Oct 25, 1978 - infants come to master grasping moving objects was studied in the present work under .... 90% of the possible times), but at the higher speeds the number of. TABLE. 1 ..... could go straight for it without any intermediate stops.
JOURNAL
OF EXPERIMENTAL
Observations
CHILD
PSYCHOLOGY
28, 158-173 (1979)
on the Development Moving Objects
of Reaching for
CLAES VON HOFSTEN AND KARIN LINDHAGEN University
of Uppsala,
Sweden
How infants come to master reaching for moving objects was studied in a situation where the distance to and the velocity of the moving object varied. Eleven infants participated in the study. They were from 12 to 24 weeks old at the first session, were seen at 3-week intervals until 30 weeks old, and were finally seen once at 36 weeks old. The following behaviors were observed from video recordings: frequency of fixated and followed motions, latency to first goaldirected behavior, type of goal-directed behaviors, and type of reaches. It was found that by the time the infant masters reaching for stationary objects he will also successfully reach for moving ones. Eighteen-week-old infants caught the object as it moved at 30 cm/set. The results suggest a basic human capacity to time-coordinate one’s behavior with external events, and to foresee in one’s actions future positions of moving objects.
Visually directed reaching is one of the most prominent developments during the infant’s first half-year. This achievement depends on the development of coordinated perception and mobility, and it marks the first step toward mastery over objects and tools. In an everchanging environment an important aspect of this development is how the infant comes to grips with moving objects. Although the development of reaching is a classical topic in developmental psychology, it has so far been concerned only with stationary objects (Halverson, 1931; Piaget, 1953; White, Castle, & Held, 1964; Bruner & Koslowsky, 1972). The present study is an attempt to bridge this gap of knowledge. Grasping a moving object is much more complex than grasping a stationary one. First, one must perceive not only spatially but also in terms We wish to thank the parents and children for their helpful cooperation and their enthusiasm in the laboratory, and the nurses at Bamavlrdscentralen, Hildur Ottelinsgatan, Uppsala, for their cooperation in recruiting subjects. Funds for this investigation were provided by grants to Claes von Hofsten from the Swedish Council for Research in the Humanities and Social Sciences. Requests for reprints should be sent to Claes von Hofsten, Department of Psychology, Box 227, S-751 04, Uppsala, Sweden. 158 0022-0%5/79/040158-16$02.00/O Copyright @ 1979 by Academic Press. Inc. All rights of reproduction in any form reserved.
DEVELOPMENT
OF REACHING
FOR
MOVING
OBJECTS
159
of motion. Second, hand motion must be coordinated with object motion. If the hand starts moving too late, it will probably miss the object. Finally, the hand will ideally be aimed at the point where it will meet the object rather than the point where the object is seen when the reach is initiated. Thus, one must predict the object’s future location. These three capacities are all well developed in normal adults. The timing and prediction of position involved in, for instance, skilled tennis is a good example. The question is when and how these basic perceptuomotor functions develop. Catching moving targets is biologically fundamental. For example, it is critically important to any predatory animal, however primitive. Conceivably, this complex capacity might at least partly build on prewired components in the human infant. Detecting and responding to motion and change is clearly present in very young human infants. Neonates smoothly track a moving series of vertical stripes (Brazelton, Scholl, & Robey, 1966), and they have also been found to suppress their nonnutritive sucking in response to an intermittent moving visual stimulus (Haith. 1966). Visual motion thus seems to be a powerful stimulus for young infants, who are commonly described as being captured by it (Tronick & Clanton, 1971; Volkmann & Dobson, 1976). The first evidence of predictive reactions to moving objects appears between 6 and 8 weeks (White et al., 1964), when the motion of the eyes begins to anticipate stimulus motion rather than lagging behind (as in peripheral pursuit). Eight-week-old infants’ eye movements also anticipate the reappearance of an object disappearing behind a screen (Bower, Broughton, & Moore, 1971). It seems, then, that at the time of his first reach, at 4-5 months (White et al., 1964) the infant can to a degree predict future locations of a moving object, To successfully grasp it, however, the infant must also coordinate predictive ability with motor function. How infants come to master grasping moving objects was studied in the present work under variable distance to and velocity of the moving object. METHOD
Subjects Five male and six female, healthy, full-term infants completed the longitudinal program. They were 12 to 24 weeks old at the first session and were seen at 3-week intervals until age 30 weeks, and finally once at 36 weeks (all ages -+ 1 week). Three infants were followed from 12 weeks, 1 from 15 weeks, 2 from 18 weeks, 2 from 21 weeks, and 3 from 24 weeks. An additional 13 females and 8 males were seen for one or more sessions but were premature (N = 3), or did not take part in the longitudinal study because of either time problems (N = 9) or fussing at more than one session (N = 9).
