Human locomotion through natural environments requires precise coordination between the biomechan... more Human locomotion through natural environments requires precise coordination between the biomechanics of the bipedal gait cycle and the eye movements that gather the information needed to guide foot placement. However, little is known about how the visual and locomotor systems work together to support movement through the world. We developed a system to simultaneously record gaze and full-body kinematics during locomotion over different outdoor terrains. We found that not only do walkers tune their gaze behavior to the specific information needed to traverse paths of varying complexity but that they do so while maintaining a constant temporal look-ahead window across all terrains. This strategy allows walkers to use gaze to tailor their energetically optimal preferred gait cycle to the upcoming path in order to balance between the drive to move efficiently and the need to place the feet in stable locations. Eye movements and locomotion are intimately linked in a way that reflects the integration of energetic costs, environmental uncertainty, and momentary informational demands of the locomotor task. Thus, the relationship between gaze and gait reveals the structure of the sensorimotor decisions that support successful performance in the face of the varying demands of the natural world.
To walk efficiently over complex terrain, humans must use vision to tailor their gait to the upco... more To walk efficiently over complex terrain, humans must use vision to tailor their gait to the upcoming ground surface without interfering with the exploitation of passive mechanical forces. We propose that walkers use visual information to initialize the mechanical state of the body before the beginning of each step so the resulting ballistic trajectory of the walker's center-of-mass will facilitate stepping on target footholds. Using a precision stepping task and synchronizing target visibility to the gait cycle, we empirically validated two predictions derived from this strategy: (1) Walkers must have information about upcoming footholds during the second half of the preceding step, and (2) foot placement is guided by information about the position of the target foothold relative to the preceding base of support. We conclude that active and passive modes of control work synergis-tically to allow walkers to negotiate complex terrain with efficiency, stability, and precision. human locomotion | visual control | foot placement | biomechanics | inverted pendulum H umans and other animals are remarkable in their ability to take advantage of what is freely available in the environment to the benefit of efficiency, stability, and coordination in movement. This opportunism can take on at least two forms, both of which are evident in human locomotion over complex terrain: (i) harnessing external forces to minimize the need for self-generated (i.e., muscular) forces (1), and (ii) taking advantage of passive stability to simplify the control of a complex movement (e.g., ref. 2). In the ensuing section, we explain how walkers exploit external forces and passive stability while walking over flat, obstacle-free terrain.* We then generalize this account to walking over irregular surfaces by explaining how walkers can adapt gait to terrain variations while still reaping the benefits of the available mechanical forces and inherent stability. This account leads to hypotheses about how and when walkers use visual information about the upcoming terrain and where that information is found. We derive several predictions from these hypotheses and then put them to the test in three experiments. Passive Control in Human Walking The basic movement pattern of the human gait cycle arises primarily from the phasic activation of flexor and extensor muscle groups by spinal-level central pattern generators, regulated by sensory signals from lower limb proprioceptors and cutaneous feedback from the plantar surface of the foot. This low-level neuromuscular circuitry serves to maintain the rhythmic physical oscillations that define locomotor behavior (see ref. 3 for review). This section will provide an overview of the basic biomechanics of the bipedal gait cycle to show how these inherent physical dynamics contribute to the passive stability and energetic efficiency of human locomotion. During the single support phase of the bipedal gait cycle, when only one foot is in contact with the ground, a walker shares the physical dynamics of an inverted pendulum. The body's center of mass (COM) acts as the bob of the pendulum and is supported above a single point of rotation in the planted foot (4, 5). At the onset of the single support phase—that is, at " toe off, " when the nonsupporting leg breaks contact with ground—momentum carries the COM up to a maximum height at midstance and then down again until the swinging foot contacts the ground, which is called " heel strike " (Fig. 1A). As the COM increases its height during the first half of the single support phase, some of the kinetic energy of the COM is transferred into potential energy, which is then transferred back into kinetic energy as the COM drops down to its original height in the latter half of the step. In the ideal case, the exchange between kinetic and potential energy is perfectly lossless and symmetric , and human walking closely approximates an idealized inverted pendulum acting conservatively. As such, humans can harness gravity and inertia and exploit the inverted pendulum dynamics of their bodies to traverse the distance traveled during the single support phase at a minimal cost in terms of work and muscle force (6, 7). Of course, in reality walking does incur costs. A primary determinant of the metabolic cost of walking is restoration of the energy lost when the swinging foot contacts the ground (refs. 8 and 9, but see ref. 10). Ground contact by the swinging foot marks the end of the single-support phase and the beginning of the double-support phase, during which the COM must be redirected from a downward trajectory to the upward trajectory it will need for the coming step (11). Upon ground contact, the force applied by the leading leg has a horizontal component opposite to the direction of motion of the COM; that is, the leading leg performs negative work on the COM. This collision is Significance The physical dynamics of the body are central to the generation and maintenance of the human gait cycle. The ability to exploit the force of gravity and bodily inertia increases the energetic efficiency of locomotion by minimizing the need for internally generated muscular forces and simplifies control by obviating the need to actively guide each body segment. Here we explore how these principles generalize to situations in which foot placement is constrained, as when walking over a rocky trail. Walkers can exploit external forces to efficiently traverse extended stretches of complex terrain provided that visual information about the up-coming ground surface is available during a particular (critical) phase of the gait cycle between midstance of the preceding step and toe-off. *The forthcoming description of the human gait cycle is an adaptation of the " simplest walking model, " which is an attempt to develop the most parsimonious description of bipedal walking that still captures key characteristics of human locomotion (13, 71, 72). Although more complete musculoskeletal models can be invaluable for determining the contributions of individual muscles to locomotor behavior (63), such models are compu-tationally expensive and conceptually difficult to analyze. By abstracting the highly complex action of human locomotion down to the simplest possible biomechanical model, it is possible to explore how the control of human locomotion is organized around the underlying physical dynamics of bipedal gait (16, 73–75).
