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When people are required to indicate the vanishing location of a moving object, systematic biases forward, in the direction of motion, and downward, in the direction of gravity, are usually found. Both these displacements, called representational momentum and representational gravity, respectively, are thought to reflect anticipatory internal mechanisms aiming to overcome neural delays in the perception of motion. We challenge this view. There may not be such a single mechanism. Although both representational momentum and representational gravity follow a specific time-course, compatible with an anticipation of the object's dynamics, they do not seem to be commensurable with each other, as they are differentially modulated by relevant variables, such as eye movements and strength of motion signals. We found separate response components, one related to overt motor localization behaviour and one limited to purely perceptual judgement. Representational momentum emerged only for the motor localization task, revealing a motor overshoot. In contrast, representational gravity was mostly evident for spatial perceptual judgements. We interpret the results in support of a partial dissociation in the mechanisms that give rise to representational momentum and representational gravity, with the former but not the latter strongly modulated by the enrolment of the motor system.

The remembered vanishing location of a moving target has been found to be displaced downward in the direction of gravity (representational gravity) and more so with increasing retention intervals, suggesting that the visual spatial updating recruits an internal model of gravity. Despite being consistently linked with gravity, few inquiries have been made about the role of vestibular information in these trends. Previous experiments with static tilting of observers' bodies suggest that under conflicting cues between the idiotropic vector and vestibular signals, the dynamic drift in memory is reduced to a constant displacement along the body's main axis. The present experiment aims to replicate and extend these outcomes while keeping the observers' bodies unchanged in relation to physical gravity by varying the gravito-inertial acceleration using a short-radius centrifuge. Observers were shown, while accelerated to varying degrees, targets moving along several directions and were required to indicate the perceived vanishing location after a variable interval. Increases of the gravito-inertial force (up to 1.4G), orthogonal to the idiotropic vector, did not affect the direction of representational gravity, but significantly disrupted its time course. The role and functioning of an internal model of gravity for spatial perception and orientation are discussed in light of the results.

The vista paradox is the illusion in which an object seen through a window appears to shrink in apparent size (and appears farther away) as the observer approaches the window. Paradoxically, the distal object appears smaller as its visual angle increases. We investigated the effect in four experiments varying object size, distance, point of fixation, and texture of the frame and of the object. In the first experiment, we tried to confirm the illusion and to test the robustness of the phenomenon. In the second experiment, we manipulated where subjects fixated (on the frame or on the object) as well as the texture of the object and the frame. Fixation was essential for the illusion: fixating the frame led to an apparent shrinking of the object, whereas fixation on the object did not. Texture of the frame intensified the apparent shrinking of the object. In a third experiment, we separated the point of fixation from the frame in a between-subjects design. Finally, in Experiment 4, we showed that the paradox does not require a frame, but it requires a fixation on a location different from the object. That is, the window or frame is dispensable for the vista paradox, but fixation is critical., ((c) 2015 APA, all rights reserved).)

With regard to impending object-object collisions, observers may use different sources of information to judge time to contact (tC). We introduced changes of the observer's vantage point to test among three sets of hypotheses: (1) Observers may use a distance-divided-by-velocity algorithm or, alternatively, elaborated τ-formulae, all of which give exact tC information; (2) observers may use simple τ-formulae (i.e., formulae of the type: visual angle divided by its own first temporal derivative); (3) observers may capitalize on non-τ variables. Hypotheses (2) and (3) imply specific patterns of errors. We presented animated, impending collisions between a moving object and a stationary pole to naïve observers. The moving object either was a square tile or a small dot of fixed size. Participants viewed these events in a prediction-motion paradigm from different vantage points, covering a full circle around the setting. As accuracy of responses varied sinusoidally with viewing angle, irrespective of the type of object used, we conclude that observers mainly responded to the perspective view of the gap between object and pole, and less to the object's changing visual angle, or τ. Results are discussed with regard to evolutionary demands and issues of generalization., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

