Human Movement Science 28 (2009) 818–832

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Human Movement Science

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Motor perseveration during an ‘‘A not B” task in childrenwith intellectual disabilities

Eliane Mauerberg-deCastro a,*, Márcia V. Cozzani b, Suelen D. Polanczyk c,Adriana I. de Paula a, Camila S. Lucena a, Renato Moraes d

a São Paulo State University at Rio Claro, Brazilb Social College of Bahia at Salvador, Brazilc Gama Filho University at São Paulo, Brazild University of São Paulo at São Paulo, Brazil

a r t i c l e i n f o a b s t r a c t

Article history:Available online 20 October 2009

PsycINFO classification:28002820

Keywords:‘‘A not B” errorMotor perseverationIntellectual disability

0167-9457/$ - see front matter � 2009 Elsevier B.Vdoi:10.1016/j.humov.2009.07.010

* Corresponding author. Address: Laboratory ofUniversity, Av. 24-A, 1515, Rio Claro, SP 13506-900

E-mail address: mauerber@rc.unesp.br (E. Maue

This study was designed to identify perseverative reaching tenden-cies in children with intellectual disabilities (ID), over a period of1 year, by using a version of the Piagetian ‘‘A not B” task modifiedby Smith, Thelen, Titzer, and McLin (1999). Nine children (4.8 yearsold at the beginning of the study) with intellectual disabilities (ID)(eight with mild ID; one with moderate ID) were assessed every3 months for approximately 1 year, totaling four assessments.The results indicate that in a majority of the cases perseverationwas resilient, and that the visual system decoupled from the reach-ing, especially towards the later assessment periods at the end ofthe year. Across assessment periods variability seemed to increasein each trial (A1 through B2) for reached target. These individuals,vulnerable to distraction and attention and to short-term memorydeficits, are easily locked into rigid modes of motor habits. They aresusceptible to perseveration while performing simple task contextsthat are typically designed for 10- to 12-month-old, normally-developing infants, therefore creating strong confinements to sta-ble, rigid modes of elementary forms of behavior.

� 2009 Elsevier B.V. All rights reserved.

. All rights reserved.

Action and Perception, Department of Physical Education, São Paulo State, Brazil. Tel.: +55 19 3526 4333; fax: +55 19 3526 4321.rberg-deCastro).

E. Mauerberg-deCastro et al. / Human Movement Science 28 (2009) 818–832 819

1. Introduction

During the pre-verbal stages of development, manipulation, reaching, and bimanual coordinationtypically are target behaviors for assessment tools that measure not only maturational status, but cog-nition as well (Bushnell & Boudreau, 1993; Streri, Lhote, & Dutilleul, 2000). The hands can reveal muchabout an infant’s cognitive skills. In order to observe the cognitive and maturational status of infantsand children, Jean Piaget (1954)—one of the most important theorists on the evolution of cognition—focused on manipulative and reaching tasks as important aspects of assessing cognitive development.His choices of assessment tools included manipulative tasks that investigated what babies’ handscould do in relation to their bodies and surrounding objects. His many contributions include organiz-ing cognitive tasks—those which he believed reveal what infants know. The canonical ‘‘A not B” task issuch a tool. It works thusly:

In a quiet room, an 8-month-old girl observes an experimenter as he hides, under one of two cups afew inches apart, a toy that the child eagerly wants to reach. This cup is assigned as the ‘‘A” loca-tion. After a few seconds, the baby girl is allowed to retrieve the toy, which she does. Again, theexperimenter plays the game of hiding the toy, and she repeats this effort five or six times. Then,suddenly, he changes the game and places the toy under a second cup, located next to location ‘‘A”,which he calls location ‘‘B”. The infant watches this event very attentively. After a few seconds, sheis allowed to search for the toy. However, the location she visits does not reveal the toy. Interest-ingly, she was unable to avoid going back to look for the toy at location ‘‘A”, even though she clearlysaw the experimenter hide her toy in the ‘‘B” location (Cozzani & Mauerberg-deCastro, 2005).

