LATERALITY, 2011, 16 (5), 565585
Handedness for grasping objects and pointing and the development of language in 14-month-old infants Rana Esseily, Anne-Yvonne Jacquet, and Jacqueline Fagard Laboratoire Psychologie de la Perception, Universite´ Paris Descartes, Paris, France
The goal of this study was to evaluate the relationship between object-related handedness and handedness for communicative gestures. We observed 22 infants aged 14 months on a baby laterality test consisting of grasping objects in different conditions, on a pointing task with targets placed out of reach at different spatial positions from left to right, and on word understanding and word production. Results show that 77% of infants pointed to the left, middle, and right targets. The majority of infants were right-handed for pointing*except for the far left target*and, to a lesser extent, for grasping objects, but there was no significant relation between the two measures of handedness. The frequency of pointing tended to be related to the number of words understood, and infants right-handed for pointing understood and produced significantly more words than non-right-handed pointers. These results are interpreted as confirming the link between pointing and language development, and as showing that communicative gesture lateralisation is not a mere consequence of object-related handedness, at least during development. Whether lateralised communicative gesture reinforces a pre-existing tendency to use the right hand to interact with objects remains an open question.
Keywords: Handedness; Grasping; Pointing; Language; Development.
Many asymmetries can be traced far back in evolution, such as directional tendencies deduced from the wounds in 600 million-year-old trilobite fossils (Babcock, 1993), lateralisation of vocal cord control in 170-million-year-old
Address correspondence to: Jacqueline Fagard, Laboratoire Psychologie de la Perception, Universite´ Paris Descartes, CNRS UMR 8158, 45 rue des Sts Pe`res, 75006 Paris, France. E-mail:
[email protected] This study was supported by the ANR program, contract no. 08-3_311472. The authors wish to thank S. Kern for her French adaptation of the MacArthur language test, and J. Vauclair and H. Cochet for their helpful comments on the paper. # 2010 Psychology Press, an imprint of the Taylor & Francis Group, an Informa business http://www.psypress.com/laterality DOI: 10.1080/1357650X.2010.499911
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species (Bauer, 1993; Nottebohm, 1971), Broca’s area asymmetry seen in endocasts made from 2 million-year-old fossil skulls (Holloway, 1983), or predominant right-handedness inferred from 2 million-year-old artefacts (Rugg & Mullane, 2001; Toth, 1985). The question of how these asymmetries co-evolved is still largely unanswered. One particular association that is still much debated is the one between handedness and language lateralisation. Even during human development, it is still unclear whether infants become right-handed because of their left-hemisphere specialization for language, or whether they speak predominantly with their left hemisphere because of a less-specific capacity of this hemisphere, for instance for processing sequential events, that would become evident in manipulation prior to language. A third possibility could be that manual and language lateralisation evolve independently during early development. The goal of the present study was to tackle this question by looking at the relationship between asymmetry for object-related actions and asymmetry for communicative gestures in infants, at the age when communicative gestures begin to represent a significant mode of communication. It is known that asymmetries favouring the left hemisphere for languagerelated areas appear already in utero (Chi, Dooling, & Gilles, 1977) and that a right-ear advantage for linguistic stimuli is present in neonates (Bertoncini et al., 1989; Molfese, Freeman, & Palermo, 1975). But at the same time signs of handedness also appear already in utero (Hepper, Shahidullah, & White, 1991) and foetus handedness relates to handedness at 1012 years of age (Hepper, Wells, & Lynch, 2005). There are few systematic and comprehensive studies comparing handedness in early communicative gestures and in object-related actions, as we discuss below. If when communicative gesture emerges, which happens several months after the infant is able to grasp and manipulate objects, there is a strong relationship between handedness with objects, not yet very strongly biased towards the right, and hand choice for communicative gestures, then it may be hypothesised that the infant is influenced in his or her choice of communicative hand by the habit of using that hand for grasping objects. In contrast if, when communicative gesture emerges, hand choice for it is already right-handed to a greater extent than hand choice for object grasping at the same age, then it could be hypothesised that, during development, lateralisation for early communicative gestures is not a simple consequence of handedness for objects.
POINTING IN INFANTS Young children use gestures to communicate before they can use words to speak (Bates, Camaioni, & Volterra, 1975). When language appears, gestures are often used in combination with words (Iverson & Goldin-Meadow, 2005).
