A neural network model for development of reaching and pointing based on the interaction of forward and inverse transformations

Pointing is one of the communicative actions that infants acquire during their first year of life. Based on a hypothesis that early pointing is triggered by emergent reaching behavior toward objects placed at out‐of‐reach distances, we proposed a neural network model that acquires reaching without explicit representation of ‘targets’. The proposed model controls a two‐joint arm in a horizontal plane, and it learns a loop of internal forward and inverse transformations; the former predicts the visual feedback of hand position and the latter generates motor commands from the visual input through random generation of the motor commands. In the proposed model, the motor output and visual input were represented by broadly tuned neural units. Even though explicit ‘targets’ were not presented during learning, the simulation successfully generated reaching toward visually presented objects at within‐reach and out‐of‐reach distances.     

The role of locomotor posture and experience on handedness and footedness in infancy

This research examined the development of handedness and footedness in infancy. We measured footedness by documenting the limb infants used to "lead-out" as they crawled or walked down a path several times. We measured handedness by documenting the hand infants used to reach for a goal at the end of each trial. Study 1 showed that locomotor skill affects limb lateralization preferences and that most 13-month-old infants demonstrated a side preference for reaching and leading out "in the moment." Study 2 longitudinally examined the developmental trajectories of handedness and footedness as they related to locomotor posture and experience. The findings suggest high variability in the developmental trajectory of footedness, with frequent fluctuation between side preferences. The developmental trajectory of handedness was more stable over the transition between locomotor milestones. The transition between crawling and walking decreased the proportion of infants demonstrating side preferences for leading out and the onset of walking decreased the proportion of infants demonstrating side preferences for reaching. These findings demonstrate the importance of making behavioral observations at multiple time scales to understand underlying developmental trajectories, specifically the stability or instability of the motor system associated with the acquisition of new motor abilities.     

Is continuous visual monitoring necessary in visually guided locomotion?

Subjects were asked to walk to targets that were up to 21 m away, either with vision excluded during walking or under normal visual control. Over the entire range, subjects were accurate whether or not vision was available as long as no more than approximately 8 sec elapsed between closing the eyes and reaching the target. If more than 8 sec elapsed, (a) this had no influence on distances up to 5 m, but (b) distances between 6-21 m were severely impaired. The results are interpreted to mean that two mechanisms are involved in guidance. Up to 5 m, motor programs of relatively long duration can be formulated and used to control activity. Over greater distances, subjects internalized information about the environment in a more general form, independently of any particular set of motor instructions, and used this to control activity and formulate new motor programs. Experiments in support of this interpretation are presented.     

Self-Optimization of Walking in Nondisabled Children and Children With Spastic Hemiplegic Cerebral Palsy

Children voluntarily adopt a frequency and movement pattern for walking. The force-driven harmonic oscillator (FDHO) model was used in this study for accurate prediction of the preferred walking frequency of nondisabled children and children with spastic hemiplegic cerebral palsy. Four potential optimality criteria with which the preferred walking pattern was forced to comply were examined: minimization of physiological costs, maximization of mechanical energy conservation, minimization of asymmetry in lower limb movements and minimization of variability of interlimb and intralimb coordination. Age and gender-matched nondisabled children (n = 6) and children with spastic hemiplegic cerebral palsy (n = 6) were tested under six frequency conditions of walking at a constant speed on a treadmill. For the nondisabled children, the results indicated that their preferred walking frequency could be accurately predicted by the FDHO model. They freely adopted a walking pattern that minimized physiological costs, asymmetry, and variability of inter- and intralimb coordination. For the children with spastic hemiplegic cerebral palsy, the prediction of preferred overground walking frequency required that the FDHO model be modified to account for muscle mass and leg length discrepancies between limbs and increased stiffness. Most of the children achieved the same optimality goals as the nondisabled when walking at the preferred frequency. However, the children were found to use different mechanisms to attain these goals: for example, a steeper increase observed in physiological cost at higher frequencies; a lowered center of gravity of the body, which allowed for angular symmetry; and greater variability of between-joint coordination in the nonaffected limb and less variability in the affected limb.     

The influences of tropical climate on imagined walking time

This study investigated the effects of a Tropical Climate (TC) on actual and imagined walking times. Participants had to execute and imagine walking 3 distances with or without a 10-kg weight in either a TC or an Air Conditioning (AC) condition. The motor imagery quality was evaluated by computing the isochrony index. The results revealed that movement times were shorter for imagined walking than for actual walking and increased with the distance of the paths in both cases. By contrast, the effect of the load increased actual walking times for the 10- and 15-meter distances without affecting imagined walking times. Importantly, the isochrony index was negatively affected by the increase in load and distance and the effects of distance were amplified in a TC. These findings showed that the environmental climate in which motor imagery arises can modulate the subject’s ability to build up the temporal characteristics of simulated actions.     

