Infant Behavior & Development 32 (2009) 44–58

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Infant Behavior and Development

Seeing and touching: The role of sensory-motor experience onthe development of infant reaching

Daniela Corbetta a,∗, Winona Snapp-Childsb

a Department of Psychology, The University of Tennessee – Knoxville, United Statesb Integrative Neuroscience Program, Department of Health & Kinesiology, Purdue University, United States

a r t i c l e i n f o

Article history:Received 4 January 2006Received in revised form 27 August 2008Accepted 17 October 2008

Keywords:InfantsReachingGraspingVisionTouchSensory-motor experience

a b s t r a c t

Researchers agree that infants must learn from prior sensory-motor experiences to plan,perform, and fine-tune their actions to the environment. Yet, little is known about theactual influences of these experiences on the development of infants’ perception and action.This study investigated how repeated experiences of seeing, reaching for, touching, grasp-ing, and manipulating objects of same sizes and textures contributed to the refinementof subsequent object-oriented motor responses in 6–9-month old infants. In addition, tounderstand whether infants relied on vision, touch, or both to tailor their motor response toobjects, we analyzed the reach-to-grasp sequences. Results show that the youngest infantsdid not benefit from the repeated experiences. Seemingly stereotypical motor responsesappeared to interfere with the process of perceptual-motor mapping. The older infantsrelied more effectively on prior experience, on touch initially and then vision, to matchtheir motor responses to objects. Consistent with a dynamic systems approach, we inter-pret the observed developmental progression as a change in tensions between perceptionand action.

© 2008 Elsevier Inc. All rights reserved.

Object exploration plays a central role in the early development of perception, action, and memory. By seeing and touch-ing objects, by bringing them to the mouth, and by manipulating them, infants can learn about their physical properties,they can remember their specific characteristics, and use this newly acquired knowledge to plan future actions. Althoughthese sensory-motor experiences are considered crucial for early development, very little research has attempted to assesstheir impact on behavioral change. To what extent do object manipulations and explorations contribute to learning anddevelopment? Do they influence infants’ future decisions and action planning on objects? This research begins to addressthese questions in relation to the development of infant reaching.

The emergence of reaching in early infancy offers a unique window to investigate the impact of sensory-motor experienceon infant learning and development. When infants begin to reach for objects at around 4 months of age, their motor patternsare poorly adapted to the physical properties of objects. Infants, at first, have rudimentary grasping abilities (Halverson,1931; von Hofsten & Lindhagen, 1979), they swap at the toys (Thelen, Corbetta, Kamm, Spencer, Schneider & Zernicke, 1993),and cannot quite adjust their reaching response and hand configuration to accommodate the size, texture, or orientationof objects ahead of time (Corbetta, Thelen, & Johnson, 2000; Fagard, 2000; Fagard & Pezé, 1997; Lockman, Ashmead, &Bushnell, 1984; von Hofsten & Fazel-Zandy, 1984; Witherington, 2005). Nonetheless, once they begin to contact objects with

∗ Corresponding author at: Department of Psychology, The University of Tennessee, Austin Peay Building, Knoxville, TN 37996-0900, United States.Tel.: +1 865 974 3346; fax: +1 865 974 3330.

E-mail address: dcorbett@utk.edu (D. Corbetta).

0163-6383/$ – see front matter © 2008 Elsevier Inc. All rights reserved.doi:10.1016/j.infbeh.2008.10.004

D. Corbetta, W. Snapp-Childs / Infant Behavior & Development 32 (2009) 44–58 45

Fig. 1. (A) Chain of successive perceptions and actions contributing to increase and update the perception–action match between an actor and its envi-ronment. (B) Studies on reaching only investigated the relation between seeing and reaching. (C) Studies on grasping only examined the relation betweentouching and grasping.

their hands, they have the opportunity to explore, discover, and manipulate them in new ways. To what extent do these earlyinfant/object interactions influence the shaping of future actions?

One way to address this question is to track and study the evolution of reaching and grasping patterns over time as infantssee, reach for, touch, grasp and manipulate objects repeatedly. Indeed, any specific encounter with a particular object andthe sensory-motor experiences that ensue from such encounter has the potential to provide an opportunity for learning.In turn, this learning experience can be used to modify the organization of the next action on this particular object. Inadults, for example, it is easy to think how certain object manipulations following a reach can alter the form and planningof the next reaching response. Imagine that you are transferring grocery bags from the cart of the store to the trunk of yourcar. You begin by grasping the first paper bag from its edge with one hand. As you lift it, you feel that the bag is heavierthan you thought and that it may tear apart. To prevent loosing your groceries, you bring your second hand to secure yourhandling of the bag and move it safely to the trunk of your car. This example illustrates that our initial action prediction aboutan object (often based on visual information) can change after one touches and/or manipulates that object. It also showsthat vision and touch can complement one another when in contact with the object. But more importantly, this exampleillustrates that as adults, we learn from these sensory-motor experiences, we remember their outcome, and we rely on themto plan and modify our next action. Imagine that you are preparing to move the second paper bag to your trunk. Because youremember your prior experience at manipulating the first bag, you decide, this time, to grasp the second bag securely with twohands.

This example points to a fundamental process of change; that development and learning may simply be related to the rep-etition of sensory-motor experiences involving seeing, reaching, touching, grasping, and manipulating objects successively.This chain of perceptions and actions can contribute to increase and update the perception–action match between an actorand its environment (see Fig. 1A). One question central to this research is to understand when and how such updating processtakes place in infancy. Do prior perceptual-motor experiences always influence infants’ next future decisions to reach? Or,does it take many attempts for such change to take place? If it does take many attempts, what is the process underlyingsuch protracted change? And at which age do infants begin taking advantage of prior object manipulations to modify theiractions? The answer to these questions is poorly known because prior studies on reaching and grasping focused almost exclu-sively on pieces and parts of the perceptual-motor cycle presented in Fig. 1A. Studies on reaching, for instance, investigatedmainly the relation between seeing and reaching to capture infants’ object-oriented anticipatory abilities (Fig. 1B). Studieson grasping focused more specifically on the relation between touch and grasp to assess infants’ prehensile skills (Fig. 1C).From these studies, two distinct findings were reported in relation to infants’ ability to match their motor response to objects.Reaching studies consistently found that infants begin orienting and pre-shaping their motor response as a function of thevisually perceived properties of objects at around 7–8 months of age (Corbetta et al., 2000; Fagard, 2000; Fagard & Pezé,1997; Lockman et al., 1984; Piérault-Le Bonniec, 1985; von Hofsten & Fazel-Zandy, 1984; von Hofsten & Rönnqvist, 1988;Witherington, 2005). Some grasping studies, in contrast, reported that infants are capable of shaping their grip configurationto match the size of the objects as early as 4 months old (Barrett, Traupman, & Needham, 2008; Newell, McDonald, Scully,& Baillargeon, 1989; Siddiqui, 1995). This developmental decalage between grasping and reaching skills is quite puzzling. If

