- MITTELSTAEDT. Max-Planck-Institut fu¨r Verhaltensphysiologie, D-82319 Seewiesen, Germany
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.
This contribution does not deal with the development of postural performance, but with the acquisition and transformation of the information on which perception and control of posture are based. It concentrates on those features of postural information processing which are functionally necessary, hence constrain and guide its development—and thus it may also yield some guidelines for developmental research.
We shall start with the essential discrimination between ‘posture’, that is, the orientation of the body and its parts to the vertical, and ‘position’, that is, the orientation of one part to another part of the body. Posture will be defined as the orientation to the earth-vertical, that is, to the line through the mass centroid of the earth and that of the body or the head, with the latter respectively as origins of right-handed body- or head-fixed coordinate systems, the Z-axis, in the normal posture, pointing upward, and the X-axis forward.
How can information about the orientation of the earthvertical be obtained? First, through gravity. However, since it is indistinguishable from forces caused by linear acceleration, mechanoreceptors cannot discriminate between tilt and translation. This may be an advantage for keeping balance, but a severe hazard for the consistency of space perception. The ambiguity may be resolved by concomitant measurement of rotatory acceleration, but merely within dynamic limits (33,9,10,17). Hence, accurate information is only available in the absence of translatory acceleration. Visual objects that are oriented earth-vertically or earthhorizontally, like trees, buildings, or the horizon at sea provide a second source of information. Their orientation has been shown to affect the subjective visual vertical (SVV), albeit with large inter-individual variance, even completely missing in some subjects. In contrast, textures of vertical and horizontal lines have a strong and consistent effect in all subjects, although it is 908- or 1808-periodic, and thus ambiguous (2,23,4). Here also, dynamic information via rotatory visual flow may increase efficiency. Again, however, the information about the earth-vertical rests on the static polarity of the source. Two organs that may yield the desired information are situated within the head, namely the vestibular system and the eyes.