Controlling posture to see the world: The integration of visual task demands and postural sway in sitting and standing infants.

Our ability to integrate posture with visually demanding tasks is a critical aspect of motor behavior flexibility. When looking at a small object, excessive body movements impair an individual's ability to visually attend to the object. To overcome this problem, we adjust our postural sway to successfully focus on the object. The goal of the current study was to assess whether infants also adjust postural sway when engaged in a challenging visual task. The participants, 19 independently sitting infants (Sitters) and 21 newly independently standing infants (Standers), sat or stood on a force plate while viewing differently sized images displayed on a monitor (smaller images: 8 × 6.5 cm or 3 × 3 cm; larger images: 13 × 16 cm or 13 × 13 cm). Regardless of image size, Standers were less stable than Sitters with larger sway areas and faster sway velocities. Both Sitters and Standers adjusted sway area but not sway velocity, based on image size. Sitters and Standers differed in how they controlled sway dynamics. Standers but not Sitters altered sway dynamics based on image size. Overall, infants used posture-specific adaptive control strategies to make fine-grained adjustments based on image size. The development of the ability to integrate posture with a visually demanding task further emphasizes the capability of advanced complex motor behaviors during infancy, enabling infants to flexibly attend to important aspects of their environment at different postural positions.

Effect of carrying objects on walking characteristics and language abilities in 13- and 24-month-olds.

Learning to walk leads to an increase in language abilities; however, the underlying mechanisms accounting for this relation remain unclear. Investigating the quality of early gait control may offer some insights. The purpose of this study was to (a) quantify how 13-month-olds (n = 39; 39% male) and 24-month-olds (n = 39; 59% male) adapt gait based on task (carrying vs. not carrying objects), object characteristics (size and weight), and environmental constraints (free play vs. structured) and to (b) assess how changes in gait relate to language abilities. To assess gait differences, Midwestern participants engaged in a free-play session and structured task with toys varying in size and weight while wearing motion capture markers. Parents completed an age-appropriate version of the MacArthur-Bates Communicative Development Inventory to measure language abilities. During free play, 13-month-olds were selective when choosing what to carry basing their decisions on object weight over size and adapting their upper-body control based on weight. Neither age group modified lower-body gait due to object characteristics but did reorganize their upper-body control. These upper-body adaptations were dependent upon age, environmental context, and task. Importantly, more mature upper-body control and more time spent in motion during free play were significant predictors of 13-month-olds' language abilities. Gait improvements may offer new opportunities for language learning; increased stability may allow young children to better focus on the objects, potentially enhancing word learning opportunities. Early movements and functional behavior during free play may be important predictors underlying the association between the onset of walking and language development. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Task-specific adaptations of postural sway in sitting infants.

When engaging in manual or visual tasks while sitting, infants modify their postural sway based on concurrent task demands. It remains unclear whether these modulations are sensitive to differences in concurrent task demands (holding a toy vs. looking at a toy being held by someone else), and whether the properties of the support surface impact these adaptations. We investigated infants' ability to modify postural sway when holding a toy or visually attending to a toy someone else was holding while sitting on different support surfaces. Twenty-six independently sitting infants sat on solid and compliant surfaces placed on a force plate while looking at or holding a toy. Measures of postural sway were calculated from the center of pressure data. Visually attending to a toy was associated with less sway and lower sway velocity than when holding a toy. Surprisingly, surface compliance did not affect sway and there were no interaction effects. Whereas sway modulations may facilitate infants' performance on both manual and visual concurrent tasks, the visual task placed more constraints on the postural system leading to greater adaptations in postural sway. These findings provide insights into how infants are allocating attention and coordinating perceptual-motor information in developing sitting skills.

Association between stride time fractality and gait adaptability during unperturbed and asymmetric walking.

Human locomotion is an inherently complex activity that requires the coordination and control of neurophysiological and biomechanical degrees of freedom across various spatiotemporal scales. Locomotor patterns must constantly be altered in the face of changing environmental or task demands, such as heterogeneous terrains or obstacles. Variability in stride times occurring at short time scales (e.g., 5-10 strides) is statistically correlated to larger fluctuations occurring over longer time scales (e.g., 50-100 strides). This relationship, known as fractal dynamics, is thought to represent the adaptive capacity of the locomotor system. However, this has not been tested empirically. Thus, the purpose of this study was to determine if stride time fractality during steady state walking associated with the ability of individuals to adapt their gait patterns when locomotor speed and symmetry are altered. Fifteen healthy adults walked on a split-belt treadmill at preferred speed, half of preferred speed, and with one leg at preferred speed and the other at half speed (2:1 ratio asymmetric walking). The asymmetric belt speed condition induced gait asymmetries that required adaptation of locomotor patterns. The slow speed manipulation was chosen in order to determine the impact of gait speed on stride time fractal dynamics. Detrended fluctuation analysis was used to quantify the correlation structure, i.e., fractality, of stride times. Cross-correlation analysis was used to measure the deviation from intended anti-phasing between legs as a measure of gait adaptation. Results revealed no association between unperturbed walking fractal dynamics and gait adaptability performance. However, there was a quadratic relationship between perturbed, asymmetric walking fractal dynamics and adaptive performance during split-belt walking, whereby individuals who exhibited fractal scaling exponents that deviated from 1/f performed the poorest. Compared to steady state preferred walking speed, fractal dynamics increased closer to 1/f when participants were exposed to asymmetric walking. These findings suggest there may not be a relationship between unperturbed preferred or slow speed walking fractal dynamics and gait adaptability. However, the emergent relationship between asymmetric walking fractal dynamics and limb phase adaptation may represent a functional reorganization of the locomotor system (i.e., improved interactivity between degrees of freedom within the system) to be better suited to attenuate externally generated perturbations at various spatiotemporal scales.

