Guidance of locomotion on foot uses perceived target location rather than optic flow

The head tracks and gaze predicts: how the world's best batters hit a ball

Hitters in fast ball-sports do not align their gaze with the ball throughout ball-flight; rather, they use predictive eye movement strategies that contribute towards their level of interceptive skill. Existing studies claim that (i) baseball and cricket batters cannot track the ball because it moves too quickly to be tracked by the eyes, and that consequently (ii) batters do not – and possibly cannot – watch the ball at the moment they hit it. However, to date no studies have examined the gaze of truly elite batters. We examined the eye and head movements of two of the world’s best cricket batters and found both claims do not apply to these batters. Remarkably, the batters coupled the rotation of their head to the movement of the ball, ensuring the ball remained in a consistent direction relative to their head. To this end, the ball could be followed if the batters simply moved their head and kept their eyes still. Instead of doing so, we show the elite batters used distinctive eye movement strategies, usually relying on two predictive saccades to anticipate (i) the location of ball-bounce, and (ii) the location of bat-ball contact, ensuring they could direct their gaze towards the ball as they hit it. These specific head and eye movement strategies play important functional roles in contributing towards interceptive expertise.

Does optic flow parsing depend on prior estimation of heading?

We have recently suggested that neural flow parsing mechanisms act to subtract global optic flow consistent with observer movement to aid in detecting and assessing scene-relative object movement. Here, we examine whether flow parsing can occur independently from heading estimation. To address this question we used stimuli comprising two superimposed optic flow fields comprising limited lifetime dots (one planar and one radial). This stimulus gives rise to the so-called optic flow illusion (OFI) in which perceived heading is biased in the direction of the planar flow field. Observers were asked to report the perceived direction of motion of a probe object placed in the OFI stimulus. If flow parsing depends upon a prior estimate of heading then the perceived trajectory should reflect global subtraction of a field consistent with the heading experienced under the OFI. In Experiment 1 we tested this prediction directly, finding instead that the perceived trajectory was biased markedly in the direction opposite to that predicted under the OFI. In Experiment 2 we demonstrate that the results of Experiment 1 are consistent with a positively weighted vector sum of the effects seen when viewing the probe together with individual radial and planar flow fields. These results suggest that flow parsing is not necessarily dependent on prior estimation of heading direction. We discuss the implications of this finding for our understanding of the mechanisms of flow parsing.

Attentional demands of cued walking in healthy young and elderly adults

Acoustic and visual cues are frequently used in gait rehabilitation. Attuning the steps to the cues is attentionally demanding. We examined the attentional demands of walking to two types of cues using a probe reaction time (RT) task. The steps were cued by either metronome beeps or visual stepping stones projected on a treadmill. The coupling between gait and these cues was assessed using a perturbation paradigm. In view of age-related changes in attentional demands of motor control, both elderly and young adults were tested. RTs were determined for walking to the two types of cues, as well as for three control conditions, viz. uncued walking, standing, and sitting. For all conditions, RTs were higher for elderly adults. However, the difference between elderly and young adults did not vary over conditions. Uncued walking required more attention than did standing and sitting. The attentional demands were further elevated during cued walking, with larger RTs for walking to visual stepping stones than to metronome beeps. Because the coupling to the cues was superior in the stepping stones condition, this type of cues seems to aid cued walking by allocating higher levels of attention to task-relevant information (viz. future footfall positions). Hence, the observed differences between the two cueing types may be associated with the natural dependence of gait on visual information.

Do walkers follow their heads? Investigating the role of head rotation in locomotor control

Eye and head rotations are normally correlated with changes in walking direction; however, it is unknown whether they play a causal role in the control of steering. The objective of the present study was to answer two questions about the role of head rotations in steering control when walking to a goal. First, are head rotations sufficient to elicit a change in walking direction? Second, are head rotations necessary to initiate a change in walking direction or guide steering to a goal? To answer these questions, participants either walked toward a goal located 7 m away or were cued to steer to the left or right by 37°. On a subset of trials, participants were either cued to voluntarily turn their heads to the left or right, or they underwent an involuntary head perturbation via a head-mounted air jet. The results showed that large voluntary head turns (35°) yielded slight path deviations (1°-2°) in the same or opposite direction as the head turn, depending on conditions, which have alternative explanations. Involuntary head rotations did not elicit path deviations despite comparable head rotation magnitudes. In addition, the walking trajectory when turning toward an eccentric goal was the same regardless of head orientation. Steering can thus be decoupled from head rotation during walking. We conclude that head rotations are neither a sufficient nor a necessary component of steering control, because they do not induce a turn and they are not required to initiate a turn or to guide the locomotor trajectory to a goal.

