Control of adaptive locomotion: effect of visual obstruction and visual cues in the environment

Visual information regarding obstacle position and size is used for planning and controlling adaptive gait. However, the manner in which visual cues in the environment are used in the control of gait is not fully known. This research examined the effect of obstacle position cues on the lead and trail limb trajectories during obstacle avoidance with and without visual information of the lower limbs and obstacle (termed visual exproprioception). Eight subjects stepped over obstacles under four visual conditions: full vision without obstacle position cues, full vision with position cues, goggles without position cues and goggles with position cues. Goggles obstructed visual exproprioception of the lower limbs and the obstacle. Position cues (2 m tall) were placed beside the obstacle to provide visual cues regarding obstacle position. Obstacle heights were 2, 10, 20 and 30 cm. When wearing goggles and without position cues, a majority of the dependent measures (horizontal distance, toe clearance and lead stride length) increased for the 10, 20 and 30 cm obstacles. Therefore lower limb–obstacle visual exproprioception was important for the control of both limbs, even though with normal vision the trail limb was not visible during obstacle clearance. When wearing goggles, the presence of position cues, which provided on-line visual exproprioception of the self relative to the obstacle position in the anterior–posterior direction, returned lead and trail foot placements to full vision values. Lead toe clearance was not affected by the position cues, trail clearance decreased but was greater than values observed during full vision. Therefore, visual exproprioception of obstacle location, provided by visual cues in the environment, was more relevant than visual exproprioception of the lower limbs for controlling lead and trail foot placement.     

Memory-guided obstacle crossing: more failures were observed for the trail limb versus lead limb

During adaptive locomotion, vision is used to guide the lead limb; however, the individual must rely on knowledge of obstacle height and position, termed obstacle memory, to guide the trail limb. Previous research has demonstrated that visual sampling of the obstacle during approach was adequate to provide obstacle height information, but online visual update of distance to the obstacle was required to plan and implement appropriate foot placement. Our purpose was to determine whether obstacle height memory, coupled with a visible obstacle position cue, could successfully guide the foot during obstacle crossing. Subjects first stepped over an obstacle for 25 trials; then, the obstacle was removed, but its position was marked with high-contrast tape; subjects were instructed to step over the obstacle as if it was still there (termed “virtual obstacle”) for 25 trials. No changes in foot placement were observed; therefore, the position cue provided salient online information to guide foot placement. Average failure rates (subject would have contacted the virtual obstacle if it was present) were 9 and 47 % (lead and trail limb, respectively). Therefore, action was impaired for both limbs when guided by obstacle height memory, but action was impaired to a greater extent for the trail limb. Therefore, viewing the obstacle during approach appears to facilitate the memory needed to guide obstacle crossing, particularly for the trail limb. This is likely because the lead limb is visible in the peripheral visual field during crossing, but the trail limb is not.     

The effects of distant and on-line visual information on the control of approach phase and step over an obstacle during locomotion

One of the goals of this study was to examine the nature and role of distant visual information sampled during locomotion in the feedforward control of leading and trailing limb while an individual is required to step over an obstacle in the travel path. In addition we were interested in whether or not on-line visual information available while the limb (lead or trail) is stepping over the obstacle influences limb trajectory control and whether the information provided during lead limb cross would be used to calibrate movement of the trail limb. Towards this end, we manipulated availability of vision following an initial dynamic sampling period during the approach phase in proximity to the obstacle and during the lead and trail limb stepping over the obstacle. Ten participants completed 40 trials of obstacle crossing in 8 testing conditions. Initial dynamic visual sampling was sufficient to ensure successful task performance in the absence of vision in the approach phase and during both lead and trail limb stepping over the obstacle. Despite successful task performance, foot placement of the lead and trail limb before obstacle crossing and limb elevation over the obstacle were increased after withdrawal of vision in the approach area. Furthermore, the correlation between toe clearance and foot placement was diminished. While both limbs require feedforward visual information to control the step over the obstacle, only lead limb elevation was influenced by availability of on-line visual information during obstacle crossing. Results were in agreement with the notion of primacy of information inherent in the optic array over those from static samples of the environment in guiding locomotion. It is suggested that the expected proprioceptive feedback information associated with the limb posture before the obstacle, reconstructed using visual memory from dynamic sampling of the environment, mismatched with those from the actual limb position. Accordingly, participants adopted a different strategy that enabled them to clear the obstacle with a higher safety margin.Financial assistance was provided by a grant from the Office of Naval Research, USA, NSERC/Canada, and CAPES/Brazil. We would like to thank Milad G. Ishac, Mike Greig, Zinat Shafaei-Shirazi, and Candida T. Goncalves for their assistance     