160
VON
HOFSTEN
AND
LINDHAGEN
Most parents were contacted through a child health center serving a predominantly middle-class and student area in Uppsala; a few were the authors’ colleagues or students. Parents were paid 20 SwCr (about $5) for each session. Apparatus
The infant sat in a semireclining seat on a table to which the motionproducing device was attached. To allow three-dimensional analysis of hand movements, the situation was recorded by two SONY AVC-3250 CE television cameras, one above and one in front of the infant, as illustrated in Fig. 1. The two pictures were fed via a mixer into a SONY AV-3670 CE videorecorder. The resulting picture is shown in Fig. 2. The stimulus object was the front part of a Rapala Deep Diver wobbler, hook and spoon removed, its head end pointing toward the infant. It was yellow, orange, and golden, with small yellow and black eyes in front and with tufts of red yarn stuck into the sides. Its diameter was 1.9 cm (3.5 cm including yarn tufts) and its length was 3.9 cm. This object proved very attractive to most infants, and they often reached for it every time it passed in front of them. The object was plugged into the end of a 55-cm-long horizontal metal rod, attached via a felt coupling to the perpendicular shaft of an electric motor with variable speed and direction. The rod was also rotated around its own axis by a rubber ring around it, which rested on a horizontal surface (See Fig. 3). Thus the object moved in a horizontal circular path of approximately 115 cm diameter while at the same time rotating around itself, and stopped moving when it was grasped. The object was placed at the infant’s nose height by moving up or down the board on which the motor with the rod was attached to the table. The distance was adjusted by moving the infant seat along the table.
u
TV 1
FIG.
TV 2 c
1.
Experimental
situation.
DEVELOPMENT
FIG.
Experimental
OF REACHING
FOR MOVING
OBJECTS
161
2. Resulting picture on TV screen.
Conditions
The object was placed about 11 or 16 cm from the infant’s eyes. It moved at 3.4, 15, or 30 cm/set, or rested right in front of the infant. Thus, there were eight experimental conditions. Each infant received all conditions, except the lZweek-olds, who only received the 1 l-cm conditions. In each condition a maximum of three reaches were recorded.
WOBBLER
RUBBER RING COUPLING
FIG. 3. Cross section of motor and rotating rod.
162
VON
HOFSTEN
AND
LINDHAGEN
Procedure
The infant was brought into the laboratory by a parent (or parents) who sat behind or beside the child during testing. When alert and quiet, the infant was placed by the parent in the infant seat, and the height and distance of the object were adjusted. For each motion condition the object was placed randomly to one side and was then moved back and forth until the infant grasped it, whereupon it was gently removed and again placed to one side. This was repeated until three reaches were obtained, or until the object had passed in front of the child at least six times. If the child did not fixate and follow the object with his eyes, the experimenter (in front of the infant) tried to attract the infants’ attention to the object, e.g., by gently shaking it. If the infant’s hands remained raised into the visual field after the object passed, it was stopped until the hands were again in resting position. At the end of each motion condition the infant was given time to reach for the object while it was standing still in front, until three no-motion reaches had been obtained at each distance. The motion conditions were run consecutively in a prearranged random order. Only if the infant fussed or cried was a pause made and the parent was asked to try to soothe the infant. Pacifiers were discouraged, but were allowed if nothing else would keep the child quiet in the seat. Sometimes the infant remained passive during the first one or two conditions but started to reach for the object regularly from then on. The passive conditions were then rerun after the last motion condition. Typically, however, the infant started trying to grasp the object immediately after being placed in the seat. The whole session took lo-20 min. RESULTS Coding and Reliability
For each infant and condition the following measures were extracted from the video recordings: frequency of fixated and followed motions, latency to first goal-directed behavior, type of goal-directed behaviors, and type of reaches (further description of measures below). The first few recordings were classified by both authors together to establish criteria for classification. Six recordings were then classified independently by the two authors, and interobserver reliability was found to be .80 or better for the above-mentioned measures. Intraobserver reliability for the main coder (KL), measured for two sessions coded twice at an interval of several months, was above .90 for all measures. Furthermore, all recordings were reviewed for reclassification of type of grasp. The reliability was .99 for the classification of reach versus follow, and .94 for the designation of a reach as grasp, touch, or miss, as calculated for the 10 sessions on 3 randomly chosen tapes.