Human locomotion through a multiple obstacle environment: strategy changes as a result of visual field limitation

This study investigated how human locomotion through an obstacle environment is influenced by visual field limitation. Participants were asked to walk at a comfortable pace to a target location while avoiding multiple vertical objects. During this task, they wore goggles restricting their visual field to small (S: 40°×25°), medium (M: 80°×60°), large (L: 115°×90°), or unlimited (U) visual field sizes. Full-body motion capture was used to extract for each trial the mean speed, pathlength, mean step width, magnitude of head rotation and head mean angular speed. The results show that compared with the U condition, the M and L conditions caused participants to select a wider path around the obstacles without slowing down or altering step width. However, the S condition did slow down the participants, and increased both their step width and path length. We conclude that only for the S condition, balancing problems were substantial enough to spend more energy associated with increased step width. In all cases, participants choose to optimize safety (collision avoidance) at the cost of spending more energy.

Perceiving the present: systematization of illusions or illusion of systematization?

Mark Changizi et al. (2008) claim that it is possible systematically to organize more than 50 kinds of illusions in a 7 × 4 matrix of 28 classes. This systematization, they further maintain, can be explained by the operation of a single visual processing latency correction mechanism that they call "perceiving the present" (PTP). This brief report raises some concerns about the way a number of illusions are classified by the proposed systematization. It also poses two general problems-one empirical and one conceptual-for the PTP approach.

Invariance of locomotor trajectories across visual and gait direction conditions

We studied the influence of vision (walking with or without vision) and of gait direction (walking forward or backward) on goal-oriented locomotion in humans. Subjects had to walk, in a free environment, from a given position and orientation towards a distant arrow which constrained their final position and orientation. We found that the average trajectories were mostly similar across the tested conditions, which suggests that locomotor trajectories are generated at a high cognitive level and, to some extent, independently of the detailed sensory and motor implementation levels. The variability profiles around the average trajectories were similar across the gait direction conditions but differed greatly across the visual conditions, indicating the existence of motor-independent and vision-dependent control mechanisms. Taken together, our observations argue further in favour of a top-down implementation of goal-oriented locomotion, where the control of locomotion is specified at the level of whole-body trajectories and then implemented through specific motor strategies.

The brain weights body-based cues higher than vision when estimating walked distances

Optic flow is the stream of retinal information generated when an observer's body, head or eyes move relative to their environment, and it plays a defining role in many influential theories of active perception. Traditionally, studies of optic flow have used artificially generated flow in the absence of the body-based cues typically coincident with self-motion (e.g. proprioceptive, efference copy, and vestibular). While optic flow alone can be used to judge the direction, speed and magnitude of self-motion, little is known about the precise extent to which it is used during natural locomotor behaviours such as walking. In this study, walked distances were estimated in an open outdoor environment. This study employed two novel complementary techniques to dissociate the contributions of optic flow from body-based cues when estimating distance travelled in a flat, open, outdoor environment void of distinct proximal visual landmarks. First, lenses were used to magnify or minify the visual environment. Second, two walked distances were presented in succession and were either the same or different in magnitude; vision was either present or absent in each. A computational model was developed based on the results of both experiments. Highly convergent cue-weighting values were observed, indicating that the brain consistently weighted body-based cues about twice as high as optic flow, the combination of the two cues being additive. The current experiments represent some of the first to isolate and quantify the contributions of optic flow during natural human locomotor behaviour.

