Laurette Hay

Availability of visual and proprioceptive afferent messages and postural control in elderly adults

The ability of young and elderly adults to keep a stable upright posture while facing changes in the availability of visual and/or propriomuscular information was investigated. The two sensory sources of information were alternatively available and withdrawn, jointly and separately, during 10-s alternating sequences. Vision was modified by means of liquid-crystal goggles, and proprioception was altered by means of tendon vibration of both antagonistic ankle muscles. Elderly adults were less stable than young adults when vision was withdrawn. Both groups were greatly affected when proprio-muscular inputs were altered by vibration. Under constant visual conditions and following a propriomuscular perturbation (i.e., vibration), elderly adults were unable to take advantage of the reinsertion of propriomuscular inputs. They showed a transient, decreased stability and were unable to fully recover during a 10-s period, whereas young adults were able to rapidly integrate the information to stabilize their posture. When both propriomuscular and visual inputs were withdrawn and concurrently reinserted, the elderly adults did not show a transitory increase in the velocity of the center of foot pressure. The present results extend findings on the inability of elderly adults to reconfigure rapidly the postural set following reinsertion of sensory inputs. The results also suggest that elderly adults have difficulties in taking advantage of sensory redundancy in postural control.

Timing and accuracy of visually directed movements in children: Control of direction and amplitude components

The reaction times (RTs), movement times (MTs), and final accuracy of hand movements directed towards visual goals were measured in 6-, 8-, and 10-year-old children, using tasks in which direction and amplitude components of movement were distinctly required. The tasks were performed with and without visual feedback of the limb. RTs decreased with age, and were shorter in directional than in amplitude task, in all ages. MTs were the longest at age 8 in both tasks, equally short at ages 6 and 10 in the directional task, the shortest at age 10, and intermediate at age 6, when amplitude had to be regulated. In the amplitude task, the target distance generally affected MTs under both visual conditions, but to a lower degree at age 10 than in the two younger groups. Movement accuracy, which was in all cases higher with visual feedback, showed different developmental trends among the two spatial components: directional accuracy was not different among the three groups of age, whereas amplitude accuracy showed a nonmonotonic development in the nonvisual condition, with an increase between age 6 and age 10, and the lowest level at age 8. In the visual condition, amplitude accuracy did not change with age. The specification of direction seems therefore to predominantly load the preparatory stage of the response. Amplitude specification seems to be more dependent on on-going regulations and to undergo a longer and more complex development, with a critical period around age 8 when a greater propensity for a feedback-based control appears on the two components. With increasing age, amplitude tends to be specific to a greater extent by a feedforward process.

Feedforward versus feedback control in children and adults subjected to a postural disturbance

Abstractu2002Any action performed by standing subjects is generally accompanied by compensatory postural activities, which reduce or abolish the postural disturbance generated by the movements and keep the subjects’ center of gravity within the supporting base. These postural activities are triggered by either anticipatory and/or feedback-based control processes, depending on the information available and on the behavioral context. To investigate the respective involvement of these two components in postural control during development, we studied the extent to which the postural equilibrium of children (3- to 10-year-olds) and adults was disturbed by the same physical event, an unloading, depending on whether it was initiated by the subject or externally imposed. The subjects were standing on a force platform with their eyes closed, holding a load (5% of their own body weight) in their hands, with arms vertical and forearms horizontal. Two conditions were applied: (1) the subjects voluntarily released the load and (2) the load was unpredictably removed. The unloading resulted in a backward movement of the center of pressure, which was smaller with self-initiated than imposed disturbances in all age groups. This difference varied depending mainly on the age-related changes in the relative amplitude of the self-initiated disturbance, which decreased between 3- to 5-, and 6- to 8-year-olds (who showed no marked postural instability after self-initiated unloading), and increased again in the two older groups (9- to 10-year-olds and adults), in which it also became more consistent . It was concluded that feedforward control becomes more efficient as children grow up, but that its relative contribution to postural control does not show a monotonic pattern of development.