DEVELOPMENT
Attention-Catching
OF
Properties
REACHING
FOR
MOVING
of the Moving
163
OBJECTS
Object
To assess the moving object’s ability to capture the infants’ attention, for each condition the number of times the object was fixated and followed by the eyes was counted, starting with the first instance of following. In the overwhelming majority of cases the infants fixated and followed the object every time it passed, once they had caught sight of it (see Table 1). There was little difference between conditions. The youngest infants tended to fixate less often, but only the difference between 12- and 24-week-olds was significant. (Chi-square tests (Siegel, 1956) were performed on all differences found between age groups or conditions. All differences discussed here and below were significant at the .05 level or better, unless otherwise stated. Note also that unless it is stated that 12-week-olds are included, the results are only for 15 to 36-week-olds.) Elicitation ofReaching and Other Goal-Directed Behaviors by the Moving Object The infants showed great interest in the object not only by looking but also by reaching for it or by making other goal-directed motions. They typically made their first attempt the first time they fixated and followed the object. There were clear differences between speed conditions, however. The slowest motion elicited goal-directed behavior on the first fixated motion more than 90% of the time, the medium speed about 70% of the time, and the fastest speed only about 50% of the time. There was no difference between the two distances. (Since this was nearly always true and there were no interactions between distance and speed or age, unless otherwise stated data were pooled over distances.) The total number of goal-directed behaviors is shown in Fig. 4. The slowest speed elicited a large number of attempts at all ages (over 90% of the possible times), but at the higher speeds the number of TABLE NUMBER OF FIXATED AND FROM AND INCLUDING
1
FOLLOWED MOTIONS THE FIRST FIXATED Age
Number of fixated motions
AMONG THE FIRST SIX MOTIONS, ONE, AT DIFFERENT AGES
(weeks)
12
15
18
21
24
27
30
36
0 l-3
4 3
2 3
2 4
1 5
0 0
0 6
0 3
0 4
9 28
4-6 Total number of instances
2 9
12 17
30 36
32 38
66 66
54 60
62 65
5.5 59
313 350
Total
164
VON HOFSTEN
AND LINDHAGEN
P.9.
5.
.I
I
15
18
21
24
27
30
&
AGE (WEEKS)
FIG. 4. Number of goal-directed behaviors at each speed and age, expressed as proportion of maximum number of possible behaviors, i.e., three per experimental condition.
goal-directed activities increased with age, faster for the medium speed (from about 50% at 15 weeks to about 90% by 24 weeks), and more slowly for the fastest speed (from about 20% at 15 weeks to 85% by 30 weeks). Comment. The object attracted the infants’ attention in all conditions. Goal-directed activities were usually elicited already on the first fixated motion, and they were more common the slower the speed. Types of Goal-Directed
Behavior
The first three goal-directed motions in each condition were classified according to the following system of categories, based on Bruner and Koslowski’s (1972) classification of infants’ movements toward a stationary object, but modified for use with moving objects. Reaching (r). The infant reaches for the object and grasps or touches it, or misses it but arrests the hand in midair, with the arm stretched as if to try to touch the object. Following (f). The hand follows the object but is not stretched out far enough to touch it and is not arrested. Forward swiping (fs). Very fast ballistic movement forward/upward and then downward. Forward extension (fe). The hand is extended forward but does not follow the motion of the object sideways. Upward extension (ue). The arm and hand are lifted upward but not extended forward. Both upper and lower arm are moved. Hand lifting (hl). The upper arm and elbow rest on the body or seat, and the infant lifts the lower arm and hand, or only the hand. Hand opening (ho). The fisted hand opens. The arm is not lifted.