Visual-vestibular influences on locomotor adjustments for stepping over an obstacle

Combined visual and vestibular influences on locomotor control, particularly in changing environments, are little understood. We studied such influences on body orientation and foot trajectory control during level walking and obstacle avoidance. Six young adults walked on the level and over an obstacle while vision was present or occluded as well as while vestibular information was intact or perturbed using galvanic vestibular stimulation (GVS). The occlusion of vision caused a slowing of gait during obstacle avoidance as well as increased clearance of the leading limb over the obstruction. GVS caused lateral deviations in head and trunk roll angles as well as in foot and trunk displacements, but these lateral deviations were the same during both level walking and obstacle avoidance. In addition, GVS had no effect at all on sagittal plane factors such as speed, foot proximity to the obstacle and vertical clearance over the obstacle. Overall, there is a complex visual control of bilateral obstacle avoidance, but the lack of differences in GVS effects between level and obstructed walking shows that vestibular information is not upregulated for obstacle avoidance. In addition, the robust indifference of anterior foot placement and body displacement to significant lateral deviations from GVS suggests an orthogonally based sensori-locomotor control.     

Characteristics of single and double obstacle avoidance strategies: a comparison between adults and children

Activities of daily living often require us to negotiate several obstacles in the travel path. To date, there is little work investigating how adults accomplish such tasks, and there is even less known about multiple obstacle avoidance strategies used by children. The current work will expand our knowledge about the role of vision in adults and children when avoiding two obstacles placed in their travel path under altered ambient lighting. Healthy 7-year old children (n=10; aged 7.51±0.2 years) and adults (n=10; aged 22.76±1.7 years) were instrumented with infrared markers (Optotrak, NDI) placed on anatomical landmarks and asked to walk along a ten meter path under three conditions: unobstructed, single obstacle, or double obstacle. These trials were performed under two lighting conditions: Full (simulating standard office lighting) and Low (simulating a dark hallway lit by nightlights). Data analyses included lead and trail clearance values, step length, step width and step velocity, take-off distance and Horizontal toe Displacement at Apex (HDA) which was defined as the distance between the horizontal position of the toe to the leading edge of the obstacle when the toe reaches its peak height. Adults were able to maintain consistent behaviour regardless of the number of obstacles in the travel path. Children, however, adjusted their foot placement for the second obstacle. This indicates that having multiple obstacles in the travel path is a more challenging task for 7-year old, and suggests that children at this age may not have fully developed anticipatory locomotor strategies. Children had larger clearance values than adults for the lead foot crossing the obstacle under all obstacle and lighting conditions, and consistently used larger HDA values than adults. Together, these findings suggest that children adopt more cautious strategies than adults in complex environments. Additionally, children decreased walking velocity, increased step width and decreased their step length in a Low light environment. These changes are all indicators of a more careful avoidance strategy, which implies that children at this age rely heavily on visual information to guide foot placements in a complex environment.     