The memory for the vanishing location of a horizontally moving target is usually displaced forward in the direction of motion (representational momentum) and downward in the direction of gravity (representational gravity). Moreover, this downward displacement has been shown to increase with time (representational trajectory). However, the degree to which different kinematic events change the temporal profile of these displacements remains to be determined. The present article attempts to fill this gap. In the first experiment, we replicate the finding that representational momentum for downward-moving targets is bigger than for upward motions, showing, moreover, that it increases rapidly during the first 300 ms, stabilizing afterward. This temporal profile, but not the increased error for descending targets, is shown to be disrupted when eye movements are not allowed. In the second experiment, we show that the downward drift with time emerges even for static targets. Finally, in the third experiment, we report an increased error for upward-moving targets, as compared with downward movements, when the display is compatible with a downward ego-motion by including vection cues. Thus, the errors in the direction of gravity are compatible with the perceived event and do not merely reflect a retinotopic bias. Overall, these results provide further evidence for an internal model of gravity in the visual representational system.

Background: When people are asked to indicate the vanishing location of a moving target, errors in the direction of motion (representational momentum) and in the direction of gravity (representational gravity) are usually found. These errors possess a temporal course wherein the memory for the location of the target drifts downwards with increasing temporal intervals between target's disappearance and participant's responses (representational trajectory)., Objective: To assess if representational trajectory is a body-referenced or a world-referenced phenomenon., Methods: A behavioral localization method was employed with retention times between 0 and 1400 ms systematically imposed after the target's disappearance. The target could move horizontally (rightwards or leftwards) or vertically (upwards or downwards). Body posture was varied in a counterbalanced order between sitting upright and lying on the side (left lateral decubitus position)., Results: In the upright task, the memory for target location drifted downwards with time in the direction of gravity. This time course did not emerge for the decubitus task, where idiotropic dominance was found., Conclusions: The dynamic visual representation of gravity is neither purely body-referenced nor world-referenced. It seems to be modulated instead by the relationship between the idiotropic vector and physical gravity.

When people are instructed to locate the vanishing location of a moving target, systematic errors forward in the direction of motion (M-displacement) and downward in the direction of gravity (O-displacement) are found. These phenomena came to be linked with the notion that physical invariants are embedded in the dynamic representations generated by the perceptual system. We explore the nature of these invariants that determine the representational mechanics of projectiles. By manipulating the retention intervals between the target's disappearance and the participant's responses, while measuring both M- and O-displacements, we were able to uncover a representational analogue of the trajectory of a projectile. The outcomes of three experiments revealed that the shape of this trajectory is discontinuous. Although the horizontal component of such trajectory can be accounted for by perceptual and oculomotor factors, its vertical component cannot. Taken together, the outcomes support an internalization of gravity in the visual representation of projectiles.

In a seminal article, Todd (Journal of Experimental Psychology: Human Perception and Performance 7:795-810, 1981) reported a difference threshold of about 50 ms to discriminate the times of arrival of two differently sized objects that simultaneously approached head-on at constant but different velocities. Subsequent investigators, however, have often found much higher thresholds. We did one complete replication of Todd's experiment, and then modified his stimuli and experimental regime, which we hypothesized may have been responsible for some of the discrepancies reported in the literature. Unlike Todd and most other researchers, we exclusively used untrained observers. Several of our participants performed almost as well as the trained observers used by Todd and others, but the performance of most of our participants fell short of this standard. Furthermore, thresholds were affected by the experimental regimes, with large differences between objects' sizes and speeds compromising performance. Analyses of the response patterns revealed that the responses were driven mainly by the objects' relative apparent sizes.

