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We examined whether the apparent size of an object is scaled to the morphology of the relevant body part with which one intends to act on it. To be specific, we tested if the visually perceived size of graspable objects is scaled to the extent of apparent grasping ability for the individual. Previous research has shown that right-handed individuals perceive their right hand as larger and capable of grasping larger objects than their left. In the first 2 experiments, we found that objects looked smaller when placed in or judged relative to their right hand compared to their left. In the third experiment, we directly manipulated apparent hand size by magnifying the participants' hands. Participants perceived objects to be smaller when their hand was magnified than when their hand was unmagnified. We interpret these results as demonstrating that perceivers use the extent of their hands' grasping abilities as "perceptual rulers" to scale the apparent size of graspable objects. Furthermore, hand size manipulations did not affect the perceived size of objects too big to be grasped, which suggests that hand size is only used as a scaling mechanism when the object affords the relevant action, in this case, grasping.

Previous research on perceiving spatial layout has found that people often exhibit normative biases in their perception of the environment. For instance, slant is typically overestimated and distance is usually underestimated. Surprisingly, however, the perception of height has rarely been studied. The present experiments examined the perception of height when viewed from the top (e.g., looking down), or from the bottom (e.g., looking up). Multiple measures were adapted from previous studies of horizontal extents to assess the perception of height. Across all of the measures, a large, consistent bias was found: vertical distances were greatly overestimated, especially from the top. Secondary findings suggest that the overestimation of distance and size that occurs when looking down from a high place correlates with reports of trait- and state-level fear of heights, suggesting that height overestimation may be due, in part, to fear.

Previous research has suggested that perceived distances are scaled by the action capabilities of the body. The present studies showed that when "reachability" is constrained due to a difficult grasp required to pick up an object, perceived distance to the object increases. Participants estimated the distances to tools with handle orientations that made them either easy or difficult to grasp with their dominant and nondominant hands. Right-handed participants perceived tools that were more difficult to grasp to be farther away than tools that were easier to grasp. However, perceived distance did not differ in left-handed participants. These studies suggest that, when reaching toward a target, the distance to that target is scaled in terms of how far one can effectively reach, given the type of reaching posture that is executed. Furthermore, this effect is modulated by handedness.

Perception is influenced by the perceiver's ability to perform intended actions. For example, when people intend to reach with a tool to targets that are just beyond arm's reach, the targets look closer than when they intend to reach without the tool (J. K. Witt, D. R. Proffitt, & W. Epstein, 2005). This is one of several examples demonstrating that behavioral potential affects perception. However, the action-specific processes that are involved in relating the person's abilities to perception have yet to be explored. Four experiments are presented that implicate motor simulation as a mediator of these effects. When a perceiver intends to perform an action, the perceiver runs a motor simulation of that action. The perceiver's ability to perform the action, as determined by the outcome of the simulation, influences perceived distance.

Recent research demonstrates neurologic and behavioral differences in people's responses to the space that is within and beyond reach. The present studies demonstrated a perceptual difference as well. Reachability was manipulated by having participants reach with and without a tool. Across 2 conditions, in which participants either held a tool or not, targets were presented at the same distances. Perceived distances to targets within reach holding the tool were compressed compared with targets that were beyond reach without it. These results suggest that reachability serves as a metric for perception. The 3rd experiment found that reachability only influenced perceived distance when the perceiver intended to reach. These experiments suggest that the authors perceive the environment in terms of our intentions and abilities to act within it.

Motion fields differ in the degree to which they can serve as hierarchical frames of reference. To assess the strength of hierarchical motion organisation, we measured the ability of human observers to extract the motion of a target dot relative to a background of moving dots. The task was to discriminate circular from elliptical relative motions for the target dot (2AFC). Observers performed above chance for translational and divergence backgrounds, but were at chance level for rotational backgrounds. This finding, however, was based on random configurations of dots, where locations and trajectories were unrelated. When spatial constraints were introduced, observers used a different strategy and performed above chance also for rotational backgrounds. Both the effect of motion transformations and the effect of spatial information can be understood if we assume that the task is to find the most likely mechanical description of an object in a three-dimensional environment.

The influence of color as a surface feature versus its influence as stored knowledge in object recognition was assessed. Participants decided whether a briefly presented and masked picture matched a test name. For pictures and words referring to similarly shaped objects, semantic color similarity (SCS) was present when picture and word shared the same prototypical color (e.g., purple apple followed by cherry). Perceptual color similarity (PCS) was present when the surface color of the picture matched the prototypical color of the named object (e.g., purple apple followed by blueberry). Response interference was primarily due to SCS, despite the fact that participants based similarity ratings on PCS. When uncolored objects were used, SCS interference still occurred, implying that the influence of SCS did not depend on the presence of surface color. The results indicate that, relative to surface color, stored color knowledge was more influential in object recognition.

The accuracy of depth judgments that are based on binocular disparity or structure from motion (motion parallax and object rotation) was studied in 3 experiments. In Experiment 1, depth judgments were recorded for computer simulations of cones specified by binocular disparity, motion parallax, or stereokinesis. In Experiment 2, judgments were recorded for real cones in a structured environment, with depth information from binocular disparity, motion parallax, or object rotation about the y-axis. In both of these experiments, judgments from binocular disparity information were quite accurate, but judgments on the basis of geometrically equivalent or more robust motion information reflected poor recovery of quantitative depth information. A 3rd experiment demonstrated stereoscopic depth constancy for distances of 1 to 3 m using real objects in a well-illuminated, structured viewing environment in which monocular depth cues (e.g., shading) were minimized.

