- Email: [email protected]
Non-specific directional adaptation to asymmetrical visual-vestibular stimulation
Cognitive Brain Research 7 Ž1999. 507–510
Short communication
Non-specific directional adaptation to asymmetrical visual-vestibular stimulation I. Viaud-Delmon
a,)
, Y.P. Ivanenko
a,b
, R. Grasso a , I. Israel ¨
a
a
b
LPPA, CNRS-College ` de France, 11 Place Marcelin Berthelot, 75005 Paris, France Institute for Information Transmission Problems, Russian Academy of Sciences, Bolshoy Karetny 19, Moscow 101447, Russian Federation Accepted 17 November 1998
Abstract Subjective estimates of passive whole-body rotations in darkness were evaluated before and after exposure to asymmetrical incoherent visual–vestibular stimulation ŽVVS.. Two subjects who showed large capacity for adaptation to symmetrical incoherent VVS were enrolled in the study. Strikingly, after 45 min of asymmetrical left–right VVS, perception of rotation decreased equally for rotations to the right and to the left indicating that the calibration of vestibular sensory input for spatial orientation did not undergo a directional specific control. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Spatial orientation; Vestibular system; Visual–vestibular interaction; Virtual reality; Adaptation; Human
The bilateral symmetry of sensory and motor organs is a fundamental feature of the body and is reflected in the organisation of brain processing. Previous studies have generally demonstrated the possibility of independent left and right side adaptation and learning both for motor control w3x and for perception of motion w1,2,6x. In normal subjects, long-lasting vestibular stimulation in only one direction can elicit habituation and asymmetrical oculomotor and perceptual vestibular responses w2x. However, a common feature of the studies testing asymmetrical recalibration w2,3,6x was that the stimulation was provided solely to one side of the body. In a previous experiment, we have shown w5,7x that long-term exposure to incoherent symmetrical visual–vestibular stimulation ŽVVS. for whole-body rotations Žvisual and body rotation of different amplitudes. caused a re-calibration of the vestibular input at the perceptual level. Here, we address the question as to whether the vestibular perception of body rotation can be modified by bilateral asymmetrical VVS. This paradigm may help uncover the physiological principles underlying the bilateral organisation of the vestibular function for spatial processing and multi-sensory interaction. The experimental design consisted of three phases: Ži. a pre-test in which turning perception in darkness was as-
)
Corresponding author. [email protected]
Fax:
q33-1-44-27-13-82;
E-mail:
sessed after passive whole-body rotations, Žii. a VVS phase, and Žiii. a post-test in which turning perception was evaluated again. A whole experimental session took about 2.5 h. Ži. Vestibular stimulation in the form of passive wholebody angular displacements Ž458, 908, 1358, 1808 to the left and to the right, triangular velocity profile, angular accelerations 118rs 2 . was administered in random order to blindfolded subjects. The subjects wore headphones delivering white noise. Subjective estimates of rotation were assessed by means of an angular pointer which consisted of a 15 cm bar rotating about a pivot ŽFig. 1A, Ref. w4x.. A radial grid was attached to a square platform just below the bar such that the experimenter could check the subject response with an accuracy of 18. The platform was placed horizontally on the subject’s knees. Before the experiment we asked the blindfolded subjects to point towards the 12 h of the clock Ži.e., 08, 308, 608, . . . , etc.. in random order. This preliminary calibration showed that the pointing error did not exceed 108. Žii. In the VVS phase, we used a virtual reality set-up to provide visual stimuli coherent in direction Žbut not in amplitude. with the rotation of the subject’s body w5,7x. The subject was seated on a mobile robot ŽFig. 1A. and was immersed into the centre of a virtual square room by means of a head-mounted LCD visual display ŽFig. 1B,C.. We used interactive computer graphics and an ultra-sound sensor system ŽFig. 1C. to track head orientation in the
0926-6410r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 6 4 1 0 Ž 9 8 . 0 0 0 5 2 - 4
508
I. Viaud-Delmon et al.r CognitiÕe Brain Research 7 (1999) 507–510
Fig. 1. Experimental set-up. ŽA. Subjects were seated on a PC-controlled rotating robot. During the pre- and post-tests they used an angular pointer to indicate the amplitude of imposed rotations in darkness. ŽB. During VVS phase, subjects were immersed into the centre of a virtual square room by means of a head-mounted visual display. The floor and the walls of the room were coloured and structured in order to facilitate subject’s perception of rotation in the virtual environment. ŽC. An ultra-sound sensor system was situated above the head in order to track head angular displacements in the horizontal plane.
