The limits of visual mass perception

The theory of direct perception suggests that observers can accurately judge the mass of a box picked up by a lifter shown in a point-light display. However, accurate perceptual performance may be limited to specific circumstances. The purpose of the present study was to systematically examine the factors that determine perception of mass, including display type, lifting speed, response type, and lifter's strength. In contrast to previous research, a wider range of viewing manipulations of point-light display conditions was investigated. In Experiment 1, we first created a circumstance where observers could accurately judge lifts of five box masses performed by a lifter of average strength. In Experiments 2–5, we manipulated the spatial and temporal aspects of the lift, the judgement type, and lifter's strength, respectively. Results showed that mass judgement gets worse whenever the context deviates from ideal conditions, such as when only the lifted object was shown, when video play speed was changed, or when lifters of different strength performed the same task. In conclusion, observers' perception of kinetic properties is compromised whenever viewing conditions are not ideal.     

The relative influences of movement kinematics and extrinsic object characteristics on the perception of lifted weight

Humans are able to perceive unique types of biological motion presented as point-light displays (PLDs). Thirty years ago, Runeson and Frykholm (Human Perception and Performance, 7(4), 733, 1981, Journal of Experimental Psychology: General, 112(4), 585, 1983) studied observers’ perceptions of weights lifted by actors and identified that the kinematic information in a PLD is sufficient for an observer to form an accurate perception of the object weight. However, research has also shown that extrinsic object size characteristics also influence the perception of object weight (Gordon, Forssberg, Johansson, & Westling in Experimental Brain Research, 83(3), 477–482, 1991). This study addresses the relative contributions of these two types of visual information to observers’ perceptions of lifted weight, through an experiment in which participants viewed an actor lifting boxes of various sizes (small, medium, or large) and weights (25, 50, or 75 lb) under four PLD conditions—box-at-rest, moving-box, actor-only, and actor-and-box—and one full-vision video condition, and then provided a weight estimate for each box lifted. The results indicated that lift kinematics and box size contributed independently to weight perception. Interestingly, the most robust weight differentiations were elicited in the conditions in which both types of information were presented concurrently, despite their converse natures. Furthermore, full-vision video presentation, which contained visual information beyond kinematics and object information, elicited the best estimates.     

Observing how others lift light or heavy objects: time-dependent encoding of grip force in the primary motor cortex

During movement observation, corticomotor excitability of the observer's primary motor cortex (M1) is modulated according to the force requirements of the observed action. Here, we explored the time course of observation-induced force encoding. Force-related changes in M1-excitability were assessed by delivering transcranial magnetic stimulations at distinct temporal phases of an observed reach-grasp-lift action. Temporal changes in force-related electromyographic activity were also assessed during active movement execution. In observation conditions in which a heavy object was lifted, M1-excitability was higher compared to conditions in which a light object was lifted. Both during observation and execution, differential force encoding tended to gradually increase from the grasping phase until the late lift phase. Surprisingly, however, during observation, force encoding was already present at the early reach phase: a time point at which no visual cues on the object's weight were available to the observer. As the observer was aware that the same weight condition was presented repeatedly, this finding may indicate that prior predictions concerning the upcoming weight condition are reflected by M1 excitability. Overall, findings may provide indications that the observer's motor system represents motor predictions as well as muscular requirements to infer the observed movement goal.     

Perceptual weight judgments when viewing one's own and others' movements under minimalist conditions of visual presentation

Across five experiments, we investigated the parameters involved in the observation and in the execution of the action of lifting an object. The observers were shown minimal information on movements, consisting of either the working-point displacement only (ie two points representing the hand and object) or additional configural information on the kinematics of the trunk, shoulder, arm, forearm, and hand, joined by a stick diagram. Furthermore, displays showed either a participant's own movements or those of another person, when different weights were lifted. The participants' task was to estimate the weight of the lifted objects. The results revealed that, although overall performance was not dependent on the visual conditions (working point versus stick diagram) or ownership conditions (self versus other), the kinematic cues used to perform the task differed as a function of these conditions. In addition, the kinematic parameters relevant for action observation did not match those relevant for action execution. This was confirmed in experiments by using artificially altered movement samples, where the variations in critical kinematic variables were manipulated separately or in combination. We discuss the implications of these results for the roles of motor simulation and visual analysis in action observation.     

