Global Optical Flow Research Articles

Active Regulation of Speed during a Simulated Low-Altitude Flight Task

This study examined active regulation of speed during a low-altitude flight task as a function of global optical flow rate, speed, and the presence or absence of a concurrent altitude disturbance. The results showed that altitude clearly had an impact on speed control. Control of speed was much more difficult when altitude disturbances were present. Even in the no altitude disturbance conditions, performance tended to be best at lower altitudes. Consistent with previous research, the results suggest that speed and altitude changes have additive effects on speed judgments. This is inconsistent with the simple global optical flow rate hypothesis that had suggested multiplicative effects. However, it is consistent with the general notion that judgments of self motion are based on properties of optical flow fields (i.e. angles and angular rates) that depend on distance and motion relative to textured surfaces.

Responsiveness to terrestrial optic flow in infancy: Does locomotor experience play a role?

Human infants show a peak in postural compensation to optic flow at approximately nine months of age. The current experiment tested whether the magnitude of visual–postural coupling in 9-month-olds increases when terrestrial optic flow is added to a moving room. A secondary objective was to explore whether locomotor experience plays any role in enhancing responsiveness to the additional terrestrial information. Ninety-one infants (experienced creepers, nascent creepers, and prelocomotors) were exposed to two conditions of optic flow: global optic flow ( G) and global optic flow minus terrestrial optic flow ( G − T). The additional terrestrial optic flow led to significantly higher visual–postural coupling. Consistent with previous findings, locomotor experience had no effect on responsiveness to the G − T condition, though there was weak evidence that the nascent creepers were more strongly influenced by the difference between flow conditions than the other infants. Unexpectedly, the prelocomotor females showed significantly lower visual–postural coupling than the prelocomotor males. These findings support the notion that the ground provides an important source of information for the control of posture and locomotion. The findings also suggest that locomotor experience most likely helps to functionalize smaller (partial), rather than larger (global), optic flow fields for postural control.

Learning to control collisions: The role of perceptual attunement and action boundaries.

The authors investigated the role of perceptual attunement in an emergency braking task in which participants waited until the last possible moment to slam on the brakes. Effects of the size of the approached object and initial speed on the initiation of braking were used to identify the optical variables on which participants relied at various stages of practice. In Experiments 1A and 1B, size and speed effects that were present early in practice diminished but were not eliminated as participants learned to initiate braking at a rate of optical expansion that varied with optical angle. When size and speed were manipulated together in Experiment 2, the size effect was quickly eliminated, and participants learned to use a 3rd optical variable (global optic flow rate) to nearly eliminate the speed effect. The authors conclude that perceptual attunement depends on the range of practice conditions, the availability of information, and the criteria for success.

Analog Integrated 2-D Optical Flow Sensor

I present a new focal-plane analog very-large-scale-integrated (aVLSI) sensor that estimates optical flow in two visual dimensions. Its computational architecture consists of a two-layer network of locally connected motion units that collectively estimate the optimal optical flow field. The applied gradient-based optical flow model assumes visual motion to be translational and smooth, and is formulated as a convex optimization problem. The model also guarantees that the estimation problem is well-posed regardless of the visual input by imposing a bias towards a preferred motion under ambiguous or noisy visual conditions. Model parameters can be globally adjusted, leading to a rich output behavior. Varying the smoothness strength, for example, can provide a continuous spectrum of motion estimates, ranging from normal to global optical flow. The non-linear network conductances improve the resulting optical flow estimate because they reduce spatial smoothing across large velocity differences and minimize the bias for reliable stimuli. Extended characterization and recorded optical flow fields from a 30 × 30 array prototype sensor demonstrate the validity of the optical flow model and the robustness and functionality of the computational architecture and its implementation.

