Information integration in judgements of time to contact

Neural mechanisms for integrating time and visual velocity cues in a prediction motion task: An fNIRS study

Human beings use accurate estimates of the time-to-collision of moving objects effortlessly in everyday life. In the laboratory, researchers typically apply prediction motion (PM) tasks to investigate motion processing. In the PM tasks, time structure refers to the ratio of travel time between the visible segment (first segment) and occluded segment (second segment). The condition of T = 1.0, which indicates that the time spent moving is the same across the two segments, is called equal time structure. The present study investigated the neural mechanisms of time and visual velocity information in prediction motion using functional near-infrared spectroscopy (fNIRS). Experiment 1 showed that when visual velocity was not available, participants performed better in equal time structure conditions than in unequal time structure conditions. Moreover, the left inferior parietal lobe (IPL) showed higher activation under equal time structure conditions. Experiment 2 showed that participants also performed better in equal time structure conditions when visual velocity was available. Both the left IPL and superior parietal lobe (SPL) exhibited stronger activation under equal time structure conditions in Experiment 2. A comparison between the two experiments showed that participants integrated time structure and visual velocity to estimate arrival time of the moving object. The fNIRS data indicated that the left SPL could be involved in information integration when judging arrival time.

Auditory affective content facilitates time-to-contact estimation of visual affective targets

Reacting to a moving object requires an ability to estimate when a moving object reaches its destination, also referred to as the time-to-contact (TTC) estimation. Although the TTC estimation of threatening visually moving objects is known to be underestimated, the effect of the affective content of auditory information on visual TTC estimation remains unclear. We manipulated the velocity and presentation time to investigate the TTC of a threat or non-threat target with the addition of auditory information. In the task, a visual or an audiovisual target moved from right to left and disappeared behind an occluder. Participants' task was to estimate the TTC of the target, they needed to press a button when they thought that the target contacted a destination behind the occluder. Behaviorally, the additional auditory affective content facilitated TTC estimation; velocity was a more critical factor than presentation time in determining the audiovisual threat facilitation effect. Overall, the results indicate that exposure to auditory affective content can influence TTC estimation and that the effect of velocity on TTC estimation will provide more information than presentation time.

Overestimated time-to-collision for quiet vehicles: Evidence from a study using a novel audiovisual virtual-reality system for traffic scenarios

To avoid collision, pedestrians intending to cross a road need to estimate the time-to-collision (TTC) of an approaching vehicle. Here, we present a novel interactive audiovisual virtual-reality system for investigating how the acoustic characteristics (loudness and engine type) of vehicles influence the TTC estimation. Using acoustic recordings of real vehicles as source signals, the dynamic spatial sound fields corresponding to a vehicle approaching in an urban setting are generated based on physical modeling of the sound propagation between vehicle and pedestrian and are presented via sound field synthesis. We studied TTC estimation for vehicles with internal combustion engine and for loudness-matched electric vehicles. The vehicle sound levels were varied by 10 dB, independently of the speed, presented TTC, and vehicle type. In an auditory-only condition, the cars were not visible, and lower loudness of the cars resulted in considerably longer TTC estimates. Importantly, the loudness of the cars also had a significant effect in the same direction on the TTC estimates in an audiovisual condition, where the cars were additionally visually presented via interactive virtual-reality simulations. Thus, pedestrians use auditory information when estimating TTC, even when full visual information is available. At equal loudness, the TTC judgments for electric and conventional vehicles were virtually identical, indicating that loudness has a stronger effect than spectral differences. Because TTC overestimations can result in risky road crossing decisions, the results imply that vehicle loudness should be considered as an important factor in pedestrian safety.

