Visual guidance of intercepting a moving target on foot

Direct perception of action-scaled affordances: the shrinking gap problem

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.

Visual control of prey-capture flight in dragonflies

Interacting with a moving object poses a computational problem for an animal's nervous system. This problem has been elegantly solved by the dragonfly, a formidable visual predator on flying insects. The dragonfly computes an interception flight trajectory and steers to maintain it during its prey-pursuit flight. This review summarizes current knowledge about pursuit behavior and neurons thought to control interception in the dragonfly. When understood, this system has the potential for explaining how a small group of neurons can control complex interactions with moving objects.

Quantitative Relation between Server Motion and Receiver Anticipation in Tennis: Implications of Responses to Computer-Simulated Motions

The purpose of this study was to determine the quantitative relationships between the server's motion and the receiver's anticipation using a computer graphic animation of tennis serves. The test motions were determined by capturing the motion of a model player and estimating the computational perturbations caused by modulating the rotation of the player's elbow and forearm joints. Eight experienced and eight novice players rated their anticipation of the speed, direction, and spin of the ball on a visual analogue scale. The experienced players significantly altered some of their anticipatory judgment depending on the percentage of both the forearm and elbow modulations, while the novice players indicated no significant changes. Multiple regression analyses, including that of the racket's kinematic parameters immediately before racket – ball impact as independent variables, showed that the experienced players demonstrated a higher coefficient of determination than the novice players in their anticipatory judgment of the ball direction. The results have implications on the understanding of the functional relation between a player's motion and the opponent's anticipatory judgment during real play.

Synchronizing self and object movement: how child and adult cyclists intercept moving gaps in a virtual environment

Two experiments examined how 10- and 12-year-old children and adults intercept moving gaps while bicycling in an immersive virtual environment. Participants rode an actual bicycle along a virtual roadway. At 12 test intersections, participants attempted to pass through a gap between 2 moving, car-sized blocks without stopping. The blocks were timed such that it was sometimes necessary for participants to adjust their speed in order to pass through the gap. We manipulated available visual information by presenting the target blocks in isolation in Experiment 1 and in streams of blocks in Experiment 2. In both experiments, adults had more time to spare than did children. Both groups had more time to spare when they were required to slow down than when they were required to speed up. Participants' behavior revealed a multistage interception strategy that cannot be explained by the use of a monotonic control law such as the constant bearing angle strategy. The General Discussion section focuses on possible sources of changes in perception-action coupling over development and on task-specific constraints that could underlie the observed interception strategy.

Parkinson's disease as a disconnection syndrome

Parkinson's disease (PD) is a major neurodegenerative disorder that is usually considered in terms of midbrain and basal ganglia dysfunction. Regarding PD instead as a disconnection syndrome may prove beneficial to understanding aspects of cognition, perception, and other neuropsychological domains in the disease. PD is usually of unilateral onset, providing evidence of intrahemispheric dissociations and an imbalance in the usual relative strengths of the right and left hemispheres. Hence, in order to appreciate the neuropsychology of PD, it is important to apply to this disease our understanding of hemispheric lateralization effects and within-hemisphere circuitry from brainstem to higher-order association cortex. The focus of this review is on the relevance of PD-related disconnections among subcortical and cortical structures to cognition, perception, emotion, and associated brainstem-based domains such as sleep and mood disturbance. Besides providing information on disease characteristics, regarding PD as a disconnection syndrome allows us to more completely understand normal brain-behavior relations in general.

Visual Strategies Used for Time-to-Arrival Judgments in Driving

To investigate the sources of visual information that are involved in the anticipation of collisions we recorded eye movements while participants made relative timing judgments about approaching vehicles at a junction. The avoidance of collisions is a critical aspect in driving, particularly where cars enter a line of traffic from a side road, and the present study required judgments about animations in a virtual driving environment. In two experiments we investigated the effects of (i) the angle of approach of the vehicle and the type of path (straight or curved) of the observer, and (ii) the speed of both the observer and the approaching car. Relative timing judgments depend on the angle of approach of the other vehicle (judgments are more accurate for perpendicular than for obtuse angles). Eye-movement analysis shows that visual strategies in relative timing judgments are characterised by saccadic eye movements back and forth between the approaching car and the road ahead, particularly the side line which may serve as a spatial reference point. Results suggest that observers use the distance of the car from this reference point for their timing judgments.

