Guiding locomotion in complex, dynamic environments

Exploring the challenges of avoiding collisions with virtual pedestrians using a dual-task paradigm in individuals with chronic moderate to severe traumatic brain injury

BACKGROUND

Individuals with a moderate-to-severe traumatic brain injury (m/sTBI), despite experiencing good locomotor recovery six months post-injury, face challenges in adapting their locomotion to the environment. They also present with altered cognitive functions, which may impact dual-task walking abilities. Whether they present collision avoidance strategies with moving pedestrians that are altered under dual-task conditions, however, remains unclear. This study aimed to compare between individuals with m/sTBI and age-matched control individuals: (1), the locomotor and cognitive costs associated with the concurrent performance of circumventing approaching virtual pedestrians (VRPs) while attending to an auditory-based cognitive task and; (2) gaze behaviour associated with the VRP circumvention task in single and dual-task conditions.

METHODOLOGY

Twelve individuals with m/sTBI (age = 43.3 ± 9.5 yrs; >6 mo. post injury) and 12 healthy controls (CTLs) (age = 41.8 ± 8.3 yrs) were assessed while walking in a virtual subway station viewed in a head-mounted display. They performed a collision avoidance task with VRPs, as well as auditory-based cognitive tasks (pitch discrimination and auditory Stroop), both under single and dual-task conditions. Dual-task cost (DTC) for onset distance of trajectory deviation, minimum distance from the VRP, maximum lateral deviation, walking speed, gaze fixations and cognitive task accuracy were contrasted between groups using generalized estimating equations.

RESULTS

In contrast to CTLs who showed locomotor DTCs only, individuals with m/sTBI displayed both locomotor and cognitive DTCs. While both groups walked slower under dual-task conditions, only individuals with m/sTBI failed to modify their onset distance of trajectory deviation and maintained smaller minimum distances and smaller maximum lateral deviation compared to single-task walking. Both groups showed shorter gaze fixations on the approaching VRP under dual-task conditions, but this reduction was less pronounced in the individuals with m/sTBI. A reduction in cognitive task accuracy under dual-task conditions was found in the m/sTBI group only.

CONCLUSION

Individuals with m/sTBI present altered locomotor and gaze behaviours, as well as altered cognitive performances, when executing a collision avoidance task involving moving pedestrians in dual-task conditions. Potential mechanisms explaining those alterations are discussed. Present findings highlight the compromised complex walking abilities in individuals with m/sTBI who otherwise present a good locomotor recovery.

Age-Related Changes in the Utilization of Visual Information for Collision Prediction: A Study Using an Affordance-Based Model

The ability to predict collisions with moving objects deteriorates with aging. We followed the affordance-based model to identify optical variables that older adults had difficulty using for collision prediction. We reproduced a modified version of the interception task used in Steinmetz (Steinmetz, Layton, Powell, & Fajen, 2020, "Affordance-based versus current - future accounts of choosing whether to pursue or abandon the chase of a moving target," Journal of Vision, 20(3), 8) in a virtual reality (VR) environment and newly introduced perturbation for each of three optical variables (vertical and horizontal expansions of a moving object and the bearing angle produced between participants and a moving object). We expected that perturbation would negatively affect the performance only for those who rely on the optical variable to perform the interception task effectively. We tested 18 older and 15 younger adults and showed that older participants were not negatively affected by the perturbation for the vertical and horizontal expansion of a moving object, while they showed decreased performance when the perturbation was introduced with a bearing angle. These findings suggest that predicting collisions with moving objects deteriorates with aging because the perception of object expansion is impaired with aging.

Avoidance behaviours while circumventing to the left or right of someone with different shoulder widths and facing directions: How do side, width, or orientation matter?

BACKGROUND

Collision avoidance during locomotion is influenced by a variety of situational factors. When circumventing around an inanimate object, the amount of clearance is dependent on the side of avoidance. When avoiding other pedestrians, individuals most often choose to walk behind a moving pedestrian, and avoid people differently depending on their body size. However, side of avoidance has not been evaluated with human obstacles, nor facing direction of a stationary pedestrian, nor the size of a single pedestrian. Therefore, the aim of this study is to evaluate these knowledge gaps concurrently.

RESEARCH QUESTION

How do people avoid a collision to the left-side or right-side of a single stationary pedestrian (interferer) of varying shoulder width and orientation?

METHODS

Participants (n = 11) walked along a 10 m pathway towards a goal, while a stationary interferer stood 6.5 m from the start. The interferer faced one of three directions relative to the participant (orientation); forward, leftward, or rightward, with either their normal shoulder width or enlarged width created by wearing football shoulder pads. Participants were explicitly instructed as to which side of the interferer to avoid (forced-left vs forced-right). Each participant completed 32 randomized avoidance trials. Centre of Mass separation at the time of crossing was used to examine individual's avoidance behaviours.

RESULTS

Results revealed no effect of interferer width, but a significant side of avoidance effect, where the centre of mass separation between the participant and interferer at the time of crossing was smallest when participants avoided to their left.

SIGNIFICANCE

Findings suggest that changing the facing direction or artificially increasing the shoulder width of a stationary interferer will not affect one's avoidance behaviours. However, an asymmetry in side of avoidance is maintained similar to that observed in obstacle avoidance behaviours.

The visual control of locomotion when stepping onto moving surfaces: A comparison of younger and older adults

Stepping between static and moving surfaces presents a locomotor challenge associated with increased injury frequency and severity in older adults. The current study evaluates younger and older adults' behaviours when overcoming challenges sampling moving walkway and escalator environments. Twelve younger adults (18-40 years, Male = 8) and 15 older adults (60-81 years, Male = 5) were examined using an integration of optoelectronic motion capture and mobile eye-tracking. Participants were investigated approaching and stepping onto a flat conveyor belt (static or moving; with or without surface (demarcation) lines). Specifically, the four conditions were: (i) static surface without demarcation lines; (ii) static surface with demarcation lines; (iii) moving surface without demarcation lines; and (iv) moving surface with demarcation lines. A two (age group) x two (surface-condition) x two (demarcation-condition) linear mixed-model revealed no main or interaction effects (p > .05) for perturbation magnitude, indicating participants maintained successful locomotion. However, different adaptive behaviours were identified between conditions with moving and accuracy demands (e.g., moving surfaces increased step length, demarcations reduced step length). Between subject effects identified differences between age groups. Older adults utilised different behaviours, such as earlier gaze transfer from the final approach walkway step location. Overall, the current study suggests that adaptive behaviours emerge relative to the environment's specific demands and the individual's action capabilities.