Enhancing memory of stair height by the motor experience of stepping

A concept emerging from recent studies on obstacle avoidance in quadrupeds is that working memory of the height of an obstacle established by visual information is enhanced by motor interactions with the obstacle. In this investigation, we found that this concept is valid in adult humans when viewing and walking up stairs. The main finding was that the memory of the height of stairs was enhanced when information about stair height was gained by walking up a short flight of stairs compared to when information about stair height was gained by vision alone. By measuring the maximum toe clearance when subjects step onto a stair, we observed that maximum toe clearance increased after diverting vision from the stair for a few seconds prior to stepping. Most of this increase occurred within a 2-s period between diverting vision from the stair and initiating the step. By contrast, this increase in maximum toe clearance after diverting vision from a stair was significantly reduced after subjects walked up two stairs prior to stepping onto a stair without vision. This reduction persisted for delays as long as 10?s between diverting vision from the stair and initiating the step. In four of twelve subjects, the maximum toe clearance after these long periods without vision of the stair was close to the value when steps were made with full vision of the stairs.     

The contribution of vision, proprioception, and efference copy in storing a neural representation for guiding trail leg trajectory over an obstacle

Stepping over obstacles requires vision to guide the leading leg, but direct visual information is not available to guide the trailing leg. The neural mechanisms for establishing a stored obstacle representation and thus facilitating the trail leg trajectory in humans are unknown. Twenty-four subjects participated in one of three experiments, which were designed to investigate the contribution of visual, proprioceptive, and efference copy signals. Subjects stepped over an obstacle with their lead leg, stopped, and straddled the obstacle for a delay period before stepping over it with their trail leg while toe elevation was recorded. Subsequently, we calculated maximum toe elevation and toe clearance. First, we found that subjects could accurately scale trail leg toe elevation and clearance, despite straddling an obstacle for up to 2 min, similar to quadrupeds. Second, we found that when the lead leg was passively moved over an obstacle (eliminating an efference copy signal and altering proprioception) without vision, trail leg toe elevation and clearance were reduced, and variability increased compared with when subjects actively moved their lead leg. Trail leg toe measures returned to normal when vision was provided during the passive manipulation. Finally, we found that altering lead leg proprioceptive feedback by adding mass to the ankle had no effect on trail leg toe measures. Taken together, our results suggest that humans can store a neural representation of obstacle properties for extended periods of time and that vision appears to be sufficient in this process to guide trail leg trajectory.     

Stepping over an obstacle on a compliant travel surface reveals adaptive and maladaptive changes in locomotion patterns

Adaptive human locomotion is dependent on safe clearance of obstacles encountered in the path of locomotion. When the terrain is uneven or compliant, stability along with safe obstacle clearance are competing demands presented to the central nervous system (CNS). To examine how the CNS deals with the two competing demands, six participants walked under four conditions: normal ground walking, normal ground walking with an obstacle in the travel path, compliant surface walking, and compliant surface walking with an obstacle in the travel path. Full body kinematics were measured and swing limb kinetics were derived from these measurements. Results showed that on a compliant surface, the CNS was able to decrease foot placement variability at foot contact when approaching an obstacle, similar to the normal ground terrain. Limb trajectory over the obstacle showed that toe elevation was maintained while clearance over the obstacle was lower in the compliant surface condition due to depression of the surface during push off. This illustrates that the CNS controls toe elevation, not toe clearance when stepping over an obstacle. Work done in the knee during elevation and hip during lowering was similar in the compliant and ground conditions even though a lower clearance over the obstacle was achieved in the complaint condition. This shows the inability of the CNS to account for compression of the surface prior to obstacle clearance and provides further evidence the CNS controls toe elevation, not clearance when stepping over an obstacle.An erratum to this article can be found at     

Gait modification during approach phase when stepping over an obstacle in rats

Stepping over obstacles to avoid tripping is an essential component in safe and smooth locomotion. Obstacle avoidance during locomotion is completed in several steps during the approach phase toward the obstacle and stepping over the obstacle. The purpose of this study was to investigate gait modification during the approach phase when stepping over obstacles of different heights in rats. In all four limbs, the toe height when the toe was just above the obstacle increased depending on the obstacle height, leaving a safe margin. However, the horizontal distance between toe and obstacle just prior to stepping over was not influenced by obstacle height. In the fore- and hindlimbs that served as trailing limbs, it was found that the stride length and its related swing phase duration in the final step were significantly shorter than those in both the penultimate step and overground locomotion. These results suggest that adjustment of trailing limb in the final step during the approach phase is important in preparation for the stepping movement over an obstacle.     

