The Organization of Multisegmental Pulls Made by Standing Humans: II. Submaximal Pulls

Previously, an autonomous oscillator model with three parameters was derived that describes the relationship between anterior-posterior center of mass motions and pulling force for near-maximal, bimanual pulls made by standing subjects (Michaels, Lee, & Pai, 1993). The present study evaluated the extent to which a full range of pulling forces could be fit by the model and how the model's three parameters changed with intended pulling force. How much variation in force each parameter could contribute was determined by simulating the model. Qualitative and quantitative analyses of pulls made by 6 well-practiced subjects at 5%, 10%, 20%, 40%, 60%, 80%, and 95% of their maximum pulling force revealed that the model holds well, except for the least forceful pulls of some subjects. Two parameters appeared to be controlled; one, related to the position of the center of pressure, varied most among less forceful pulls; the second, related to the position of the center of mass at the time of handle-force onset, varied most among more forceful pulls. How these parameters might be set is discussed.     

The organization of multisegmental pulls made by standing humans: I. Near-maximal pulls

Complex multisegmental movements occur when standing subjects exert forceful, impulse-like pulls on a bimanually held handle. The degrees of freedom of this task were analyzed to provide a principled basis for understanding the act's coordination. Body posture was found to be describable by only two degrees of freedom, expressible as the anterior-posterior and vertical coordinates of the center of mass (CM(AP), CM(V)). Kinetic analysis revealed that the two major contributors to pulling force depended only upon CM(AP) motion and the location of the center of the pressure. Kinematic and kinetic data from six well-practiced subjects pulling near their maxima were used to test the prediction of less intersubject variability in CM(AP) than in CM(V) variation led to different movement patterns among subjects. A dynamic model of CM(AP) motion was developed, and manipulation of its three degrees of freedom yielded CM(AP) trajectories that matched the empirical trajectories. It is suggested that the pull might be controlled with reference to these three parameters.     

College men's and women's reactions to hypothetical sexual touch varied by initiator gender and coercion level

A sample of 152 men and 152 women (mostly Caucasian) rated their reactions to a vignette in which the subjects were to imagine receiving an uninvited genital touch from a college acquaintance. The vignette was varied so that the acquaintance was of the opposite or the same gender as the subject, and the touch was either gentle or forceful. Results indicated that women anticipated strong negative effects from receiving opposite- or same-gender touch, whether gentle or forceful. Men anticipated almost no negative effects from either a gentle or forceful touch from a female acquaintance, but expected strong negative effects from a gentle or forceful touch from a male acquaintance. Regression analyses revealed that women's reactions to opposite-gender touch were mediated by beliefs in a male norm promoting casual sex, and feelings of violation and fear of harm. Men's reactions to opposite-gender touch were influenced by feelings related to sexual arousal. Men and women's reactions to same-gender touch were related to feelings of violation and harm.     

Individual differences in perceived pinch force and bite force

Six subjects made cross-modalmatches of pinch force and bite force to an electrocutaneous st4niulus. The electrocutaneous stimulus consisted ofbursts ofpulses, and the intensity ofthe stimulus was varied by changing the number of pulses per burst. All of the individual matching functions were fit well by powerfunctions. The scaling constants and exponents ofthe power functions covaried systematically with the maximum pinch force for 5 of the 6 subjects. The relationship was consistent with the hypothesis that subjects perceive their physical maxima equally, in agreement with Borg’s theory of relative perceived exertion. For both pinch force and bite force functions, the scale factors could be described by a single linear function ofthe exponents, suggesting that all of the matching functions converged at a single point with extreme values.     