Kinematics of aiming in direction and amplitude: a developmental study.

The patterns of aimed movements to visual targets were analyzed in children aged 6, 8 and 10. Tasks with direction and/or amplitude requirements were used. The tasks were performed both with and without vision. Peak velocity, acceleration and deceleration and their relative temporal occurrence were evaluated. Overall, the 6- and 10-year-olds exhibited higher peak velocity and acceleration when performing the pure directional task than when performing tasks with an amplitude or stopping requirement. On the contrary, 8-year-olds showed similar peak acceleration and velocity across all three tasks. Similarly, when performing the pure directional task, the 6- and 10-year-olds reached their peak velocity and acceleration relatively later in time than the 8-year-olds. Vision of movement increased the peak velocity in all experimental tasks and peak acceleration was increased only in the pure directional task. Thus, movement kinematics varied according to the task requirements and age. Eight-year-olds showed greater propensity to feedback control in all tasks, suggesting an over-inhibition in their approach patterns, whereas 10-year-olds tended to use feedforward processes, with a shortened deceleration phase.

Visuomanual coordination in childhood: adaptation to visual distortion

The aim of the experiment was to study the adaptive capacities of children to perform drawing movements while being visually perturbed. Children aged 5–11xa0years and a group of adults drew diamonds via information provided through a computer screen. The screen display was either upright or rotated 180°. Results showed that the absence of direct vision of the hand yielded more perturbation in the youngest group of children compared to all other groups. In spite of some initial difficulty, all children reached accurate control after five trials. When faced with spatial rotations of the visual field, youngsters were again more perturbed than others. All children showed the same rate of adaptation to visual rotations, but they differed on adaptive strategies. Five- and 7-year-olds shifted to a feedforward mode of control consisting of the production of a rapid gesture, followed by error evaluation in order to correct their next movement. Older children were characterised by a progressive integration of reafferent visual and proprioceptive information. It resulted in an increase in duration of strokes and reduced speed, meaning enhanced on-line retrieval of information. However, 9-year-old children experienced more difficulty recuperating sensory information during movement than 11-year-olds, and kept using error feedback. Finally, visuomanual coordination in children aged 11xa0years, while slightly differing from that of adults, was not yet totally mature.

Age-related differences in the reaching and grasping coordination in children: unimanual and bimanual tasks.

This study examined age-related differences in the coordinative mechanism of the reach-to-grasp movement in three groups of children aged 6, 8, and 11xa0year, and in healthy adults. Three prehension conditions were manipulated: an unimanual and a bimanual self-driven tasks in which the reaching and grasping of the object were performed by participants, and a bimanual externally-driven task, in which the experimenter brought the object into the vicinity of the participant which grasped it. Classical kinematics data—peak velocities of the reaching and the grasping, the time to onset grip opening, maximum grip opening and grip closure—were calculated. Moreover, to obtain equivalent kinematics variables for all age groups, relative time to peak velocity (% of reaching duration), relative maximum grip opening (% of object size), and percentage of the four types of phase plans between reaching velocity and grip size have been calculated for each group of age. Our main results showed (1) a high variability at age 6, (2) an age-related change between the 6- and 8-year old for almost all of the dependent variables, and (3) a significant difference between the 11-year olds and adults. In summary, at 6xa0years, the interdependence between the reaching and grasping programs was unstable. A transitory feedback-based coordination between reaching and grasping appeared at 8xa0years of age. Finally, the adults’ relationship between reaching and grasping was not attained at the age of 11.

