unaltered sensorial input/1/. In previous experiments carried out on subjects in dynamic balance conditions, which provide a large amount of proprioceptive afferences from the whole body musculature, we successfully elicited SMS-like symptoms using abnormal stimulation of the vestibular apparatus (electrical stimulation) while visual cues were normal/2/. A correlation was seen to exist between the subjects' sensitivity to the unusual stimulation and the dynamic balance deterioration. The present work aimed, inversely, to investigate on subjects in dynamic balance conditions, whether unusual visual stimulation, vestibular cues remaining normal, could induce SMS-like symptoms. MATERIALS AND METHODS Dvnamic Balance Conditions The method used to induce dynamic balance conditions has been previously described/2/. The subjects stood on a rocking platform (stabilometer), consisting of a cylinder segment (55-cm diameter) in contact with the ground by a generatrix called the pivot. The subject oscillates, according to his position on the platform, forward-backward (anteroposterior balance) or right-left (lateral balance). An optical sensor (precision : 100 Hz, 7.10-3 cm) measures the stabilometer sway, that allows to calculate the displacement of the pivot on the ground (stabilogram). An ataxiameter/3L coupling the head to an optical sensor (precision : 100 Hz, 0.02 mm) by a weighted string (20g) allows simultaneous measurement of the displacement of the head (ataxiagram) in the same plane as the pivot displacement. Several parameters were measured for both head (I-I) and pivot (P) displacement (D) : (1) the length of the linear displacement HLD and PLD (cm) ; (2) the maximum amplitude HMA and PMA (cm) ; (3) the average position around which the subject oscillated HAvP and PAvP (cm) ; (4) the ratio HLD to PLD (H:P ratio). A Fast Fourier Transform process of the recordings provided : (5) the total energy of the spectrum TE (cm2) and (6) the energy distribution (cm2 and %) into 3 frequency bands : 0-0.5 Hz, 0.5-2 Hz and 2-20 Hz i.e. low (LFE), medium (MFE) and high frequency (HFE) respectively. While the parameters 1,2,3 and 5 indicated the skill in dynamic balance, the parameters 4 and 6 informed us about the strategy implemented. (8)389
(8)390
A. Stverac et al.
Unusual Visual Stimulation (UVS) Visual input was made unusual by using 13-diopters Fresnel membrane prisms in front of each eye and by making them rotate by means of a motor in each side of the goggles (Fig. 1A) : prisms by themselves induce slight decrease of normal visual acuity (from 10:10 to 6:10) and various distorsions of the vision/4/; prism rotation elicits circular translation of each element of the visual field that does not fit any of the usual visual flow patterns described by Gibson/5/; the goggles steadily bound the prisms to the head made the distorsion in the visual input constant, whatever eye or body movements. With the prisms, motionless, vertically orientated, the bases facing the same way, the subject looking straight ahead always experienced binocular fusion but when they rotate diplopia was felt. The visual scene observed by the subject consisted of the experimental room, containing various objects before a wall situated about 3.8 m from the subject. The velocity and the direction of prism rotation were controlled by a computer which simultaneously stored the data provided by the stabilometer and the ataxiameter. Each subject underwent 2 UVS tests (fig 1B), one in anteroposterior sway and another in lateral sway conditions, divided into three periods : (1) the "Referenceperiod" (R) or "Before stimulation period" lasting 10 seconds, (2) the"Stimulation period" (STI) lasting 30 seconds and (3) the "Poststimulation period" (F) lasting 10 seconds. Fig. 1 : A. Subject wearing the goggles with rotating prisms. The visual field horizontally subtented is indicated. B. Experimental protocol.