In two experiments we investigated people's ability to judge the relative mass of two objects involved in a collision. It was found that judgments of relative mass were made on the basis of two heuristics. Roughly stated, these heuristics were (a) an object that ricochets backward upon impact is less massive than the object that it hit, and (b) faster moving objects are less massive. A heuristic model of judgment is proposed that postulates that different sources of information in any event may have different levels of salience for observers and that heuristic access is controlled by the rank ordering of salience. It was found that observers ranked dissimilarity in mass on the basis of the relative salience of angle and velocity information and not proportionally to the distal mass ratio. This heuristic model was contrasted with the notion that people can veridically extract dynamic properties of motion events when the kinematic data are sufficient for their specification.

Geometry informs us that there exist a laige number of possible connectivity patterns consistentwith a point-light display of a person walking. Yet there is only one pattern consistent with a "stickfigure" representation of the human form, and that pattern is uniquely specified by those pairwiseconnections that remain locally rigid. In this study, sensitivity to local rigidity in biomechanicaldisplays was investigated in 3- and 5-month-old infants. The results of Experiment 1 revealed thatby 5 months of age, infants discriminate a locally rigid point-light walker display from one in whichlocal rigidity is perturbed. In Experiment 2 we tested infants' sensitivity to the same stimuli whenthose stimuli were inverted. Contrary to the preceding experiment, the results revealed no evidenceof discrimination. Taken together, these findings suggest that infants are sensitive to local rigidity inbiomechanical displays but that this sensitivity is orientation specific. Possible mechanisms for thisspecificity are discussed in the context of additional constraints on the processing of biomechanicaldisplays.Among the varieties of motion in the visual world, the loco-motor patterns of people and most other ambulatory creaturesare unique. This uniqueness stems from their basic anatomicalstructure, which consists of a hierarchical nesting of limbs thatmove in pendular fashion. No doubt, the perception of theselocomotor patterns plays a significant role in the survival of thespecies. For example, recognition of the differences between theplayful gait of a friend and the stalking pattern of a predator isbasic to any animal's survival.Human adults appear to be especially sensitive to the move-ment patterns produced by other members of their species.Even when all familiarity cues are removed and the stimuli arerestricted to points of light located on the major joints and headof an unseen person, adults are still capable of recognizing thehuman form from these biomechanical motions (Cutting P Johansson, 1973; Proffitt & Bertenthal, in press).Although most of the experimental research is restricted tostimuli depicting a person walking, Johansson (1973) reportsthat adult observers are equally proficient in recognizing anumber of other biomechanical patterns, including climbing,This research wa s supported by National Institute of Child Healthand Human Development (NICHHD) Grant HD-16195 to the first twoauthors and NICHHD Career Development Award HD-00678 to the firstauthor.Portions of this article were presented at the Meetings of the Societyfor Research in Child Development in Toronto, Canada, April 1984,and at the Meetings of the International Conference on Infant Studiesin Los Angeles, April 1985.We acknowledge our appreciation to Dina Bai, Liz DiLalla, andSarah Dunn for their assistance in the collection of the data.Correspondence concerning this article should be addressed to Ben-nett I. Bertenthal, Department or Psychology, Gilmer Hall, Universityof Virginia, Charlottesville, Virginia 22903.cycling, and dancing. There is also evidence that adult observersare capable of recognizing friends, the gender of a person, andvarious dispositional characteristics of a person from these bio-mechanical motions (Cutting & Proffitt, 1981; MacArthur &Baron, 1983).Our research was designed to extend these findings by ad-dressing two related issues. One concerned whether young in-fants are sensitive to the rigid connections that exist betweenbody joints, such as the shoulder-to-elbow connection, elbow-to wrist connection, and so forth. In essence, these connectionscorrespond to the straight lines that constitute a human stick-figure interpretation of a point-light walker display. The secondissue concerned whether the detection of these rigid connec-tions was in any way dependent upon the natural and/or famil-iar orientation of the point-light walker display.

Contrary to the view that ambient light information unequivocally specifies phenomenal events, recent research suggests that natural event perception is determined by processes that pick up and combine visual and motor information. This thesis is challenged by Turvey and Solomon (1984). The present article responds to their misgivings by elaborating empirical methods and theoretical arguments of past work. A control experiment is also presented on pointing measurements of distance illusions related to esophoric shifts of eye convergence that are induced by near work. The soundness of both the empirical methodology and the theoretical arguments in support of the original thesis is upheld. Language: en

College students and children between the ages of 4 and 12 were asked to draw the path a ball would take upon exiting a curved tube. As in previous studies, many subjects erroneously predicted curvilinear paths. However, a clear U-shaped curve was evident in the data: Preschoolers and kindergartners performed as well as college students, whereas school-aged children were more likely to make erroneous predictions. A second study suggested that the youngest children's correct responses could not be attributed to response biases or drawing abilities. This developmental trend is interpreted to mean that the school-aged children are developing intuitive theories of motion that include erroneous principles. The results are related to the 'growth errors' found in other cognitive domains and to the historical development of formal theories of motion.

When making dynamical judgments, people can make effective use of only one salient dimension of information present in the event. People do not make dynamical judgments by deriving multidimensional quantities. The adequacy of dynamical judgments, therefore, depends on the degree of dimensionality that is both inherent in the physics of the event and presumed to be present by the observer. There are two classes of physical motion contexts in which objects may appear. In the simplest class, there exists only one dynamically relevant object parameter: the position over time of the object's center of mass. In the other class of motion contexts, there are additional object attributes, such as mass distribution and orientation, that are of dynamical relevance. In the former class, objects may be formally treated as extensionless point particles, whereas in the latter class some aspect of the object's extension in space is coupled into its motion. A survey of commonsense understandings showed that people are relatively accurate when specific dynamical judgments can be accurately based on a single information dimension; however, erroneous judgments are pervasive when simple motion contexts are misconstrued as being multidimensional, and when multidimensional quantities are the necessary basis for accurate judgments.