Obstacle avoidance during locomotion using haptic information in normally sighted humans

The goal of the study was to examine the accuracy and precision of control of adaptive locomotion using haptic information in normally sighted humans before and after practice. Obstacle avoidance paradigm was used to study adaptive locomotion; individuals were required to approach and step over different sizes of obstacles placed in the travel path under three sensory conditions: full vision (FV); restricted lower visual field (RLVF) using blinders on custom glass frames; and no vision (NV) using haptic information only. In the NV condition, individuals were a given an appropriate-sized cane to guide their locomotion. Footfall patterns were recorded using the GAITRite system, and lead and trail limb trajectories were monitored using the OPTOTRAK system, which tracked infrared diodes placed on the toes and the cane. Approach step lengths were reduced for the haptic condition: this slowed the forward progression and allowed greater time for haptic exploration, which ranged from 2.5 to 4 s and consisted of horizontal cane movements (to detect the width and relative location of the obstacle) and vertical cane movements (to detect the height of the obstacle). Based on feed-forward and on-line sensory (under both vision and haptic conditions) information about location of the obstacle relative to the individual, variability of foot placement reduced as the individual came closer to the obstacle, as has been shown in the literature. The only difference was that the reduction in variability of foot placement under haptic condition occurred in the last step compared with earlier under vision. Considering that the obstacle is detected only when the cane comes in contact, as opposed to vision condition when it is visible earlier, this difference is understandable. Variability and magnitude of lead and trail limb elevation for the haptic condition was higher than the RLVF and FV conditions. In contrast, only the magnitude of lead and trail limb elevation was higher in the RLVF condition when compared with the FV condition. This suggests that it is the inability of the haptic sense to provide accurate information about obstacle characteristics compared with the visual system, and not simple caution that lead to higher limb elevation. In the haptic and RLVF condition when vision was unavailable for on-line monitoring of lead limb elevation, kinesthetic information from lead limb elevation was used to fine-tune trail limb elevation. Both the control of approach phase and limb elevation findings held up even after sufficient practice to learn haptic guidance of adaptive locomotion in the second experiment. These results provide a clear picture of the efficacy of the haptic sensory system to guide locomotion in a cluttered environment.     

Visual guidance of the human foot during a step

When the intended foot placement changes during a step, either due to an obstacle appearing in our path or the sudden shift of a target, visual input can rapidly alter foot trajectory. However, previous studies suggest that when intended foot placement does not change, the path of the foot is fixed after it leaves the floor and vision has no further influence. Here we ask whether visual feedback can be used to improve the accuracy of foot placement during a normal, unperturbed step. To investigate this we measured foot trajectory when subjects made accurate steps, at fast and slow speeds, to stationary floor-mounted targets. Vision was randomly occluded in 50% of trials at the point of foot-off. This caused an increase in foot placement error, reflecting lower accuracy and higher variability. This effect was greatest for slow steps. Trajectory heading analysis revealed that visually guided corrections occurred as the foot neared the target (on average 64 mm away). They occurred closer to the target for the faster movements thus allowing less time and space to execute corrections. However, allowing for a fixed reaction time of 120 ms, movement errors were detected when the foot was approximately halfway to the target. These results suggest that visual information can be used to adjust foot trajectory during the swing phase of a step when stepping onto a stationary target, even for fast movements. Such fine control would be advantageous when environmental constraints place limitations on foot placement, for example when hiking over rough terrain.     