Monkey steering responses reveal rapid visual-motor feedback

The neural mechanisms underlying primate locomotion are largely unknown. While behavioral and theoretical work has provided a number of ideas of how navigation is controlled, progress will require direct physiolgical tests of the underlying mechanisms. In turn, this will require development of appropriate animal models. We trained three monkeys to track a moving visual target in a simple virtual environment, using a joystick to control their direction. The monkeys learned to quickly and accurately turn to the target, and their steering behavior was quite stereotyped and reliable. Monkeys typically responded to abrupt steps of target direction with a biphasic steering movement, exhibiting modest but transient overshoot. Response latencies averaged approximately 300 ms, and monkeys were typically back on target after about 1 s. We also exploited the variability of responses about the mean to explore the time-course of correlation between target direction and steering response. This analysis revealed a broad peak of correlation spanning approximately 400 ms in the recent past, during which steering errors provoke a compensatory response. This suggests a continuous, visual-motor loop controls steering behavior, even during the epoch surrounding transient inputs. Many results from the human literature also suggest that steering is controlled by such a closed loop. The similarity of our results to those in humans suggests the monkey is a very good animal model for human visually guided steering.

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 selectivity for heading in monkey area MST

The control of self-motion is supported by visual, vestibular, and proprioceptive signals. Recent research has shown how these signals interact in the monkey medio-superior temporal area (area MST) to enhance and disambiguate the perception of heading during self-motion. Area MST is a central stage for self-motion processing from optic flow, and integrates flow Weld information with vestibular self-motion and extraretinal eye movement information. Such multimodal cue integration is clearly important to solidify perception. However to understand the information processing capabilities of the brain, one must also ask how much information can be deduced from a single cue alone. This is particularly pertinent for optic flow, where controversies over its usefulness for self-motion control have existed ever since Gibson proposed his direct approach to ecological perception. In our study, we therefore, tested macaque MST neurons for their heading selectivity in highly complex flow Welds based on the purely visual mechanisms. We recorded responses of MST neurons to simple radial flow Welds and to distorted flow Welds that simulated a self-motion plus an eye movement. About half of the cells compensated for such distortion and kept the same heading selectivity in both cases. Our results strongly support the notion of an involvement of area MST in the computation of heading.

A Contribution of Area 5 of the Posterior Parietal Cortex to the Planning of Visually Guided Locomotion: Limb-Specific and Limb-Independent Effects

We tested the hypothesis that area 5 of the posterior parietal cortex (PPC) contributes to the planning of visually guided gait modifications. We recorded 121 neurons from the PPC of two cats during a task in which cats needed to process visual input to step over obstacles attached to a moving treadmill belt. During unobstructed locomotion, 64/121 (53%) of cells showed rhythmic activity. During steps over the obstacles, 102/121 (84%) of cells showed a significant change of their activity. Of these, 46/102 were unmodulated during the control task. We divided the 102 task-related cells into two groups on the basis of their discharge when the limb contralateral to the recording site was the first to pass over the obstacle. One group (41/102) was characterized by a brief, phasic discharge as the lead forelimb passed over the obstacle (Step-related cells). These cells were recorded primarily from area 5a. The other group (61/102) showed a progressive increase in activity prior to the onset of the swing phase in the modified limb and frequently diverged from control at least one step cycle before the gait modification (Step-advanced cells). Most of these cells were recorded in area 5b. In both groups, some cells maintained a fixed relationship to the activity of the contralateral forelimb regardless of which limb was the first to pass over the obstacle (limb-specific cells), whereas others changed their phase of activity so that they were always related to activity of the first limb to pass over the obstacle, either contralateral or ipsilateral (limb-independent cells). Limb-independent cells were more common among the Step-advanced cell population. We suggest that both populations of cells contribute to the gait modification and that the discharge characteristics of the Step-advanced cells are compatible with a contribution to the planning of the gait modification.

