<p>In community mobility, walking is commonly completed with other concurrent tasks, described as locomotor multitasks. Many locomotor multitasks rely on vision for both gait and concurrent tasks. When each of the individual tasks uses the same perceptual modality (e.g. vision), structural interference occurs. Structural interference is different from capacity interference, which refers to tasks competing for limited cognitive resources. While locomotor multitask studies have demonstrated that completing the locomotor multitask typically leads to performance impairment in gait and/or the concurrent task, the wide range of tasks has confounded the ability to fully understand how structural and capacity interference affect multitask performance. Thus, the purpose of this dissertation was to delineate how structural interference (Study 1) and capacity interference (Study 2) affect gait multitask performance. To facilitate comparison across studies, the two studies (Study 1 and Study 2) in this dissertation used the same gait task – obstacle crossing – and the same cognitive task – a visual discrete reaction time (RT) task. A discrete RT task was completed while approaching to an obstacle, where visual information regarding obstacle is being gathered to plan for the successful obstacle crossing. In Study 1, to determine if structural interference affects performance impairment in young and older adults, gaze diversion was manipulated by the RT task location (gaze diverted to the obstacle, and gaze diverted away from the obstacle). The RT task was also completed while standing to strengthen the interpretation that any performance impairments were due to structural interference. Study 1 results indicated that structural interference affects both gait and cognitive task performance. Structural interference demonstrated performance impairments in both young and older adults, but the strategies were different. Young adults were more likely adopt gait behavior that increased the risk of tripping when gaze was diverted away from the obstacle (high structural interference), but older adults demonstrated a strategy that decreased the risk of trip when gaze was diverted to the obstacle (low structural interference). This finding highlights the critical role of vision in adaptive gait. In study 2, to determine if capacity interference affects performance impairment in young adults, both gait and cognitive task were manipulated while structural interference was held constant; gait task was manipulated by obstacle height (level walking, 15% leg length height, and 30% leg length height obstacle), and cognitive tasks were three RT tasks (Simple RT, Choice RT, Simon RT). The baseline for each gait task (without RT task) and cognitive task (while seating) was also measured. Capacity interference demonstrated that task prioritization strategy was different for gait challenge versus cognitive challenge in young adults. As gait task difficulty increased, gait task was prioritized. Conversely, as cognitive task difficulty increased, cognitive task was prioritized. This finding highlights that young adults have the ability to flexibly allocate the resources to accomplish the multitask. Lastly, an interesting finding from two studies (Study 1 and Study 2) was when interference is applied during the planning phase – during the approach to the obstacle – structural interference has a greater effect on obstacle crossing performance than capacity interference.</p>

Falls in the elderly are a major cause of mortality and morbidity. Elderly people with visual impairment have been found to be at increased risk of falling, with poor visual acuity in one eye causing greater risk than poor binocular visual acuity. The present study investigated whether monocular refractive blur, at a level typically used for monovision correction, would significantly reduce stereoacuity and consequently affect gait parameters when negotiating a raised surface. Fourteen healthy subjects (25.8 ± 5.6 years) walked up to and on to a raised surface, under four visual conditions; binocular, +2DS blur over their non-dominant eye, +2DS blur over their dominant eye and with their dominant eye occluded. Analysis focussed on foot positioning and toe clearance parameters. Monocular blur had no effect on binocular acuity, but caused a small decline in binocular contrast sensitivity and a large decline in stereoacuity (p < 0.01). Vertical toe clearance increased under monocular blur or occlusion (p < 0.01) with a significantly greater increase under blur of the dominant eye compared with blur of the non-dominant eye (p < 0.01). Increase in toe clearance was facilitated by increasing maximum toe elevation (p < 0.01). Findings indicate that monocular blur at a level typically used for monovision correction significantly reduced stereoacuity and consequently the ability to accurately perceive the height and position of a raised surface placed within the travel path. These findings may help explain why elderly individuals with poor visual acuity in one eye have been found to have an increased risk of falling.

Yes / Descending kerbs during locomotion involves the regulation of appropriate foot placement before the kerb-edge and foot clearance over it. It also involves the modulation of gait output to ensure the body-mass is safely and smoothly lowered to the new level. Previous research has shown that vision is used in such adaptive gait tasks for feedforward planning, with vision from the lower visual field (lvf) used for online updating. The present study determined when lvf information is used to control/update locomotion when stepping from a kerb. Methodology/Principal Findings: 12 young adults stepped down a kerb during ongoing gait. Force sensitive resistors (attached to participants' feet) interfaced with an high-speed PDLC 'smart glass' sheet, allowed the lvf to be unpredictably occluded at either heel-contact of the penultimate or final step before the kerb-edge up to contact with the lower level. Analysis focussed on determining changes in foot placement distance before the kerb-edge, clearance over it, and in kinematic measures of the step down. Lvf occlusion from the instant of final step contact had no significant effect on any dependant variable (p>0.09). Occlusion of the lvf from the instant of penultimate step contact had a significant effect on foot clearance and on several kinematic measures, with findings consistent with participants becoming uncertain regarding relative horizontal location of the kerb-edge. Conclusion/Significance: These findings suggest concurrent feedback of the lower limb, kerb-edge, and/or floor area immediately in front/below the kerb is not used when stepping from a kerb during ongoing gait. Instead heel-clearance and pre-landing-kinematic parameters are determined/planned using lvf information acquired in the penultimate step during the approach to the kerb-edge, with information related to foot placement before the kerb-edge being the most salient.