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Pointing is one of infants’ main communicative gestures (see Capirci & Volterra, 2008, for a review). A distinction is commonly made between protodeclarative pointing, whereby infants use an object as a tool to obtain adult attention, and protoimperative pointing, whereby infants use adults as a tool to obtain an object, or indicate a request towards an object (Liszkowski, Carpenter, Henning, Striano, & Tomasello, 2004). This distinction is somewhat validated by the fact that apes, who can point to request for out-of-reach food (Leavens & Hopkins, 1998), only produce pointing gestures that serve imperative functions (Tomasello, Carpenter, & Liszkowski, 2007). We decided to design a set-up likely to trigger predominantly protodeclarative pointing, rather than protoimperative pointing, the latter being sometimes considered as contaminated with intended grasping. Protodeclarative pointing appears some time before the end of the first year (Butterworth & Morissette, 1996). Pointing consists first in the extension of the arm, and reaches its mature index-finger-extended version before the seventh month. In one study a quarter of the sample used pointing at 9 months, and two thirds at 14 months (Murphy, 1978). According to other studies, a majority of 12-month-olds use pointing (Lempers, Flavell, & Flavell, 1977; Liszkowski et al., 2004). Few studies have focused on the hand used for pointing but, in general, use of the right hand has been observed for communicative gestures in children, as in adults, as we will see further below.
HANDEDNESS FOR GRASPING According to some authors, signs of future hand preference can be observed in utero during thumb sucking (Hepper et al., 1991), and in newborns’ asymmetric tonic neck reflex (ATNR) (Michel, 1981). As soon as mature voluntary grasping is observed, at around 5 months of age, infants show hand preference at least to some extent, as indicated by both cross-sectional (Cornwell, Harris, & Fitzgerald, 1991; Fagard & Lockman, 2005; Fagard & Marks, 1998; Gesell & Ames, 1947; Hawn & Harris, 1983; Michel & Harkins, 1986; Peters, 1983) and longitudinal studies (Corbetta & Thelen, 1999; Coryell & Michel, 1978; McCormick & Maurer, 1988; Michel & Harkins, 1986). Even though the proportion of infants without clear hand preference greatly surpasses that of adults, and handedness fluctuates during the first months of object grasping, there are nonetheless more infants with right-hand preference than with left-hand preference (Fagard, 1998), and the frequency of children without preference decreases with age (Dellatolas, Tubert Bitter, Curt, & de Agostini, 1997). Neuroimaging studies support these early asymmetries, showing leftward asymmetries in the cortico-spinal tract in infants from 1 to 4 months of age (Dubois et al., 2009).
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HANDEDNESS FOR GESTURING AND FOR OBJECT-RELATED ACTIONS Preferential use of the right hand for gesturing has been observed in adults (Kimura, 1973b). The preference is enhanced when gesturing is accompanied by a vocalisation (Kimura, 1973a). In infants, Hannan and Fogel (1987) report significantly more right-hand than left-hand gestures during the 38 instances of pointing in their single-case study (Hannan & Fogel, 1987). A few studies have compared handedness for symbolic and communicative gestures and for other actions. These studies showed that symbolic activities and communicative gestures elicit more right-hand bias than noncommunicative actions in infants and toddler (Bates, O’Connell, Vaid, Sledge, & Oakes, 1986). A right-hand bias for pointing seems to emerge at 12 months of age, at the same time when vocalisations accompanying those gestures also increases (Blake, Orourke, & Borzellino, 1994). A recent study has compared the pattern of hand preference for pointing gestures and for object manipulation in 123 infants and toddlers aged 10 to 40 months (Vauclair & Imbault, 2009). The authors observed that not only right-handers but also left-handers and ambidextrous participants, as evaluated with objects, tended to use their right hand for pointing. In addition, the correlation between the two indices increased during significant periods in language development, like vocabulary spurts and syntax improvement. Other indications that communicative gestures share a common brain mechanism with language come from sign language studies. Behavioural (Grossi, Semenza, Corazza, & Volterra, 1996) and brain-imaging data (Corina, San Jose-Robertson, Guillemin, High, & Braun, 2003; Emmorey, Mehta, & Grabowski, 2007) indicate that signing in the majority of deaf people is controlled by the left hemisphere. Furthermore, sign language communicators use their right hand more than their left hand, especially those who use the right hand preferentially for non-signing actions (Bonvillian, Garber, & Dell, 1997; Vaid, Bellugi, & Poizner, 1989). A related study on 6- to 14-monthold signing children showed that right-handedness was stronger earlier for signing than for object manipulation (Bonvillian & Richards, 1993). In the study presented here we tested infants at 14 months, an age when most infants use pointing, for pointing and for grasping. To verify that pointing reflects language development we also tested infants for language comprehension and production, and we checked for a difference in language development between pointing and non-pointing infants, and between infants lateralised for pointing and those not lateralised. If pointing is a proto-language controlled by the language brain area, then early pointers, in particularly early right-hand pointers, might also be expected to be early language users.
