Distinct mechanisms for the representation of moving and static objects

The visual system has been suggested to integrate different views of an object in motion. We investigated differences in the way moving and static objects are represented by testing for priming effects to previously seen ("known") and novel object views. We showed priming effects for moving objects across image changes (e.g., mirror reversals, changes in size, and changes in polarity) but not over temporal delays. The opposite pattern of results was observed for objects presented statically; that is, static objects were primed over temporal delays but not across image changes. These results suggest that representations for moving objects are: (1) updated continuously across image changes, whereas static object representations generalize only across similar images, and (2) more short-lived than static object representations. These results suggest two distinct representational mechanisms: a static object mechanism rather spatially refined and permanent, possibly suited for visual recognition, and a motion-based object mechanism more temporary and less spatially refined, possibly suited for visual guidance of motor actions.     

Memory for moving and static images

Despite the substantial interest in memory for complex pictorial stimuli, there has been virtually no research comparing memory for static scenes with that for their moving counterparts. We report that both monochrome and color moving images are better remembered than static versions of the same stimuli at retention intervals up to one month. When participants studied a sequence of still images, recognition performance was the same as that for single static images. These results are discussed within a theoretical framework which draws upon previous studies of scene memory, face recognition, and representational momentum.     

Development of naive beliefs about moving objects: The straight-down belief in action

The naive belief that carried objects fall straight down when released has become known as the straight-down belief in the literature of intuitive physics. The present data show that, without formal instruction, many children revise the straight-down belief between 8 and 12 years of age. In Experiment 1, 6-, 8-, and 12-year-old children repeatedly tried to hit a target on the floor or on a table (without feedback) by dropping a ball while moving and judged the optimal release points. Whereas, in their judgments, the majority of the 6- and 8-year-old children exhibited the erroneous straight-down belief, most 12-year-olds gave correct forward answers. In their actions, children who held the straight-down belief dropped the ball significantly later than children who exhibited correct judgmental knowledge. The results of three additional experiments provide converging evidence for the claim that children use their naive beliefs to plan their actions. The data further suggest that the straight-down belief does not directly stem from a perceptual illusion. Age differences concerning the ability to execute action plans as intended are documented and discussed in relation to conceptual development.     

Visible persistence of moving objects

A single line was presented in a succession of orientations, each orientation separated by a fixed angle and by a fixed interval of time, and subjects reported the number of successive lines that appeared to rotate together. The perceived number of rotating lines increased linearly with the rate of stimulus presentation, with a slope that was proportional to the spatial separation. The linear functions obtained in this first experiment predicted the results of a second experiment in which subjects adjusted the spatial and temporal variables to a discrimination threshold for seeing two rotating lines. If the slope of the linear functions is considered to be an estimate of the duration of visible persistence, then these results suggest that the visible persistence of a briefly presented stimulus increases with the distance separating that stimulus from other stimuli.     

Observations on the development of reaching for moving objects.

How infants come to master reaching for moving objects was studied in a situation where the distance to and the velocity of the moving object varied. Eleven infants participated in the study. They were from 12 to 24 weeks old at the first session, were seen at 3-week intervals until 30 weeks old, and were finally seen once at 36 weeks old. The following behaviors were observed from video recordings: frequency of fixated and followed motions, latency to first goal-directed behavior, type of goal-directed behaviors, and type of reaches. It was found that by the time the infant masters reaching for stationary objects he will also successfully reach for moving ones. Eighteen-week-old infants caught the object as it moved at 30 cm/sec. The results suggest a basic human capacity to time-coordinate one's behavior with external events, and to foresee in one's actions future positions of moving objects.     

Predictive reaching for moving objects by human infants

To what degree do infants use a predictive strategy when reaching for moving objects? This question was studied longitudinally in five infants from 18 to 36 weeks of age. The aiming of 356 reaches were analyzed by a technique that took into consideration the three-dimensional properties of the reaches. Each reach was divided into ballistic steps and the aiming of each step was calculated and compared with an optimal value. It was found that the infants studied had an ability to reach for fast moving objects in a predictive way. Further, the results show that the predictive ability is remarkably good in the lowest age groups which suggests that it is, at least partly, prewired. What develops seems mainly to be the mobility aspects of reaching which makes for more economical and flexible reaching. Older infants reach successfully for the fast moving object also with a nonpredictive chasing strategy.     