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infants can adapt their grip configuration to objects’ size from the age of 4 months, why does it take another 3–4 months forthem to develop and transfer this ability to reaching?

Newell et al. (1989) suggested an answer to this developmental decalage by stressing the type of sensory informationthat infants use to adapt their motor response to objects. These authors explained that at an early age, infants are able toadapt their grip configuration to objects because, similarly to our grocery bag example, once in contact with the objectsthey can use haptic information to supplement visual information and adjust their hand on the object. During the reachingphase, however, which precedes object contact, young infants do not have access to haptic information; they fail makingappropriate reaching adjustments because they cannot anticipate their response based on vision alone until much later. Butagain, following Newell et al.’s explanation, why does the haptic experience take so long to transfer to reaching? Don’t younginfants learn from these object encounters to improve their reaching before the age of 8 months?

We think that Newell et al. (1989) have provided a partial account of the observed changes in object-directed behaviorsoccurring between the 4 and 8 months old period. Newell et al.’s explanation only stresses the perceptual side of reachingand grasping and neglects changes that may be occurring on the motor side. By stressing a change in perceptual modalitiesalone, these authors make the assumption that young infants already have a wide range of motor responses available toaccommodate perceptual information; all they lack to make the same adjustments in reaching are anticipatory abilities.But, studies that concentrated more specifically on the motor side of the development of reaching and grasping presenta very different developmental picture—one that puts more emphasis on limited motor flexibility and lack of arm controlduring the early stages of reaching and grasping (Clearfield & Thelen, 2001; Corbetta et al., 2000; Fagard, 2000; Halverson,1931; Thelen et al., 1993; von Hofsten, 1979, 1991; von Hofsten & Lindhagen, 1979). These studies seem to agree that object-related perceptual-motor matching during the early months following the onset of reaching and grasping is quite imperfect.Halverson (1931), for example, who provided one of the first detailed longitudinal descriptions of the development of graspingin infancy, reported undifferentiated “primitive squeezes” or palmar grasps during the weeks following reach onset, and foundthat the emergence of fine, 2 or 3 digit, grip configurations comparable to those described by Newell et al. (1989) in 4-monthold infants did not occur until much later in the first year of life. This lack of adjustment in early grip configuration was alsodocumented in some more recent studies (Corbetta et al., 2000; Fagard & Jacquet, 1996; Fagard, 2000). In particular, Corbettaet al. (2000) who used objects of different size and texture to assess early reaching and grasping abilities in 5–9-month oldscould not find evidence that young infants relied on haptic information to shape their grip configurations. From that report,only the 8- and 9-month olds were beginning to demonstrate such ability. Furthermore, these authors analyzed the sequenceof patterns of reaching and grasping trial after trial. They discovered that the response patterns of the young infants wereconstrained by systemic one- or two-handed motor tendencies that seemed to conflict with the process of perceptual-motormapping. Even though all infants were given time to touch and explore objects after every trial, the young ones seemed tocontinue to reach for and grasp objects in the same stereotypical manner on the next trials, as they did on the prior trials,regardless of object size or texture. In other words, if we behaved like those infants in our grocery example, we would havelearned nothing from grasping and handling the first bag and would have continued to reach for the second bag as we didfor the first one – with one hand – at the risk of losing the groceries again. On the basis of these observations, Corbetta etal. (2000) concluded that early developmental changes in perception–action matching were not related to a shift in the useof perceptual modalities as suggested by Newell et al. (1989), but rather, were linked to the dissipation of early one- andtwo-handed motor constraints. As these early intrinsic motor tendencies dissolved at around 8 or 9 months of age, infantsbecame better at matching both their reaching and grasping responses to the “seen” and “felt” properties of objects.

.The purpose of this study is to further address the developmental puzzles and controversies brought about by the studiesreviewed above by investigating how repeated experiences of seeing, reaching for, touching, grasping, and manipulatingobjects of same sizes and textures can influence the formation of subsequent motor patterns in 6–9-month old infants.One problem with the Corbetta et al. (2000) study is that infants were exposed to objects that were gradually increasingor decreasing in size over successive trials, thus, infants were never exposed to the same-sized object twice in a row. It ispossible that this progressive variation in object size has made it difficult for young infants to use knowledge from priorobject encounters to modify their reach for the next object. As a result, they may have responded with systemic one- andtwo-handed responses over the successive trials. A first goal of this study is to provide more of the same experience usingsame-sized objects to better assess whether young infants are capable of learning or taking advantage of prior grasping andobject manipulations to update and plan their next reaching response. Infants will be given time to explore and manipu-late each grasped object during each trial and subsequent analyses will examine how these experiences will contribute tomodify the reaching responses over time. A second goal is to gain more insights into the process relating to the observeddevelopmental decalage between reaching and grasping. By analyzing the reach-to-grasp sequences, as they succeed oneanother (as highlighted in Fig. 1A) rather than separately, we aim to establish whether infants begin to adjust their gripconfiguration first, before shaping their reaching response, or, do it later, or, adjust both reaching and grasping following asimilar developmental pattern. This reach-to-grasp analysis will inform us directly on whether infants rely on touch first,vision first, or both to make their object-oriented action decisions.

