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 ? or ? 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. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Haptically perceiving the distances reachable with hand-held objects.

Nine experiments are reported on the ability of people to perceive the distances reachable with hand-held rods that they could wield by movements about the wrist but not see. An observed linear relation between perceived and actual reaching distances with the rods held at one end was found to be unaffected by the density of the rods, the direction relative to the body in which they were wielded, and the frequency at which they were wielded. Manipulating (a) the position of an attached weight on an otherwise uniformly dense rod and (b) where a rod was grasped revealed that perceived reaching distance was governed by the principle movement(s) of inertia (I) of the hand-rod system about the axis of rotation. This dependency on moment of inertia (I) was found to hold even when the reaching distance was limited to the length of rod extending beyond an intermediate grasp. An account is given of the haptic subsystem (hand–muscles–joints–nerves) as a smart perceptual instrument in the Runeson (1977) sense, characterizable by an operator equation in which one operator functionally diagonalizes the inertia and strain tensors. Attunement to the invariants of the inertia tensor over major physical transformations may be the defining property of the haptic subsystem. This property is discussed from the Gibsonian (ecological) perspectives of information as invariants over transformations and of intentions as extraordinary constraints on natural law. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Selective perception by dynamic touch

Perceiving the length of a rod by dynamic touch is tied to the inertia tensor Iij, a quantification of its resistance to rotational acceleration. Perception of the portion extending in front of the grasp has previously been ascribed to decomposing one component of Iij by attention. The tensorial nature of dynamic touch suggests that this ability must be anchored wholly in the tensor. Three experiments show that perceived partial length is a function of two components of the tensor, one tied primarily to magnitude and the other tied primarily to direction, whereas perceived whole length is a function of a magnitude component alone. Dynamic touch is characterized in terms of a haptic perceptual instrument that softly assembles to exploit Iij differently depending on the intention, producing 1:1 maps that are appropriately scaled for each intention.     

Spinors and selective dynamic touch.

Nonvisual perception of a hand-held object's length through wielding is known to be a function of the object's inertia tensor about a fixed rotation point in the wrist. The tensor is insufficient, however, to support perceptions of 2 different extents of an object in 2 different directions. The attitude spinor represents the tilt of the object's inertia ellipsoid relative to the hand in 2 oppositely oriented directions of rotation. Predictions about selective dynamic touch based on the spinor's 2-orientedness were confirmed in experiments in which participants wielded in 1 hand either (a) 2 rods, and attempted to perceive the length of one of them, or (b) 1 rod, and attempted to perceive the length of a part. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Anticipatory load torques produced by voluntary movements.

The stability of an object held between the finger and thumb depends on friction developed by grip force, normal to the contact surfaces, to overcome tangential load force. Previous research has shown that in lifting an object, grip force rises with the increase in gravitational load force as the hand takes the weight and that in moving an object, grip force is adjusted to meet movement-induced inertial load force. Those results demonstrated the anticipatory nature of coordination of grip force with load force. Whether grip force anticipates load torque was studied in this research. When participants were constrained to use grasp points where the grasp axis was manifestly distant from object center of mass, it was found that they made grip force adjustments in anticipation of load torques that tended to destabilize an object as a result of lifting or moving it. These adjustments imply use of information about object center of mass in movement planning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Role of the inertia tensor in perceiving object orientation by dynamic touch.

Ss wielded an occluded L-shaped rod and attempted to perceive the direction in which the rod was pointing with respect to the hand. The pattern of the rod's different resistances to rotation in different directions, quantified by the inertia tensor, changes systematically with the rod's orientation. Perception of orientation by wielding is possible if the tissue deformation consequences of the rod's inertia tensor are detectable. It was shown that perceived orientation was a linear function of actual orientation for both free and restricted wielding and for rods of different-size branches. The eigenvectors of the inertia tensor were implicated as the basis for this haptic perceptual capability. Results are discussed in reference to information–perception specificity and its implications for effortful or dynamic touch. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Grip-load force coupling: A general control strategy for transporting objects.

