Visual prediction of collision with natural and nonnatural motion functions

A movement with constant velocity looks fast in the beginning and later slows down, whereas a certain type of accelerated motion (natural motion) looks constant throughout. It was predicted that early occlusion of a constant motion would lead to overestimation of velocity whereas late occlusion would not. With natural motion, there would be no such difference. Constant and natural motions together with constant deceleration and constant acceleration motions were tested in a modified prediction-of-collision experiment. The results agree well with the predictions. It was concluded that the phenomena previously found are operative also in a more complex perceptual task where the observer’s attention is not focused on velocity directly. The visual system seems to achieve perception of partly occluded motion by applying a natural motion function rather than constant velocity. Acquaintance with the phenomena does not seem to alter the way they are perceived.     

Velocity not acceleration of self-motion mediates vestibular-visual interaction

We investigated the influence of vestibular stimulation with different angular accelerations and velocities on the perception of visual motion direction. Constant accelerations resulting in different angular velocities and constant angular velocities obtained at different accelerations were combined in twenty healthy subjects. Random-dot kinematograms with coherently moving pixels and randomly moving pixels were used as visual stimuli during whole-body rotations. The smallest percentage of coherently moving pixels leading to a clear perception of motion direction was taken as the perception threshold. Perception thresholds significantly increased with increasing angular velocity. Increased acceleration, however, had no significant effect on the perception thresholds. We conclude that the achieved angular velocity, and not acceleration, is the predominant factor in the processing of vestibular-visual interaction.     

Task dependent processing of visual information about target acceleration

The objective of this study was to investigate the sensitivity of the perceptual and motor systems to target acceleration information using verbal magnitude estimations of target acceleration and manual interception of these targets. The results showed that in the perceptual task the participants were responding mainly to acceleration threshold values, which is acceleration as a function of initial, final, and average velocities, rather then to the absolute accelerations. When manually intercepting the targets the participants responded mainly to the absolute acceleration value and target initial velocity. Thus, these results suggest that target motion can be processed in the ventral (perception) and dorsal (action) visual streams however different motion characteristics are processed in these streams depending on the required output.     

Perceiving concealment in relationships between parents and adolescents: Links with parental behavior

Although concealment in relationships is commonplace, little is known about its implications for the target of concealment. Two large‐scale studies among adolescents and their parents tested the central hypothesis that parents’ perception of child concealment predicts poorer parenting behaviors toward their child. Further, we investigated whether actual child concealment adds to the prediction of parenting behaviors through an interaction with parental perception of concealment. Study 1 yielded evidence for the hypothesized link, which was independent of actual concealment. Study 2 largely replicated these results for perceptions of both concealment and lying while controlling for perceptions of disclosure. Overall, these results suggest that parents’ perception of child concealment coincides with poorer parenting behaviors, regardless of actual child concealment.     

Perception of acceleration with short presentation times: can acceleration be used in interception?

To investigate whether visual judgments of acceleration could be used for intercepting moving targets, we determined how well subjects can detect acceleration when the presentation time is short. In a differential judgment task, two dots were presented successively. One dot accelerated and the other decelerated. Subjects had to indicate which of the two accelerated. In an absolute judgment task, subjects had to adjust the motion of a dot so that it appeared to move at a constant velocity. The results for the two tasks were similar. For most subjects, we could determine a detection threshold even when the presentation time was only 300 msec. However, an analysis of these thresholds suggests that subjects did not detect the acceleration itself but that they detected that a target had accelerated on the basis of the change in velocity between the beginning and the end of the presentation. A change of about 25% was needed to detect acceleration with reasonable confidence. Perhaps the simplest use of acceleration for interception consists of distinguishing between acceleration and deceleration of the optic projection of an approaching ball to determine whether one has to run backward or forward to catch it. We examined the results of a real ball-catching task (Oudejans, Michaels, & Bakker, 1997) and found that subjects reacted before acceleration could have been detected. We conclude that acceleration is not used in this simple manner to intercept moving targets.     

Perception-action coupling and expertise in interceptive actions

The goal of this experiment was to show that expertise in interceptive actions can be explained by a shorter delay in movement regulation. In this contribution, we tested tennis experts and non-experts using a simulated interceptive task. The experimental device simulated linear motion of an object toward a target on a horizontal runway. Participants had to intercept the simulated moving object with their right hand holding a cart that could slide along a horizontal track perpendicular to the runway. Three different velocity conditions were used: a constant velocity condition that maintained the initial velocity (2m/s) constant until arriving on the target; the decelerated and accelerated velocity conditions, in which the velocity suddenly changed (400 ms before its arrival on the target) from 2 to 1m/s or 3m/s, respectively. Timing accuracy and movement correction after the unexpected velocity change were analysed. The experts were more accurate in the decelerative case (-29 and -124 ms respectively), in the accelerative case (69 and 116 ms respectively), but not in the constant velocity case (2 and 13 ms respectively). Findings can be explained by the shorter visuo-motor delay (VMD: the time required to adapt the movement to the new velocity) for the experts (162 ms) than for the non-experts (221 ms). This shorter VMD offers more time to adapt the interceptive movement to the new velocity. These results can be interpreted as an optimization of the perception-action coupling with expertise.     