B II°s [STI [P s
During the tests, the subject stood on the stabilometer, eyes open, wearing the goggles. The prisms were rotated only during STI. The velocity, 45°/second, corresponding to a 0.125 Hz frequency, belonged to the frequency range described as provocative/6/. Thirty normal voluntary subjects were investigated, 15 males and 15 females of 31.6+7.2 and 32.9:t:12.9 years of age respectively. At the end of the test, the subjects were asked to fill in a questionnaire about the symptoms they felt during the test and immediately after. The symptoms, according to their relevance as SMS symptom, were given a coefficient and classed. The classification was based on Graybiel's scale/7/. The subjects were distributed into three categories according to their score, calculated by adding the coefficients of each sign felt : over 9 : category A, between 2 and 9: category B and less than 2 : category C. RESULTS SMS-like Svmotoms UVS elicited various signs of discomfort by 22 of the 30 subjects (73%). The number of subjects reporting each sign is found in Figure 2. The signs of the strongest discomfort appeared during the early seconds of prism rotation. Nevertheless, all the tests were carried out till the end : no subject asked for the rotation of the prisms to be interrupted. After the prism rotation had stopped, the discomfort persisted and was sometimes enhanced. Some signs such as dizziness, yawns and headache appeared more after the stimulation than during it. According to their score, ten subjects were classed in category A, they are the subjects strongly disturbed by the stimulation. Twelve subjects were classed in category B, and 8 subjects in category C.
Space Motion Sickness Test
(8)391
Fig. 2 : Number of subjects(n=30) relatingthe various SMS-like symptoms during and following the UVS tests. The signs are ordered according to their severity coefficient (in brackets). Symptoms (Coefficient) Nausea (4) Gastric awareness (3)
,t,:_.__,
[-7
Vertigo (3)
During Stimulation After Stimulation
General malaise (3) Drowsiness (2) Dizziness (2) Yawn (2) Warm Hushes (2) Cold sweating (2) Headache (2) Visual troubles(2) Pallor (2) Anxiety (2) Sens. imbalance No symptom/~/
|
o
1"11
~o
~o
Number of subjects
Dynamic Balance Disturbances The circuLar translation of the visual field induced stereotyped balance disturbances. Figure 3 illustrates the anteroposterior sway recordings of one subject (n°22) during an UVS test, similar modifications occurred in lateral sway. During the reference period, the pivot oscillated with small amplitude (PMA : 2.43 cm) at a medium-frequency (50% of the total energy) around a balance position (0.43 cm backward). When the prisms were rotated, the pivot oscillated with a large amplitude (laMA : 11.4 cm) at the same low frequency as the prisms (0.125 Hz, 80% of the TE). The average position around which the subject oscillated (1.03 cm backward) was very close to the reference one (0.43 cm backward). Higher frequency oscillations (0.9 Hz), belonging to the medium-frequency range (18% of the TE), were superimposed on the low-frequency reactions of the stabilogram. Only the low-frequency reactions did appear on the ataxiagram, the medium-frequency ones being attenuated by the body structures between the feet and the head. The H:P ratio (0.48) provides an index of this attenuation. The medium-frequency oscillations appeared to a lesser extent in the anteroposterior sway than in lateral sway where low-frequency oscillations were not so apparent. Fig. 3 : Ataxlagram (upper curve) and stabilogram (lower curve) of subject N ° 22 during an UVS test, in anteroposterior sway. Rotation of the 13-diopters Fresnel prismatic glasses : 0.125 Hz. The 0 position corresponded to head and pivot position when the platform was horizontal. Forward
T Backward
Amplitude (cm) +10"3
-1
0
10 R
20
30 STI
40 50 Time (sec) P
(8)392
A. S6verac et al.
The dynamic balance disturbances consequent to UVS, presented by the 30 subjects, induced the increase of the parameter values of the stabilogram : length of the linear displacement, maximum amplitude and total energy. The increases were confirmed by analysis of the variance with repeated measurements. However, the analysis also testing the incidence of the "category" grouping factor (categories A, B and C) on the measurements did not evidence a significant difference of the 3 parameters from one category of subjects to another. On the contrary, the categories of subjects did differ in their H:P ratio and the distribution of the energy into the three frequency bands. Fig. 4 : Mean H:P ratio for each category during the 5 periods of an UVS test, in anteroposterior and lateral sway. ~"indicates the significance (p<0.001) of the difference between the 5 periods of U-VS tests and * indicates the significance (p<0.05) of the difference found between categories.
•
o
Category of subjects A B C
0.$
Category of subjects •
•
l
.~. 0.6
,
,,
0.6
,,
0.4
0.4
0.2.
