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HK453 - Mo nuol ConIml Deve/oprnenf ofReoIo/Iing onol...

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Unformatted text preview: Mo nuol ConIml , Deve/oprnenf ofReoIo/Iing onol Grasping Chapter objectives This chopIeI will do the following: , '1 Outline IlIe moin mile stones achieved In reoching ond gros I In the Infants IIIsI__ 0 DISCUSS rote-lImI g vonobles IlIoI offeo oleVelopmenI of I Ing and grospIng ' DeSCIIbe eoIly I. 0 Discuss monuol osyfm meI'IIes found In I‘he IIs yeoIr ond look oI IlIe ,__ reloIIonslIIp Io loIeI hond preference ’ '3‘? ’5 l0? Infant Motor Development lthough walking is often considered the primary motor milestone -. Aim an infant’s development, the ability to grasp and manipulate objects is of great significance in everyday life, as these skills are needed in order to function on a daily basis. For example, using a knife and fork, spoon, or chopsticks for eating requires manual dexterity that is often not developed until well into childhood. The foundation for these complex manipulative skills is established in the first year of life. The infant appears to have few reaching and grasping skills at birth, and it is nearly six months before he or she can competently reach and grasp an object. In this chapter, the processes that lead to the develop- ment of prehension are described, along with a discussion of manual asymmetries and whether these are linked to the later development of handedness. EE'” a“: jpt' EEI‘ Prehension, or manual control, can be divided into reaching and grasping. Reaching is concerned with moving the hand from its initial location to the target location, whereas grasping requires the shaping of the hand around the object. These two types of motor behavior are thought to be very different. For example, in his visuomotor channels hypothesis, Jeannerod (1981, 1984, 1996) described two distinct or parallel phases of prehension. The first phase is considered the trans- port phase, in which the hand is transported to the target area, and requires the greatest amount of time (70—80% of the total movement time). The second, or grasp, phase of the movement occurs when the hand is shaped and placed in the appropriate orientation to facilitate the gripping of the object. J eannerod’s model has been very influential in shaping research on reaching and “grasping. The reaching and grasping components of prehension are thought to be controlled by separate neuromotor mechanisms (e. g., J eannerod, 1988; Sugden & Keogh, 1990; von Hofsten, 1986). Jeannerod (1996) argued that there must be different structures and modes of action for the gross motor reaching action compared with the fine motor manipulation phase. Whereas reaching is a more primitive behavior utilizing proximal arm joints, grasping requires distal joints for the precise finger and hand movements that have evolved in primates and humans. Jeannerod provides neurophysiological evidence that these two action components can be dissociated through lesioning of the motor and parietal cortex. Although they are parallel processes, the two components of prehen- sion share a common time course. That is, although the molding of the hand' 1n preparation for the grip is part of the manipulation component, this occurs during the reaching or transportation phase. Therefore, as the two components are functionally interrelated, it is artificial to isolate them (J eannerod, 1996). Despite this, for ease of presentation, most texts on reaching and grasping have separated these two com- ponents, as does this one. This separation is particularly useful when Monuol Control 103 one is investigating prehension in infancy, as reaching and grasping emerge at different times in the infant’s first year. ReachingIntheFlrstFewMonths _ . , , Early reaching movements occur prior to what is usually termed suc- cessful reaching, that is, when the infant can reach and grasp an object of interest. This early reaching behavior in newborn infants is often termed prereaching (Trevarthen, 1982). Researchers have been interested in studying reaching in the new— born for several reasons. One is to understand the developmental profile of reaching behavior. This type of research was popular in the first half of the 20th century when the maturational theorists mapped the stages of motor development. Another issue that has been of considerable interest to researchers is whether these reach- ing movements are spontaneous or coincidental or whether they can be classed as “intentional.” This issue can be linked back to early theorists such as Piaget (1953), who argued that infants had no intent (and no intelligence) in the first few months and therefore that any movements that appeared intentional were simply coincidental. This has been investi- gated in the newborn infant with two main types of studies, those on hand-to-mouth actions and those on newborns’ reaching responses to an object. Hond-thouth Movements Reaching movements are present pre- natally, as