To address these goals, we used a method from the dynamic systems approach. We first probed the reaching responses of6–9-month old infants with different sized objects to determine their reaching tendencies (one- or two-handed). If infantsresponded consistently with two hands, we offered them identically sized small objects for several trials to see if they wouldshift from two- to one-handed reaching. Likewise, if during probing, they responded mainly with one hand, we offered themlarge objects to see if they would shift from one- to two-handed reaching. We analyzed the sequence of responses over time

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following two developmental time scales: the short time scale as infants responded to each successive object presentationwithin a same session, and the larger developmental time scale as infants of different ages responded to the same task. Theseanalyses directly informed us about infants’ learning or adaptive potentials at the different ages. In addition, to further assessto which extent infants relied on haptic information to grasp objects and used prior sensory-motor experience to make theirnext decision to reach, we also offered infants, as Corbetta et al. (2000) did, large pompons made of yarn. Large pomponslook as if they require two hands to be grasped, but once touched infant can find out that they can grab and hold them withone hand. With this condition we were able to examine further if infants could adapt their grip configuration on contactand if they were capable of using such manipulatory experience to modify their reach for the next pompon to one hand. Wefocused our analyses on the rate of object–response matching in reaching and grasping, pattern change between reachingand grasping, pattern change over the consecutive trials, and the type of object manipulation performed after grasping todetect whether infants were sensitive to the size and texture properties of objects.

1. Methods

1.1. Participants

Forty infants (20 girls and 20 boys), aged 6, 7, 8, and 9 months (±7 days) participated in this study. Each group had 10infants, with an equal number of boys and girls. An additional 14 infants were also tested, but were not included in thestudy because they lost interest in the task before the full protocol was completed. Infants were recruited from the GreaterLafayette area in Indiana by birth announcement records published in the local newspaper. All infants were born full-termwith no known physical or psychological impairments. Most infants were White. This research was approved by the PurdueUniversity Institutional Review Board.

1.2. Apparatus

The experimental apparatus consisted of a specially designed infant chair that provided stable support to the trunk andupper body while allowing free arm and leg movements (see Fig. 2). The chair was designed to fit infants 5–18 months oldand had a padded Velcro strap that secured the infants in the seat. A small pillow was placed on the top of the infant seatand provided a cushion for the infants’ heads. During testing, the chair was reclined 15 degrees from vertical.

All sessions were videotaped using two Sony Super High 8 Handycams that provided two diagonal side views of the frontof the infants. A Videonics Digital Video Mixer (FOCUS Enhancements, Inc., Campbell, CA) was used to merge and synchronizethe views from both cameras on the same screen; this provided a full view of the infants’ manual activities from both sidesat the same time.

Fig. 2. (A) Picture of a 6-month old infant in the infant seat looking and touching at a large object before grasping it. (B) Picture of the same infant holdinga large pompon with one hand.

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Three different sets of objects were used during experimentation: small solid balls, large solid balls, and large pompons.The small solid balls were 5 cm in diameter, varied only in color and could be easily grasped and held by the youngest infantswith one hand. The large solid balls were 13 cm in diameter, varied only in color and required two hands to be grasped (seeFig. 2A). The large pompons were made from yarn and were also 13 cm in diameter. They varied only in color and could beeasily grasped and held by the youngest infants with one hand (see Fig. 2B).

1.3. Procedure

At their arrival to the laboratory, the parents were explained the goal and procedure of the study. Once they agreed toparticipate and signed the consent form, parents helped secure their child in the infant seat. An experimenter sat in frontof the infant and presented the objects to the infant one-at-a-time, at midline, and at waist level. At each trial, the infantswere allowed time (typically 30 s) to hold, explore, mouth and manipulate the objects before the next object presentation.If the infant did not grasp the object or dropped it before the 30 s elapsed, the experimenter picked the object up and heldit in front of the child allowing object exploration to take place (see Fig. 2A). All infants were tested under 3 conditions:

Probing condition: This condition was run first and was designed to assess the preferred reaching patterns of the infants.We used 4 solid balls, 2 small and 2 large, that we presented either in an increasing or decreasing order. The order ofpresentation was counterbalanced between participants. Two trained experimenters independently coded the reachingresponses of the infants as unimanual or bimanual as they were reaching for the object. A reaching tendency was definedwhen the experimenters’ coding were matching and infants were reaching either unimanually or bimanually for at least 3out of the 4 object presentations (75%).1

Solid objects condition: This condition was run next and designed to assess the effect of repeated exposure to an object withphysical properties not matching the infants’ preferred reaching tendencies. In other words, if during the probing condition,the infants reached for both the small and large objects predominantly with two hands, we presented them with small solidballs to see if they could switch to a unimanual reaching response. If during the probing condition, the infant reached forboth the small and large objects predominantly with one hand, we presented them with large balls to see if they could shiftto two-handed reaching patterns. Infants were given 10 successive trials of identically sized solid objects (small or large)depending on their preferred tendencies. Infants were allowed to manipulate, mouth, and explore the objects after everytrial for about 30 s.

Large pompon condition: This condition was designed to assess whether infants use haptic information to adjust theirreaching and grasping patterns to objects. The pompons are large and should trigger two-handed reaching responses, but atthe same time they are easy to grasp and hold with one hand. We reasoned that if infants switch to one-handed reaching andgrasping patterns after having touched, held, and experienced these soft objects, that would indicate that infants can rely onhaptic perception and haptic memory of prior object encounters to adjust their goal-oriented responses. We presented 10individual large pompons to the infants to reach for and grasp. As in the solid condition, we allowed 30 s for object explorationbetween each trial. Pompons were always tested last following the solid object condition.

1.4. Behavioral coding

From the videotaped recordings, two independent coders analyzed the reaching and grasping responses as unimanual orbimanual, and tracked the occurrence of object mouthing and one-handed object holding.

