VISUAL INFLUENCE ON HUMAN LOCOMOTION - MODULATION TO CHANGES IN OPTICFLOW

The effect of an optic flow pattern on human locomotion was studied in subjects walking on a self-driven treadmill. During walking an optic flow pattern was presented, which gave subjects the illusion of walkin g in a tunnel. Visual stimulation was achieved by a closed-loop system in which optic flow and treadmill velocity were automatically adjuste d to the intended walking velocity (WV). Subjects were instructed to k eep their WV constant. The presented optic flow velocity was sinusoida lly varied relative to the WV with a cycle period of 2 min. The indepe ndent variable was the relative optic flow (rOF), ranging from -1, i.e ., forward flow of equal velocity as the WV, and 3, i.e., backward flo w 3 times faster than WV. All subjects were affected by rOF in a simil ar way. The results showed, firstly, an increase in stride-cycle varia bility that suggests a larger instability of the walking pattern than in treadmill walking without optic flow; and, secondly, a significant modulating effect of rOF on the self-chosen WV. Backward flow resulted in a decrease, whereas forward flow induced an increase of WV. Within the analyzed range, a linear relationship was found between rOF and W V. Thirdly, WV-related modulations in stride length (SL) and stride fr equency (SF) were different from normal walking: whereas in the latter a change in WV is characterized by a stable linear relationship betwe en SL and SF (i.e., an approximately constant SL to SF ratio), optic f low-induced changes in WV are closely related to a modulation of SL (i .e., a change of SL-SF ratio). Fourthly, this effect of rOF diminished by about 45% over the entire walking distance of 800 m. The results s uggest that the adjustment of WV is the result of a summation of visua l and leg-proprioceptive velocity informations. Visual information abo ut ego-motion leads to an unintentional modulation of WV by affecting specifically the relationship between SL and SF. It is hypothesized th at the space-related parameter (SL) is influenced by visually perceive d motion information, whereas the temporal parameter (SF) remains stab le. The adaptation over the entire walking distance suggests that a sh ift from visual to leg-proprioceptive control takes place.