Inhibition of return for objects and locations in static displays

When orienting attention, inhibition mechanisms prevent the return of attention to previously examined stimuli. This inhibition of the return of attention (IOR) has been shown to be associated additively with location- and object-based representations. That is, when static objects are attended, IOR is associated with both the object and the location cued, and hence IOR is larger than when only spatial location is attended. Recently McAuliffe, Pratt, and O'Donnell (2001) failed to observe such additive effects except under a narrow set of conditions (at short cue-target intervals and using mixed blocks in which object- and pure location-based effects were probed in the same display). The present study shows that additive IOR effects are observed under conditions that violate all of these boundary conditions. The results also show that IOR is modulated by internal structural properties of objects. These findings are cosistent with the hypothesis that IOR operates over functionally independent object- and location-based frames of reference.     

Effects of spatially directed attention on visual encoding

The purpose in this study was to distinguish among possible mechanisms by which focused attention facilitates visual perceptual processing in a cued discrimination task. In two experiments, subjects verified the presence of an X in masked, briefly presented, four-letter arrays. On most trials, subjects were precued to the location of the stimulus array (valid-cue condition); however, sometimes a nonstimulus location was cued (invalid-cue condition). The exposure duration of the stimulus array was varied. In Experiment 1, there was a large effect of cue condition on hit probability, but no effect of cue condition on false alarm probability. In Experiment 2, there was a large effect of cue condition on d'. In both experiments, the stimulus duration needed to reach any given performance level was greater by a constant factor for stimuli in the invalid-cue than it was in the valid-cue condition. This suggests that visual information is acquired (or utilized) more rapidly from attended than from unattended locations.     

The position of moving objects.

Moving objects occupy a range of positions during the period of integration of the visual system. Nevertheless, a unique position is usually observed. We investigate how the trajectory of a stimulus influences the position at which the object is seen. It has been shown before that moving objects are perceived ahead of static objects shown at the same place and time. We show here that this perceived position difference builds up over the first 500 ms of a visible trajectory. Discontinuities in the visual input reduce this buildup when the presentation frequency of a stimulus with a duration of 42 ms falls below 16 Hz. We interpret this relative mislocalization in terms of a spatiotemporal-filtering model. This model fits well with the data, given two assumptions. First, the position signal persists even though the objects are no longer visible and, second, the perceived distance is a 500 ms average of the difference of these position signals.     

Monocular discrimination of rigidly and nonrigidly moving objects

Wemeasured thresholds for the monocular discrimination of rigidly and nonrigidly moving objects defined by motion parallax. The retinal projections of rigidly moving objects are subject to certain constraints. By applying smooth 2-D transformations to the projections of rigidly moving objects, we created stimuli in which these constraints were affected. Thresholds for (generic) nonrigid transformations that in theory can be detected from rigid ones by processing pairs of views depended not only on the extent to which the rigidity constraints were affected, but also on the structure and the movement of the simulated object. Nonrigid transformations under which every three successive views had a rigid interpretation were not discriminable from rigid transformations, except in cases where the distortions were very large. Under the rigidity assumption, this would mean that a large class of nonrigidly moving objects is erroneously perceived as rigidly moving.     

Human heading judgments in the presence of moving objects

When moving toward a stationary scene, people judge their heading quite well from visual information alone. Much experimental and modeling work has been presented to analyze how people judge their heading for stationary scenes. However, in everyday life, we often move through scenes that contain moving objects. Most models have difficulty computing heading when moving objects are in the scene, and few studies have examined how well humans perform in the presence of moving objects. In this study, we tested how well people judge their heading in the presence of moving objects. We found that people perform remarkably well under a variety of conditions. The only condition that affects an observer’s ability to judge heading accurately consists of a large moving object crossing the observer’s path. In this case, the presence of the object causes a small bias in the heading judgments. For objects moving horizontally with respect to the observer, this bias is in the object’s direction of motion. These results present a challenge for computational models.     

Perceiving curvilinear heading in the presence of moving objects

Four experiments were directed at understanding the influence of multiple moving objects on curvilinear (i.e., circular and elliptical) heading perception. Displays simulated observer movement over a ground plane in the presence of moving objects depicted as transparent, opaque, or black cubes. Objects either moved parallel to or intersected the observer's path and either retreated from or approached the moving observer. Heading judgments were accurate and consistent across all conditions. The significance of these results for computational models of heading perception and for information in the global optic flow field about observer and object motion is discussed.     

Amodal completion of moving objects by pigeons

In a series of four experiments, we explored whether pigeons complete partially occluded moving shapes. Four pigeons were trained to discriminate between a complete moving shape and an incomplete moving shape in a two-alternative forced-choice task. In testing, the birds were presented with a partially occluded moving shape. In experiment 1, none of the pigeons appeared to complete the testing stimulus; instead, they appeared to perceive the testing stimulus as incomplete fragments. However, in experiments 2, 3, and 4, three of the birds appeared to complete the partially occluded moving shapes. These rare positive results suggest that motion may facilitate amodal completion by pigeons, perhaps by enhancing the figure - ground segregation process.     