Reaching was categorized as unimanual and bimanual as a function of the number of hands extended toward the targetbefore object contact (Corbetta & Thelen, 1996). Unimanual reaches were those in which the infant extended one arm toreach for the object while the other arm remained inactive or was activated after the first arm had contacted the target.Bimanual reaches were those in which the infant extended both arms toward the target, revealing coactivation of the arms.Both arms could start at the same time or with a delay. This code was independent of the number of hands used to contactthe target.

Grip configuration was categorized as unimanual or bimanual depending on the number of hands used to take the objectfrom the experimenter’s hand. A unimanual grip occurred when the infant used one hand to remove the object from theexperimenter’s hand and a bimanual grip occurred when the infant used both hands to take the object. Grip configurationwas coded independently from the number of hands used for reaching.

Object manipulation and exploration following grasping: This coding was aimed at determining whether infants were per-forming size- or texture-related activities on the objects after having successfully grasped the toy. Indeed, post-graspingactivities could provide additional sensory-motor experience that infants could use to plan their next reach and grasp. Wefocused on object mouthing and object holding because both behaviors could inform the child about object size and/or tex-ture, but also because these behaviors were the most frequently observed across babies. Thus, for each trial, we documentedwhether infants brought the object to the mouth for exploration and if so we considered this an occurrence of object mouthing.

1 In a pilot study we probed infants with 4 ball sizes of 5, 7, 10, and 13 cm that we presented in an increasing and decreasing order to a total of 8 objectpresentations. This more extended probing led to results similar to the shorter probing we used in the study. On the basis of this pilot data, we chose toadopt the shorter probing because it was a quicker assessment and it increased the likelihood that infants would complete the entire testing protocol.

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Table 1Distribution of participants as a function of their response patterns during probing by age.

Response type 6 months 7 months 8 months 9 months

Bimanual 6 5 8 8Unimanual 3 2 0 1Matching 0 2 1 1Mixed 1 1 1 0

We also documented whether infants changed to or continued to hold the object with one hand after they grasped it. If infantsdid so without assistance from either their other hand or body part, we considered this an occurrence of one-handed holding.

Coding reliability was performed randomly on 20% of our data sample. Coding agreements in the probing, solid objects,and large pompons conditions for the type of reach, the type of grasp, the occurrences of object mouthing and occurrences ofone-handed holding were consistently above 92%. Because all data are proportions and frequencies and most did not followa normal distribution, we used non-parametric tests for all statistical analyses.

2. Results

2.1. Probing

During probing, 27 infants responded mainly bimanually, 6 responded mainly unimanually, 4 used one- and two-handedreaching patterns that matched the object sizes, and 3 used an equal number of one- and two-handed reaching patterns thatwere inconsistent with object size (see age distributions in Table 1). As a result, 27 infants were tested with small objectsand 6 were tested with large objects in the solid object condition. The remaining 7 infants who did not display a preferredreaching response during probing were not included in our subsequent analyses. As shown in Table 1, these 7 infants withno preferred reaching tendencies were spread among all 4 age groups and were not typical of one age. Of the 33 infants whodisplayed a preferred reaching tendency, 25 (76%) used the same consistent response over the 4 probing trials, and 8 (24%)used the same response over 3 out of the 4 probing trials. These 8 infants were also spread among all 4 age groups.

2.2. Solid objects condition

2.2.1. Matching reaching and grasping to object sizeDid infants change reaching patterns during testing when the object sizes were not matching their preferred reaching

tendencies? And did they adjust grip configuration despite their reaching tendencies? Fig. 3 displays the rate of reachingpatterns (unimanual or bimanual) observed for the 27 infants tested with the small objects (top graph) and the 6 infantstested with the large solid objects (bottom graph). This figure shows that, overall, the preferred motor tendencies identifiedduring probing continued to characterize the reaching patterns of the infants. Infants tested with the small objects tendedto reach more bimanually and infants tested with the large objects tended to reach more unimanually.

To assess whether response adaptation changed with age and identify whether reaching and grasping patterns displayeddiffering adaptation rates, we grouped all infants together and calculated the percent of response matching for reachingand grasping separately. Responses were considered matching when the infants tested with small objects used one hand forreaching and/or grasping and when the infants tested with large objects used two hands for reaching and/or grasping. Fig. 4reveals that object–response matching was very low at 6 months old (24% for reaching and 23% for grasping). Nonetheless, thisrate increased significantly with age for both reaching (Kruskal–Wallis, H(3) = 7.978, p = 0.046) and grasping (Kruskal–Wallis,H(3) = 7.905, p = 0.048). Paired comparisons between reaching and grasping revealed that infants displayed significantlygreater response match in grasping compared to reaching (Wilcoxon Signed Ranks Test, T+ = 386.50, n = 22, p = 0.001, two-tailed).2 This increase in response match for grasping that began to appear at 7 months was particularly significant in the 8months old group (Wilcoxon Signed Ranks Test, T+ = 36.00, n = 8, p = 0.011, two-tailed).

2.2.2. Reach-to-grasp pattern sequenceTo gain a better understanding of how infants at the different age groups adjusted their response to object size and infer

whether and which type of sensory information (visual or haptic) they used to do so, we performed a finer analysis withineach trial to examine how patterns of reaching and grasping succeeded one another. Note that this analysis was performedonly on the 27 infants who were presented with small objects in the solid condition, because the 6 infants presented withlarge objects did not have the option of grasping objects with one hand, and thus, displayed response patterns that fell onlyin 2 of the 4 possible pattern successions defined below:

Both reach and grasp match (R-unimanual/G-unimanual): This form of patterning was identified when infants reached forand grasped the small objects using one single arm and hand. This type of response would indicate that the infant was able

2 N represents the number of infants who showed a higher proportion of response matching for grasping compared to reaching.

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Fig. 3. Unimanual and bimanual reaching tendencies of the infants tested with small solid objects (top graph) and large solid objects (bottom graph) byage. The top graph displays the reaching patterns of the infants who displayed bimanual tendencies during probing and the bottom graph displays the oneswho displayed unimanual tendencies during probing.

Fig. 4. Mean percentage and standard errors of object–response match for reaching and grasping by age in the solid object condition. Matching responseswere one-handed for the infants who were presented with small objects and two-handed those who were presented with large objects.

