Changes in reaching and grasping objects of ... - Wiley Online Library

British]ournal ofDevelopmenta1 Psychology (1996), 14,6578 Printed in Great Britain 0 1996 The British Psychological Society

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Changes in reaching and grasping objects of different sizes between 7 and 13 months of age -

J. Fagard“ and A. Y. Jacquet Laboratoire de Psychobiologie dr Diveloppement, Paris, France Twenty-four 7- to 13-month-old infants were videotaped when grasping or attempting to grasp objects of different sizes ranging in height from 7 to 17.5 cm. We investigated how early during the action sequence young infants use their two hands to grasp large objects, in order to determine the age at which they anticipate the need for a bimanual strategy from perception of object size. Second hand onset was also measured to examine whether the second hand could be recruited at any time during an action. The results show that most movements start unimanually and that the second hand is activated later. The younger infants were more successful when grasping large objects with two hands than with one hand, but they did not show more bimanual reaching with large as compared to small objects before 11 months of age. These findings suggest that young infants d o not perceive the same action relevant information to drive an action as older infants and that infants shift from a correction to an anticipation strategy as they grow older. The results fit with the idea that, regarding features like size, relevant visuomanual experience may be important for full coordination of the constituent perceptual and motor skills. In addition, after a unimanual initiation, the younger infants tended to activate their second hand only after the first one had completed its course. In contrast, more intermanual flexibility was observed in the older infants. This reflects an increasing range of coordinated patterns during bimanual actions around the end of the first year of life.

The hand is the main tool we use to act on the environment. Most skills require the cooperative use of both hands. Bimanual coordination has received a great deal of attention over the last 10 years, mainly in adults. The study presented here concerns the development of bimanual reaching and grasping of large objects during infancy. Do infants initiate their grasping movements of small objects with one hand and of large objects with both hands as soon as they try to grasp large objects? Or do infants learn to adapt their grasping strategy to object size through experience? Infants start reaching successfully for an object around 3 or 4 months of age but grasping is often unsuccessful, even at 7 months of age (Fetters & Todd, 1987). Studies on the development of reaching and grasping show that progress is associated with an increased capacity to direct the arm towards the object, and increased hand and finger control, which results in shaping the hand before the object is touched (Halverson, 1931; Twitchell, 1970; von Hofsten & Ronnqvist, 1988). It is likely that the capacity to use and coordinate the hands for securing the object may also be important for successful grasping. A few studies have examined the relationship between grasping strategy and object size. According to Bower (1972), some adaptation to object size can be observed even at

’’ Requests for reprints should be addrcssed to J. Fagard, Laboratoire “Cognition et DCveloppcment”, 28 rue Scrpcnte, 75006 Paris, France.

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a pre-reaching stage; interhand separation is greater with large as compared to small objects. However, in Bower’s study, interhand separation was measured only at the time of contact and differences could passively result from contact with the object itself rather than from initial movement parameters. In a more recent study, Newell, Scully, McDonald & Baillargeon (1989) used 1.25, 2.50 and 8.50 cm objects and showed that grasping is unimanual for small objects and bimanual for large objects even at 4 months of age. However, in the Newell et al. (1989) study, in which infants ranging from 4 to 8 months old were compared, bilaterality was also coded at the time of grasping rather than during the reaching movement. The observation of bilaterality at the time of grasping is not informative concerning the infant’s reaching strategy. If the second hand is activated after the first hand has reached and touched the object, then feedback from haptic stimulation may induce corrections to the original action pattern, which was ill adapted to the stimulus on the basis of exclusively visual clues. In contrast, if the movement is bilateral from the start or before the object is touched, the infant must have initiated a bilateral strategy after only looking at the stimulus. One study analysed the uni- versus bilateral preparatory component of infants’ reaching in relation to object size; Rochat, Clifton, Litovsky & Perris (1989) showed that 6-month-old infants adjust their reaching patterns according to the size of the object. However, Rochat et al. used objects that contrasted greatly in size (5 vs. 30 cm), which precludes firm conclusions about fine adaptation of reaching to object size. Therefore, it is not clear how finely infants can tune their reaching movements to object size when they start grasping objects successfully. In the present study, we examined how precisely infants adapt their approach to an object based on its size, and whether such adaptation is observed as soon as they grasp objects successfully or whether it results from experience. The issue behind this question relates to the coupling between perception and action during skill development. By 6 or 7 months of age, infants normally have experienced visually objects of different sizes. In addition, at this age, infants have enough control over their proximal system to reach bimanually (Lockman, Ashmead & Bushnell, 1984; Rochat & Stacy, 1989; White, Castle & Held, 1964). In terms of grasping experience, however, observations of infant toys suggest that the first objects infants are offered to play with are rather small. If, as Newell et al. (1989) suggested based on their results, infants differentiate one- and two-hand movement as a function of object size at an early stage of the development of grasping, then the visual system provides action relevant information (concerning object size) for the reaching movement without an initial stage in which infants correct their lack of anticipation through feedback from the result of their action. Although this was not precisely Newell et a1.S’ view, this result would fit with the Gibsonian idea that experience is not a prerequisite for detecting affordances in the environment, infants being able to perceive them as soon as the relevant perceptual and motor skills are developed (Gibson, 1977).