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METHOD Participants A total of 22 children aged 14 months (14m 2d to 16m 25d; mean14m 20d, SD0m 20d) were tested for object grasping and for pointing at five targets positioned on the wall from extreme left to extreme right. The participants included 8 girls and 14 boys. Infants were recruited from a local list of families who expressed interest in participating in studies of infant development. Prior parental consent was granted before observing the infants. Five children did not point or pointed no more than four times, and were not included in the analyses of handedness for pointing; 17 children pointed at least once to the right, to the left and to the middle position. Of these 17 children, 11 pointed to each of the five positions, allowing comparison across all positions. Given this small number of children having pointed to all positions, we added a separate analysis by pooling positions 1 and 2 (left positions) and positions 4 and 5 (right positions).
Procedures and materials All children were given a baby grasping-laterality test (Sacco, Moutard, & Fagard, 2006), and a test for handedness at pointing. In addition, they were evaluated for language. We also noted whether the infant was an independent walker or not. Testing occurred in a university infant testing room. Infants were seated at a table in the parent’s lap. Once infants were judged to be accustomed to their surroundings, testing began. Handedness for grasping. The baby laterality test comprised five items to test simple grasping, two items to test precision grasping, and one item to test grasping involving bimanual strategies. Objects for testing simple grasp† ing were small baby toys: three Playmobil figurines, one hand-shake toy (maracas), and a teether. For precision grasping the same material as in Fagard and Lockman (2005) was used. The tasks consisted in taking a very thin red tube, 6 mm in diameter, inserted in a slightly shorter transparent tube from which only the top protruded, and a small horse inserted in a container 30 mm in height. To favour a unimanual grasp, these two objects were presented so that the infants could not grasp the container, but only the object inside. Finally, grasping involving a bimanual strategy was tested using the ‘‘tube-container’’ task (Fagard & Jacquet, 1989). The object consists of a small plastic tube inside a wooden container. To succeed, the infant must hold the wooden container with one hand, the passive hand, and pull off the transparent tube with the other hand, the active hand. The object was
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given in such a way that the infants were obliged to first grasp and hold the container with one hand and use the other hand to pull out the tube. The infants were considered to be right-handed for this task if they used their right hand to pull the tube, and as left-handed for this task if they used their left hand to pull it, regardless of which hand was used first for grasping. The entire baby laterality test thus comprised eight items. All objects were presented within reaching distance of the infant at a midline position. Handedness for pointing. To evaluate handedness in pointing we presented five puppets to the infants, one at each of five possible positions, through a hole made in the white sheet lining the wall facing the infant, in a set-up inspired by Liszkowski (2005) (see Figure 1). The white sheet was 2 m by 1.80 m, and the five holes were 48 cm apart. Infants were seated between a parent and an observer who encouraged them to indicate the puppet when they did not point spontaneously, at a distance of two metres from the screen, so that the angle between adjacent holes was about 258. Infants received a maximum of 25 presentations, five per position, in a randomised order within blocks of five presentations containing one at each position. We chose to present the puppets far enough away that their pointing could not be part of a reaching movement. Language evaluation. Language production and comprehension was evaluated using the short version of the French adaptation of the MacArthur 1
3 48 cm 4
2
5
2m50 11°5
Figure 1. Diagram of the five targets for testing pointing.
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language test (Bovet et al., 2005). The parents filled out the form*a list of 81 words whereon they must mark off each word their infant produces and/ or understands*with the help of an experimenter at the beginning of the session. Two variables were considered: the number of words understood, and the number of words produced.