movements of the hand to the mouth have been identified in the fetus as early as 10 weeks gestational age (de Vries et al., 1982). De Vries and colleagues used ultrasound to investigate these types of movements. This procedure allowed the researchers to investigate the infants’ hands in relation to their mouth. As shown in figure 5.1, the hands are well formed by 12 weeks gestational age, and their posi- tion can be easily tracked. Given the level of brain development at this early stage, it is clear that these first movements would not be intentional but would be cofisidered spontaneous movements. Figure 5.1 The honols can be cleorly seen in relo- ’Hand—to-mouth movements continue tion to the mouth in this ultrasound of o lQ-week-old after birth (ROChat, 1993), and according tetus. to a purely reflexive perspective (e.g., Piaget, l 04 Infant Motor Development 1953; Wyke, 1975), these reaching movements are unintentional and uncoordinated. That is, infants do not develop intent, in particular, eye—hand coordination, until several months later. However, there is now considerable evidence suggesting that this may not be the case. For example, Butterworth and Hopkins (1988) used frame-by—frame Video analysis to demonstrate that newborn infants-have some form of hand-to—mouth coordination. They observed instances in which new— born infants opened their mouths as the hand approached the face, suggesting that newborn reaching movements may have intent. To understand the function of this early hand—mouth coordination, Rochat and colleagues (1988) examined the relationship of this response to sucrose delivery to the mouth. They found significant increases in the number and duration of hand-to-mouth contacts, compared to baseline recordings, with the delivery of a sucrose solution to the mouth. Duration of hand-to—mouth contact almost doubled with the introduction of the sucrose solution into the mouth. This was evidence that this relationship can be an active, coordinated response in the newborn, and not just a chance occurrence. Rochat and colleagues suggested that the introduction of the sucrose solution initiated the suckling mechanism and that the hand may provide something for the infant to suck on once this sucking or feeding system is engaged. In the following few months, the functional role of hand—mouth coordination changes considerably. At around two months of age, once infants can grasp an object placed in their hand, they attempt to bring the object to the mouth for oral/haptic exploration. By four to five months, infants tend to bring the object within their field of Vision to look at it before drawing it to the mouth (Rochat, 1993). Oral exploration, as shown in figure 5.2, continues to play an important role in the infant’s discovery of his or her environment during the first year. Reaching to an Obiect Early researchers such as Trevar- than (1974, cited in von Hofsten, 1986) and Bower and colleagues (1970) found evidence ffir a coor- dinated reaching movement in the newborn. Bower and colleagues’ evi- dence was based on Video analysis Figure 5.2 Infants enioy oral exploration of obiects. of neonates (less than a month old) Manual Control reaching for objects. The authors investigated both orientation of the arm toward the object and intent. Their findings provided evidence that, given the appropriate postural support, newborn infants can produce surprisingly efficient prereaching movements. One of the “landmark” studies of reaching in infants was carried out by Claes von Hofsten (1982) using a two—camera system to plot the course of hand movements in young infants (see figure 5.3). He inves- ' tigated the arm movements of 14 newborn infants aged between five and nine days. The infants were strapped into a chair that provided the appropriate postural control, and both arm and eye movements were observed as a brightly colored ball of yarn was moved along a horizontal path in front of them. Von Hofsten (1982) argued that many earlier studies failed to find evidence for voluntary reaching because they did not take into account the orienting response. That is, when a new or interesting object or event is presented, there is a reduction in motor activity as the infant attends to this novel stimulus. By recording eye move- ments as well as hand movements, von Hofsten could determine the number of reaching movements produced when the infant fixated on the object, in contrast to movements that occurred when the infant was not looking at the object. As predicted, there were significantly more movements when the object was absent rather than present, supporting the hypothesis relating to the orienting response. How- ever, there were also significantly more forward-extended movements when the infants were fixating on the object than when they were not, implying some intentional relationship between the infant and the Figure 5.3 The experimental setup used by von Hofsten (1982), showing the two cameras and the View of the infant trom each camera. Adapted from von Hofsten, 1982. 