Veering in human locomotion: the role of the effectors

Without visual information, human subjects are not able to maintain displacement in a straight line. This tendency is called veering. The goal of this paper was to investigate the origin of veering in a population of subjects who were homotropic during walking (i.e. veering consistently in the same direction on repeated trials). Three types of locomotion were compared, each one required a specific set of effecters: (i) walking (LEG); (ii) propelling on a wheelchair (ARM) and (iii) verbally ordering a second person pushing the wheelchair (VERB). After 15 m displacement, all subjects (n = 8) exhibited large deviations from the initial direction (2.5 m in LEG, 3.2 m in ARM and 4 m in VERB). We also observed that ail participants were homotropic in ARM, but only half of them continued to veer in the same direction than in LEG. By contrast in VERB, deviations occurred randomly. We concluded that systematic deviations occurring in two-limb displacements originate from a peripheral mechanism (slight different properties of the right and left limbs) rather than a central mechanism (systematic bias in the perceived body trajectory). (C) 1999 Published by Elsevier Science Ireland Ltd. All rights reserved.