0.2
0.01
0.0
RSTIP RSTIP RSTIP ANTEROPOSTERIOR SWAY
RSTIP RSTIP RSTIP LATERAL SWAY
Figures 4 and 5 represent the modifications of the mean H:P ratio and the modifications of the energy distribution for each category, respectively. During R, the 2 parameters were not statistically different from one category to another in anteroposterior balance but in lateral bal~r~e H:P ratio was significantly higher in A and B (0.551 and 0.552 respectively) than in C (0.4) and the content of low-frequency energy was higher in categories A and B than in C. During STI, a significant modification of the H:P ratio was observed in anteroposterior sway for each category while in lateral sway, it was modified for catego|'y A only, indeed, in B and C, the ratio remained unchanged during the 5 periods. The proportion of stabUogram LFE increased both in anteroposterior and lateral sway, in the latter case, pivot LFE was lower in category C than in the other categories. Fig. 5 : Modifications of the mean of the energy distribution of the stabilogram per category during the 5 periods of a unusual visual stimulation test in anteropostefior and lateral balance.
10 r~ e~ 8 r~ 6 4
A
Category of subjects B C
~ 100" r~ ~80.
A
Category of subjects B C
z
~60. 40,
201 Oi R STI P R STI P R STI P R STI P R STI P R STI P ANTEROPOSTERIOR SWAY LATERAL SWAY Low Frequency Energy m l Medium Frequency Energy [ - 7 High Frequency Energy
Space Motion Siclmcss Test
(8)393
DISCUSSION Unusual Visual Information in Dynamic Balance : an Efficient Means of Elicitinc, SMS-like Svmmoms under Normom'avitv The circular translation of the visual scene induced a feeling not concordant with the proprioceptive input (vestibular, muscular, tendinous and articular). This sensori-motor mismatch resulted in head and body oscillations and SMS-like symptoms. Vertigo, dizziness, gastric awareness and nausea were frequently reported by subjects during and following the tests. The provocative character of visuovestibular conflict induced by such an unusual visual stimulation was successfully demonstrated. The Balance Swate~v : an Index of Subiects Susceutibilitv to Sensorial Conflict
The dynamic balance skill, assessed by measurements of the linear displacement of the stabilometer, was not relevant alone to differentiate subjects highly sensitive to sensorial conflict arising from altered visual input whereas it had previously been demonstrated to be an accurate indicator of individual susceptibility to SMS induction by eleclrical vestibular stimulation/2/. The balance strategy, assessed by the ratio H/P and the distribution of the stabilogram energy into three frequency bands allowed differentiation of the 3 categories of subjects. Two hypotheses can be proposed to explain the immunity of category C subjects : (1) the lower H:P ratio of these subjects indicates an efficient reduction of head movements allowing the avoidance of malaise : the movements being known to aggravate motion sickness elicited during tests/8/and space flights/9,10/; (2) the lower content of low-frequency energy in the stabilogram of the C subjects could evidence that they scarcely used vision to balance themselves and thus they were not sensitive to the unusual stimulation of this channel. As a matter of fact, several authors have shown that vision, as well as the vestibular apparatus, is involved in low-frequency balance reactions/11,12/. CONCLUSION Matching unusual visual information with normal proprioception was obtained in subjects in dynamic balance conditions with the aim of eliciting SMS-like symptoms. The circular translation of the visual scene was efficient to create a malaise in 73% of cases and provoked large b~lance reactions in all cases : the movements of the head and the pivot followed the prism rotation frequency i.e. 0.125 Hz. The method used to measure dynan~ic balance allowed the objective identification of subjects sensitive to the visuo-proprioceptive conflict on the basis of the strategy they used to balance themselves : they presented more low-frequency balance reactions than the other subjects. This work confirms that the use of dynamic balance conditions under conflicting sensorial stimulation is an efficient way of eliciting, under normogravity, SMS-like symptoms and that the analysis of the strategies of dynamic balance could be useful in the selection of subjects sensitive to scnsori-motor conflict, susceptible to develop SMS.
This work was supported by grants from CNES (Centre National d'Etudes Spatiales), DASSAULT AVIATION (France) and Fondation pour la Recherche Mddicale. REFERENCES 1. W.E. Thornton, T.P. Moore, S.L. Pool and J. Vanderplocg. Clinical Characterization and Etiology of Space Motion Sickness. Aviat. Space Environ. Med. 58 (9, Suppl.): A1-8 (1987). 2. A. Stverac. Electrical vestibular stimulation and space motion sickness. Acta Astronautica, 28, 401-408 (1992). 3. B.M. Wright. A simple mechanical ataxia-meter. Proceedings of the physiological society (1971). 4. R.N. Haber and M. Hershenson. In : The psychology of visual perception. Holt, Rinehart and Winston, Inc, New-York (1973).