105 toe tntont Motor Development object. A second experiment demonstrated that when the infant was fixating on the object, the arm movements were aimed more toward the object compared with when there was no fixation. These results provided compelling evidence to suggest that the newborn has some form of prefunctional eye—hand coordination at birth. Von Hofsten suggested that this may serve as an attentional mechanism rather than manipulative behavior at this early age. Newborn reaching is considered to be similar to the first transport phase found in adults (von Hofsten, 1993). The arm movement appears ” to be monitored by proprioception and involves the extension of the arm with the fingers extending. There is, however, no flexion of the hand or zooming in on the target; because of a synergistic coupling of the arm and hand, the newborn infant cannot grasp objects. Furthermore, there appears to be a considerable amount of muscle coactivity in these early arm movements (Hadders-Algra et al., 1992). Using electromyography (EMG) to monitor the muscle activity, Hadders—Algra and colleagues found that this coactivation appeared to continue up to around three months of age, with coactivation of the antagonists initially present more than 70% of the time. However, there were substantial changes in the nature of the EMG activity after three months of age. The phasic muscle activity became shorter, the amplitude was attenuated, and there was a reduction in the tonic background activity. These changes were thought to be due to a decrease in the co-contraction of antagonist muscle groups with a corresponding increase in reciprocal activation (Hadders-Algra et al., 1992). It was suggested that as a result of spinal and supraspinal reorganization, there was a reduction in the motor unit sensitivity that would account for the observed changes in EMG activity. Spencer and Thelen (2000) investigated this further by examining the changes in muscle activity thaf‘toccurred during the transition from early prereaching movements to successful reaching. They measured the muscle activity in the biceps, triceps, anterior deltoid, and trapezius muscles in four infants presented with a toy at their midline. Video analysis was used to determine where the infants moved their arms in relation to their body and the toy. Although the authors found no evidence of the co—contraction between the muscle groups that was observed by Hadders-Algra and colleagues (1992), they found that different muscle groups were involved in prereach— ing as opposed to reaching. Movements prior to successful reaching involved primarily either the biceps or the triceps muscle. The inves— tigators suggested that these simple movements, which flex the elbow and shoulder joints, are useful in guiding the hand to and away from the mouth. As described earlier, these are common reaching move- ments found in the first few months. Once successful reaching was achieved, however, the deltoid and trapezius muscles demonstrated increased activity. These muscles are important for guiding the arm to objects in the midline. Manual Control 107 ammmgtswniaem Successful reaching, that is, successfully transporting the hand so that it makes contact with an object, usually occurs around three to four months of age (von Hofsten & Lindhagen, 1979). Unlike the smooth, controlled reaches of an adult, these first successful movements appear jerky, with multipeaked speed—time trajectories (Fetters & Todd, 1987) and poor control of the hand. These early movements are composed of a series of accelerations and decelerations described by von Hofsten and Lindhagen as multiple movement units. As the infant becomes older and more proficient at reaching, the number of movement units made in each reaching movement declines. The Role of Sensory Information Mathew and Cook (1990) carried out an extensive kinematic analysis of reaching movements on three groups of infants with mean ages of 4.5, 6, and 7.5 months. With increasing age, the infants became more successful with their reaches, although the youngest group was already successful in making contact with the object 71% of the time. There were also improvements in the time taken to achieve the reach, as well as a simplification of the movement pattern indicated by a smoothing of the speed profile, with a more direct path of the hand to the target. Mathew and Cook’s detailed analysis of the movement curves provided evidence to suggest that there was continuous correction of the move- ment as a result of sensory feedback, although they did not investigate what type of feedback was being used to correct the hand trajectory. In adults, both vision and proprioception are essential for accurate reaching (J eannerod, 1 996). Propfioception provides positional informa- tion on the relative joint angles, whereas vision is required to provide information on the position of the limb in relation to the target. An essential part of this mechanism appears to be the vision of the limb prior to the movement. However, the