Examined the coupling of grip force and load force during point-to-point and cyclic arm movements while holding an object in a variety of grips, including 1- and 2-handed grips and "inverted" grips. In all grips, grip force is modulated in phase with fluctuations in load force that are induced by the arm movement. The tight temporal coupling between grip force and load force seen when moving an object held in a precision grip (J. R. Flanagan et al, 1993) is observed in other grips. The control of precision grip force during whole-body jumping movements was also investigated. Grip force was modulated in phase with changes in load force induced by jumping even though the arm's joint angles were fixed. The tight temporal coupling between grip force and load force during object transport reflects a general control strategy that is not specific to any particular grip or mode of transport. Models of the coordination of grasp and transport in prehensile behaviors are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Wrist action affects precision grip force

When moving objects with a precision grip, fingertip forces normal to the object surface (grip force) change in parallel with forces tangential to the object (load force). We investigated whether voluntary wrist actions can affect grip force independent of load force, because the extrinsic finger muscles cross the wrist. Grip force increased with wrist angular speed during wrist motion in the horizontal plane, and was much larger than the increased tangential load at the fingertips or the reaction forces from linear acceleration of the test object. During wrist flexion the index finger muscles in the hand and forearm increased myoelectric activity; during wrist extension this myoelectric activity increased little, or decreased for some subjects. The grip force maxima coincided with wrist acceleration maxima, and grip force remained elevated when subjects held the wrist in extreme flexion or extension. Likewise, during isometric wrist actions the grip force increased even though the fingertip loads remained constant. A grip force "pulse" developed that increased with wrist force rate, followed by a static grip force while the wrist force was sustained. Subjects could not suppress the grip force pulse when provided visual feedback of their grip force. We conclude that the extrinsic hand muscles can be recruited to assist the intended wrist action, yielding higher grip-load ratios than those employed with the wrist at rest. This added drive to hand muscles overcame any loss in muscle force while the extrinsic finger flexors shortened during wrist flexion motion. During wrist extension motion grip force increases apparently occurred from eccentric contraction of the extrinsic finger flexors. The coactivation of hand closing muscles with other wrist muscles also may result in part from a general motor facilitation, because grip force increased during isometric knee extension. However, these increases were related weakly to the knee force. The observed muscle coactivation, from all sources, may contribute to grasp stability. For example, when transporting grasped objects, upper limb accelerations simultaneously produce inertial torques at the wrist that must be resisted, and inertial loads at the fingertips from the object that must be offset by increased grip force. The muscle coactivation described here would cause similarly timed pulses in the wrist force and grip force. However, grip-load coupling from this mechanism would not contribute much to grasp stability when small wrist forces are required, such as for slow movements or when the object's total resistive load is small.     

Effects of irrelevant stimulus orientation on visually guided grasping movements.

The present study investigated the effects of irrelevant stimulus orientation on visually guided grasping movements. Participants had to grasp a rectangular object at either the ends or the sides, depending on the color of a visual stimulus. In this task, correspondence between stimulus orientation and object orientation (stimulus-object congruency) and correspondence between stimulus orientation and hand orientation (stimulus-hand congruency) varied independently. Two experiments, with different sets of object orientations, revealed a consistent pattern of results. In particular, there were significant effects of stimulus-hand congruency, suggesting that perceiving an object activates congruently oriented hand movements. However, stimulus-object congruency had no effects, indicating that participants did not benefit from a preactivation of object orientation in the present task. The pattern of congruency effects implies that the cognitive representation, which is affected by irrelevant visual information, entails only those object or response features that are needed to select and control a response. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Body-scaled transitions in human grip configurations.

This article reports two experiments that were set up to examine the preferred human grip configuration used to displace cubes that varied in length (Lc), mass (Mc), and density (ML3). In particular, the authors sought to provide a more precise test of a dimensional relation between the object and the hand that had previously been shown to predict the grip configuration used to transport an object from one location to another. The experiments examined 2 grip transitions (from 3 digits to 4 digits and from 1 hand to 2 hands) within 2 sets of object conditions. In Experiment 1, cubes with a low density and a small increment in size (1 mm) were used, whereas in Experiment 2, cubes with 2 fixed sizes and small increments in mass were used. The results showed that the body-scaled equation K ?=?log Lc?+?(log Mc la ?+? bMh?+? cL h), where Mh and Lh are the anthropometric measures of the hand mass and length and a, b, and c are empirical constants, is the body-scaled information that predicts the grip configurations used to displace objects. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Impact of group consensus and social support on stimulus meaning: Mediation of conformity by cognitive restructuring.