Many animals avoid dark, approaching objects seen against a lighter background but show no or weaker reactions to stimuli with inverted contrast. We investigated whether human observers would respond differently to such stimuli in terms of estimated time-to-arrival. We varied luminances of an approaching, light or dark disk and a plain, grey background, and for several conditions, continuously adjusted calibrations so as to keep contrast and/or overall lightness constant. Since no effects were found, we conclude that humans are able to discard luminance and contrast for the task at hand. Generally, however, performance was affected by different, consecutive regimes of feedback: Initially, without feedback, observers responded inconsistently and much too late; they improved after correct feedback, and in a third block of trials with pseudo-random feedback, they responded increasingly early without reverting to the initial level of uncertainty. We discuss our findings with regard to implications for neural mechanisms, put them in the context of evolutionary considerations, and propose continuative animal behavioral studies., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

We assessed the effect of the coherence of optic flow on time-to-passage judgments in order to investigate the strategies that observers use when local expansion information is reduced or lacking. In the standard display, we presented a cloud of dots whose image expanded consistent with constant observer motion. The dots themselves, however, did not expand and were thus devoid of object expansion cues. Only the separations between the dots expanded. Subjects had to judge which of two colored target dots, presented at different simulated depths and lateral displacements would pass them first. Image velocities of the target dots were chosen so as to correlate with time-to-passage only some of the time. When optic flow was mainly incoherent, subjects' responses were biased and relied on image velocities rather than on global flow analysis. However, the bias induced by misleading image velocity cues diminished as a function of the coherence of the optic flow. We discuss the results in the context of a global tau mechanism and settle a debate whether local expansion cues or optic flow analysis are the basis for time-to-passage estimation., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

When making parallel time-to-contact (TTC) estimates of two approaching objects, the two respective TTC estimates interfere with one another in an asymmetric fashion. The TTC of the later-arriving object is systematically overestimated, while the estimated TTC for the first-arriving object is as accurate as in a condition presenting only a single object. This asymmetric interference points to a processing bottleneck that could be due to early (e.g., during the estimation of the TTC from the optic flow) or late (e.g., during the timing of the response or the motor execution) constraints in the TTC estimation process. We used a Sperling-like prediction-motion task to differentiate between these two possibilities. Participants produced an absolute estimate of the TTC of only one of two objects approaching a target line. The target object to which the response was to be made was indicated by an auditory cue that occurred either at motion-onset or at the instant at which the two objects disappeared from the screen (occlusion-onset). The cue at motion-onset should disengage visual processing of the irrelevant stimulus. The cue at occlusion-onset, in contrast, requires visual processing of both relevant and irrelevant stimulus until occlusion. A single-object condition was introduced as a control condition. Results show symmetric interference in the motion-onset condition. In the occlusion-onset condition however, the results were congruent with asymmetric interference. Thus, the processing bottleneck in TTC estimation is originating at the earlier stages., (2011 Elsevier B.V. All rights reserved.)

We investigated the effect of local texture motion on time-to-contact (TTC) estimation. In Experiment 1, observers estimated the TTC of a looming disk with a spiral texture pattern in a prediction-motion task. Rotation of the spiral texture in a direction causing illusory contraction resulted in a significant TTC overestimation, relative to a condition without texture rotation. This would be consistent with an intrusion of task-irrelevant local upon task-relevant global information. However, illusory expansion did not cause a relative TTC underestimation but rather also a tendency towards overestimation. In Experiment 2, a vertical cylinder moved on the frontoparallel plane. Observers judged its TTC with a finish line. The cylinder was textured with stripes oriented in parallel to its longitudinal axis. It was either not rotating, rotating such that the stripes moved towards the finish line (i.e., in the same direction as the contour), or rotating such that the stripes moved away from the finish line. Both types of texture motion caused TTC overestimation compared to the static condition. Experiment 3 showed that the different effects of task-relevant and task-irrelevant texture motion are not a mere procedural effect of the prediction-motion task. In conclusion, task-irrelevant local motion and global motion are neither averaged in a simple manner nor are they processed independently.