Wider is better but sharper is not: optimizing the image of camera-monitor systems

The replacement of rear-view mirrors with camera-monitor systems introduces new opportunities for design, such as altering the image quality and the rearward field-of-view. We investigated how the image quality and field-of-view might affect the distance and time-to-contact estimation of other vehicles. Eighty-six subjects estimated either their egocentric distance to a stationary vehicle (Experiment I) or the time-to-contact to an approaching vehicle (Experiment II). Throughout the experiments, the pixel density and either the field-of-view or the viewing condition varied. A larger field-of-view increased distance estimation accuracy and confidence. Reduced pixel density led to larger estimates. In contrast, reduced pixel density and simulated dirt shortened time-to-contact estimates. This is compatible with a safety strategy applied under conditions of impaired vision. Moreover, a limited benefit was observed for higher pixel densities. Therefore, camera-monitor systems with large field-of-view and a pixel density of around 300 ppi could ensure accurate TTC and distance estimation. Practitioner summary: A camera's field-of-view and image quality are important parameters for camera-monitor systems. In two experiments, we investigated the effects of these two parameters on rearward distance and time-to-contact estimation. Whereas a larger field-of-view improved distance estimation accuracy, increasing the pixel density had a limited effect in the estimation of time-to-contact.

Estimating time-to-contact when vision is impaired

Often, we have to rely on limited information when judging time-to-contact (TTC), as for example, when driving in foul weather, or in situations where we would need reading glasses but do not have them handy. However, most existing studies on the ability to judge TTC have worked with optimal visual stimuli. In a prediction motion task, we explored to what extent TTC estimation is affected by visual stimulus degradation. A simple computer-simulated object approached the observer at constant speed either with clear or impaired vision. It was occluded after 1 or 1.5 s. The observers extrapolated the object’s motion and pressed a button when they thought the object would have collided with them. We found that dioptric blur and simulated snowfall shortened TTC-estimates. Contrast reduction produced by a virtual semi-transparent mask lengthened TTC estimates, which could be the result of distance overestimation or speed underestimation induced by the lower contrast or the increased luminance of the mask. We additionally explored the potential influence of arousal and valence, although they played a minor role for basic TTC estimation. Our findings suggest that vision impairments have adverse effects on TTC estimation, depending on the specific type of degradation and the changes of the visual environmental cues which they cause.

Pedestrians Accept Shorter Distances to Light Vehicles Than Dark Ones When Crossing the Street

Does the brightness of an approaching vehicle affect a pedestrian's crossing decision? Thirty participants indicated their street-crossing intentions when facing approaching light or dark vehicles. The experiment was conducted in a real daylight environment and, additionally, in a corresponding virtual one. A real road with actual cars provides high face validity, while a virtual environment ensures the scenario's precise reproducibility and repeatability for each participant. In both settings, participants judged dark vehicles to be a more imminent threat-either closer or moving faster-when compared with light ones. Secondary results showed that participants accepted a significantly shorter time-to-contact when crossing the street in the virtual setting than on the real road.

Road crossing decisions in real and virtual environments: A comparative study on simulator validity

Virtual reality (VR) is a valuable tool for the assessment of human perception and behavior in a risk-free environment. Investigators should, however, ensure that the used virtual environment is validated in accordance with the experiment's intended research question since behavior in virtual environments has been shown to differ to behavior in real environments. This article presents the street crossing decisions of 30 participants who were facing an approaching vehicle and had to decide at what moment it was no longer safe to cross, applying the step-back method. The participants executed the task in a real environment and also within a highly immersive VR setup involving a head-mounted display (HMD). The results indicate significant differences between the two settings regarding the participants' behaviors. The time-to-contact of approaching vehicles was significantly lower for crossing decisions in the virtual environment than for crossing decisions in the real one. Additionally, it was demonstrated that participants based their crossing decisions in the real environment on the temporal distance of the approaching vehicle (i.e., time-to-contact), whereas the crossing decisions in the virtual environment seemed to depend on the vehicle's spatial distance, neglecting the vehicle's velocity. Furthermore, a deeper analysis suggests that crossing decisions were not affected by factors such as the participant's gender or the order in which they faced the real and the virtual environment.

Assessing pedestrian safety across modalities via a simulated vehicle time-to-arrival task

Pedestrians must use a variety of visual and auditory cues when determining safe crossing opportunities. Although vision has received a bulk of the attention in research on pedestrian safety, the examination of both vision and audition are important to consider. Environmental, intrapersonal, and cognitive qualities of a pedestrian context may limit the use of one or both perceptual modalities. Across two experiments, we examined the impact of perceptual constraints on pedestrian safety by measuring the accuracy of vehicle time-to-arrival estimates in a virtual environment when vehicles were only visible, only audible, or both visible and audible. In both experiments, participants estimated the time-to-arrival of vehicles moving at one of two speeds (8-kph, 40-kph). In the second experiment, we introduced ambient traffic noises to examine the impact of environmentally relevant traffic noises on pedestrian perception. Results suggest seeing a vehicle is more advantageous than hearing a vehicle when interacting with traffic, especially in the presence of ambient sound. Both experiments resulted in more accurate time-to-arrival estimates in the visual and mixed conditions than in the auditory-only condition. Implications for pedestrian safety and future research are discussed.

