The purpose of the study was to investigate whether older adults show different modulations in locomotion compared with younger adults in response to availability of vision, optic flow speed, and lateral flow asymmetry. The result in Trial 1 corroborates previous findings showing that, in terms of walking speed and stride length, older and younger adults were affected differentially by the availability of vision (Choy et al., 2003
; Cromwell et al., 2001
; Hay et al., 1996
; Manchester et al., 1989
). However, in Trials 2 and 3, the effects of flow speed and lateral flow asymmetry were not influenced by age. Our findings, complementing previous studies (Anderson et al., 1998
; Konczak, 1994
; Schubert et al., 2005
), did not support a greater dependence on optic flow information in older adults during walking. Additionally, no age-related difference was found for the rod and frame test. Taken together, the findings suggest that the ability to integrate optic flow information into the multimodal system for assessment of walking speed and heading direction was comparable in older and younger adults. When vision was deprived, adapted walking performance in older adults emerged possibly due to changes in vestibular or somatosensory function or previous experiences, making them walk more cautiously. We are aware that some participants in the older group were younger than 65. Could the insignificant interaction effect between age and optic flow speed be attributed to the existence of subgroups in the older group? We, thus, divided the older group into two by a cutoff age of 65 years and ran the statistical analyses again. The findings with three groups were not different from those with two groups, indicating that this possibility can be ruled out.
Confirming previous results (Prokop et al., 1997
; Schubert et al., 2005
; Varraine et al., 2002
), changes in absolute value of optic flow speed were linearly and negatively correlated with variations in walking speed, stride frequency, and RPI. The finding can be interpreted as that optic flow speed presented in Trials 2 and 3 was compared with the flow speed perceived during training (i.e., −0.8 m/s). When the flow speed was faster/slower than −0.8 m/s, participants felt that they were walking more quickly/slowly than the trained speed, and they modulated locomotion inadvertently or intentionally to keep their walking speed as instructed. Interestingly, even when changes in flow speed occurred in the unilateral wall only, the effects of optic flow speed could still be observed. One possible explanation is that the perceived flow speed might be the average of the flow speeds from the two walls. More experiments are needed to understand how optic flow speed is perceived when flow speeds are different in two visual fields.
With respect to the effects of lateral flow asymmetry, participants drifted 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. This agrees with data from previous studies (Duchon & Warren, 2002
; Srinivasan et al., 1991
), suggesting that when optic flow speeds are different in two visual fields, participants consider their trajectory as curved and would drift to a balance point at which the flow speed in both sides appears equal.
In all, 30 of 33 participants retained a pattern of 1:1 arm-to-leg frequency ratio across all conditions and did not show any transitions with the manipulations of optic flow speed. This finding suggests that optic flow speed may not be an appropriate control parameter for changes in interlimb coordination during walking. However, it may also be the case that walking overground at 0.8 m/s reduced the likelihood of finding effects of optic flow speed manipulations on interlimb coordination. It appears that more stable patterns (i.e., 1:1 arm-to-leg frequency ratio) were observed during overground walking at 0.8 m/s than during treadmill walking at the same speed, and there is a possibility that the transition point during overground walking is at a lower walking speed compared with treadmill walking. Future work on interlimb coordination during overground walking with a systematic manipulation of walking speed will provide further insight into coordination dynamics.
Older adults walked significantly faster than younger adults in Trials 2 and 3, but not in Trial 1. There are two possible explanations. First, the reported preferred walking speed was between 0.82 and 1.38 m/s for older adults and around 1.20 m/s for younger adults (Malatesta et al., 2004
; McGibbon & Krebs, 2001
; Patel et al., 2006
). Because the trained walking speed might not be the preferred walking speed for all the participants, older adults who could not maintain the muscle strength needed for smooth walking at 0.8 m/s, or found the exposure to the VR environment challenging (Giphart et al., 2007
), might have unintentionally walked at their preferred speed to reduce gait variability (i.e., to improve stability). Second, it has been reported that the perceived speed reduced after prolonged exposure to optic flow (Krekelberg, van Wezel, & Albright, 2006
; Smith, 1985
). It is possible that the perceived speed in older adults was lower than that in younger adults after adaptation, thereby inducing higher walking speeds in older adults. Further studies are needed to test these hypotheses.
In the current study, we provide evidence that older adults are able to integrate optic flow information into the multimodal system to monitor their walking speed and heading direction in much the same manner as younger adults. Future research on the fall-prone elderly is needed to understand whether perception of optic flow is degraded in this population and how they use the optic flow information to guide locomotion.