horizontal plane and to update the rotation of the virtual room proportionally to the robot angular motion. About 200 rotations, alternating to the left and right, were imposed during 45 min of VVS. For asymmetrical VVS, the amplitude ratio between actual rotation of the subject’s body and virtual rotation Žgain G . was different for motion to the right and to the left: to achieve a 908 rotation in the virtual room, the robot had to be rotated by 1808 to the right Ž G R s 0.5. but by 908 to the left Ž G L s 1.. The subject was exposed to two conditions, alternately Ževery 10 min.. In the first condition Žpassive rotation. the robot was remotely controlled. In the second condition Žself-driven rotation. the subject was instructed to control robot rotation with a joystick and to perform 908 turns with respect to the virtual room Žturns to the right corresponded to a real 1808 robot rotation, and turns to the left to a real 908 robot rotation.. This second condition was applied in order to motivate the subjects to pay more attention to their orientation in the virtual room and to prevent declining alertness. In most cases, subjects performed this task in a single motion step with an approximately trapezoidal angular velocity profile. For symmetrical VVS, G R s G L s 0.5. Žiii. Same as pre-test. The virtual reality set-up used in the VVS phase had the following technical specifications. The LCD display ŽVR4, Virtual Research Systems, Santa Clara, CA. had a monoc-
ular field of view of 488 by 368, a total resolution of 742 = 230 red, green, and blue pixels, and was refreshed at 30 frames per second ŽNTSC interlaced standard.. The subject’s horizontal head orientation was measured by an ultra-sound system ŽLogitech, Fremont, CA. which had a reporting rate of 50 Hz. The image generator ŽIndigo 2 Extreme, Silicon Graphics. took the head angular position information from the tracker and output the corresponding image to the display, with a 30 Hz update rate Žthe total latency between sensor motion and complete image display was estimated at 150 ms.. We performed experiments with asymmetrical VVS on 2 subjects who previously showed good adaptation for symmetrical VVS w5x. Subjects were undergraduate students Žmales. who gave their written, informed consent to the study and were paid for their participation. They had no history of neurological or vestibular disorders. None of them was aware of the characteristics of vestibular stimuli. Experiments with different gains were performed on separate occasions, 1 month apart. Fig. 2 shows the results obtained in subject 1. The points represent the slope of the pre-test and post-test angular stimulus–response regression line. Both after symmetrical and asymmetrical VVS there was a significant decrease in estimates of imposed rotations in darkness ŽTable 1.. However, after asymmetrical VVS, the decrease was the same for rotations to the right and to the left,
I. Viaud-Delmon et al.r CognitiÕe Brain Research 7 (1999) 507–510
509
Fig. 2. Turning estimates in darkness before and after adaptation to conflicting symmetrical and asymmetrical VVS. The slope of stimulus–response regression line Žforced through the origin. was used as an estimate of individual turning perception. The amplitude of visual Ždotted lines. and vestibular Žsolid lines. stimulation during VVS phase is indicated at the bottom of each diagram: the visual input was always 908 but the concurrent rotation of the subject’s body was either 1808 for G s 0.5 or 458 for G s 2. G is the gain of VVS discrepancy and corresponds to the ratio between visual and vestibular stimulation. For asymmetrical VVS: G L —gain for left rotations, G R —gain for right rotations.