Angle of Camera View Influences Resumption Lag in a Visual-Motor Task

OBJECTIVE

To determine whether top-view and side-view camera angles, which putatively impose different cognitive demands, differentially affect the resumption lag in a visual-motor task relevant to laparoscopic surgery.

BACKGROUND

Prior research showed that the time to resume a primary task after performing an interrupting task (resumption lag) increases with increases in the subjective workload of the primary task. Camera views used in laparoscopic surgery provide different views of the anatomy and have different cognitive costs and associated levels of workload.

METHOD

Participants completed a peg transfer task while interrupted with a mental rotation task of different durations and angles of stimulus rotation.

RESULTS

Participants required significantly more time to resume the peg transfer task when using a side view than a top view and when interrupted for a longer duration. Participants' ratings of subjective workload were consistent with these patterns of performance data; the side view resulted in longer resumption lags and was rated as greater in mental demand. Additionally, the time needed to resume the peg transfer task decreased across trials for both views.

CONCLUSION

More time is required to resume an interrupted visual-motor task when it is more cognitively demanding than when it is less cognitively demanding possibly due to needing more time to learn the visual-motor mapping of the task higher in cognitive demand.

APPLICATION

Training for laparoscopic surgery should include interruptions to allow surgeons to practice resuming a surgery-related task after an interruption and consequently shorten the time needed to resume the surgery-related task.

Impending Collision Judgment from an Egocentric Perspective in Real and Virtual Environments: A Review

The human egocentric perception of approaching objects and the related perceptual processes have been of interest to researchers for several decades. This article gives a literature review on numerous studies that investigated the phenomenon when an object approaches an observer (or the other way around) with the goal to single out factors that influence the perceptual process. A taxonomy of metrics is followed by a breakdown of different experimental measurement methods. Thereinafter, potential factors affecting the judgment of approaching objects are compiled and debated while divided into human factors (e.g., gender, age, and driving experience), compositional factors (e.g., approaching velocity, spatial distance, and observation time), and technical factors (e.g., field of view, stereoscopy, and display contrast). Experimental findings are collated, juxtaposed, and critically discussed. With virtual-reality devices having taken a tremendous developmental leap forward in the past few years, they have been able to gain ground in experimental research. Therefore, special attention in this article is also given to the perception of approaching objects in virtual environments and put in contrast to the perception in reality.

Auditory pitch glides influence time-to-contact judgements of visual stimuli

A common experimental task used to study the accuracy of estimating when a moving object arrives at a designated location is the time-to-contact (TTC) task. The previous studies have shown evidence that sound motion cues influence TTC estimates of a visual moving object. However, the extent to which sound can influence TTC of visual targets still remains unclear. Some studies on the crossmodal correspondence between pitch and speed suggest that descending pitch sounds are associated with faster speeds compared to ascending pitch sounds due to an internal model of gravity. Other studies have shown an opposite pitch-speed mapping (i.e., ascending pitch associated with faster speeds) and no influence of gravity heuristics. Here, we explored whether auditory pitch glides, a continuous pure tone sound either ascending or descending in pitch, influence TTC estimates of a vertically moving visual target and if any observed effects are consistent with a gravity-centered or gravity-unrelated pitch-speed mapping. Subjects estimated when a disc moving either upward or downward at a constant speed reached a visual landmark after the disc disappeared behind an occluder under three conditions: with an accompanying ascending pitch glide, with a descending pitch glide, or with no sound. Overall, subjects underestimated TTC with ascending pitch glides and overestimated TTC with descending pitch glides, compared to the no-sound condition. These biases in TTC were consistent in both disc motion directions. These results suggest that subjects adopted a gravity-unrelated pitch-speed mapping where ascending pitch is associated with faster speeds and descending pitch associated with slower speeds.