Position encoding of the centres of global structure: separate form and motion processes

The retinal flow of information during locomotion provides cues to instantaneous heading. Reconciliation of observer trajectory with the internal representation of the environment implies that the positions of the centre of structure of global motion can be localized relative to points in the visual field. Humans are also sensitive to global structure in Glass patterns, which can approximate temporally integrated optic flow. Encoding of the position of centre of structure of Glass patterns could augment the motion information. However, Glass-like pattern structure could also be present in the texture of objects and indicate their centres, raising the question of whether the centres of form and motion patterns are encoded separately. Psychophysical methods are used to examine ability to localize the centres of structure of Glass and global dot motion (GDM) patterns. Radial and concentric Glass patterns are localized more precisely than spiral Glass patterns but performance in localizing the centres of radial, concentric and spiral GDM patterns is equally precise. Also, Glass patterns centres can be localized at signal levels close to their threshold for discrimination from wholly incoherent patterns but GDM pattern centres cannot, suggesting that detectors for looming and rotating stimuli exist that do not rely on the provision of a centre of structure. Collectively, our results provide evidence for independent encoding of the positions of the centres of structure of global motion and form patterns. These positions can be accurately and precisely localized within the visual field. While the centres of structure of a number of form patterns can be simultaneously encoded, allowing their positions to be compared, only a single focus of expansion for optic flow is returned.

Perception of scene-relative object movement: Optic flow parsing and the contribution of monocular depth cues

We have recently suggested that the brain uses its sensitivity to optic flow in order to parse retinal motion into components arising due to self and object movement (e.g. Rushton, S. K., & Warren, P. A. (2005). Moving observers, 3D relative motion and the detection of object movement. Current Biology, 15, R542-R543). Here, we explore whether stereo disparity is necessary for flow parsing or whether other sources of depth information, which could theoretically constrain flow-field interpretation, are sufficient. Stationary observers viewed large field of view stimuli containing textured cubes, moving in a manner that was consistent with a complex observer movement through a stationary scene. Observers made speeded responses to report the perceived direction of movement of a probe object presented at different depths in the scene. Across conditions we varied the presence or absence of different binocular and monocular cues to depth order. In line with previous studies, results consistent with flow parsing (in terms of both perceived direction and response time) were found in the condition in which motion parallax and stereoscopic disparity were present. Observers were poorer at judging object movement when depth order was specified by parallax alone. However, as more monocular depth cues were added to the stimulus the results approached those found when the scene contained stereoscopic cues. We conclude that both monocular and binocular static depth information contribute to flow parsing. These findings are discussed in the context of potential architectures for a model of the flow parsing mechanism.

On the open-loop and feedback processes that underlie the formation of trajectories during visual and nonvisual locomotion in humans

We investigated the nature of the control mechanisms at work during goal-oriented locomotion. In particular, we tested the effects of vision, locomotor speed, and the presence of via points on the geometric and kinematic properties of locomotor trajectories. We first observed that the average trajectories recorded in visual and nonvisual locomotion were highly comparable, suggesting the existence of vision-independent processes underlying the formation of locomotor trajectories. Then by analyzing and comparing the variability around the average trajectories across different experimental conditions, we were able to demonstrate the existence of on-line feedback control in both visual and nonvisual locomotion and to clarify the relations between visual and nonvisual control strategies. Based on these insights, we designed a model in which maximum-smoothness and optimal feedback control principles account, respectively, for the open-loop and feedback processes. Taken together, the experimental and modeling findings provide a novel understanding of the nature of the motor, sensory, and "navigational" processes underlying goal-oriented locomotion.

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.

Optic flow processing for the assessment of object movement during ego movement

The vast majority of research on optic flow (retinal motion arising because of observer movement) has focused on its use in heading recovery and guidance of locomotion. Here we demonstrate that optic flow processing has an important role in the detection and estimation of scene-relative object movement during self movement. To do this, the brain identifies and globally discounts (i.e., subtracts) optic flow patterns across the visual scene-a process called flow parsing. Remaining motion can then be attributed to other objects in the scene. In two experiments, stationary observers viewed radial expansion flow fields and a moving probe at various onscreen locations. Consistent with global discounting, perceived probe motion had a significant component toward the center of the display and the magnitude of this component increased with probe eccentricity. The contribution of local motion processing to this effect was small compared to that of global processing (experiment 1). Furthermore, global discounting was clearly implicated because these effects persisted even when all the flow in the hemifield containing the probe was removed (experiment 2). Global processing of optic flow information is shown to play a fundamental role in the recovery of object movement during ego movement.