indicating that asymmetry was not maintained and that re-calibration was transferred also to the side which underwent coherent VVS Ž G L s 1.. Subject 2 behaved similarly although the change in his response was smaller relative to subject 1 Ž28% vs. 49%, respectively, after symmetrical VVS, and 27% vs. 41%, after asymmetrical VVS.. This subject displayed a relative asymmetry in turning estimates before the VVS. He underestimated rotations to the right relative to those to the left. After asymmetrical VVS Ž G L s 1, G R s 0.5. the slopes of stimulus–response regression line decreased on both sides but to a relatively greater extent on the left side, that is, where VVS was coherent. After the whole experiment, subjects were asked about their sensations during rotations in the visual virtual room Žthe VVS phase.. Interestingly, they did not report any conflict or incoherence during both symmetrical and asymmetrical incoherent VVS. In order to check whether the observed post-effect in our subjects simply depended on the presence of a longlasting exposure to visual-vestibular discrepancy, we performed another experiment by imposing the reciprocal ratio between virtual rotation and actual rotation of the subject’s body Ž G s 2, i.e., visual rotations were the dou-
ble of the vestibular ones.. In this case, turning estimates increased Žthough the effect of G s 2 is not the reciprocal of that induced by G s 0.5., indicating that adaptation of rotation perception depended on the gain of the visualvestibular discrepancy and was not solely due to habituation to long-lasting rotatory stimuli. The present results suggest that the vestibular perception of whole-body rotations cannot be calibrated independently on one side or the other if the stimulation occurs in both directions and the subjects are unaware of potential anisotropies in the virtual environment. This finding and the fact that subjects did not perceive directional asymmetry in spite of the large discrepancy between visual and vestibular stimuli might reflect the strength of perceptual assumptions about external and internal symmetry which may constrain the interpretation of sensory input. It is worth remarking that, at the vestibular level, conspicuous asymmetries in left–right discharge may indicate pathological conditions. We suggest that the observed non-specific directional adaptation to asymmetric VVS might be one natural compensation mechanism whereby the central nervous system can overcome the effect of unilateral vestibular deficits.
Table 1 Values of the slope Ž"standard error of the estimate. of stimulus–response regression line Žforced through the origin. before and after adaptation to conflicting symmetrical and asymmetrical VVS ŽR—rotations to the right, L—rotations to the left. Subject
G s 0.5—symmetrical VVS Before
1 2
G L s 1 and G R s 0.5—asymmetrical VVS After
Before
After
L
R
L
R
L
R
L
R
1.05 Ž"0.08. 1.11 Ž"0.12.
0.95 Ž"0.04. 0.82 Ž"0.05.
0.44 Ž"0.09. 0.85 Ž"0.03.
0.58 Ž"0.04. 0.54 Ž"0.09.
0.93 Ž"0.08. 0.81 Ž"0.07.
0.89 Ž"0.06. 0.66 Ž"0.07.
0.56 Ž"0.05. 0.57 Ž"0.04.
0.51 Ž"0.03. 0.51 Ž"0.05.
510
I. Viaud-Delmon et al.r CognitiÕe Brain Research 7 (1999) 507–510
Acknowledgements This work was supported by HFSP: RG71r96B, the GIS-Sciences de la Cognition ŽFrance., and SmithKline Beecham; Y. Ivanenko by a grant of the Fondation pour la Recherche Medicale ŽFrance.. The authors thank P. Leboucher for technical help.
References w1x J. Bloomberg, G. Melvill Jones, B. Segal, Adaptive modification of vestibularly perceived rotation, Exp. Brain Res. 84 Ž1991. 47–56. w2x W.E. Collins, Task-control of arousal and the effects of repeated
w3x
w4x
w5x
w6x
w7x
unidirectional angular acceleration on human vestibular responses, Acta Otolaryng. Suppl. 190 Ž1964. 3–34. M. Ioffe, J. Massion, N. Gantchev, M. Dufosse, M.A. Kulikov, Coordination between posture and movement in a bimanual load-lifting task: is there a transfer?, Exp. Brain Res. 109 Ž1996. 450–456. Y.P. Ivanenko, R. Grasso, I. Israel, ¨ A. Berthoz, Spatial orientation in humans: perception of angular whole-body displacements in two-dimensional trajectories, Exp. Brain Res. 117 Ž1997. 419–427. Y.P. Ivanenko, I. Viaud-Delmon, I. Siegler, I. Israel, ¨ A. Berthoz, The vestibulo-ocular reflex and angular displacement perception in darkness in humans: adaptation to a virtual environment, Neurosci. Lett. 241 Ž1998. 167–170. J.J. Rieser, H.L. Pick Jr., D.H. Ashmead, A.E. Garing, Calibration of human locomotion and models of perceptual–motor organization, J. Exp. Psychol. Hum. Percept. Perform. 21 Ž1995. 480–497. I. Viaud-Delmon, Y.P. Ivanenko, A. Berthoz, R. Jouvent, Sex, lies and virtual reality, Nature Neuroscience 1 Ž1. Ž1998. 15–16.