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Table 2Mean proportions of successive response patterns between reaching (R) and grasping (G) as they occurred within each trial as a function of age and typeof movement sequence for the solid object condition (analysis performed only on infants tested with small objects).

Age Non-adaptive responses Adaptive responses

All bimanual Opposite Touch-based Visually basedR-bimanual; G-bimanual R-unimanual; G-bimanual R-bimanual; G-unimanual R-unimanual; G-unimanual

6 months (n = 6) 63.16 10.53 8.77 17.547 months (n = 5) 53.06 12.24 30.61 4.088 months (n = 8) 43.04 0.00 53.16 3.809 months (n = 8) 30.67 8.00 29.33 32.00

to judge the size of the object based on vision alone and was able to anticipate and select the appropriate motor response toget the object.

Reach does not match but grasp matches (R-bimanual/G-unimanual): This form of response corresponds to when infantsdisplay their preferred non-adaptive motor tendency during reaching (i.e. reaching with two hands for a small object), butthen adjust their grip configuration to a one-handed pattern to match the size of the object. Such a response would be typicalof infants who cannot make the appropriate size judgment based on vision alone but can rely on haptic information once incontact with the object to adjust their grip configuration.

Both reach and grasp do not match (R-bimanual/G-bimanual): This is a non-adaptive form of patterning. During this response,infants never match their reach and grasp to the size of the object and perform the whole movement using two hands. Such aresponse could be a stereotypical pattern dominated by motor tendencies where infants neither take into account the visualnor the felt properties of the objects.

Reach matches but grasp does not match (R-unimanual/G-bimanual): This last form is expected to happen rather infrequently.It would correspond to reaching for a small object with one hand, but then switching to two hands for grasping. This typeof response could reflect an attempt to secure a one-handed grasp, even though grasping with one hand is not a problem atthe age range we tested.

Table 2 presents the frequencies of these 4 types of reach-to-grasp sequence patterns as a function of age for the solidobject condition. To highlight the age trends present in these data and facilitate the reading of this table, we marked in boldthe mode, or pattern category containing the highest frequency, within each age group. Table 2 shows that the 6- and 7-month olds produced a greater rate of stereotyped responses than the other 2 ages, where both reach and grasp were mainlybimanual and not matching the size of the object. At 8 months, a shift occurred showing that touched-based responses,where infants adjusted their grip configuration to the size of object on contact began to emerge as a new pattern. Finally,the 9-month olds displayed a growing rate of visually based responses, revealing that those infants were increasingly ableto anticipate and match their reaching and grasping response to the size of the objects. This developmental transition fromall bimanual, to touch-based at 8 months, and visually based responses at 9 months was significant (Chi-square (9) = 92.314,p < 0.001).

2.3. Pompons condition

2.3.1. Matching reaching and grasping to object textureAll computations for this condition were performed on the last 9 trials. Because infants never had prior exposure to large

pompons, we considered the first trial as a practice trial in which the child could gain knowledge about and experiencethe texture properties of the object. Recall that this condition was designed to assess whether infants would rely on hapticinformation and learn from prior experience that despite their large appearance, pompons can be reached for and held withone hand.

Fig. 5 displays the reaching patterns predominantly used by the infants in the pompons condition as a function of theobject size they were exposed to during the solid testing phase. The top graph reveals that infants who were bimanual duringprobing continued to show little adaptation and preferred reaching with two hands for the pompons despite having reachedfor small solid objects for 10 trials before. Likewise, infants with unimanual reaching tendencies who were tested with largeobjects in the solid object condition continued to reach with one hand for the pompons (except for one 9 months old infant).From these graphs, it appears that 5 infants at 6 and 7 months old may have relied more on haptic memory to adapt theirreaching responses to the pompons, but in reality these infants were already strongly unimanual during probing and the solidobject condition; it is more likely that they kept using their preferred reaching tendency in this condition as well. Similarmovement stereotyping throughout conditions was already reported in Corbetta et al. (2000).

Again, to analyze age trends and compare how reaching and grasping responses differed from one another, we grouped allsubjects and computed their rate of one-handed responses by age. Fig. 6 shows that, overall, infants did not increase signif-icantly their rate of one-handed reaching (Kruskal–Wallis, H(3) = 4.677, p = 0.197) or grasping (Kruskal–Wallis, H(3) = 2.803,p = 0.423) with age. Because the unimanual babies in the 6 and 7 months old groups may have skewed the age trend,we repeated the analysis with the bimanual babies only. This second analysis, again, revealed no significant age trend inresponse adaptation toward a greater use of one-handed reaching and grasping for the pompons (Reaching: Kruskal–Wallis,H(3) = 2.995, p = 0.392; grasping: Kruskal–Wallis, H(3) = 3.711, p = 0.294).

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Fig. 5. Unimanual and bimanual reaching tendencies of the infants when tested in the pompons condition. The top graph displays the reaching patternsof the infants who displayed bimanual tendencies during probing and the bottom graph displays the ones who displayed unimanual tendencies duringprobing.

Fig. 6. Mean percentage and standard errors of object–response match for reaching and grasping by age in the large pompons condition. Responses wereconsidered matching when infants used one hand for reaching and grasping. Such response suggested that infants relied on haptic information or memoryto tailor their response.

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Table 3Mean proportions of successive response patterns between reaching(R) and grasping (G) as they occurred within each trial as a function of age and type ofmovement sequence for the large pompons condition (analysis performed only on infants tested with small object in the solid condition).

Age Non-adaptive responses Adaptive responses

All bimanual Opposite Touch-based Haptic-memoryR-bimanual; G-bimanual R-unimanual; G-bimanual R-bimanual; G-unimanual R-unimanual; G-unimanual

6 months (n = 6) 58.33 10.42 22.92 8.337 months (n = 5) 42.22 20.00 28.89 8.898 months (n = 8) 47.76 0.00 44.78 7.469 months (n = 8) 37.14 4.29 34.29 25.71

Interestingly, however, comparisons between reaching and grasping responses revealed that as a whole infants sig-nificantly increased their rate of unimanual responses for grasping compared to reaching (Wilcoxon Signed Ranks Test,T+ = 303.50, n = 20, p = 0.022, two-tailed). This increase in grip adaptation for the pompons was particularly significant in the8 and 9 months old groups (8 months: Wilcoxon Signed Ranks Test, T+ = 28.00, n = 7, p = 0.018, two-tailed; 9 months: WilcoxonSigned Ranks Test, T+ = 42.50, n = 8, p = 0.017, two-tailed) (see endnote 2).