A neural model of how the brain computes heading from optic flow in realistic scenes

Visually-based navigation is a key competence during spatial cognition. Animals avoid obstacles and approach goals in novel cluttered environments using optic flow to compute heading with respect to the environment. Most navigation models try either explain data, or to demonstrate navigational competence in real-world environments without regard to behavioral and neural substrates. The current article develops a model that does both. The ViSTARS neural model describes interactions among neurons in the primate magnocellular pathway, including V1, MT(+), and MSTd. Model outputs are quantitatively similar to human heading data in response to complex natural scenes. The model estimates heading to within 1.5 degrees in random dot or photo-realistically rendered scenes, and within 3 degrees in video streams from driving in real-world environments. Simulated rotations of less than 1 degrees /s do not affect heading estimates, but faster simulated rotation rates do, as in humans. The model is part of a larger navigational system that identifies and tracks objects while navigating in cluttered environments.

Estimating distance in real and virtual environments: Does order make a difference?

In this investigation, we examined how the order in which people experience real and virtual environments influences their distance estimates. Participants made two sets of distance estimates in one of the following conditions: (1) real environment first, virtual environment second; (2) virtual environment first, real environment second; (3) real environment first, real environment second; or (4) virtual environment first, virtual environment second. In Experiment 1, the participants imagined how long it would take to walk to targets in real and virtual environments. The participants' first estimates were significantly more accurate in the real than in the virtual environment. When the second environment was the same as the first environment (real-real and virtual-virtual), the participants' second estimates were also more accurate in the real than in the virtual environment. When the second environment differed from the first environment (real-virtual and virtual-real), however, the participants' second estimates did not differ significantly across the two environments. A second experiment, in which the participants walked blindfolded to targets in the real environment and imagined how long it would take to walk to targets in the virtual environment, replicated these results. These subtle yet persistent order effects suggest that memory can play an important role in distance perception.

Behavioral responses of big brown bats to dives by praying mantises

Insectivorous echolocating bats face a formidable array of defenses employed by their airborne prey. One such insect defense is the ultrasound-triggered dive, which is a sudden, rapid drop in altitude, sometimes all the way to the ground. Although many previous studies have investigated the dynamics of such dives and their effect on insect survival rate, there has been little work on how bats may adapt to such an insect defense employed in the middle of pursuit. In this study we investigated how big brown bats (Eptesicus fuscus) adjust their pursuit strategy when flying praying mantises (Parasphendale agrionina) execute evasive, ultrasound-triggered dives. Although the mantis dive occasionally forced the bat to completely abort its chase (25% trials), in a number of cases (75% trials) the bat followed the mantis into the dive. In such cases the bat kept its sonar beam locked onto the target and maneuvered to maintain the same time efficient strategy it adopted during level flight pursuit, though it was ultimately defeated by the dive. This study suggests that although the mantis dive can be effective in evading the bat, it does not always deter the bat from continuing pursuit and, given enough altitude, the bat can potentially capture diving prey using the same flight strategy it employs to intercept prey in level flight.

Perception of approaching and retreating floor-projected shapes in a large, immersive, multimedia learning environment

Perception of floor-projected moving geometric shapes was examined in the context of the Situated Multimedia Arts Learning Laboratory (SMALLab), an immersive, mixed-reality learning environment. As predicted, the projected destinations of shapes which retreated in depth (proximal origin) were judged significantly less accurately than those that approached (distal origin). Participants maintained similar magnitudes of error throughout the session, and no effect of practice was observed. Shape perception in an immersive multimedia environment is comparable to the real world. One may conclude that systematic exploration of basic psychological phenomena in novel mediated environments is integral to an understanding of human behavior in novel human-computer interaction architectures.

Cortical dynamics of navigation and steering in natural scenes: Motion-based object segmentation, heading, and obstacle avoidance

Visually guided navigation through a cluttered natural scene is a challenging problem that animals and humans accomplish with ease. The ViSTARS neural model proposes how primates use motion information to segment objects and determine heading for purposes of goal approach and obstacle avoidance in response to video inputs from real and virtual environments. The model produces trajectories similar to those of human navigators. It does so by predicting how computationally complementary processes in cortical areas MT(-)/MSTv and MT(+)/MSTd compute object motion for tracking and self-motion for navigation, respectively. The model's retina responds to transients in the input stream. Model V1 generates a local speed and direction estimate. This local motion estimate is ambiguous due to the neural aperture problem. Model MT(+) interacts with MSTd via an attentive feedback loop to compute accurate heading estimates in MSTd that quantitatively simulate properties of human heading estimation data. Model MT(-) interacts with MSTv via an attentive feedback loop to compute accurate estimates of speed, direction and position of moving objects. This object information is combined with heading information to produce steering decisions wherein goals behave like attractors and obstacles behave like repellers. These steering decisions lead to navigational trajectories that closely match human performance.