Data collection and coding All trials were videotaped: from the videotape record, observers scored the hand infants grasped the object with, whether they produced pointing, and if so with which hand. Inter-rater agreement, based on two independent observers scoring 40% of the sample, averaged 98% perfect agreements for grasping, and 92% perfect agreement for pointing. Handedness for grasping. From the raw numbers of right-handed (RhG), left-handed (LhG), and bimanual strategies (BimG) out of the eight items, a grasping laterality index (GLI) was calculated based on the following formula: P P RhG LhG P (RhG BimG LhG) The GLI allowed us to categorise the infants as right-handed when they had a GLI]0.5, and as left-handed when they had a GLI50.5. Otherwise they were considered as non-lateralised. This is a common criterion in such studies (Sacco et al., 2006; Vauclair & Imbault, 2009). Handedness for pointing. A gesture was considered as pointing whenever the child pointed to the object while looking at it, during the whole gesture or in alternation with looks to the parent. When infants pointed to the object after the object disappeared, but before the next object appeared, we scored it as a pointing gesture. Pointing was usually made with the index finger, but we scored any arm extension toward the object, even with the whole hand, as pointing. For most of the pointing gestures, only one hand was used; in these cases coding for hand use was easy (RhP or LhP). Very few pointing gestures were made bimanually (BimP). A session with 25 trials is quite long for a 14-month-old, and we had to stop before the end for some children. A pointing laterality index (PLI) was calculated using the same formula as for grasping: P P RhP LhP P (RhP BimP LhP)
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Since not all infants had the same number of trials per target position, all calculations were made from the frequency of pointing gestures calculated as the number of pointing gestures divided by the number of trials for a given position. ANOVA and repeated-measures ANOVA were performed on continuous data, whereas chi-square analyses were used to test for differences in categories. When the effects were not significant, we calculated the effect size (Corroyer & Rouanet, 1994), using Cohen’s d or 82 statistic (Cohen, 1977).
RESULTS Handedness for grasping The mean GLI (grasping laterality index) for the whole sample was .37 (from 1 to 1; SD.58). Based on the GLI, 14 of the 22 children were categorised as right-handers (63.6%), three were categorised as left-handers (13.6%), and five were not lateralised (22.7%). The 17 infants compared for handedness during grasping and pointing had a mean GLI of .49 (from .5 to 1; SD.46). Among these 17 infants, 12 infants were categorised as right-handers (70.6%), one infant was categorised as left-hander (5.9%), and four infants were categorised as non-lateralised (23.5%).
Frequency of pointing gestures We analysed pointing as a function of target position, first for its frequency, then for the hand used to point. The number of trials per child varied between 7 and 25, with a mean of 15.6 (SD5.1; median15). The infants did not point on every trial. There were large inter-individual differences in the frequency of pointing. The total number of pointing gestures per infant, for all positions combined, varied between 0 and 24, with a mean number of 9.45 (SD6.8; median7). One infant did not point at all, one infant pointed once, one infant pointed three times, and two infants pointed four times: these five infants were excluded from pointing laterality analyses because they did not point at all or because they did not point to both sides of presentation. Out of the 17 remaining infants, for some analyses we considered only the 11 infants who pointed to each target, whereas for other analyses we also included the six infants who pointed at least to the middle, left and right. Among the 17 children analysed for handedness at pointing, the number of trials per child varied between 7 and 25, with a mean number of 16.3 (SD 5.3; median15). For these 17 children the number of pointing gestures per infant varied between 5 and 24, with a mean number of 11.5 (SD6.3;
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median9). When the frequency of trials was calculated by dividing the number of pointing gestures by the number of trials, we found that infants made between .3 and 1 pointing gesture per trial, with a mean number of 0.69 pointing gesture (SD0.24; median0.73). The frequency of pointing gestures per trial for each position separately varied between .62 (position 4) and .74 (position 3) (see Table 1). A repeated measures ANOVA calculated on the frequency of pointing gestures as a function of position (N5) showed that the difference between positions was not significant and the effect is small, F(4, 76)0.48, p.75; d0.15.