(8)394
A. S~era¢ et aL
5. J.J. Gibson. The senses considered as percetual systems. Boston Houghton Mifflin (1966). 6. K.E. Money. Motion Sickness. Physiological reviews, 50, N°I, 1-39 (1970) 7. A. Graybiel, Ch. D. Wood, E.F. Miller and D.B. Cramer. Diagnostic criteria for grading the severity of acute motion sickness, Aerospace Medicine, 39, 453-455 (1968). .
J_R. Lackner and A. Graybiel. Head movements in non-terrestrial force environnements elicit motion sickness: implications for the etiology of space motion sickness, Aviat. Space Environ. Med., 57, 443-448 (1986).
9. W.J. Ockels, F. Furrer and E. MessetschmidL Simulation of space-adaptation syndrome on Earth, ESA Journal, Vol. 13, 235-239 (1989). 10. C.M. Oman, B.K. Lichtenberg, K.E. Money and RX. McCoy. M.I.T./Canadian vestibular experiments on the Spacelab-1 mission: 4. Space motion sickness : symptoms, stimuli, and predictability, Exp. Brain Res., 64, 316-334 (1986). 11. H. C. Diener, J. Dichgans, W. Bruzek and H. Selinka. Stabilization of human posture during induced oscillations of the body.For.p.Brain Research, 45 : 126-132, (1982). 12. W.N.J.C. Van Asw~, C.C.A.M. Gielen and JJ. Denier van der Gon. Postural movements induced by rotation of visual scenes. J. Opt. Soc. Am., Vol. 5, N ° 10, 1781-1789 (1988).
Pergamon
UNUSUAL VISUAL STIMULATION IN DYNAMIC BALANCE CONDITIONS: PROPOSAL FOR A SPACE MOTION SICKNESS TEST Alexandra Stverac, Paul Bessou and Bernard Pages Laboratoire de Physiologie, U.R.A. CNRS 649, Facultd de Mddecine, Universit~ Paul Sabatier, 133 Route de Narbonne, 31062 Toulouse, France
ABSTRACT We previously demonstrated the efficiency of normal vision/unusual vestibular cues conflict to induce motion sickness. In the present study, we investigate whether, inversely, unusual visual information/normal vestibular function conflict also elicited motion sickness. The experiments were again carried out in dynamic balance conditions to increase proprioceptive input. Circular translation of the visual field with diplopia were produced by rotating Fresnel prismatic glasses. The stimulation triggered SMS-like symptoms and dynamic balance disturbance. A positive relationship was found between discomfort and balance disturbance. Unusual visual information should therefore be included in Space Motion Sickness susceptibility testing. INTRODUCTION
Space Motion Sickness (SMS) is most often attributed to a conflict between unusual information sent by certain sensorial organs and the input awaited by the CNS on the basis of the other, unaltered sensorial input/1/. In previous experiments carried out on subjects in dynamic balance conditions, which provide a large amount of proprioceptive afferences from the whole body musculature, we successfully elicited SMS-like symptoms using abnormal stimulation of the vestibular apparatus (electrical stimulation) while visual cues were normal/2/. A correlation was seen to exist between the subjects' sensitivity to the unusual stimulation and the dynamic balance deterioration. The present work aimed, inversely, to investigate on subjects in dynamic balance conditions, whether unusual visual stimulation, vestibular cues remaining normal, could induce SMS-like symptoms. MATERIALS AND METHODS Dvnamic Balance Conditions The method used to induce dynamic balance conditions has been previously described/2/. The subjects stood on a rocking platform (stabilometer), consisting of a cylinder segment (55-cm diameter) in contact with the ground by a generatrix called the pivot. The subject oscillates, according to his position on the platform, forward-backward (anteroposterior balance) or right-left (lateral balance). An optical sensor (precision : 100 Hz, 7.10-3 cm) measures the stabilometer sway, that allows to calculate the displacement of the pivot on the ground (stabilogram). An ataxiameter/3L coupling the head to an optical sensor (precision : 100 Hz, 0.02 mm) by a weighted string (20g) allows simultaneous measurement of the displacement of the head (ataxiagram) in the same plane as the pivot displacement. Several parameters were measured for both head (I-I) and pivot (P) displacement (D) : (1) the length of the linear displacement HLD and PLD (cm) ; (2) the maximum amplitude HMA and PMA (cm) ; (3) the average position around which the subject oscillated HAvP and PAvP (cm) ; (4) the ratio HLD to PLD (H:P ratio). A Fast Fourier Transform process of the recordings provided : (5) the total energy of the spectrum TE (cm2) and (6) the energy distribution (cm2 and %) into 3 frequency bands : 0-0.5 Hz, 0.5-2 Hz and 2-20 Hz i.e. low (LFE), medium (MFE) and high frequency (HFE) respectively. While the parameters 1,2,3 and 5 indicated the skill in dynamic balance, the parameters 4 and 6 informed us about the strategy implemented. (8)389
