A step and a hop on the M�ller-Lyer: illusion effects on lower-limb movements

Effect of the Visual Illusion on Stepping-Over Action and Its Association with Gaze Behavior

An adequate foot clearance height while stepping over an obstacle is important for safety in daily life. In the present study, we examined whether visual illusions affect foot clearance during a stepping-over action, and whether this is further influenced by gaze behavior. Twelve participants stepped over an obstacle placed four meters away under conditions of three different obstacle characteristics: white, horizontal, or vertical lines. We measured the participants' foot clearances during the step-over action and their gaze behavior during the approaching phase. Participants stepped significantly higher over the obstacles in the vertical lines (illusion) condition. The duration of gaze fixation on the obstacle positively correlated with increased foot clearance in the vertical condition, suggesting that the effect of the visual illusion on foot clearance was enhanced by prolonged gaze fixation. Conversely, prolonged fixation negatively correlated with foot clearance in the white (control) condition, implying that a cautious perception of an obstacle may contribute to efficient stepping-over action.

Manipulable objects can potentiate pointing and unusual grasping besides habitual grasping behaviors

People are faster to perform a precision grip when they see a cherry (i.e., a small graspable object) than to perform a power grip, and the reverse holds true when they see an apple (i.e., a large graspable object). This potentiation effect supports that object representations could include motor components that would be simulated when a graspable object is seen. However, the nature of these motor components remains unclear. The embodied account posits that seeing an object only potentiates the most frequent actions associated with it (i.e., usual actions). In contrast, the size-coding account posits that seeing an object potentiates any actions associated to spatial codes compatible with those associated to the objects. We conducted three experiments to disentangle these two alternative accounts. We especially varied the nature of the responses while participants saw either large or small graspable objects. Our results showed a potentiation effect when participants performed the usual grasping actions (Experiment 1: power and precision grip) but also when they performed unusual grasping actions (Experiment 2: grasping between the thumb and little finger) and even when they had to perform non-grasping actions (Experiment 3: pointing actions). By supporting the size-coding account, our contribution underlines the need for a better understanding of the nature of the motor components of object representations and for using a proper control condition (i.e., pointing action) before arguing that the embodied account convincingly explains the potentiation effect of grasping behaviors.

Vertical-horizontal illusory effects with gaze restrictions do not change length estimations using the lower limb

Gaze direction and use of visual feedback can affect illusory influences over perceptions and manual length size estimates of the vertical-horizontal (V-H) illusion, in which the vertical, bisecting segment of an inverted T (IT) appears longer than the horizontal, bisected segment. We questioned whether V-H illusory influences would also exist for the lower limb. Participants stepped forward in an attempt to make the toe-to-toe distance of their dominant foot equal to a short or long bisecting segment length of a vertically projected IT. Performances under three gaze conditions included: maintaining gaze on the IT intersection throughout a trial for target fixation (TF); viewing the intersection for 4 s then looking down and performing the step for movement fixation (MF); and viewing the intersection for 4 s then maintaining gaze on the remembered location of the intersection and performing the step for remembered target fixation (RTF). Variables included step displacement, peak velocity (PV), and normalized ground reaction force amplitude (GRFampN), as well as time to peak and peak amplitude of the center of pressure (COPtime and COPamp, respectively). Main effects of gaze on PV, GRFampN, COPtime, and COPamp revealed lower values for MF compared to TF and RTF, which did not exist for step displacement. No significant correlations existed between step displacement and other variables across participants. Together, we found evidence to suggest differences between movement planning and movement completion. Exploitation of deceptive visual cues can guide step planning and early step execution, but do not guide final step estimations.

The effects of Müller-Lyer illusion on toe clearance height in stair ascent

BACKGROUND

The appearance of a stair can impact the safe negotiation of stair climbing. Increasing toe clearance height is a common strategy for reducing the risk of tripping when stepping onto a raised surface.

RESEARCH QUESTION

Can the Müller-Lyer illusion cause people to increase their toe clearance height when walking over a stair?

METHODS

Indoors, 15 healthy individuals (9 women and 6 men) walked up a 3-step staircase with each step 60 cm wide, 18 cm tall, and 30.5 cm deep. Outdoors, 253 people walked up a 2-step staircase with each step 6 m wide, 15 cm tall, and 38 cm deep. Four visual conditions were displayed at both the indoor and outdoor locations: fins out, fins in, line, and control.