Kinesthetic compensation for sensorimotor rearrangements

The authors report a new sensorimotor phenomenon in which participants use hand-sensed kinesthetic information to compensate for rotational sensorimotor rearrangements. This compensation benefits from conscious awareness and is related to hand posture. The technique can reduce control inefficiency with some misalignments by as much as 64%. The results support Y. Guiard's (1987) suggestion that in bimanual tasks one hand provides an operational frame of reference for the other hand as in a closed kinematic chain. Results with right-handed participants show that the right and left hands are equally effective at providing such a cue. A constant-angular-targeting-error model, similar to that used for hand movements by H. Cunningham and I. Vardi (1990) and for walking by S. K. Rushton, J. M. Harris, M. R. Lloyd, and J. P. Wann (1998), is used to model the trajectories of targeting hand movements demonstrating the phenomenon. The model provides a natural parameter of the error.

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.

Dual-task costs while walking increase in old age for some, but not for other tasks: an experimental study of healthy young and elderly persons

BACKGROUND

It has been suggested in the past that the ability to walk while concurrently engaging in a second task deteriorates in old age, and that this deficit is related to the high incidence of falls in the elderly. However, previous studies provided inconsistent findings about the existence of such an age-related dual-task deficit (ARD). In an effort to explain this inconsistency, we explored whether ARD while walking emerges for some, but not for other types of task.

METHODS

Healthy young and elderly subjects were tested under five different combinations of a walking and a non-walking task. The results were analysed jointly with those of a previous study from our lab, such that a total of 13 task combinations were evaluated. For each task combination and subject, we calculated the mean dual-task costs across both constituent tasks, and quantified ARD as the difference between those costs in elderly and in young subjects.

RESULTS

An analysis of covariance yielded no significant effects of obstacle presence and overall task difficulty on ARD, but a highly significant effect of visual demand: non-walking tasks which required ongoing visual observation led to ARD of more than 8%, while those without such requirements led to near-zero ARD. We therefore concluded that the visual demand of the non-walking task is critical for the emergence of ARD while walking.

CONCLUSION

Combinations of walking and concurrent visual observation, which are common in everyday life, may contribute towards disturbed gait and falls during daily activities in old age. Prevention and rehabilitation programs for seniors should therefore include training of such combinations.

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.

Non-linear gaining in precision aiming: making Fitts' task a bit easier

The role of information in the processes underlying kinematic trajectory-formation was examined by manipulating the relation between effector space (movement of a hand-held stylus on a graphics tablet) and task space (movement of a cursor on a screen where targets were presented) in a precision aiming task with five different levels of task difficulty. Movement patterns were found to evolve as a function of the flow of information in task space, with participants (N=13) producing more rapid and more fluent movements when the mapping between spaces included the softening-spring characteristics typical of behavioural patterns at higher levels of task difficulty. We conclude that the kinematic changes (movement time and pattern) observed when task difficulty increases result from informational influences. Information affects behavioural dynamics at the level of the parameters without affecting the underlying dynamical structure.

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.

Bayesian motion estimation accounts for a surprising bias in 3D vision

Determining the approach of a moving object is a vital survival skill that depends on the brain combining information about lateral translation and motion-in-depth. Given the importance of sensing motion for obstacle avoidance, it is surprising that humans make errors, reporting an object will miss them when it is on a collision course with their head. Here we provide evidence that biases observed when participants estimate movement in depth result from the brain's use of a "prior" favoring slow velocity. We formulate a Bayesian model for computing 3D motion using independently estimated parameters for the shape of the visual system's slow velocity prior. We demonstrate the success of this model in accounting for human behavior in separate experiments that assess both sensitivity and bias in 3D motion estimation. Our results show that a surprising perceptual error in 3D motion perception reflects the importance of prior probabilities when estimating environmental properties.