Piaget used this hide-and-seek ‘‘A not B” reaching task to demonstrate that 8-month-old infantswho fail to correctly find a hidden object from a new location—even though they had successfully res-cued it from a place in which it had been formally hidden—do so because they can not understand anindependence between objects and their own actions on them. Piaget’s task represents a cognitive ap-proach to the understanding of infant development.

There were, however, differing interpretations that rebutted the idea that cognitive factors inter-fere, producing this search tendency. Diamond, Cruttenden, and Neiderman (1994) claimed that the‘‘A not B” error was the result of the infant’s weak memory for the new location, and, also, that therewas a strong influence of a gestural motor memory, which was created by the infant’s successivereaching toward one specific direction. Diedrich, Thelen, Smith, and Corbetta (2000) reclaimed thisinterpretation, and added the notion that repetitive reaching creates a perseveration raised from mo-tor memory. They suggested that this was not a cognitive phenomenon, but instead was a motor ten-dency that confined the inappropriate gesture to a very strong attractor. Thelen and her colleaguesdistinguished their motor task from Piaget’s by using a reaching task with two ‘‘A” and ‘‘B” visible tar-gets in order to demonstrate that reaching for the lids, as opposed to reaching for an object hiddenunder a cover—as in the Piagetian task—was sufficient to capture the ‘‘A not B” error (without the needto actually hide an object in location A, then in B). Typically, around the age of 10 months, normally-developing infants return to reaching for the ‘‘A” lid after being cued to reach for the ‘‘B” lid. In thisway, and utilizing a dynamical systems’ paradigm, they opposed the classical cognitive interpretationabout the ‘‘A not B” error, which indicates that infants, in certain ages, do not understand that objectscontinue to exist, even when they cannot be seen or acted upon directly (Thelen, Schöner, Scheiner, &Smith, 2001). To them, the task context variations offered an embodied view of the error phenomenon(for details, read Thelen et al., 2001). For example, besides the use of visible targets, they also demon-strated that when delays between the hiding of the toy and its recovery are manipulated, the behaviortends to change. The longer the delay, the more likely the infants are to continue to make the mistake(Diamond, 1985; Diedrich, Highlands, Spahr, Thelen, & Smith, 2001). The number of searches in the‘‘A” location also is important with regard to increasing or decreasing the likelihood of error. If infantsmake numerous attempts in the ‘‘A” location, before the toy is subsequently hidden in the ‘‘B” location,they surely will go back to the ‘‘A” location (Lew, Hopkins, Owen, & Green, 2007). The new interpre-tation of this phenomenon was that motor memory caused this perseveration (Corbetta & Thelen,1996; Diedrich et al., 2000). In other words, the performance of a repetitive reaching task, in whichthe hand movement is made repetitively in one direction, creates a strong synergy, which attracts

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the motion to the old pattern, even if the next trial is in a different location. Unless there is a strongdistinction in the input (i.e., the ‘‘B” target possesses a clear visual difference) to call the baby’s atten-tion, the error persists. Apparently, changing the target location only slightly, and locating it next tothe previous one, is not enough for the baby, at this age with a limited attention span, to make a dis-tinction. Infants tend to repeat the things they learn, which reveals much about their perception of theenvironment.

2. Motor perseveration: does it play a role in developmental delays?

It is widely known that individuals with developmental delays, such as when Down syndrome andother intellectual disability conditions are present, experience difficulties in performing motor skills(Charlton, Ihsen, & Oxley, 1996; Jobling & Mon-Williams, 2000; Mauerberg-deCastro & Angulo-Kinzler, 2000). By the age of 4 months, infants typically are able to reach and visually guide theirhands to objects. However, babies with Down syndrome show delays in reaching for objects and ex-hibit poor manipulative actions, and, in contrast with their non-disabled peers, they also have poorerperformance when adjusting their hands to the shape and size of objects (Charlton, Ibsen, & Lavelle,2000; Hogg & Moss, 1983). Charlton et al. (1996) attribute these differences to the peculiar use of vi-sual information by the former. When reaching or using their hands to manipulate objects, childrenwith intellectual disabilities often are unable to closely entrain the visual search with the activitiesof their hands.