RESULTS

While the fins out stairs were perceived taller than all the other displayed stair conditions, the toe clearance height was not different between display conditions when tested indoors. However, toe clearance height was greater in the fins out condition when compared to all other conditions for the outdoor stairs location.

SIGNIFICANCE

In the natural environment when walkers were oblivious to themselves being filmed and not having any knowledge of the task demand, the Müller-Lyer illusion significantly impacted their toe clearance height.

Visuomotor control of leaping over a raised obstacle is sensitive to small baseline displacements

The limb kinematics used for stepping or leaping over an obstacle are determined primarily by visual sensing of obstacle position and geometry. In this study, we demonstrate that changes are induced in limb kinematics even when obstacle geometry is manipulated in a way that does not introduce a mechanical requirement for a change of limb trajectory nor increase risk of collision. Human participants performed a running leap over a single raised obstacle bar. Kinematic changes were measured when an identical second bar was introduced at a ground level underneath the obstacle and displaced by a functionally insignificant distance along the axis of travel. The presence or absence of a baseline directly beneath the highest extremity had no significant effect on limb kinematics. However, displacing the baseline horizontally induced a horizontal translation of limb trajectory in the direction of the displacement. These results show that systematic changes to limb trajectories can occur in the absence of a change in sensed mechanical constraints or optimization. The nature of visuomotor control of human leaping may involve a continuous mapping of sensory input to kinematic output rather than one responsive only to information perceived to be mechanically relevant.

The role of perception and action on the use of allocentric information in a large-scale virtual environment

In everyday life, our brain constantly builds spatial representations of the objects surrounding us. Many studies have investigated the nature of these spatial representations. It is well established that we use allocentric information in real-time and memory-guided movements. Most studies relied on small-scale and static experiments, leaving it unclear whether similar paradigms yield the same results on a larger scale using dynamic objects. We created a virtual reality task that required participants to encode the landing position of a virtual ball thrown by an avatar. Encoding differed in the nature of the task in that it was either purely perceptual (“view where the ball landed while standing still”—Experiment 1) or involved an action (“intercept the ball with the foot just before it lands”—Experiment 2). After encoding, participants were asked to place a real ball at the remembered landing position in the virtual scene. In some trials, we subtly shifted either the thrower or the midfield line on a soccer field to manipulate allocentric coding of the ball’s landing position. In both experiments, we were able to replicate classic findings from small-scale experiments and to generalize these results to different encoding tasks (perception vs. action) and response modes (reaching vs. walking-and-placing). Moreover, we found that participants preferably encoded the ball relative to the thrower when they had to intercept the ball, suggesting that the use of allocentric information is determined by the encoding task by enhancing task-relevant allocentric information. Our findings indicate that results previously obtained from memory-guided reaching are not restricted to small-scale movements, but generalize to whole-body movements in large-scale dynamic scenes.

Dynamic modulation of illusory and physical target size on separate and coordinated eye and hand movements

In everyday behavior, two of the most common visually guided actions-eye and hand movements-can be performed independently, but are often synergistically coupled. In this study, we examine whether the same visual representation is used for different stages of saccades and pointing, namely movement preparation and execution, and whether this usage is consistent between independent and naturalistic coordinated eye and hand movements. To address these questions, we used the Ponzo illusion to dissociate the perceived and physical sizes of visual targets and measured the effects on movement preparation and execution for independent and coordinated saccades and pointing. During independent movements, we demonstrated that both physically and perceptually larger targets produced faster preparation for both effectors. Furthermore, participants who showed a greater influence of the illusion on saccade preparation also showed a greater influence on pointing preparation, suggesting that a shared mechanism involved in preparation across effectors is influenced by illusions. However, only physical but not perceptual target sizes influenced saccade and pointing execution. When pointing was coordinated with saccades, we observed different dynamics: pointing no longer showed modulation from illusory size, while saccades showed illusion modulation for both preparation and execution. Interestingly, in independent and coordinated movements, the illusion modulated saccade preparation more than pointing preparation, with this effect more pronounced during coordination. These results suggest a shared mechanism, dominated by the eyes, may underlie visually guided action preparation across effectors. Furthermore, the influence of illusions on action may operate within such a mechanism, leading to dynamic interactions between action modalities based on task demands.