Handedness in pointing Pointing was largely unimanual. Infants rarely pointed towards the puppet with both hands. Out of the 196 pointed gestures observed in total for the 17 infants who pointed at least once each to the right, the left, and the middle, only 9 gestures were bimanual, whereas 135 were made with the right hand and 52 were made with the left hand. All positions considered, these infants produced a frequency of .59 right-handed pointing gestures per trial (SD0.32) and .26 left-handed pointing gestures per trial (SD0.27). Thus we observed more than twice as many pointing gestures with the right hand than with the left hand. For these 17 infants, we pooled targets 1 and 2 (left targets) and targets 4 and 5 (right targets). It can be seen in Table 2 that the relative frequency of right- and left-hand pointing varies with target. An ANOVA on the frequency of right-hand pointing with repeated measures for target indicates that the effect is significant, F(2, 32)5.03; pB.01. A post-hoc LSD test indicates that the target effect is due to the difference between left targets and middle target. TABLE 1 Mean number of trials and mean number of pointing gestures per trial for the 17 infants who pointed at least once to the left, to the right, and to the middle position
Target 1 2 3 4 5 All
Mean number of trials 3.5 3.6 3.2 3.1 3.1 3.3
(0.8) (1.5) (1.2) (1.2) (1.3) (1.1)
Mean frequency of pointing gestures per trial .67 .64 .74 .62 .71 .69
(.2) (.3) (.3) (.3) (.3) (.24)
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ESSEILY, JACQUET, FAGARD TABLE 2 Mean frequency of left-handed (LH) and right-handed (RH) pointing gestures as a function of target location Mean frequency of LH pointing gestures out of all pointing gestures
Targets 12 3 45 All
.45 .20 .31 .26
Mean frequency of RH pointing gestures out of all pointing gestures
(.6) (.4) (.6) (.27)
.45 .76 .57 .59
(.42) (.4) (.7) (.32)
For the 11 children who pointed at each of the targets, the decrease in lefthand pointing from targets 1 to 5 was significant, F(4, 40)3.9; pB.01. A post-hoc LSD test indicates that the target effect is due to the difference between target 1 and the four other targets. Similarly, the increase in righthand pointing from targets 1 to 5 was also significant, F(4, 40)2.9; pB.05. A post-hoc LSD test indicates that the target effect is due to the difference between target 1 and targets 3 to 5 (see Figure 2).
Comparison between handedness for grasping and handedness for pointing
Mean frequency of pointing gestures per trial
The PLI (pointing laterality index) was calculated for the 17 infants, but only for target 3, the middle target: this is the only target that could be RH
LH
1 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0 1
2
3
4
5
Target position
Figure 2. Mean frequency of right- and left-handed pointing gestures per trial as a function of target position (N 11).
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directly compared with grasping, which was tested in the middle. The PLI was .56 (from 1 to 1; SD.78). Based on the PLI, 13 children were categorised as right-handers for pointing (76.5%), 3 were categorised as lefthanders (17.6%), and 1 was not lateralised (5.9%). Among the 12 infants who were right-handed for grasping, 10 were righthanded for pointing (83.3%) (see Table 3). The only child who was lefthanded for grasping was right-handed for pointing. Finally, among the four infants who were categorised as non-lateralised for grasping, two were righthanded for pointing, one was left-handed, and one was not lateralised. A chi-square test calculated on handedness for grasping as a function of handedness for pointing was not significant and with only a small effect, x2(4)4.07, p.40; 82 .24.
Language development and pointing We checked for a relationship between the frequency of pointing gestures and language development. The two variables for language were the number of words understood and number of words produced by the infant. All 22 infants were considered for this analysis. The mean number of words understood was 39.3 (between 4 and 69, SD20.8), and the mean number of words produced was 4.6 (between 0 and 13, SD3.7). The infants who were excluded from the pointing analyses because of their absence or very low frequency of pointing gestures understood fewer words (23.4, SD20) than infants who pointed at least five times (44, SD18). To check whether TABLE 3 Number of infants as a function of (a) handedness for Grasping, (b) Pointing (c) handedness for Pointing All infants (N 22) (a) Handedness for Grasping (N of words understood/produced) (b) Pointing category (N of words understood/ produced) (c) Handedness for Pointing (frequency of pointing)
8 NRH (3 LH5 NL) (36.9/3.1) 3 NP (2 LHG1 NLG) (21/1.67)
14 RH (40.7/5.5)
5P (1 LHG4 NLG) (46.4 /4) 2 NRHP (.83)
3 RHP (.72)
2 NP (27/3.5)
12 P (43/5.8) 2 10 NRHP RHP (.67) (.65)
Number of infants as a function of (a) handedness for Grasping (RH right-handers; NRH non-right-handers; LHG: left-hander for grasping; NLG: non-lateralised for grasping), and (b) as a function of Pointing (NPnon-pointers; P pointers)*in parentheses are the numbers of words understood and produced*and (c) handedness for Pointing (in parentheses is the frequency of pointing).
