Does perceptual-motor calibration generalize across two different forms of locomotion? Investigations of walking and wheelchairs

Jumping and leaping estimations using optic flow

Optic flow provides information on movement direction and speed during locomotion. Changing the relationship between optic flow and walking speed via training has been shown to influence subsequent distance and hill steepness estimations. Previous research has shown that experience with slow optic flow at a given walking speed was associated with increased effort and distance overestimation in comparison to experiencing with fast optic flow at the same walking speed. Here, we investigated whether exposure to different optic flow speeds relative to gait influences perceptions of leaping and jumping ability. Participants estimated their maximum leaping and jumping ability after exposure to either fast or moderate optic flow at the same walking speed. Those calibrated to fast optic flow estimated farther leaping and jumping abilities than those calibrated to moderate optic flow. Findings suggest that recalibration between optic flow and walking speed may specify an action boundary when calibrated or scaled to actions such as leaping, and possibly, the manipulation of optic flow speed has resulted in a change in the associated anticipated effort for walking a prescribed distance, which in turn influence one's perceived action capabilities for jumping and leaping.

Understanding mechanisms of generalization following locomotor adaptation

Our nervous system has the remarkable ability to adapt our gait to accommodate changes in our body or surroundings. However, our adapted walking patterns often generalize only partially (or not at all) between different contexts. Here, we sought to understand how the nervous system generalizes adapted gait patterns from one context to another. Through a series of split-belt treadmill walking experiments, we evaluated different mechanistic hypotheses to explain the partial generalization of adapted gait patterns from split-belt treadmill to overground walking. In support of the credit assignment hypothesis, our experiments revealed the central finding that adaptation involves recalibration of two distinct forward models. Recalibration of the first model generalizes to overground walking, suggesting that the model represents the general movement dynamics of our body. On the other hand, recalibration of the second model does not generalize to overground walking, suggesting the model represents dynamics specific to treadmill walking. These findings reveal that there is a predefined portion of forward model recalibration that generalizes across context, leading to overall partial generalization of walking adaptation.

Perceiving distance in virtual reality: theoretical insights from contemporary technologies

Decades of research have shown that absolute egocentric distance is underestimated in virtual environments (VEs) when compared with the real world. This finding has implications on the use of VEs for applications that require an accurate sense of absolute scale. Fortunately, this underperception of scale can be attenuated by several factors, making perception more similar to (but still not the same as) that of the real world. Here, we examine these factors as two categories: (i) experience inherent to the observer, and (ii) characteristics inherent to the display technology. We analyse how these factors influence the sources of information for absolute distance perception with the goal of understanding how the scale of virtual spaces is calibrated. We identify six types of cues that change with these approaches, contributing both to a theoretical understanding of depth perception in VEs and a call for future research that can benefit from changing technologies. This article is part of the theme issue 'New approaches to 3D vision'.

Visually directed action

When people throw or walk to targets in front of them without visual feedback, they often respond short. With feedback, responses rapidly become approximately accurate. To understand this, an experiment is performed with four stages. 1) The errors in blind walking and blind throwing are measured in a virtual environment in light and dark cue conditions. 2) Error feedback is introduced and the resulting learning measured. 3) Transfer to the other response is then measured. 4) Finally, responses to the perceived distances of the targets are measured. There is large initial under-responding. Feedback rapidly makes responses almost accurate. Throw training transfers completely to walking. Walk training produces a small effect on throwing. Under instructions to respond to perceived distances, under-responding recurs. The phenomena are well described by a model in which the relation between target distance and response distance is determined by a sequence of a perceptual, a cognitive, and a motor transform. Walk learning is primarily motor; throw learning is cognitive.

The effects of testing environment, experimental design, and ankle loading on calibration to perturbed optic flow during locomotion

Calibration is the process by which the execution of actions becomes scaled to the (changing) relationship between environmental features and the actor's action capabilities. Though much research has investigated how individuals calibrate to perturbed optic flow, it remains unclear how different experimental factors contribute to the magnitude of calibration transfer. In the present study, we assessed how testing environment (Experiment 1), an adapted pretest-calibration-posttest design (Experiment 2), and bilateral ankle loading (Experiment 3) affected the magnitude of calibration to perturbed optic flow. We found that calibration transferred analogously to real-world and virtual environments. Although the magnitude of calibration transfer found here was greater than that reported by previous researchers, it was evident that calibration occurred rapidly and quickly plateaued, further supporting the claim that calibration is often incomplete despite continued calibration trials. We also saw an asymmetry in calibration magnitude, which may be due to a lack of appropriate perceptual-motor scaling prior to calibration. The implications of these findings for the assessment of distance perception and calibration in real-world and virtual environments are discussed.

Calibration of perception fails to transfer between functionally similar affordances

Prior work shows that the calibration of perception and action transfers between actions depending on their functional similarity: Practising (and thus calibrating perception of) one affordance will also calibrate perception for an affordance with a similar function but not for an affordance with a disparate function. We tested this hypothesis by measuring whether calibration transferred between two affordances for passing through openings: squeezing sideways through doorways without becoming stuck and fitting sideways through doorways while avoiding collision. Participants wore a backpack to alter affordances for passage and create a need for perceptual recalibration. Calibration failed to transfer between the two actions (e.g., practising squeezing through doorways calibrated perception of squeezing but not fitting). Differences between squeezing and fitting affordances that might have required different information for perception and recalibration are explored to understand why calibration did not transfer. In light of these results, we propose a revised hypothesis-calibration transfers between affordances on the basis of both functional and informational similarity.