Impact of optic flow perception and egocentric coordinates on veering in Parkinson's disease

Spatial navigation is a complex process requiring integration of visuoperceptual information. The present study examined how visuospatial function relates to navigational veering in Parkinson's disease, a movement disorder in which visuospatial cognition is affected by the degeneration of the basal ganglia and resulting dysfunction of the parietal lobes. We hypothesized that patients whose initial motor symptoms start on the left versus right side of the body (LPD, predominant right-hemisphere dysfunction; RPD, predominant left-hemisphere dysfunction) would display distinct patterns of navigational veering associated with the groups' dissimilar visuospatial profiles. Of particular interest was to examine the association of navigational veering (lateral deviation along the medio-lateral axis) with perception of egocentric coordinates and of radial optic flow patterns, both of which are mediated by the parietal lobes. Thirty-one non-demented Parkinson's disease patients (16 LPD, 15 RPD) and 18 healthy control (HC) adults received visuospatial tests, of whom 23 Parkinson's disease patients and 17 HC also underwent veering assessment. The participants were examined on three visual-feedback navigation conditions: none (eyes closed), natural, and optic flow supplied by a virtual-reality headset. All groups veered to the left when walking with eyes closed, women with Parkinson's disease more so than the other participants. On the navigation assessments with visual feedback, only LPD patients deviated right of centre. On tests of visuospatial function, the perceived midline was shifted rightward in LPD (men and women), increasingly so with the addition of visual input. In contrast, men with RPD showed leftward deviation. RPD patients and HC perceived optic flow in the left hemifield as faster than in the right hemifield, with a trend for the opposite pattern for LPD. Navigational veering in LPD was associated with deviation of the perceived egocentric midline and not with perception of optic flow speed asymmetries, and in RPD it was also associated with visual dependence, though in fact LPD subjects were more visually dependent than those with RPD. Our results indicate that (i) parietal-mediated perception of visual space is affected in Parkinson's disease, with both side of motor symptom onset and gender affecting spatial performance, and (ii) visual input affects veering.

Testing the role of expansion in the prospective control of locomotion

The constant bearing angle (CBA) strategy is a prospective strategy that permits the interception of moving objects. The purpose of the present study is to test this strategy. Participants were asked to walk through a virtual environment and to change, if necessary, their walking speed so as to intercept approaching targets. The targets followed either a rectilinear or a curvilinear trajectory and target size was manipulated both within trials (target size was gradually changed during the trial in order to bias expansion) and between trials (targets of different sizes were used). The curvature manipulation had a large effect on the kinematics of walking, which is in agreement with the CBA strategy. The target size manipulations also affected the kinematics of walking. Although these effects of target size are not predicted by the CBA strategy, quantitative comparisons of observed kinematics and the kinematics predicted by the CBA strategy showed good fits. Furthermore, predictions based on the CBA strategy were deemed superior to predictions based on a required velocity (V (REQ)) model. The role of target size and expansion in the prospective control of walking is discussed.

Strategies used to walk through a moving aperture

The objectives of the study were to determine what strategy (pursuit or interception) individuals used to pass through an oscillating target and to determine if individuals walked towards where they were looking. Kinematic and gaze behaviour data was collected from seven healthy female participants as they started at one of five different starting positions and walked 7 m towards an oscillating target. The target was a two-dimensional 70 cm aperture made by two-76 cm wide doors and oscillated between two end posts that were 300 cm apart. In order to quantify the objectives, target-heading angles [Fajen BR, Warren WH. Behavioral dynamics of steering, obstacle avoidance, and route selection. J Exp Psychol Hum Percept Perform 2003;29(2):343-62; Fajen BR, Warren WH. Visual guidance of intercepting a moving target on foot. Perception 2004;33:689-715] were calculated. Results showed that the participants used neither an interception nor a pursuit strategy to successfully pass through the moving aperture. The participants steered towards the middle of the pathway prior to passing through the middle of the aperture. A cross correlation between the horizontal gaze locations and the medial/lateral (M/L) location of the participants' center of mass (COM) was performed. The results from the cross correlation show that during the final 2s prior to crossing the aperture, the participants walked where they were looking. The findings from this study suggest that individuals simplify a task by decreasing the perceptual load until the final stages. In this way the final stages of this task were visually driven.