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Number of words understood
this difference was significant, we calculated an ANOVA on the number of words understood as a function of category for pointing (2; pointers vs non pointers). To control for handedness, we added handedness for grasping as another independent variable (2; right-handed vs non-right-handed). The results indicated that the effect of pointing on the number of words understood is almost significant, F(1, 18)3.73, p.06, and the effect is large (d0.94). Handedness did not have a significant effect, F(1, 18)0.01, p.90, and the effect is very small (d0.04). There was no significant interaction between handedness and pointing, F(1, 18)0.19, p.66. The infants who were excluded from the pointing analyses because of their absence or very low frequency of pointing gestures also produced fewer words (2.4, SD3) than infants who pointed at least five times (5.3, SD 3.6). To check whether this difference was significant, we calculated an ANOVA on the number of words understood as a function of inclusion for pointing analyses (2; pointers vs non pointers). We added handedness for grasping as another dependant variable (2; right-handed vs non-righthanded) to control for handedness. The results indicated that the effect of pointing on the number of words produced is not significant, F(1, 18)1.49, p.23, but the effect is large (d0.57). Handedness did not have a significant effect, F(1, 18)0.92, p.35, but the effect is large (d0.44). There was no significant interaction between handedness and pointing (p1). Finally we looked at the relationship between hand preference for pointing and language development. The 13 infants who were categorised as righthanded for pointing at the middle target understood more words than the four infants categorised as non-right-handed for pointing (respectively 49.8 and 25 words; see Figure 3). We calculated an ANOVA on the number of words understood as a function of handedness category for pointing (2; 60 50 40 30 20 10 0 NRH
RH
Handedness category for pointing Figure 3. Mean number of words understood as a function of handedness for pointing (NRH: non-right-handers; RH: right-handers, N 17).
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right-handed vs non-right-handed). In order to control for frequency of pointing we added it as a co-variable. The effect of handedness for pointing on the number of words understood was significant, F(1, 14)7.6, pB.02. There was no effect of frequency of pointing, F(1, 14)0.004, p.95; d 0.03. Similarly, infants who used their right hand for pointing at the middle target produced more words than infants who used their left hand (respectively 6.4 and 2.3 words). An ANOVA on the number of words produced as a function of handedness for pointing, with frequency of pointing as a co-variable, indicated a significant effect of handedness for pointing on the number of words produced, F(1, 14)6.5, pB.05, but no effect of the frequency of pointing, F(1, 14)0.18, p.67; d0.22. Considering all targets, a Pearson correlation indicated that frequency of pointing with the right hand was significantly correlated to the number of words understood (r.74; pB.05) and to the number of words produced (r.45; pB.05). In contrast to this relationship between hand preference for pointing and language development, we found no significant relationship between hand preference for grasping and language development. The 12 pointing infants who had been categorised as right-handed for grasping understood no more words (43) than the 5 pointing infants categorised as non-right-handed (46.4) (see Table 3). An ANOVA on the number of words understood as a function of handedness category for grasping (2; RH vs NRH), with frequency of pointing as a co-variable, indicated no significant effect of handedness and the effect is small, F(1, 14)0.16, p.69; d0.21. There was no effect of the frequency of pointing, F(1, 14)0.14, p.71; d0.19. Similarly, the number of words produced tends to be the same for infants, independent of their handedness for grasping (5.8 and 4 respectively for right-handed and non-right-handed for grasping). An ANOVA on the number of words produced as a function of handedness category for grasping (2; RH vs NRH), with frequency of pointing as a co-variable, indicated no significant effect for handedness but the effect is large, F(1, 14)0.88, p.36; d 0.50. There was no effect of the frequency of pointing, F(1, 14)0.05, p .83; d0.12. To see whether frequency and lateralisation of pointing were specifically associated with language development, or whether it reflected a general advance in the child’s development, we also checked whether postural state was associated with pointing. To evaluate postural state, we simply distinguished between infants who walked independently and those who did not. This variable was unrelated to pointing, whether we considered the frequency of pointing for all infants, F(1,14)0.28, p.60; d0.27, or the lateralisation of pointing for the infants who pointed. x2(2)0.86, p.65; 82 .05.