Perception of time to contact of slow- and fast-moving objects using monocular and binocular motion information

The role of the monocular-flow-based optical variable τ in the perception of the time to contact of approaching objects has been well-studied. There are additional contributions from binocular sources of information, such as changes in disparity over time (CDOT), but these are less understood. We conducted an experiment to determine whether an object's velocity affects which source is most effective for perceiving time to contact. We presented participants with stimuli that simulated two approaching squares. During approach the squares disappeared, and participants indicated which square would have contacted them first. Approach was specified by (a) only disparity-based information, (b) only monocular flow, or (c) all sources of information in normal viewing conditions. As expected, participants were more accurate at judging fast objects when only monocular flow was available than when only CDOT was. In contrast, participants were more accurate judging slow objects with only CDOT than with only monocular flow. For both ranges of velocity, the condition with both information sources yielded performance equivalent to the better of the single-source conditions. These results show that different sources of motion information are used to perceive time to contact and play different roles in allowing for stable perception across a variety of conditions.

Effects of Auditory Patterns on Judged Displacements of an Occluded Moving Object

Using displays in which a moving disk disappeared behind an occluder, we examined whether an accompanying auditory rhythm influenced the perceived displacement of the disk during occlusion. We manipulated a baseline rhythm, comprising a relatively fast alternation of equal sound and pause durations. We had two different manipulations to create auditory sequences with a slower rhythm: either the pause durations or the sound durations were increased. In the trial, a disk moved at a constant speed, and at a certain point moved behind an occluder during which an auditory rhythm was played. Participants were instructed to track the occluded disk, and judge the expected position of the disk at the moment that the auditory rhythm ended by touching the judged position on a touch screen. We investigated the influence of the auditory rhythm, i.e., ratio of sound to pause duration, and the influence of auditory density, i.e., the number of sound onsets per time unit, on the judged distance. The results showed that the temporal characteristics affected the spatial judgments. Overall, we found that in the current paradigm relatively slow rhythms led to shorter judged distance as compared to relatively fast rhythms for both pause and sound variations. There was no main effect of auditory density on the judged distance of an expected visual event. That is, whereas the speed of the auditory rhythm appears crucial, the number of sound onsets per time unit as such, i.e., the auditory density, appears a much weaker factor.

Does Binocular Disparity or Familiar Size Information Override Effects of Relative Size on Judgements of Time to Contact?

Previous studies indicate that non-tau sources of depth information, such as pictorial depth cues, can influence judgements of time to contact (TTC). The effect of relative size on such judgements, the size-arrival effect, is particularly robust. However, earlier studies of the size-arrival effect did not include binocular disparity or familiar size information. The effects of these cues on relative TTC judgements were measured. Results suggested that disparity can eliminate the size-arrival effect but that the amount of disparity needed to do so is greater than typical stereoacuity thresholds. In contrast, familiar size eliminated the size-arrival effect even when disparity information was not available. Furthermore, disparity contributed more to performance when familiar size was present than when it was absent. Consistent with previous studies, TTC judgements were influenced by multiple sources of information. The present results suggested further that familiar size is one such source of information and that familiar size moderates the influence of binocular disparity information.

Auditory Rhythms Influence Judged Time to Contact of an Occluded Moving Object

We studied the expected moment of reappearance of a moving object after it disappeared from sight. In particular, we investigated whether auditory rhythms influence time to contact (TTC) judgments. Using displays in which a moving disk disappears behind an occluder, we examined whether an accompanying auditory rhythm influences the expected TTC of an occluded moving object. We manipulated a baseline auditory rhythm — consisting of equal sound and pause durations — in two ways: either the pause durations or the sound durations were increased to create slower rhythms. Participants had to press a button at the moment they expected the disk to reappear. Variations in pause duration (Experiments 1 and 2) affected expected TTC, in contrast to variations in sound duration (Experiment 3). These results show that auditory rhythms affect expected reappearance of an occluded moving object. Second, these results suggest that temporal auditory grouping is an important factor in TTC.