Rapid recalibration based on optic flow in visually guided action

Action capabilities are always subject to limits. Whether on foot or in a vehicle, people can only move so fast, slow down so quickly, and turn so sharply. The successful performance of almost any perceptual-motor task requires actors to learn and continually relearn their ever-changing action capabilities. Such learning can be considered an example of perceptual-motor calibration. The present study includes two experiments designed to address basic questions about the nature of this calibration process. Subjects performed a simulated braking task, using a foot pedal to slow down to a stop in front of an obstacle in the path of motion. At one point in the experiment, the strength of the brake was increased or decreased unbeknownst to subjects, and behavior before and after the change in brake strength was analyzed for evidence of recalibration. Experiment 1 showed that actors rapidly recalibrate following a change in brake dynamics, even when they are unaware of the change. In Experiment 2, the scene turned black one second after braking was initiated. Subjects still recalibrated following the change in brake strength, suggesting that information in the sensory consequences of the initial brake adjustment is sufficient for recalibration, even in the absence of feedback about the outcome (i.e., in terms of final position error) of the task. Discussion focuses on the critical but often overlooked role of calibration in continuously controlled visually guided action, and the nature of the information used for recalibration.

A unified fielder theory for interception of moving objects either above or below the horizon

A unified fielder theory is presented that explains how humans navigate to intercept targets that approach from either above or below the horizon. Despite vastly different physical forces affecting airborne and ground-based moving targets, a common set of invariant perception-action principles appears to guide pursuers. When intercepting airborne projectiles, fielders keep the target image rising at a constant optical speed in a vertical image plane and moving in a constantoptical direction in an image plane that remains perpendicular to gaze direction. We confirm that fielders use the same strategies to intercept grounders. Fielders maintained a cotangent of gaze angle that decreases linearly with time (accounting for 98.7% of variance in ball speed) and a linear optical trajectory along an image plane that remains perpendicular to gaze direction (accounting for 98.2% of variance in ball position). The universality of maintaining optical speed and direction for both airborne and ground-based targets supports the theory that these mechanisms are domain independent.

Behavioral dynamics of intercepting a moving target

From matters of survival like chasing prey, to games like football, the problem of intercepting a target that moves in the horizontal plane is ubiquitous in human and animal locomotion. Recent data show that walking humans turn onto a straight path that leads a moving target by a constant angle, with some transients in the target-heading angle. We test four control strategies against the human data: (1) pursuit, or nulling the target-heading angle beta, (2) computing the required interception angle beta (3) constant target-heading angle, or nulling change in the target-heading angle beta and (4) constant bearing, or nulling change in the bearing direction of the target psi which is equivalent to nulling change in the target-heading angle while factoring out the turning rate (beta - phi) We show that human interception behavior is best accounted for by the constant bearing model, and that it is robust to noise in its input and parameters. The models are also evaluated for their performance with stationary targets, and implications for the informational basis and neural substrate of steering control are considered. The results extend a dynamical systems model of human locomotor behavior from static to changing environments.

A Feedback-Controlled Interface for Treadmill Locomotion in Virtual Environments

Virtual environments (VEs) allow safe, repeatable, and controlled evaluations of obstacle avoidance and navigation performance of people with visual impairments using visual aids. Proper simulation of mobility in a VE requires an interface, which allows subjects to set their walking pace. Using conventional treadmills, the subject can change their walking speed by pushing the tread with their feet, while leveraging handrails or ropes (self-propelled mode). We developed a feedback-controlled locomotion interface that allows the VE workstation to control the speed of the treadmill, based on the position of the user. The position and speed information is also used to implement automated safety measures, so that the treadmill can be halted in case of erratic behavior. We compared the feedback-controlled mode to the self-propelled mode by using speed-matching tasks (follow a moving object or match the speed of an independently moving scene) to measure the efficacy of each mode in maintaining constant subject position, subject control of the treadmill, and subject pulse rates. Additionally, we measured the perception of speed in the VE on each mode. The feedback-controlled mode required less physical exertion than self-propelled. The average position of subjects on the feedback-controlled treadmill was always within a centimeter of the desired position. There was a smaller standard deviation in subject position when using the self-propelled mode than when using the feedback-controlled mode, but the difference averaged less than six centimeters across all subjects walking at a constant speed. Although all subjects underestimated the speed of an independently moving scene at higher speeds, their estimates were more accurate when using the feedback-controlled treadmill than the self-propelled.