No / The aim of the present study was to determine adaptive gait changes in long-term wearers of monovision correction contact lenses by comparing gait parameters when wearing monovision correction to those observed when wearing binocular distance correction contact lenses. Methods: Gait and toe clearance parameters were measured in eleven participants (53.5 ± 4.6 years, median monovision wearing time 5 years) as they repeatedly walked up to and onto a raised surface with either monovision or distance correction. Results: Compared to distance correction, monovision resulted in a large reduction in stereoacuity from 17¿ to 87¿, a slower walking velocity (p = 0.001), a reduced horizontal toe clearance of the step edge (p = 0.035) and, for trials when monovision correction occurred first, a 33% greater variability in vertical toe clearance (p = 0.021). Variability in some gait data was large due to certain study design features and learning effects. Conclusion: A slower walking velocity with monovision correction suggests participants became more cautious, likely as a result of the significantly reduced stereoacuity. The decreased horizontal toe clearance and increased vertical toe clearance variability suggests that monovision correction may cause a greater likelihood of hitting step edges and tripping during everyday gait. Recommended study design features are suggested for future adaptive gait studies to increase the precision of the data and to attempt to minimize the effects of learning from somatosensory feedback.

The majority of previous research investigating the role of vision in controlling adaptive gait has predominantly focused on over-ground walking or obstacle negotiation. Thus there is a paucity of literature investigating visuomotor control of step descent. This thesis addressed the importance of the lower visual field (lvf) in regulating step descent landing control, and determined when visual feedback is typically used in regulating landing control prior to/during step descent. When step descents were completed from a stationary starting position, with the lvf occluded or degraded, participants adapted their stepping strategy in a manner consistent with being uncertain regarding the precise location of the foot/lower leg relative to the floor. However, these changes in landing control under conditions of lvf occlusion were made without fundamentally altering stepping strategy. This suggests that participants were able to plan the general stepping strategy when only upper visual field cues were available. When lvf was occluded from either 2 or 1 step(s) prior to descending a step during on-going gait, stepping strategy was only affected when the lvf was occluded in the penultimate step. Findings suggest that lvf cues are acquired in the penultimate step/few seconds prior to descent and provide exproprioceptive information of the foot/lower leg relative to the floor which ensures landing is regulated with increased certainty. Findings also highlight the subtle role of online vision used in the latter portion of step descent to 'fine tune' landing control.

<p> A key mechanism to maintain balance during walking is the foot placement strategy,  where the person steps in the direction of an impending fall. On a clear walkway, the foot  placement strategy translates to maintaining a consistent relationship between the center of mass  state and the base of support (a body-centric constraint on foot placement), which is reflected in  a consistent step length. However, to safely navigate in the community, foot placement must  maintain certain spatial relations with environmental features as well (environmental constraints on foot placement). For stepping over obstacles, the environmental constraint takes the form of  targeting. That is, the feet must be placed at precise locations relative to the obstacle to minimize  the likelihood of tripping.  My dissertation focused on proactive adaptations to foot placements while navigating  cluttered environments. I developed the interstep covariation (ISC) index that quantifies the covariation between consecutive foot placements relative to stationary, visible environmental  features (an obstacle and a visual target). The across-step (or group) changes in this index  indicate how the two constraints (body-centric and environmental) on foot placement are  managed during adaptive gait tasks. I quantified how the ISC index changed (1) across steps  while approaching and crossing an obstacle, (2) due to healthy aging and (3) when the proximity  of two environmental features was systematically altered. Specifically, in Study 1, the ISC index  was quantified for the obstacle crossing step for healthy younger and older adults. In Study 2, proactive changes in the ISC index as healthy young adults approached and crossed an obstacle were characterized. In Study 3, the changes in the dynamics of the across-step ISC index due to  an additional visual stepping target in the approach to the obstacle were identified.  I found that there exists a covariance strategy that healthy adults use to navigate the  environment safely and successfully. First, I found that individuals prioritize the environmental  constraint at the expense of the body-centric constraint when the environment poses a larger risk  to balance (the obstacle), or to satisfy a specified constraint (stepping on a visual target). Second,  I found that the shift in prioritization is proactive, i.e., it occurs while approaching an obstacle.  The strategy to shift priorities is influenced by age (Study 1), environmental features (Study 2  and Study 3), and the proximity of two environmental features (Study 3). These studies add to  the current understanding of foot placement control by demonstrating how this well-known and 15 fundamental strategy to maintain balance while walking is systematically influenced by the  environment and task constraints. These findings can be further extended to study proactive and  reactive adaptations during walking in different populations.   </p>