Effects of Object Texture on Precontact Movement Time in Human Prehension

Using kinematic data in a precision-grip reaching task, Weir, MacKenzie, Marteniuk, and Cargoe (1991) concluded that prior to contact with an object, its texture does not affect the course of grasping. The present study used their task of reaching for and lifting a slippery-, normal- (polished metal), or rough-surfaced dowel. This occurred under the original, blocked condition, in which textures were held constant within a series of trials, and under a new, randomized condition, in which textures varied randomly from trial to trial. Performance was also examined over more extended periods of practice. Reaction time and precontact movement time were directly measured. In contrast to the results of Weir et al., 1991, reaching for the slippery dowel resulted in slower movement time. This effect was found both early and late in practice for the randomized condition; it was found only in late practice for the blocked condition. These effects can be attributed to the greater geometric and dynamic precision required for lifting a slippery object.     

The role of predictability of the magnitude of a perturbation in control of vertical posture when catching an object

The predictability of perturbation magnitude plays an important role in control of standing posture. The aim of the study was to examine anticipatory (APAs) and compensatory (CPAs) postural adjustments in response to catching objects of uncertain mass. Twenty adults caught the same object with either light or heavy weight placed in it. Electromyographic activity of eight trunk and leg muscles, displacements of the center of pressure, and angular displacement of the shoulder joint were recorded and analyzed during the APAs and CPAs intervals. When the subjects experienced repeated catching of the object with the same weight, they estimated the object mass beforehand and generated APAs more precisely. When the object mass changed unpredictably, they generated APAs based on the most recent catch and needed four to six trials to optimize APAs and CPAs. The muscle co-contraction was a primary pattern for catching the object of uncertain mass. The results of the study suggest that catching the object of uncertain mass is a challenging task that involves co-contraction of postural muscles to maintain balance.     

The Consistent Acceleration of Lifted Objects

In examining films of lifting movements in a study of the size-weight illusion (Davis & Roberts, 1976), a consistency was noted in the values obtained for the maximum accelerations of the objects lifted. While at first surprising, this finding can be embedded significantly in theories relating to kinesthetic illusions and the perception of weight and to theories on the control of general physical movement. This study was designed to confirm its existence. Twenty-four subjects were filmed lifting four objects differing in size, shape, substance, color, and weight. The film was analyzed frame-by-frame and the data were subjected to a two-way analysis of variance. Subjects, while differing from one another, were consistent in the maximum accelerations they applied to the three heaviest of the four objects. The accelerations of the lightest object differed significantly from the accelerations of the other three, but it seems likely that this was due to the experimental task itself.     

Mechanical advantage in the size-weight illusion

When a hand-held object is lifted by wrist flexion, the lifting system composed of muscles, bones, and the object lifted constitutes a third-class lever. Therefore, objects require greater lifting force as they are supported further distally The small can of the DeMoors size-weight illusion cans is usually supported further distally than the large one, possibly influencing their relative perceived weight When Ss are required to lift the small can through a shorter lever than the large one, there is a significant shift of judgments toward a reversal of the SWI in a paired-comparison situation. It is concluded that mechanical advantage does influence weight judgments and that biomechanical factors should be considered whenever weight judgments are made.     