Muscular proprioception contributes to the control of interceptive actions

The authors proposed a model of the control of interceptive action over a ground plane (Chardenon, Montagne, Laurent, & Bootsma, 2004). This model is based on the cancellation of the rate of change of the angle between the current position of the target and the direction of displacement (i.e., the bearing angle). While several sources of visual information specify this angle, the contribution of proprioceptive information has not been directly tested. In this study, the authors used a virtual reality setup to study the role of proprioception when intercepting a moving target. In a series of experiments, the authors manipulated proprioceptive information by using the tendon vibration paradigm. The results revealed that proprioception is crucial not only to locate a moving target with respect to the body but also, and more importantly, to produce online displacement velocity changes to intercept a moving target. These findings emphasize the importance of proprioception in the control of interceptive action and illustrate the relevance of our model to account for the regulations produced by the participants.

The dynamics of perception and action

How might one account for the organization in behavior without attributing it to an internal control structure? The present article develops a theoretical framework called behavioral dynamics that integrates an information-based approach to perception with a dynamical systems approach to action. For a given task, the agent and its environment are treated as a pair of dynamical systems that are coupled mechanically and informationally. Their interactions give rise to the behavioral dynamics, a vector field with attractors that correspond to stable task solutions, repellers that correspond to avoided states, and bifurcations that correspond to behavioral transitions. The framework is used to develop theories of several tasks in which a human agent interacts with the physical environment, including bouncing a ball on a racquet, balancing an object, braking a vehicle, and guiding locomotion. Stable, adaptive behavior emerges from the dynamics of the interaction between a structured environment and an agent with simple control laws, under physical and informational constraints.

Prospective strategies underlie the control of interceptive actions

The purpose of this study was to test whether a constant bearing angle strategy could account for the displacement regulations produced by a moving observer when attempting to intercept a ball following a curvilinear path. The participants were asked to walk through a virtual environment and to change, if (deemed) necessary, their walking speed so as to intercept a moving ball that followed either a rectilinear or a curvilinear path. The results showed that ball path curvature did indeed influence the participants' displacement kinematics in a way that was predicted by adherence to a constant bearing angle strategy mode of control. Velocity modifications were found to be proportional to the magnitude of target curvature with opposing curvatures giving rise to mirror displacement velocity changes. The role of prospective strategies in the control of interceptive action is discussed.

Echolocating bats use a nearly time-optimal strategy to intercept prey

Acquisition of food in many animal species depends on the pursuit and capture of moving prey. Among modern humans, the pursuit and interception of moving targets plays a central role in a variety of sports, such as tennis, football, Frisbee, and baseball. Studies of target pursuit in animals, ranging from dragonflies to fish and dogs to humans, have suggested that they all use a constant bearing (CB) strategy to pursue prey or other moving targets. CB is best known as the interception strategy employed by baseball outfielders to catch ballistic fly balls. CB is a time-optimal solution to catch targets moving along a straight line, or in a predictable fashion—such as a ballistic baseball, or a piece of food sinking in water. Many animals, however, have to capture prey that may make evasive and unpredictable maneuvers. Is CB an optimum solution to pursuing erratically moving targets? Do animals faced with such erratic prey also use CB? In this paper, we address these questions by studying prey capture in an insectivorous echolocating bat. Echolocating bats rely on sonar to pursue and capture flying insects. The bat's prey may emerge from foliage for a brief time, fly in erratic three-dimensional paths before returning to cover. Bats typically take less than one second to detect, localize and capture such insects. We used high speed stereo infra-red videography to study the three dimensional flight paths of the big brown bat, Eptesicus fuscus, as it chased erratically moving insects in a dark laboratory flight room. We quantified the bat's complex pursuit trajectories using a simple delay differential equation. Our analysis of the pursuit trajectories suggests that bats use a constant absolute target direction strategy during pursuit. We show mathematically that, unlike CB, this approach minimizes the time it takes for a pursuer to intercept an unpredictably moving target. Interestingly, the bat's behavior is similar to the interception strategy implemented in some guided missiles. We suggest that the time-optimal strategy adopted by the bat is in response to the evolutionary pressures of having to capture erratic and fast moving insects.