DEVELOPMENT
OF
REACHING
FOR
MOVING
165
OBJECTS
Backward extension (be). The arm is moved backward or backward and upward. Midline breaking (mb). The hand is pulled away from the midline after having been engaged in clasping or other midline activity. Adduction to midline (am). The hand is brought from a position at the side to the midline close to the body. Hovering (h). The hand hovers within a few centimeters of the object but does not touch the object; there is no grasping. Other (0). The above were the categories foreseen before classification. The only other kind of motion encountered among all subjects consisted of bringing the hand back toward the body, but not all the way to the midline, from a backward or sideways extended position. Table 2 shows the distribution of the various types of goal-directed behaviors. The most common was reaching. On over half of the possible occasions the infants reached for the object, and reaches accounted for over 60% of the goal-directed behaviors made. It occurred four times as often as the second most usual category, which was following. Handlifting, forward extension, and upward extension were also rather common. There were few swipes and the remaining types were rarely seen. The proportion of reaching increased with age from less than 10% at 15 weeks to 80% at 36 weeks, whereas the proportion of following remained fairly constant at U-20% across the age groups. The number of reaches was higher the slower the speed; the reverse was true for following. For the slowest speed, the proportion of reaches TABLE 2 TYPES OF GOAL-DIRECTED AGE GROUPS (NUMBER
DISTRIBUTION OF THE VARIOUS SPEED FOR DIFFERENT
Type Age (weeks)
Speed (cmlsec)
12-18
3.4 15 30 3.4 15 30 3.4 15 30 3.4 15 30 All
21-27
30-36
12-36
12-36 a Abbreviations
r 28 10 9 119 77 39 113 87 51 260 174 99 533 explained
f
fs
4 3 88 4 0 5 2 29 1 39 2 0 2 17 0 32 0 9 7 54 9 75 2 138 18 in text.
of goal-directed ue
fe
hl
3 5 5 2 230 1 3 8 3 9 1 6 9122 410202 1 1 4 1 3 2 2 12 6 7 15 10 9 14 17 7 25 34 23 54 61
BEHAVIORS OF INSTANCES)
AT EACH
activitya ho 0 0 1
1 1 0 2 3 2 7
be 3 2 0 1 10 11 0 1 0 4 4 1 9
mb 3 3 1 0
0 0 0 3 3 2 8
am 2 100 0 4 001 100 0 0 0 6 1 1 8
h
o
Total
2
0
0 0
0 0
0 0 0 2 0 0 2
0 0 0 0 1 0 1
58 39 26 145 138 119 122 112 103 325 289 248 862
166
VON HOFSTEN
AND LINDHAGEN
hit its maximum at 24 weeks, but for the medium and fast speeds it continued to increase until 30 or 36 weeks. Comment. In sum, there were great differences among speeds, with more goal-directed behaviors, and among them more reaches but fewer followings, at lower than at higher speeds. This held for all age groups, but the difference was larger for the younger ages, there being fewer attempts at the medium and high speeds. Number
of Reaches
A maximum of three reaches per condition were counted, including reaches both among and after the first three goal-directed motions. There were 711 reaches distributed over age and condition as shown in Table 3. The number of reaches, expressed as proportions of the maximum number possible (three per condition), are shown in Fig. 5. The total number of reaches per infant across all conditions increased monotonically with age. There were more reaches the slower the speed. The slowest speed elicited 300 out of 342 possible reaches, the medium speed 238 reaches, and the fast speed 170 reaches, i.e., 88, 70, and 50%, respectively. There was also a difference between distances in this case at all ages and all speeds: there were more reaches toward the near than the far object (at 11 cm there were 395 out of 513 possible reaches, at 16 cm 313 reaches, i.e., 77 and 61%, respectively). Type of Contact
with Object
The reaches were classified into four categories: (1) grasps and holds the object, (2) grasps the object but lets go within about a second, (3) touches the object, and (4) misses. The number of grasps, especially “grasp and hold,” increased with age, the number of touches decreased with age, and the number of misses remained at the same low level (about 10%) across all age groups (See Fig. TABLE NUMBER