Audiovisual Integration of Time-to-Contact Information for Approaching Objects

Previous studies of time-to-collision (TTC) judgments of approaching objects focused on effectiveness of visual TTC information in the optical expansion pattern (e.g., visual tau, disparity). Fewer studies examined effectiveness of auditory TTC information in the pattern of increasing intensity (auditory tau), or measured integration of auditory and visual TTC information. Here, participants judged TTC of an approaching object presented in the visual or auditory modality, or both concurrently. TTC information provided by the modalities was jittered slightly against each other, so that auditory and visual TTC were not perfectly correlated. A psychophysical reverse correlation approach was used to estimate the influence of auditory and visual cues on TTC estimates. TTC estimates were shorter in the auditory than the visual condition. On average, TTC judgments in the audiovisual condition were not significantly different from judgments in the visual condition. However, multiple regression analyses showed that TTC estimates were based on both auditory and visual information. Although heuristic cues (final sound pressure level, final optical size) and more reliable information (relative rate of change in acoustic intensity, optical expansion) contributed to auditory and visual judgments, the effect of heuristics was greater in the auditory condition. Although auditory and visual information influenced judgments, concurrent presentation of both did not result in lower response variability compared to presentation of either one alone; there was no multimodal advantage. The relative weightings of heuristics and more reliable information differed between auditory and visual TTC judgments, and when both were available, visual information was weighted more heavily.

Effects of vehicle speed on flight initiation by Turkey vultures: implications for bird-vehicle collisions

The avoidance of motorized vehicles is a common challenge for birds in the modern world. Birds appear to rely on antipredator behaviors to avoid vehicles, but modern vehicles (automobiles and aircraft) are faster than natural predators. Thus, birds may be relatively ill-equipped, in terms of sensory capabilities and behaviors, to avoid vehicles. We examined the idea that birds may be unable to accurately assess particularly high speeds of approaching vehicles, which could contribute to miscalculations in avoidance behaviors and ultimately cause collisions. We baited turkey vultures (Cathartes aura) to roads with animal carcasses and measured flight initiation distance and effective time-to-collision in response to a truck driving directly towards vultures from a starting distance of 1.13 km and at one of three speeds: 30, 60, or 90 kph (no vultures were struck). Flight initiation distance of vultures increased by a factor of 1.85 as speed increased from 30 to 90 kph. However, for 90-kph approaches there was no clear trend in flight initiation distance across replicates: birds appeared equally likely to initiate escape behavior at 40 m as at 220 m. Time-to-collision decreased by a factor of 0.62 with approach speeds from 30 to 90 kph. Also, at 90 kph, four vehicle approaches (17%) resulted in near collisions with vultures (time-to-collision ≤1.7 s), compared to none during 60 kph approaches and one during 30 kph approaches (4%). Our findings suggest that antipredator behaviors in turkey vultures, particularly stimulus processing and response, might not be well tuned to vehicles approaching at speeds ≥90 kph. The possible inability of turkey vultures to react appropriately to high-speed vehicles could be common among birds, and might represent an important determinant of bird-vehicle collisions.

Effects of Size on Collision Perception and Implications for Perceptual Theory and Transportation Safety

People avoid collisions when they walk or drive, and they create collisions when they hit balls or tackle opponents. To do so, people rely on the perception of depth (perception of objects’ locations) and time-to-collision (perception of when a collision will occur), which are supported by different information sources. Depth cues, such as relative size, provide heuristics for relative depth, whereas optical invariants, such as tau, provide reliable time-to-collision information. One would expect people to rely on invariants rather than depth cues, but the size-arrival effect shows the contrary: People reported that a large far approaching object would hit them sooner than a small near object that would have hit first. This effect of size on collision perception violates theories of time-to-collision perception based solely on the invariant tau and suggests that perception is based on multiple information sources, including heuristics. The size-arrival effect potentially can lead drivers to misjudge when a vehicle would arrive at an intersection and is considered a contributing factor in motorcycle accidents. In this article, I review research on the size-arrival effect and its theoretical and practical implications.