The consistent acceleration of lifted objects: implications for kinesthetic illusions and the perception of weight

In examining films of lifting movements in a study of the size-weight illusion (Davis & Roberts, 1976) a consistency was noted in the values obtained for the maximum accelerations of the objects lifted. While at first surprising, this finding can be embedded significantly in theories relating to kinesthetic illusions and the perception of weight and to theories on the control of general physical movement. This study was designed to confirm its existence. Twenty-four subjects were filmed lifting four objects differing in size, shape, substance, color, and weight. The film was analyzed frame-by-frame and the data were subjected to a two-way analysis of variance. Subjects, while differing from one another, were consistent in the maximum accelerations they applied to the three heaviest of the four objects. The accelerations of the lightest object differed significantly from the accelerations of the other three, but it seems likely that this was due to the experimental task itself.     

Lifting movements in the size-weight illusion

The occurrence of the size-weight illusion is related to the manner in which-objects are lifted. When the SWI occurs, the larger of two objects of equal objective weight is usually lifted with greater acceleration, deceleration, and maximum velocity than the smaller one, but to approximately the same height. These differences are not present when the cans feel equally heavy. The relationship of lifting movements to judgments is consistent with the known behavior of proprioceptors which provide sensory input about muscular and movement events.     

Effects of anticipation certainty on preparatory brain activity and anticipatory postural adjustments associated with voluntary unilateral arm movement while standing

We examined the effects of anticipation certainty concerning which voluntary movement is required in response to a stimulus while standing on preparatory brain activity and anticipatory postural adjustments (APAs). Ten right-handed adults abducted their left or right arm rapidly in response to a visual imperative stimulus, based on the type of stimulus. A warning cue, which did or did not contain information about the side of arm abduction, was presented 2000ms before the imperative stimulus. Preparatory brain activity before arm abduction was quantified by the mean amplitude of the contingent negative variation 100ms before the imperative stimulus (late CNV amplitude). Compared with the low anticipation condition, in the high anticipation condition the following results were obtained only in the case of right arm abduction: (1) larger late CNV amplitude, (2) earlier postural muscle activities with respect to the focal muscle of arm abduction, and (3) smaller peak displacement of center of pressure during the abduction. These findings suggest that high anticipation of voluntary movement of dominant arm to a stimulus while standing influences preparatory brain activity before the movement, resulting in earlier APAs and thus smaller disturbance of postural equilibrium during the movement.     

Longitudinal Development of the Automatic Postural Response in Infants

In this longitudinal study, the development of automatic postural responses elicited during stance following perturbation of the support surface was investigated. Infants (N = 9) unable to stand without support were tested initially: follow-up tests were performed until the infants were able to stand and walk independently. Surface electromyographic recordings of leg and trunk muscle activity following a postural perturbation induced by a forward or backward translation of the support surface were made for each infant. Muscle onset latencies following the perturbation and the proportion of trials in which muscle activity was recorded were determined. First, infants activated appropriate muscles either in isolation or in pairs and then combined these muscles into functional synergies. Although activation of all three postural muscles was recorded in infants before they were able to stand and walk independently, the three-muscle response was not consistent in the youngest children. The proportion of trials eliciting muscle activity continued to increase (p <.O5) after infants began to walk independently, with postural muscle activity recorded in virtually every trial by late independent walking. Thus. the automatic postural response elicited during stance was shown to begin with activity in single or paired muscles, followed by activation of the postural muscles in functional synergies. These data illustrate the progressive development of an effective sensory-motor organization.     

Activity of wrist muscles elicited during imposed or voluntary movements about the elbow joint