Information for step length adjustment in running

The purpose of this study was to directly test the hypothesis that the tau parameter, as introduced by Lee et al. [Lee, D. N., Lishman, J. R., & Thomson, J. A. (1982). Regulation of gait in long jumping. Journal of Experimental Psychology: Human Perception and Performance, 8, 448–459] and Warren et al. [Warren, W. H., Young, D. S., & Lee, D. N. (1986). Visual control of step length during running over irregular terrain. Journal of Experimental Psychology: Human Perception and Performance, 12, 259–266], is the primary information used to regulate step length in running toward a ground target. Visual information available to research participants performing a facsimile of a long jump approach run was manipulated, and the effects on task performance and running gait were observed. Task performance and running gait were unaffected or minimally affected by (a) a perturbation of the normal optical expansion of the target, (b) elimination of global optical flow – including the focus of expansion, and (c) elimination or severe degradation of visual information about distance from the target as well as running velocity. The findings are inconsistent with notions that time-to-arrival with a ground target while running is predominantly specified optically by (a) local tau, (b) global tau, or (c) a distance/velocity computational strategy. A multisensory tau hypothesis regarding informational support for step length adjustment in running is offered.

Optical flow and viewpoint change modulate the perception and memorization of complex motion

Participants observed a point-light character (PLC) performing a gymnastic movement. They either memorized the final PLC orientation from the initial viewpoint, to match it to a test posture (memory task), or judged whether the biological motion appeared continuous (perceptual task), despite a viewpoint change. The observer could be either static or virtually in motion (pan or track) while looking at the movement from the initial viewpoint. The presence of a spatial layout during virtual self-motion induced a global optical flow specifying the translational component of the PLC movement, rendering the event more predictable for the participants. A representational momentum effect was observed in the memory task, suggesting that when a visual stimulation, such as a PLC motion, is abruptly stopped, its dynamics survive. In contrast, structural and transformational invariants specifying the PLC motion were sufficient to solve the perceptual task accurately. Finally, both the remembering of the final posture and the perception of continuity degraded with an increase in viewpoint change due to tilt/slant posture orientation matching, indicating that orientation processes interfered with event perception.

Drivers' Gaze Patterns in Braking From Three Different Approaches to a Crash Barrier

In our previous research, we found that drivers when engaged in straight-road driving, and also braking, tend to look in the direction of their intended movement regardless of speed or expertise (Rogers, Kadar, & Costall, this issue). Furthermore, gaze direction is highly constrained in braking, suggesting that speed control might be based on global optic flow. This study further investigates gaze when drivers are braking in front of a vertical barrier and either approaching the barrier orthogonally or from the right or the left. These three different directions of approach entail three different potential points of collision: the nose of the car, or the front right or left wheel. In all conditions, drivers directed their gaze in the direction of their movement. However, in the case of nonorthogonal approach, the drivers at the very end of their approach shifted their gaze away from their direction of travel and toward the potential collision point on the barrier. On the basis of these findings, we propose that drivers tend to rely on global optic flow to control their speed, but can also control speed by the local rate of optic expansion around the point of fixation (i.e., the point of potential collision).

Calibration, Information, and Control Strategies for Braking to Avoid a Collision.

This study explored visual control strategies for braking to avoid collision by manipulating information about speed of self-motion. Participants watched computer-generated displays and used a brake to stop at an object in the path of motion. Global optic flow rate and edge rate were manipulated by adjusting eye-height and ground-texture size. Stopping distance, initiation of braking, and the magnitude of brake adjustments were influenced by both optical variables, but global optic flow rate had a stronger effect. A new model is introduced according to which braking is controlled by keeping the perceived ideal deceleration, based in part on global optic flow rate, within a "safe" region between 0 and the maximum deceleration of the brake.

Judgments of Speed of Self-Motion: Modeling the Relative Effects of Speed and Altitude Change

Empirical studies consistently show that judgments of the speed of self-motion are influenced by changes in the altitude of an observer. In general, a given actual speed is judged to be relatively faster from a lower altitude and relatively slower from a higher altitude. Similarly, loss of altitude can be accompanied by a false perception of increasing speed, while increases in altitude can be accompanied by a false perception of loss in speed. The direction of these effects is consistent with the Global Optical Flow Rate (GOFR) Hypothesis. However, the pattern of effects is not proportional to the ratio of velocity to altitude as predicted by the GOFR Hypothesis. An alternative model is presented that predicts additive effect of speed and altitude changes on speed perception.

Visual guidance of intercepting a moving target on foot.