Association Between Self-Perceived General Human-Likeness During Walking and Walking Speed in Stroke Patients: A Preliminary Study

The difference between the walking speeds of stroke patients and the general population may influence the self-perception of patients, who perceive their walk as lacking general human-likeness. Perception toward human-likeness during walking is defined here as the feeling that one can walk as intended, just like healthy people. Such negative subjective experiences may curb their social participation. However, the perception associated with walking speed in stroke patients is poorly understood. The main purpose of this study was to investigate the relationship between walking speed and perception toward general human-likeness during walking in stroke patients. Thirty-two post-stroke patients were enrolled in this cross-sectional study. Patients performed 10-m walk tests at comfortable and fast speeds and answered questions about their perceived human-like walking after completing the walk (“How much did you feel your walking resembled the human-likeness during walking of general people?”). We found a significant positive correlation between perception toward human-likeness during walking and walking speed at both comfortable and fast speeds. To the best of our knowledge, this report is the first to suggest that walking speed may correlate with self-perception. Our findings may help understand the underlying mechanism in patients perceiving less human-likeness during walking.

Coordinating Clearance and Postural Reorientation When Avoiding Physical and Virtual Pedestrians

Community ambulation requires efficient locomotor adaptations to avoid collisions with other pedestrians. Virtual reality (VR) offers the possibility to experimentally manipulate the environment, allowing researchers to safely assess locomotor responses during pedestrian interactions. In this paper, locomotor adjustments in response to interferers approaching from different directions were examined in both a physical and a virtual environment. Further analysis compared the extent to which locomotor adjustments were influenced by different circumvention strategies. To achieve these goals, twelve healthy young participants were assessed while walking towards a target and avoiding pedestrians approaching from the left, middle, or right, in both VR and the real world. Results showed that in VR, participants walked with slower velocities, maintained larger minimum distances, and reached larger trajectory deviations. Additionally, trajectory deviations and postural reorientations were executed earlier in VR. There were additional differences according to whether participants had to give way to a head-on approaching interferer or choose to pass in front or behind a diagonally approaching interferer. Compared to the other circumvention strategies, passing in front was associated with faster walking speeds as well as smaller and later trajectory deviations and postural reorientations. Lastly, while a cephalocaudal sequence of segment reorientation was observed in both environments, no difference between the onset of head and thorax reorientation was observed in VR. In conclusion, obstacle clearance and postural reorientation are modulated by the environment and circumvention strategies. These modulations should be considered when designing experiments or clinical interventions.

Egocentric Chunking in the Predictive Brain: A Cognitive Basis of Expert Performance in High-Speed Sports

What principles and mechanisms allow humans to encode complex 3D information, and how can it be so fast, so accurately and so flexibly transformed into coordinated action? How do these processes work when developed to the limit of human physiological and cognitive capacity—as they are in high-speed sports, such as alpine skiing or motor racing? High-speed sports present not only physical challenges, but present some of the biggest perceptual-cognitive demands for the brain. The skill of these elite athletes is in many ways an attractive model for studying human performance “in the wild”, and its neurocognitive basis. This article presents a framework theory for how these abilities may be realized in high-speed sports. It draws on a careful analysis of the case of the motorsport athlete, as well as theoretical concepts from: (1) cognitive neuroscience of wayfinding, steering, and driving; (2) cognitive psychology of expertise; (3) cognitive modeling and machine learning; (4) human-in-the loop modellling in vehicle system dynamics and human performance engineering; (5) experimental research (in the laboratory and in the field) on human visual guidance. The distinctive contribution is the way these are integrated, and the concept of chunking is used in a novel way to analyze a high-speed sport. The mechanisms invoked are domain-general, and not specific to motorsport or the use of a particular type of vehicle (or any vehicle for that matter); the egocentric chunking hypothesis should therefore apply to any dynamic task that requires similar core skills. It offers a framework for neuroscientists, psychologists, engineers, and computer scientists working in the field of expert sports performance, and may be useful in translating fundamental research into theory-based insight and recommendations for improving real-world elite performance. Specific experimental predictions and applicability of the hypotheses to other sports are discussed.

Load Carriage Modes and Limb Crossing Patterns Altered Gait during Obstacle Negotiation

Personal and environmental factors both increase the likelihood of falling injuries while negotiating obstacles. Eighteen male participants (seven older, eleven young) were recruited to walk over an obstacle with and without loads on their hands to study the effects of age, load carriage modes, and limb crossing patterns on gait during obstacle negotiation. Participants initiated tasks with either their dominant or non-dominant leg. Step length (SL), toe clearance (TC), step velocity (V), and step width (SW) were extracted from four critical steps. Results showed that during obstacle negotiation (1) older adults had more TC than younger adults, (2) hand loads affected SL and TC, (3) gait parameters are dissimilar between the dominant limb and non-dominant limb.

On the psychological origins of tool use

The ubiquity of tool use in human life has generated multiple lines of scientific and philosophical investigation to understand the development and expression of humans' engagement with tools and its relation to other dimensions of human experience. However, existing literature on tool use faces several epistemological challenges in which the same set of questions generate many different answers. At least four critical questions can be identified, which are intimately intertwined-(1) What constitutes tool use? (2) What psychological processes underlie tool use in humans and nonhuman animals? (3) Which of these psychological processes are exclusive to tool use? (4) Which psychological processes involved in tool use are exclusive to Homo sapiens? To help advance a multidisciplinary scientific understanding of tool use, six author groups representing different academic disciplines (e.g., anthropology, psychology, neuroscience) and different theoretical perspectives respond to each of these questions, and then point to the direction of future work on tool use. We find that while there are marked differences among the responses of the respective author groups to each question, there is a surprising degree of agreement about many essential concepts and questions. We believe that this interdisciplinary and intertheoretical discussion will foster a more comprehensive understanding of tool use than any one of these perspectives (or any one of these author groups) would (or could) on their own.

Gaze behavior during pedestrian interactions in a community environment: a real-world perspective

Locomotor adaptations, as required for community walking, rely heavily on the sense of vision. Little is known, however, about gaze behavior during pedestrian interactions while ambulating in the community. Our objective was to characterize gaze behavior while walking in a community environment and interacting with pedestrians of different locations and directions. Twelve healthy young individuals were assessed as they walked in a shopping mall from a pre-set location to a goal located 20 m ahead. Eye movements were recorded with a binocular eye-tracker and temporal distance factors were assessed using wearable sensors from a full-body motion capture system. Participants exhibited more numerous and longer gaze episodes on pedestrians (GEP) that were walking in the same direction as themselves vs. those that were in the opposite direction. The relative durations of GEPs, however, showed no significant differences between pedestrians walking in the same vs. opposite direction. Longer durations of GEPs were also observed for centrally located pedestrians compared to those located on either side, but this was the case only for pedestrians that were walking in the same direction as participants. In addition, pedestrians in the centre, and even more so those on the right, were fixated at farther distances compared to those on the left. Results indicate that healthy young individuals modulate their gaze behavior as a function of the location and direction of pedestrians when ambulating in a community environment. The observed modulation is interpreted as being caused by an interplay between collision risk, pedestrian visibility, presence of leaders and social conventions (right-sided circulation). Present results also establish baseline measures for the quantification of defective visuomotor strategies in individuals with mobility disorders.