Visual cues for manual control of headway

The ability to maintain appropriate gaps to objects in one's environment is important when navigating through a three-dimensional world. Previous research has shown that the visual angle subtended by a lead/approaching object and its rate of change are important variables for timing interceptions, collision avoidance, continuous regulation of braking, and manual control of headway. However, investigations of headway maintenance have required participants to maintain a fixed distance headway and have not investigated how information about own-speed is taken into account. In the following experiment, we asked participants to use a joystick to follow computer-simulated lead objects. The results showed that ground texture, following speed, and the size of the lead object had significant effects on both mean following distances and following distance variance. Furthermore, models of the participants' joystick responses provided better fits when it was assumed that the desired visual extent of the lead object would vary over time. Taken together, the results indicate that while information about own-speed is used by controllers to set the desired headway to a lead object, the continuous regulation of headway is influenced primarily by the visual angle of the lead object and its rate of change. The reliance on visual angle, its rate of change, and/or own-speed information also varied depending on the control dynamics of the system. Such findings are consistent with an optimal control criterion that reflects a differential weighting on different sources of information depending on the plant dynamics. As in other judgements of motion in depth, the information used for controlling headway to other objects in the environment varies depending on the constraints of the task and different strategies of control.

Threatening pictures induce shortened time-to-contact estimates

The ability to estimate the time remaining until collision occurs with an approaching object (time-to-collision, TTC) is crucial for any mobile animal. In the present study, we report three experiments examining whether higher level cognitive factors, represented by affective value of approaching objects, could affect judgments of TTC. A theory of TTC estimates based purely on the optical variable tau does not predict an influence of the affective value of an approaching object. In Experiments 1 and 2, we compared TTC estimates of threatening and neutral pictures that approached our participants on a screen and disappeared from view before a collision would have occurred. Images were taken from the International Affective Picture System. Threatening pictures-in particular, the picture of a frontal attack-were judged to collide earlier than neutral pictures. In Experiment 3, the approaching stimuli were faces with different emotional expressions. TTC tended to be underestimated for angry faces. We discuss these results, considering the roles of affective and cognitive mechanisms modulating TTC estimation and general time perception.

Judgments of approach speed for motorcycles across different lighting levels and the effect of an improved tri-headlight configuration

The misperception of vehicle approach speed is a key contributory factor to road traffic crash involvement. Past research has indicated that individuals use the rate of visual looming to calculate the time to passage (TTP) of a vehicle, and that smaller vehicles loom to a lesser extent than larger vehicles. Despite a disproportionate number of fatal injuries occurring on the road after dark, and a higher than average number of accidents involving automobile drivers violating the right of way of a motorcyclist occurring in low light conditions, there has been very little consideration of the accuracy of TTP for smaller and larger vehicles under low levels of luminance. We investigated drivers' judgments of motorcycle and car approach speeds across a number of levels of luminance within a virtual city scene, as well as the effectiveness of a tri-headlight formation on motorcycle speed judgments. The accuracy of car approach speed judgments were not affected by changes in lighting conditions, but speed judgments for the solo headlight motorcycle became significantly less accurate as lighting reduced in the early night and night-time conditions. Incorporation of a tri-headlight formation onto the standard motorcycle frame resulted in improved accuracy of approach speed judgments, relative to the solo headlight motorcycle, as ambient light levels reduced. The practical implications of the findings are discussed in terms of road safety and motorcycle design.

People favour imperfect catching by assuming a stable world

The visual angle that is projected by an object (e.g. a ball) on the retina depends on the object's size and distance. Without further information, however, the visual angle is ambiguous with respect to size and distance, because equal visual angles can be obtained from a big ball at a longer distance and a smaller one at a correspondingly shorter distance. Failure to recover the true 3D structure of the object (e.g. a ball's physical size) causing the ambiguous retinal image can lead to a timing error when catching the ball. Two opposing views are currently prevailing on how people resolve this ambiguity when estimating time to contact. One explanation challenges any inference about what causes the retinal image (i.e. the necessity to recover this 3D structure), and instead favors a direct analysis of optic flow. In contrast, the second view suggests that action timing could be rather based on obtaining an estimate of the 3D structure of the scene. With the latter, systematic errors will be predicted if our inference of the 3D structure fails to reveal the underlying cause of the retinal image. Here we show that hand closure in catching virtual balls is triggered by visual angle, using an assumption of a constant ball size. As a consequence of this assumption, hand closure starts when the ball is at similar distance across trials. From that distance on, the remaining arrival time, therefore, depends on ball's speed. In order to time the catch successfully, closing time was coupled with ball's speed during the motor phase. This strategy led to an increased precision in catching but at the cost of committing systematic errors.