Extent-of-motion thresholds under subject-relative and object-relative conditions

The experiment was designed to discover the threshold extent of motion at medium speeds amounting to 41, 82, and 164 min./sec., and to compare the perception of motion arising from subject-relative displacement with the perception of motion arising from object-relative displacement. Extent thresholds were found while velocity was kept constant. Different groups of ten Ss were used for each displacement velocity, and for each S the extent threshold was twice obtained by the method of constant stimuli, once under subjectrelative and once under object-relative displacement conditions. Sensitivity to brief displacements of a continuously visible target was high; average thresholds ranged from 1.0 to 4.4 min. under the various conditions employed. The thresholds were higher for subject-relative conditions and the slower displacement velocities and lower for objectrelative conditions and faster displacements.     

Extent-of-motion thresholds under subject-relative and object-relative conditions

The experiment was designed to discover the threshold extent of motion at medium speeds amounting to 41, 82, and 164 min./sec., and to compare the perception of motion arising from subject-relative displacement with the perception of motion arising from object-relative displacement. Extent thresholds were found while velocity was kept constant. Different groups of ten Ss were used for each displacement velocity, and for each S the extent threshold was twice obtained by the method of constant stimuli, once under subjectrelative and once under object-relative displacement conditions. Sensitivity to brief displacements of a continuously visible target was high; average thresholds ranged from 1.0 to 4.4 min. under the various conditions employed. The thresholds were higher for subject-relative conditions and the slower displacement velocities and lower for objectrelative conditions and faster displacements.     

Individuation of visual objects over time

How does an observer decide that a particular object viewed at one time is actually the same object as one viewed at a different time? We explored this question using an experimental task in which an observer views two objects as they simultaneously approach an occluder, disappear behind the occluder, and re-emerge from behind the occluder, having switched paths. In this situation the observer either sees both objects continue straight behind the occluder (called "streaming") or sees them collide with each other and switch directions ("bouncing"). This task has been studied in the literature on motion perception, where interest has centered on manipulating spatiotemporal aspects of the motion paths (e.g. velocity, acceleration). Here we instead focus on featural properties (size, luminance, and shape) of the objects. We studied the way degrees and types of featural dissimilarity between the two objects influence the percept of bouncing vs. streaming. When there is no featural difference, the preference for straight motion paths dominates, and streaming is usually seen. But when featural differences increase, the preponderance of bounce responses increases. That is, subjects prefer the motion trajectory in which each continuously existing individual object trajectory contains minimal featural change. Under this model, the data reveal in detail exactly what magnitudes of each type of featural change subjects implicitly regard as reasonably consistent with a continuously existing object. This suggests a simple mathematical definition of "individual object:" an object is a path through feature-trajectory space that minimizes feature change, or, more succinctly, an object is a geodesic in Mahalanobis feature space.     

Perceptions of depth elicited by occluded and shearing motions of random dots

A computer-controlled display of random dots was used to study perceptions of depth. In this display, a field of stationary random dots surrounded a rectangular area in which random dots moved with uniform velocity in a single direction. The boundaries of this rectangle did not move. When dot motion was perpendicular to the longer boundary of the rectangle (occluded motion), the rectangle seemed to be behind the stationary background surround. Motion parallel to the longer boundary of the rectangle (shearing motion) made it appear in front of the surround. The relative lengths of the sides of the rectangle determined which effect predominated. Thus, for motion perpendicular to the long axis of the rectangle the occlusion predominated and naive subjects reported that the central area seemed farther away than the surround. For shearing motion parallel to the long axis, the subjects reported that the rectangle was closer than the surround and the strength of both effects also depended on the length-to-width ratio of the rectangle. If there was occluded motion along the long axis, as the length-to-width ratio increased so did the likelihood that subjects would report seeing the rectangle behind the surround. Conversely, with shearing motion along the long axis, increasing the length-to-width ratio increased the likelihood that the rectangle would appear unambiguously in front of the surround. Some subjects integrated the two cues with the resulting perception being a rotating cylinder. The occlusion effect was stronger than the shearing effect.(ABSTRACT TRUNCATED AT 250 WORDS)     

The Optics and Actions of Catching Fly Balls: Zeroing Out Optical Acceleration

Advancing or retreating so as to maintain a projectile's constant vertical optical velocity was suggested by Chapman (1968) as a possible basis for locomotion in ball catching. Three experiments examined this thesis. In Experiments I and 2, the positions of balls and catchers were videotaped to see if the movements of the catchers canceled optical acceleration. Such canceling was indeed observed until just prior to the catch for hand-thrown balls (Experiment 1). The monocular availability of the information predicts success with monocular viewing, confirmed in Experiment 2 with machine-thrown balls. In Experiment 3, observers judged whether a ball (represented as a moving dot on a computer screen) would land at, in front of, or behind them. Performance was above chance, but only some observers used acceleration. Together, the experiments provide broad, though not unequivocal, support for the utilization of optical acceleration to guide locomotion in catching.     

