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Nathalie Bonneton, Daniel Mellier
To cite this version:
Nathalie Bonneton, Daniel Mellier. The Isochronic Component of Pointing Movements in children
aged from 26 to 40 months. Current Psychology Letters/Behaviour, Brain and Cognition, de Boeck
Université, 2001. �hal-01682407�
Nathalie Bonneton and Daniel Mellier University of Rouen, PSYCO (EA 1780)
Abstract
This study looks at the isochronic component of pointing movements. Eleven children between the ages of 26 and 36 months at the beginning of the study performed a pointing task twice, at a 4-month interval. The location of the target was defined by its distance from the child (13 or 20 cm) and its direction with respect to the child's mid-body line (5 positions, ranging from 90° to the right and 90° to the left). A kinetic analysis of movement time and mean speed showed that isochrony can account for the organization of movement during the third year: mean speed was increased when the distance to cover was longer, in such a way that movement time remained constant.
This compensation mechanism was found already at 26 months, regardless of where the target was located.
Keywords: Motor representations, Movement, Space, Isochrony, Development
1. Introduction
Studies on the planning and control of motor actions in adults have shown that movement parameters exhibit a number of systematic variations. The principle of isochrony accounts for one of these invariants. This control mechanism, which keeps movement time constant (i.e., the actual duration of the movement exclusive of response latency) no matter what distance must be covered, has been largely validated in modern-day research on the kinematics of writing (Viviani & Terzuolo, 1980) and on the control of other motor tasks such as pointing and reaching (Jeannerod, 1984; Ghez, Gordon, Ghilardi, Christakos, & Cooper, 1990). Movement isochrony is a testimony to the motor system's ability to integrate distance into execution speed. This principle also governs the mental simulation of motor actions (Decety & Michel, 1989). In this respect many authors consider it like a motor rule used by motor representation.
The renewed interest of psychologists in the study of writing (for a review, see Zesiger, 1995) and the development of motor actions (for a review, see Mellier & Bullinger, 1999) brought together the neurobiological and psychocognitive approaches, with a new focus on how motor representations are built. In this joint approach, mental representations of motor activity are thought to act as "action simulators" that enable the subject to pre-program future actions, whether imagined or executed (Jeannerod, 1994, 1999). This view is based on the idea that there are mental links between the properties of the target,and the properties of the movement. Its proponents go along with Paillard (1986) in saying that a motor action is successful if the cognitive-motor system responds adequately to three types of questions: WHAT (identifying the target of the action), WHERE (locating the target of the action), and HOW (ordering and timing the action sequence).
In this "prescriptive" view, the perceptual localization stage precedes the encoding of spatial data into motor coordinates. As such, a motor representation contains information about how far away and in what direction the target is located. Abidance by the rule of isochrony for a class of movements implies that the distance parameter is encoded in the motor representation in terms of a variation in movement speed, but no variation in its duration.
The question raised here is whether this encoding rule also governs children's motor representations, and if so, at what age. The idea is to find out if the actions being acquired at all stages of development achieve perceptual-motor processing efficiency through reliance upon the movement-time constancy strategy. The present analysis will be limited to pointing and reaching tasks.
Research on children ages 4 to 8 years has shown that spatial information describing a target is not always handled in the same way by the cognitive-motor system. According to Hay (1987, 1990), for example, the mechanisms that control movement direction (as defined with respect to the subject's own body) are quite elementary and are operative early in life, whereas distance control mechanisms are not acquired until later. Thus, the processes responsible for controlling a movement's direction and amplitude are differentiated during the formation of pointing and reaching movements. Direction is already programmed by the time the hand starts to move, whereas distance is controlled as the action is taking place. In short, in the development of movement spatialization, amplitude is regarded as more complex than direction, and takes more time to define (Hay, 1990).
The principle of isochrony in child movements was studied by Vinter and Mounoud (1991) in a circle drawing task performed in several contexts, with circles of different sizes. A comparison of the dynamic
parameters of the drawing movements the children made to depict a bear, to produce a series of increasingly large circles, or to simply draw circles of different sizes without seriation, showed that the isochrony principle was more strictly obeyed when the goal was to make larger and larger circles. Thus, the planning process was based not only on the movement's amplitude, but also on the final task goal (seriation of amplitude). From a developmental standpoint, the results showed that the principle of isochrony is not applied in a steadily increasing way between the ages of 5 and 10. The authors found a U-shaped developmental curve with less compensation at about the age of 7.
