Veering in human locomotion: the role of the effectors

Without visual information, human subjects are not able to maintain displac ement 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, eac h one required a specific set of effecters: (i) walking (LEG); (ii) propell ing on a wheelchair (ARM) and (iii) verbally ordering a second person pushi ng the wheelchair (VERB). After 15 m displacement, all subjects (n = 8) exh ibited 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 homotropi c in ARM, but only half of them continued to veer in the same direction tha n in LEG. By contrast in VERB, deviations occurred randomly. We concluded t hat systematic deviations occurring in two-limb displacements originate fro m a peripheral mechanism (slight different properties of the right and left limbs) rather than a central mechanism (systematic bias in the perceived b ody trajectory). (C) 1999 Published by Elsevier Science Ireland Ltd. All ri ghts reserved.