A fundamental question about locomotion in the presence of moving objects is whether movements ar... more 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.
Many locomotor tasks involve interactions with moving objects. When observer (i.e., self-)motion ... more Many locomotor tasks involve interactions with moving objects. When observer (i.e., self-)motion is accompanied by object motion, the optic flow field includes a component due to self-motion and a component due to object motion. For moving observers to perceive the movement of other objects relative to the stationary environment, the visual system could recover the object-motion component – that is, it could factor out the influence of self-motion. In principle, this could be achieved using visual self-motion information, non-visual self-motion information, or a combination of both. In this study, we report evidence that visual information about the speed (Experiment 1) and direction (Experiment 2) of self-motion plays a role in recovering the object-motion component even when non-visual self-motion information is also available. However, the magnitude of the effect was less than one would expect if subjects relied entirely on visual self-motion information. Taken together with previous studies, we conclude that when self-motion is real and actively generated, both visual and non-visual self-motion information contribute to the perception of object motion. We also consider the possible role of this process in visually guided interception and avoidance of moving objects.
The aim of this study was to investigate the perception of possibilities for action (i.e., afford... more The aim of this study was to investigate the perception of possibilities for action (i.e., affordances) that depend on one's movement capabilities, and more specifically, the passability of a shrinking gap between converging obstacles. We introduce a new optical invariant that specifies in intrinsic units the minimum locomotor speed needed to safely pass through a shrinking gap. Detecting this information during self-motion requires recovering the component of the obstacles' local optical expansion attributable to obstacle motion, independent of self-motion. In principle, recovering the obstacle motion component could involve either visual or non-visual self-motion information. We investigated the visual and non-visual contributions in two experiments in which subjects walked through a virtual environment and made judgments about whether it was possible to pass through a shrinking gap. On a small percentage of trials, visual and non-visual self-motion information were independently manipulated by varying the speed with which subjects moved through the virtual environment. Comparisons of judgments on such catch trials with judgments on normal trials revealed both visual and non-visual contributions to the detection of information about minimum walking speed. To safely and efficiently navigate through complex, dynamic environments, people must choose actions and control their movements in a way that takes their locomotor capabilities into account. For example, when stepping off a curb, a pedestrian may need to decide whether to go now ahead of an approaching vehicle or wait until it passes. Similarly, a child playing a game of tag may need to decide whether to go to the left or right around a stationary obstacle to intercept another player. In both cases, the possible actions (i.e., the affordances) are partly determined by the person's locomotor capabilities. If people were unable to perceive affor-dances in a way that takes their locomotor capabilities into account , they would sometimes choose actions that are beyond their capabilities and therefore have no chance of succeeding, and other times fail to choose beneficial actions that are within their capabilities. Such decisions lead to energetically inefficient behavior and increase the risk of injury (e.g., see Plumert, Kearney, & Cremer, 2004). Thus, the ability to take one's movement capabilities into account is essential for safe and efficient locomotor control. The aim of this study is to explore how people perceive affor-dances in a way that takes their locomotor capabilities into account. Locomotor capabilities constrain affordances in a variety of tasks, but will be investigated in the present study within the context of perceiving whether it is possible to walk through a shrinking gap between a pair of converging obstacles. We begin this paper by summarizing previous research on the visual guidance of locomotion through gaps. We then introduce a new optical invariant that specifies the passability of a shrinking gap, and present the results of two experiments designed to investigate the detection of this information.
The aim of this study was to examine how visual information is used to control stepping during lo... more 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.
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Papers by Jon Matthis