How trunk turns affect locomotion when you are not looking where you go

How well do we maintain heading direction during walking while we look at objects beside our path by rotating our eyes, head, or trunk? Common experience indicates that it may be fairly hazardous not to look where you are going. In the present study, 12 young adults walked on a treadmill while they followed a moving dot along a horizontal line with their gaze by rotating primarily either their eyes, head, or trunk for amplitudes of up to 25 degrees . During walking the movement of the center of pressure (COP) was monitored using force transducers under a treadmill. Under normal light conditions, the participants showed little lateral deviation of the COP from the heading direction when they performed the eye or head movement task during walking, even when optic flow information was limited. In contrast, trunk rotations led to a doubling of the COP deviation in the mediolateral direction. Some of this deviation was attributed to foot rotation. Participants tended to point their feet in the gaze direction when making trunk turns. The tendency of the feet to be aligned with the trunk is likely to be due to a preference to have feet and body in the same orientation. Such alignment is weaker for the feet with respect to head position and it is absent with respect to eye position. It is argued that feet and trunk orientation are normally tightly coupled during gait and that it requires special abilities to move both segments independently when walking.

Evidence for flow-parsing in radial flow displays

Retinal motion of objects is not in itself enough to signal whether or how objects are moving in the world; the same pattern of retinal motion can result from movement of the object, the observer or both. Estimation of scene-relative movement of an object is vital for successful completion of many simple everyday tasks. Recent research has provided evidence for a neural flow-parsing mechanism which uses the brain's sensitivity to optic flow to separate retinal motion signals into those components due to observer movement and those due to the movement of objects in the scene. In this study we provide further evidence that flow-parsing is implicated in the assessment of object trajectory during observer movement. Furthermore, it is shown that flow-parsing involves a global analysis of retinal motion, as might be expected if optic flow processing underpinned this mechanism.

Steering behaviour can be modulated by different optic flows during walking

Optic flow is a typical pattern of visual motion that can be used to control locomotion. While the ability to discriminate translational or rotational optic flows have been extensively studied, how these flows control steering during locomotion is not known. The goal of this study was to compare the steering behaviour of subjects subjected to rotational, translational, or combined (rotational added to translational) optic flows with a focus of expansion (FOE) located to the right, left, or straight ahead. Ten healthy young subjects were instructed to walk straight in a virtual room viewed through a helmet mounted display while the location of the FOE was randomly offset. Horizontal trajectory of the body's centre of mass (CoM), as well as rotations of the head, trunk and foot were recorded in coordinates of both the physical and virtual worlds. Results show that subjects experienced a mediolateral shift in CoM opposite to the FOE location, with larger corrections being observed at more eccentric FOE locations. Head and body segment reorientations were only observed for optic flows containing a rotational component. CoM trajectory corrections in the physical world were also of small magnitude, leading to deviation errors in the virtual world. Altogether, these results suggest a profound influence of vision, especially due to the pattern of visual motion, on steering behaviours during locomotion.

Perceptually guided action: a step in the right direction

To walk to a target you need to know where it is. A recent study provides new insight into how the brain ensures you don't head off in the wrong direction.

As Go the Feet … : On the Estimation of Attentional Focus from Stance

The estimation of the direction of visual attention is critical to a large number of interactive systems. This paper investigates the cross-modal relation of the position of one's feet (or standing stance) to the focus of gaze. The intuition is that while one CAN have a range of attentional foci from a particular stance, one may be MORE LIKELY to look in specific directions given an approach vector and stance. We posit that the cross-modal relationship is constrained by biomechanics and personal style. We define a stance vector that models the approach direction before stopping and the pose of a subject's feet. We present a study where the subjects' feet and approach vector are tracked. The subjects read aloud contents of note cards in 4 locations. The order of `visits' to the cards were randomized. Ten subjects read 40 lines of text each, yielding 400 stance vectors and gaze directions. We divided our data into 4 sets of 300 training and 100 test vectors and trained a neural net to estimate the gaze direction given the stance vector. Our results show that 31% our gaze orientation estimates were within 5°, 51% of our estimates were within 10°, and 60% were within 15°. Given the ability to track foot position, the procedure is minimally invasive.

