| The purpose of this study was to investigate how systematic manipulations of optic flow speed in both hemifields, either symmetrically (Experiment 1) or asymmetrically (Experiment 2) across hemifields would influence locomotion in older (N = 17) and younger adults (N = 16). Participants were instructed to walk overground at 0.8 m/s in a virtual hallway (i.e., a hallway that was visually specified in an immersive virtual reality environment). In Experiment 1, participants were presented with optic flow symmetrically at either higher, lower, or equal speeds compared to -0.8 m/s (minus sign indicating that the optic flow was programmed to move in a direction opposite to the participant's walking direction). In Experiment 2, the optic flow speed in one hemifield (left or right) was always equal to -0.8 m/s, while the optic flow speed in the other hemifield was either higher, lower or equal to -0.8 m/s. The kinematics of arm and leg movements during walking was recorded with an Optotrak 3020 system in the sagittal plane. The results of Experiment 1 indicated that walking speed and stride frequency were linearly and negatively correlated with the absolute value of symmetric optic flow speed in both groups. Experiment 2 replicated this result and showed that walking speed and stride frequency can still be modulated even when the variation of optic flow speed occurred in unilateral hemifield only. Concerning the heading direction, the results of Experiment 1indicated that symmetric optic flow speeds specified straight walking. In Experiment 2, participants showed a significant drift away from the wall that was moving faster, and the degree of drift was positively related to the difference of optic flow speeds between the two walls. The findings suggest that heading direction during walking can be specified by the degree of symmetry of the optic velocity field. In both experiments, significant age by optic flow speed interaction effects on walking speed and heading direction were not observed, implying that older and younger adults were not differentially affected by the manipulation of optic flow speed during overground walking. |