Regular physical activity modulates perceived visual speed when running in treadmill-mediated virtual environments

In virtual reality, visual speed is usually underestimated relative to locomotor speed. Here we investigated how physical activity and fitness affect perceived visual speed when running in a treadmill-mediated virtual environment. Thirty healthy participants (ten sedentary individuals, ten team sport players and ten expert runners) ran on a treadmill at two different speeds (8, 12km/h) in front of a moving virtual scene. Participants were asked to match the speed of the visual scene to their running speed (i.e. treadmill speed), indicating for each trial whether the scene was moving slower or faster than the treadmill. The speed of the visual scene was adjusted according to the participant’s response using a staircase until visual and running speeds were perceived as equivalent. More sedentary participants underestimated visual speed relative to their actual running speed. Specifically, visual speed had to exceed running speed to be perceived as equivalent. The underestimation of visual speed was speed-dependent, and it was significantly larger for sedentary participants than for team sports players and expert runners. The volume of physical activity per week was found to be the best predictor of visual speed perception for both running speeds, while the perceived effort constituted a good predictor only at 8km/h. Physical fitness, on the other hand turned out to be a poor predictor of visual speed perception. Therefore, in order to enhance users’ engagement and their adherence to physical activity programs, the development of “personalized” treadmill-mediated virtual environments should take into account users’ personal characteristics to provide the most natural and engaging feedback possible.

Locomotive Recalibration and Prism Adaptation of Children and Teens in Immersive Virtual Environments

As virtual reality expands in popularity, an increasingly diverse audience is gaining exposure to immersive virtual environments (IVEs). A significant body of research has demonstrated how perception and action work in such environments, but most of this work has been done studying adults. Less is known about how physical and cognitive development affect perception and action in IVEs, particularly as applied to preteen and teenage children. Accordingly, in the current study we assess how preteens (children aged 8-12 years) and teenagers (children aged 15-18 years) respond to mismatches between their motor behavior and the visual information presented by an IVE. Over two experiments, we evaluate how these individuals recalibrate their actions across functionally distinct systems of movement. The first experiment analyzed forward walking recalibration after exposure to an IVE with either increased or decreased visual flow. Visual flow during normal bipedal locomotion was manipulated to be either twice or half as fast as the physical gait. The second experiment leveraged a prism throwing adaptation paradigm to test the effect of recalibration on throwing movement. In the first experiment, our results show no differences across age groups, although subjects generally experienced a post-exposure effect of shortened distance estimation after experiencing visually faster flow and longer distance estimation after experiencing visually slower flow. In the second experiment, subjects generally showed the typical prism adaptation behavior of a throwing after-effect error. The error lasted longer for preteens than older children. Our results have implications for the design of virtual systems with children as a target audience.

Action ability modulates time-to-collision judgments

Time-to-collision (TTC) underestimation has been interpreted as an adaptive response that allows observers to have more time to engage in a defensive behaviour. This bias seems, therefore, strongly linked to action preparation. There is evidence that the observer’s physical fitness modulates the underestimation effect so that people who need more time to react (i.e. those with less physical fitness) show a stronger underestimation effect. Here we investigated whether this bias is influenced by the momentary action capability of the observers. In the first experiment, participants estimated the time-to-collision of threatening or non-threatening stimuli while being mildly immobilized (with a chin rest) or while standing freely. Having reduced the possibility of movement led participants to show more underestimation of the approaching stimuli. However, this effect was not stronger for threatening relative to non-threatening stimuli. The effect of the action capability found in the first experiment could be interpreted as an expansion of peripersonal space (PPS). In the second experiment, we thus investigated the generality of this effect using an established paradigm to measure the size of peripersonal space. Participants bisected lines from different distances while in the chin rest or standing freely. The results replicated the classic left-to-right gradient in lateral spatial attention with increasing viewing distance, but no effect of immobilization was found. The manipulation of the momentary action capability of the observers influenced the participants’ performance in the TTC task but not in the line bisection task. These results are discussed in relation to the different functions of PPS.

Testing the mechanisms underlying improved distance judgments in virtual environments

Virtual environments (VEs) presented via head-mounted displays are typically perceived as smaller in scale than intended. Visual-motor experience in VEs can reduce this underestimation of distance, though the mechanisms underlying this improved accuracy of distance estimates are unknown. To address this question, we created a mismatch between biomechanical and visual indicators of self-movement within the VE, and assessed the effect on distance and size judgments. Our results suggest that visual-motor feedback influences subsequent distance judgments by recalibrating perceptual-motor relationships, but we found no evidence that perceived size, which was substantially underestimated, changed as a function of this feedback. In contrast to recent studies that suggest that feedback in VEs causes a broad rescaling ofvirtual space, our results are consistent with a visual-motor recalibration account for much of the improvement in distance judgments following VE experience.