Optic flow drives human visuo-locomotor adaptation

Two strategies can guide walking to a stationary goal: (1) the optic-flow strategy, in which one aligns the direction of locomotion or "heading" specified by optic flow with the visual goal; and (2) the egocentric-direction strategy, in which one aligns the locomotor axis with the perceived egocentric direction of the goal and in which error results in optical target drift. Optic flow appears to dominate steering control in richly structured visual environments, whereas the egocentric- direction strategy prevails in visually sparse environments. Here we determine whether optic flow also drives visuo-locomotor adaptation in visually structured environments. Participants adapted to walking with the virtual-heading direction displaced 10 degrees to the right of the actual walking direction and were then tested with a normally aligned heading. Two environments, one visually structured and one visually sparse, were crossed in adaptation and test phases. Adaptation of the walking path was more rapid and complete in the structured environment; the negative aftereffect on path deviation was twice that in the sparse environment, indicating that optic flow contributes over and above target drift alone. Optic flow thus plays a central role in both online control of walking and adaptation of the visuo-locomotor mapping.

The pop out of scene-relative object movement against retinal motion due to self-movement

An object that moves is spotted almost effortlessly; it "pops out". When the observer is stationary, a moving object is uniquely identified by retinal motion. This is not so when the observer is also moving; as the eye travels through space all scene objects change position relative to the eye producing a complicated field of retinal motion. Without the unique identifier of retinal motion an object moving relative to the scene should be difficult to locate. Using a search task, we investigated this proposition. Computer-rendered objects were moved and transformed in a manner consistent with movement of the observer. Despite the complex pattern of retinal motion, objects moving relative to the scene were found to pop out. We suggest the brain uses its sensitivity to optic flow to "stabilise" the scene, allowing the scene-relative movement of an object to be identified.

Response characteristics of visual altitude control system in Bombus terrestris

Frequency response characteristics of bumblebees to vertical visual oscillations were measured and analyzed. We measured the vertical force of the bees at four oscillation frequencies (0.9, 1.8, 3.6 and 7.4 Hz), and summarized their response characteristics in terms of amplitude and phase differences. The amplitude was almost constant throughout the examined frequency domain, whereas the phase gradually lagged with increasing frequency. In order to view the relationship between the input (visual oscillation) and output (response of the bee) more clearly as a control system, we compared them in the same dimension; we calculated hypothetical positions of the tethered bees on the basis of the measured variation in the vertical force, and compared them with the visual stripe positions. The resultant gain and phase data were plotted on a Bode plot. A transfer function was identified from the Bode plot, revealing that the response characteristics of the measured system could be represented as a simple expression. The dynamic control characteristics of the bumblebees were analyzed on the basis of the frequency response data. First, we showed that the measured system possesses a substantial stability margin. This means that the control system has substantial damping characteristics, and was suitable for stable flight control. In addition, our results showed that the measured bumblebee system possesses superior steady state and quick-response characteristics in comparison with a human pilot-vehicle system. Such excellence in both the steady state and transient characteristics (i.e. damping and quick response characteristics) provide the evidence that bumblebees can effectively control their flight with stability and maneuverability.

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.

Heading assessment by "tunnel vision" patients and control subjects standing or walking in a virtual reality environment

Virtual reality locomotion simulators are a promising tool for evaluating the effectiveness of vision aids to mobility for people with low vision. This study examined two factors to gain insight into the verisimilitude requirements of the test environment: the effects of treadmill walking and the suitability of using controls as surrogate patients. Ten "tunnel vision" patients with retinitis pigmentosa (RP) were tasked with identifying which side of a clearly visible obstacle their heading through the virtual environment would lead them, and were scored both on accuracy and on their distance from the obstacle when they responded. They were tested both while walking on a treadmill and while standing, as they viewed a scene representing progress through a shopping mall. Control subjects, each wearing a head-mounted field restriction to simulate the vision of a paired patient, were also tested. At wide angles of approach, controls and patients performed with a comparably high degree of accuracy, and made their choices at comparable distances from the obstacle. At narrow angles of approach, patients' accuracy increased when walking, while controls' accuracy decreased. When walking, both patients and controls delayed their decisions until closer to the obstacle. We conclude that a head-mounted field restriction is not sufficient for simulating tunnel vision, but that the improved performance observed for walking compared to standing suggests that a walking interface (such as a treadmill) may be essential for eliciting natural perceptually-guided behavior in virtual reality locomotion simulators.

