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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.

The aim of this study was to examine how visual information is used to control stepping during locomotion over terrain that demands precision in the placement of the feet. More specifically, we sought to determine the point in the gait cycle at which visual information about a target is no longer needed to guide accurate foot placement. Subjects walked along a path while stepping as accurately as possible on a series of small, irregularly spaced target footholds. In various conditions, each of the targets became invisible either during the step to the target or during the step to the previous target. We found that making targets invisible after toe off of the step to the target had little to no effect on stepping accuracy. However, when targets disappeared during the step to the previous target, foot placement became less accurate and more variable. The findings suggest that visual information about a target is used prior to initiation of the step to that target but is not needed to continuously guide the foot throughout the swing phase. We propose that this style of control is rooted in the biomechanics of walking, which facilitates an energetically efficient strategy in which visual information is primarily used to initialize the mechanical state of the body leading into a ballistic movement toward the target foothold. Taken together with previous studies, the findings suggest the availability of visual information about the terrain near a particular step is most essential during the latter half of the preceding step, which constitutes a critical control phase in the bipedal gait cycle. Language: en

Studying human behavior in the natural context of everyday visual tasks--including locomotor tasks such as driving--can reveal visual strategies or even suggest underlying visual mechanisms. This paper reviews empirical and theoretical work in the past 20 years (1994-2014) on the visual control of steering a vehicle along a winding path-one of the most comprehensively studied forms of visually guided locomotion in humans. The focus is on on-road studies of visual behavior and what they can reveal about the visual strategies in curve driving. Theoretical models and results from simulator studies are discussed where they have direct relevance to the interpretation of on-road data. For the past 20 years, the point of departure in studies of curve driving has been tangent point orientation, and tangent point models (models based on tracking the tangent point) have become established as the default account of how vision is used in curve negotiation. More recent studies have questioned the generality of the tangent point hypothesis, however, arguing that in addition to (or instead of) the tangent point, drivers target visual reference points on their future path. Ecological validity of real-world studies often comes at the cost of methodological challenges that make the data difficult to interpret in terms of underlying mechanisms, and the limitations of existing data and the complementary roles of real-world and laboratory studies are discussed.

When people walk together in groups or crowds they must coordinate their walking speed and direction with their neighbors. This paper investigates how a pedestrian visually controls speed when following a leader on a straight path (one-dimensional following). To model the behavioral dynamics of following, participants in Experiment 1 walked behind a confederate who randomly increased or decreased his walking speed. The data were used to test six models of speed control that used the leader's speed, distance, or combinations of both to regulate the follower's acceleration. To test the optical information used to control speed, participants in Experiment 2 walked behind a virtual moving pole, whose visual angle and binocular disparity were independently manipulated. The results indicate the followers match the speed of the leader, and do so using a visual control law that primarily nulls the leader's optical expansion (change in visual angle), with little influence of change in disparity. This finding has direct applications to understanding the coordination among neighbors in human crowds.

The visual strategies for the control of steering toward a goal include aligning one's instantaneous direction of travel (i.e., heading; J. J. Gibson, 1950) or the future path (J. P. Wann & D. K. Swapp, 2000) specified by optic flow with the target, equating the time to closure of the target-heading angle with the time to passage of the target (tau equalization, B. Fajen, 2001), or using the target egocentric direction and steering to center the target in the straight ahead or cancel the target optical drift (S. K. Rushton, J. M. Harris, M. Lloyd, & J. P. Wann, 1998). Supporting evidences for the use of these strategies in guiding steering or walking toward a goal were reported, but no consensus has been reached. In this study, by presenting participants with displays in which target egocentric direction was fixed and thus unavailable for steering to force participants to rely on information from optic flow for the control of self-motion, we systematically examined the use of the optic flow-based strategies in the visual control of steering toward a goal. We found that participants steered to align their heading with the target, supporting the use of the heading strategy. We found no evidence to support the use of the path or the tau-equalization strategy in the visual control of steering toward a goal.

The normal human gait cycle is to a significant degree driven by the passive physical forces acting on the structure of the body, but must be adjusted on the basis of visual information when walking over complex terrain. Visual information about the layout of flat terrain features is picked up two steps ahead in order to enact adjustments that secure foot placement on the basis of passive physical forces, which suggests that the use of visual information is constrained by the biomechanics of walking. When the terrain also contains raised obstacles, foot placement must be controlled with respect to an obstacle's location ahead of time, but foot elevation, which depends on obstacle size, requires additional muscle activation in the lower limbs throughout the crossing steps. As such, information about an obstacle's size may be needed at a different time than information about obstacle location. In the present study, we tested this prediction by instructing subjects to step over obstacles in a virtual environment viewed through a head-mounted display and manipulating the availability of visual information about the obstacle's size. The obstacle's location was always visible, but its size was not apparent until subjects reached a visibility window located at different points during approach. The size of the obstacle and timing of the visibility window varied between trials. Performance measures and gait parameters from an initial experiment suggest that information about an obstacle's size is not necessary until just prior to obstacle crossing. As such, information about obstacle size may be picked up later than information about obstacle location in order to raise the feet on the basis of active adjustments to the gait cycle during crossing. We discuss how these findings support the broader theoretical claim that the use of visual information is constrained by the biomechanics of walking. Meeting abstract presented at VSS 2015. Language: en

