Geometries and Dynamics of a Rod Determine How It Is Used for Reaching

Displacing an object with a hand-held rod provided a simple paradigm for studying tool use. The authors asked how reaching was affected by manipulations of rod properties. Adults held a rod (length = .10 to 1.5 m), with its tip in the air; walked toward an object on a table; chose a place to stop; and displaced the object with the rod's tip. In 3 experiments (Ns = 9, 22, and 17 participants), the authors manipulated rod length, mass, and mass distribution to determine whether and how geometric and dynamic properties affected the chosen distance and the posture. Both the chosen stopping distance and the postures were well accommodated to rod characteristics. Postural adaptations took place only in the arm, which was organized as a synergy. Predictably, rod length explained most of the variance, but small and reliable differences in both distance and posture depended on mass and mass distribution. The chosen distance anticipated not only rod length but also the upcoming posture needed to control the rod.     

Geometric,But Not Kinetic,Properties of Tools Affect the Affordances Perceived by Toddlers

We asked what rod properties affected children's reaching range. Children, 2 to 4 years old, held a rod (length 10-60 cm) with the tip in the air, walked toward a toy on a table, chose a place to stop, and displaced the toy with the rod's tip. In 2 experiments rod length, mass, and mass distribution were manipulated to determine whether and how geometric and kinetic properties affected chosen distance and posture. Chosen distance depended only on the length of the rod. Postures were affected by length and mass properties of the rod. Not all adaptations in posture were prospectively reflected in the distance. Although we found variations over age, we did not find clear developmental trends. The results are discussed in broader perspectives of development of affordances and tool use.     

Variations of tool and task characteristics reveal that tool-use postures are anticipated

The authors examined anticipation in tool use, focusing on tool length and tool-use posture. Adults (9 women and 9 men in each experiment) held a rod (length 0.4-0.8 m), with the tip upward; walked toward a cube; chose a place to stop; and displaced the cube with the rod's tip. In 2 experiments, rod length, mass, and mass distribution, and the size of the cube were manipulated. Chosen distance depended on rod length and cube size. Because effects of cube size on distance resulted only from postural changes related to required control, distance anticipated displacement posture. A postural synergy comprising legs and trunk provided a stable platform for the displacement. An arm synergy was less extended for small cubes, longer rods, and handle-weighted rods. Selected distance anticipated those postures.     

Perceived arm posture and remote haptic perception of whether an object can be stepped over

The authors investigated whether the perceived posture of the arm plus a hand-held object influences remote haptic perception of whether an object can be stepped over. Blindfolded participants (N = 20) determined whether they could step over bars at different heights by exploring the bars with a T-shaped rod. The rod was weighted so that the perceived posture of the arm-plus-rod was shifted up, down, or left unchanged. Both the leg length of the participant and the perceived posture of the arm-plus-rod systematically influenced perception of whether the bar could be stepped over. The results highlight the role of perceived posture of the arm-plus-rod in remote haptic perception and have potential implications for the design of navigation aids for the visually impaired.     

Assume a Spherical Chicken: Analytic Constraints,Inertia Tensor Information,and Wielded Rod Length Perception

Information associated with the inertia tensor is the preeminent explanation for haptic perception of object properties, notably wielded rod length. Critics counter that tensorial-based information requires non-tensorial supplementation (mass, torque). However, those critiques omit important constraints. With relevant constraints included, the inertia tensor alone completely specifies rod length. I list constraints inherent (but tacit) in haptic rod length perception, and show that object properties associated with the inertia tensor are invariant, even with constraints removed, by involving (a) longitudinal moment equivalents for rod mass and (b) derivatives of moments with respect to varying rotation axes. Analytic outcomes show tensorial-based information is a robust basis for wielded rod length perception, and suggest open questions for empirical exploration.     