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DISCUSSION A total of 22 infants aged 14 months were analysed for pointing to distant targets at different positions, from the extreme left to the extreme right. In addition the infants were evaluated for handedness for grasping and language development. For grasping, 63.6% of the infants were right-handed, 13.6% left-handed, and 22.7% were not lateralised. These figures are in accordance with previous studies on infants’ handedness, which show that infants as young as this age are already more often right-handed than not right-handed, but that this tendency is apparent more at the population level than at individual level, many infants remaining non-lateralised (Fagard, 2004; Michel, Tyler, Ferre, & Sheu, 2006). We observed that the majority of infants pointed towards the puppets. This is in accordance with previous studies showing that a large majority of infants point by 14 months (Butterworth & Morissette, 1996; Murphy, 1978). The frequency of pointing did not vary with target position. Overall, the right hand was used twice as often for pointing as the left hand. When we pooled the lateral targets and compared left targets with middle and right targets, we found that the mean frequency of right-hand pointing was more or less twice the mean frequency of left-hand pointing for the middle and the right position, whereas for the left position, right- and left-handed pointing were equally frequent. This asymmetry between the right and left targets was also obvious when each of the five targets was considered separately for the 11 infants who pointed at each target. For these infants, for extreme-left target, left- and right-hand pointing occurred with the same frequency, but for the second-left target there was more right-handed than left-handed pointing. These results are in accordance with other studies showing that the spatial position of the target influences hand choice, but also that the right hand is more likely to point into the left visual field than the left hand is to point into the right visual field (Butterworth, Franco, McKenzie, Graupner, & Todd, 2002). It is interesting to relate these results to results on handedness for grasping, which show that infants tend to grasp objects presented to the left with the left hand and rarely with the right hand (Fagard, 1998). This result might reflect a stronger left-hemisphere involvement in pointing than in grasping. We also looked at the relationship between the two measures of handedness. Among the 17 infants who pointed, 85% of the infants categorised as right-handers for grasping were also right-handers for pointing at the middle target. However, the only left-handed infant for object grasping and two of the four infants not lateralised for grasping were right-handed for pointing. Thus the two types of handedness were relatively independent. This result is congruent with Vauclair and Imbault’s showing no significant correlation between handedness for manipulation and handedness for pointing
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outside the periods of language milestones, and in particular in 10- to -17month-old infants (Vauclair & Imbault, 2009). We observed relationships between pointing and language development. First, infants who were excluded from the pointing analyses because of their absence, or near-absence, of pointing, tended to understand fewer words than infants who pointed at least five times, even after controlling for handedness. Second, infants categorised as right-handed for pointing at the middle target understood and produced significantly more words than infants categorised as non-right-handed for pointing, even after controlling for frequency of pointing. No such relationship was found for using the right hand for grasping. These results are in accordance with other results suggesting that gestures can be considered as a ‘‘proto-conversation’’ (Petitto, Holowka, Sergio, Levy, & Ostry, 2004; Trevarthen, 1996), and that manual gestures predict later success in language (Iverson & Goldin-Meadow, 2005). It is interesting to consider several of these results together: (1) infants point to the leftmost target as often with their right hand as with their left hand, and to the less-leftward target with the right hand more than with the left one; (2) the frequency of right-handers tends to be greater for pointing than for grasping; (3) the infant who was left-handed for grasping, and half of the infants who were non-lateralised for grasping, used their right hand for pointing at the middle target; and (4) pointing, in particular pointing with the right hand, is associated with language development. In addition, this advantage associated with lateralised pointing was not merely an indication of generally advanced development, since no such correlation was found between postural milestones and pointing. These results, including the lack of significant relationship between handedness for object grasping and pointing, suggest that hand choice for pointing is not simply a consequence of hand choice for object-related action, and is more strongly biased towards the right than manual grasping. Can we relate these findings to the more general question of how handedness and language lateralisation co-evolved during evolution? If a causal link is assumed between the emergence of handedness and language lateralisation, it may be wondered whether (hypothesis 1) humans started to communicate with the left hemisphere because they used their left hemisphere controlled right hand as the dominant hand, or whether (hypothesis 2) they became right-handed because communication depended on the left hemisphere. Whether communication started as gestures, as believed by the defenders of the gestural origin of language, or as vocalisations, as believed by the defenders of the vocal origin of language, is still another question. Both hypotheses 1 and 2 are compatible with a gestural origin of language, according to which articulate language developed out of gestural communication before ceasing to depend directly on the hand.