Motor adaptation to different dynamic environments is facilitated by indicative context stimuli

When humans are exposed to external forces while performing arm movements, they adapt by compensating for these novel forces. The basis of this learning process is thought to be a neural representation that models the relation between all forces acting upon the system and the kinematic effects they produce, called inverse dynamic model (IDM). The present study investigated whether and how the predictability of a given external force affects the selection of an appropriate motor response to compensate for such force. Adult human subjects (N=32) held a handle that could rotate around the elbow joint and learned to perform goal-directed forearm flexion movements, while an external velocity-dependent negative damping force was applied that assisted forearm movement. Subjects were randomly assigned to two groups. In the associative group, the applied damping force was always associated with a specific initial position. Thus, after initial learning, the force application became predictable. In the non-associative group, where the same movements were performed, the applied force was independent of the initial position, so that no association between force and location could be formed. We found that only the associative group significantly reduced target error when damping was present. That is, the location cue aided these subjects in generating dynamic responses in the appropriate limb. Our results indicate that motor adaptation to different dynamic environments can be facilitated by indicative stimuli.     

Illusory gravitational forces affect aimed limb movements

The authors report on 2 experiments in which participants produced rapid wrist-rotation movements in a clockwise direction to move a cursor from a home position into a target region. The display was constructed such that the target could be depicted above, below, or beside the home position, so that the clockwise wrist rotation could produce upward, downward, or rightward movements of the cursor. By parsing the movements into component submovements, the authors found that upward movements were consistently less forceful, as evidenced by shorter primary submovements, compared with downward and horizontal movements. Those results suggested a new virtual gravity illusion: Participants apparently overcompensated for the absence of the anticipated effects of gravity by adjusting the initial force they used to propel the cursor toward the target.     

Spatial memory in rats under restricted viewing conditions

In a procedure devised by J. A. Walker and D. S. Olton (Learning and Motivation, 1979, 10, 73–84), rats were placed on two arms of a four-arm radial maze and then were placed in the center of the maze to test how accurately they could choose the alleys on which they had not been placed. In three experiments, the conditions under which animals viewed the environment from the arms were varied. In Experiment 1, both the extent of spatial view and the exposure time were varied factorially in a within-subjects design; animals viewed the environment down a tunnel or had a 180° or 360° view, and subjects were allowed to view the environment for either 2 or 20 sec. In Experiment 2, a between-subjects design was used, in which different groups of subjects were tested repeatedly under either the tunnel, 180°, or 360° conditions. Both experiments showed that animals could avoid the arms previously visited at no better than a chance level of accuracy in the tunnel viewing condition but could perform with progressively better accuracy at the 180 and 360° viewing conditions. Animals also were more accurate in Experiment 1 after viewing for 20 sec than after viewing for 2 sec. Experiment 3 involved a procedure in which restricted viewing conditions were used both during arm placements and testing. Animals tested under tunnel viewing eventually achieved above-chance performance with this procedure, but did not exceed chance as rapidly as groups tested with 45 and 90° views of the environment. These results suggest that animals can learn about their position in a spatial environment through observation and that an animal's ability to locate its position is directly related to the extent of the surrounding environment it can see and the length of time it is allowed to look. The implications of these findings for list and map hypotheses of spatial memory representation are discussed.     

Human observing: maintained by negative informative stimuli only if correlated with improvement in response efficiency.

Two experiments investigated the effect of observing responses that enabled college students to emit more efficient distributions of reinforced responses. In Experiment 1, the gains of response efficiency enabled by observing were minimized through use of identical low-effort response requirements in two alternating variable-interval schedules. These comprised a mixed schedule of reinforcement; they differed in the number of money-backed points per reinforcer. In each of three choices between two stimuli that varied in their correlation with the variable-interval schedules, the results showed that subjects preferred stimuli that were correlated with the larger average amount of reinforcement. This is consistent with a conditioned-reinforcement hypothesis. Negative informative stimuli--that is, stimuli correlated with the smaller of two rewards--did not maintain as much observing as stimuli that were uncorrelated with amount of reward. In Experiment 2, savings in effort made possible by producing S- were varied within subjects by alternately removing and reinstating the response-reinforcement contingency in a mixed variable-interval/extinction schedule of reinforcement. Preference for an uncorrelated stimulus compared to a negative informative stimulus (S-) decreased for each of six subjects, and usually reversed when observing permitted a more efficient temporal distribution of the responses required for reinforcement; in this case, the responses were pulls on a relatively high-effort plunger. When observing the S- could not improve response efficiency, subjects again chose the control stimulus. All of these results were inconsistent with the uncertainty-reduction hypothesis.     