Influence of semantic consistency and perceptual features on visual attention during scene viewing in toddlers

Conceptual representations of everyday scenes are built in interaction with visual environment and these representations guide our visual attention. Perceptual features and object-scene semantic consistency have been found to attract our attention during scene exploration. The present study examined how visual attention in 24-month-old toddlers is attracted by semantic violations and how perceptual features (i. e. saliency, centre distance, clutter and object size) and linguistic properties (i. e. object label frequency and label length) affect gaze distribution. We compared eye movements of 24-month-old toddlers and adults while exploring everyday scenes which either contained an inconsistent (e.g., soap on a breakfast table) or consistent (e.g., soap in a bathroom) object. Perceptual features such as saliency, centre distance and clutter of the scene affected looking times in the toddler group during the whole viewing time whereas looking times in adults were affected only by centre distance during the early viewing time. Adults looked longer to inconsistent than consistent objects either if the objects had a high or a low saliency. In contrast, toddlers presented semantic consistency effect only when objects were highly salient. Additionally, toddlers with lower vocabulary skills looked longer to inconsistent objects while toddlers with higher vocabulary skills look equally long to both consistent and inconsistent objects. Our results indicate that 24-month-old children use scene context to guide visual attention when exploring the visual environment. However, perceptual features have a stronger influence in eye movement guidance in toddlers than in adults. Our results also indicate that language skills influence cognitive but not perceptual guidance of eye movements during scene perception in toddlers.     

The influence of time constraints on posture choices during an end-state comfort task

People adopt comfortable postures for the end states of motor actions (end-state comfort; Rosenbaum & Jorgensen, 1992). The choice to end comfortably often elicits adoption of uncomfortable beginning states, demonstrating that a sequence of movement is planned in advance of movement onset. Many factors influence the choice of comfortable end-state postures including the greater precision and speed afforded by postures at joint angle mid-ranges (Short & Cauraugh, 1999). To date, there has been little evaluation of the hypothesis that postures are chosen based on minimizing the time spent in uncomfortable postures. The aim of this experiment was to examine how the relative time required to hold beginning and end-state postures influenced the choice of posture. Participants moved a two-toned wooden dowel from one location to another with the requirement to grasp the object and place a specified color down. Participants completed four conditions where no postures were held, only one posture was held, or both postures were held. We predicted more thumb-up postures for positions held longer regardless of whether these postures were at the end or beginning state. Results verified that the constraint of holding the initial posture led to decreased end-state comfort supporting the hypothesis that estimation of time spent in postures is an important constraint in planning. We also note marked individual differences in posture choices, particularly when the object was moved to the left.     

Holding an object one is looking at: Kinesthetic information on the object’s distance does not improve visual judgments of its size

Visual judgments of distance are often inaccurate. Nevertheless, information on distance must be procured if retinal image size is to be used to judge an object’s dimensions. In the present study, we examined whether kinesthetic information about an object’s distance—based on the posture of the arm and hand when holding it—influences the object’s perceived size. Subjects were presented with a computer simulation of a cube. This cube’s position was coupled to that of a rod in the subject’s hand. Its size was varied between presentations. Subjects had to judge whether the cube they saw was larger than, smaller than, or the same size as a reference. On some presentations, a small difference was introduced between the positions of the rod and of the simulated cube. When the simulated cube was slightly closer than the rod, subjects judged the cube to be larger. When it was farther away, they judged it to be smaller. We show that these changes in perceived size are due to alterations in the cube’s distance from the subject rather than to kinesthetic information.     

Mechanical invariants are implicated in dynamic touch as a function of their salience in the stimulus flow

The authors investigated the mechanical basis of length perception through dynamic touch using specially designed rods in which the various moments of mass distribution (mass, static moment, and rotational inertia) were varied independently. In a series of 4 experiments, exploration style and rod orientation were manipulated such that the relative salience of moments of mass distribution varied markedly. Results showed that perceived length was based on the most salient moments. The authors concluded that the notion of salience is crucial for understanding the implication of moments of mass distribution in length perception and that it should play a pivotal role in developing an encompassing theory of dynamic touch.     

Shaping of Reach-to-Grasp Kinematics by Intentions: A Meta-Analysis

When we reach to grasp something, we need to take into account both the properties of the object we are grasping and the intention we have in mind. Previous research has found these constraints to be visible in the reach-to-grasp kinematics, but there is no consensus on which kinematic parameters are the most sensitive. To examine this, a systematic literature search and meta-analyses were performed. The search identified studies assessing how changes in either an object property or a prior intention affect reach-to-grasp kinematics in healthy participants. Hereafter, meta-analyses were conducted using a restricted maximum likelihood random effect model. The meta-analyses showed that changes in both object properties and prior intentions affected reach-to-grasp kinematics. Based on these results, the authors argue for a tripartition of the reach-to-grasp movement in which the accelerating part of the reach is primarily associated with transporting the hand to the object (i.e., extrinsic object properties), the decelerating part of the reach is used as a preparation for object manipulation (i.e., prepare the grasp or the subsequent action), and the grasp is associated with manipulating the object's intrinsic properties, especially object size.     

