Gravitational acceleration as a cue for absolute size and distance?

The psychophysics of bouncing: Perceptual constraints, physical constraints, animacy, and phenomenal causality

In the present study we broadly explored the perception of physical and animated motion in bouncing-like scenarios through four experiments. In the first experiment, participants were asked to categorize bouncing-like displays as physical bounce, animated motion, or other. Several parameters of the animations were manipulated, that is, the simulated coefficient of restitution, the value of simulated gravitational acceleration, the motion pattern (uniform acceleration/deceleration or constant speed) and the number of bouncing cycles. In the second experiment, a variable delay at the moment of the collision between the bouncing object and the bouncing surface was introduced. Main results show that, although observers appear to have realistic representations of physical constraints like energy conservation and gravitational acceleration/deceleration, the amount of visual information available in the scene has a strong modulation effect on the extent to which they rely on these representations. A coefficient of restitution >1 was a crucial cue to animacy in displays showing three bouncing cycles, but not in displays showing one bouncing cycle. Additionally, bouncing impressions appear to be driven by perceptual constraints that are unrelated to the physical realism of the scene, like preference for simulated gravitational attraction smaller than g and perceived temporal contiguity between the different phases of bouncing. In the third experiment, the visible opaque bouncing surface was removed from the scene, and the results showed that this did not have any substantial effect on the resulting impressions of physical bounce or animated motion, suggesting that the visual system can fill-in the scene with the missing element. The fourth experiment explored visual impressions of causality in bouncing scenarios. At odds with claims of current causal perception theories, results indicate that a passive object can be perceived as the direct cause of the motion behavior of an active object.

Trial-by-trial feedback fails to improve the consideration of acceleration in visual time-to-collision estimation

When judging the time-to-collision (TTC) of visually presented accelerating vehicles, untrained observers do not adequately account for acceleration (second-order information). Instead, their estimations only rely on vehicle distance and velocity (first-order information). As a result, they systemically overestimate the TTC for accelerating objects, which represents a potential risk for pedestrians in traffic situations because it might trigger unsafe road-crossing behavior. Can training help reduce these estimation errors? In this study, we tested whether training with trial-by-trial feedback about the signed deviation of the estimated from the actual TTC can improve TTC estimation accuracy for accelerating vehicles. Using a prediction-motion paradigm, we measured the estimated TTCs of twenty participants for constant-velocity and accelerated vehicle approaches, from a pedestrian's perspective in a VR traffic simulation. The experiment included three blocks, of which only the second block provided trial-by-trial feedback about the TTC estimation accuracy. Participants adjusted their estimations during and after the feedback, but they failed to differentiate between accelerated and constant-velocity approaches. Thus, the feedback did not help them account for acceleration. The results suggest that a safety training program based on trial-by-trial feedback is not a promising countermeasure against pedestrians' erroneous TTC estimation for accelerating objects.

Gravity and Known Size Calibrate Visual Information to Time Parabolic Trajectories

Catching a ball in a parabolic flight is a complex task in which the time and area of interception are strongly coupled, making interception possible for a short period. Although this makes the estimation of time-to-contact (TTC) from visual information in parabolic trajectories very useful, previous attempts to explain our precision in interceptive tasks circumvent the need to estimate TTC to guide our action. Obtaining TTC from optical variables alone in parabolic trajectories would imply very complex transformations from 2D retinal images to a 3D layout. We propose based on previous work and show by using simulations that exploiting prior distributions of gravity and known physical size makes these transformations much simpler, enabling predictive capacities from minimal early visual information. Optical information is inherently ambiguous, and therefore, it is necessary to explain how these prior distributions generate predictions. Here is where the role of prior information comes into play: it could help to interpret and calibrate visual information to yield meaningful predictions of the remaining TTC. The objective of this work is: (1) to describe the primary sources of information available to the observer in parabolic trajectories; (2) unveil how prior information can be used to disambiguate the sources of visual information within a Bayesian encoding-decoding framework; (3) show that such predictions might be robust against complex dynamic environments; and (4) indicate future lines of research to scrutinize the role of prior knowledge calibrating visual information and prediction for action control.

