Are avatars treated like human obstacles during aperture crossing in virtual environments?

RESEARCH OBJECTIVE: The current study set out to determine whether individuals walking in a virtual reality environment pass through apertures made of two avatars differently than apertures created by two pole obstacles, as previously observed between pole and human obstacles in real-world environments. METHODS: Eleven healthy young adults wore a head-mounted virtual reality display, walked along a 10 m path and passed through a virtual aperture located 5 m from the starting location. Participants were instructed to avoid colliding with the obstacles when passing through the aperture. The experiment was conducted in a block design, where the aperture was either created by two pole obstacles or by two avatars. In both conditions, the width of the aperture ranged between 1.0-1.8x each participant's shoulder width. RESULTS: Regardless of whether the aperture was created by the virtual poles or the avatars, participants rotated their shoulders for all aperture sizes and results found no significant differences in shoulder rotation angle, onset of rotation, walking speed or velocity at time of crossing between the two types of obstacles. Therefore, it appears that the differences in avoidance behaviours observed in real-world settings between people and pole obstacles is not translated to a virtual reality environment. SIGNIFICANCE: It is possible that during experiments in which the avatars do not move, they do not possess human-like qualities suggested to be responsible for the increased caution used when walking through real human obstacles and instead, are treated as any ordinary obstacle.

Action strategies for walking through multiple, misaligned apertures.

When avoiding obstacles, path selection is thought to be determined by the attraction of the end-goal. However for aperture crossing, it is unclear whether the attraction point originates in the center of the aperture or at the end-goal, as previous experiments align the aperture with the end-goal. The purpose of the current study was to decipher the possible location of the attraction point, by evaluating crossing behaviour for multiple, misaligned apertures. Participants were instructed to walk through three separate apertures while en route to an end-goal. The first and last apertures were fixed such that they were both either 0.9× or 1.7× shoulder width (SW) while the second aperture was either 0.9, 1.3 or 1.7× SW and shifted 25, 50 or 75cm off the midline. Findings revealed that the attraction of the end-goal, and not the middle of the aperture, guided crossing behaviour. The spatial margin decreased as the size of the shift increased. Furthermore, the frequency of rotation increased as the aperture was shifted away from midline, regardless of the aperture size. Since rotations would not normally occur for all of these aperture sizes when aligned with the end-goal, these results suggest that rotations were produced in an attempt to keep one's trajectory as close to the midline as possible. Therefore, not only does the attraction of the goal guide path trajectory, but individuals will choose to reduce the spatial margin and rotate the shoulders when walking through misaligned apertures, likely in attempt to maintain the straightest possible path.

Does the passability of apertures change when walking through human versus pole obstacles?

The current study set out to evaluate how individuals walk through apertures created by different stationary obstacles. Specifically, we examined whether the passability of apertures differed between human and pole obstacles by quantifying aperture crossing behaviors such as the critical point. Participants walked an 8m path toward a visible goal located at the end. Two obstacles were positioned 5m from the starting location and participants were instructed to pass between the obstacles without hitting them. The distance between the obstacles ranged between 1.0 and 1.8× the participant's shoulder width. Results revealed that, when the obstacles were humans, individuals rotated their shoulders more frequently at larger apertures, as evidenced by a larger critical point (1.7 vs 1.3 for poles), initiated shoulder rotations earlier, rotated to a larger degree, left a wider clearance between their shoulders and the obstacles at the time of crossing, and walked slower when approaching and passing through the obstacles compared to when the obstacles were poles. Furthermore, correlational analyses revealed that the amount of change between an individual's critical point for the poles and the critical point for the human obstacles was related to social risk-taking and changes in walking speed. Therefore, it appears that the passability of apertures changes when walking between two people versus two objects such that more space and greater caution are needed for human obstacles. It is possible that the greater caution observed for human obstacles is to account for the personal space needs of others that do not exist in the same extent for poles and that the degree of caution is related to social factors.

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.

The effects of specific athletic training on path selection while running.