Development of postural adaptation to arm raising

Abstract. We studied the development of the coordination between posture and movement by analyzing the shifts of the center of pressure (CoP) associated with arm raising. Three groups of children aged 3–5xa0years, 6–8xa0years, and 9–10xa0years and an adult group were tested. The subjects were required to raise their arms to the horizontal position while standing still, with their hands free or loaded (5% of the body weight). The arm movements were recorded by a TV-image processor, and the changes in position of the CoP were measured by a force platform and analyzed before, during, and after the arm movement. The data show that the CoP moved forward during arm raising, that additional load induced a greater shift in all age groups, and that the relative amplitude of the shift decreased with age. The greatest changes occurred between ages 3–5xa0years and 6–8xa0years. The pre- and postmovement CoP shift suggests qualitative changes in the postural adaptation to movement between these two age groups: the anticipatory postural adjustments moved from a supporting function to a compensatory function, yielding an increasing functional convergence between the feedforward and the feedback modes of postural control, and an increasing rapidness in recovering postural stability after arm movement. The postural behavior shown by the 9- to 10-year-old children and by the adults in the arms-free condition suggests an increased tolerance to unbalance when postural oscillations do not jeopardize static equilibrium.

Response delay and spatial representation in pointing movements.

Pointing movements decrease in accuracy when target information is removed before movement onset. This time effect was analyzed in relation with the spatial representation of the target location, which can be egocentric (i.e. in relation to the body) or exocentric (i.e. in relation to the external world) depending on the visual environment of the target. The accuracy of pointing movements performed without visual feedback was measured in two delay conditions: 0 and 5-s delay between target removal and movement onset. In each delay condition, targets were presented either in the darkness (egocentric localization) or within a structured visual background (exocentric localization). The results show that pointing was more accurate when targets were presented within a visual background than in the darkness. The time-related decrease in accuracy was observed in the darkness condition, whereas no delay effect was found in the presence of a visual background. Therefore, contextual factors applied to a simple pointing action might induce different spatial representations: a short-lived sensorimotor egocentric representation used in immediate action control, or a long-lived perceptual exocentric representation which drives perception and delayed action.

The control of goal-directed movements in children: Role of proprioceptive muscle afferents

Abstract The purpose of this study was to evaluate, using the vibration technique, the extent to which children rely on propriomuscular inputs to control their goal-directed movements on-line. Children aged 5, 7, 9, 11 years and adults performed pointing movements by sliding a stylus towards targets without any visual feedback while vibration (30 Hz and 70 Hz) was being applied to the distal tendon of their biceps muscle. The results show that in all the subjects the 70 Hz vibration induced a deviation of the limb trajectory in the adduction direction (towards the contralateral side of the body), which was maximum at 5 years of age, decreased between 5 and 7, and levelled out later on. The 30 Hz frequency did not affect the trajectory except in the 5-year-olds. Neither the duration of the pointing movements nor the trajectory parameters were affected by any of the vibration conditions. The effects of target distance on the movement time were found to decrease after 9 years of age, as did the number of velocity peaks and the relative time to peak, whereas the peak velocity increased after 11 years of age. In conclusion, in all the age-groups studied, the additional propriomuscular messages were interpreted like an error signal in controlling the movement trajectory in the absence of visual feedback, particularly in the younger children, whereas the older children and adults seem to have relied more on a feedforward process.

Contribution of the propriomuscular channel to movement coding in children: A study involving the use of vibration-induced kinaesthetic illusion

Abstract The specific contribution of the propriomuscular afferent channel to the coding of movement was investigated in children aged 5, 7 and 9 years by applying vibration to muscle tendons, which is known to elicit kinaesthetic illusions in adults in a motionless limb. Vibrations at various frequencies ranging from 20 to 130 Hz were applied to the distal tendon of the right triceps brachii. The subject had to simultaneously copy with the left forearm the kinaesthetic sensation thus induced. The ability of the children to discriminate between various real movement speeds (2, 5 and 10°/s) was established using the same method. In all the children, the vibratory stimulation evoked an illusory sensation of elbow flexion, the velocity of which depended on the subjects age and the vibration frequency. At 5 years of age, the perceived velocity was highest and did not depend on the vibration frequency; it was lower at 7 and 9. It began to depend on the vibration frequency at the age of 9, when higher values were obtained with the median than with the extreme frequencies, as in adults. The results indicate that propriomuscular information contributes to conscious movement perception at each of the ages considered here, but that the processing of this information varies with age as children become aware of the quantitative (movement speed) content of the proprioceptive messages.