OF REACHES
3
AT EACH
AGE
AND
CONDITION
Age (weeks) Speed (cmhec)
Distance (cm)
3.4
11 16 11 16 11 16
15 30
Total Number of infants
12
15
18
21
24
27
30
36
Total
3
6 0 3 1 0 0 10 4
15 12 11 8 9 5 60 6
18 17 11 11 10 8 75 8
30 26 27 20 18 8 129 11
28 28 24 18 21 12 131 11
33 30 33 22 22 15 155 11
29 27 23 27 24 18 148 11
162 141 132 106 104 66 711
0 0 3 3
DEVELOPMENT
OF REACHING
FOR MOVING
OBJECTS
FIG. 5. Number of reaches at each speed and age, expressed as proportion number of possible reaches, i.e., three per experimental condition.
167
of maximum
6). The relative proportion of grasping and touching was about the same at all speeds with grasps almost twice as common. “Grasp and hold” occurred equally often at the different speeds, but more often at 11 than at 16 cm. “Grasp and let go” occurred more often at the slowest speed than at the medium or fast, with no difference between distances. Touches were equally common at all speeds, but occurred more often at the far than the near distance. Misses showed no systematic differences between conditions. There was no effect of practice in the experimental situation on success in grasping the object. Infants making their first visit at 18 weeks were as successful as infants making their second or third visit, and the
I5
I6
21
24
27
30
36
FIG. 6. Type of contact with object. Proportion of total number of reaches for “grasp,” “touch,” and “miss.”
168
VON HOFSTEN
AND LINDHAGEN
same held true for beginners as compared to experienced infants at 21 and 24 weeks. Commenr. The low number of misses in all conditions does not indicate that all conditions were equally easy, but rather that the infants did not even attempt to reach in the more difficult conditions. As described above, the total number of goal-directed motions was also lower in the faster conditions. The high number of “grasp and let go” at the slowest speed reflects the playful, often repeated grasping and letting go, which the infants would commonly engage in at the slowest speed, but which was not possible at the medium or fast speed. The grasps and touches were further classified as: (1) grasps directly, (2) touches, then grasps, (3) touches and keeps the hand near the object, so that it would have been possible to grasp the object, and (4) touches the object in passing without a chance to grasp it. Both kinds of grasps increased with age, direct grasping at first more slowly (See Table 4). “Touch then grasp” was more common than direct grasping, especially at ages 24-27 weeks. There were also differences between speeds. “Touch then grasp” was more common at the slowest speed than at the two faster speeds, whereas direct grasping was more common at the medium speed. “Touch in passing” decreased steeply from 89% of contacts at 15 weeks to 16% at 18 weeks. This kind of touch was more common the faster the speed, the reverse being true for touches with possibility of grasping. Comment. At the lowest speed the infants would often play with the object while it passed slowly from side to side, touching, grasping and letting go, etc., before finally grasping it and trying to mouth it. At the medium speed, the object moved too quickly to allow such playful manipulation. At the highest speed, there was little time to correct the motion path of the hand to achieve a perfect hit, and often the infant would stop the object by touching it with the side of the hand or with one finger and only then go on to grasp it. The touches with possibility of grasping at the TABLE TYPE
OF GRASP
OR TOUCH
AT DIFFERENT
4 SPEEDS
(NUMBER
OF INSTANCES)
Speed (cm/set) Type of grasp/touch
3.4
15
30
Total
Grasp Touch then grasp Touch, possible to grasp Touch in passing Total
41 47 6 42 136
115 166 41 48 370
71 103 42 54 270