Three-dimensional transformations for goal-directed action

Much of the central nervous system is involved in visuomotor transformations for goal-directed gaze and reach movements. These transformations are often described in terms of stimulus location, gaze fixation, and reach endpoints, as viewed through the lens of translational geometry. Here, we argue that the intrinsic (primarily rotational) 3-D geometry of the eye-head-reach systems determines the spatial relationship between extrinsic goals and effector commands, and therefore the required transformations. This approach provides a common theoretical framework for understanding both gaze and reach control. Combined with an assessment of the behavioral, neurophysiological, imaging, and neuropsychological literature, this framework leads us to conclude that (a) the internal representation and updating of visual goals are dominated by gaze-centered mechanisms, but (b) these representations must then be transformed as a function of eye and head orientation signals into effector-specific 3-D movement commands.

The effect of the visual characteristics of obstacles on risk of tripping and gait parameters during locomotion

PURPOSE

Injuries from falls are a serious health issue. Approaches to preventing falls should consider increasing relevant visual information of an obstacle. Obstacle parameters, such as position and height, may be specified by the visible structure of an obstacle. The present study examined the relationship between visible structure of an obstacle and locomotor behaviour. This relationship may be modified as a function of experience with navigating obstacles. Since workers at construction sites must navigate through cluttered environments with varied obstacles, these workers may have superior skills at avoiding obstacles. Therefore, the effect of work experience was also examined.

METHODS

Nine construction workers and 10 age- and gender-matched control subjects participated. Subjects stepped over obstacles in an 8 m walkway. Three different obstacles were examined, arranged according to a hierarchy ranging from most to least visible structure: a solid obstacle, a three-edge outline obstacle and a top-edge obstacle. The obstacles were 10, 20 or 30 cm high. In addition, visual information was decreased with goggles which obstructed the lower visual field, removing information of the obstacle and foot-relative-to-obstacle in the two steps before the obstacle. All conditions were presented randomly.

RESULTS

Higher risk of contact and higher lead and trail toe clearance variability were observed for the top-edge obstacle. Higher risk of contact was observed when the lower visual field was obstructed and for the 30 cm obstacle. Work experience did not influence risk of contact. Construction workers had lower trail toe clearances and lower trail toe clearance variability for the 10 cm obstacle, but were not different from controls for the 30 cm obstacle.

CONCLUSIONS

Decreased visible structure of an obstacle resulted in increased gait variability and increased risk of contact. The changes are consistent with decreased accuracy of the sensory-to-motor transformation used to control the lead and trail limb during obstacle crossing when only the top-edge was visible. There is some evidence that construction workers were better able to transform the visual information to motor actions, as reflected by decreased gait variability, but these findings were not supported by decreased risk of obstacle contact.

Locomotor adaptation versus perceptual adaptation when stepping over an obstacle with a height illusion

BACKGROUND

During locomotion, vision is used to perceive environmental obstacles that could potentially threaten stability; locomotor action is then modified to avoid these obstacles. Various factors such as lighting and texture can make these environmental obstacles appear larger or smaller than their actual size. It is unclear if gait is adapted based on the actual or perceived height of these environmental obstacles. The purposes of this study were to determine if visually guided action is scaled to visual perception, and to determine if task experience influenced how action is scaled to perception.

METHODOLOGY/PRINCIPAL FINDINGS

Participants judged the height of two obstacles before and after stepping over each of them 50 times. An illusion made obstacle one appear larger than obstacle two, even though they were identical in size. The influence of task experience was examined by comparing the perception-action relationship during the first five obstacle crossings (1-5) with the last five obstacle crossings (46-50). In the first set of trials, obstacle one was perceived to be 2.0 cm larger than obstacle two and subjects stepped 2.7 cm higher over obstacle one. After walking over the obstacle 50 times, the toe elevation was not different between obstacles, but obstacle one was still perceived as 2.4 cm larger.

CONCLUSIONS/SIGNIFICANCE

There was evidence of locomotor adaptation, but no evidence of perceptual adaptation with experience. These findings add to research that demonstrates that while the motor system can be influenced by perception, it can also operate independent of perception.