The effects of familiar size and object trajectories on time-to-contact judgements

Many interceptive actions involve interactions with objects that are familiar to the observer and have known sizes. Two experiments investigated how known size influences observers' perception of time-to-contact (T(c)). Participants made T(c) judgements of objects that were either ambiguously sized, standard-size in identity/familiarity, or off-size in identity/familiarity, and simulated as approaching on linear trajectories (Experiment 1), or linear versus parabolic trajectories (Experiment 2). In Experiment 1, T(c) judgements were influenced by the size of the object in the three object identity/familiarity conditions; the greatest size effect occurred in the off-size condition compared to the ambiguous size and standard-size conditions. The results of Experiment 2 replicated these results and found that size effects were not reduced with displays simulating parabolic trajectories, that is, displays simulating ecologically valid free-falling objects. Taken together, the finding that T(c) judgements are influenced by object identity/familiarity does not provide support for the tau hypothesis, nor the hypothesis that T(c) judgements are based solely on optic expansion rates. However, the results do provide support for the proposition that T(c) judgements are based on a combination of rate of retinal image expansion and object identity/familiarity information, the latter information requiring observers to have prior experience with, or knowledge about, the objects.

Judgements of time to contact are affected by rate of appearance of visible texture

More fine-grained texture becomes visible if the distance between an observer and an object or surface is reduced. This article illustrates with a schematic example that the inverse rate of the relative appearance of visible texture provides information about time to contact if the observer has a constant visual acuity and the texture has a certain scale-independent structure. An experiment is reported in which texture appearance was manipulated. Participants were asked to make forced-choice time-to-contact judgements. A small but significant effect indicates that the judgements were affected by the rate of appearance of the texture. It is concluded that observers use this type of information.

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.

Detection of collision events on curved trajectories: optical information from invariant rate-of-bearing change

Previous research (Andersen & Kim, 2001) has shown that a linear trajectory collision event (i.e., a collision between a moving object and an observer) is specified by objects that expand and maintain a constant bearing (the object location remains constant in the visual field). In the present study, we examined the optical information for detecting a collision event when the trajectory was of constant curvature. Under these conditions, a collision event is specified by expansion of an object and a constant rate-of-bearing change. Three experiments were conducted in which trajectory curvature and display duration were varied while time to contact, speed, and initial image position of the collision objects were maintained. The results indicated that collision detection performance decreased with an increase in trajectory curvature and decreased with a decrease in display duration, especially for highly curved trajectories. In Experiment 3, we found that the presentation of a constant rate-of-bearing change in noncollision stimuli resulted in an increase in the false alarm rate. These results demonstrate that observers can detect collision events on curved trajectories and that observers utilize bearing change information.

Effects of texture and shape on perceived time to passage: knowing "what" influences judging "when"

In the present study, we examined whether it is easier to judge when an object will pass one's head if the object's surface is textured. There are three reasons to suspect that this might be so: First, the additional (local) optic flow may help one judge the rate of expansion and the angular velocity more reliably. Second, local deformations related to the change in angle between the object and the observer could help track the object's position along its path. Third, more reliable judgments of the object's shape could help separate global expansioncaused by changes in distance from expansion due to changes in the angle between the object and the observer. We can distinguish among these three reasons by comparing performance for textured and uniform spheres and disks. Moving objects were displayed for 0.5-0.7 sec. Subjects had to decide whether the object would pass them before or after a beep that was presented 1 sec after the object started moving. Subjects were not more precise with textured objects. When the disk rotated in order to compensate for the orientation-related contraction that its image would otherwise undergo during its motion, it appeared to arrive later, despite the fact that this strategy increases the global rate of expansion. We argue that this is because the expected deformation of the object's image during its motion is considered when time to passage is judged. Therefore, the most important role for texture in everyday judgments of time to passage is probably that it helps one judge the object's shape and thereby estimate how its image will deform as it moves.

Intercepting free falling objects: better use Occam's razor than internalize Newton's law

Several studies have recently provided empirical data supporting the view that gravity has been embodied in a quantitative internal model of gravity thereby permitting access to exact time-to-contact (TTC) when intercepting a free falling object. In this review, we discuss theoretical and methodological concerns with the experiments that supposedly support the assumption of a predictive and accurate model of gravity. Having done so, we then propose that only a "qualitative implicit physics knowledge" of the effects of gravity is used as an approximate pre-information that influences timing of interceptive actions in the specific case of free falling objects. Clear evidence remains to be provided to define how this knowledge is combined with optical information for on-line timing of interceptive actions.