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Newell et al. opposed maturational and cognitive theories with a dynamic approach; through the latter perspective, they see the emergence of a bimanual configuration for large objects as a consequence of task constraints. Such constraints would channel the dynamics to produce new behavioural configurations without calling for central prescriptions or object representations guiding the action (Newell et al., 1989; Thelen, Kelso & Fogel, 1987).

Changes in reaching and grasping objects

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O n the other hand, the data could show that infants initially approach large objects with one hand and activate their second hand only after touching the object but increasingly approach large objects bimanually with age and practice; this would support the idea that anticipatory action as a function of the visually perceived properties of the stimulus has to be learned through active experience. Such results would fit with a constructivist point of view, in which infants become progressively able to plan movement as new coding capacities appear allowing them to integrate new physical dimensions of the object to guide their action and therefore to develop new perceptualmotor procedures (Mounoud, 1983, 1993). A related issue addressed in the current study concerns changes in intermanual temporal coordination during the second semester of life, as the capacity for bimanual cooperation develops. Can the second hand be recruited at any time during an action? O r is the second hand activation initially restricted either to movement initiation (as is the case for synchronous bilateral movements), or to the end of the first hand movement (as part of a sequence of successive movements)? Although some investigations have reported bimanual cooperation in infants as young as 4 to 5 months of age (Rochat, 1989), there is rapid improvement in the ability to use the two hands in complementary patterns around the end of the first year of life (Diamond, 1991; Fagard, 1994; Fagard & Jacquet, 1989; Ramsay & Weber, 1986). Before this age, infants either move their two hands with Complementary but successive movement patterns (Diamond, 1991; Fagard, 1994) or exhibit undifferentiated simultaneous patterns to complete a bimanual goaldirected action. Many studies have shown the tendency of bimanual movements to be initiated simultaneously and to be temporally coupled (Kelso, Putnam & Goodman, 1983; see Fagard, 1991 for a review). These studies support the idea that a bilateral linkage underlying interlimb coordination is an inherent property of the motor system. This coupling between the limbs can be considered as a functional constraint. The guiding hypothesis for the present study is that synchronized or mirror bimanual synergies would be observed before simultaneous non-synchronized synergies, and that the flexibility between the two limbs engaged in bimanual synergy might increase between 7 and 13 months of age, as bimanual coordination develops. To address both issues (adaptation of the reaching movement to object size and flexibility within the bimanual synergy), we presented 7- to 13-month-old infants with objects of different size and analysed the whole sequence from initiation of the movement after object presentation until the object was properly secured or the unsuccessful attempt at grasping had been abandoned. In order to avoid biasing the conclusions by eliminating trials where infants used a strategy that was poorly adapted to object size, all trials were analysed, including unsuccessful ones. We chose the 7- to 13-month age range in order to observe infants before the progress in bimanual cooperation which occurs towards the end of the first year of life, and because we noticed that infants who are younger than 7 months of age frequently refuse to reach for large objects.

