Development of infant reaching behaviors: Kinematic changes in touching and hitting

This longitudinal study investigated the development of reaching in typical infants, from age 4 to 8 months, and described the pattern of hand kinematics underlying changes in the characteristics of infants’ actions while reaching for a target. Thirteen infants were followed biweekly. Two reaching behaviors emerged during the infants’ free interactions with the target, touching and hitting. Changes over time were documented for the number of movement units, straightness index, distance, peak velocity and time to peak velocity of the hand for touches and hits. We observed increases in the numbers of touches and hits and changes in hand kinematics over time; the distance traveled by the hand was greater for hitting compared to touching. These kinematic changes were specific to the movement patterns that infants adopted to reach to the target.     

Executive Functioning Demands of the Object Retrieval Task for 8-month-old Infants

At 8 to 11 months of age, infants are more successful in negotiating opaque relative to transparent barriers. However, 7-month-old infants have more difficulty with opaque barriers relative to semitransparent barriers. Here, 8-month-old infants spent more time in ineffective direct reaches with more the transparent barriers (Experiments 1 & 2) and were faster with the fully opaque barrier (Experiment 2). This demonstration of the graded effects of transparency confirms the working memory and/or response inhibition demands of the object retrieval task.     

Diminished joint coordination with aging leads to more variable hand paths

Differences in joint coordination between arms and due to aging were studied in healthy young and older adults reaching to either a fixed, central target or to the same target when it could unexpectedly change location after reach initiation. Joint coordination was investigated by artificially removing the covariation of each joint’s motions with other joints’ motions. Uncontrolled manifold analysis was used to partition joint configuration variance into variance reflecting motor abundance (V_UCM) and variance causing hand path variability (V_ORT). The extent to which V_ORT, related to the consistency of the hand path, increased after removing a joint’s covariation indicated the strength of its coordination with other joints. Young adults exhibited stronger indices of joint coordination, evidenced by a larger increase in V_ORT after removing joint covariation than for older adults. This effect was more striking for the dominant right compared to the left arm for young adults, but not for older adults, especially with target uncertainty. The results indicate that interjoint coordination in young adults leads to less hand path variability compared to older adults.     

Motion planning strategies in human control of non-rigid objects with internal degrees of freedom

The paper deals with modeling of human-like reaching movements in dynamic environments. A simple but not trivial example of reaching in a dynamic environment is the rest-to-rest manipulation of a multi-mass flexible object with the elimination of residual vibrations. Two approaches to the prediction of reaching movements are formulated in position and force actuation settings. In the first approach, either the position of the hand or the hand force is specified by the lowest order polynomial satisfying the boundary conditions of the reaching task. The second approach is based on the minimization of either the hand jerk or the hand force-change, with taking into account the dynamics of the flexible object. To verify the resulting four mathematical models, an experiment on the manipulation of a ten-masses flexible object of low stiffness is conducted. The experimental results show that the second approach gives a significantly better prediction of human movements, with the minimum hand force-change model having a slight but consistent edge over the minimum hand jerk one.     

Visual cues and perceived reachability

A rather consistent finding in studies of perceived (imagined) compared to actual movement in a reaching paradigm is the tendency to overestimate at midline. Explanations of such behavior have focused primarily on perceptions of postural constraints and the notion that individuals calibrate reachability in reference to multiple degrees of freedom, also known as the whole-body explanation. The present study examined the role of visual information in the form of binocular and monocular cues in perceived reachability. Right-handed participants judged the reachability of visual targets at midline with both eyes open, dominant eye occluded, and the non-dominant eye covered. Results indicated that participants were relatively accurate with condition responses not being significantly different in regard to total error. Analysis of the direction of error (mean bias) revealed effective accuracy across conditions with only a marginal distinction between monocular and binocular conditions. Therefore, within the task conditions of this experiment, it appears that binocular and monocular cues provide sufficient visual information for effective judgments of perceived reach at midline.     

Does late preterm birth impact trunk control and early reaching behavior?

