Origin and processing of postural information.

This contribution surveys the sources and the processing of spatial information about posture, that is, about the orientation of the body and its parts with respect to the vertical (whereas 'position' designates their orientation to each other). Postural information is, to a considerable extent, gained by sense organs in the head. Hence information gained by the mobile eyes and the pitched-up labyrinths is first transformed from a retinal and otolithic into a head-fixed frame of reference, then from head- to trunk-fixed coordinates, and, finally, from a trunk-fixed to an exocentric frame of reference. To that end the position of eyes and otoliths to the head, of the head to the trunk, and of the trunk to the rest of the world must be known, deduced by efference copies or measured by proprioceptors. It is shown that the perceived relation of the visual world to the vertical is exclusively determined by sense organs in the head, whereas body posture is also directly measured by recently discovered graviceptors in the human trunk. It appears that the proprioceptors mediate perception of position, but not, or only indirectly, of posture.