On Earth, gravity accelerates freely moving objects downward, whereas upward-moving objects are being decelerated. Do humans take internalised knowledge of gravity into account when estimating time-to-contact (TTC, the time remaining before the moving object reaches the observer)? To answer this question, we created a motion-prediction task in which participants saw the initial part of an object's trajectory moving on a collision course prior to an occlusion. Observers had to judge when the object would make contact with them. The visual scene was presented with a head-mounted display. Participants lay either supine (looking up) or prone (looking down), suggestive of the ball either rising up or falling down toward them. Results showed that body posture had a significant effect on time-to-contact estimation, but only when occlusion times were long (2.5 s). The effect was also rather small. This lack of immediacy in the posture effect suggests that TTC estimation is initially robust toward the effect of gravity, which comes to bear only as more time is allowed for post-processing of the visual information.

In many situations, it is necessary to predict when a moving object will reach a given target even though the object may be partially or entirely occluded. Typically, one would track the moving object with eye movements, but it remains unclear whether ocular pursuit facilitates accurate estimation of time-to-contact (TTC). The present study examined this issue using a prediction-motion (PM) task in which independent groups estimated TTC in a condition that required fixation on the arrival location as an object approached, or a condition in which participants were instructed to pursue the moving object. The design included 15 TTC ranging from 0.4 to 1.5 s and three object velocities (2.5, 5, 10 deg/s). Both constant error and variable error in TTC estimation increased as a function of actual TTC. However, for the fixation group only, there was a significant effect of object velocity with a relative overestimation of TTC for the slower velocity and underestimation for the faster velocity. Further analysis indicated that the velocity effect exhibited by the fixation group was consistent with participants exhibiting a relatively constant misperception for each level of object velocity. Overall, these findings show that there is an advantage in the PM task to track the moving object with the eyes. We explain the different pattern of TTC estimation error exhibited when fixating and during pursuit with reference to differences in the available retinal and/or extra-retinal input.

The accuracy of time-to-contact (TTC) judgments for single approaching objects is well researched, however, close to nothing is known about our ability to make simultaneous TTC judgments for two or more objects. Such complex judgments are required in many everyday situations, for instance when crossing a multi-lane street or when engaged in multi-player ball games. We used a prediction-motion paradigm in which participants simultaneously estimated the absolute TTC of two objects, and compared the performance to a standard single-object condition. Results showed that the order of arrival of the two objects determined the accuracy of the TTC estimates: Estimation of the first-arriving object was unaffected by the added complexity compared to the one-object condition, whereas the TTC of the second-arriving object was systematically overestimated. This result has broad implications for complex everyday situations. We suggest that it is akin to effects observed in experiments on the psychological refractory period (PRP) and that the proactive interference of the first-arriving object indicates a bottleneck or capacity sharing at the central stage., (2010 Elsevier B.V. All rights reserved.)

Although observers know about the law of reflection, their intuitive understanding of spatial locations in mirrors is often erroneous. Hecht et al. (2005) proposed a two-stage mirror-rotation hypothesis to explain these misconceptions. The hypothesis involves an egocentric bias to the effect that observers behave as if the mirror surface were rotated by about 2 degrees to be more orthogonal than is the case. We test four variants of the hypothesis, which differ depending on whether the virtual world, the mirror, or both are taken to be rotated. We devised an experimental setup that allowed us to distinguish between these variants. Our results confirm that the virtual world--and only the virtual world--is being rotated. Observers had to perform a localization task, using a mirror that was either fronto-parallel or rotated opposite the direction of the predicted effect. We were thus able to compensate for the effect. The positions of objects in mirrors were perceived in accordance with the erroneous conception that the virtual world behind the mirror is slightly rotated and that the reconstruction is based on the non-rotated fronto-parallel mirror. A covert rotation of the mirror by about 2 degrees against the predicted effect was able to compensate for the placement error.