Since the ‘‘A not B” error can induce different rates of perseveration in infants and children just bychanging the aspects and salience of the task, we can assume that the status of development plays animportant role in whether or not perseveration can be broken. Therefore, it appears that it would bevaluable to investigate how children with intellectual disabilities respond under such tasks so that wecan try to understand the extent of what we call ‘‘developmental delay.”

The questions, then, are:

1. Do infants or children with intellectual disabilities (ID) perseverate at all, and, if so, for how long?2. What can the reaching behaviors such as those displayed during the performance of the ‘‘A not B”

task teach us about the development of these children? We suspect that when children with IDstart to perform simple repetitive motor tasks such as reaching, both the absence of target contrastand a history of gesture repetition limit the reaching decisions that should be coherent with thetask context. Attention and memory are important factors in breaking old movement patternsand assisting with the emergence of new adaptive processes (i.e., correct goal-directed movementpatterns). In addition, it is widely known that infants and children with ID have limited attentionspans (Reid, 1986; Sherrill, 2004), restricted short-term memory (Hoover & Horgan, 1990), and,also, poor motor behavior repertoires (Block, 1991; Rarick, 1980; Sherrill, 2004). Therefore, we pre-dicted that, in the absence of contrasting differences between targets during simple reaching tasks,these children would be more resilient than their non-disabled peers with regard to breaking per-severative patterns.

3. What role does vision play in reaching behaviors during the performance of the ‘‘A not B” task? Wepredicted that coupling between the looking and reaching movements would be inconsistent. Thiscan be assessed through the ways in which they look at the target while reaching; this strategyreveals how attention levels play a role in motor perseveration.

We claimed, in resonance with the literature (Spencer, Smith, & Thelen, 2001), that the motormemory trace created by previous reaches toward the ‘‘A” location would affect the memory traceto the ‘‘B” location by subsidizing the latter, and that this phenomenon would be relatively steadyacross longitudinal testing. Observing the motor strategies of children with ID as they perform direc-ted tasks can tell us about the relationship between higher-order (e.g., attention and memory) andlow-order mechanisms (e.g., motor synergy of reaching). Examples of motor strategies can be identi-fied during the coupling of the eyes with the hand as individuals reach toward a target. We predicted

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that the children with ID would show weak coupling between the eyes and hand while reaching to-ward either the ‘‘A” or the ‘‘B” targets.

The purpose of this study was to identify whether or not children with ID exhibit perseverativebehavior when performing an ‘‘A not B” modified task. If so, an additional objective was to observewhether or not the perseverative tendency is resilient with regard to developmental changes; thatis, whether it would last throughout year-long periodical assessments. The performance of childrenwith ID, as it related to a modified ‘‘A not B task,” was assessed every 3 months (Smith et al., 1999).The task, which used ‘‘lids-only” as a target for reaching, is suitable to test the ‘‘A not B” error phenom-enon of normally-developing infants, aged 8–10 months (Marcovitch & Zelazo, 1999).

The error that occurs during the ‘‘A not B” reaching task reflects the status of the basic perceptual-motor processes that are typically involved in the act of visually-elicited reaching for objects (Diedrichet al., 2000). Therefore, understanding the perseverative behavior of children with ID can help us tounderstand the process of achieving motor stability through perseveration. Additionally, it can helpus understand the role that flexibility plays on the adaptation mechanism of action systems, as wellas what the developmental implications are regarding age-appropriate contexts for the ‘‘A not B” task.