Multimodal Perception of Reachability Expressed Through Locomotion

We investigated the information that supports perception of whether an object is within reach using a locomotor task. Participants adjusted their own position relative to a fixed target by stepping or by propelling a wheelchair until they judged it to be within reach. The to-be-reached object was presented in virtual reality. The display of the target was driven in real time as a function of the observer's movement, thus depicting a stationary virtual object at a definite distance only through the relation across optical and nonoptical patterns of stimulation. We asked participants to judge the distance they could reach with their unaided hand or when holding a rod that extended their effective reach. They could see neither their body nor the rod thereby limiting available visual information about “reachability.” As expected, our results showed that despite the limited information that was available, participants' locomotor adjustments were influenced by (a) their simulated distance from the target, (b) their arm length, and (c) the presence or absence of the rod. The type of motion (stepping or wheelchair) had little influence. However, judgment accuracy was influenced by participants' initial simulated distance from the target. We compare the performance obtained in our locomotor judgment task with previous studies that have used different methods for measuring perceived reaching-ability. We discuss perceptual information that could have supported performance within the framework of the global array.     

Perceptual behavior: recurrence analysis of a haptic exploratory procedure

Various object properties are perceptible by wielding. We asked whether the dynamics of wielding differed as a function of the to-be-perceived property. Wielding motions were analyzed to determine if they differed under the intention to perceive or not perceive rod length (experiment 1), to perceive object height versus object width (experiment 2), and to perceive the length forward of where the rod was grasped versus the position of the grasp (experiment 3). Perceiving these different properties is known to depend on different components of the object's inertia tensor. Analyses of the subtle recurrent patterns in the phase space of the hand motions revealed differences in wielding across the different perceptual intentions. Haptic exploratory procedures may exhibit distinct exploratory dynamics.     

Attentionally splitting the mass distribution of hand-held rods

Two experiments on the length-perception capabilities of effortful or dynamic touch differed only in terms of what the subject intended to perceive, while experimental conditions and apparatus were held constant. In each trial, a visually occluded rod was held as still as possible by the subject at an intermediate position. For two thirds of the trials, a weight was attached to the rod above or below the hand. In Experiment 1, in which the subject's task was to perceive the distance reachable with the portion of the rod forward of the hand, perceived extent was a function of the first moment of the mass distribution associated with the forward portion of the rod, and indifferent to the first moment of the entire rod. In Experiment 2, in which the task was to perceive the distance reachable with the entire rod if it was held at an end, the pattern of results was reversed. These results indicate the capability of selective sensitivity to different aspects of a hand-held object's mass distribution, without the possibility of differential exploration specific to these two tasks. Results are discussed in relation to possible roles of differential information, intention, and self-organization in the explanations of selective perceptual abilities.     

Geometric specification of dynamics: learning to visually perceive kinetic quantities from static images

In a single experiment, perceivers viewed computer-generated hammer-like objects and estimated a kinetic property. Each hammer comprised a cylinder of some diameter and height, attached to a handle-a second cylinder of fixed diameter and variable length. On 5 blocks of trials, three groups of perceivers reported a property reflecting mass, torque, or moment of inertia. Blocks 2-4 were training blocks on which numerical feedback was given. On blocks 6 and 7, perceivers were asked to report the kinetic properties not reported on blocks 1-5. We were interested in how geometrical object properties affected the perception of kinetic properties, and how that perception changed with experience. To determine how geometrical object properties informed reports of kinetics, a continuous information space of head diameter and height, and handle length was created, and perceivers' loci within that space were determined. Perceivers began at various distances from the optimum for the to-be-reported property and were closer to the optimum after training. On blocks 6 and 7, performance did not reflect transfer from training on another property. The results were interpreted in the context of the theory of direct learning.     

Transfer of calibration in dynamic touch: What do perceivers learn when they learn about length of a wielded object?

Successfully performing everyday behaviours requires that perceptions and actions are properly calibrated to environmental properties. In three experiments, the authors tested whether calibration was specific to perception of a particular property of a wielded object from a particular grasp position on that object. The experiments investigate whether transfer of recalibration occurs across changes in grasp position (Experiment 1), object property (Experiment 2), and grasp position and object property (Experiment 3). The results suggest that a complete transfer of recalibration occurred in each case. Such results are consistent with recent research on dynamic touch and suggest that rather than recalibrating participants to one particular property of the wielded objects, feedback about a particular property served to recalibrate participants to the properties of the rod set as a whole.     