To examine the coordination of muscles during multijoint movement, we compared the response of wrist muscles to perturbations about the elbow joint with their activation during a volitional elbow movement. The purpose was to test the following two predictions: (a) Responses can occur in muscles not stretched by the perturbation, as has been reported for other multijoint systems; and (b) the motor pattern in response to a perturbation mimics an opposing volitional motor pattern across the two joints. We recorded the electromyographic (EMG) activity of elbow and wrist muscles as well as the flexion/extension motions at the elbow and wrist joints during individual trials that either involved a response to a torque perturbation that extended the elbow or required volitional elbow flexion. The results of this study confirmed that responses were elicited in the nonstretched wrist muscles when the elbow joint was perturbed. The same motor sequence of elbow and wrist flexors was present for both the volitional and perturbation task (with the forearm supinated), regardless of whether the wrist joint was immobilized or freely moving. The findings suggest that the nervous system relies on the purposeful coupling of elbow and wrist flexors to counter the inertial effects during the unrestricted voluntary movement, even though the coupling does not appear to be purposeful during the perturbation or with the wrist immobilized. The coupling of elbow and wrist flexors, however, was not rigidly fixed, as evidenced by muscle onsets that adapted over repeated perturbation trials and a reversal of the wrist muscle activated (wrist extensor) when the forearm was pronated. Hence, the coupling of muscle activities can be modified quantitatively when not beneficial and can be altered qualitatively with different initial configurations of the arm.     

Age Group Differences in Postural Adjustments Associated With a Stepping Task

In this study, differences among age groups in the postural adjustments associated with a stepping task were identified. Twenty subjects from each of 3 age groups, children (8-12 years), young adults (25-35 years), and older adults (65ndash73 years), performed the task in 2 movement contexts: place and step. In place, the subject simply lifted the foot and placed it on the step. In step, the subject lifted the foot, placed it on the step, and stepped up onto the step. Latencies of postural and focal muscle activation were determined by using surface electromyography and pressure switches. Center of pressure (CP) data were obtained by using a force platform. Subjects in all 3 age groups consistently demonstrated postural adjustments before movement initiation. Children displayed longer postural latencies than young adults as well as disproportionately large values for CP path length. Older adults showed prolonged postural-focal latencies and decreased CP excursions compared with the 2 younger age groups. These results suggest that maturation of coordination between posture and movement may not be fully complete in 8- to l2-year-olds and that increased restraint characterizes the performance of postural adjustments in healthy persons over 65 years of age.     

Time course of action representations evoked during sentence comprehension

The nature of hand-action representations evoked during language comprehension was investigated using a variant of the visual–world paradigm in which eye fixations were monitored while subjects viewed a screen displaying four hand postures and listened to sentences describing an actor using or lifting a manipulable object. Displayed postures were related to either a functional (using) or volumetric (lifting) interaction with an object that matched or did not match the object mentioned in the sentence. Subjects were instructed to select the hand posture that matched the action described in the sentence. Even before the manipulable object was mentioned in the sentence, some sentence contexts allowed subjects to infer the object's identity and the type of action performed with it and eye fixations immediately favored the corresponding hand posture. This effect was assumed to be the result of ongoing motor or perceptual imagery in which the action described in the sentence was mentally simulated. In addition, the hand posture related to the manipulable object mentioned in a sentence, but not related to the described action (e.g., a writing posture in the context of a sentence that describes lifting, but not using, a pencil), was favored over other hand postures not related to the object. This effect was attributed to motor resonance arising from conceptual processing of the manipulable object, without regard to the remainder of the sentence context.     

Effects of surface texture on weight perception when lifting objects with a precision grip

In this paper, we show that, when lifting an object using a precision grip with the distal pads of the thumb and index finger at its sides, the perceived weight depends on the object’s surface texture. The smoother the surface texture, the greater the perceived weight. We suggest that a smoother object is judged to be heavier because the grip force, normal to the surface, required to prevent it from slipping is greater. The possibility of there being an influence of surface texture per se is excluded by a second experiment that employed a variant of the precision grip in which the thumb supports the weight of the object from underneath. With the grip oriented in this way, there is no need to match grip force to surface texture and, under these conditions, there is no effect of surface texture on weight perception. In the first two experiments, the test and comparison weights were lifted successively by the same hand. In a third experiment, the effect of surface texture was replicated for sequential lifts made with separate hands. Thus, the effect is not restricted to comparisons made with the same hand.     