How do people walk to a moving target, and what visual information do they use to do so? Under a pursuit strategy, one would head toward the target's current position, whereas under an interception strategy, one would lead the target, ideally by maintaining a constant target-heading angle (or constant bearing angle). Either strategy may be guided by the egocentric direction of the target, local optic flow from the target, or global optic flow from the background. In four experiments, participants walked through a virtual environment to reach a target moving at a constant velocity. Regardless of the initial conditions, they walked ahead of the target for most of a trial at a fairly constant speed, consistent with an interception strategy (experiment 1). This behavior can be explained by trying to maintain a constant target-heading angle while trying to walk a straight path, with transient steering dynamics. In contrast to previous results for stationary targets, manipulation of the local optic flow from the target (experiment 2) and the global optic flow of the background (experiments 3 and 4) failed to influence interception behavior. Relative motion between the target and the background did affect the path slightly, presumably owing to its effect on perceived target motion. We conclude that humans use an interception strategy based on the egocentric direction of a moving target.

Robustness of the tuning of fly visual interneurons to rotatory optic flow.

The sophisticated receptive field organization of motion-sensitive tangential cells in the visual system of the blowfly Calliphora vicina matches the structure of particular optic flow fields. Hypotheses on the tuning of particular tangential cells to rotatory self-motion are based on local motion measurements. So far, tangential cells have never been tested with global optic flow stimuli. Therefore we measured the responses of an identifiable neuron, the V1 tangential cell, to wide-field motion stimuli mimicking optic flow fields similar to those the fly encounters during particular self-motions. The stimuli were generated by a "planetarium-projector," casting a pattern of moving light dots on a large spherical projection screen. We determined the tuning curves of the V1-cell to optic flow fields as induced by the animal during 1) rotation about horizontally aligned body axes, 2) upward/downward translation, and 3) a combination of both components. We found that the V1-cell does not respond as specifically to self-rotations, as had been concluded from its receptive field organization. The neuron responds strongly to upward translation and its tuning to rotations is much coarser than expected. The discrepancies between the responses to global optic flow and the predictions based on the receptive field organization are likely due to nonlinear integration properties of tangential neurons. Response parameters like orientation, shape, and width of the tuning curve are largely unaffected by changes in rotation velocity or a superposition of rotational and translational optic flow.

Visual search asymmetries in motion and optic flow fields.

In visual search, items defined by a unique feature are found easily and efficiently. Search for a moving target among stationary distractors is one such efficient search. Search for a stationary target among moving distractors is markedly more difficult. In the experiments reported here, we confirm this finding and further show that searches for a stationary target within a structured flow field are more efficient than searches for stationary targets among distractors moving in random directions. The structured motion fields tested included uniform direction of motion, a radial flow field simulating observer forward motion, and a deformation flow field inconsistent with observer motion. The results using optic flow stimuli were not significantly different from the results obtained with other structured fields of distractors. The results suggest that the local properties of the flow fields rather than global optic flow properties are important for determining the efficiency of search for a stationary target.

Image velocity, not tau, explains arrival-time judgments from global optical flow.

Image velocity, not tau, explains arrival-time judgments from global optical flow.

Perception of Egospeed from Absolute and Relative Motion

Perceiving the speed of self-motion or egospeed directly from the view out a window is important in some vehicular control tasks. Recent research found that visual perception of egospeed is primarily based on global optical flow rate, which is directly related to the magnitude of both absolute and relative motion. The purpose of the present study was to determine whether the magnitude of relative motion affects egospeed estimates independently of the magnitude of absolute motion. Observers estimated their perceived egospeed during simulations of flight over three parallel planes. We manipulated the degree of relative motion by varying the interplane distance, the spacing between the two outer planes and the center plane. The results indicate that relative motion affects perception of egospeed independently of absolute motion. Hence, it is important to consider the magnitude of relative motion when designing head-up displays that present optical flow to augment speed information available through the windscreen.

Control of Egospeed: Influence of Discontinuity Rate and Global Optical Flow Rate

Control of Egospeed: Influence of Discontinuity Rate and Global Optical Flow Rate

Image velocity, not tau, explains arrival-time judgments from global optical flow.