Multimodal immersive trail making-virtual reality paradigm to study cognitive-motor interactions

BACKGROUND

Neuropsychological tests of executive function have limited real-world predictive and functional relevance. An emerging solution for this limitation is to adapt the tests for implementation in virtual reality (VR). We thus developed two VR-based versions of the classic Color-Trails Test (CTT), a well-validated pencil-and-paper executive function test assessing sustained (Trails A) and divided (Trails B) attention-one for a large-scale VR system (DOME-CTT) and the other for a portable head-mount display VR system (HMD-CTT). We then evaluated construct validity, test-retest reliability, and age-related discriminant validity of the VR-based versions and explored effects on motor function.

METHODS

Healthy adults (n = 147) in three age groups (young: n = 50; middle-aged: n = 80; older: n = 17) participated. All participants were administered the original CTT, some completing the DOME-CTT (14 young, 29 middle-aged) and the rest completing the HMD-CTT. Primary outcomes were Trails A and B completion times (t_A, t_B). Spatiotemporal characteristics of upper-limb reaching movements during VR test performance were reconstructed from motion capture data. Statistics included correlations and repeated measures analysis of variance.

RESULTS

Construct validity was substantiated by moderate correlations between the'gold standard' pencil-and-paper CTT and the VR adaptations (DOME-CTT: t_A 0.58, t_B 0.71; HMD-CTT: t_A 0.62, t_B 0.69). VR versions showed relatively high test-retest reliability (intraclass correlation; VR: t_A 0.60-0.75, t_B 0.59-0.89; original: t_A 0.75-0.85, t_B 0.77-0.80) and discriminant validity (area under the curve; VR: t_A 0.70-0.92, t_B 0.71-0.92; original: t_A 0.73-0.95, t_B 0.77-0.95). VR completion times were longer than for the original pencil-and-paper test; completion times were longer with advanced age. Compared with Trails A, Trails B target-to-target VR hand trajectories were characterized by delayed, more erratic acceleration and deceleration, consistent with the greater executive function demands of divided vs. sustained attention; acceleration onset later for older participants.

CONCLUSIONS

The present study demonstrates the feasibility and validity of converting a neuropsychological test from two-dimensional pencil-and-paper to three-dimensional VR-based format while preserving core neuropsychological task features. Findings on the spatiotemporal morphology of motor planning/execution during the cognitive tasks may lead to multimodal analysis methods that enrich the ecological validity of VR-based neuropsychological testing, representing a novel paradigm for studying cognitive-motor interactions.

The Untapped Potential of Virtual Reality in Rehabilitation of Balance and Gait in Neurological Disorders

Dynamic systems theory transformed our understanding of motor control by recognizing the continual interaction between the organism and the environment. Movement could no longer be visualized simply as a response to a pattern of stimuli or as a demonstration of prior intent; movement is context dependent and is continuously reshaped by the ongoing dynamics of the world around us. Virtual reality is one methodological variable that allows us to control and manipulate that environmental context. A large body of literature exists to support the impact of visual flow, visual conditions, and visual perception on the planning and execution of movement. In rehabilitative practice, however, this technology has been employed mostly as a tool for motivation and enjoyment of physical exercise. The opportunity to modulate motor behavior through the parameters of the virtual world is often ignored in practice. In this article we present the results of experiments from our laboratories and from others demonstrating that presenting particular characteristics of the virtual world through different sensory modalities will modify balance and locomotor behavior. We will discuss how movement in the virtual world opens a window into the motor planning processes and informs us about the relative weighting of visual and somatosensory signals. Finally, we discuss how these findings should influence future treatment design.

Developmental differences across the lifespan in the use of perceptual information to guide action-based decisions

Perceptual information about unfolding events is important for guiding decisions about when and how to move in real-world action situations. As an exemplary case, road-crossing is a perceptual-motor task where age has been shown to be a strong predictor of risk due to errors in action-based decisions. The present study investigated age differences between three age groups (Children: 10-12 years old; Adults: 19-39 years old; Older Adults: 65 + year olds) in the use of perceptual information for selection, timing, and control of action when crossing a two-way street in an immersive, interactive virtual reality environment. Adults and children selected gaps to cross that were consistent with the use of a time-based information variable (tau), whereas older adults tuned less into the time-based variable (tau) to guide road-crossing decisions. For action initiation and control, children and adults also showed a strong ability to precisely time their entry with respect to the lead vehicle maximising the available time to cross and coordinating walking movements with the tail vehicle to ensure they were not on a collision course. In contrast, older adults delayed action initiation and showed difficulty coordinating self-movement with the approaching vehicle. This study and its results tie together age-based differences in the three components of action decision-making (selection, timing and control) within a unified framework based on perceptual information. The implications of these age-related differences in action decisions and crossing behaviours are discussed in the context of road safety.

Adding Haptic Feedback to Virtual Environments With a Cable-Driven Robot Improves Upper Limb Spatio-Temporal Parameters During a Manual Handling Task

Physical interactions within virtual environments are often limited to visual information within a restricted workspace. A new system exploiting a cable-driven parallel robot to combine visual and haptic information related to environmental physical constraints (e.g. shelving, object weight) was developed. The aim of this study was to evaluate the impact on user movement patterns of adding haptic feedback in a virtual environment with this robot. Twelve healthy participants executed a manual handling task under three conditions: 1) in a virtual environment with haptic feedback; 2) in a virtual environment without haptic feedback; 3) in a real physical environment. Temporal parameters (movement time, peak velocity, movement smoothness, time to maximum flexion, time to peak wrist velocity) and spatial parameters of movement (maximum trunk flexion, range of motion of the trunk, length of the trajectory, index of curvature and maximum clearance from the shelf) were analysed during the reaching, lowering and lifting phases. Our results suggest that adding haptic feedback improves spatial parameters of movement to better respect the environmental constraints. However, the visual information presented in the virtual environment through the head mounted display appears to have an impact on temporal parameters of movement leading to greater movement time. Taken together, our results suggest that a cable-driven robot can be a promising device to provide a more ecological context during complex tasks in virtual reality.

Effects of Initial Starting Distance and Gap Characteristics on Children's and Young Adults' Velocity Regulation When Intercepting Moving Gaps

OBJECTIVE

This study investigated how children and young adults regulate their velocity when crossing roads under varying traffic conditions.

BACKGROUND

To cross roads safely, pedestrians must adapt their movements to the moving vehicles around them while tightly coupling their movement to visual information.

METHOD

Using an Oculus Rift, 16 children and 16 young adults walked on a treadmill and intercepted gaps between two simulated moving vehicles in an immersive virtual environment. We varied the participants' initial distance from the curb to the interception point, as well as gap characteristics, including gap size and vehicle size.

RESULTS

Varying the initial distance led to systematic adjustments in participants' approach velocities. The inter-vehicle gap and the vehicle size affected the crossing position induced by the initial distance. However, participants did not systematically scale their positions according to the initial distance in narrow gap. Notably, children did not finely tune their movements when they approached wide gap from a closer distance or when they approached the large vehicle from closer distance.

CONCLUSION

Children were less precise in coupling their movements to the moving vehicle in complex traffic environments. In particular, large moving vehicles approaching at closer distances can pose risks when children cross roads.