Incomplete inactivation and rapid recovery of voltage-dependent sodium channels during high-frequency firing in cerebellar Purkinje neurons

The ability of individuals to adapt locomotion to constraints associated with the complex environments normally encountered in everyday life is paramount for survival. Here, we tested the ability of 24 healthy young adults to adapt to a rightward prism shift (～11.3°) while either walking and stepping to targets (i.e., precision stepping task) or stepping over an obstacle (i.e., obstacle avoidance task). We subsequently tested for generalization to the other locomotor task. In the precision stepping task, we determined the lateral end-point error of foot placement from the targets. In the obstacle avoidance task, we determined toe clearance and lateral foot placement distance from the obstacle before and after stepping over the obstacle. We found large, rightward deviations in foot placement on initial exposure to prisms in both tasks. The majority of measures demonstrated adaptation over repeated trials, and adaptation rates were dependent mainly on the task. On removal of the prisms, we observed negative aftereffects for measures of both tasks. Additionally, we found a unilateral symmetric generalization pattern in that the left, but not the right, lower limb indicated generalization across the 2 locomotor tasks. These results indicate that the nervous system is capable of rapidly adapting to a visuomotor mismatch during visually demanding locomotor tasks and that the prism-induced adaptation can, at least partially, generalize across these tasks. The results also support the notion that the nervous system utilizes an internal model for the control of visually guided locomotion.     

Factors leading to obstacle contact during adaptive locomotion

During everyday life, healthy adults occasionally trip over an obstacle that they knew was there. These ‘spontaneous’ trips can provide insight into the circumstances leading to trips and falls. The goal of this study was to describe the errors in foot placement and/or foot elevation that resulted in a spontaneous contact with a fixed, visible obstacle in young, healthy adults. Fifteen subjects stepped over an obstacle (height set to 25 % leg length) placed in the middle of an 8 m walkway, up to 300 times. Three subjects never contacted the obstacle and 12 subjects contacted the obstacle 1–4 times, totaling 24 contacts in 3,843 trials (0.6 %). Most of the contacts (92 %) were with the trail limb. Minimum foot clearance of the trail limb (trail MFC) decreased linearly (average slope of ?1 mm/trial) with repeated trials. The majority of subjects (70 %) continued the linear decrease of trail MFC until they contacted the obstacle. The remaining contacts resulted from an apparent misjudgment of foot placement and/or foot elevation. Following contact, trail MFC increased 75 % in the subsequent trials and remained elevated at least up to 30 trials post-contact, but the trajectory of the unperturbed lead limb did not change, further supporting the idea of independent control for the lead and trail limbs during obstacle crossing. Possible causes of the progressive decrease in trail MFC until obstacle contact are considered.     

Stepping over obstacles: anticipatory modifications in children with and without Down syndrome

The purpose of this study was to explore the mechanism of anticipatory control of gait in relation to the perception of an obstacle. Typically developing (TD) children (4–7 years of age) and children with Down syndrome (5–6 years of age) walked and stepped over obstacles of two different heights—a subtle obstacle that was placed at a very low distance from the floor (1% of total body height) and an obvious obstacle that was placed at a much higher distance from the floor (15% of total body height). Spatial and temporal measures of the gait cycle were analyzed. TD children showed increased variability in pre-obstacle step lengths only in response to the higher obstacle. Children with DS showed a decrease in variability in response to the higher obstacle and marked qualitative changes in their gait cycle. Both groups of children were able to scale toe clearance with obstacle height. These results show that TD young children can make task-specific anticipatory adjustments by modulating step length and toe clearance. Children with DS show appropriate scaling of toe clearance and are beginning to show the emergence of anticipatory responses under specific environmental conditions.     