2.3.2. Reach-to-grasp sequence for the pomponsThis analysis was also performed on the 27 infants who were tested with small objects in the solid object condition.

Indeed, those infants were the ones displaying preferred bimanual patterns during probing, therefore they would provide astronger test to assess whether they rely on haptic information to shift to a one-handed reaching. As shown in Table 3, thereach-to-grasp sequences for the pompons followed an age trend similar to the solid objects condition, except that the trendappears protracted compared to the solid object condition. At all ages, the response pattern most frequently used was thetwo-handed reach followed by a two-handed grasp. Interestingly, however, a shift toward greater one-handed responses canbe observed again at 8 and 9 months old. Both the 8 and 9 months old groups showed an increase in touch-based responsescompared to the 6- and 7-month olds, and the 9-month olds also performed the highest rate of one-handed reaches for thepompons, presumably based on haptic memory of the seen object, compared to the other age groups. This overall age trendwas again significant (Chi-square (9) = 58.24, p < 0.001).

2.4. Pattern succession across repeated trials

When infants shifted response from their preferred pattern, on which trial did it happen and did infants maintain the newresponse afterward? In the solid object condition, infants displayed their first change in reaching on average on the seventhtrial (S.D. = 4.07) and change in grasping on average on the third trial (S.D. = 3.06). In the large pompons condition, infantsdisplayed their first one-handed reaching pattern on average on the sixth trial (S.D. = 3.88) and grasped the pompons withone hand for the first time on average on the third trial (S.D. = 2.84). These data confirm that, overall, changes in graspingpreceded changes in reaching.

Once infants experienced this new motor solution, did they maintain this response for the remainder of the trials? Resultsshow that it was not the case. Once they produced the new response, they repeated this response for a low average of 28%(S.D. = 35) of the remaining trials for reaching and for an average of 46% (S.D. = 33) of the remaining trials for grasping. Why arethese rates of object–response match so low? Why aren’t infants continuing to use the same response once they tried it andwere able to see that it worked? To answer these questions we examined the sequences of patterning of reaching and graspingas they reached for and grasped the objects successively trial after trial. Some examples of these trial-dependent movementpatterns are presented in Fig. 7, for 4 infants (one from each age group) for the solid and pompons object conditions. These 4infants displayed bimanual tendencies during probing and therefore were all tested with small objects in the solid condition.As shown in Fig. 7, all infants began reaching and grasping on the first trials with two hands. Eventually, as the same-sized orsame texture object kept being presented, they shifted their reaching pattern (black circle with solid lines), and/or graspingpattern (white circles with dotted lines) to one hand. Interestingly, these graphs reveal that when that shift occurred, the newresponse never remained stable. Infants returned to their previous preferred response either almost immediately (infants 602and 705), a few trials later (infants 802 and 909), or kept fluctuating back and forth between the new and old patterns (infant602). The only groups that seemed to reveal a slightly more steady maintenance of the new response were the 8-montholds for grasping and the 9-month olds for reaching and grasping, but even these infants displayed a tendency to return totheir initial preferred pattern despite same size and same texture trial repetition. We believe that these fluctuating patternsreflect competing tensions between perception and action. The infants are trying to break their individual intrinsic motortendencies to accommodate the ‘seen’ and ‘felt’ characteristics of object, but their previous history of preferred reachingpatterns pulls their motor response back to their original form.

2.5. Post-grasping object holding and mouthing

After infants grasped the objects and held them in their hands, did they display behaviors that matched the size andtexture properties of the objects? Such behaviors could provide additional sensory-motor experience and make available

54 D. Corbetta, W. Snapp-Childs / Infant Behavior & Development 32 (2009) 44–58

Fig. 7. Examples of sequences of reaching and grasping patterns during the 10 testing trials in the small objects and pompons conditions. Each pair of graphscorresponds to an infant from a different age group. Pairs of graphs are arranged in increasing order from 6 (top graphs) to 9 months old (bottom graphs).The solid lines correspond to the reaching pattern (one- or two-handed) used for each successive trial and the dashed lines correspond to the graspingresponses.

Table 4Mean percentages of one hand grasping compared to one hand holding during object manipulation and Wilcoxon signed ranks tests by age group andobject condition.

Age Grasp mean Hold mean T+ N p, two-tailed

Solid objects (small only)All ages (n = 27) 45.56 75.56 289.5 23 0.0006 months (n = 6) 25.00 55.00 21.0 6 0.0267 months (n = 5) 34.00 70.00 10.0 4 0.0688 months (n = 8) 56.25 87.50 28.0 7 0.0189 months (n = 8) 57.50 82.50 24.0 6 0.090

PomponsAll ages (n = 33) 42.27 57.27 264.5 20 0.0066 months (n = 9) 32.00 56.89 26.5 6 0.0347 months (n = 7) 33.14 58.71 10.0 4 0.0688 months (n = 8) 48.38 58.38 19.0 5 0.3959 months (n = 9) 54.22 55.56 24.0 6 0.351

further information about the objects properties. Infants could potentially assimilate and use this information for reachingand grasping on the next trial, moreover, documenting such behaviors directly informed us about infants’ sensitivity to objectsphysical characteristics and whether they experienced them. To examine this, we tracked how many times infants managedto hold the small objects and pompons in one hand (one hand holding) following grasping. This behavior indeed could reflecta continued adjustment of the grip configuration after having the object in hand. We also tracked how many times infantsbrought the object to the mouth (object mouthing). Distinct rates of object mouthing between solid and pompons wouldindicate that infants are sensitive to the texture properties of objects.