Integration of speed and time for estimating time to contact

Existing theories suggest that reacting to dynamic stimuli is made possible by relying on internal estimates of kinematic variables. For example, to catch a bouncing ball the brain relies on the position and speed of the ball. However, when kinematic information is unreliable one may additionally rely on temporal cues. In the bouncing ball example, when visibility is low one may benefit from the temporal information provided by the sound of the bounces. Our work provides evidence that humans rely on such temporal cues and automatically integrate them with kinematic information to optimize their performance. This finding reveals a hitherto unappreciated role of the brain’s timing mechanisms in sensorimotor function.

To coordinate movements with events in a dynamic environment the brain has to anticipate when those events occur. A classic example is the estimation of time to contact (TTC), that is, when an object reaches a target. It is thought that TTC is estimated from kinematic variables. For example, a tennis player might use an estimate of distance (d) and speed (v) to estimate TTC (TTC = d/v). However, the tennis player may instead estimate TTC as twice the time it takes for the ball to move from the serve line to the net line. This latter strategy does not rely on kinematics and instead computes TTC solely from temporal cues. Which of these two strategies do humans use to estimate TTC? Considering that both speed and time estimates are inherently uncertain and the ability of the human brain to combine different sources of information, we hypothesized that humans estimate TTC by integrating speed information with temporal cues. We evaluated this hypothesis systematically using psychophysics and Bayesian modeling. Results indicated that humans rely on both speed information and temporal cues and integrate them to optimize their TTC estimates when both cues are present. These findings suggest that the brain’s timing mechanisms are actively engaged when interacting with dynamic stimuli.

Using Blur to Affect Perceived Distance and Size

We present a probabilistic model of how viewers may use defocus blur in conjunction with other pictorial cues to estimate the absolute distances to objects in a scene. Our model explains how the pattern of blur in an image together with relative depth cues indicates the apparent scale of the image's contents. From the model, we develop a semiautomated algorithm that applies blur to a sharply rendered image and thereby changes the apparent distance and scale of the scene's contents. To examine the correspondence between the model/algorithm and actual viewer experience, we conducted an experiment with human viewers and compared their estimates of absolute distance to the model's predictions. We did this for images with geometrically correct blur due to defocus and for images with commonly used approximations to the correct blur. The agreement between the experimental data and model predictions was excellent. The model predicts that some approximations should work well and that others should not. Human viewers responded to the various types of blur in much the way the model predicts. The model and algorithm allow one to manipulate blur precisely and to achieve the desired perceived scale efficiently.

Dynamic action in virtual environments: Constraints on the accessibility of action knowledge in children and adults

In a series of three experiments, we probed the accessibility of action knowledge in different versions of a virtual environment (VE) with 7-year-old children and adults. Using a PHANToMTM haptic interface, participants performed a virtual throwing task in which they tried to propel a ball from a table to hit a target on the ground. In Experiments 1 and 2, the virtual scene was presented on a computer monitor, and, in Experiment 3, it was projected by using a video projector so that the vertical and horizontal dimensions and the spatial location of the VE corresponded to the real-world dimensions. Results indicate that action knowledge is accessible even in a nonimmersive VE, but also suggest that the need to recalibrate perceptual-motor mappings constrains the accessibility of this kind of intuitive knowledge.