Short communication
Non-specific directional adaptation to asymmetrical visual-vestibular stimulation I. Viaud-Delmon
a,)
, Y.P. Ivanenko
a,b
, R. Grasso a , I. Israel ¨
a
a
b
LPPA, CNRS-College ` de France, 11 Place Marcelin Berthelot, 75005 Paris, France Institute for Information Transmission Problems, Russian Academy of Sciences, Bolshoy Karetny 19, Moscow 101447, Russian Federation Accepted 17 November 1998
Abstract Subjective estimates of passive whole-body rotations in darkness were evaluated before and after exposure to asymmetrical incoherent visual–vestibular stimulation ŽVVS.. Two subjects who showed large capacity for adaptation to symmetrical incoherent VVS were enrolled in the study. Strikingly, after 45 min of asymmetrical left–right VVS, perception of rotation decreased equally for rotations to the right and to the left indicating that the calibration of vestibular sensory input for spatial orientation did not undergo a directional specific control. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Spatial orientation; Vestibular system; Visual–vestibular interaction; Virtual reality; Adaptation; Human
The bilateral symmetry of sensory and motor organs is a fundamental feature of the body and is reflected in the organisation of brain processing. Previous studies have generally demonstrated the possibility of independent left and right side adaptation and learning both for motor control w3x and for perception of motion w1,2,6x. In normal subjects, long-lasting vestibular stimulation in only one direction can elicit habituation and asymmetrical oculomotor and perceptual vestibular responses w2x. However, a common feature of the studies testing asymmetrical recalibration w2,3,6x was that the stimulation was provided solely to one side of the body. In a previous experiment, we have shown w5,7x that long-term exposure to incoherent symmetrical visual–vestibular stimulation ŽVVS. for whole-body rotations Žvisual and body rotation of different amplitudes. caused a re-calibration of the vestibular input at the perceptual level. Here, we address the question as to whether the vestibular perception of body rotation can be modified by bilateral asymmetrical VVS. This paradigm may help uncover the physiological principles underlying the bilateral organisation of the vestibular function for spatial processing and multi-sensory interaction. The experimental design consisted of three phases: Ži. a pre-test in which turning perception in darkness was as-
)
Corresponding author. [email protected]
Fax:
q33-1-44-27-13-82;
E-mail:
sessed after passive whole-body rotations, Žii. a VVS phase, and Žiii. a post-test in which turning perception was evaluated again. A whole experimental session took about 2.5 h. Ži. Vestibular stimulation in the form of passive wholebody angular displacements Ž458, 908, 1358, 1808 to the left and to the right, triangular velocity profile, angular accelerations 118rs 2 . was administered in random order to blindfolded subjects. The subjects wore headphones delivering white noise. Subjective estimates of rotation were assessed by means of an angular pointer which consisted of a 15 cm bar rotating about a pivot ŽFig. 1A, Ref. w4x.. A radial grid was attached to a square platform just below the bar such that the experimenter could check the subject response with an accuracy of 18. The platform was placed horizontally on the subject’s knees. Before the experiment we asked the blindfolded subjects to point towards the 12 h of the clock Ži.e., 08, 308, 608, . . . , etc.. in random order. This preliminary calibration showed that the pointing error did not exceed 108. Žii. In the VVS phase, we used a virtual reality set-up to provide visual stimuli coherent in direction Žbut not in amplitude. with the rotation of the subject’s body w5,7x. The subject was seated on a mobile robot ŽFig. 1A. and was immersed into the centre of a virtual square room by means of a head-mounted LCD visual display ŽFig. 1B,C.. We used interactive computer graphics and an ultra-sound sensor system ŽFig. 1C. to track head orientation in the
0926-6410r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 6 4 1 0 Ž 9 8 . 0 0 0 5 2 - 4
508
I. Viaud-Delmon et al.r CognitiÕe Brain Research 7 (1999) 507–510
Fig. 1. Experimental set-up. ŽA. Subjects were seated on a PC-controlled rotating robot. During the pre- and post-tests they used an angular pointer to indicate the amplitude of imposed rotations in darkness. ŽB. During VVS phase, subjects were immersed into the centre of a virtual square room by means of a head-mounted visual display. The floor and the walls of the room were coloured and structured in order to facilitate subject’s perception of rotation in the virtual environment. ŽC. An ultra-sound sensor system was situated above the head in order to track head angular displacements in the horizontal plane.