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A gestural origin for language is one of the theories about language evolution. It was first proposed by Condillac in the eighteenth century, and has received renewed interest relatively recently (Corballis, 2003, 2010; Kimura, 1976; Vauclair, 2004; for a review see Capirci & Volterra, 2008). The proponents of this theory offer many arguments in its favour: the close cortical representation of manual and oral movements in the primary cortex, the ventral premotor cortex, the intraparietal sulcus and inferior parietal areas (Rizzolatti et al., 1988); the fact that the homologue of Broca’s area in monkeys is implicated in manual gestures and not in vocalisations (Gentilucci, Benuzzi, Gangitano, & Grimaldi, 2001); the fact that manual gestures are frequently associated with language in different cultures (Goldin-Meadow, 1999); and the fact that specific gestures often accompany progress in language learning in infants, such as hand clasping or rhythmic movements with duplicated babbling (Petitto & Marentette, 1991), pointing with word comprehension (Bates & Dick, 2002), or symbolic actions with object naming (Iverson & Goldin-Meadow, 2005). The hypothesis of hand dominance leading language lateralisation (hypothesis 1 above) would imply that right-handedness for action influenced the hand used to communicate. Since the appearance of Homo sapiens most people have been right-handed, and the right hand is mainly controlled by the left hemisphere. Thus, if gestural communication emerged due to the necessity to share tool use skills, for instance (Gibson, 1993), the gestures to model the action to an observer would have given more importance to the preferred right hand, controlled by the left hemisphere, than to the left. Articulate language, which developed thereafter and became more or less liberated from the hands, then ended up being controlled mainly by the left hemisphere as well (Hewes, 1973). The second hypothesis presents right-handedness for action as a consequence of right-handedness for communication. Early gestures may have been lateralised because of the left-hemispheric dominance for perception and production of species-typical communicative signals, encouraging the use of the right hand for object manipulation as well. Another possibility, proposed by Corballis (2003) could be that early gestures would have been symmetrical, and the incorporation of sounds to gestures caused manual asymmetries because of the left-cerebral dominance for vocal language, ‘‘the interaction between manual and vocal programming [allowing] the vocal asymmetry to create a manual one’’ (Corballis, 2003, p. 205). For Annett (1978) or McManus (2002), a genetic mutation might have created the lefthemispheric dominance underlying both asymmetries. One argument to support the idea that right-handedness is a consequence of the emergence of language has been the long-time belief that population-level right-handedness is unique to human populations (Corballis, 1997). However, there is now evidence that non-human primates show population-level right-handedness
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in some species like the chimpanzees, not so much in the wild (McGrew, Marchant, & Hunt, 2007) but at least in captivity (Hopkins, Wesley, Izard, Hook, & Schapiro, 2004). In addition chimpanzees use the right hand more consistently for communicative gestures than for object manipulation (Hopkins et al., 2005) and this has also been shown in baboons (Meguerditchian & Vauclair, 2006; Vauclair, Meguerditchian, & Hopkins, 2005), but not in all studies of captive bonobos (Harrison & Nystrom, 2008), and not in orangutans and gorillas (Hopkins, 2006). Even though it seems difficult nowadays to agree with Haeckel’s theory that ontogeny recapitulates phylogeny, it might be interesting to relate our results to the two hypotheses about the temporal relationship between the development of the two kinds of asymmetries. In fact, at least in terms of ontogeny, neither of them is supported by our results. As concluded also by Vauclair and Imbault (2009), our results rather suggest some degree of independence between the two systems. It could be the case that a tendency to use the right hand more than the left for object-related actions is induced by early asymmetries of the cortical tract (Dubois et al., 2009) before communication emerges, but is later reinforced when gestural communication and language develop. It would be interesting to perform a longitudinal study to see if, as pointing emerges, right-handedness increases*especially given that hand preference fluctuates during the first months of life (Fagard, 1998), that an increase in right-handedness accompanies several language milestones (Locke, Bekken, McMinn-Larson, & Wein, 1995; Ramsay, 1980), and that the correlation between asymmetries increases during language milestones (Vauclair & Imbault, 2009). Manuscript received 1 December 2009 Revised manuscript received 28 April 2010 First published online 9 December 2010
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