Borelli@Saints-Pères : Isabelle Israel

Isabelle ISRAEL

presents her work at the Borelli@Saints-Pères seminar.

Title: "Otoliths : linear multisensory perception and illusions".

Abstract: The talk will describe several psychophysical tests where participants are moved in darkness and are required to estimate this self-motion.

The otoliths are the sensors of the vestibular system that detect linear acceleration, including gravity, while the semi-circular canals detect angular self-motion. The vestibular system is the main sensory organ that detects the idiothetic signals (Mittelstaedt), allowing path integration. Indeed theoretically, through double time-integration of the otolith acceleration signal, the distance covered during passive linear transport could be estimated.

These tests showed that :

1.  With the cart and vestibular patients, we found that the otoliths are necessary to estimate self-transport distance (Israël & Berthoz, J. Neurophysiol. 1989).
2. With the sled we found that travelled distances are overestimated, due to otoliths overexcitation by acceleration step (Israël et al., J. Neurophysiol. 1993).
3. With the robot we found that velocity profiles were reproduced but time is not necessary to estimate or compute distance (Berthoz et al. Science 1995).
4. Time duration estimation is disturbed by self-motion (Capelli et al., Curr Psy Lett 2007).
5. During circular trajectory, if there is no semi-circular canal input the otoliths alone cannot help the subjects to perceive non-linear motion (Ivanenko et al., J. Physiol. 1997).

Subjects had an accurate perception of their 2D motion only when the orientation of the body was coherent with motion heading. The occurrence of illusory trajectories reflects some fundamental properties of the internal model for motion perception. The results strongly suggest that the brain is unable to process the fine directional dynamics of otolith input to reconstruct motion in space (and that our brain did not invent office chair...). Even for the pure otolith-somatosensory stimulation in the absence of semicircular canal input, the subjects perceived linear or zigzag trajectories rather than circular ones.