Isabelle Israël

Visual and vestibular factors influencing vestibular navigation

In order to elucidate the role of the “starting point” in path integration, normal subjects underwent a self-rotational task in a motor-driven turntable rotating around an earth-vertical axis. They were passively rotated (“stimulus”) and had to return to the starting point, controlling the direction and velocity of the turntable by means of a joystick (“response”). The test included conditions with an earth-fixed target (EFT) as starting point, shown before the stimulus, and conditions without EFT presentation. The subjects response always took place in total darkness. Subjects succeeded in returning to the starting point in all conditions but were more precise (i.e. had smaller variability of responses) with the EFT than in the other conditions. The larger data scatter (inaccuracy) in these latter tasks was directly related to the return peak velocity, whereas with EFT there was no relationship between amplitude and velocity of the return motion. These results suggest that the presentation of the starting point (the EFT) allows a real time integration to take place, thereby improving accuracy during self-controlled motion in the dark. Five subjects were also tested with the same rotational paradigm in total darkness throughout, but with the head in a different position during stimulus and response motions. Thus, motion detection was performed by different semicircular canals during stimulus and response. The conditions used were head upright during stimulus, hyper-extended backward during response motion, and head backward during stimulus and upright during response motion. It was found that the accuracy during these tasks did not differ from that during stimulus/response motion without change in the upright or backward head position. These data indicate that estimates of trajectory are, within limits, independent of canal plane.

Self-motion perception during a sequence of whole-body rotations in darkness

Abstract. The main aim of this study was to examine how postrotatory effects, induced by passive whole-body rotations in darkness, could alter the perception of motion and eye movements during a subsequent rotation. Perception of angle magnitude was assessed in a reproduction task: blindfolded subjects were first submitted to a passive rotation about the earth-vertical axis on a mobile robot. They were then asked to reproduce this angle by controlling the robot with a joystick. Stimulus rotations ranged from 80° to 340°. Subjects were given one of two delay instructions: after the stimulus, they either had to await the end of postrotatory sensations before starting reproduction (condition free delay, FD), or they had to start immediately after the end of the stimulus rotation (no delay, ND). The delay in FD was used as an incidental measure of the subjective duration of these sensations. Eye movements were recorded with an infrared measuring system (IRIS). Results showed that in both conditions subjects accurately reproduced rotation angles, though they did not reproduce the stimulus dynamics. Peak velocities reached in ND were higher than in FD. This difference suggests that postrotatory effects induced a bias in the perception of angular velocity in the ND condition.

The vestibulo-ocular reflex and angular displacement perception in darkness in humans: adaptation to a virtual environment

The vestibulo-ocular reflex (VOR) and angular displacement perception were measured in 25 healthy humans in darkness before and after exposure to incoherent visual-vestibular stimulation (VVS): 45 min of repeated passive 180 degrees whole-body rotations around the vertical axis concurrent with only 90 degrees rotation in a visual virtual square room. Large inter-individual variability was observed for both VOR gain and turning estimates. The individual VOR gains were not correlated with perceived angles of rotation either before or after VVS. After VVS, the angular displacement perception decreased by 24+/-16% while the VOR gain did not change significantly. The results suggest that adaptive plasticity in turning perception and adaptive plasticity in VOR might be independent of one another.

Shift of the beating field of vestibular nystagmus: an orientation strategy?

We investigated in humans whether the shift of the beating field, which is often observed during vestibular nystagmus, could be related to some strategy of orientation. Eye movements were measured with an infrared system during an experiment on self-motion perception in the dark. Subjects were asked to rotate, by means of a joystick, a mobile robot on which they were seated in order to reproduce a previously imposed passive rotation. We suggest that the shift of the ocular beating field is the manifestation of two different orientation strategies based on allocentric and egocentric reference frames, respectively. It is also proposed that subjects who preferably used the first strategy exhibited large shifts of the beating field, while the others who probably used egocentric memory did not exhibit any shift.

Commanding the direction of passive whole-body rotations facilitates egocentric spatial updating

In conditions of slow passive transport without vision, even tenuous inertial signals from semi-circular canals and the haptic-kinaesthetic system should provide information about changes relative to the environment provided that it is possible to command the direction of the bodys movements voluntarily. Without such control, spatial updating should be impaired because incoming signals cannot be compared to the expected sensory consequences provided by voluntary command. Participants were seated in a rotative robot (Robuter) and learnt the positions of five objects in their surroundings. They were then blindfolded and assigned either to the active group (n=7) or to the passive group (n=7). Members of the active group used a joystick to control the direction of rotation of the robot. The acceleration (25 degrees /s2) and plateau velocity (9 degrees /s) were kept constant. The participants of the passive group experienced the same stimuli passively. After the rotations, the participants had to point to the objects whilst blindfolded. Participants in the active group significantly outperformed the participants in the passive group. Thus, even tenuous inertial cues are useful for spatial updating in the absence of vision, provided that such signals are integrated as feedback associated with intended motor command.