APPLICATION

These findings suggest the need for an intervention program to improve children's skill in perceiving larger vehicles and timing their movements when crossing roads. We suggest using an interactive virtual reality system to practice this skill.

Increased Risk of Musculoskeletal Injury Following Sport-Related Concussion: A Perception-Action Coupling Approach

Recent studies have concluded that athletes have increased risk of musculoskeletal injury following sport-related concussion. While an underlying explanation is still unknown, perceptual-motor control may be implicated in this increased risk. Some authors have purported that indirect perception (i.e., a "top-down" view of neuromuscular control) may be disrupted following sport-related concussion. Direct perception theory states that the athlete and environment are inextricably linked in a continuous perception-action coupling loop. That is, the athlete is able to directly perceive opportunities for action (e.g., "affordances") in the environment. Based on these notions, the aim of the current paper was to introduce a theoretical model that argues that sport-related concussion may dysregulate the direct perception process, potentially increasing behavioral risk of musculoskeletal injury during sport. Our model is integrated with a sport-related concussion clinical treatment model, which highlights individualized profiles that characterize the heterogeneous response to sport-related concussion. These profiles have a typical constellation of symptoms (e.g., anxiety, fatigue, ocular dysfunction, etc.), which themselves have been associated with disrupted perception-action coupling, independent of sport-related concussion. Therefore, we argue that athletes who have not re-established perception-action coupling loops following sport-related concussion may be at increased risk of subsequent musculoskeletal injury.

Limb movements of another pedestrian affect crossing distance but not path planning during virtual over ground circumvention

Circumventing another pedestrian is a frequent daily activity. The literature has provided a better understanding about anticipatory locomotor adjustments based on global (whole body) movement, but how local limb movements of another pedestrian affect one's circumvention is not well known. The purpose of this study was to understand how local limb movements of another pedestrian affect the planning and execution to circumvent them. Ten healthy young adults (24.5 ± 3.0 years) were immersed in a virtual environment representing a shopping mall. Participants walked to a shop located directly in front of them while circumventing a virtual agent which, when present, approached from straight ahead with one of four different locomotor patterns: 1-Normal locomotor movements; 2- No arm movements; 3- No leg movements; 4- No arm and leg movements (gliding to them). Circumvention trajectory, minimum clearance and coordination of head and trunk rotations along with center of mass lateral displacement were examined. Nonparametric Analysis of Longitudinal Data was used to compare variables across the different conditions. Minimum clearance was smaller for normal locomotor movements compared to all other conditions, but coordination of body movements and onset of circumvention trajectory deviations remained unchanged. These results suggest that global body movement is sufficient to plan circumvention trajectories in a predictable avoidance task, but clearance safety margins are influenced by the local limb movements of another pedestrian.

Collision avoidance behaviours when circumventing people of different sizes in various positions and locations

The current study examined whether young adults' avoidance behaviours differed when circumventing a larger versus smaller interferer. It was expected that avoidance behaviours (repulsion) would be affected by the interferer's size (i.e., greater repulsion for larger body size). Participants (n = 20) walked along an 8 m pathway towards a goal while avoiding either a larger or smaller sized male interferer who stood stationary facing forward, backward, left, or right and were located 2, 4, or 6 m from the participants' starting position. Results revealed that there was an effect of interferer body size (personal-characteristics) and orientation (situational-characteristics) on M-L clearance between the interferer and participant at the time of crossing, suggesting that repulsion magnitudes are scaled to an interferer's closest body surface.

Locomotor Coordination, Visual Perception and Head Stability during Running

Perception and action are coupled such that information from the perceptual system is related to the dynamics of action in order to regulate behavior adaptively. Using running as a model of a cyclic behavior, this coupling involves a continuous, cyclic relationship between the runner’s perception of the environment and the necessary adjustments of the body that ultimately result in a stable pattern of behavior. The purpose of this paper is to illustrate how individuals relate visual perception to rhythmic locomotor coordination patterns in conditions during which foot–ground collisions and visual task demands are altered. We review the findings of studies conducted to illustrate how humans change their behavior to maintain head stability during running with and without various degrees of visual challenge from the environment. Finally, we show that the human body adapts specific segment/joint configuration and coordination patterns to maintain head stability, both in the lower extremity and upper body segments, together with an increase in coordinative variability. These results indicate that in human locomotion, under higher speed (running) and visual task demands, systematic adaptations occur in the rhythmic coupling between the perceptual and movement systems.

Affordance-based versus current-future accounts of choosing whether to pursue or abandon the chase of a moving target

Affordance-based control and current-future control offer competing theoretical accounts of the visual control of locomotion. The aim of this study was to test predictions derived from these accounts about the necessity of self-motion (Experiment 1) and target-ground contact (Experiment 2) in perceiving whether a moving target can be intercepted before it reaches an escape zone. We designed a novel interception task wherein the ability to perceive target catchability before initiating movement was advantageous. Subjects pursued a target moving through a field in a virtual environment and attempted to intercept the target before it escaped into a forest. Targets were catchable on some trials but not others. If subjects perceived that they could not reach the target, they were instructed to immediately give up by pressing a button. After each trial, subjects received a point reward that incentivized them to pursue only those targets that were catchable. On the majority of trials, subjects either pursued and successfully intercepted the target or chose not to pursue at all, demonstrating that humans are sensitive to catchability while stationary. Performance also degraded when the target was floating rather than in contact with the ground. Both findings are incompatible with the current-future account and support the affordance-based account of choosing whether to pursue moving targets.

Reading text messages at different stages of pedestrian circumvention affects strategies for collision avoidance in young and older adults

BACKGROUND

Reading text messages is associated with accidents while walking in community places.

RESEARCH QUESTION

To what extent does reading text messages at different stages of obstacle circumvention affect avoidance strategies while walking in young vs. older adults?

METHODS

Sixteen healthy young and 14 older adults were assessed while walking and viewing a virtual environment (VE) simulating a subway station with three virtual pedestrians positioned 7.5 m away from the participant in the centre (0°), left and right (±40°). As participants advanced 0.5 m towards a target in the far space, a virtual pedestrian randomly approached them. Text messages were delivered at onset of pedestrian movement (0.5 m; early message) or during obstacle circumvention (2.5 m; late message). A Vicon motion capture system captured trajectory displacement while walking.

RESULTS

In both age groups, accuracy of message report (AMR) was reduced for early compared to late messages (p < 0.001), although older adults showed larger deterioration (p < 0.001) compared to younger participants. Locomotor outcomes (obstacle clearance, onset time of avoidance and walking speed) showed no differences between young and older participants (p > 0.05). Early messages led to slower walking speed (p < 0.001) and more frequent collisions compared to late messages and the no-message condition. Late messages yielded faster walking speed (p < 0.001) and onset time of avoidance (p < 0.02) compared to the other conditions.

SIGNIFICANCE

Results indicate that the stage of an avoidance strategy at which text messages are received impacts on pedestrian circumvention, with early messages posing a greater challenge to collision avoidance. In older adults, the reduced AMR suggests larger dual-task interference and prioritization of the walking task. The lack of further walking speed reduction in older vs. young adults may put them at greater risk of collisions in crowded, unpredictable community environments.