findings from several studies have suggested that vision of the hand is not essential for successful reach- ing in infants. For example, Clifton and colleagues (1994) developed a testing procedure in which a toy was presented to the infants in dark- ness so they could not see their arm or hand. The infants could identify the toy’s location either through the use of glow-in-the-dark paint or through sound (from a noisy toy), or both. In all these conditions, the reach was unaffected by the loss of vision of the hand. This suggests that proprioception may play a more significant role in reaching by infants compared with adults. The Role of Posture Woollacott (1993) argued that before prehension can be established, a preparatory or postural stage of development is necessary to provide lO8 lntant Motor Development Figure 5.4 An illustration of the tour ditterent postural a stabilizing framework for the appropriate manipulative skills. It has consistently been demonstrated that if young infants are placed in the appropriate posture or given appropriate support, they are capable of reaching and grasping at a much earlier age than generally acknowl- edged. The study by von Hofsten (1982) presented earlier is a good example. Another example is provided by a study conducted by Savels— bergh and van der Kamp (1994). They found that infants aged between 12 and 19 weeks produced more reaching movements in the vertical position compared to the supine position. The authors attributed this difference to a greater difficulty in producing the appropriate force to compensate for the gravitational torque generated in the supine position as opposed to the vertical position. That is, frequency of occurrence not only is dependent on the infant’s own physical constraints but is also related to external environmental influences. Good head control is considered a necessary precursor for the development of reaching. In a longitudinal study examining the devel- opment of reaching in four infants, Thelen and Spencer (1998) noted that successful reaching was achieved in all infants several weeks after they had produced good head control. Rochat (1992) pointed out the importance of postural control in understanding unimanual and bimanual reaching. He allocated 32 infants aged five to eight months into two groups, those who either had or had not achieved independent sitting. Infant reaching was videotaped while the infant was in one of four different postures, namely sitting, sitting reclined, supported upright slightly prone, and supine, as illustrated in figure 5.4. Rochat provided evidence for a trend from sym- metrical and synergistic bimanual reach- ing to asymmetrical, unimanual reaching. Howeveivthere were distinct differences in the reach structure and hand prepara- tion for the two groups of infants. When the infants had stable postural control while sitting (i.e., the group that could sit independently), they were more likely to reach with one hand in all four postures. This result was interpreted according to Bernstein’s principle of degrees of free— dom. When infants have not mastered postural control, they tend to reduce the degrees of freedom by forming synergies ’ between the two arms. Once stability is mastered, these synergies are no longer needed and unimanual arm ,rnovements g“ Reclined Supine conditions used by Rochat (V992) to examine the intlu- become more prevalent. ence of posture on reaching control. An important mileStOIle in the dCVCIOp- Reprinted from Rochat, 1992. ment of reaching skills is the ability to sit Manual Control l 09 independently, which occurs at around six months of age. As shown in figure 5.5, once the infant has achieved a stable sitting posture, she is capable of using both hands to manipulate a toy. The Role of Previous Experience Another important factor is the infant’s previous experience. Thelen and colleagues (1993) argued that when young infants attempt to reach for an object they desire, they need to adapt their “current ongo- ing spontaneous and intentional movements to the specific, new task of reaching and grasping” (p. 1059). This emphasizes the importance of the individual’s ,, early experience, not just with reaching movements , j 3 but also with spontaneous movements. Considering . i that arm movements are evident in the 10—Week-old fetus, infants already have a long history of arm if F‘ 5 5 Th b’l’ t , | t movements before they achieve successful reach— 'gur§- '-. e O my 0 mon'pgge ing. Recent research has demonstrated that early g» on oblect IS enhanced by 0 stable sntlng , . . , :- posture. experience can affect the emergence and fac111tat10n of early Infant reachlng (e. g., Lobo et al., 2004; Thelen 81 Spencer, 1998). Thelen and colleagues (1993) examined rate-limiting variables in the development of reaching in a longitudinal study involving four infants. Reaching movements were examined at several different stages, namely, two weeks before the infants achieved successfu...
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