Mark Andrew Hollands

This study investigates if there are age- and falls-risk related differences in the length of time individuals need following fixation of a stepping target in order to step accurately onto it. This aim was achieved by manipulating the timing and location of stepping target presentation and comparing the effects on stepping performance between young adults, older adults characterised as having a low risk of falling and older adults characterised as having increased risk of falling (N = 10 in each group). Eye and lower limb kinematics were recorded using an eye tracker interfaced with a 3D motion analysis system. Temporal and spatial characteristics of eye and stepping movements were analysed and compared between groups and conditions in which participants had either <1, 2 or 3 s, following target fixation, in order to view and respond to target presentations. Comparisons were made between steps to centrally or laterally positioned targets (125% of individual participant’s normal step width). The results showed that high-risk older adults required significantly more time than low-risk older and younger adults in order to plan and execute medio-lateral stepping adjustments. A reduced ability to make rapid sideways stepping adjustments to avoid obstacles or step on safe areas may contribute towards trips and falls in these individuals. Possible neural mechanisms underlying this group-related decline in performance are discussed.

The authors quantitatively described gaze behavior of young (n = 10) and older (n = 10) adults during stair negotiation, which is information that is crucial for understanding the underlying visuomotor control of stair walking and the effects of aging on this control. Both age groups spent the majority of time looking at central aspects of the stairs approximately 3 steps ahead. Older adults showed less variability in the extent to which they looked ahead (p < .05), and all participants fixated the stairs for briefer periods during descent as opposed to ascent (p < .001). Older adults fixated stairs significantly longer than did young adults before stepping onto the stairs (p < .05). The authors conclude that adults need central visual information describing future stepping locations and that there are age-related differences in visual sampling that reflect changes in the visuomotor control processes subserving locomotion.

The aim of the present study was to investigate whether there are age-related changes in the ability of individuals to use vision to plan (feedforward control) and guide (on-line control) foot placement during locomotion. This aim was achieved by constraining the availability of vision and comparing the effects on the stepping performances of older and young adults during a precision stepping task. We experimentally controlled the availability of visual information such that: (1) vision was only available during each stance phase of the targeting limb, (2) vision was only available during each swing phase of the targeting limb or (3) vision was always available. Our visual manipulations had relatively little effect on younger adults’ stepping performance as demonstrated by their missing the target on less than 10% of occasions. However, there were clear visual condition-related differences in older adults’ stepping performance. When vision was only available during the stance phase of the targeting limb, older adults demonstrated significantly larger foot placement error and associated task failure rate (23%) than trials in which vision was always available (10%). There was an even greater increase in older adults’ foot placement error and task failure rate (42%) during trials in which vision was only available in the swing phase than the other visual conditions. These findings suggest that older adults need vision at particular times during the step cycle, to effectively pre-plan future stepping movements. We discuss the evidence that these age-related changes in performance reflect decline in visual and visuomotor CNS pathways.

Although previous studies have demonstrated the existence of coordinated eye and head movements during gaze shifts, none has studied the temporal and spatial characteristics of the various body segments during gaze transfers that require whole body movements. Without this information it is not possible to determine the extent of the interaction between the oculomotor control system and the motor control systems responsible for moving other body parts. Presented here is a detailed analysis of the timing and kinematic characteristics of participants’ (N=5) eye, head, upper body and feet during rotation of their body to align with light cues positioned at eccentric locations (45, 90, and 135°, left and right of centre). For all rotation amplitudes there was a clear sequence of body segment orientation (eye, head, upper body and feet) consistent with previous studies of locomotor steering and significant correlations between the onset latency times of the eyes and all body segments. There were also significant correlations between temporally aligned kinematic profiles of the feet and the eye in space for all movement amplitudes. The extent of correlation was significantly lower for displacement profiles of the feet versus head and of the feet versus upper body. These findings demonstrate substantial eye-foot coordination during a novel whole-body rotation paradigm and provide evidence that the output of the motor systems responsible for moving the feet is heavily influenced by the motor systems responsible for generating and coordinating eye and head movements to peripheral targets.