A pilot study of a dynamical systems approach to examining changes in static balance of adolescents

In a dynamical systems model, movement patterns are dictated by several variables, called control parameters. The goal of this pilot study was to assess whether changes on a static balance task can be described by a dynamical systems model with body inertial properties as one of the potential control parameters. Three aspects of a dynamic system were examined in relation to a 2-ft. static balance task: a relation between the changes in the balance pattern and the control parameter, a relation between the stability of the balance pattern and the stability under perturbed conditions (1-ft. balance task), and during the perturbation lack of relation between the balance pattern and the control parameters. Nine adolescent boys, 15.3 +/- 1.0 yr. old were examined twice over a 14-wk. period. During each testing session, participants' body mass, moments of inertia, and radius from the ankle to the center of mass were calculated, after which 1- and 2-ft. balance tasks were performed. Center of pressure coordinates were recorded using a Kistler force plate. The body parameters were used to calculate the natural frequency of the body to represent the control parameter. Significant relations among all three aspects of a dynamic system examined in both the lateral and anterior-posterior axes were found. This investigation was designed for exploratory purposes and limited to correlational analysis; therefore, no concrete conclusions could be drawn. The results, however, suggest a dynamical systems approach to the study of static balance patterns may be possible.     

Comparison of kinematic and kinetic methods for computing the vertical motion of the body center of mass during walking

The vertical excursion of the body center of mass (BCOM) was calculated using three different techniques commonly used by motion analysis laboratories. The sacral marker method involved estimating vertical BCOM motion by tracking the position of a reflective marker that was placed on the sacrum of subjects as they walked. The body segmental analysis technique determined the vertical motion of the BCOM from a weighted average of the vertical positions of the centers of mass of individual body segments for each frame of kinematic data acquired during the data trial. Anthropomorphic data from standard tables were used to determine the mass fractions and the locations of the centers of mass of each body segment. The third technique involved calculating BCOM vertical motion through double integration of force platform data. Data was acquired from 10 able-bodied, adult research subjects--5 males and 5 females--walking at speeds of 0.8, 1.2, 1.6, and 2.0 m/s. A repeated measures ANOVA indicated that at the slowest walking speed the vertical excursions calculated by all three techniques were similar, but at faster speeds the sacral marker significantly (p < 0.001) overestimated the vertical excursion of the BCOM compared with the other two methods. The body segmental analysis and force platform techniques were in agreement at all walking speeds. Discrepancies between the sacral marker method and the other two techniques were explained using a simple model; the reciprocal configuration of the legs during double support phase significantly raises the position of the BCOM within the trunk at longer step lengths, corresponding to faster walking speeds. The sacral marker method may provide a reasonable approximation of vertical BCOM motion at slow and freely selected speeds of able-bodied walking. However, the body segmental analysis or force platform techniques will probably yield better estimates at faster walking speeds or in persons with gait pathologies.     

Handedness in a virtual haptic environment: Assessments from kinematic behavior and modeling

This study evaluated hand asymmetries in performance of a dexterous, controlled task under haptic feedback. Participants punctured a virtual membrane with a pushing or pulling movement, using the left or right hand. For pulling movements, the dominant (right) hand exhibited faster average stopping latency and shorter skidding distance. When the kinematic data were fit to a three-phase model previously applied to this task (Klatzky et al., 2013), the right hand exhibited faster force decay attributable to biomechanical factors. Analyses of the aggregated performance measures and model parameters showed that the left and right hands are associated with two different distributions, supporting handedness effects. Furthermore, while the majority of participants expressed right-hand dominance, which was consistent with their self-reported hand preferences, others showed partial or no dominance. This approach could potentially be extended to quantify and differentiate individuals with difficulties in manual behavior due to abnormal motor control (e.g., dyspraxia), progressive deterioration (e.g., Parkinson's syndrome) or improvement (neural regrowth after transplant).     