The time-to-passage (TTP; i.e., the time) until an object passes an observer is optically specified by global tau, a variable that operates on the expansion rate of the angle subtended by an object relative to the observer's heading. M. K. Kaiser and L. Mowafy (1993) provided evidence for observers' sensitivity to global tau in a 3-D cloud of point lights. This interpretation is challenged, and it is suggested that TTP judgments are based on a related but much simpler variable, the image velocity of the object. The present study reexamined several factors that are relevant for the extraction of global tau. When global tau and image velocity were brought into conflict by varying the lateral offsets of the targets, observers showed a strong tendency to rely on the latter variable. Other factors that are supposed to affect TTP judgments only if observers relied on global tau, such as flow-field density and gazemovement angle, did not affect performance.

The utility of motion parallax information for the perception and control of heading.

Two experiments in which participants were given control over the direction of computer-simulated self-motion were conducted. Environments were designed to evaluate the functionality of simple and multiple motion parallax as well as a separation ratio (sigma; indexing the separation of 2 objects in depth) for the perception and control of heading. Results provide a 1st indication of optimizing performance in the top end of the global optical flow velocity range available during human bipedal self-motion. The introduction of sigma, developed to explain performance improvements with decreasing distance to the target, was able to account for most of the performance differences among all simulated environments. The rate of change in horizontal optical separation between at least 2 discontinuities was identified as a likely candidate for the optical foundation of the perception and control of heading during target approach.

Use of Global Optical Flow Rate and Discontinuity Rate Depends on Their Validity as Determinants of Egospeed

Previous research found both discontinuity rate and global optical flow rate contribute to the perception of egospeed but differed in estimates of which source of information dominated. This research examined two methodological differences that may explain the conflicting results: frame rate, and the relative validity of discontinuity rate and flow rate as determinants of egospeed. Observers estimated egospeed perceived during simulations of self-motion. Discontinuity rate and flow rate were manipulated independently by varying velocity and altitude. Frame rates were consistent with Dyre (1997) and the validity of discontinuity rate and flow rate as determinants of egospeed was varied. We found the relative use of discontinuity rate and flow rate was related to the validity of these sources of information. Frame rate was not an important factor. These results suggest the visual system is sensitive to multiple sources of information for perceiving egospeed and adapts to the most optimal source.

Emulating the Visual Receptive-Field Properties of MST Neurons with a Template Model of Heading Estimation

We have proposed previously a computational neural-network model by which the complex patterns of retinal image motion generated during locomotion (optic flow) can be processed by specialized detectors acting as templates for specific instances of self-motion. The detectors in this template model respond to global optic flow by sampling image motion over a large portion of the visual field through networks of local motion sensors with properties similar to those of neurons found in the middle temporal (MT) area of primate extrastriate visual cortex. These detectors, arranged within cortical-like maps, were designed to extract self-translation (heading) and self-rotation, as well as the scene layout (relative distances) ahead of a moving observer. We then postulated that heading from optic flow is directly encoded by individual neurons acting as heading detectors within the medial superior temporal (MST) area. Others have questioned whether individual MST neurons can perform this function because some of their receptive-field properties seem inconsistent with this role. To resolve this issue, we systematically compared MST responses with those of detectors from two different configurations of the model under matched stimulus conditions. We found that the characteristic physiological properties of MST neurons can be explained by the template model. We conclude that MST neurons are well suited to support self-motion estimation via a direct encoding of heading and that the template model provides an explicit set of testable hypotheses that can guide future exploration of MST and adjacent areas within the superior temporal sulcus.

Perception of Accelerating Self-Motion: Global Optical Flow Rate Dominates Discontinuity Rate

Owen, Wolpert, and Warren (1984) proposed that egospeed may be perceived from global optical flow rate, discontinuity rate, or both. Previous psychophysical research found that both sources of information influence judgments of acceleration and control of egospeed, but that discontinuity rate dominated. However, the validity of these results is questionable due to problems with the visual stimuli used, such as confounding of discontinuity rate with proximal flow rate and low frame rates. The current study examined the relative contributions of global optical flow rate and discontinuity rate to perception of accelerating self-motion with stimuli that lacked these problems. I found that global optical flow rate accounted for 60% of the variability in acceleration judgments, compared with 0.86% for discontinuity rate. This result indicates that discontinuity rate exerts only a minor influence, and global optical flow rate is the primary basis for perception of accelerating self-motion, and hence, egospeed.