The theory of direct perception suggests that observers can accurately judge the mass of a box picked up by a lifter shown in a point-light display. However, accurate perceptual performance may be limited to specific circumstances. The purpose of the present study was to systematically examine the factors that determine perception of mass, including display type, lifting speed, response type, and lifter's strength. In contrast to previous research, a wider range of viewing manipulations of point-light display conditions was investigated. In Experiment 1, we first created a circumstance where observers could accurately judge lifts of five box masses performed by a lifter of average strength. In Experiments 2-5, we manipulated the spatial and temporal aspects of the lift, the judgement type, and lifter's strength, respectively. Results showed that mass judgement gets worse whenever the context deviates from ideal conditions, such as when only the lifted object was shown, when video play speed was changed, or when lifters of different strength performed the same task. In conclusion, observers' perception of kinetic properties is compromised whenever viewing conditions are not ideal.

The use of virtual reality is nowadays common in many studies in the field of human perception and movement control, particularly in interceptive actions. However, the ecological validity of the simulation is often taken for granted without having been formally established. If participants were to perceive the real situation and its virtual equivalent in a different fashion, the generalization of the results obtained in virtual reality to real life would be highly questionable. We tested the ecological validity of virtual reality in this context by comparing the timing of interceptive actions based upon actually falling objects and their simulated counterparts. The results show very limited differences as a function of whether participants were confronted with a real ball or a simulation thereof. And when present, such differences were limited to the first trial only. This result validates the use of virtual reality when studying interceptive actions of accelerated stimuli.

The effects of moving task-irrelevant objects on time-to-contact (TTC) judgments were examined in 5 experiments. Observers viewed a directly approaching target in the presence of a distractor object moving in parallel with the target. In Experiments 1 to 4, observers decided whether the target would have collided with them earlier or later than a standard (absolute identification task). A contrast effect was observed: If the distractor arrived later than the target, it caused a bias toward early responses, relative to the condition without a distractor. The early-arriving distractor had no significant effect. The pattern of results was unaltered when potentially confounding information from individual visual cues was removed. The availability of stereoscopic information reduced the effect. The contrast effect was also observed if target and distractor were abstract geometric objects rather than simulations of real-world vehicles, rendering less likely a simple safety strategy activated by a potentially threatening distractor. Experiment 5 showed that the effect of the late-arriving distractor generalized to a prediction-motion task. The results indicate that task-irrelevant information in the background has to be considered in revision of time-to-contact theory.

When motion in the frontoparallel plane is temporally sampled, it is often perceived to be slower than its continuous counterpart. This finding stands in contrast to humans' ability to extrapolate and anticipate constant-velocity motion. We investigated whether this sampling bias generalizes to motion in the sagittal plane (i.e., objects approaching the observer). We employed a paradigm in which observers judged the arrival time of an oncoming object. We found detrimental effects of time sampling on both perceived time to contact and time to passage. Observers systematically overestimated the time it would take a frontally approaching object to intersect their eye plane. To rule out artifacts inherent in computer simulation, we replicated the experiment, using real objects. The bias persisted and proved to be robust across a large range of temporal and spatial variations. Energy and pooling mechanisms are discussed in an attempt to understand the effect.

Current theories of arrival time have difficulty explaining performance in the common but neglected case of nonlinear approach. Global tau, a variable supposed to guide time-to-passage (TTP) judgments of objects approaching on linear trajectories, does not apply to circular movement. However, TTP judgments are surprisingly accurate in such cases. We simulated movement through a three-dimensional cloud of point-lights on various circular trajectories. Arrival-time judgments were found to be above chance when observers had to determine which of two expansionless targets would pass them first. Similar to the inside bias observed in heading studies on circular trajectories, observers showed a strong bias to select the target on the inside of their own curved motion path as passing by first. Analysis of the projected target motion revealed that targets on the inside had lower optical velocities and relatively high optical acceleration rates. Empirical TTP judgments agreed best with a strategy based on relative optical velocity changes.