Probing the limits of tool competence: Experiments with two non-tool-using species (Cercopithecus aethiops and Saguinus oedipus)

Non-human animals vary in their ability to make and use tools. The goal of the present study was to further explore what, if anything, differs between tool-users and non-tool-users, and whether these differences lie in the conceptual or motor domain. We tested two species that typically do not use tools-cotton top tamarins (Saguinus oedipus) and vervet monkeys (Cercopithecus aethiops)-on problems that mirrored those designed for prolific tool users such as chimpanzees. We trained subjects on a task in which they could choose one of two canes to obtain an out-of-reach food reward. After training, subjects received several variations on the original task, each designed to examine a specific conceptual aspect of the pulling problem previously studied in other tool-using species. Both species recognized that effective pulling tools must be made of rigid materials. Subsequent conditions revealed significant species differences, with vervets outperforming tamarins across many conditions. Vervets, but not tamarins, had some recognition of the relationship between a tool's orientation and the position of the food reward, the relationship between a tool's trajectory and the substance that it moves on, and that tools must be connected in order to work properly. These results provide further evidence that tool-use may derive from domain-general, rather than domain-specific cognitive capacities that evolved for tool use per se.     

Comparison and evaluation of two common methods to measure center of mass displacement in three dimensions during gait

Center of mass displacement during gait has frequently been used as an indicator of gait efficiency or as a complement to standard gait analysis. With technological advances, measuring the center of mass as the centroid of a multi-segment system is practical and feasible, but must first be compared to the well-established Newtonian computation of double-integrating the ground reaction force. This study aims to verify that the kinematic centroid obtained from a commonly-used model (Vicon Peak Plug-In-Gait) provides at least as reliable measurements of center of mass displacement as those obtained from the ground reaction forces. Gait data was collected for able-bodied children and children with myelomeningocele who use larger lateral center of mass excursions during gait. Reasonable agreement between methods was found in fore-aft and vertical directions, where the methods' excursions differed by an average of less than 10 mm in either direction, and the average RMS differences between methods' computed curves were 6 and 13 mm. Particularly good agreement was observed in the lateral direction, where the calculated excursions differed by an average of less than 2 mm and the RMS difference was 5 mm. Error analyses in computing the center of mass displacement from ground reaction forces were performed. A 5% deviation in mass estimation increased the computed vertical excursion twofold, and a 5% deviation in the integration constant of initial velocity increased the computed fore-aft excursions by 10%. The suitability of calculating center of mass displacement using ground reaction forces in a patient population is questioned. The kinematic centroid is susceptible to errors in segment parameters and marker placement, but results in plausible results that are at least within the range of doubt of the better-established ground reaction force integration, and are more useful when interpreting 3-D gait data.     

Cognitive imitation in 2-year-old children (<Emphasis Type="Italic">Homo sapiens</Emphasis>): a comparison with rhesus monkeys (<Emphasis Type="Italic">Macaca mulatta</Emphasis>)

Here we compare the performance of 2-year-old human children with that of adult rhesus macaques on a cognitive imitation task. The task was to respond, in a particular order, to arbitrary sets of photographs that were presented simultaneously on a touch sensitive video monitor. Because the spatial position of list items was varied from trial to trial, subjects could not learn this task as a series of specific motor responses. On some lists, subjects with no knowledge of the ordinal position of the items were given the opportunity to learn the order of those items by observing an expert model. Children, like monkeys, learned new lists more rapidly in a social condition where they had the opportunity to observe an experienced model perform the list in question, than under a baseline condition in which they had to learn new lists entirely by trial and error. No differences were observed between the accuracy of each species' responses to individual items or in the frequencies with which they made different types of errors. These results provide clear evidence that monkeys and humans share the ability to imitate novel cognitive rules (cognitive imitation).     