Vertical gaze angle: absolute height-in-scene information for the programming of prehension

One possible source of information regarding the distance of a fixated target is provided by the height of the object within the visual scene. It is accepted that this cue can provide ordinal information, but generally it has been assumed that the nervous system cannot extract "absolute" information from height-in-scene. In order to use height-in-scene, the nervous system would need to be sensitive to ocular position with respect to the head and to head orientation with respect to the shoulders (i.e. vertical gaze angle or VGA). We used a perturbation technique to establish whether the nervous system uses vertical gaze angle as a distance cue. Vertical gaze angle was perturbed using ophthalmic prisms with the base oriented either up or down. In experiment 1, participants were required to carry out an open-loop pointing task whilst wearing: (1) no prisms; (2) a base-up prism; or (3) a base-down prism. In experiment 2, the participants reached to grasp an object under closed-loop viewing conditions whilst wearing: (1) no prisms; (2) a base-up prism; or (3) a base-down prism. Experiment 1 and 2 provided clear evidence that the human nervous system uses vertical gaze angle as a distance cue. It was found that the weighting attached to VGA decreased with increasing target distance. The weighting attached to VGA was also affected by the discrepancy between the height of the target, as specified by all other distance cues, and the height indicated by the initial estimate of the position of the supporting surface. We conclude by considering the use of height-in-scene information in the perception of surface slant and highlight some of the complexities that must be involved in the computation of environmental layout.     

Children use different anticipatory control strategies than adults to circumvent an obstacle in the travel path

Carrying out the daily activities of work and play requires the ability to integrate available sensory information in order to navigate complex, potentially cluttered, environments. The expression of locomotor adjustment behaviour is still maturing during mid- to late-childhood (Grasso et al. in Neurosci Biobehav Rev 22(4): 533–539, 1998a; McFadyen et al. in Gait Posture 13:7–16, 2001), which raises the question, do children coordinate their body segments differently than adults when circumventing an obstacle in their travel path? Healthy young children (n=5; age 10.3±1.5 years) and adults (n=6; age 26.3±2.9 years) were asked to walk at their natural pace during unobstructed walking, as well as during the avoidance to the right or left of a cylindrical obstacle located in the travel path 3 m from the initial starting position. Fourteen infrared markers were fixed to participants and tracked using the Optotrak motion analysis system (60 Hz; Northern Digital Inc, Canada). Data analyses included center of mass (COM) clearance from the obstacle, gait speed, angular movement of the head and trunk (yaw, pitch and roll) and medial–lateral (M-L) COM displacement. Onset of change in these variables from unobstructed walking was also calculated as the time from OBS crossing. Although there were no differences in when adults or children altered their M-L COM trajectory, adults reoriented their head and trunk segments at the same time as their COM while children reoriented their head and trunk prior to changing COM direction. A comparison of foot placement data for this task indicated that while adults changed their gait patterns well in advance of obstacle crossing, children initiated M-L adjustments to gait patterns just prior to OBS crossing. Vallis and McFadyen (Exp Brain Res 152 (3):409–414, 2003) indicated that during circumvention of an obstacle, adults coordinate body segments for a single transient change in COM trajectory while maintaining the underlying travel direction. The present data suggest, however, that children partition obstacle avoidance into two tasks, initially steering with proactive movement of the head and trunk segments and finally making adjustments to their gait trajectory, via stride and step width changes, to ensure adequate obstacle clearance just prior to obstacle crossing. This study demonstrates different anticipatory control strategies used by children as compared to adults to circumvent obstacles in the travel path. The different head and trunk anticipatory segmental coordination suggests that children gather visual information differently when circumventing an obstacle in their travel path and are more dependent on visual input to guide their circumvention strategy.     

Keep looking ahead? Re-direction of visual fixation does not always occur during an unpredictable obstacle avoidance task