Transfer of calibration in dynamic touch: what do perceivers learn when they learn about length of a wielded object?

Successfully performing everyday behaviours requires that perceptions and actions are properly calibrated to environmental properties. In three experiments, the authors tested whether calibration was specific to perception of a particular property of a wielded object from a particular grasp position on that object. The experiments investigate whether transfer of recalibration occurs across changes in grasp position (Experiment 1), object property (Experiment 2), and grasp position and object property (Experiment 3). The results suggest that a complete transfer of recalibration occurred in each case. Such results are consistent with recent research on dynamic touch and suggest that rather than recalibrating participants to one particular property of the wielded objects, feedback about a particular property served to recalibrate participants to the properties of the rod set as a whole.     

Perceiving extents of rods by wielding: haptic diagonalization and decomposition of the inertia tensor

We report two experiments on the length-perception capabilities of the hand-related haptic subsystem. On each trial, a visually occluded rod was wielded by the subject at a position intermediate between its two ends. The position was either 1/2 or 3/4 of the rod's length. On two-thirds of the trials, a weight was attached to the rod at a point either above or below its center of gravity and not coincident with the hand's position. In Experiment 1, the subject's task was to perceive the distance reachable with the portion of the rod extending beyond the position of the grasp. In the second experiment, the subject's task was to perceive the distance reachable with the entire rod if it were held at its proximal end. In Experiment 1, perceived reaching distance was a function of the moment of inertia of the amount of rod forward of the grasp about an axis through the proximal end of the rod segment. In Experiment 2, perceived reaching distance was a function of the moment of inertia of the entire rod about the given axis of rotation intermediate between the rod's ends. The results are discussed in terms of (a) the notion of smart perceptual instruments capitalizing on invariant properties of the inertia tensor and (b) how the haptic decomposition of moments of inertia follows the principle of equivalence of forces.     

Principles of part-whole selective perception by dynamic touch extend to the torso

The haptic subsystem of dynamic touch expresses a novel form of part-whole selective perception. When wielding a nonvisible rod grasped at some intermediate point along its length, an individual can attend to and report the length of a part of the rod (e.g., the segment forward of the hand) or the length of the whole rod. Both perceptions relate to the rod's mass moments about the point of grasp but in systematically different ways. Previous demonstrations of this part-whole selectivity have been in respect to rods grasped by hand or attached to a foot. The authors demonstrated the part-whole selectivity for nonvisible rods attached to the shoulder girdle and wielded primarily by movements of the trunk with benchmark performance provided by the same rods grasped and wielded by hand. Their results suggest that part-whole selectivity is a haptic capability general to the body.     

Haptic perception of partial-rod lengths with the rod held stationary or wielded

Three experiments on the haptic perception of partial-rod lengths are reported. The rods were gripped between the two ends and held horizontal. The subjects held the rods stationary; the distribution of mass of the segment in front of the hand was fixed, while the distribution of mass of the segment behind the hand was varied. Perceived forward length was found to be significantly affected by the distribution of mass of the backward segment. Similar results were obtained when the rods were wielded. The results indicated that partial-rod lengths are specified by functions of mechanical perturbations acting on the hand, and not ay the breaking up of the first moment of mass or the moment of inertia of the rod by attention as suggested previously by others. The results are also discussed with respect to invariant detection and attention.     

The effect of density and diameter on haptic perception of rod length

Three experiments on the effect of density and diameter on haptic perception of rod length are reported. In Experiment 1, the subjects wielded visually occluded rods of different densities. Perceived length was found to be affected by the density of the rod regardless of the actual length. In Experiment 2, three aluminum rods of different lengths with handles of four different diameters were wielded. Perceived length of the rod was found to be shorter as the diameter of the handle with which it was wielded increased. A diameter—length illusion was thereby produced. In Experiment 3, visually occluded rods of different diameters but of the same moment of inertia about thex-axis were wielded with the right hand, and tubes of different diameters were felt with the left hand. The subjects were instructed that their right hand was grasping a handle, and that the actual diameter of the rod could be felt with the left hand. Rods were perceived to be shorter if a larger diameter was felt with the left hand. The results showed that perceived length is not just a function of actual rod length, and that it is not accounted for by inertia only. The results are further discussed in terms of the nature of invariants and the effect of knowledge on perception.