Can we correctly perceive the reaching range of others?

Little is known about our ability to predict the motor capacities of other people. This study explored how observers perceive the reaching range of another person by asking them to judge the reachability of a moving object. Experiments 1 and 2 found that observers overestimated the reaching ranges of both short and tall people. Estimates were larger when objects moved towards rather than away from the person. Experiment 3 showed that perceived reaching range depended on the observer's viewing perspective. Compared with previous findings, these results suggest that observers use different cognitive operations to mentally simulate actions for themselves vs. others. They also inform the current debate about motor imagery and action understanding.     

Infants’ perception of depth from cast shadows

Five- and 7-month-old infants viewed displays in which cast shadows provided information that two objects were at different distances. The 7-month-olds reached preferentially for the apparently nearer object under monocular-viewing conditions but exhibited no reaching preference under binocularviewing conditions. These results indicate that 7-month-old infants perceive depth on the basis of cast shadows. The 5-month-olds did not reach preferentially for the apparently nearer object and, therefore, exhibited no evidence of sensitivity to cast shadows as depth information. In a second experiment, 5-month-olds reached preferentially for the nearer of two objects that were similar to those used in the first experiment but were positioned at different distances from the infant. This result indicated that 5-month-olds have the motor skills and motivation necessary to exhibit a reaching preference under the conditions of this study. The results are consistent with the hypothesis that depth perception based on cast shadows first appears between 5 and 7 months of age.     

Infants' perception of depth from cast shadows

Five- and 7-month-old infants viewed displays in which cast shadows provided information that two objects were at different distances. The 7-month-olds reached preferentially for the apparently nearer object under monocular-viewing conditions but exhibited no reaching preference under binocular-viewing conditions. These results indicate that 7-month-old infants perceive depth on the basis of cast shadows. The 5-month-olds did not reach preferentially for the apparently nearer object and, therefore, exhibited no evidence of sensitivity to cast shadows as depth information. In a second experiment, 5-month-olds reached preferentially for the nearer of two objects that were similar to those used in the first experiment but were positioned at different distances from the infant. This result indicated that 5-month-olds have the motor skills and motivation necessary to exhibit a reaching preference under the conditions of this study. The results are consistent with the hypothesis that depth perception based on cast shadows first appears between 5 and 7 months of age.     

Mechanical Efficiency and Metabolic Cost as Measures of Learning a Novel Gross Motor Task

“Efficiency,” or economy of movement with respect to energy expended in achieving the goal of the task, is implicit in many definitions of skilled performance. This study examined changes in mechanical efficiency and transport efficiency on a novel gross motor skill. The subjects were 5 physically fit adult males who were asked to perform 20 3-min trials walking on hands and feet (crawling) on a motor-driven treadmill at constant speed (0.76 mis). Transport efficiency, the metabolic cost of transporting the body mass a given distance at constant speed, improved significantly over practice trials. Mechanical efficiency, derived from the mechanical power output of individual body segments, showed an overall improvement of 13.7% by the last day of practice. Even though this improvement was not statistically significant it appears to be greater than that expected due to physiological training effects. The efficiency measures correlated significantly with changes in limb kinematics. It was concluded that with practice subjects tailored their movement pattern to produce energy efficient adaptations to task constraints. These findings provide empirical support for theoretical perspectives that have emphasized biological principles in the organization of motor coordination and control.     

Mechanical efficiency and metabolic cost as measures of learning a novel gross motor task

"Efficiency, " or economy of movement with respect to energy expended in achieving the goal of the task, is implicit in many definitions of skilled performance. This study examined changes in mechanical efficiency and transport efficiency on a novel gross motor skill. The subjects were 5 physically fit adult males who were asked to perform 20 3-min trials walking on hands and feet (crawling) on a motor-driven treadmill at constant speed (0.76 m/s). Transport efficiency, the metabolic cost of transporting the body mass a given distance at constant speed, improved significantly over practice trials. Mechanical efficiency, derived from the mechanical power output of individual body segments, showed an overall improvement of 13.7% by the last day of practice. Even though this improvement was not statistically significant it appears to be greater than that expected due to physiological training effects. The efficiency measures correlated significantly with changes in limb kinematics. It was concluded that with practice subjects tailored their movement pattern to produce energy efficient adaptations to task constraints. These findings provide empirical support for theoretical perspectives that have emphasized biological principles in the organization of motor coordination and control.     