Visual information about the environment, especially fixation of key objects such as obstacles, is critical for safe locomotion. However, in unpredictable situations where an obstacle suddenly appears it is not known whether central vision of the obstacle and/or landing area is required or if peripheral vision is sufficient. We examined whether there is a re-direction of visual fixation from an object fixated ahead to a suddenly appearing obstacle during treadmill walking. Furthermore, we investigated the temporal relationship between the onset of muscle activity to avoid the obstacle and saccadic eye and head movements to shift fixation. Eight females (mean ± SD; age = 24.8 ± 2.3 years) participated in this experiment. There were two visual conditions: a central vision condition where participants fixated on two obstacles attached to a bridge on the treadmill and a peripheral vision condition where participants fixated an object two steps ahead. There were two obstacle release conditions: only an obstacle in front of the left foot was released or an obstacle in front of either foot could be released. Only trials when the obstacle was released in front of the left foot were analyzed such that the difference in the two obstacle conditions was whether there was a choice of which foot to step over the obstacle. Obstacles were released randomly in one of three phases during the step cycle corresponding to available response times between 219 and 462 ms. We monitored eye and head movements along with muscle activity and spatial foot parameters. Performance on the task was not different between vision conditions. The results indicated that saccades are rarely made (< 18% of trials) and, when present, are initiated ∼ 350 ms after muscle activity for limb elevation, often accompanied by a downward head movement, and always directed to the landing area. Therefore, peripheral vision of a suddenly appearing obstacle in the travel path is sufficient for successful obstacle avoidance during locomotion: visual fixation is generally not re-directed to either the obstacle or landing area.     

Age-related differences in stepping performance during step cycle-related removal of vision

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.     

Importance of binocular vision in foot placement accuracy when stepping onto a floor-based target during gait initiation

This study investigated the importance of binocular vision to foot placement accuracy when stepping onto a floor-based target during gait initiation. Starting from stationary, participants placed alternate feet onto targets sequentially positioned along a straight travel path with the added constraint that the initial target (target 1) could move in the medio-lateral (M-L) direction. Repeated trials when target 1 remained stationary or moved laterally at the instant of lead-limb toe-off (TO) or 200 ms after TO (early swing) were undertaken under binocular and monocular viewing. Catch trials when target 1 shifted medially were also undertaken. Foot-reach kinematics, foot trajectory corrections and foot placement accuracy for the step onto target 1 were determined via 3D motion analyses. Peak foot-reach velocity and initial foot-reach duration were unaffected by vision condition but terminal foot-reach duration was prolonged under monocular conditions (p = 0.002). Foot trajectory alteration onsets were unaffected by vision condition, but onsets occurred sooner when the target shifted in early swing compared to at TO (p = 0.033). M-L foot placement accuracy decreased (p = 0.025) and variability increased (p = 0.05) under monocular conditions, particularly when stepping onto the moving target. There was no difference between vision conditions in A-P foot placement accuracy. Results indicate that monocular vision provides sufficient information to determine stepping distance and correctly transport the foot towards the target but binocular vision is required to attain a precise M-L foot placement; particularly so when stepping onto a moving target. These findings are in agreement with those found in the reaching and grasping literature, indicating that binocular vision is important for end-point precision.     

Spatial orientation of attention and obstacle avoidance following concussion

Re-injury to the brain during recovery from an initial concussion leads to increased probability of permanent brain damage or death. Recovery from concussion has been proposed to be ongoing even up to a month post-injury. The goal of the current study was to investigate the relationship between the visuospatial orientation of attention and obstacle avoidance during gait in individuals that have recently suffered a concussion (mTBI) over a month post-injury. MTBI subjects and matched control subjects performed the attentional network test (ANT), designed to isolate several different components of attention. Obstacle crossing during gait with and without a concurrent attention dividing task was also performed. Reaction times from the ANT and obstacle clearance measurements were the main dependent variables. We observed that concussed individuals had statistically more obstacle contacts than controls. The ability to orient attention in space was also statistically deficient immediately after a concussion, and this was correlated with lower obstacle clearances of the leading foot. Similar correlations could also be found between both leading and trailing foot avoidance and spatial orientation of attention in participants with concussion when attention was divided during obstacle crossing, and these relationships gradually weakened as recovery progressed. By contrast, spatial attention and obstacle clearance were not significantly correlated in control subjects. These findings indicate that patients with mTBI who display greater spatial attention deficits cross over the obstacle with a lower clearance than patients with less or without spatial attention deficits, leading to an increased probability of obstacle contact.