Three experiments with a total of 163 undergraduates tested the hypothesis that a person reinterprets the meaning of the stimulus object when facing unpopular responses from a unanimous group and that this change in meaning leads to a shift in response toward the group's position. In Exp I, several opinion items were presented, and either Ss observed unpopular responses, supposedly made earlier by a unanimous group (UG) or by a group having 1 dissenter (social support), or they observed no response at all (control). Ss merely gave their interpretation of the meaning of a key word or phrase in each opinion statement—they did not give their own opinions. Results show that UG Ss gave more uncommon meanings to the stimuli than did Ss in the other 2 conditions. Exp II ruled out the possibility that the shift in meaning was due to Ss' adhering to the interpretation they assumed to be held by the majority. In Exp III, Ss were exposed to scores representing the meanings that were produced by the unanimous group and control conditions in Exp I. Results show that after observing the consensually produced meanings for these items, the Ss shifted their own opinions toward the position held by the UG in Exp I. (13 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Feeling more than we can know: Exposure effects without learning.

It is generally assumed that some form of learning, such as recognition, both precedes and mediates the affective response. The present study questioned that assumption by seeking to obtain positive exposure effects in the absence of stimulus recognition. To unconfound affect and learning effects, 2 experiments with 48 undergraduate females employed a dichotic-listening procedure for stimulus exposure, in which the critical stimuli were exposed on the unattended channel. Reliable attitudinal enhancement toward objectively familiar stimuli was obtained in both experiments, even though recognition levels were low (Exp I) or at chance levels (Exp II). Subjective stimulus familiarity was not a significant predictor of affect in either experiment. It is concluded that positive feelings toward a previously encountered object are not dependent on consciously knowing or perceiving that the object is familiar. (36 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Perceiving the width and height of a hand-held object by dynamic touch.

The haptic perceptual subsystem of dynamic touch is prominent in manipulating and transporting objects, providing a nonvisible awareness of their linear dimensions. The hypothesis that perceptions of object width and height by dynamic touch are different functions of the inertia tensor is addressed. In two experiments heights and widths of nonvisible wielded objects were judged separately. Experiment 1 used solid rectangular parallelepipeds of different sizes; Experiment 2 used objects of identical mass and linear dimensions but nonidentical inertia ellipsoids. Width and height perceptions of comparable reliability and accuracy were found to vary as distinct functions of the objects' inertial eigenvalues. Discussion focused on the notion of tangible shape and on the selectivity of attention within dynamic touch. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Individual finger forces acting on a grasped object during shaking actions

Individual finger grip forces acting on a hand-held object were examined during shaking tasks with a five-finger precision grip. The subjects (n = 13) shook a force transducer-equipped grip object (mass = 400 g) in vertical, horizontal, and mediolateral directions at an average movement speed of 33 cm/s (moderate) and 66 cm/s (fast). In addition, grip forces were examined while the subjects (n = 10) held the object in front of the body and walked or ran in place. It was found that the grip forces for all the fingers changed temporally and spatially coupling with the acceleration of the object resulting from shaking. The results suggest that grip force control is accomplished in an active and anticipatory fashion. Regardless of the shaking direction and speed, among the four fingers the absolute grip force in the index finger was largest, followed by the middle, ring, and little finger forces. The index finger therefore plays a primary role in grip force control during shaking. The percent force contribution by each finger varied depending on the direction of shaking. Contributions of the ring and little fingers were larger when shaken in the horizontal and mediolateral directions than they were in the vertical direction. The results suggest that different finger co-ordination is required in relation to shaking direction. Changes in shaking speed from moderate to fast changed the grip forces for all the fingers. During walking and running, grip force control similar to that during active vertical shaking was required to hold the object safely in the hand.     

Aging and immunoglobulin isotype patterns in oral tolerance

The haptic perceptual sybsystem of dynamic touch is prominent in manipulating and transporting objects, providing a nonvisible awareness of their linear dimensions. The hypothesis that perceptions of object width and height by dynamic touch are different functions of the inertia tensor is addressed. In two experiments heights and widths of nonvisible wielded objects were judged separately. Experiment 1 used solid rectangular parallelepipeds of different sizes; Experiment 2 used objects of identical mass and linear dimensions but nonidentical inertia ellipsoids. Width and height perceptions of comparable reliability and accuracy were found to vary as distinct functions of the objects' inertial eigenvalues. Discussion focused on the notion of tangible shape and on the selectivity of attention within dynamic touch.     

Self-image bias in person perception.

Conducted 3 experiments using a total of 283 male and female high school students. Exp I demonstrated that the more desirable the self-rating on a personality characteristic, the more central that characteristic is in perceiving others. This self-image bias in person perception was hypothesized to reflect the defense mechanism protecting high self-evaluation. In Exp II it appeared that, consistent with this defense interpretation, there was lower self-image bias among Ss in a condition that reduced defensiveness by using objective self-awareness techniques. Exp II also suggested an alternative to this defense explanation: a purely cognitive process whereby self-image mediates external evaluative stimuli and centrality of characteristics. Exp III attempted to test this alternative explanation; results support a cognitive interpretation of self-image bias in person perception. (36 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Inertia tensor and weight-percept models of length perception by static holding.