These findings do not support Hay's (1990) conclusion that isochrony cannot be achieved for all actions in the motor repertoire. On a pointing task, this author has concluded that the isochrony mechanism is not in place until the age of 10.
To our knowledge, the issue of speed/amplitude compensation has never been documented for the age range between 26 and 40 months. Yet, if we see development as consisting of the successive "re-elaboration"
(Mounoud, 1981) or "redescription" (Karmiloff-Smith, 1992) of representations and thus of action programs, then the age range that ends with the sensorimotor stage (Piaget, 1936) deserves particular attention because it marks the onset of access to new forms of intelligence. It would seem worthwhile to fill in the gaps in our knowledge of developmental patterns (1) by finding out if isochrony-based control is part of action planning before the age of 4 and (2) by determining whether the updating of this rule evolves in a non-monotonic fashion that reflects psychological reorganization during this period, or whether the repertoire of cognitive strategies evolves steadily.
The present study explores the way in which the cognitive-motor system of children between the ages of 26 and 40 months represents and integrates the distance parameter in a pointing task.
2. Method
2.1 Subjects
Eleven children (6 girls and 5 boys) were observed in two sessions held four months apart. They were 26 to 36 months old at the beginning of the study and 30 to 40 months at the end. The youngest ones were observed at their child-care center and the oldest ones, at their preschool. We made sure that the observation and data collection conditions were the same in all sessions.
It seemed like a good idea to examine the same population twice rather than setting up independent groups. Even this minimal amount of longitudinal observation, albeit on a small sample, would make us more confident about our conclusions regarding the development of motor actions.
2.2 Task and procedure
The child's task was to hold a 8-cm long stick (a "Q-tip"), dip it in a flat container of colored powder (facial makeup), and move it to a target (a piece of white paper measuring 1 sq. cm) located 13 or 20 cm from the (fixed) starting point. The container was placed on the table straight in front of the child. The direction of the target varied across trials. On each trial, the target was laid in one of five positions (0°, 45°, 90°, 135°, and 180°) marked off along one of two semi-circles (13 cm or 20 cm radius) drawn on the table but invisible to the child. The 90°
mark was straight in front of the child. For all trials, the starting point and the target can be simultaneously perceived by the child.
Ten independent trials were run (5 directions x 2 distances). Testing was done in a constant random order.
The child was seated at a child-size table, and the task was performed on the horizontal plane.
The experimenter explained the task to the child as follows: "You have a small dish of colored powder in front of you. You're going to get a little bit of the powder on the stick and then put it on a little piece of paper that I'll set on the table." So as not to influence the child's choice of which hand to use, the experimenter laid the stick directly in front of the child before the task began. Eight children spontaneously chose their right hand. Two children worked with their left hand, and one child changed hands during the task.
After making sure the hand preference did not have a notable effect on the movement parameters, the data was divided up according to whether the action was carried out in the ipsilateral or contralateral field with respect to the executing hand. The 0° location was homolateral to the executing hand.
All subjects were observed in two sessions held four months apart so we make sure that conditions for task and procedure was maintain constant.
2.3 Data collection and processing
The children were filmed full-face from above, at a fixed focal distance for all shots. The videotapes were analyzed using image processing software (Cosserat, Tavernier, & Gabillaud, 1994) that supplied x-y coordinates for the various elements in the frames. The sampling rate was 10 images per second. The successive locations of the end of the stick relative to the movement's starting and ending points were used to reconstruct the movement's spatial and temporal organization. The measures chosen were movement time (actual duration of the movement between exit from of container and contact with the target) and mean speed.
The grasping and moving of the stick to the powder container were not included. These movements served to contextualize the task. The data analyzed only pertained to the movement of the stick from the container to the paper target.
3. Results
The measures (movement time, mean speed) were processed in an analysis of variance with repeated measurements. This analysis assessed the statistical effects of the following variables: observation session (2 categories) x distance (2 categories) x direction (5 categories).
3.1 Analysis of movement time
Table I gives the data (in seconds) as a function of target direction and distance, for each observation session. For all conditions taken together, we can see that movement time decreased significantly between the first and second sessions (F(1,5) = 6.52, p = .05). Comparisons among the various categories of the direction and distance variables indicated no statistically significant differences. This was true for both observation sessions.