Discriminating constant from variable angular velocities in structure from motion

We investigated accuracy in discriminating between constant and variable angular velocities for orthographic projections of three-dimensional rotating objects. The reported judgments of “constant” or “variable” angular velocity were only slightly influenced by the projected angular velocities, but they were greatly affected by the variations of the deformation, a first-order component of the optic flow. When viewing either a rotating ellipsoidal volume or a planar surface that accelerated and decelerated over the course of rotation, observers’ tendencies to report a variable angular velocity were increased when the temporal phase of the acceleration pattern increased the range of variation of the median deformation; the tendencies were decreased when the same acceleration pattern was used to decrease the range of variation of the median deformation. These results provide evidence contrary to the hypothesis that the visual system performs a mathematically correct analysis of the optic flow.     

The latency of circular vection during different accelerations of the optokinetic stimulus

Subjects were seated inside a full-field optokinetic cylinder which was accelerated with values between. 1 and 100 deg/sec2. Subjects indicated when motion was first detected. Latency for onset of self-motion shows a minimum of around 5 deg/sec2 and increases for lower and faster accelerations of the visual surround. In the low acceleration range, up to 5 deg/sec2, all movement is perceived as circular vection, that is, self-rotation. With higher accelerations, motion of the visual surround is perceived initially; over seconds, this gradually transforms to circular vection. Velocity estimation during low acceleration is better than during comparable vestibular acceleration. During subject rotation in the light, that is, when both the visual and vestibular inputs combine to generate a velocity signal, detection of motion has the shortest latency and represents actual velocity over a wider range than it does with each stimulus alone.     

An acceleration illusion caused by underestimation of stimulus velocity during pursuit eye movements: Aubert-Fleischl revisited

When the eyes pursue a fixation point that sweeps across a moving background pattern, and the fixation point is suddenly made to stop, the ongoing motion of the background pattern seems to accelerate to a higher velocity. Experiment I showed that this acceleration illusion is not caused by the sudden change in (i) the relative velocity between background and fixation point, (ii) the velocity of the retinal image of the background pattern, or (iii) the motion of the retinal image of the rims of the CRT screen on which the experiment was carried out. In experiment II the magnitude of the illusion was quantified. It is strongest when background and eyes move in the same direction. When they move in opposite directions it becomes less pronounced (and may disappear) with higher background velocities. The findings are explained in terms of a model proposed by the first author, in which the perception of object motion and velocity derives from the interaction between retinal slip velocity information and the brain's 'estimate' of eye velocity in space. They illustrate that the classic Aubert-Fleischl phenomenon (a stimulus seems to be moving slower when pursued with the eyes than when moving in front of stationary eyes) is a special case of a more general phenomenon: whenever we make a pursuit eye movement we underestimate the velocity of all stimuli in our visual field which happen to move in the same direction as our eyes, or which move slowly in the direction opposite to our eyes.     

Reprogramming of Interceptive Actions: Time Course of Temporal Corrections for Unexpected Target Velocity Change

The authors investigated the time course of reprogramming of the temporal dimension of motor acts in a task requiring interception of a moving target. The target moved at a constant velocity on a monitor screen; in part of the trials, target velocity was unexpectedly increased or decreased. Those modifications were produced at different moments during target displacement, leaving periods of time from 100 to 800 ms for movement timing correction. The authors assessed the effects of probability of target velocity change (25% vs. 50%), uncertainty about direction of velocity change (unidirectional vs. bidirectional), and direction of velocity change (increase vs. decrease). Analysis of 24 participants' arm acceleration showed that fast adjustments took place between 100 and 200 ms after target velocity change similarly for all uncertainty conditions. Analysis of temporal error indicated that the combination of high probability of target velocity change and certainty on direction of target velocity change led to the most successful movement timing reprogramming. For the other experimental conditions, temporal accuracy was still poor when a period of 800 ms was available for correction. Movement reprogramming was a continuous process that was more efficient for target velocity increase than for target velocity decrease.     