horizontal plane and to update the rotation of the virtual room proportionally to the robot angular motion. About 200 rotations, alternating to the left and right, were imposed during 45 min of VVS. For asymmetrical VVS, the amplitude ratio between actual rotation of the subject’s body and virtual rotation Žgain G . was different for motion to the right and to the left: to achieve a 908 rotation in the virtual room, the robot had to be rotated by 1808 to the right Ž G R s 0.5. but by 908 to the left Ž G L s 1.. The subject was exposed to two conditions, alternately Ževery 10 min.. In the first condition Žpassive rotation. the robot was remotely controlled. In the second condition Žself-driven rotation. the subject was instructed to control robot rotation with a joystick and to perform 908 turns with respect to the virtual room Žturns to the right corresponded to a real 1808 robot rotation, and turns to the left to a real 908 robot rotation.. This second condition was applied in order to motivate the subjects to pay more attention to their orientation in the virtual room and to prevent declining alertness. In most cases, subjects performed this task in a single motion step with an approximately trapezoidal angular velocity profile. For symmetrical VVS, G R s G L s 0.5. Žiii. Same as pre-test. The virtual reality set-up used in the VVS phase had the following technical specifications. The LCD display ŽVR4, Virtual Research Systems, Santa Clara, CA. had a monoc-
ular field of view of 488 by 368, a total resolution of 742 = 230 red, green, and blue pixels, and was refreshed at 30 frames per second ŽNTSC interlaced standard.. The subject’s horizontal head orientation was measured by an ultra-sound system ŽLogitech, Fremont, CA. which had a reporting rate of 50 Hz. The image generator ŽIndigo 2 Extreme, Silicon Graphics. took the head angular position information from the tracker and output the corresponding image to the display, with a 30 Hz update rate Žthe total latency between sensor motion and complete image display was estimated at 150 ms.. We performed experiments with asymmetrical VVS on 2 subjects who previously showed good adaptation for symmetrical VVS w5x. Subjects were undergraduate students Žmales. who gave their written, informed consent to the study and were paid for their participation. They had no history of neurological or vestibular disorders. None of them was aware of the characteristics of vestibular stimuli. Experiments with different gains were performed on separate occasions, 1 month apart. Fig. 2 shows the results obtained in subject 1. The points represent the slope of the pre-test and post-test angular stimulus–response regression line. Both after symmetrical and asymmetrical VVS there was a significant decrease in estimates of imposed rotations in darkness ŽTable 1.. However, after asymmetrical VVS, the decrease was the same for rotations to the right and to the left,
I. Viaud-Delmon et al.r CognitiÕe Brain Research 7 (1999) 507–510
509
Fig. 2. Turning estimates in darkness before and after adaptation to conflicting symmetrical and asymmetrical VVS. The slope of stimulus–response regression line Žforced through the origin. was used as an estimate of individual turning perception. The amplitude of visual Ždotted lines. and vestibular Žsolid lines. stimulation during VVS phase is indicated at the bottom of each diagram: the visual input was always 908 but the concurrent rotation of the subject’s body was either 1808 for G s 0.5 or 458 for G s 2. G is the gain of VVS discrepancy and corresponds to the ratio between visual and vestibular stimulation. For asymmetrical VVS: G L —gain for left rotations, G R —gain for right rotations.