The efficacy of the Microsoft KinectTM to assess human bimanual coordination.

The Microsoft Kinect has been used in studies examining posture and gait. Despite the advantages of portability and low cost, this device has not been used to assess interlimb coordination. Fundamental insights into movement control, variability, health, and functional status can be gained by examining coordination patterns. In this study, we investigated the efficacy of the Microsoft Kinect to capture bimanual coordination relative to a research-grade motion capture system. Twenty-four healthy adults performed coordinated hand movements in two patterns (in-phase and antiphase) at eight movement frequencies (1.00-3.33 Hz). Continuous relative phase (CRP) and discrete relative phase (DRP) were used to quantify the means (mCRP and mDRP) and variability (sdCRP and sdDRP) of coordination patterns. Between-device agreement was assessed using Bland-Altman bias with 95 % limits of agreement, concordance correlation coefficients (absolute agreement), and Pearson correlation coefficients (relative agreement). Modest-to-excellent relative and absolute agreements were found for mCRP in all conditions. However, mDRP showed poor agreement for the in-phase pattern at low frequencies, due to large between-device differences in a subset of participants. By contrast, poor absolute agreement was observed for both sdCRP and sdDRP, while relative agreement ranged from poor to excellent. Overall, the Kinect captures the macroscopic patterns of bimanual coordination better than coordination variability.

A model to investigate the mechanisms underlying the emergence and development of independent sitting.

When infants first begin to sit independently, they are highly unstable and unable to maintain upright sitting posture for more than a few seconds. Over the course of 3 months, the sitting ability of infants drastically improves. To investigate the mechanisms controlling the development of sitting posture, a single-degree-of-freedom inverted pendulum model was developed. Passive muscle properties were modeled with a stiffness and damping term, while active neurological control was modeled with a time-delayed proportional-integral-derivative (PID) controller. The findings of the simulations suggest that infants primarily utilize passive muscle stiffness to remain upright when they first begin to sit. This passive control mechanism allows the infant to remain upright so that active feedback control mechanisms can develop. The emergence of active control mechanisms allows infants to integrate sensory information into their movements so that they can exhibit more adaptive sitting.

Do actions speak louder than knowledge? Action manipulation, parental discourse, and children's mental state understanding in pretense.

Studies on pretense mental state understanding in young children have produced inconsistent findings. These findings could potentially emerge from the confounding influences of action manipulation or the failure to examine possible influences on individual children's performances. To address these issues, we created a task in which 68 3- and 4-year-olds viewed two actors, side by side, on a monitor. Children were told that one actor was knowledgeable about a specific animal, whereas the other actor was not. The actors performed identical movements that were either related or unrelated to the animal they were mimicking or engaged in different behaviors contradictory to their knowledge. Saliency of action was also manipulated by presenting either dynamic images or a paused frame of the actors' behavior (i.e., the static condition). Children performed similarly on the dynamic and static conditions. Children selected the knowledgeable actor more often in the unrelated and related trials but were not as successful at selecting the knowledgeable actor when the actor's knowledge contradicted the actor's behavior. Therefore, by 3 years of age, some children may understand that pretend play involves mental representations and appreciate that the mind influences a pretender's behavior. To investigate the observed individual differences, we also examined children and parents as they engaged in reading and pretense activities prior to data collection. The frequency of parents' cognitive mental state utterances strongly predicted performance on the mental state task. Individual differences in performance as a result of parental language and executive functioning abilities are discussed.

Sitting infants alter the magnitude and structure of postural sway when performing a manual goal-directed task.