Apertures that are smaller than 1.3 times the shoulder width (SW) require that individuals make an adjustment to their normal walking behavior [6]. When given a choice, individuals will choose to avoid apertures smaller than this ratio, rather than rotate their shoulders and walk through [7]. Research has yet to determine whether this choice in path selection can be influenced by the speed at which one approaches the aperture or by experience/training. Therefore, the current study investigated whether approach speed and/or specific athletic training influences the choice in path selection. Specifically-trained athletes (n=6) and non-trained (n=6) young adults ran toward a visible goal placed at the end of the path and avoided an aperture (created by two poles) placed along the midline of the path. The separation between the poles ranged between 0.6 and 1.8 times each participant's SW, in increments of 0.2. Participants were permitted to either run through or around the aperture to get to the end goal. Results demonstrated that regardless of training experience, participants ran around apertures smaller than 1.4× the SW and ran through apertures larger than this ratio. Increased approach speed (i.e., running) therefore appears to elicit similar aperture crossing behaviors as walking [2,3,6,7]. Additionally, when faced with the choice to run around or to run through apertures, individuals who are specifically-training to run through small spaces chose similar paths as individuals who are not trained to do so. Therefore, specific training does not appear to influence voluntary path selection.

Is the critical point for aperture crossing adapted to the person-plus-object system?

When passing through apertures, individuals scale their actions to their shoulder width and rotate their shoulders or avoid apertures that are deemed too small for straight passage. Carrying objects wider than the body produces a person-plus-object system that individuals must account for in order to pass through apertures safely. The present study aimed to determine whether individuals scale their critical point to the widest horizontal dimension (shoulder or object width). Two responses emerged: Fast adapters adapted to the person-plus-object system by maintaining a consistent critical point regardless of whether the object was carried while slow adapters initially increased their critical point (overestimated) before adapting back to their original critical point. The results suggest that individuals can account for increases in body width by scaling actions to the size of the object width but people adapt at different rates.

Direction of single obstacle circumvention in middle-aged children.

When required to walk around a stationary object, adults use the location of the goal to set up their locomotor axis and obstacles presented along the locomotor axis will repel the individual towards the side that affords more space [1]. Research has yet to examine whether children can identify the locomotor axis and choose their paths accordingly. Therefore, the current study examined the factors that influence the direction in which children choose to deviate around a single obstacle and whether the presence or absence of a goal influences path selection and trajectory. Ten children (age: 7.1 years±0.8) walked along a 9 m path and avoided a single obstacle that was located in one of three locations (midline, 15 cm to the right or 15 cm to the left). On half the trials, an end-goal was visible from the start of the path while the other half of the trials had no visible goal. The results demonstrate that: (1) children are able to perceive and move towards more open space but are more variable when the end-goal is not visible; (2) children are capable of maintaining an elliptical-shaped protective envelope when avoiding a single obstacle regardless of whether or not the locomotor axis is established; and (3) although children are capable of choosing paths that afford the most space, the manner in which they arrive at their goal is not driven by factors similar to adults.

Action strategies used by children to avoid two vertical obstacles in non-confined space.

Information used to determine the action strategies necessary to successfully pass through apertures is based on the dimensions of the individual and the mover's action capabilities (Warren in J Exp Psychol 10:683-703, 1984; Warren and Whang in J Exp Psychol 13:371-383, 1987). Previous research has demonstrated that when children must pass through small spaces, they will produce a shoulder rotation at apertures 1.6 times their shoulder width and smaller (i.e., critical point) and their avoidance strategies are based more on dynamic than geometric measures (Snapp-Childs and Bingham in Exp Brain Res 198:527-533, 2009; Wilmut and Barnett in Exp Brain Res 210:185-194, 2011). The question remains as to whether similar strategies exist when children are given a choice in their obstacle avoidance strategy. The current study aimed to determine the action strategies employed by children when confronted with a non-confined obstacle avoidance task. Specifically, the study intended to identify the aperture width that elicited a change in action (e.g., a shoulder rotation or a change in travel path). Children (N = 12, mean age = 7.1 years, ±0.2) were instructed to walk along a 10-m path toward a visible goal located at the end of the pathway and avoid colliding with the two vertical obstacles placed halfway (5 m) down the path on either side of the midline. The space between the obstacles ranged between 0.6 and 1.8 times the participant's shoulder width (presented in increments of 0.2). Results revealed that when the aperture was too small for straight passage, children choose to circumvent the obstacles rather than rotate their shoulders the majority of the time. However, unlike young and older adults (Hackney and Cinelli in Gait Posture 37:93-97, 2013a, Exp Brain Res 225:419-429, 2013b), this strategy was not used consistently. Instead, changes in travel path were highly variable both across participants and within trials. This variability suggests that a true critical point cannot be established for children in this non-confined task. Variable actions at the time of crossing were significantly correlated with the medial-lateral center of mass variability during the approach to the obstacles. These results further support the idea that children's actions may be more affected by dynamic factors than geometric measures.