Behavioral Dynamics of Pedestrians Crossing between Two Moving Vehicles

This study examines the human behavioral dynamics of pedestrians crossing a street with vehicular traffic. To this end, an experiment was constructed in which human participants cross a road between two moving vehicles in a virtual reality setting. A mathematical model is developed in which the position is given by a simple function. The model is used to extract information on each crossing by performing root-mean-square deviation (RMSD) minimization of the function from the data. By isolating the parameter adjusted to gap features, we find that the subjects primarily changed the timing of the acceleration to adjust to changing gap conditions, rather than walking speed or duration of acceleration. Moreover, this parameter was also adjusted to the vehicle speed and vehicle type, even when the gap size and timing were not changed. The model is found to provide a description of gap affordance via a simple inequality of the fitting parameters. In addition, the model turns out to predict a constant bearing angle with the crossing point, which is also observed in the data. We thus conclude that our model provides a mathematical tool useful for modeling crossing behaviors and probing existing models. It may also provide insight into the source of traffic accidents.

Effects of Age on Obstacle Avoidance while Walking and Deciphering Text versus Audio Phone Messages

BACKGROUND

Widely popular among young, and more recently older adults, mobile phones are increasingly used while walking. Knowledge of the impact of phone message modality (e.g., text vs. audio) on the ability to avoid collisions with other pedestrians, however, remains limited.

OBJECTIVES

This study aimed to investigate the extent to which the circumvention of an approaching pedestrian is affected by text versus audio phone messages in healthy young and older adults.

METHOD

Sixteen young (aged 24 ± 3 years) and 14 older adults (aged 68 ± 4.5 years) were tested while walking and viewing a virtual environment depicted as a subway station in a helmet-mounted display. As they walked, one of three virtual humans randomly approached from the center (0°), right (+40°), or left (+40°). Phone messages, when present, were delivered at obstacle displacement onset and presented either as text messages on a virtual phone or as audio messages delivered through earphones. Participants were instructed to avoid collisions with pedestrians and to fully report the message content at the end of trials.

RESULTS

Both groups showed decreased accuracy of message report (AMR), slower walking speed, and more collisions in response to text versus audio messages. Compared to young adults, older adults showed greater reduction in AMR, more collisions, and similar speed adaptation in the presence of text messages. In both age groups, no significant differences in walking speed emerged between the audio message and the no-message condition, but only older adults experienced collisions and reduced AMR with the audio messages. Obstacle clearance and the onset time of avoidance strategy were not affected by message condition.

CONCLUSIONS

Results suggest that coping with text messages while walking leads to greater risk of collision and alters message deciphering accuracy, while audio messages stand out as a safer and more efficient alternative for on-the-go communication. In general, older adults experienced larger motor-cognitive interference than younger adults, resulting in reduced AMR and more collisions without further changes in gait adaptation. Consequently, older adults failed to prioritize their safety when attending to phone messages while walking.

Measuring Pedestrian Collision Detection With Peripheral Field Loss and the Impact of Peripheral Prisms

Purpose

Peripheral field loss (PFL) due to retinitis pigmentosa, choroideremia, or glaucoma often results in a highly constricted residual central field, which makes it difficult for patients to avoid collision with approaching pedestrians. We developed a virtual environment to evaluate the ability of patients to detect pedestrians and judge potential collisions. We validated the system with both PFL patients and normally sighted subjects with simulated PFL. We also tested whether properly placed high-power prisms may improve pedestrian detection.

Methods

A virtual park-like open space was rendered using a driving simulator (configured for walking speeds), and pedestrians in testing scenarios appeared within and outside the residual central field. Nine normally sighted subjects and eight PFL patients performed the pedestrian detection and collision judgment tasks. The performance of the subjects with simulated PFL was further evaluated with field of view expanding prisms.

Results

The virtual system for testing pedestrian detection and collision judgment was validated. The performance of PFL patients and normally sighted subjects with simulated PFL were similar. The prisms for simulated PFL improved detection rates, reduced detection response times, and supported reasonable collision judgments in the prism-expanded field; detections and collision judgments in the residual central field were not influenced negatively by the prisms.

Conclusions

The scenarios in a virtual environment are suitable for evaluating PFL and the impact of field of view expanding devices.

Translational Relevance

This study validated an objective means to evaluate field expansion devices in reproducible near-real-life settings.

Avoidance behaviours of young adults during a head-on collision course with an approaching person

Individuals use visual information to guide their avoidance behaviours. More specifically, individuals may directly perceive the time prior to colliding with an approaching obstacle (i.e., time to contact, TTC) to determine when to avoid. In addition, individuals use body-scaled information to control their movements. These avoidance behaviours differ when avoiding a human obstacle compared to an inanimate object. As such, the purpose of this experiment was to examine the avoidance behaviours of individuals during a head-on collision course with an approaching person. Young adults (N = 20, [Formula: see text] = 22.25 ± 1.5 years, 10 males) were instructed to walk along a 10 m path towards a goal located along the midline. A female confederate positioned along the midline walked towards the participants to one of the four predetermined final positions: (1) along the midline in the participants' starting position; (2) stopped along the midline 2.5 m from her starting position; (3) to the left of the participants' starting position; and (4) to the right of the participants' starting position. Results revealed when the path of the confederate was certain, individuals used a greater TTC to determine when to change their path in comparison with when the path of the confederate was uncertain. Males were found to avoid significantly earlier (i.e., larger TTC) than females. However, following a change in path, sex did not impact the avoidance behaviours of the groups, but rather, the environment was the regulating factor.

When flow is not enough: evidence from a lane changing task

Humans are able to estimate their heading on the basis of optic flow information and it has been argued that we use flow in this way to guide navigation. Consistent with this idea, several studies have reported good navigation performance in flow fields. However, one criticism of these studies is that they have generally focused on the task of walking or steering towards a target, offering an additional, salient directional cue. Hence, it remains a matter of debate as to whether humans are truly able to control steering in the presence of optic flow alone. In this study, we report a set of maneuvers carried out in flow fields in the absence of a physical target. To do this, we studied the everyday task of lane changing, a commonplace multiphase steering maneuver which can be conceptualized without the need for a target. What is more (and here is the crucial quirk), previous literature has found that in the absence of visual feedback, drivers show a systematic, asymmetric steering response, resulting in a systematic final heading error. If optic flow is sufficient for controlling navigation through our environment, we would expect this asymmetry to disappear whenever optic flow is provided. However, our results show that this asymmetry persisted, even in the presence of a flow field, implying that drivers are unable to use flow to guide normal steering responses in this task.