The aims of the study were to 1) verify the level of trunk control longitudinally and reaching behavior while sitting in two positions in late preterm (LPT) and full-term (FT) infants, 2) determine whether the level of trunk control relates to reaching outcomes. Twenty LPT infants and 36 infants born FT were assessed via three in-lab visits: at 6, 7, and 8 months. At each visit, the Segmental Assessment of Trunk Control (SATCo) and reaching assessment were performed, where the infants were positioned sitting in the ring and at 90° of flexion of hips, knees, and ankles. Accurate manual support to the trunk was provided in each visit. LPT infants presented a lower level of trunk control over time. LPT infants presented a higher percentage of unimanual reaches and successful grasping at 7 months' visit, and a higher number of reaches at 8 months' visit compared to FT infants. The sitting positions did not influence reaching performance. The level of trunk control relates to functional reaching strategies only in FT infants. This study might provide insights for clinicians for understanding the level of trunk control, the importance of reaching behaviors for exploration, and considering these behaviors as strategies for intervention.     

Maturational and situational determinants of reaching at its onset

At 3 months of age, reaching behavior was measured in a group of 10 girls and 10 boys born at term. The assessments were carried out on the average 2 days after reaching onset. Reaching kinematics was measured in both supine and reclined positions. Girls reached more than boys, had straighter reaching trajectories and movements of shorter durations as well as fewer movement units. The reclined position gave rise to straighter trajectories in both girls and boys. Several anthropometric parameters were measured. Girls had less length and volume of the forearm than boys but similar upper arm volumes. There was a weak relation between kinematic and anthropometric variables.     

Toy-oriented changes in hand and joint kinematics during the emergence of purposeful reaching

Infants first consistently reach for objects between 3 and 5 months of age. In the months before reaching, infants produce a variety of arm movements. The relationship between these early arm movements and the emergence of purposeful reaching is still unclear. The purpose of the present study was to determine how groups of non-reaching, nearly reaching, and newly reaching infants changed the kinematics of their spontaneous arm movements in the presence of a toy. Five infants in each of these groups were observed with a high-speed motion capture system during trials with and without a toy present. Kinematic analyses examined 3D hand, shoulder, and elbow motions. Our results suggest that with a toy present, non-reachers altered their movement quantity whereas near- and new-reachers altered their movement quality through spatio-temporal dissociation and reorientation of the arm. When comparing the changes across groups we observed three preliminary patterns of toy-oriented changes. Our results join other studies to strengthen the relationship between early arm movements and purposeful reaching. Future longitudinal studies are now required to begin to fully understand the complex process by which infants adapt their early arm movements for purposeful behaviors.     

A study of reaching actions in walking infants

Acquiring motor skills transforms the perceptual and cognitive world of infants and expands their exploratory engagement with objects. This study investigated how reaching is integrated with walking among infant walkers (n = 23, 14.5–15.5 months). In a walk-to-reach paradigm, diverse object retrieval strategies were observed. All infants were willing to use their upper and lower bodies in concert, and the timing of this coordination reflected features of their environment. Infants with an older walking age (months since walking onset) retrieved items more rapidly and exploited their non-reaching hand more effectively during object retrieval than did same-age infants with a younger walking age. This suggests that the actions of the upper- and lower-body are flexibly integrated and that this integration may change across development. Mechanisms that shape sophisticated upper-body use during upright object retrieval are discussed. Infants flexibly integrate emerging motor skills in the service of object retrieval in ways not previously documented.     

The development of reaching behavior in low-risk preterm infants

This longitudinal study investigated the development of reaching behavior in the seated position in preterm infants at the ages of 5–7 months by analyzing kinematic variables (straightness and adjustment indexes, movement unit, mean and final velocities). The correlation between kinematic variables and grasping was verified. The participants were nine low-risk preterm infants with no cerebral lesions. Ten fullterm infants served as control. In both groups, kinematic variables remained unchanged over age, except for the adjustment index, which was higher at 6 months in the preterm group. Successful grasping increased in both groups over age and it was shown to be negatively correlated with mean velocity in the preterm infants. At the ages of 6 and 7 months, preterms showed lower mean and final velocities and higher adjustment index when compared with fullterms. The relative constancy in the kinematic variables suggests that, after having explored the action possibilities during the acquisition phase, the infants selected an adaptative pattern to perform the reaching movements. Slower movements and greater adjustments may be functional strategies of preterms to achieve successful grasps.