Changes in pinch force with bidirectional load forces

The forces used to grasp an object were measured while positive (push) and negative (pull) load forces were applied to the hand under varying frictional conditions. Subjects held between the tips of their thumb and index finger a manipulandum composed of two symmetrically mounted disks. The manipulandum was connected to the stage of an electromagnetic linear motor that generated load forces under computer control. In the first experiment, subjects held the position of the manipulandum constant while the motor generated forces in first the positive and then the negative direction. The motor force at which the manipulandum slipped from the fingers was measured in the second experiment. In both experiments, friction was varied by changing the surface (sandpaper, suede, or plastic) of the manipulandum disks. The pinch forces produced by subjects were linearly related to changes in motor force in both the positive and negative directions, with the slope of this relation varying as a function of the surface properties of the manipulandum. The modulation of pinch force with motor force was influenced, however, by the direction of the load force; higher forces were produced in response to negative load forces. Slip forces varied as a function of pinch force and surface texture; higher forces were associated with materials with lower coefficients of friction. These findings suggest that the friction between the skin and an object being grasped changes as a function of the direction of force that the object applies to the skin, possibly due to the anisotropic nature of glabrous skin, and that this mechanical property contributes to variations in pinch force.     

Does effort influence inequity aversion in cotton-top tamarins (Saguinus oedipus)?

The human sense of fairness entails sensitivity not just to equality, the equal division of resources, but also to merit, the relationship between an individual’s share of resources and how hard they worked for their share. Recent evidence suggests that our sensitivity to equality has deep phylogenetic roots: several nonhuman animal species show an aversion to unequal reward distributions. However, the extent to which nonhuman animals share sensitivity to merit remains poorly understood, largely because previous studies have failed to properly manipulate work effort in inequity aversion tasks. Here, we tested whether cotton-top tamarins (Saguinus oedipus) would exhibit a differential response to inequity when acquiring rewards was either (1) effortful or (2) effortless. Subjects engaged in a pulling task in which they had an opportunity to deliver a disadvantageously unequal distribution of food to themselves and a partner (one piece for self, four pieces for partner). We made delivery effortful by adding a weight to the pulling handle. Critically, effort was calibrated to each individual. Results show that individuals varied markedly in their response to effort, highlighting the importance of manipulating work effort at the individual level. Overall, subjects showed little aversion to inequity. However, subjects were slightly less likely to accept inequity when doing so was effortful, although this effect was pronounced in only one individual. Our findings suggest a new method for capturing individual variation in effort and for studying the roots of the concept of merit in nonhuman animals.     

The Timing Effects of Accent Production in Periodic Finger-Tapping Sequences

When subjects are required to produce short sequences of equally paced finger taps and to accentuate one of the taps, the interval preceding the forceful tap is shortened and the one that immediately follows the accent is lengthened. Assuming that the tapping movements are triggered by an internal clock, one explanation attributes the mistiming of the taps to central factors: The momentary rate of the clock is accelerated or decelerated as a function of motor preparation to, respectively, increase or decrease the movement force. This hypothesis predicts that the interre-sponse intervals measured between either tap movement onsets or movement terminations (taps) will show the same timing pattern. A second explanation for the observed interval effects is that the tapping movements are triggered by a regular internal clock but the timing of the successive taps is altered because the forceful movement is completed in less time than the other tap movements are. This “peripheral” hypothesis predicts regular timing of movement onsets but distorted timing of movement terminations. In the present study, the trajectories of the movements performed by subjects were recorded and the interresponse intervals were measured at the beginning and the end of the tapping movements. The results of Experiment 1 showed that neither model can fully explain the interval effects: The fast forceful movements were initiated with an additional delay that took into account the small execution time of these movements. Experiment 2 reproduced this finding and showed that the timing of the onset and contact intervals did not evolve with the repetition of trial blocks. Therefore, the assumption of an internal clock that would trigger the successive movements must be rejected. The results are discussed in the framework of a modified two-stage model in which the internal clock, instead of triggering the tapping movements, provides target time points at which the movements have to produce their meaningful effects, that is, contacts with the response key. The timing distortions are likely to reflect both peripheral and central components.     