Healthy young adults implement distinctive avoidance strategies while walking and circumventing virtual human vs. non-human obstacles in a virtual environment

This study examines how three types of obstacles (cylinder, virtual human and virtual human with footstep sounds) affect circumvention strategies of healthy young adults. Sixteen participants aged 25.2 ± 2.5 years (mean ± 1SD) were tested while walking overground and viewing a virtual room through a helmet mounted display. As participants walked towards a stationary target in the far space, they avoided an obstacle (cylinder or virtual human) approaching either from the right (+40°), left (-40°) or head-on (0°). Obstacle avoidance strategies were characterized using the position and orientation of the head. Repeated mixed model analysis showed smaller minimal distances (p = 0.007) while avoiding virtual humans as compared to cylinders. Footstep sounds added to virtual humans did not modify (p = 0.2) minimal distances compared to when no sound was provided. Onset times of avoidance strategies were similar across conditions (p = 0.06). Results indicate that the nature of the obstacle (human-like vs. non-human object) matters and can modify avoidance strategies. Smaller obstacle clearances in response to virtual humans may reflect the use of a less conservative avoidance strategy, due to a resemblance of obstacles to pedestrians and a recall of strategies used in daily locomotion. The lack of influence of footstep sounds supports the fact that obstacle avoidance primarily relies on visual cues and the principle of 'inverse effectiveness' whereby multisensory neurons' response to multimodal stimuli becomes weaker when the unimodal sensory stimulus (vision) is strong. Present findings should be taken into consideration to optimize the ecological validity of VR-based obstacle avoidance paradigms used in research and rehabilitation.

Changes in perception-action tuning over long time scales: How children and adults perceive and act on dynamic affordances when crossing roads

This investigation examined developmental change in how children perceive and act on dynamic affordances when crossing roads on foot. Six- to 14-year-olds and adults crossed roads with continuous cross-traffic in a large-screen, immersive pedestrian simulator. We observed change both in children's gap choices and in their ability to precisely synchronize their movement with the opening of a gap. Younger children were less discriminating than older children and adults, choosing fewer large gaps and more small gaps. Interestingly, 12-year-olds' gap choices were significantly more conservative than those of 6-, 8-, 10-, and 14-year-olds, and adults. Timing of entry behind the lead vehicle in the gap (a key measure of movement coordination) improved steadily with development, reaching adultlike levels by age 14. Coupled with their poorer timing of entry, 6-, 8-, and 10-year-olds' gap choices resulted in significantly less time to spare and more collisions than 14-year-olds and adults. Time to spare did not differ between 12-year-olds, 14-year-olds, and adults, indicating that 12-year-olds' more conservative gap choices compensated for their poorer timing of entry. The findings show that children's ability to perceive and act on dynamic affordances undergoes a prolonged period of development, and that older children appear to compensate for their poorer movement timing skills by adjusting their gap decisions to match their crossing actions. Implications for the development of perception-action tuning and road-crossing skills are discussed. (PsycINFO Database Record

Modeling spatial navigation in the presence of dynamic obstacles: a differential games approach

Obstacle circumvention strategies can be shaped by the dynamic interaction of an individual (evader) and an obstacle (pursuer). We have developed a mathematical model with predictive and emergent components, using experimental data from seven healthy young adults walking toward a target while avoiding collision with a stationary or moving obstacle (approaching head-on, or diagonally 30° left or right) in a virtual environment. Two linear properties from the predictive component enable the evader to predict the minimum distance between itself and the obstacle at all times, including the future intersection of trajectories. The emergent component uses the classical differential games model to solve for an optimal circumvention while reaching the target, wherein the locomotor strategy is influenced by the obstacle, target, and the evader velocity. Both model components were fitted to a different set of experimental data obtained from five poststroke and healthy participants to derive the minimum predicted distance (predictive component) and obstacle influence dimensions (emergent component) during circumvention. Minimum predicted distance between evader and pursuer was kept constant when the evader was closest to the obstacle in all participants. Obstacle influence dimensions varied depending on obstacle approach condition and preferred side of circumvention, reflecting differences in locomotor strategies between poststroke and healthy individuals. Additionally, important associations between model outputs and observed experimental outcomes were found. The model, supported by experimental data, suggests that both predictive and emergent processes can shape obstacle circumvention strategies in healthy and poststroke individuals. NEW & NOTEWORTHY Obstacle circumvention during goal-directed locomotion is modeled with a new mathematical approach comprising both predictive and emergent elements. The major novelty is using differential games solutions to illustrate the dynamic interactions between the individual as an evader and the approaching obstacle as a pursuer. The model is supported by experimental evidence that explains the behavior along the continuum of locomotor adaptation displayed by healthy subjects and individuals with stroke.

Evaluation of a conceptual framework for predicting navigation performance in virtual reality

Previous research in spatial cognition has often relied on simple spatial tasks in static environments in order to draw inferences regarding navigation performance. These tasks are typically divided into categories (e.g., egocentric or allocentric) that reflect different two-systems theories. Unfortunately, this two-systems approach has been insufficient for reliably predicting navigation performance in virtual reality (VR). In the present experiment, participants were asked to learn and navigate towards goal locations in a virtual city and then perform eight simple spatial tasks in a separate environment. These eight tasks were organised along four orthogonal dimensions (static/dynamic, perceived/remembered, egocentric/allocentric, and distance/direction). We employed confirmatory and exploratory analyses in order to assess the relationship between navigation performance and performances on these simple tasks. We provide evidence that a dynamic task (i.e., intercepting a moving object) is capable of predicting navigation performance in a familiar virtual environment better than several categories of static tasks. These results have important implications for studies on navigation in VR that tend to over-emphasise the role of spatial memory. Given that our dynamic tasks required efficient interaction with the human interface device (HID), they were more closely aligned with the perceptuomotor processes associated with locomotion than wayfinding. In the future, researchers should consider training participants on HIDs using a dynamic task prior to conducting a navigation experiment. Performances on dynamic tasks should also be assessed in order to avoid confounding skill with an HID and spatial knowledge acquisition.

A Neural Model of MST and MT Explains Perceived Object Motion during Self-Motion

When a moving object cuts in front of a moving observer at a 90° angle, the observer correctly perceives that the object is traveling along a perpendicular path just as if viewing the moving object from a stationary vantage point. Although the observer's own (self-)motion affects the object's pattern of motion on the retina, the visual system is able to factor out the influence of self-motion and recover the world-relative motion of the object (Matsumiya and Ando, 2009). This is achieved by using information in global optic flow (Rushton and Warren, 2005; Warren and Rushton, 2009; Fajen and Matthis, 2013) and other sensory arrays (Dupin and Wexler, 2013; Fajen et al., 2013; Dokka et al., 2015) to estimate and deduct the component of the object's local retinal motion that is due to self-motion. However, this account (known as "flow parsing") is qualitative and does not shed light on mechanisms in the visual system that recover object motion during self-motion. We present a simple computational account that makes explicit possible mechanisms in visual cortex by which self-motion signals in the medial superior temporal area interact with object motion signals in the middle temporal area to transform object motion into a world-relative reference frame. The model (1) relies on two mechanisms (MST-MT feedback and disinhibition of opponent motion signals in MT) to explain existing data, (2) clarifies how pathways for self-motion and object-motion perception interact, and (3) unifies the existing flow parsing hypothesis with established neurophysiological mechanisms.