(8)390
A. Stverac et al.
Unusual Visual Stimulation (UVS) Visual input was made unusual by using 13-diopters Fresnel membrane prisms in front of each eye and by making them rotate by means of a motor in each side of the goggles (Fig. 1A) : prisms by themselves induce slight decrease of normal visual acuity (from 10:10 to 6:10) and various distorsions of the vision/4/; prism rotation elicits circular translation of each element of the visual field that does not fit any of the usual visual flow patterns described by Gibson/5/; the goggles steadily bound the prisms to the head made the distorsion in the visual input constant, whatever eye or body movements. With the prisms, motionless, vertically orientated, the bases facing the same way, the subject looking straight ahead always experienced binocular fusion but when they rotate diplopia was felt. The visual scene observed by the subject consisted of the experimental room, containing various objects before a wall situated about 3.8 m from the subject. The velocity and the direction of prism rotation were controlled by a computer which simultaneously stored the data provided by the stabilometer and the ataxiameter. Each subject underwent 2 UVS tests (fig 1B), one in anteroposterior sway and another in lateral sway conditions, divided into three periods : (1) the "Referenceperiod" (R) or "Before stimulation period" lasting 10 seconds, (2) the"Stimulation period" (STI) lasting 30 seconds and (3) the "Poststimulation period" (F) lasting 10 seconds. Fig. 1 : A. Subject wearing the goggles with rotating prisms. The visual field horizontally subtented is indicated. B. Experimental protocol.
B II°s [STI [P s
During the tests, the subject stood on the stabilometer, eyes open, wearing the goggles. The prisms were rotated only during STI. The velocity, 45°/second, corresponding to a 0.125 Hz frequency, belonged to the frequency range described as provocative/6/. Thirty normal voluntary subjects were investigated, 15 males and 15 females of 31.6+7.2 and 32.9:t:12.9 years of age respectively. At the end of the test, the subjects were asked to fill in a questionnaire about the symptoms they felt during the test and immediately after. The symptoms, according to their relevance as SMS symptom, were given a coefficient and classed. The classification was based on Graybiel's scale/7/. The subjects were distributed into three categories according to their score, calculated by adding the coefficients of each sign felt : over 9 : category A, between 2 and 9: category B and less than 2 : category C. RESULTS SMS-like Svmotoms UVS elicited various signs of discomfort by 22 of the 30 subjects (73%). The number of subjects reporting each sign is found in Figure 2. The signs of the strongest discomfort appeared during the early seconds of prism rotation. Nevertheless, all the tests were carried out till the end : no subject asked for the rotation of the prisms to be interrupted. After the prism rotation had stopped, the discomfort persisted and was sometimes enhanced. Some signs such as dizziness, yawns and headache appeared more after the stimulation than during it. According to their score, ten subjects were classed in category A, they are the subjects strongly disturbed by the stimulation. Twelve subjects were classed in category B, and 8 subjects in category C.