Table 4 displays the rate of one hand holding performed following grasping compared to the rate of one-handed graspingfollowing reaching for the solid and pompon conditions for all infants by age.3 This table shows that overall infants continued

3 For the solid object condition, this analysis was performed only with the 27 infants who were presented with small objects. Indeed, infants who werepresented with the large objects did not have the option to hold them with one hand. As per previous analyses, occurrences of one-hand holding for thepompons condition did not include the 1st practice trial.

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Fig. 8. Mean percentage and error bars of object mouthing produced after grasping the object as a function of age and object condition.

to adjust their grip configuration after having grasped the object by increasing significantly their rate of one-handed behaviorwhen holding the object compared to grasping per se (Solid: Wilcoxon Signed Ranks Test, T+ = 289.5, n = 23, p = 0.0001, two-tailed; Pompons: Wilcoxon Signed Ranks Test, T+ = 264.5.0, n = 20, p = 0.006, two-tailed). This increase in one-handed objectholding compared to one-handed grasps was present at all ages, but was significant only in the younger groups (see Table 4).Significance between one hand holding and one hand grasping decreased with age due to the fact that the 8- and 9-monthold infants were more successful at adjusting their hand configuration to object size during grasping, thereby reducing thedifferences in response rate with one hand holding. What is most notable in this data is the result of the younger infants.Even though those infants demonstrated a poor ability at adjusting their hand configuration to object size during grasping,they revealed much greater size and texture sensitivity once they had the objects in their hands.

Fig. 8 displays the rate of object mouthing by age and by object condition. It shows that infants mouthed the solidobjects significantly more than the pompons (Wilcoxon Signed Ranks Test, W = −407.0, n = 24, p = 0.0001, two-tailed). Therate of object mouthing per se did not change significantly with age in neither the solid nor the pompon condition. However,within each age group infants consistently displayed a greater proportion of mouthing for the solid objects compared to thepompons. These differences were especially significant at 7 months (Wilcoxon Signed Ranks Test, W = −27.0, n = 6, p = 0.028,two-tailed), at 8 months (Wilcoxon Signed Ranks Test, W = −35.0, n = 7, p = 0.017, two-tailed), and 9 months of age (WilcoxonSigned Ranks Test, W = −27.0, n = 6, p = 0.028, two-tailed). These data confirm that infants were sensitive to the texturalproperties of objects and mouthed objects discriminately.

3. Conclusions

This research had two main goals. The first goal was to examine how repeated experiences of seeing, reaching for, touching,grasping, and manipulating objects of same sizes and textures influenced the formation of subsequent motor responses in6–9-month old infants. Results revealed that this developmental process is quite protracted and that infants are limited intheir ability of taking advantage of prior encounters with objects to modify their preferred motor patterns. Unlike adults whocan immediately learn from a prior object manipulation and change response on the next trial, infants seem to need manytrials to alter response and seem to need even more practice before being able to maintain and reproduce the new responsesteadily. Most striking was the fact that infants’ intrinsic reaching tendencies appeared to resist throughout all testingconditions despite repeated encounters and manipulations of the solid and soft objects. We would like to acknowledge,however, that the lower rate of reaching adaptation in the pompons condition may have been linked to the size of theobjects. Despite being graspable with one hand, the pompons were of a large size and therefore could have been justifiablyperceived as two-handed reaching objects. Furthermore, the large size of the pompons, compounded with the fact that mostinfants in the study displayed preferred bimanual reaching tendencies, could have hampered infants’ attempts to switchresponse and reach for those objects with one hand despite the that fact that 8 and 9 months old increased their rate ofone-handed grasping for those same objects. This leads us to the second goal of the study.

The second goal of this study was to understand when and which type of sensory information (visual, haptic, or both)6–9-month old infants used to tailor their one- and two-handed reach and grip configurations to object size and texture.

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Fig. 9. Behavior emerges from tensions between perception and action. The word “ACTION” is in bigger letters to indicate that the action pole may be astronger attractor during early development.

Regarding this second goal, results have shown that infants began to use haptic information to adjust their grip configurationto object size and texture before they began relying on visual information alone to adjust their reaching response. Thatdevelopmental sequence in the use of sensory information in the development of reaching and grasping was consistentwith the one proposed by Newell et al. (1989). However, the present data which specifically documented both patterns ofreaching and grasping within infants and within trials, revealed that the adjustment of the grip emerged around 8 monthsof age. This particular finding is not in accord with Newell et al. (1989) and Siddiqui (1995) who reported haptic adjustmentin grip configuration as early as 4 months old. Rather, results from this study are more in line with other studies showingthat adjustments in object-directedness and object-related hand configurations begin to occur more systematically around7–8 months of age (Corbetta et al., 2000; Fagard, 2000; Fagard & Pezé, 1997; Lockman et al., 1984; Piérault-Le Bonniec, 1985;von Hofsten & Fazel-Zandy, 1984; von Hofsten & Rönnqvist, 1988). What do these findings tell us about the developmentaland sensory-motor processes underlying changes in infant reaching?

To begin to address this question we would like to discuss first the response patterns of the younger infants, because wethink that they are particularly informative. The 6- and 7-month old infants revealed rather sporadic and limited changesin their motor patterns. If they reached for a small object with two arms, they were also more likely to grasp it with twohands, and if they showed a tendency to reach with two hands during probing, they were more likely to continue to reachwith two hands during testing compared to their older peers. This pattern of response that we referred to as seeminglystereotypical is consistent with the interpretation that young infants motor patterns are constrained by intrinsic motortendencies that may compete with the process of perceptual-motor mapping (Corbetta et al., 2000). From a dynamic systemapproach, behavioral pattern formation can be seen as the product of continuous tensions between perception and action,where either perception, or action, or both can represent poles of attraction with varying levels of strength (see Fig. 9). Asa result, change in the development of infant reaching can be viewed as the result of changing tensions between actionand perception where the strength of either one or both of those two attracting poles can vary and evolve as a functionof time. In the behavior studied in this paper, it is possible that the infants’ intrinsic tendencies (i.e. using more one- ortwo-handed patterns for reaching) corresponded to strong action attractors that could have interfered with the effectiveintegration of perceptual information. Perception and action may be competing with one another particularly as infants arereaching, then grasping, while perceiving the object’s physical characteristics at the same time. The fact that motor patternsfluctuated back and forth between one- and two-handed responses as illustrated in Fig. 7 is suggestive that such tensionsand competition between perception and action may have been taking place; infants could have been trying to break theirindividual intrinsic motor tendencies to accommodate the seen and felt characteristics of object, but intermittently returnedto their original motor pattern. The fact that those motor tendencies were observed consistently across all testing conditionswould be consistent with the interpretation that those tendencies formed strong attractor poles pulling the infant back totheir initially preferred mode of response.