Does counter-habitual behavior carry psychological costs?

Experience sampling studies have shown that people act out of character a lot of the time. These findings have raised the question of potential costs of counter-habitual behavior. The present experience sampling study (N = 242; measurement occasions = 4342) tested, for five behavioral dimensions derived from the Big Five theory, whether self-reported counter-habitual behavior is related to psychological costs in everyday life. The results mostly supported the view that engaging in desirable counter-habitual behaviors is beneficial, though some evidence for counter-habitual costs was found for self-control. Overall, the results suggest that the state-content significance hypothesis better accounts for everyday life behavioral, affective, and self-regulatory processes than the views highlighting the importance of acting according to one’s “true self”.     

Embodied Perception and the Economy of Action

ABSTRACT— Perception informs people about the opportunities for action and their associated costs. To this end, explicit awareness of spatial layout varies not only with relevant optical and ocular-motor variables, but also as a function of the costs associated with performing intended actions. Although explicit awareness is mutable in this respect, visually guided actions directed at the immediate environment are not. When the metabolic costs associated with walking an extent increase—perhaps because one is wearing a heavy backpack—hills appear steeper and distances to targets appear greater. When one is standing on a high balcony, the apparent distance to the ground is correlated with one's fear of falling. Perceiving spatial layout combines the geometry of the world with behavioral goals and the costs associated with achieving these goals.     

Walking While Memorizing: Age-Related Differences in Compensatory Behavior

This study investigated predictions of the life-span theory of selection, optimization, and compensation, focusing on different patterns of task priority during dual-task performance in younger and older adults. Cognitive (memorizing) and sensorimotor (walking a narrow track) performance were measured singly, concurrently, and when task difficulty was manipulated. Use of external aids was measured to provide another index of task priority. Before dual-task testing, participants received extensive training with each component task and external aid. Age differences in dual-task costs were greater in memory performance than in walking, suggesting that older adults prioritized walking over memory. Further, when given a choice of compensatory external aids to use, older adults optimized walking, whereas younger adults optimized memory performance. The results have broad implications for systemic theories of cognitive and sensorimotor aging, and the costs and benefits of assistive devices and environmental support for older populations.     

Observed reach trajectory influences executed reach kinematics in prehension

Previous studies have demonstrated that the observation of action can modulate motor performance. This literature has focused on manipulating the observed goal of the action, rather than examining whether action observation effects could be elicited by changing observed kinematics alone. In the study presented here, observed reach trajectory kinematics unrelated to the goal of the action were manipulated in order to examine whether observed movement kinematics alone could influence the action of the observer. Participants observed an experimenter grasp a target object using either a normal or an exaggeratedly high reaching action (as though reaching over an invisible obstacle). When participants observed the experimenter perform actions with a high reach trajectory, their own movements took on aspects of the observed action, showing greater wrist height throughout their reaching trajectory than under conditions in which they observed normal reaching actions. The data are discussed in relation to previous findings which suggest that kinematic aspects of observed movements can prime action through kinematic or intention based matching processes.     

Development of the Correspondence Between Real and Imagined Fine and Gross Motor Actions

The development of the correspondence between real and imagined motor actions was investigated in 2 experiments. Experiment 1 evaluated whether children imagine body position judgments of fine motor actions in the same way as they perform them. Thirty-two 8-year-old children completed a task in which an object was presented in different orientations, and children were asked to indicate the position of their hand as they grasped and imagined grasping the object. Children’s hand position was almost identical for the imagined- and real-grasping trials. Experiment 2 replicated this result with 8-year-olds as well as 6-year-olds and also assessed the development of the correspondence of the chronometry of real and imagined gross motor actions. Sixteen 6-year-old children and seventeen 8-year-old children participated in the fine motor grasping task from Experiment 1 and a gross motor task that measured the time it took for children to walk and imagine walking different distances. Six-year-olds showed more of a difference between real and imagined walking than did 8-year-olds. However, there were strong correlations between real and imagined grasping and walking for both 6- and 8-year-old children, suggesting that by at least 6 years of age, motor imagery and real action may involve common internal representations and that motor imagery is important for motor control and planning.     