Successful locomotion over complex terrain such as a rocky trail requires walkers to place each step on a safe foothold with high spatiotemporal precision while also taking the future terrain into account. We have previously shown that humans guide foot placement using a visual control strategy that exploits the physical dynamics of the bipedal gait cycle (e.g. Matthis, Barton & Fajen, 2014). However, this work was limited to locomotion over virtual terrain projected onto flat surfaces within the laboratory. In the current project, we develop a research protocol to allow accurate and integrated tracking of the eye, head, and feet during locomotion over real-world complex and rocky terrain, a protocol that has been previously limited by numerous technical challenges. This protocol allows us to test predictions derived from the results of previous lab-based studies on the visual control of foot placement in real-world terrains, and opens new opportunities to study the visual control of locomotion outside a laboratory setting. Our protocol utilizes a Positive Science mobile eye tracker with a GoPro scene camera and inertial measurement units (IMUs) attached to the head, trunk, and feet. Each IMU integrates the output of temperature-calibrated, tri-axial accelerometers, gyroscopes, and magnetometers to determine the sensor's orientation within a world-centered reference frame. The orientation quaternions produced by the head and trunk-mounted IMUs were used to rotate the gaze vectors recorded by the eye tracker in order to specify gaze fixations within a body-centered reference frame and relate gaze location to foot placement. In a pilot study, subjects walked across different types of real-world terrain while performing different distraction tasks. Preliminary results reveal that foot placement in complex terrain is highly demanding of attentional resources, and that the control strategies used to guide foot placement vary dramatically according to the difficulty of the terrain being traversed. Meeting abstract presented at VSS 2015. Language: en

1) Motivation When a person walks over complex terrain such as a rocky trail, obstacles and other impediments may render desirable foothold locations unavailable, so visual information from the upcoming terrain must be utilized in order to select safe footholds from among the available options. Our goal is to understand the way in which humans use visual information to guide foothold selection and foot placement when walking over complex terrain.

Most reaching arm movements have amplitude and direction constraints. Here we investigated the interdependence of these movement parameters in terms of visual control. To do so, we asked human adults to look and reach toward targets such that, in a first experiment, both movement amplitude and direction had to be controlled. Randomly, hand visual feedback was shifted near arm movement onset to influence movement direction, movement amplitude or both. Because the visual shifts occurred during ocular saccades, they were not consciously perceived. The rapid reaching movements (mean duration = 334 ms) were slightly influenced by the visual shifts (approximately 15% and 8% of visual adjustment for movement direction and amplitude, respectively). Moreover, directional adjustments varied according to amplitude adjustments (and vice-versa). We thus examined, in a second experiment, the effect of relaxing the requirement to control movement amplitude. Asking participants to control only movement direction led to substantial directional adjustments (49%) based on shifted hand visual feedback. Overall, these findings indicate that the control of movement amplitude constrains the online adjustments of movement direction and that the mechanisms controlling movement amplitude and direction are not independent.

Eye-movements have long been considered a problem when trying to understand the visual control of locomotion. They transform the retinal image from a simple expanding pattern of moving texture elements (pure optic flow), into a complex combination of translation and rotation components (retinal flow). In this article we investigate whether there are measurable advantages to having an active free gaze, over a static gaze or tracking gaze, when steering along a winding path. We also examine patterns of free gaze behavior to determine preferred gaze strategies during active locomotion. Participants were asked to steer along a computer-simulated textured roadway with free gaze, fixed gaze, or gaze tracking the center of the roadway. Deviation of position from the center of the road was recorded along with their point of gaze. It was found that visually tracking the middle of the road produced smaller steering errors than for fixed gaze. Participants performed best at the steering task when allowed to sample naturally from the road ahead with free gaze. There was some variation in the gaze strategies used, but sampling was predominantly of areas proximal to the center of the road. These results diverge from traditional models of flow analysis.

The visually guided interception of a moving target is a fundamental visuomotor task that humans can do with ease. But how humans carry out this task is still unclear despite numerous empirical investigations. Measurements of angular variables during human interception have suggested three possible strategies: the pursuit strategy, the constant bearing angle strategy, and the constant target-heading strategy. Here, we review previous experimental paradigms and show that some of them do not allow one to distinguish among the three strategies. Based on this analysis, we devised a virtual driving task that allows investigating which of the three strategies best describes human interception. Crucially, we measured participants' steering, head, and gaze directions over time for three different target velocities. Subjects initially aligned head and gaze in the direction of the car's heading. When the target appeared, subjects centered their gaze on the target, pointed their head slightly off the heading direction toward the target, and maintained an approximately constant target-heading angle, whose magnitude varied across participants, while the target's bearing angle continuously changed. With a second condition, in which the target was partially occluded, we investigated several alternative hypotheses about participants' visual strategies. Overall, the results suggest that interceptive steering is best described by the constant target-heading strategy and that gaze and head are coordinated to continuously acquire visual information to achieve successful interception.