Method

Participants Twenty-four children (12 girls, 12 boys) were tested during one of the regular monthly check-ups to which all families living in France are entitled for the first two years of life. Groups were composed of

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Y; Jacquet

six 7-month-olds (30 to 33 weeks; mean age = 32 weeks), six 9-month-olds (38 to 42 weeks; mean age = 40 weeks), six 11-month-olds (48 to 51.5 weeks; mean age = 50 weeks) and six 13-month-old children (57 to 60 weeks; mean age = 59 weeks). All infants were Africans or north Africans residing in France. All were full term, healthy babies.

Procedure Infants were tested in a quiet room, seated in either a small infant chair or on their mother’s lap2 if they fussed in the chair. Prior to each object presentation, the experimenter made sure that the infants could move their hands freely (not in the mouth or inadvertently restrained). Objects were presented at a midline position on a narrow plastic table by the experimenter seated in front of the infant. The height of the table was adjustable so that the object could always be presented at the infant’s armpit height. To avoid lateral bias, the experimenter presented all objects bimanually. The object was gently taken from the infant immediately after it was grasped (trials lasted no more than five seconds after the infants started to reach for the object). A set of graded object sizes was used. The objects were five flasks of identical shape and colour but of increasing size, whose diameter x height measurements were respectively 3.5 x 7 cm, 5 x 8 cm, 6 x 12 cm, 8 x 13.5 cm, and 10 x 17.5 cm. The objects weighed between 10 and 50 g. The three largest objects were difficult to grasp unimanually, even for the oldest infants.’ The flasks of different sizes were presented randomly in two successive series (two presentations for each object). In addition to the five flasks, each infant was presented with one object that could only be manipulated successfully by using both hands. The rationale was to test whether changes observed in approaching the large flasks were functionally related to an increased capacity for bimanual coordination. The test object was a small plastic tube in a wooden container (total size = 12 x 2 cm), with only the bright cap visible. The task was to extract the plastic tube from the container, which required bimanual coordination such that one hand held the container while the other removed the tube.

Data analyses Sessions were videotaped using a 24 frames/s videocamera positioned in front of the subject. Each session was analysed frame by frame and coding started with the presentation of the object and ended with the end of the grasping phase. The criterion for movement initiation was the first move towards the object while looking at it. Each of the following events was used for coding: initiation of the movement, object touching and object grasping. Reaching (or approach) relates to the time between initiation and touch. Bimanual coding for initiation and grasping was defined as trials where the two hands simultaneously started towards or grasped the object within the sume videoframe, i.e. less than 40 ms apart. Bimanual coding for reaching was defined as trials where the two hands moved simultaneously before the first hand touched the object, whether initiations were synchronized or not, and whether grasping was bimanual or not. The movement was coded as successful when the object was actually grasped, and as not successful when the child attempted to grasp the object but dropped it. Some trials were missing due to infants’ refusal to complete a session. In addition, trials were discarded when infants were not interested in the object, made no attempt to grasp it or only contacted the object to push it off the tray deliberately (out of 240 theoretical trials, 21 were lost; rate of attrition = 8.75 per cent); 11 infants had missing trials and misses were roughly equally distributed across objects and age groups. Inter-rater reliabilities were tested on the videotapes of five randomly selected participants. The comparisons indicated 89 per cent inter-rater agreement for initiation time, touch and grasp and 99 per cent agreement for success and failure. The mother was then asked to hold the infant firmly at the waist and put the infant in as symmetrical a position as possible. Only three infants needed to be held this way. The maximum index-thumb width of the first 15 participants was measured. Similar to Newell et al. (1989), no significant differences were found for hand width in infants of different ages. Therefore, this variable was discarded.

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Changes in reaching and grasping objects

The two presentations of each object were combined in the analysis. The effects of the independent variables o n binary dependent variables were tested using a chi-square test and the interaction effects between independent variables were tested using linear logistic regression analysis. A Cochran test for related samples was used to test the object-size effect.

Results Success versus failure

Age (X2(3) = 31.10, p