SIGNIFICANCE STATEMENT

To intercept targets, we must perceive the motion of objects that move independently from us as we move through the environment. Although our self-motion substantially alters the motion of objects on the retina, compelling evidence indicates that the visual system at least partially compensates for self-motion such that object motion relative to the stationary environment can be more accurately perceived. We have developed a model that sheds light on plausible mechanisms within the visual system that transform retinal motion into a world-relative reference frame. Our model reveals how local motion signals (generated through interactions within the middle temporal area) and global motion signals (feedback from the dorsal medial superior temporal area) contribute and offers a new hypothesis about the connection between pathways for heading and object motion perception.

Visual Tracking: Hot Pursuit with Tiny Eyes

Flying insect predators intercept their aerial prey with deadly precision. New research reveals that a tiny robber fly, with a brain smaller than a pinhead, achieves this using the same visual mechanism that we ourselves employ to catch a passing ball.

A Novel Interception Strategy in a Miniature Robber Fly with Extreme Visual Acuity

Our visual system allows us to rapidly identify and intercept a moving object. When this object is far away, we base the trajectory on the target’s location relative to an external frame of reference [1]. This process forms the basis for the constant bearing angle (CBA) model, a reactive strategy that ensures interception since the bearing angle, formed between the line joining pursuer and target (called the range vector) and an external reference line, is held constant [2, 3, 4]. The CBA model may be a fundamental and widespread strategy, as it is also known to explain the interception trajectories of bats and fish [5, 6]. Here, we show that the aerial attack of the tiny robber fly Holcocephala fusca is consistent with the CBA model. In addition, Holcocephala fusca displays a novel proactive strategy, termed “lock-on” phase, embedded with the later part of the flight. We found the object detection threshold for this species to be 0.13°, enabled by an extremely specialized, forward pointing fovea (∼5 ommatidia wide, interommatidial angle Δφ = 0.28°, photoreceptor acceptance angle Δρ = 0.27°). This study furthers our understanding of the accurate performance that a miniature brain can achieve in highly demanding sensorimotor tasks and suggests the presence of equivalent mechanisms for target interception across a wide range of taxa.

Video Abstract

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Holcocephala fusca robber flies capture prey with a constant bearing angle strategy

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The approach is proactively altered to “lock on” on to prey within 29 cm distance

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The retina spatial resolution is 0.28°, but the object detection threshold is 0.13°

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The fly’s stereopsis range is estimated to be ∼26 cm

The effects of obstacle proximity on aperture crossing behaviours

Affordance theory and behavioural dynamics have been used as theoretical constructs to explain how individuals interact with the environment in order to avoid obstacles. Features of obstacle distance and multiple obstacle avoidance have been discussed in unique studies, yet the interactions of these environmental features have yet to be explored. The purpose of this study was to asses the effects of obstacle distance, relative to the goal, on aperture crossing strategies. Kinematics and gaze behaviours were assessed in a cohort of female young adults (N = 24, 21.3 ± 1.4 years). Results identified that participants chose to navigate through gaps of 1.3× shoulder width or greater, regardless of obstacle distance. However, safety margin in the anterior-posterior direction was found to increase with increased obstacle distance, suggesting unique environmental affordances for each obstacle distance. Therefore, although decision making on whether to navigate through, or around, the aperture appears to be unaffected by obstacle location, specific environmental features result in unique kinematic behaviours. Such behaviours fit within, and add merit to, the tenets of both affordance theory and behavioural dynamics.

Head and Tibial Acceleration as a Function of Stride Frequency and Visual Feedback during Running

Individuals regulate the transmission of shock to the head during running at different stride frequencies although the consequences of this on head-gaze stability remain unclear. The purpose of this study was to examine if providing individuals with visual feedback of their head-gaze orientation impacts tibial and head accelerations, shock attenuation and head-gaze motion during preferred speed running at different stride frequencies. Fifteen strides from twelve recreational runners running on a treadmill at their preferred speed were collected during five stride frequencies (preferred, ±10% and ±20% of preferred) in two visual task conditions (with and without real-time visual feedback of head-gaze orientation). The main outcome measures were tibial and head peak accelerations assessed in the time and frequency domains, shock attenuation from tibia to head, and the magnitude and velocity of head-gaze motion. Decreasing stride frequency resulted in greater vertical accelerations of the tibia (p<0.01) during early stance and at the head (p<0.01) during early and late stance; however, for the impact portion the increase in head acceleration was only observed for the slowest stride frequency condition. Visual feedback resulted in reduced head acceleration magnitude (p<0.01) and integrated power spectral density in the frequency domain (p<0.01) in late stance, as well as overall of head-gaze motion (p<0.01). When running at preferred speed individuals were able to stabilize head acceleration within a wide range of stride frequencies; only at a stride frequency 20% below preferred did head acceleration increase. Furthermore, impact accelerations of the head and tibia appear to be solely a function of stride frequency as no differences were observed between feedback conditions. Increased visual task demands through head gaze feedback resulted in reductions in head accelerations in the active portion of stance and increased head-gaze stability.

Development of Visual Motion Perception for Prospective Control: Brain and Behavioral Studies in Infants

During infancy, smart perceptual mechanisms develop allowing infants to judge time-space motion dynamics more efficiently with age and locomotor experience. This emerging capacity may be vital to enable preparedness for upcoming events and to be able to navigate in a changing environment. Little is known about brain changes that support the development of prospective control and about processes, such as preterm birth, that may compromise it. As a function of perception of visual motion, this paper will describe behavioral and brain studies with young infants investigating the development of visual perception for prospective control. By means of the three visual motion paradigms of occlusion, looming, and optic flow, our research shows the importance of including behavioral data when studying the neural correlates of prospective control.

Optic flow improves adaptability of spatiotemporal characteristics during split-belt locomotor adaptation with tactile stimulation

Human locomotor adaptation requires feedback and feed-forward control processes to maintain an appropriate walking pattern. Adaptation may require the use of visual and proprioceptive input to decode altered movement dynamics and generate an appropriate response. After a person transfers from an extreme sensory environment and back, as astronauts do when they return from spaceflight, the prolonged period required for re-adaptation can pose a significant burden. In our previous paper, we showed that plantar tactile vibration during a split-belt adaptation task did not interfere with the treadmill adaptation however, larger overground transfer effects with a slower decay resulted. Such effects, in the absence of visual feedback (of motion) and perturbation of tactile feedback, are believed to be due to a higher proprioceptive gain because, in the absence of relevant external dynamic cues such as optic flow, reliance on body-based cues is enhanced during gait tasks through multisensory integration. In this study, we therefore investigated the effect of optic flow on tactile-stimulated split-belt adaptation as a paradigm to facilitate the sensorimotor adaptation process. Twenty healthy young adults, separated into two matched groups, participated in the study. All participants performed an overground walking trial followed by a split-belt treadmill adaptation protocol. The tactile group (TC) received vibratory plantar tactile stimulation only, whereas the virtual reality and tactile group (VRT) received an additional concurrent visual stimulation: a moving virtual corridor, inducing perceived self-motion. A post-treadmill overground trial was performed to determine adaptation transfer. Interlimb coordination of spatiotemporal and kinetic variables was quantified using symmetry indices and analyzed using repeated-measures ANOVA. Marked changes of step length characteristics were observed in both groups during split-belt adaptation. Stance and swing time symmetries were similar in the two groups, suggesting that temporal parameters are not modified by optic flow. However, whereas the TC group displayed significant stance time asymmetries during the post-treadmill session, such aftereffects were absent in the VRT group. The results indicated that the enhanced transfer resulting from exposure to plantar cutaneous vibration during adaptation was alleviated by optic flow information. The presence of visual self-motion information may have reduced proprioceptive gain during learning. Thus, during overground walking, the learned proprioceptive split-belt pattern is more rapidly overridden by visual input due to its increased relative gain. The results suggest that when visual stimulation is provided during adaptive training, the system acquires the novel movement dynamics while maintaining the ability to flexibly adapt to different environments.