227 316 89 144 776
DEVELOPMENT
OF REACHING
FOR MOVING
169
OBJECTS
lowest speed also often represented playful manipulation. In this condition the infants may have found repeated touching more interesting, perhaps more challenging, than grasping. Higher speeds precluded this kind of touching. Sex Differences There were clear sex differences. Girls were more likely than boys to attempt goal-directed behaviors and to reach for the object. When reaching they grasped the object more often than the boys, but this difference had disappeared in the oldest groups (see Table 5). There was no difference in proportion of misses. DISCUSSION
The ability to grasp moving objects is an early achievement. Indeed, by the time the infant masters reaching for stationary objects he will also successfully reach for moving ones. In the present study one 15week-old infant caught the object as it moved at 15 cm/set. At 18 weeks several infants caught it as it moved at 30 cm/set, at which speed in 1 set the object moves twice the length of the infant’s arm. To be able to catch it the infant has to start reaching for it while it is still out of reach. He has a TABLE SEX DIFFERENCES
5
IN NUMBER OF GOAL-DIRECTED AND TYPE OF CONTACT WITH OBJECT
BEHAVIORS, NUMBER OF REACHES, (NUMBER OF INSTANCES)
Female (a) Number of goal-directed Number made Max. number possible
behaviors (12-36 weeks) 518 585
Male
Significant*
350 468
Yes p < .ool
p < .ool
(b) Number of reaches (12-36 weeks) Number made Max. number possible
468 585
242 468
(c) Among Number Number Number Total
155 60 13 236
30 28 11 69
145 59 24 228
114 42 16 172
reaches 12-24 weeks of grasps of touches of misses
27-36 weeks Number of grasps Number of touches Number of misses Total * p < .05, x2 test.
Yes
Yes
p
/:’
l
Lz
t
2
FIG. 8. Two examples of reaches performed by a 26-week-old girl (Ea). Both reaches are right-hand ones. In the upper row the speed ofthe object was 30 cmisec and the reach was completed in 0.7 sec. In the lower row the speed Of the object was 3.4 cm/set and the reach was completed in I .9 sec. The trajectory of the hand and in the upper row the trajectory of the object are marked at each 1OSmsec interval.
---c----4-----+
X
DEVELOPMENT
OF REACHING
FOR MOVING
OBJECTS
173
REFERENCES Bower, T. G. R. Development in infancy. San Francisco: Freeman, 1974. Bower, T. G. R. Infant development. San Francisco: Freeman, 1977. Bower, T. G. R., Broughton, J., & Moore, M. K. Development of the object concept as manifested in changes in the tracking behavior of infants between 7 and 20 weeks of age. Journal of Experimental Child Psychology, 1971, 11, 182-193. Brazelton, T. B., Scholl, M. L., & Robey, J. S. Visual response in the newborn. Pediatrics, 1966, 37, 284-290. Bruner, J. S. Volition, skill, and tools. In L. J. Stone, T. H. Smith, and L. B. Murphy (Eds.), The competent infant. London: Tavistock, 1974. Bnmer, J. S., & Koslowski, B. Visually preadapted constituents of manipulating action. Perception, 1972, 1, 3-14. Gesell, A., & Thompson, H. Infant behavior: Its genesis and growth. New York: McGraw-Hill, 1934. Haith, M. M. The response of the human newborn to visual movement. Journal of Experimental Child Psychology, 1966, 3, 235-243. Halverson, H. M. Study of prehension in infants. Genetic Psychology Monographs, 1931, 10, 107-285. Piaget, J. The origin of intelligence in the child. London: Routledge & Kegan Paul, 1953. Siegel, S. Nonparametric statistics for the behavioral sciences. New York: McGraw-Hill, 1956. Tronick, E., & Clanton, C. Infant looking patterns. Vision Research, 1971, 11, 1479-1486. Volkmann, F. C., & Dobson, M. V. Infant responses of ocular fixation to moving stimuli. Journal of Experimental Child Psychology, 1976, 22, 86-99. White, B. L., Castle, P., & Held, R. Observations on the development of visua.lly directed reaching. Child Development. 1964, 35, 349-364. RECEIVED: March 21, 1978: REVISED: October 25, 1978.