Throwing and catching balls or other objects is a generally highly practiced skill; however, conceptual as well as perceptual understanding of the mechanics that underlie this skill is surprisingly poor. In 5 experiments, we investigated conceptual and perceptual understanding of simple ballistic motion. Paper-and-pencil tests revealed that up to half of all participants mistakenly believed that a ball would continue to accelerate after it left the thrower's hand. Observers also showed a remarkable tolerance for anomalous trajectory shapes. Perceptual judgments based on graphics animations replicated these erroneous beliefs for shallow release angles. Observers' tolerance for anomalies tended to decrease with their distance from the actor. The findings are at odds with claims of the naive physics literature that liken intuitive understanding to Aristotelian or medieval physics theories. Instead, observers seem to project their intentions to the ball itself (externalization) or even feel that they have power over the ball when it is still close.

The World-Wide Web offers a potentially interesting tool to collect data from a large and heterogeneous audience. While questionnaires have become rather common on the Internet, its potential reaches far beyond text processing. In principle, it is possible not only to perform interactive, dynamic experiments on the Web, but also to include graphical animation and time-critical responses, such as reaction times. We implemented a visual motion extrapolation task on the Web using the programming language Java, which can be interpreted by standard Web browsers such as Netscape or Internet Explorer. The data collected with this method turned out to be reliable and differed little from data obtained in a controlled laboratory setting, with the exception of conditions with fixation instruction. Thus, the Web can, generally speaking, be used for data collection of large sample sizes. The strengths and weaknesses of dynamic visual simulation experiments on the Internet are discussed.

Extended objects presented instantaneously appeared to propagate or grow into their final shape. A strong bias was found toward perceiving straight lines to grow away from the observer's point of fixation. If attention and fixation were not directed to the same point of the visual field, both influenced illusory motion directionality, and if another small object was present on the screen at a location that was neither fixated nor attended to, the object that was instantaneously presented seemed to grow away from this point. Thus, three factors are involved in illusory-motion directionality: biases toward perceiving objects spreading away from the fixation point, away from an attended area, and away from preexisting objects. The experiments indicate that a satisfactory explanation of the illusion has to include bottom-up and top-down processes. They also challenge existing theories of motion detection.

Four experiments were carried out to investigate observers' abilities to judge rolling motions. The experiments were designed to assess whether two important aspects of such motions are appreciated: the kinematic coupling of rotation and translation, and the dynamic effects of gravity. Different motion contexts of rolling wheels were created using computer-generated displays. The first experiment involved wheels rolling down an inclined plane. Observers spontaneously appreciated the anomaly of wheels that failed to accelerate, but they were not able to differentiate between different acceleration functions. Moreover, their judgments were almost exclusively based on the translation component of the rolling motion, neglecting the rotation component. In a second experiment it was found that observers could accurately estimate the perimeter of various objects. Thus, their inability to consider rotation information is not attributable to misperceptions of the geometry of wheels. In a third experiment the finding that rolling wheels appear to overrotate was replicated; however, findings from this experiment also showed, together with those from a fourth experiment, that observers are able to make very accurate judgments about translation-rotation coupling in rolling wheels when information is provided about the orientation of the wheel and the texture of the surface on which it rolls.

The motions of objects in the environment reflect underlying dynamical constraints and regularities. The conditions under which people are sensitive to natural dynamics are considered. In particular, the article considers what determines whether observers can distinguish canonical and anomalous dynamics when viewing ongoing events. The extent to which such perceptual appreciations are integrated with and influence common-sense reasoning about mechanical events is examined. It is concluded that animation evokes accurate dynamical intuitions when there is only 1 dimension of information that is of dynamical relevance. This advantage is lost when the observed motion reflects higher dimension dynamics or when the kinematic information is removed or degraded.

Five experiments were designed to investigate the influence of three-dimensional (3-D) orientation change on apparent motion. Projections of an orientation-specific 3-D object were sequentially flashed in different locations and at different orientations. Such an occurrence could be resolved by perceiving a rotational motion in depth around an axis external to the object. Consistent with this proposal, it was found that observers perceived curved paths in depth. Although the magnitude of perceived trajectory curvature often fell short of that required for rotational motions in depth (3-D circularity), judgments of the slant of the virtual plane on which apparent motions occurred were quite close to the predictions of a model that proposes circular paths in depth.