Vestibular Nuclei and Cerebellum Put Visual Gravitational Motion in Context

Animal survival in the forest, and human success on the sports field, often depend on the ability to seize a target on the fly. All bodies fall at the same rate in the gravitational field, but the corresponding retinal motion varies with apparent viewing distance. How then does the brain predict time-to-collision under gravity? A perspective context from natural or pictorial settings might afford accurate predictions of gravity's effects via the recovery of an environmental reference from the scene structure. We report that embedding motion in a pictorial scene facilitates interception of gravitational acceleration over unnatural acceleration, whereas a blank scene eliminates such bias. Functional magnetic resonance imaging (fMRI) revealed blood-oxygen-level-dependent correlates of these visual context effects on gravitational motion processing in the vestibular nuclei and posterior cerebellar vermis. Our results suggest an early stage of integration of high-level visual analysis with gravity-related motion information, which may represent the substrate for perceptual constancy of ubiquitous gravitational motion.

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.

The inversion effect in biological motion perception: evidence for a "life detector"?

If biological-motion point-light displays are presented upside down, adequate perception is strongly impaired. Reminiscent of the inversion effect in face recognition, it has been suggested that the inversion effect in biological motion is due to impaired configural processing in a highly trained expert system. Here, we present data that are incompatible with this view. We show that observers can readily retrieve information about direction from scrambled point-light displays of humans and animals. Even though all configural information is entirely disrupted, perception of these displays is still subject to a significant inversion effect. Inverting only parts of the display reveals that the information about direction, as well as the associated inversion effect, is entirely carried by the local motion of the feet. We interpret our findings in terms of a visual filter that is tuned to the characteristic motion of the limbs of an animal in locomotion and hypothesize that this mechanism serves as a general detection system for the presence of articulated terrestrial animals.

Visual perception and interception of falling objects: a review of evidence for an internal model of gravity

Prevailing views on how we time the interception of a moving object assume that the visual inputs are informationally sufficient to estimate the time-to-contact from the object's kinematics. However, there are limitations in the visual system that raise questions about the general validity of these theories. Most notably, vision is poorly sensitive to arbitrary accelerations. How then does the brain deal with the motion of objects accelerated by Earth's gravity? Here we review evidence in favor of the view that the brain makes the best estimate about target motion based on visually measured kinematics and an a priori guess about the causes of motion. According to this theory, a predictive model is used to extrapolate time-to-contact from the expected kinetics in the Earth's gravitational field.

Task-Specific Knowledge of the Law of Pendulum Motion in Children and Adults

The present experiment investigated children and adults’ knowledge of the pendulum law under different task conditions. The question asked was whether adults and fourth-graders knew that the period of a pendulum is a function of pendulum length but is independent of its mass. The task was to judge the period on a rating scale (judgment task), to imagine the swinging pendulum and indicate the corresponding time interval (imagery task), or to adjust the period of a dynamically presented pendulum (perception task). Normative consideration of pendulum length as the only relevant factor was primarily found in the perception task and, for adults, in the imagery task, whereas in the judgment task, children and adults frequently considered the irrelevant dimension of mass. Most children showed poor imagery performance. Preceding adjustment (perception task) and rating (judgment task) had no differential influence on subsequent imagery performance.

Judging Time Intervals Using a Model of Perceptuo-Motor Control

Estimating a time interval and temporally coordinating movements in space are fundamental skills, but the relationships between these different forms of timing, and the neural processes that they incur, are not well understood. While different theories have been proposed to account for time perception, time estimation, and the temporal patterns of coordination, there are no general mechanisms which unify these various timing skills. This study considers whether a model of perceptuo-motor timing, the τGUIDE, can also describe how certain judgements of elapsed time are made.

To evaluate this, an equation for determining interval estimates was derived from the τGUIDE model and tested in a task where participants had to throw a ball and estimate when it would hit the floor. The results showed that in accordance with the model, very accurate judgements could be made without vision (mean timing error 19.24 msec), and the model was a good predictor of skilled participants' estimate timing. It was concluded that since the τGUIDE principle provides temporal information in a generic form, it could be a unitary process that links different forms of timing.