Cue Reliability and a Landmark Stability Heuristic Determine Relative Weighting Between Egocentric and Allocentric Visual Information in Memory-Guided Reach

It is not known how egocentric visual information (location of a target relative to the self) and allocentric visual information (location of a target relative to external landmarks) are integrated to form reach plans. Based on behavioral data from rodents and humans we hypothesized that the degree of stability in visual landmarks would influence the relative weighting. Furthermore, based on numerous cue-combination studies we hypothesized that the reach system would act like a maximum-likelihood estimator (MLE), where the reliability of both cues determines their relative weighting. To predict how these factors might interact we developed an MLE model that weighs egocentric and allocentric information based on their respective reliabilities, and also on an additional stability heuristic. We tested the predictions of this model in 10 human subjects by manipulating landmark stability and reliability (via variable amplitude vibration of the landmarks and variable amplitude gaze shifts) in three reach-to-touch tasks: an egocentric control (reaching without landmarks), an allocentric control (reaching relative to landmarks), and a cue-conflict task (involving a subtle landmark “shift” during the memory interval). Variability from all three experiments was used to derive parameters for the MLE model, which was then used to simulate egocentric–allocentric weighting in the cue-conflict experiment. As predicted by the model, landmark vibration—despite its lack of influence on pointing variability (and thus allocentric reliability) in the control experiment—had a strong influence on egocentric–allocentric weighting. A reduced model without the stability heuristic was unable to reproduce this effect. These results suggest heuristics for extrinsic cue stability are at least as important as reliability for determining cue weighting in memory-guided reaching.

A Judd illusion in far-aiming: evidence of a contribution to action by vision for perception

The present study addresses the role of vision for perception in determining the location of a target in far-aiming. Participants (N = 12) slid a disk toward a distant target embedded in illusory Judd figures. Additionally, in a perception task, participants indicated when a moving pointer reached the midpoint of the Judd figures. The number of hits, the number of misses to the left and to the right of the target, the sliding error (in mm) and perceptual judgment error (in mm) served as dependent variables. Results showed an illusory bias in sliding, the magnitude of which was comparable to the bias in the perception of target location. The determination of target location in far-aiming is thus based on relative metrics. We argue that vision for perception sets the boundary constraints for action and that within these constraints vision for action autonomously controls movement execution, but alternative accounts are discussed as well.

Does my step look big in this? A visual illusion leads to safer stepping behaviour

Background

Tripping is a common factor in falls and a typical safety strategy to avoid tripping on steps or stairs is to increase foot clearance over the step edge. In the present study we asked whether the perceived height of a step could be increased using a visual illusion and whether this would lead to the adoption of a safer stepping strategy, in terms of greater foot clearance over the step edge. The study also addressed the controversial question of whether motor actions are dissociated from visual perception.

Methodology/Principal Findings

21 young, healthy subjects perceived the step to be higher in a configuration of the horizontal-vertical illusion compared to a reverse configuration (p = 0.01). During a simple stepping task, maximum toe elevation changed by an amount corresponding to the size of the visual illusion (p<0.001). Linear regression analyses showed highly significant associations between perceived step height and maximum toe elevation for all conditions.

Conclusions/Significance

The perceived height of a step can be manipulated using a simple visual illusion, leading to the adoption of a safer stepping strategy in terms of greater foot clearance over a step edge. In addition, the strong link found between perception of a visual illusion and visuomotor action provides additional support to the view that the original, controversial proposal by Goodale and Milner (1992) of two separate and distinct visual streams for perception and visuomotor action should be re-evaluated.

The visual control of stepping operates in real time: Evidence from a pictorial illusion

Recent studies have suggested important similarities between the visual control of upper and lower limb actions despite the different effectors, time scales, and regions of space associated with the two types of actions. Drawing on previous findings from upper limb studies, we show here that the effect of a Muller-Lyer illusion on stepping amplitude is greatly increased when the target display is occluded as compared to when it is visible during the reaction time period (i.e., brief delay and 3-s delay conditions versus full vision and open-loop conditions). The present results suggest that the control of stepping is mediated by different systems when the target is visible as compared to when it is occluded immediately before movement initiation; when target vision is not available during the reaction time period, the control of stepping is more highly dependent upon a non-veridical, perceptual representation of the target environment. These findings are consistent with the idea that, much like grasping, vision during the reaction time period is critical for the optimal control of stepping; that is, the visual control of stepping operates in real time.