Perception of heading without retinal optic flow

How do we determine where we are heading during visually controlled locomotion? Psychophysical research has shown that humans are quite good at judging their travel direction, or heading, from retinal optic flow. Here we show that retinal optic flow is sufficient, but not necessary, for determining heading. By using a purely cyclopean stimulus (random dot cinematogram), we demonstrate heading perception without retinal optic flow. We also show that heading judgments are equally accurate for the cyclopean stimulus and a conventional optic flow stimulus, when the two are matched for motion visibility. The human visual system thus demonstrates flexible, robust use of available visual cues for perceiving heading direction.

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.

Changing lanes: inertial cues and explicit path information facilitate steering performance when visual feedback is removed

Can driver steering behaviors, such as a lane change, be executed without visual feedback? In a recent study with a fixed-base driving simulator, drivers failed to execute the return phase of a lane change when steering without vision, resulting in systematic final heading errors biased in the direction of the lane change. Here we challenge the generality of that finding. Suppose that, when asked to perform a lane (position) change, drivers fail to recognize that a heading change is required to make a lateral position change. However, given an explicit path, the necessary heading changes become apparent. Here we demonstrate that when heading requirements are made explicit, drivers appropriately implement the return phase. More importantly, by using an electric vehicle outfitted with a portable virtual reality system, we also show that valid inertial information (i.e., vestibular and somatosensory cues) enables accurate steering behavior when vision is absent. Thus, the failure to properly execute a lane change in a driving simulator without a moving base does not present a fundamental problem for feed-forward driving behavior.

Do migraineurs have difficulty judging direction of simulated heading?

Some migraineurs have increased thresholds for the detection of global dot motion. We investigated whether migraineurs show consequential abnormalities in the determination of direction of self-motion (heading) from simulated optic flow. The ability to determine heading from optic flow is likely to be necessary for optimal determination of self-motion through the environment. Twenty-five migraineurs and 25 controls participated. Global dot motion coherence thresholds were assessed, in addition to performance on two simulated heading tasks: one with a symmetrical flow field, and the second with differing velocity of optic flow on the left and right sides of the participant. While some migraineurs demonstrated abnormal global motion coherence thresholds, there was no difference in performance on the heading tasks at either simulated walking (5 km/h) or driving (50 km/h) speeds. Increased global motion coherence thresholds in migraineurs do not result in abnormal judgements of heading from 100% coherent optic flow.

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.

Resolution of complex motion detectors in the central and peripheral visual field

We examine how local direction signals are combined to compute the focus of radial motion (FRM) in random dot patterns and examine how this process changes across the visual field. Equivalent noise analysis showed that a loss in FRM accuracy was largely attributable to an increase in local motion detector noise with little or no change in efficiency across the visual field. The minimum separation for discriminating the foci of two overlapping optic flow patterns increased in the periphery faster than predicted from the resolution for a single FRM. This behavior requires that observers average numerous local velocities to estimate the FRM, which enables resistance to internal and external noise and endows the system with the property of position invariance. However, such pooling limits the precision with which multiple looming objects can be discriminated, especially in the peripheral visual field.

Galvanic Vestibular Stimulation Modifies Vection Paths in Healthy Subjects

The present study aimed at determining whether vestibular inputs contribute to the perception of the direction of self-motion. This question was approached by investigating the effects of binaural bipolar galvanic vestibular stimulation (GVS) on visually induced self-motion (i.e., vection) in healthy subjects. Stationary seated subjects were submitted to optokinetic stimulation inducing either forward or upward linear vection. While perceiving vection, they were administered trapezoidal GVS of different intensities and ramp durations. Subjects indicated the shape and direction of their perceived self-motion path throughout the experiment by a joystick, and after each trial by the manipulation of a 3D mannequin. Results show that: 1) GVS induced alterations of the path of vection; 2) these alterations occurred more often after GVS onset than after GVS offset; 3) the occurrence of vection path alterations after GVS onset depended on the intensity of GVS but not on the steepness of the GVS variation; 4) the vection path deviated laterally according to either an oblique or a curved path; and 5) the vection path deviated toward the cathode side after GVS onset. It is the first time that vestibular information, already known to contribute to the induction of vection, is shown to modify self-motion perception during the course of vection.