BLINDFOLDED LINEAR TRANSPORT DISTANCE ESTIMATE : ACTION FOR TIME.

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Preprint submitted on 21 Nov 2002

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BLINDFOLDED LINEAR TRANSPORT DISTANCE ESTIMATE : ACTION FOR TIME.

Isabelle Israël, Diane Sablé, Cyril Laurent

To cite this version:

Isabelle Israël, Diane Sablé, Cyril Laurent. BLINDFOLDED LINEAR TRANSPORT DISTANCE

ESTIMATE : ACTION FOR TIME.. 2002. �hal-00000068�

BLINDFOLDED LINEAR TRANSPORT DISTANCE ESTIMATE : ACTION FOR TIME.

I. Israël, D. Sablé, C. Laurent

LPPA - Collège de France, Place Marcelin Berthelot, 75005 PARIS, FRANCE

The otoliths of the vestibular system detect the linear gravitoinertial acceleration. It has thus been suggested that passive linear self-motion distance in darkness could be estimated, through a double time-integration of the acceleration signal. However several results have revealed a strong overestimate of passively travelled linear distance in darkness, suggesting that the brain is unable to correctly double-integrate the otoliths (and somatosensory) signals. Here we demonstrate that self-driven linear transport distance in darkness is not overestimated. While similar sensory inputs are involved in both active (self-driven) and passive situations, the discrepancy could follow from higher (cognitive) systems, implying internal model, efferent copy and/or top-down processes, but we suggest that the solution is (in) time.

As the vestibular system is the only human sensory organ dedicated to self-motion perception, through the detection of acceleration, and since Mittelstaedt and Mittelstaedt

claimed that self- motion sensory signals are used during motion for spatial navigation, we investigated the role of the vestibular otolithic organs in self-motion linear distance estimate. In a preliminary

experiment

we showed that passively travelled distance can be estimated (on 0.4-0.8 m right-left displacements): the gain (eye tracking movements / head traveled distance) was 0.96 ± 0.35 (n = 8 subjects, mean ± SD), but that a functioning vestibular (i.e. otolithic) system is required for this ability. However, in this experiment the subjects eyes were used as a pointing or tracking tool, and vergence could not be measured, shading some doubt on the results. Thus, in a subsequent experiment

, subjects had to estimate the distance of their linear self-motion toward a previously seen target, by indicating with a push-button when they reached this memorized target position (0.8 - 2.4 m), during motion. Subjects greatly overestimated their self-motion (gain = perceived target distance / traveled distance = 3.02 ± 1.08, n = 7 subjects). We here wanted to extend this result to more ecological distances (5 - 15 m).

The subjects (11 healthy volunteers), seated on a mobile robot, first looked at the experimenter (the visual target) positioned 5, 10 or 15 m ahead. Then the subjects were blindfolded, earphones delivered a white noise, and the robot moved straight ahead, travelling 17 m. During self-motion, the subjects had to push a button when they reached the previously seen target, at 5, 10 or 15 m distance from the starting point. They had beforehand performed some training trials, in light and in darkness. Therefore, the subjects had been familiarized with robot motion, its velocity and the distances travelled. However, during the experimental trials they pushed the button again too early, and their gain (target / travelled distance) was 1.68 ± 0.40, n = 11.

Such overestimate had also been found in the previous experiment with shorter distances, but not with the eye movements tracking experiment. We then wondered whether the active (tracking) factor could be what improved self-motion distance estimate, by reducing the gain, and we performed a new experiment with actively self-driven passive linear motion estimate.

The experimental protocol was very similar to the previous one, with the same target and target

distances, and using the same mobile robot and earphones. However, instead of pushing a button,

the subjects (10 healthy volunteers) had to drive themselves up to the previously seen target, in

darkness, with a joystick controlling robot velocity. In this experiment, the subjects used a quasi-

constant velocity (by fully pushing the joystick at motion onset and keeping it in the same tilted

position throughout the whole distance). As a matter of consequence, the subjects reached the target closer than in the passive condition, with a gain (target / travelled distance) of 1.14 ± 0.19.

Mittelstaedt and Mittelstaedt

, using passive motion at constant velocity, obtained correct results at a velocity close to 1.0 m/s. On the other hand, Harris et al.

used passive triangular velocity self-motion and obtained an overestimate close to that of our passive condition.

Our subjects confessed they were either counting (something) or using time, during both main conditions (trials ranged between 5.5 s and 30.5 s). Therefore, we believe that subjects did need constant velocity in order to estimate the travelled distance, through its time duration, and when they could control motion velocity they simply chose the easiest way to compute time.

Reference List

1. M. L. Mittelstaedt and H. Mittelstaedt, Naturwiss. 67, 566-567 (1980).

2. I. Israël and A. Berthoz, J.Neurophysiol. 62(1), 247-263 (1989).

3. I. Israël, N. Chapuis, S. Glasauer, O. Charade, A. Berthoz, J.Neurophysiol. 70, 1270-1273 (1993).

4. M. L. Mittelstaedt and H. Mittelstaedt, Exp.Brain Res. 139, 318-332 (2001).

5. L. R. Harris, M. Jenkin, D. C. Zikovitz, Exp.Brain Res. 135, 12-21 (2000).