Dissociation between visual perception of allocentric distance and visually directed walking of its extent

Walking without vision to previously viewed targets was compared with visual perception of allocentric distance in two experiments. Experimental evidence had shown that physically equal distances in a sagittal plane on the ground were perceptually underestimated as compared with those in a frontoparallel plane, even under full-cue conditions. In spite of this perceptual anisotropy of space, Loomis et al (1992 Journal of Experimental Psychology. Human Perception and Performance 18 906-921) found that subjects could match both types of distances in a blind-walking task. In experiment 1 of the present study, subjects were required to reproduce the extent of allocentric distance between two targets by either walking towards the targets, or by walking in a direction incompatible with the locations of the targets. The latter condition required subjects to derive an accurate allocentric distance from information based on the perceived locations of the two targets. The walked distance in the two conditions was almost identical whether the two targets were presented in depth (depth-presentation condition) or in the frontoparallel plane (width-presentation condition). The results of a perceptual-matching task showed that the depth distances had to be much greater than the width distances in order to be judged to be equal in length (depth compression). In experiment 2, subjects were required to reproduce the extent of allocentric distance from the viewing point by blindly walking in a direction other than toward the targets. The walked distance in the depth-presentation condition was shorter than that in the width-presentation condition. This anisotropy in motor responses, however, was mainly caused by apparent overestimation of length oriented in width, not by depth compression. In addition, the walked distances were much better scaled than those in experiment 1. These results suggest that the perceptual and motor systems share a common representation of the location of targets, whereas a dissociation in allocentric distance exists between the two systems in full-cue conditions.     

Observed reach trajectory influences executed reach kinematics in prehension

Previous studies have demonstrated that the observation of action can modulate motor performance. This literature has focused on manipulating the observed goal of the action, rather than examining whether action observation effects could be elicited by changing observed kinematics alone. In the study presented here, observed reach trajectory kinematics unrelated to the goal of the action were manipulated in order to examine whether observed movement kinematics alone could influence the action of the observer. Participants observed an experimenter grasp a target object using either a normal or an exaggeratedly high reaching action (as though reaching over an invisible obstacle). When participants observed the experimenter perform actions with a high reach trajectory, their own movements took on aspects of the observed action, showing greater wrist height throughout their reaching trajectory than under conditions in which they observed normal reaching actions. The data are discussed in relation to previous findings which suggest that kinematic aspects of observed movements can prime action through kinematic or intention based matching processes.     

Bimanual interference associated with the selection of target locations

Four experiments were conducted to identify the locus of interference observed during the preparation of bimanual reaching movements. Target locations were specified by color, and the right-hand and left-hand targets could be either the same or a different color. Movements of different amplitudes (Experiment 1) or different directions (Experiment 2) to targets of the same color were initiated more quickly than symmetric movements to targets of different colors. These results indicate that costs observed during bimanual movements arise during target selection rather than during motor programming. Experiments 3 and 4 further examined the interference associated with target selection. Reaction time costs were found with unimanual movements when the target was presented among distractors associated with responses for the other hand. Interference observed during bimanual reaching appears to reflect difficulty in segregating the response rules assigned to each hand.     

Infants return to two-handed reaching when they are learning to walk

The authors examined whether infants of about 1 year return to 2-handed reaching when they begin to walk independently. Infants (N = 9) were followed longitudinally before, during, and after their transition to upright locomotion. Every week, the infants' reaching responses and patterns of interlimb coordination were screened in 3 tasks involving different adaptive reaching responses. Before the onset of upright locomotion, the infants responded to each task adaptively. Following walking onset, they increased their rate of 2-handed responses in all tasks. The 2-handed responses declined when the infants gained better balance control. The results suggest that infants' return to 2-handed reaching is experience dependent. Those findings are discussed in terms of the integration of new developing motor skills into existing cognitive and motor repertoires.     

Braking reaching movements: a test of the constant tau-dot strategy under different viewing conditions

Following F. Zaal and R. J. Bootsma (1995), the authors studied whether the decelerative phase of a reaching movement could be modeled as a constant tau-dot strategy resulting in a soft collision with the object. Specifically, they investigated whether that strategy is sustained over different viewing conditions. Participants (N = 11) were required to reach for 15- and 50-mm objects at 2 different distances under 3 conditions in which visual availability of the immediate environment and of the reaching hand were varied. Tau-dot estimates and goodness-of-fit were highly similar across the 3 conditions. Only within-participant variability of tau-dot estimates was increased when environmental cues were removed. That finding suggests that the motor system uses a tau-dot strategy involving the intermodal (i.e., visual, proprioceptive, or both) specification of information to regulate the decelerative phase of reaching under restricted viewing conditions. The authors provide recommendations for improving the derivation of tau;(x) estimates and stress the need for further research on how time-to-contact information is used in the regulation of the dynamics of actions such as reaching.