S. J. Lederman, S. R. Ganeshan, and R. E. Ellis (1996) reported an experiment demonstrating that for occluded rods of equal mass and length but different diameters length perception by static holding was larger for rods of smaller diameter. They concluded that participants inferred length from illusory weight percepts. However, rods of equal mass and length that differ in diameter also differ in the eigenvalues of their respective inertia tensors. In the present experiments, the authors manipulated the diameters (Experiment 1) and the inertial eigenvalues (Experiments 4 and 5) of statically held objects. As has been shown with wielded objects, perceived length was a function of the eigenvalues. Additional experiments failed to confirm the expectation from the weight-percept model that perceived length maps to the estimated weight (Experiments 2 and 3). Physical quantities, not psychological quantities, seem to explain length perception by static holding. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Control of grasp stability when humans lift objects with different surface curvatures

In previous investigations of the control of grasp stability, humans manipulated test objects with flat grasp surfaces. The surfaces of most objects that we handle in everyday activities, however, are curved. In the present study, we examined the influence of surface curvature on the fingertip forces used when humans lifted and held objects of various weights. Subjects grasped the test object between the thumb and the index finger. The matching pair of grasped surfaces were spherically curved with one of six different curvatures (concave with radius 20 or 40 mm; flat; convex with radius 20, 10, or 5 mm) and the object had one of five different weights ranging from 168 to 705 g. The grip force used by subjects (force along the axis between the 2 grasped surfaces) increased with increasing weight of the object but was modified inconsistently and incompletely by surface curvature. Similarly, the duration and rate of force generation, when the grip and load forces increased isometrically in the load phase before object lift-off, were not influenced by surface curvature. In contrast, surface curvature did affect the minimum grip forces required to prevent frictional slips (the slip force). The slip force was smaller for larger curvatures (both concave and convex) than for flatter surfaces. Therefore the force safety margin against slips (difference between the employed grip force and the slip force) was higher for the higher curvatures. We conclude that surface curvature has little influence on grip force regulation during this type of manipulation; the moderate changes in slip force resulting from changes in curvature are not fully compensated for by changes in grip force.     

Grip reorganization during wrist transport: the influence of an altered aperture

Past studies have examined the coupling of reach and grasp components during prehensile movements. Many of these studies have supported the view that these components reflect the output of two parallel, though temporally coupled, motor programs. When the grip aperture is Altered prior to the onset of prehension from its usual, normally flexed position to one of maximal finger extension, our previous work has shown that the grasp component appears to reorganize itself during the reach. This reorganization, consisting of a brief closing and reopening of the grip aperture, only slightly influenced the temporal components of the wrist transport. The present experiment continues this research theme by examining the characteristics of grip aperture reorganization through the comparison of the kinematics of prehension components during movements to two different size objects under normal and Altered grip aperture conditions. It was hypothesized that if the grip reorganization is task dependent it should be related to object size. The experiment found that in the Altered grip condition reorganization did occur, as indicated by a slight closing and reopening of the aperture without influencing the transport of the wrist. The amplitude of and the time to the observed inflection point in the aperture time course were related to object size. The velocity of grip closing for the large object showed double peaks, with the first substantially smaller than the second. Moreover, for the small object, the velocity of grip aperture closing also was double peaked, but the difference between peaks was less pronounced. These changes in grip velocity suggest that the grip reorganization is related to object size. No effect of Altered aperture was observed on the transport component. For both object sizes in the Altered condition, the final peak velocity of grip aperture was statistically significantly correlated with transport time and time to peak deceleration. In contrast, such correlations were not observed for the initial peak velocity of the grip aperture. Furthermore, time to maximum grip aperture was correlated with both time to peak wrist velocity and time peak to wrist deceleration. Thus, as the reach progressed toward the object, the grip and transport components became more interdependent. The results are consistent with the notion that, when a well-practiced, coordinated act such as prehension is confronted with an Altered grip posture at the onset of the reach, the grip can be reorganized during the transport to preserve the relative timing between them. Thus these data add to the growing awareness that not only is there temporal coupling between the reach and grasp components but that these components may be integrated by higher-order control mechanism.