indicating that asymmetry was not maintained and that re-calibration was transferred also to the side which underwent coherent VVS Ž G L s 1.. Subject 2 behaved similarly although the change in his response was smaller relative to subject 1 Ž28% vs. 49%, respectively, after symmetrical VVS, and 27% vs. 41%, after asymmetrical VVS.. This subject displayed a relative asymmetry in turning estimates before the VVS. He underestimated rotations to the right relative to those to the left. After asymmetrical VVS Ž G L s 1, G R s 0.5. the slopes of stimulus–response regression line decreased on both sides but to a relatively greater extent on the left side, that is, where VVS was coherent. After the whole experiment, subjects were asked about their sensations during rotations in the visual virtual room Žthe VVS phase.. Interestingly, they did not report any conflict or incoherence during both symmetrical and asymmetrical incoherent VVS. In order to check whether the observed post-effect in our subjects simply depended on the presence of a longlasting exposure to visual-vestibular discrepancy, we performed another experiment by imposing the reciprocal ratio between virtual rotation and actual rotation of the subject’s body Ž G s 2, i.e., visual rotations were the dou-
ble of the vestibular ones.. In this case, turning estimates increased Žthough the effect of G s 2 is not the reciprocal of that induced by G s 0.5., indicating that adaptation of rotation perception depended on the gain of the visualvestibular discrepancy and was not solely due to habituation to long-lasting rotatory stimuli. The present results suggest that the vestibular perception of whole-body rotations cannot be calibrated independently on one side or the other if the stimulation occurs in both directions and the subjects are unaware of potential anisotropies in the virtual environment. This finding and the fact that subjects did not perceive directional asymmetry in spite of the large discrepancy between visual and vestibular stimuli might reflect the strength of perceptual assumptions about external and internal symmetry which may constrain the interpretation of sensory input. It is worth remarking that, at the vestibular level, conspicuous asymmetries in left–right discharge may indicate pathological conditions. We suggest that the observed non-specific directional adaptation to asymmetric VVS might be one natural compensation mechanism whereby the central nervous system can overcome the effect of unilateral vestibular deficits.
Table 1 Values of the slope Ž"standard error of the estimate. of stimulus–response regression line Žforced through the origin. before and after adaptation to conflicting symmetrical and asymmetrical VVS ŽR—rotations to the right, L—rotations to the left. Subject
G s 0.5—symmetrical VVS Before
1 2
G L s 1 and G R s 0.5—asymmetrical VVS After
Before
After
L
R
L
R
L
R
L
R
1.05 Ž"0.08. 1.11 Ž"0.12.
0.95 Ž"0.04. 0.82 Ž"0.05.
0.44 Ž"0.09. 0.85 Ž"0.03.
0.58 Ž"0.04. 0.54 Ž"0.09.
0.93 Ž"0.08. 0.81 Ž"0.07.
0.89 Ž"0.06. 0.66 Ž"0.07.
0.56 Ž"0.05. 0.57 Ž"0.04.
0.51 Ž"0.03. 0.51 Ž"0.05.
510
I. Viaud-Delmon et al.r CognitiÕe Brain Research 7 (1999) 507–510
Acknowledgements This work was supported by HFSP: RG71r96B, the GIS-Sciences de la Cognition ŽFrance., and SmithKline Beecham; Y. Ivanenko by a grant of the Fondation pour la Recherche Medicale ŽFrance.. The authors thank P. Leboucher for technical help.
References w1x J. Bloomberg, G. Melvill Jones, B. Segal, Adaptive modification of vestibularly perceived rotation, Exp. Brain Res. 84 Ž1991. 47–56. w2x W.E. Collins, Task-control of arousal and the effects of repeated
w3x
w4x
w5x
w6x
w7x
unidirectional angular acceleration on human vestibular responses, Acta Otolaryng. Suppl. 190 Ž1964. 3–34. M. Ioffe, J. Massion, N. Gantchev, M. Dufosse, M.A. Kulikov, Coordination between posture and movement in a bimanual load-lifting task: is there a transfer?, Exp. Brain Res. 109 Ž1996. 450–456. Y.P. Ivanenko, R. Grasso, I. Israel, ¨ A. Berthoz, Spatial orientation in humans: perception of angular whole-body displacements in two-dimensional trajectories, Exp. Brain Res. 117 Ž1997. 419–427. Y.P. Ivanenko, I. Viaud-Delmon, I. Siegler, I. Israel, ¨ A. Berthoz, The vestibulo-ocular reflex and angular displacement perception in darkness in humans: adaptation to a virtual environment, Neurosci. Lett. 241 Ž1998. 167–170. J.J. Rieser, H.L. Pick Jr., D.H. Ashmead, A.E. Garing, Calibration of human locomotion and models of perceptual–motor organization, J. Exp. Psychol. Hum. Percept. Perform. 21 Ž1995. 480–497. I. Viaud-Delmon, Y.P. Ivanenko, A. Berthoz, R. Jouvent, Sex, lies and virtual reality, Nature Neuroscience 1 Ž1. Ž1998. 15–16.