Apparent velocity of motion aftereffects in central and peripheral vision

Adapting to a drifting grating (temporal frequency 4 Hz, contrast 0.4) in the periphery gave rise to a motion aftereffect (MAE) when the grating was stopped. A standard unadapted foveal grating was matched to the apparent velocity of the MAE, and the matching velocity was approximately constant regardless of the visual field position and spatial frequency of the adapting grating. On the other hand, when the MAE was measured by nulling with real motion of the test grating, nulling velocity was found to increase with eccentricity. The nulling velocity was constant when scaled to compensate for changes in the spatial 'grain' of the visual field. Thus apparent velocity of MAE is constant across the visual field, but requires a greater velocity of real motion to cancel it in the periphery. This confirms that the mechanism underlying MAE is spatially-scaled with eccentricity, but temporally homogeneous. A further indication of temporal homogeneity is that when MAE is tracked, by matching or by nulling, the time course of temporal decay of the aftereffect is similar for central and for peripheral stimuli.     

Implied velocity and acceleration induce transformations of visual memory

In this study, the phenomenon of representational momentum (Freyd & Finke, 1984) is investigated in cases where visual memories are distorted by implied motions of the elements of a pattern. Our theory predicts that these memory distortions should be sensitive not only to the direction of the implied motions but also to changes in the implied velocity. Subjects observed a sequence of dot-pattern displays that implied that the dots were moving at either a constant velocity or constant acceleration, but in separate directions. Discrimination functions for recognizing the final pattern in the sequence revealed that subjects' memories had shifted forward, corresponding to small continuations of the implied motions. The induced memory shifts increased in size as the implied velocity and acceleration of the dots increased, but were eliminated when the display sequence implied a deceleration of the dots to a final velocity of zero. These findings suggest that mentally extrapolated motion may have some of the same inertial properties as actual physical motion.     

Judgments of synchrony between auditory and moving or still visual stimuli.

The flash-lag effect is a visual illusion wherein intermittently flashed, stationary stimuli seem to trail after a moving visual stimulus despite being flashed synchronously. We tested hypotheses that the flash-lag effect is due to spatial extrapolation, shortened perceptual lags, or accelerated acquisition of moving stimuli, all of which call for an earlier awareness of moving visual stimuli over stationary ones. Participants judged synchrony of a click either to a stationary flash of light or to a series of adjacent flashes that seemingly bounced off or bumped into the edge of the visual display. To be judged synchronous with a stationary flash, audio clicks had to be presented earlier--not later--than clicks that went with events, like a simulated bounce (Experiment 1) or crash (Experiments 2-4), of a moving visual target. Click synchrony to the initial appearance of a moving stimulus was no different than to a flash, but clicks had to be delayed by 30-40 ms to seem synchronous with the final (crash) positions (Experiment 2). The temporal difference was constant over a wide range of motion velocity (Experiment 3). Interrupting the apparent motion by omitting two illumination positions before the last one did not alter subjective synchrony, nor did their occlusion, so the shift in subjective synchrony seems not to be due to brightness contrast (Experiment 4). Click synchrony to the offset of a long duration stationary illumination was also delayed relative to its onset (Experiment 5). Visual stimuli in motion enter awareness no sooner than do stationary flashes, so motion extrapolation, latency difference, and motion acceleration cannot explain the flash-lag effect.     

Changes in motion perception following oculomotor smooth pursuit adaptation

The hypothesis that oculomotor smooth pursuit (SP) adaptation is accompanied by alterations in velocity perception was tested by assessing coherence thresholds, using random-dot kinematograms before and after the adaptation paradigm. The results showed that the sensitivity to coherent motion at 10 deg/sec (the initial target velocity during adaptation) was reduced after the SP adaptation, ending up at a level that was between those normally observed for velocities of 10 and 20 deg/sec. This is consistent with an overestimation of the velocity of the coherent motion and suggests that SP adaptation alters not only the oculomotor output, but also the perception of target velocity.     

Identifying contours from occlusion events

Surface contours specified by occlusion events that varied in density, velocity, and type of motion (rotation or translation) were examined in four experiments. As a fourth experimental factor, there were both figure-motion trials (the occluding surface moved over a stationary background) and background-motion trials (the background moved behind a stationary surface) in each experiment. Displays contained line patterns and rotary motion (Experiment 1), line patterns and translatory motion (Experiment 2), textured surfaces and rotary motion (Experiment 3), and textured surfaces and translatory motion (Experiment 4). Results indicate that contour identifications are more accurate with translation than with rotation, and that background-motion trials are generally easier than figure-motion trials. Although density in all experiments affected identifications in both background- and figure-motion trials, velocity did so in Experiment 4 only. In Experiments 1, 2, and 3, velocity affected identifications in background-motion trials but not in figure-motion trials. In Experiments 3 and 4, the rate of accretion and deletion of texture was a poor predictor of identification accuracy. These results are not consistent with previous accounts of contour perception from occlusion events, and may reflect an involvement of ocular pursuit as a mechanism for registering contour information.