In typical daily life, adults routinely adapt posture so that balance can be maintained while other goal-directed activities are performed. Interestingly, newly standing infants also control posture based on the demands of a task. It is unknown if the ability to properly adapt postural movements as a goal-directed task is performed emerges soon after the acquisition of independent stance or if it is present at earlier key postural milestones, such as independent sitting. In this study, the postural sway patterns of independently sitting infants were compared while either holding or not holding a toy. Infants exhibited less postural sway when holding the toy. This reduction in sway allowed infants to look at and stabilize the toy in their hand. Thus, the ability to adjust postural movements while performing a concurrent goal-directed task emerges long before the acquisition of independent stance.

Newly standing infants increase postural stability when performing a supra-postural task.

Independent stance is one of the most difficult motor milestones to achieve. Newly standing infants exhibit exaggerated body movements and can only stand for a brief amount of time. Given the difficult nature of bipedal stance, these unstable characteristics are slow to improve. However, we demonstrate that infants can increase their stability when engaged in a standing goal-directed task. Infants' balance was measured while standing and while standing and holding a visually attractive toy. When holding the toy, infants stood for a longer period of time, exhibited less body sway, and more mature postural dynamics. These results demonstrate that even with limited standing experience, infants can stabilize posture to facilitate performance of a concurrent task.

Task-dependent postural control throughout the lifespan.

Routine activities performed while standing and walking require the ability to appropriately and continuously modulate postural movements as a function of a concurrent task. Changes in task-dependent postural control contribute to the emergence, maturation, and decline of complex motor skills and stability throughout the lifespan.

The control of posture in newly standing infants is task dependent.

The postural sway patterns of newly standing infants were compared under two conditions: standing while holding a toy and standing while not holding a toy. Infants exhibited a lower magnitude of postural sway and more complex sway patterns when holding the toy. These changes suggest that infants adapt postural sway in a manner that facilitates visually fixating on and stabilizing the toy in their hand. When simply standing, infants exhibited postural sway patterns that appeared to be more exploratory in nature. Exploratory sway patterns may allow infants to learn the affordances of their new standing posture. These results demonstrate that newly standing infants are capable of task-dependent postural control.

Development of the coordination between posture and manual control.

Studies have suggested that proper postural control is essential for the development of reaching. However, little research has examined the development of the coordination between posture and manual control throughout childhood. We investigated the coordination between posture and manual control in children (7- and 10-year-olds) and adults during a precision fitting task as task constraints became more difficult. Participants fit a block through an opening as arm kinematics, trunk kinematics, and center of pressure data were collected. During the fitting task, the precision, postural, and visual constraints of the task were manipulated. Young children adopted a strategy where they first move their trunk toward the opening and then stabilize their trunk (freeze degrees of freedom) as the precision manual task is being performed. In contrast, adults and older children make compensatory trunk movements as the task is being performed. The 10-year-olds were similar to adults under the less constrained task conditions, but they resembled the 7-year-olds under the more challenging tasks. The ability to either suppress or allow postural fluctuations based on the constraints of a suprapostural task begins to develop at around 10 years of age. This ability, once developed, allows children to learn specific segmental movements required to complete a task within an environmental context.

Self-directed action affects planning in tool-use tasks with toddlers.

Toddlers grasp a tool more effectively when it is self-directed (e.g., spoon) than other-directed (e.g., hammer), possibly because the consequences of self-directed actions are more obvious. When the negative consequences of an inefficient grip were made equally salient, the self-directed versus other-directed differences remained.

Individual differences in executive functioning and theory of mind: An investigation of inhibitory control and planning ability.

This research examined the relative contributions of two aspects of executive function-inhibitory control and planning ability-to theory of mind in 49 3- and 4-year-olds. Children were given two standard theory of mind measures (Appearance-Reality and False Belief), three inhibitory control tasks (Bear/Dragon, Whisper, and Gift Delay), three planning tasks (Tower of Hanoi, Truck Loading, and Kitten Delivery), and a receptive vocabulary test (Peabody Picture Vocabulary Test [PPVT-3]). Multiple regression analyses indicated that two inhibition tasks (Bear/Dragon and Whisper) were significantly related to theory of mind after accounting for age, receptive vocabulary, and planning. In contrast, the planning tasks did not share unique variance with theory of mind. These results increase our understanding of the specific nature of executive function-theory of mind relations during early childhood.

Evidence of motor planning in infant reaching behavior.

When adults reach for an object, kinematic measures of their approach movement are affected by what they intend to do after grasping it. We examined whether such future intended actions would be reflected in the approach-to-grasp phase of infant reaching. Twenty-one 10-month-old infants were encouraged to either throw a ball into a tub or fit it down a tube. Kinematic measures of the approach phase of the reach toward the ball were obtained using a motion analysis system. Infants, like adults, reached for the ball faster if they were going to subsequently throw it as opposed to using it in the precision action. The perceptual aspects of the ball were the same and cannot account for these kinematic differences. Infants appear to be planning both segments of their actions in advance. Our findings provide evidence for a level of sophistication in infant motor planning not reported before.