Visual control of action without retinal optic flow

In everyday life, the optic flow associated with the performance of complex actions, like walking through a field of obstacles and catching a ball, entails retinal flow with motion energy (first-order motion). We report the results of four complex action tasks performed in virtual environments without any retinal motion energy. Specifically, we used dynamic random-dot stereograms with single-frame lifetimes (cyclopean stimuli) such that in neither eye was there retinal motion energy or other monocular information about the actions being performed. Performance on the four tasks with the cyclopean stimuli was comparable to performance with luminance stimuli, which do provide retinal optic flow. The near equivalence of the two types of stimuli indicates that if optic flow is involved in the control of action, it is not tied to first-order retinal motion.

Unconfounding the direction of motion in depth, time to passage and rotation rate of an approaching object

Observers were presented with a set of 216 simulated approaching textured baseballs in random order. In Experiment 1 each had a different combination of time to passage (TTP), direction of motion in depth (dMID) in the vertical plane and total change in angular size (Deltatheta). In Experiments 2 and 3 each had a different combination of TTP, dMID and rate of ball rotation (RR). When required to discriminate TTP and dMID in separate experimental blocks for a non-rotating baseball (Experiment 1), observers could not discriminate dMID independently of variations in TTP but instead showed a bias towards perceiving objects approaching on a trajectory close to the nose as having a shorter TTP than objects approaching on a trajectory that would miss the face. When required to discriminate TTP, dMID and RR in separate experimental blocks (Experiment 2), TTP judgments were again influenced by dMID but could be made independently of RR. Judgments of the relative dMID were affected by variations in RR and rotation direction: for simulated overspin the (i.e., the top of the ball spins towards the observer) perceived ball trajectory was biased towards the ground whereas for simulated underspin the perceived ball trajectory was biased towards the sky. RR could be discriminated independently of both TTP and dMID. When required to make all three of these judgments simultaneously on each trial (Experiment 3) discrimination thresholds were not appreciably different from those found in Experiment 2. We conclude that TTP, dMID and RR can be estimated in parallel but not completely independently within the human visual system.

Motorcycle Accident Risk Could Be Inflated by a Time to Arrival Illusion

PURPOSE

Drivers adopt smaller safety margins when pulling out in front of motorcycles compared with cars. This could partly account for why the most common motorcycle/car accident involves a car violating a motorcyclist's right of way. One possible explanation is the size-arrival effect in which smaller objects are perceived to arrive later than larger objects. That is, drivers may estimate the time to arrival of motorcycles to be later than cars because motorcycles are smaller.

METHODS

We investigated arrival time judgments using a temporal occlusion paradigm. Drivers recruited from the student population (n = 28 and n = 33) saw video footage of oncoming vehicles and had to press a response button when they judged that vehicles would reach them.

RESULTS

In experiment 1, the time to arrival of motorcycles was estimated to be significantly later than larger vehicles (a car and a van) for different approach speeds and viewing times. In experiment 2, we investigated an alternative explanation to the size-arrival effect: that the smaller size of motorcycles places them below the threshold needed for observers to make an accurate time to arrival judgment using tau. We found that the motorcycle/car difference in arrival time estimates was maintained for very short occlusion durations when tau could be estimated for both motorcycles and cars.

CONCLUSIONS

Results are consistent with the size-arrival effect and are inconsistent with the tau threshold explanation. Drivers estimate motorcycles will reach them later than cars across a range of conditions. This could have safety implications.

Effects of intrastimulus modality change on audiovisual time-to-arrival judgments

In this research, anticipatory perception of an approaching vehicle was examined. By varying the availability of visual and acoustic media, conditions occurred in which the input to the modalities was repetitively interrupted while the presentation of the approach event continued. In these conditions, the audio and the visual signals were presented either in short and concurrent segments at regular intervals or in alternation (interleaved) at regular intervals. It was found that interrupting the signal within a single modality did not affect performance if the approach information was available in the alternate modality. These results are consistent with a modally flexible detection mechanism for the perception of approaching objects. This modal flexibility may provide some evidence that information is detected using a modality-neutral strategy.