Selection and consolidation of objects and actions

Response selection bottleneck models attribute performance costs under dual-task conditions to the human inability to select more than one response at a time. Consistent with this claim Pashler (1991) found that carrying out a speeded manual choice reaction time (RT) task does not impair the unspeeded report of a cued visual target from a masked display. In contrast, Jolicoeur and DellAcqua (1999, Experiment 2) observed pronounced interference between a speeded manual choice RT task and the unspeeded report of a small number of visually presented letters, a finding they attributed to resource sharing between response selection and stimulus consolidation. We demonstrate that comparable costs are obtained with the same task combination used by Pashler (1991) if only task order is reversed—a manipulation that is likely to increase the necessity of consolidating the target stimulus into working memory. We also found that these costs are not diminished if the location of the target to be reported is cued in advance (reducing demands on spatial focusing) and that they do not vary with the number of target features to be reported. These findings support a consolidation account of costs in dual-task performance.     

Task-specific modulations of locomotor action parameters based on on-line visual information during collision avoidance with moving objects

The objectives of this study were: (a) to determine if the control mechanism for interacting with a dynamic real environment is the same as in the virtual reality (VR) studies, and (b) to identify the action control parameters that are modulated to successfully pass through oscillating doors. The participants walked along a 14-m path towards oscillating doors (rate of change in aperture size = 44 cm/s and maximum aperture varied 70, 80, or 100 cm). The participants had to use vision to extrapolate what the aperture of the doors would be at the time of crossing and determine if a change in action parameters was necessary. If their current state did not match the required state then the participants made modifications to their actions. The results showed that individuals in a real environment used similar action modifications (i.e., velocity adjustments) as those seen in VR studies to increase success. Aside from the gradual velocity adjustments observed, there was an immergence of a different locomotor action parameter on some trials that was not seen in VR studies (i.e., shoulder rotations). These shoulder rotations occurred when the participants perceived that a velocity adjustment alone would not lead to a successful trial. These results show that participants use perception to control movement in a feedback rather than feedforward manner.     

Visual causal impressions in the perception of several moving objects

Several kinds of visual causal impressions occur when people observe stimuli involving objects in motion. It is hypothesized that these impressions occur when the visual system avoids coincidences by matching the observable kinematic features of the stimulus to a template of a physical mechanism. This was tested by constructing a stimulus in which several spatially separated objects move in a coordinated manner. Variations on this stimulus were constructed such that some were inconsistent with a possible physical mechanism that could explain the motions of the objects and others were consistent with it. Mechanism‐inconsistent variations yielded significantly lower ratings of the causal impression and mechanism‐consistent variations did not, supporting the hypothesis. Misperceptions of features of the stimulus were also reported by a majority of participants, and the nature of these misperceptions is also consistent with the hypothesis that the stimulus is interpreted as a representation of a kind of physical mechanism.     

Components of reflexive visual orienting to moving objects

Posner and Cohen (1984) and Maylor (1985) initially observed that a luminance change produces both facilitatory and inhibitory effects on subsequent detection. While Posner and Cohen claimed that the facilitatory effect was mapped in retinotopic coordinates, they showed that inhibition of return (IOR) was mapped in “environmental coordinates.” Tipper and colleagues (Tipper, Driver, & Weaver, 1991; Tipper et al., 1997; Tipper, Weaver, Jerreat, & Burak, 1994) and Abrams and Dobkin (1994b) have recently reported that IOR can be object based, but contradictory results have also been reported (Muller & von Mühlenen, 1996). Here we report six experiments showing that an uninformative peripheral cue can generate either facilitatory or inhibitory object-based effects that can tag moving objects and that can persist for several hundred milliseconds. Although the boundary conditions determining which effect will be manifest remain to be defined, the present results suggest that facilitation and inhibition are generated independently, rather than being components of the same biphasic process.     

Topography of spatially directed conditioned responding: effects of context and trial duration

Domesticated quail (Coturnix japonica) received a discrete conditioned stimulus (CS) at one end of the experimental chamber paired with the opportunity to copulate with a female quail (the unconditioned stimulus) in a goal box located 112 cm away. Approach to the CS (sign tracking) and approach to the goal area (goal tracking) were measured. The duration of exposure to the experimental context (C) was varied in Experiment 1, and the duration of the conditioning trials (T) was varied in Experiment 2 for independent groups, creating C/T ratios of 1.0, 1.5, 4.5, 45, and 180. Contrary to previous reports of a direct relation between the C/T ratio and conditioned responding, in the present experiments, a shift in the topography and stimulus control of conditioned behavior occurred. Low C/T ratios (1.0-4.5) produced goal tracking controlled by contextual cues, whereas high C/T ratios (45 and 180) produced sign tracking controlled by the discrete CS.