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1 Abt. Allg. Psychologie, University of Zuerich, Zuerich, Switzerland
2 Swiss Paraplegic Center, University Hospital Balgrist, Zuerich, Switzerland
3 Neurology Department, University Hospital Zuerich, Zuerich, Switzerland
* To whom correspondence should be addressed. E-mail: tjarchow{at}allgpsy.unizh.ch.
The perception of the body position is mainly mediated by otolith information and visual cues. It has been shown, however, that also proprioceptive sources are involved. To distinguish between the contributions of the vestibular and non-visual extravestibular information to graviception, we tested the effects of a stimulus that leaves the vestibular input unchanged, but modifies the information from sense organs located more caudal along the trunk. This was achieved by bringing subjects into a horizontal ear-down position and then rotating them about an earth-vertical axis that coincided with the interaural axis. In this paradigm, through centrifugal force, the stimulation of the vestibular and the putative extravestibular graviceptive organs in the body becomes dissociated. Healthy subjects (N=14) and paraplegic patients with lesions between T4 and T8 (N=7) adjusted themselves to the perceived horizontal right-ear down body position under two conditions: One with constant velocity rotation (ROT, velocity = 120°/s) about the earth-vertical axis of the turntable, and one without rotation (BASE). Among healthy subjects, the individual differences between BASE and ROT varied widely in both the feet-up or feet-down direction. In contrast, adjustments in paraplegic patients during ROT were always in the feet-down direction compared to BASE. A model with two extravestibular graviceptive sensors could explain our results: one sensor is located rostral to T4, and the other caudal to T8. A load on the rostral graviceptor is interpreted as a tilt of the body in the feet-up direction and shifts the adjustments of perceived body position feet-down; a load on the caudal receptor is interpreted as a tilt in the feet-down direction and shifts the perceived body position feet-up. During ROT, healthy subjects solve the discrepant inputs of both extravestibular graviceptors in a highly variable manner, while paraplegic subjects show less variability, as they are restricted to only the rostral graviceptor.
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