This interpretation is very different from the one proposed by Newell et al. (1989) who stressed perception as the factorguiding reaching and driving developmental change. As mentioned in introduction, by stressing the perception side only,one may neglect changes that may be occurring at the motor level. As we have seen here, and as reported by previous studies(Corbetta et al., 2000; Thelen et al., 1993), infants bring their own neuromotor and biomechanical constraints into the task,which can compete with the process of perceptual-motor mapping. We see such competition and the behavioral instabilityassociated with it a strong indicator that behavioral change is taking place. But as our findings suggest, such change ininfancy may require time and practice before intrinsic motor tendencies dissolve and new stable patterns form within amore balanced perception–action system.

One result that was rather puzzling and that we observed in our previous study (Corbetta et al., 2000) was that infantsseemed more apt at modulating their motor response to the felt properties of objects during manipulation after they hadgrasped the object than during grasping per se. Moreover, the fact that infants experienced and explored these objectsdiscriminately with one hand or with the mouth while holding them did not seem to have an impact on their next response.Why is that so? We think that there are two possible explanations to these findings. The first one relies on the fact that reachingand object manipulation may involve different cognitive or attentional mechanisms. Indeed, during reaching infants needto direct their hand to the object location, which is a requirement that is not present when they have acquired the object intheir hands. Additionally, previous reports have shown that infants are not very good at controlling goal-oriented movementsuntil later in the first year (Thelen, Corbetta, & Spencer, 1996; von Hofsten, 1991). It is possible therefore, that because ofthese additional requirements of object directness and the difficulty of controlling arm trajectory, infants pay more attentionto the location of the toys during reaching than to their specific physical characteristics. Specific attention to the objects’physical characteristics leading to grip adjustments or differentiated object manipulation could become available only after,

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once infants have successfully grabbed the toy. Because these constraints on attaining the toy are repeating themselves onevery trial and infants do not improve their reaching skills in the span of a few trials, they continue to fail taking advantageof prior object-oriented experiences to modulate their response on each new trial.

Another possible explanation for the observed greater size and texture sensitivity during object manipulation comparedto reaching and grasping may rely on the fact that vision and touch are not quite fully integrated until later in the first year.When observing the behavior of the infants on the video during reaching and objects manipulation, we noticed that duringreaching infants always looked at and then touched the toy at the same time (Fig. 2A), while during object manipulation theyrarely continued to look at the object while they were touching it (Fig. 2B). This prompted us to examine the literature oninfant’s vision and touch. To our great surprise we found that vision and touch do not seem to work together very well untillater in the first year. Studies on texture discrimination and recognition memory report that 6–9-month old infants performpoorly when allowed to touch and manipulate objects while looking at them at the same time (compared to visual onlyconditions and compared to 12-month olds). In particular, two studies showed that manipulating objects while looking atthem hindered infants’ recognition memory of shapes (Gottfried, Rose, & Bridger, 1978; Rose, Gottfried, & Bridger, 1979). Andanother study from Stack and Tsonis (1999) observed that while vision increased exploration attention, it did not facilitatethe haptic perception of textures. These authors also found that the discrimination of texture through the haptic modalitytook longer compared to visual discrimination tasks in 7-month olds, and that young infants needed more familiarizationtime to respond to novelty in the haptic conditions. Thus, it is possible that during reaching and grasping, while infants arelooking at and then touching the toy at the same time, they cannot quite parse the sensory information diligently to adjusttheir response pattern, and therefore continue to respond on the basis of their motor tendencies. The reason they are betterat adjusting their motor responses during object manipulation, might be because once they have grasped the object andstopped looking at it, they can adjust their grip configuration or bring the object to the mouth without needing guidancefrom the visual system. This interpretation and especially the results from the above studies (Gottfried et al., 1978; Rose etal., 1979; Stack & Tsonis, 1999) on visual and haptic perception are incompatible with Newell et al’s (1989) interpretationthat young infants need to rely on both visual and haptic perception to adjust their grip configuration. Findings from thisresearch and the work cited above all suggest that the successful integration of haptic information with visual informationbegins only around 8–9 months of age (Corbetta et al., 2000; Gottfried et al., 1978; Rose et al., 1979; Stack & Tsonis, 1999).

Clearly, more research will be needed to further examine the plausibility of these different scenarios. We acknowledgethat they may not be mutually exclusive; it is possible that the presence of coordination tendencies, the lack of controlin movement trajectory, and the difficulty to integrate visual with haptic information are all factors contributing to theseemingly stereotypical movement patterns observed in younger infants and protracted development of perception–actionmatching in infant reaching. Nonetheless, despite the fact that these factors may at first appear to limit the range of possibleresponses in young infants, we contend that they bring valuable sensory-motor experience that is fundamental to supportand drive developmental change (see also Bojczyk & Corbetta, 2004). The fact that even the youngest infants showed smalland sporadic changes in perception and action over the short time scale of 10 trials is a sign that developmental change intaking place. Only over the longer developmental time scale of weeks and months do these small changes accumulate tobecome responses that are gradually long lasting and more consistently tuned to the environment.

Acknowledgments

We are thankful to the parents and their infants for agreeing to participate in this study. We also wish to thank twoundergraduate students, Kathleen Le Guern and Jennifer Isenogle, for their valuable assistance during data collection andvideo coding. This research was supported by a summer faculty grant from the Purdue University Research Foundationawarded to Daniela Corbetta.

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