The majority of previous research investigating the role of vision in controlling adaptive gait has predominantly focused on over-ground walking or obstacle negotiation. Thus there is a paucity of literature investigating visuomotor control of step descent. This thesis addressed the importance of the lower visual field (lvf) in regulating step descent landing control, and determined when visual feedback is typically used in regulating landing control prior to / during step descent. When step descents were completed from a stationary starting position, with the lvf occluded or degraded, participants adapted their stepping strategy in a manner consistent with being uncertain regarding the precise location of the foot / lower leg relative to the floor. However, these changes in landing control under conditions of lvf occlusion were made without fundamentally altering stepping strategy. This suggests that participants were able to plan the general stepping strategy when only upper visual field cues were available. When lvf was occluded from either 2 or 1 step(s) prior to descending a step during on-going gait, stepping strategy was only affected when the lvf was occluded in the penultimate step. Findings suggest that lvf cues are acquired in the penultimate step / few seconds prior to descent and provide exproprioceptive information of the foot / lower leg relative to the floor which ensures landing is regulated with increased certainty. Findings also highlight the subtle role of online vision used in the latter portion of step descent to 'fine tune' landing control.

Impairment of stereoacuity is common in the elderly population and is found to be a risk factor for falls. The purpose of these experiments was to extend knowledge regarding impairment of binocular vision and adaptive gait. Firstly using a 3D motion analysis system to measure how impairment of stereopsis affected adaptive gait during a negotiation of a step, secondly by determining which clinical stereotest was the most reliable for measuring stereoacuity in elderly subjects and finally investigating how manipulating the perceived height of a step in both binocular and monocular conditions affected negotiation of a step. In conditions of impaired stereopsis induced by acutely presented monocular blur, both young and elderly subjects adopted a safety strategy of increasing toe clearance of the step edge, even at low levels of monocular blur (+0.50DS) and the effect was greater when the dominant eye was blurred. The same adaptation was not found for individuals with chronic monocular blur, where vertical toe clearance did not change but variability of toe clearance increased compared to full binocular correction. Findings indicate stereopsis is important for accurately judging the height of a step, and offers support to epidemiological findings that impaired stereoacuity is a risk for falls. Poor agreement was found between clinical stereotests. The Frisby test was found to have the best repeatability. Finally, a visual illusion that caused a step to be perceived as taller led to increased toe elevation. This demonstrates a potential way of increasing toe clearance when stepping up and hence increase safety on stairs. / The Study data files are unavailable online.

No / Purpose. To determine whether gait alterations due to monocular spherical lens blur were a safety strategy or driven by lens magnification. Methods. Adaptive gait and visual function were measured in 10 older adults (mean age, 74.9 4.8 years) with the participants' optimal refractive correction and when monocularly blurred with 1.00 DS and 2.00 DS lens over the dominant eye. Adaptive gait measurements for the lead and trail foot included foot position before the raised surface, toe clearance of the raised surface edge, and foot position on the raised surface. Vision measurements included binocular visual acuity, contrast sensitivity, and stereoacuity. Results. Equal levels of monocular positive and negative spherical lens blur led to very different stepping strategies when negotiating a raised surface. Positive blur lenses led to an increased vertical toe clearance and reduced distance of the lead foot position on the raised surface. Negative lenses led to the opposite of these changes. Conclusions. Findings suggest that step negotiation strategies were driven by the magnification effect provided by the spherical lenses. Steps appeared closer and larger with magnification from positive lenses and further away and smaller with minification from negative lenses and gait was adjusted accordingly. These results suggest that previously reported adaptive gait changes to monocular spherical lens blur were not safety strategies as previously suggested but driven by lens magnification. The significance of these findings in terms of prescribing large refractive changes in frail older patients is discussed.

No / Intervention trials that reduce visual impairment in older adults have not produced the expected improvements in reducing falls rate. We hypothesised that this may be caused by adaptation problems in older adults due to changes in magnification provided by new spectacles and cataract surgery. This study assessed the effects of ocular magnification on adaptive gait in young and older adults. Methods: Adaptive gait was measured in 10 young (mean age 22.3 ± 4.6 years) and 10 older adults (mean age 74.2 ± 4.3 years) with the participants' habitual refractive correction (0%) and with size lenses producing ocular magnification of ±1%, ±2%, ±3%, and ±5%. Adaptive gait parameters were measured when participants approached and stepped up onto a raised surface. Results: Adaptive gait changes in the young and older age groups were similar. Increasing amounts of magnification (+1% to +5%) led to an increased distance of the feet from the raised surface, increased vertical toe clearance and reduced distance of the lead heel position on the raised surface (p < 0.0001). Increasing amounts of minification (¿1% to ¿5%) led to the opposite of these changes (p < 0.0001). Adaptation to ocular magnification did not occur in the short term in young or older adults. Conclusion: The observed adaptive gait changes were driven by the magnification changes provided by the size lenses. The raised surface appeared closer and larger with magnification and further away and smaller with minification and gait was adjusted accordingly. Magnification may explain the mobility problems some older adults have with updated spectacles and after cataract surgery.