Reference frames for orientation anisotropies in face recognition and biological-motion perception

Both face recognition and biological-motion perception are strongly orientation-dependent. Recognition performance decreases if the stimuli are rotated with respect to their normal upright orientation. Here, the question whether this effect operates in egocentric coordinates or in environmental coordinates is examined. In addition to the use of rotated stimuli the observers were also rotated and tested both with a same-different face-recognition task and with a biological-motion detection task. A strong orientation effect was found that depended only on the stimulus orientation relative to the observer. This result clearly indicates that orientation effects in both stimulus domains operate in an egocentric frame of reference. This finding is discussed in terms of the particular requirement of extracting sophisticated information for social recognition and communication from faces and biological motion.

Causation, causal perception, and conservation laws

We investigated the perception of causation via the ability to detect conservation violations in simple events. We showed that observers were sensitive to energy conservation violations in free-fall events. Furthermore, observers were sensitive to gradually perturbed energy dynamics in such events. However, they were more sensitive to the effect of decreasing gravity than to that of increasing gravity. Displays with decreasing gravity were the only displays in which the energy profile was dominated by (apparent) potential energy, leading to an asymmetric trajectory.

Individual differences and the use of nonspecifying variables in learning to perceive distance and size: comments on McConnell, Muchisky, and Bingham (1998)

McConnell, Muchisky, and Bingham (1998) showed that observers are able to judge the distance and size of falling, rolling, and swinging balls and that performance improves after practice with feedback. They concluded that observers use information that specifies the spatial scales of the different event types--namely, event duration in combination with event-specific constants. The improvement was interpreted as the calibration of the event-specific constants. We argue that their analyses should have considered the use of optical variables that do not specify the to-be-perceived metrics and individual differences in variable use. Furthermore, we propose convergence on the more useful variables as an alternative explanation for the observed improvement. The viability of these arguments is demonstrated with an experiment in which participants are trained with feedback to judge the distance and size of freely falling balls.

Commentary on Jacobs and Michaels (2001): calibration and perceptual learning in event perception

Jacobs and Michaels (2001) have argued that increased precision in judgments of the viewing distance to a perceived event should be attributed in part to perceptual learning. They found that observers used feedback to attune to the appropriate information variables gradually. McConnell, Muchisky, and Bingham (1998) had found that observers used feedback to calibrate event-specific scaling coefficients, that the calibration of one type of event generalized to other types, and that calibration occurred suddenly. We argue that Jacobs and Michaels must be partially correct and that, in our experiments, both calibration and perceptual attunement were required for accurate and precise judgments.

Physical imagery: kinematic versus dynamic models

Physical imagery occurs when people imagine one object causing a change to a second object. To make inferences through physical imagery, people must represent information that coordinates the interactions among the imagined objects. The current research contrasts two proposals for how this coordinating information is realized in physical imagery. In the traditional kinematic formulation, imagery transformations are coordinated by geometric information in analog spatial representations. In the dynamic formulation, transformations may also be regulated by analog representations of force and resistance. Four experiments support the dynamic formulation. They show, for example, that without making changes to the spatial properties of a problem, dynamic perceptual information (e.g., torque) and beliefs about physical properties (e. g., viscosity) affect the inferences that people draw through imagery. The studies suggest that physical imagery is not so much an analog of visual perception as it is an analog of physical action. A simple model that represents force as a rate helps explain why inferences can emerge through imagined actions even though people may not know the answer explicitly. It also explains how and when perception, beliefs, and learning can influence physical imagery.

The use of time and trajectory forms as visual information about spatial scale in events

Spatial metrics are lost but temporal metrics are preserved in the mapping from events to optic flow. In inanimate events governed by gravity, temporal scale is linked to spatial scale in ways specific to particular events. We tested whether time can be used as information about spatial scale in visually recognizable events. On average, observers were able to judge object size in event displays that eliminated information other than time and trajectory forms. However, judgment variability was substantial. After feedback on one event, observers judging distance performed better and generalized training to other events. Observers are sensitive to the general form of the scaling relation, but they require feedback to attune event-specific constants.