Analysis of the three dimensional flight paths of the big brown bat reveals a similar strategy to intercept targets as used by some guided missiles.

The scaling of information to action in visually guided braking

Braking to avoid a collision can be controlled by keeping the deceleration required to stop (i.e., ideal deceleration) in the "safe" region below maximum deceleration, but maximum deceleration is not optically specified and can vary as conditions change. When brake strength was manipulated between participants using a simulated braking task, the ratio of ideal to maximum deceleration at brake onset was invariant across groups, suggesting that calibration involves scaling information about ideal deceleration in intrinsic units of maximum deceleration. Evidence of rapid recalibration was found when brake strength was manipulated within participants, and the presence of external forces that affect brake dynamics resulted in biases in performance. Discussion focuses on the role of calibration, internal models, and affordance perception in visually guided action.

Steering by hearing: a bat's acoustic gaze is linked to its flight motor output by a delayed, adaptive linear law

Adaptive behaviors require sensorimotor computations that convert information represented initially in sensory coordinates to commands for action in motor coordinates. Fundamental to these computations is the relationship between the region of the environment sensed by the animal (gaze) and the animal's locomotor plan. Studies of visually guided animals have revealed an anticipatory relationship between gaze direction and the locomotor plan during target-directed locomotion. Here, we study an acoustically guided animal, an echolocating bat, and relate acoustic gaze (direction of the sonar beam) to flight planning as the bat searches for and intercepts insect prey. We show differences in the relationship between gaze and locomotion as the bat progresses through different phases of insect pursuit. We define acoustic gaze angle, theta(gaze), to be the angle between the sonar beam axis and the bat's flight path. We show that there is a strong linear linkage between acoustic gaze angle at time t [theta(gaze)(t)] and flight turn rate at time t + tau into the future [theta(flight) (t + tau)], which can be expressed by the formula theta(flight) (t + tau) = ktheta(gaze)(t). The gain, k, of this linkage depends on the bat's behavioral state, which is indexed by its sonar pulse rate. For high pulse rates, associated with insect attacking behavior, k is twice as high compared with low pulse rates, associated with searching behavior. We suggest that this adjustable linkage between acoustic gaze and motor output in a flying echolocating bat simplifies the transformation of auditory information to flight motor commands.

Judgments of path, not heading, guide locomotion

To steer a course through the world, people are almost entirely dependent on visual information, of which a key component is optic flow. In many models of locomotion, heading is described as the fundamental control variable; however, it has also been shown that fixating points along or near one's future path could be the basis of an efficient control solution. Here, the authors aim to establish how well observers can pinpoint instantaneous heading and path, by measuring their accuracy when looking at these features while traveling along straight and curved paths. The results showed that observers could identify both heading and path accurately (approximately 3 degrees ) when traveling along straight paths, but on curved paths they were more accurate at identifying a point on their future path (approximately 5 degrees ) than indicating their instantaneous heading (approximately 13 degrees ). Furthermore, whereas participants could track changes in the tightness of their path, they were unable to accurately track the rate of change of heading. In light of these results, the authors suggest it is unlikely that heading is primarily used by the visual system to support active steering.

The perceptual support of goal-directed displacement is context-dependent

This study investigates the perceptual-motor organisation underlying the control of goal-directed displacement. We used a virtual reality set-up to study the locomotor interception of a moving ball. Subjects had to intercept moving balls by modifying displacement velocity if necessary, while the ball's place of arrival and the environment were manipulated. The results showed that subjects simultaneously managed multiple sources of information and placed priority on the most salient variables, depending on the task and environmental constraints.

Using visual direction in three-dimensional motion perception

The eyes receive slightly different views of the world, and the differences between their images (binocular disparity) are used to see depth. Several authors have suggested how the brain could exploit this information for three-dimensional (3D) motion perception, but here we consider a simpler strategy. Visual direction is the angle between the direction of an object and the direction that an observer faces. Here we describe human behavioral experiments in which observers use visual direction, rather than binocular information, to estimate an object's 3D motion even though this causes them to make systematic errors. This suggests that recent models of binocular 3D motion perception may not reflect the strategies that human observers actually use.