Long latency reflex regulation in human ballistic movement

Step torque perturbations were applied to adduct the index during either a 25° ballistic or a 25° slow abduction movement. Subjects were instructed to react as quickly and as forcefully as possible in the opposite direction to the perturbation. The resulting long latency reflex (LLR) EMG activity was studied in the first dorsal interosseus. The LLR measured when the perturbation occurred in the first part of the movement was not different in a ballistic movement than a ramp movement. In our experiments, the LLR was the same in the first as well as in the second part of a ballistic movement. The LLR observed when the perturbation was applied at the onset of the agonist EMG burst (before the onset of a ballistic movement) was either similar or reduced when recorded during the ballistic movement.     

姿势干扰强度的心理预期效应

本研究基于经典快速举臂试验与落球试验范式, 采用表面肌电信号分析技术, 研究内、外姿势干扰强度的心理预期对腰部姿势肌肉和上肢动作肌肉预期姿势调节(APAs)和补偿姿势调节(CPAs)的影响, 探讨中枢神经系统(CNS)对内、外姿势干扰的控制策略。20名健康受试者先后完成不同负荷强度的快速举臂试验和落球试验, 同步采集腰部竖脊肌、腰部多裂肌和上肢肱二头肌的表面肌电信号, 计算肌肉预激活时间和APAs与CPAs积分肌电值, 观察内、外姿势干扰强度的心理预期对中枢APAs和CPAs控制机制的影响。结果显示内部姿势干扰条件下, 干扰强度的心理预期对腰部多裂肌、腰部竖脊肌和上肢肱二头肌的APAs强度有显著影响, 而对预激活时间和CPAs强度无显著影响; 外部姿势干扰条件下, 干扰强度的心理预期对腰部多裂肌、腰部竖脊肌和肱二头肌的APAs强度有显著影响, 对肱二头肌和腰部多裂肌预激活时间有显著影响, 而对CPAs强度无显著影响。突发可预期姿势干扰条件下姿势的快速反应是一个由CNS主导的神经肌肉运动控制过程。受姿势干扰强度心理预期的影响, CNS对内、外突发姿势干扰条件下腰部姿势肌肉的活动采取了不同的控制策略。在内部姿势干扰条件下, 干扰刺激发生时间明确, CNS主要通过对APAs强度的调节来实现姿势肌肉的优化控制; 而在外部姿势干扰条件下, 干扰刺激时间不明确, CNS则通过对局部稳定肌APAs预激活时间以及局部稳定肌和整体稳定肌APAs强度的双重调节实现姿势肌肉的优化控制。干扰强度的心理预期对姿势肌肉APAs和CPAs的作用表明, 心理预期效应主要源自于CNS对局部和整体稳定肌APAs控制机制的调制。     

The coupling of head,reach and grasp movement in nine months old infant prehension

In 9-month-old-infants adjustments in the reaching pattern to sudden changes in object location were examined. An attractive ball was presented to the infants at their midline and on some trials (perturbation trials) the ball suddenly changed position 15 cm to the right or left during the reach. For the perturbed trials the movement times approximately doubled compared to the control trials and significantly fewer balls were grasped. The results indicate that infants need to finish the first movement before being able to redirect the reach to a new destination. The correlation between the latency of the head and hand adjustment to the perturbation were 0.85 and 0.78 for movements to the left and to the right, respectively, indicating a tight coupling. The time between the start of the perturbation and peak velocity (TPPV) was significantly shorter for the head movement than for the hand movement, indicating that the head is leading the hand.     

Motor strategies in lifting movements: a comparison of adult and child performance

The experiment compares the performances of children six to nine years old and adults in a simple, monoarticular lifting task. Overt behaviors, as described by the kinematic features of the movement, do not differ qualitatively in the two groups. The patterns of motor commands, as expressed by the electromyographic recordings, are however strikingly different. Adults plan the movement with a careful balance between agonist muscle activity and passive, viscoelastic forces, whereas children use both agonist and antagonist active forces. It is argued that the motor strategy adopted by adults depends upon an internal representation of the properties of the motor system and of the size/weight covariation in natural objects, and that this representation is not yet fully developed at nine years of age.