Space Motion Sickness Test
(8)391
Fig. 2 : Number of subjects(n=30) relatingthe various SMS-like symptoms during and following the UVS tests. The signs are ordered according to their severity coefficient (in brackets). Symptoms (Coefficient) Nausea (4) Gastric awareness (3)
,t,:_.__,
[-7
Vertigo (3)
During Stimulation After Stimulation
General malaise (3) Drowsiness (2) Dizziness (2) Yawn (2) Warm Hushes (2) Cold sweating (2) Headache (2) Visual troubles(2) Pallor (2) Anxiety (2) Sens. imbalance No symptom/~/
|
o
1"11
~o
~o
Number of subjects
Dynamic Balance Disturbances The circuLar translation of the visual field induced stereotyped balance disturbances. Figure 3 illustrates the anteroposterior sway recordings of one subject (n°22) during an UVS test, similar modifications occurred in lateral sway. During the reference period, the pivot oscillated with small amplitude (PMA : 2.43 cm) at a medium-frequency (50% of the total energy) around a balance position (0.43 cm backward). When the prisms were rotated, the pivot oscillated with a large amplitude (laMA : 11.4 cm) at the same low frequency as the prisms (0.125 Hz, 80% of the TE). The average position around which the subject oscillated (1.03 cm backward) was very close to the reference one (0.43 cm backward). Higher frequency oscillations (0.9 Hz), belonging to the medium-frequency range (18% of the TE), were superimposed on the low-frequency reactions of the stabilogram. Only the low-frequency reactions did appear on the ataxiagram, the medium-frequency ones being attenuated by the body structures between the feet and the head. The H:P ratio (0.48) provides an index of this attenuation. The medium-frequency oscillations appeared to a lesser extent in the anteroposterior sway than in lateral sway where low-frequency oscillations were not so apparent. Fig. 3 : Ataxlagram (upper curve) and stabilogram (lower curve) of subject N ° 22 during an UVS test, in anteroposterior sway. Rotation of the 13-diopters Fresnel prismatic glasses : 0.125 Hz. The 0 position corresponded to head and pivot position when the platform was horizontal. Forward
T Backward
Amplitude (cm) +10"3
-1
0
10 R
20
30 STI
40 50 Time (sec) P
(8)392
A. S6verac et al.
The dynamic balance disturbances consequent to UVS, presented by the 30 subjects, induced the increase of the parameter values of the stabilogram : length of the linear displacement, maximum amplitude and total energy. The increases were confirmed by analysis of the variance with repeated measurements. However, the analysis also testing the incidence of the "category" grouping factor (categories A, B and C) on the measurements did not evidence a significant difference of the 3 parameters from one category of subjects to another. On the contrary, the categories of subjects did differ in their H:P ratio and the distribution of the energy into the three frequency bands. Fig. 4 : Mean H:P ratio for each category during the 5 periods of an UVS test, in anteroposterior and lateral sway. ~"indicates the significance (p<0.001) of the difference between the 5 periods of U-VS tests and * indicates the significance (p<0.05) of the difference found between categories.
•
o
Category of subjects A B C
0.$
Category of subjects •
•
l
.~. 0.6
,
,,
0.6
,,
0.4
0.4
0.2.
0.2
0.01
0.0
RSTIP RSTIP RSTIP ANTEROPOSTERIOR SWAY
RSTIP RSTIP RSTIP LATERAL SWAY
Figures 4 and 5 represent the modifications of the mean H:P ratio and the modifications of the energy distribution for each category, respectively. During R, the 2 parameters were not statistically different from one category to another in anteroposterior balance but in lateral bal~r~e H:P ratio was significantly higher in A and B (0.551 and 0.552 respectively) than in C (0.4) and the content of low-frequency energy was higher in categories A and B than in C. During STI, a significant modification of the H:P ratio was observed in anteroposterior sway for each category while in lateral sway, it was modified for catego|'y A only, indeed, in B and C, the ratio remained unchanged during the 5 periods. The proportion of stabUogram LFE increased both in anteroposterior and lateral sway, in the latter case, pivot LFE was lower in category C than in the other categories. Fig. 5 : Modifications of the mean of the energy distribution of the stabilogram per category during the 5 periods of a unusual visual stimulation test in anteropostefior and lateral balance.
10 r~ e~ 8 r~ 6 4
A
Category of subjects B C
~ 100" r~ ~80.