Effects of room tilting on body sway: Adaptation and strategies for maintaining a standing posture

The purpose of this study was to examine the effects of a tilting room on body sway across a series of trials. Sixteen subjects were instructed to maintain an upright position in the tilting room, and their body sway was measured by a force plate. After the experimental session, subjects were interviewed about awareness, strategies and impressions. The results indicated that the forward-tilting room induced forward body sway, as would a moving room. The center of gravity in the pre-tilting period in the first trial was significantly more forward than in the following trials. From the analysis of strategy, subjects were divided into two groups: maintaining-standing strategy group and other-strategies group. While subjects who used the maintaining-standing strategy swayed more in the first trial, they had significantly reduced their body sway compared with the other group by the ninth and tenth trials. Based on these results, the effects of a moving room for adult subjects as related to the context-dependent weighting of new inputs are discussed.     

Perceived standing position after reduction of foot-pressure sensation by cooling the sole

We investigated the influence of the reduction of foot-pressure sensation by cooling the sole of the foot, at 1 degree C for 30 or 40 minutes, on the perception of standing position varied in the anteroposterior direction. The subjects were 16 healthy undergraduates. Firstly, for 4 of the subjects, cooling the sole of the foot decreased sensory information from the mechanoreceptors in the sole, by testing for an increase in the threshold for two-point discrepancy discrimination on the sole of the foot and for the disappearance of postural change with vibration to the sole. Next, the perception of standing position was measured by reproduction of a given standing reference position involving forward or backward leaning under both normal and cooled conditions of the feet. Standing position was varied in relation to the location of the center of foot pressure, defined as distance from the heel in percentage of the length of the foot. The reference positions, representing various locations of the center of foot pressure, were set at 10% increments from 20% to 80% of the length of the foot. With eyes closed, the subject first experienced the reference position and then attempted to reproduce it. The mean location of the center of foot pressure in the quiet standing posture was 45.7%. At the 40%, 50%, and 60% reference positions, those closest to quiet standing, absolute errors of reproduction were significantly larger than at other reference positions in both the normal and the cooled conditions. They were significantly larger in the cooled than in the normal condition. The 50% and 60% reference positions were reproduced significantly further forward in the cooled than in the normal condition. These results may be explained as due to an absence of marked changes in sensory information from both muscular activity and foot pressure when moving to reference positions close to the quiet standing posture.     

The Timing Effects of Accent Production in Periodic Finger-Tapping Sequences

When subjects are required to produce short sequences of equally paced finger taps and to accentuate one of the taps, the interval preceding the forceful tap is shortened and the one that immediately follows the accent is lengthened. Assuming that the tapping movements are triggered by an internal clock, one explanation attributes the rnistiming of the taps to central factors: The momentary rate of the clock is accelerated or decelerated as a function of motor preparation to, respectively, increase or decrease the movement force. This hypothesis predicts that the interresponse intervals measured between either tap movement onsets or movement terminations (taps) will show the same timing pattern. A second explanation for the observed interval effects is that the tapping movements are triggered by a regular internal clock but the timing of the successive taps is altered because the forceful movement is completed in less time than the other tap movements are. This "peripheral" hypothesis predicts regular timing of movement onsets but distorted timing of movement terminations. In the present study, the trajectories of the movements performed by subjects were recorded and the interresponse intervals were measured at the beginning and the end of the tapping movements. The results of Experiment 1 showed that neither model can fully explain the interval effects: The fast forceful movements were initiated with an additional delay that took into account the small execution time of these movements. Experiment 2 reproduced this finding and showed that the timing of the onset and contact intervals did not evolve with the repetition of trial blocks. Therefore, the assumption of an internal clock that would trigger the successive movements must be rejected. The results are discussed in the framework of a modified two-stage model in which the internal clock, instead of triggering the tapping movements, provides target time points at which the movements have to produce their meaningful effects, that is, contacts with the response key. The timing distortions are likely to reflect both peripheral and central components.