3.2 Analysis of mean speed
For all conditions pooled, the comparison of the two observation sessions (Figure 1) yielded a significant rise in mean speed after four months (F(1,3) = 19.62, p = .02 ). In addition, mean speed increased significantly with the distance to the target (13 or 20 cm.) on both the first (F(1,5) = 60.68, p = .0006) and second (F(1,7) = 19.16, p = .003) sessions. There was no significant effect of the direction on the mean speed and no interaction was found between distance and direction on either session.
4. Discussion
Comparisons of the dynamic parameters (duration and speed) of a pointing task performed by children aged from 26 to 40 months showed firstly, that movement time decreased during the four months between the two observations of the same children. The mean speed increase during development is classically viewed like an improvement of the process responsible for feedback integration (Hay, 1987). However, an additional cross-section study will enable us to understand whether this evolution is linked to a developmental effect or to a learning effect.
Moreover, target direction and distance did not have an impact on movement time at either age. These findings point to the conclusion that movement time varies little, if at all, and suggest that the principle of isochrony is being applied. Secondly, for all target directions, the mean speed of the children's movements increased with target distance, whereas movement time was the same in both observation sessions. This compensation mechanism is a testimony to the existence of an invariable temporal structure that becomes operative by the third year of life in spatially constraining tasks. At the age of 26 months, the target distance parameter is processed and integrated into the action planning process. The movement-time constancy strategy, governed by the principle of isochrony, appears to be in place by the child's second birthday and continues to be used until 40 months, with a rising curve for pointing movement speed during the four-month period studied here.
In conclusion, this study clearly showed that for pointing tasks, the duration-preference strategy (principle of isochrony) is in place by the age of 26 months and continues to take effect during the third year of life, without causing a developmental regression in the measures suggesting a non-monotonic evolution over this age range.
We are now in a position to state that at 26 months, an increase in the distance to be covered by a given movement is encoded by the cognitive-motor system as the necessity to increase the movement speed. Our conclusions disagree with those of Brown, Sepehr, Ettlinger and Skreczek (1986). In this study, a variation in target distance is not accompanied by a temporal invariance of the pointing movements executed by children aged from 1,5 to 8 years-old. However, these authors don't include the age in the movement time analysis. So, it seems difficult to make comparisons between their study and ours. Moreover, in our study, the subjects point to the target in a horizontal plane whereas it was vertical in Brown and Al 's task. This difference raises the problem of the execution constraint and a study should be undertaken in order to measure their influence on the actualization of the motor rule.
Our results support the idea that the mechanisms responsible for the parametrization of movement direction and amplitude (Hauert, 1995; Hay, 1990, 1991) are independent, insofar as speed depends on distance and age, but not on target direction. Our conclusions are also in line with Vinter and Mounoud's work (1991), where variations in target direction did not change the meaning subjects gave to the task. An additional study
where we vary the motor intention while keeping the distance and direction constant is now underway and will serve to validate this conclusion.
References
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Table 1. Movement time, by observation session and target direction and distance
1st session Direction
0° m = 1.44 sd = 0.7 min = 0.7 max = 3.8
45° m = 1.68 sd = 0.56 min = 0.8 max = 2.9
90° m = 1.7 sd = 0.91 min = 0.7 max = 4
135° m = 1.47 sd = 0.79 min = 0.7 max = 3.7
180° m = 1.45 sd = 0.47 min = 0.5 max = 2.4
Distance
13 cm m = 1.46 sd = 0.78 min = 0.7 max = 4
20 cm m = 1.64 sd = 0.67 min = 0.5 max = 3.8
2nd session Direction
0° m = 1.09 sd = 0.7 min = 0.6 max = 3.8
45° m = 1.32 sd = 0.56 min = 0.6 max = 3.8
90° m = 1.05 sd = 0.37 min = 0.6 max = 3.8
135° m=1.27 sd = 0.77 min = 0.5 max = 3.8
180° m = 1.22 sd = 0.6 min = 0.5 max = 3.8
Distance
13 cm m = 1.17 sd = 0.55 min = 0.5 max = 3.8
20 cm m = 1.22 sd = 0.66 min = 0.5 max = 3.8
Figure 1: Effect of distance to cover on mean speed
16 18 20 22 24 26 28 30
13 cm 20 cm
Target distance
mean speed (in cm/s)
obs 1 obs 2
Figure 1: Effect of distance to cover on mean speed.
Mean speed (in cm/s) Target distance