Interaction of visual and idiothetic information in a path completion task

To assess the contribution of visual and vestibular information on human navigation, five blindfolded subjects were passively displaced along two sides of a triangular path using a mobile robot. Subjects were required to complete the triangle by driving the robot to the starting point either blindfolded or in full vision in a 7×6-m and a 38×38-m room. Room dimensions exerted a significant effect on performances: in the smaller environment blindfolded responses were always too short whereas subjects correctly reached the starting point when visual feedback was allowed. On the contrary, in the larger room subjects correctly responded while blindfolded but drove significantly farther than requested in full vision. Our data show that vestibular navigation is highly sensitive to both stored (knowledge of environment) and current visual information.

Eye deviation during rotation in darkness in trait anxiety: an early expression of perceptual avoidance?

BACKGROUND Patients with dizziness and patients with panic disorder and agoraphobia share a common symptomatology. Numerous studies have investigated a potential link between anxiety and the vestibular system, but few of them have addressed the specific topic of spatial representation. METHODS Passive whole-body rotations in the horizontal plane were imposed on two groups of subjects who differed in their level of trait anxiety. Subjects were seated on a mobile robot in darkness. After each passive rotation, subjects were asked to reproduce the stimulus by driving the robot with a joystick and to perform a rotation of the same magnitude. Eye movements were recorded and analyzed. RESULTS No difference in either perception (accuracy in the reproduction task) or in VOR gain was found between the two groups of subjects. Mean eye deviation, caused by fast phases of the nystagmus, differed in the two groups. It was typically in the anticompensatory direction in the non-anxious group, and in the compensatory direction the anxious group. Such compensatory movement may be explained by an egocentric orientation strategy, that may in turn indicate a lack of interest toward the visual surroundings. CONCLUSIONS An egocentric strategy for self-orientation exhibited at a level below the threshold of awareness could reveal the existence of a physiological mode of processing leading to agoraphobic avoidance.

Path integration: is there a difference between athletes and non-athletes?

The ability to estimate distance walked when blindfolded is associated with the vestibular, proprioceptive and (loco-)motor systems. In this study, we examined subjects’ ability to walk when blindfolded to a previously seen target. We examined whether there is a difference of performance in path integration between athletes and non-athletes. Two groups of healthy volunteers took part in this experiment: 21 athletes and 20 non-athletes. Subjects were asked to walk at three different velocities (slow, normal, fast) to a target (10 m in front of them) that they had seen before being blindfolded. Increase in velocity was associated with a decrease in the distance walked for both groups. Both groups were accurate at normal velocities. Athletes were also accurate at fast velocities whereas non-athletes undershot the target. In both groups, accuracy considerably decreased at slow velocities. It seems that our perceptual system can adapt to different velocities (normal, fast) but is most strongly disrupted at low velocity. When attempting to modify walking velocity, step length is also modified, playing a determining role in the estimation of distance.

A common mechanism for the control of eye and head movements in humans

How the human brain controls the subtle coupling between eye and head movements is still debated. The brain could either coordinate two separate (eye and head) networks or use a single system involved in gaze (eye + head) control. In a recent report, a total transfer from eye to head movements was observed in a patient with congenital ophthalmoplegia. This led the authors to hypothesize that such transfer resulted from a long‐term adaptation between oculomotor and head movement systems. We report on a patient in whom a similar transfer was observed but at the acute stage of an acquired ophthalmoplegia. This case demonstrates that the transfer between head and eye movements does not necessarily require long‐term adaptation and supports the hypothesis of a common unique gaze motor command in which eye and head movements would be rapidly exchangeable. Ann Neurol 2000;47:819–822

Optic-Flow-Based Perception of Two-Dimensional Trajectories and the Effects of a Single Landmark

Human observers can detect their heading direction on a short time scale on the basis of optic flow. We investigated the visual perception and reconstruction of visually travelled two-dimensional (2-D) trajectories from optic flow, with and without a landmark. As in our previous study, seated, stationary subjects wore a head-mounted display in which optic-flow stimuli were shown that simulated various manoeuvres: linear or curvilinear 2-D trajectories over a horizontal plane, with observer orientation either fixed in space, fixed relative to the path, or changing relative to both. Afterwards, they reproduced the perceived manoeuvre with a model vehicle, whose position and orientation were recorded. Previous results had suggested that our stimuli can induce illusory percepts when translation and yaw are unyoked. We tested that hypothesis and investigated how perception of the travelled trajectory depends on the amount of yaw and the average path-relative orientation. Using a structured visual environment instead of only dots, or making available additional extra-retinal information, can improve perception of ambiguous optic-flow stimuli. We investigated the amount of necessary structuring, specifically the effect of additional visual and/or extra-retinal information provided by a single landmark in conditions where illusory percepts occur. While yaw was perceived correctly, the travelled path was less accurately perceived, but still adequately when the simulated orientation was fixed in space or relative to the trajectory. When the amount of yaw was not equal to the rotation of the path, or in the opposite direction, subjects still perceived orientation as fixed relative to the trajectory. This caused trajectory misperception because yaw was wrongly attributed to a rotation of the path: path perception is governed by the amount of yaw in the manoeuvre. Trajectory misperception also occurs when orientation is fixed relative to a curvilinear path, but not tangential to it. A single landmark could improve perception. Our results confirm and extend previous findings that, for unambiguous perception of ego-motion from optic flow, additional information is required in many cases, which can take the form of fairly minimal, visual information.