Action strategies of individuals during aperture crossing in nonconfined space.

Walking through cluttered environments is a requirement of everyday locomotion, and individuals will walk toward open space and adjust their actions in order to prevent injury. When walking in a confined space, individuals require a shoulder rotation to pass through apertures smaller than 1.3 times their shoulder widths. The current study aimed to identify the action strategies employed by young adults to avoid contacting two obstacles placed in the travel path when walking in a nonconfined space. Participants (N = 12) walked along a 10-m path towards a goal while avoiding two vertical obstacles specifically placed to create an aperture (of 0.6 to 1.8 times the participants' shoulder widths) on opposite sides of the travel path midline. Results showed that participants walked around obstacles that were separated by less than 1.4 times their shoulder width (i.e., critical point). When participants deviated from their initial travel path, they did so by maintaining a consistent protective zone, regardless of the aperture width. The protective zone had dimensions of 3.80 m in the plane of progression and of 0.30 m between themselves and the obstacles at the time of crossing. This study demonstrates that individuals use body-scaled information to control actions in nonconfined space similar to that used in confined space.

Young and older adults use body-scaled information during a non-confined aperture crossing task.

Young and older adults demonstrate differences in action when passing through confined spaces (Warren and Whang in J Exp Psychol Hum Percept Perform 13:371-383, 1987; Hackney and Cinelli in Gait Posture 33:733-736, 2011). However, it is unknown whether or not these differences in actions exist during non-confined multiple obstacle avoidance tasks. The current study aimed to determine: (1) the differences in actions between young and older adults when given a choice in path selection and (2) establish the variables that may account for these differences in action. Older adults (N = 12) and young adults (N = 12) walked along a 10-m path towards a goal and avoided two vertical poles placed halfway down the path on either side of the midline (ranging between 0.6 and 1.8× shoulder width). Results revealed that in non-confined space, both age groups use body-scaled information to determine the passability of apertures and maintain similar Critical Points to those reported in confined aperture crossing (1.4 for young adults and 1.6 for older adults). Variability of the medial-lateral centre of mass movement (i.e. how much the trunk moved side to side) between the groups most likely accounted for the larger aperture sizes (i.e. Critical Points) required by the older adults to pass through the apertures. Therefore, it appears that body-scaled information may include an individual's knowledge of both actual body size and body sway magnitude.

Older adults are guided by their dynamic perceptions during aperture crossing.

Perceptions guide actions and these actions will affect perceptions (Gibson [1]). In return, these new perceptions will affect subsequent actions. The current study aimed to determine if the action differences previously observed in young and older adults are due to differences in perception and whether perceptual judgments guide action. Young (n=10) and older adults (n=9) completed two tasks; (1) judge the passability of various sized apertures during static and dynamic conditions and (2) physically pass through similar aperture sizes. The perceptual tasks required participants to give a yes/no response as to whether they could pass through an aperture (0.9-1.8 times SW (SW)) without rotating their shoulders from a distance of 5m from the aperture. During the passage through the aperture, the participants approached the aperture (1-1.8 times the SW) along a10m path at a self-selected pace and passed through the aperture using a suitable method. Results from the aperture crossing confirmed that older adults produce shoulder rotations at larger relative aperture widths than young adults and are more variable in their shoulder rotations at each aperture width. Perceptual results indicated that older adults had similar static but different dynamic perceptions than the young adults. The observed age-related differences in dynamic perceptions were most likely the result of differences in dynamic balance control.

Action strategies of older adults walking through apertures.

The current study aimed to determine if the action strategies (i.e. Critical Point) of older adults when walking through apertures are different from those previously reported in young adults. Older adults (N=9) walked at a self selected pace along a 6-m path passing through a static door aperture (45-80 cm in 5 cm increments). Results showed that older adults rotate their shoulders more at apertures less than 1.6 times their shoulder width, indicating a more cautious approach. However, rotation magnitudes were highly variable at all aperture widths, but was not related to differences in stability. Therefore it appears that older adults use a body-scaled locomotor control strategy when passing through apertures.