The effects of narrow and elevated path walking on aperture crossing

The study investigated the impact that action capabilities have on identifying possibilities for action, particularly how postural threat influences the passability of apertures. To do this, the ability to maintain balance was challenged by manipulating the level of postural threat while walking. First, participants walked along a 7m path and passed through two vertical obstacles spaced 1.1-1.5×the shoulder width apart during normal walking. Next, postural threat was manipulated by having participants complete the task either walking on a narrow, ground level path or on an elevated/narrow path. Despite a decrease in walking speed as well as an increase in trunk sway in both the narrow and elevated/narrow walking conditions, the passability of apertures was only affected when the consequence of instability was greatest. In the elevated/narrow walking condition, individuals maintained a larger critical point (rotated their shoulders for larger aperture widths) compared to normal walking. However, this effect was not observed for the narrow path walking suggesting that the level of postural threat was not enough to impose similar changes to the critical point. Therefore, it appears that manipulating action capabilities by increasing postural threat does indeed influence aperture crossing behavior, however the consequence associated with instability must be high before both gait characteristics and the critical point are affected.

On-line and model-based approaches to the visual control of action

Two general approaches to the visual control of action have emerged in last few decades, known as the on-line and model-based approaches. The key difference between them is whether action is controlled by current visual information or on the basis of an internal world model. In this paper, we evaluate three hypotheses: strong on-line control, strong model-based control, and a hybrid solution that combines on-line control with weak off-line strategies. We review experimental research on the control of locomotion and manual actions, which indicates that (a) an internal world model is neither sufficient nor necessary to control action at normal levels of performance; (b) current visual information is necessary and sufficient to control action at normal levels; and (c) under certain conditions (e.g. occlusion) action is controlled by less accurate, simple strategies such as heuristics, visual-motor mappings, or spatial memory. We conclude that the strong model-based hypothesis is not sustainable. Action is normally controlled on-line when current information is available, consistent with the strong on-line control hypothesis. In exceptional circumstances, action is controlled by weak, context-specific, off-line strategies. This hybrid solution is comprehensive, parsimonious, and able to account for a variety of tasks under a range of visual conditions.

How Do Children Perceive and Act on Dynamic Affordances in Crossing Traffic-Filled Roads?

Successfully perceiving and acting on dynamic affordances is critical for children and adults to function. In our work, we look at how children cross roads as a model for understanding how they learn to perceive and act on dynamic affordances. Ten- to 14-year-old children and adults ride an interactive bicycling simulator through an immersive virtual environment where they cross intersections with continuous cross traffic. We consistently find developmental and individual differences in children's ability to tightly time their entry into the roadway relative to the lead car in the gap. Given that children do not adjust their gap choices to match their less precise timing abilities, children take more risks when crossing roads than adults. We conclude by discussing possible reasons for these developmental differences in movement timing.

Dynamic affordances in embodied interactive systems: the role of display and mode of locomotion

We investigated how the properties of interactive virtual reality systems affect user behavior in full-body embodied interactions. Our experiment compared four interactive virtual reality systems using different display types (CAVE vs. HMD) and modes of locomotion (walking vs. joystick). Participants performed a perceptual-motor coordination task, in which they had to choose among a series of opportunities to pass through a gate that cycled open and closed and then board a moving train. Mode of locomotion, but not type of display, affected how participants chose opportunities for action. Both mode of locomotion and display affected performance when participants acted on their choices. We conclude that technological properties of virtual reality system (both display and mode of locomotion) significantly affected opportunities for action available in the environment (affordances) and discuss implications for design and practical applications of immersive interactive systems.

Adjustments of speed and path when avoiding collisions with another pedestrian

When walking in open space, collision avoidance with other pedestrians is a process that successfully takes place many times. To pass another pedestrian (an interferer) walking direction, walking speed or both can be adjusted. Currently, the literature is not yet conclusive of how humans adjust these two parameters in the presence of an interferer. This impedes the development of models predicting general obstacle avoidance strategies in humans’ walking behavior. The aim of this study was to investigate the adjustments of path and speed when a pedestrian is crossing a non-reactive human interferer at different angles and speeds, and to compare the results to general model predictions. To do so, we designed an experiment where a pedestrian walked a 12 m distance to reach a goal position. The task was designed in such a way that collision with an interferer would always occur if the pedestrian would not apply a correction of movement path or speed. Results revealed a strong dependence of path and speed adjustments on crossing angle and walking speed, suggesting local planning of the collision avoidance strategy. Crossing at acute angles (i.e. 45° and 90°) seems to require more complex collision avoidance strategies involving both path and speed adjustments than crossing at obtuse angles, where only path adjustments were observed. Overall, the results were incompatible with predictions from existing models of locomotor collision avoidance. The observed initiations of both adjustments suggest a collision avoidance strategy that is temporally controlled. The present study provides a comprehensive picture of human collision avoidance strategies in walking, which can be used to evaluate and adjust existing pedestrian dynamics models, or serve as an empirical basis to develop new models.

Humans perceive object motion in world coordinates during obstacle avoidance

A fundamental question about locomotion in the presence of moving objects is whether movements are guided based upon perceived object motion in an observer-centered or world-centered reference frame. The former captures object motion relative to the moving observer and depends on both observer and object motion. The latter captures object motion relative to the stationary environment and is independent of observer motion. Subjects walked through a virtual environment (VE) viewed through a head-mounted display and indicated whether they would pass in front of or behind a moving obstacle that was on course to cross their future path. Subjects' movement through the VE was manipulated such that object motion in observer coordinates was affected while object motion in world coordinates was the same. We found that when moving observers choose routes around moving obstacles, they rely on object motion perceived in world coordinates. This entails a process, which has been called flow parsing (Rushton & Warren, 2005; Warren & Rushton, 2009a), that recovers the component of optic flow due to object motion independent of self-motion. We found that when self-motion is real and actively generated, the process by which object motion is recovered relies on both visual and nonvisual information to factor out the influence of self-motion. The remaining component contains information about object motion in world coordinates that is needed to guide locomotion.