A
Category of subjects B C
z
~60. 40,
201 Oi R STI P R STI P R STI P R STI P R STI P R STI P ANTEROPOSTERIOR SWAY LATERAL SWAY Low Frequency Energy m l Medium Frequency Energy [ - 7 High Frequency Energy
Space Motion Siclmcss Test
(8)393
DISCUSSION Unusual Visual Information in Dynamic Balance : an Efficient Means of Elicitinc, SMS-like Svmmoms under Normom'avitv The circular translation of the visual scene induced a feeling not concordant with the proprioceptive input (vestibular, muscular, tendinous and articular). This sensori-motor mismatch resulted in head and body oscillations and SMS-like symptoms. Vertigo, dizziness, gastric awareness and nausea were frequently reported by subjects during and following the tests. The provocative character of visuovestibular conflict induced by such an unusual visual stimulation was successfully demonstrated. The Balance Swate~v : an Index of Subiects Susceutibilitv to Sensorial Conflict
The dynamic balance skill, assessed by measurements of the linear displacement of the stabilometer, was not relevant alone to differentiate subjects highly sensitive to sensorial conflict arising from altered visual input whereas it had previously been demonstrated to be an accurate indicator of individual susceptibility to SMS induction by eleclrical vestibular stimulation/2/. The balance strategy, assessed by the ratio H/P and the distribution of the stabilogram energy into three frequency bands allowed differentiation of the 3 categories of subjects. Two hypotheses can be proposed to explain the immunity of category C subjects : (1) the lower H:P ratio of these subjects indicates an efficient reduction of head movements allowing the avoidance of malaise : the movements being known to aggravate motion sickness elicited during tests/8/and space flights/9,10/; (2) the lower content of low-frequency energy in the stabilogram of the C subjects could evidence that they scarcely used vision to balance themselves and thus they were not sensitive to the unusual stimulation of this channel. As a matter of fact, several authors have shown that vision, as well as the vestibular apparatus, is involved in low-frequency balance reactions/11,12/. CONCLUSION Matching unusual visual information with normal proprioception was obtained in subjects in dynamic balance conditions with the aim of eliciting SMS-like symptoms. The circular translation of the visual scene was efficient to create a malaise in 73% of cases and provoked large b~lance reactions in all cases : the movements of the head and the pivot followed the prism rotation frequency i.e. 0.125 Hz. The method used to measure dynan~ic balance allowed the objective identification of subjects sensitive to the visuo-proprioceptive conflict on the basis of the strategy they used to balance themselves : they presented more low-frequency balance reactions than the other subjects. This work confirms that the use of dynamic balance conditions under conflicting sensorial stimulation is an efficient way of eliciting, under normogravity, SMS-like symptoms and that the analysis of the strategies of dynamic balance could be useful in the selection of subjects sensitive to scnsori-motor conflict, susceptible to develop SMS.
This work was supported by grants from CNES (Centre National d'Etudes Spatiales), DASSAULT AVIATION (France) and Fondation pour la Recherche Mddicale. REFERENCES 1. W.E. Thornton, T.P. Moore, S.L. Pool and J. Vanderplocg. Clinical Characterization and Etiology of Space Motion Sickness. Aviat. Space Environ. Med. 58 (9, Suppl.): A1-8 (1987). 2. A. Stverac. Electrical vestibular stimulation and space motion sickness. Acta Astronautica, 28, 401-408 (1992). 3. B.M. Wright. A simple mechanical ataxia-meter. Proceedings of the physiological society (1971). 4. R.N. Haber and M. Hershenson. In : The psychology of visual perception. Holt, Rinehart and Winston, Inc, New-York (1973).
(8)394
A. S~era¢ et aL
5. J.J. Gibson. The senses considered as percetual systems. Boston Houghton Mifflin (1966). 6. K.E. Money. Motion Sickness. Physiological reviews, 50, N°I, 1-39 (1970) 7. A. Graybiel, Ch. D. Wood, E.F. Miller and D.B. Cramer. Diagnostic criteria for grading the severity of acute motion sickness, Aerospace Medicine, 39, 453-455 (1968). .
J_R. Lackner and A. Graybiel. Head movements in non-terrestrial force environnements elicit motion sickness: implications for the etiology of space motion sickness, Aviat. Space Environ. Med., 57, 443-448 (1986).
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