3. Method

3.1. Participants

Nine children with intellectual disabilities (ID) took part in this study and were assessed initially,and then every 3 months for approximately 1 year, totaling four assessments each. At the time of theirenrollment in the study, the ID children’s average age was 54 months (SD 11.3), or 4.8 years. Theywere the youngest children enrolled in the single local institution for people with intellectual disabil-ities in the city of Rio Claro, State of São Paulo, Brazil. According to the records of this special educationinstitution, all but one of the ID children have mild intellectual disabilities. Only two of the childrenhave unknown causes for the ID; all of the others have Down syndrome (DS) (Table 1). These childrenwere those that remained from a larger pool of thirteen participants. Four children were excluded fornot finishing the task or refusing to participate in two subsequent assessment trials. A signed consentform for non-paid participation was obtained from the parents or guardians of each child, and thestudy was approved by the Ethics Committee of the São Paulo State University.

3.2. Procedure

Following the procedure of the traditional Piagetian ‘‘A not B” task as modified by Smith et al.(1999), we presented the child with two identical targets (lids), 9 cm diameters each, placed side-by-side (15 cm apart), on top of a wooden box (40 � 25 � 7 cm) (Fig. 1).

The experimenter ‘‘enticed” the child to reach for and grasp at one of the two lids (called ‘‘A”), sixtimes for each child, with a 5-s delay between cueing (shaking the lid and enticing the child to grab it)and giving the stimulus to the child as a reward for reaching for it. The first time the lid was cued, itsposition was moved closer to the child in order to ensure a greater probability that the child wouldreach for it. In the successive two trials the ‘‘A” lid was placed closer and closer to the ‘‘B” lid (seeFig. 2). The four initial ‘‘A” trials were considered practice trials, but all were used during data analysis.Subsequently, the second lid, called ‘‘B,” was similarly cued two times consecutively, and the child wasallowed to reach and grasp at it. The order of ‘‘A” and ‘‘B” lid positions (left or right, relative to thechild) was randomly assigned. If the child did not perform a reach within 15 s, the trial ended anda new trial was presented. All but one child were right-handed. Some children who grabbed at bothtargets exhibited a temporal lag between the right and left reach. Out of a total of 72 possible reachesper assessment, 2 bimanual reaches occurred in assessment 1; 3 in assessment 2; and 3 in assessment4. For this group, bimanual reaches occurred only when ‘‘A” target was cued. The children reached to-ward ‘‘B” and followed with the other hand toward ‘‘A.” No such reaches were observed during thecuing of the ‘‘B” target.

Table 1Demographic information for the participants, and timetable of participation.

Participantcode

Gender ID classificationlevela

Age in months at thetime of enrollment(years in parenthesis)

ID etiology Number of times achild was assessedin 1-year period

Average intervalof assessments (months)

CED Male Mild 66 (5.6) Down syndrome 4 3.5LUV Male Mild 54 (4.6) Down syndrome 4 3.0TAA Female Mild 43 (3.7) Other 4 3.5GIS Female Mild 43 (3.7) Down syndrome 4 3.5CAO Male Moderate 73 (6.1) Down syndrome 4 3.5ROA Female Mild 54 (4.6) Down syndrome 4 3.5LEB Male Mild 41 (3.5) Other 2 3.0GAB Male Mild 51 (4.3) Down syndrome 4 3.5AMG Female Mild 61 (5.1) Down syndrome 4 3.5Mean 54 (4.8) 3.4Standard deviation 11.03 (1.04) 0.2

a ID level was classified using a multiple intelligences protocol adapted by the APAE/Brazil (i.e., perceptual, cognitive and daily living functioning, pre-school level, language and socialskills).

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Fig. 1. Task representation and cameras’ layout.

Fig. 2. Representation of the target presentation during specific input for trials ‘‘A” (i.e., A1–A6) and trials ‘‘B” (i.e., B1–B2).

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All participants were videotaped with two video cameras while performing the ‘‘A not B” task. Onecamera was located at the frontal plane, taping the eyes and reach; the other was placed perpendicularto the side of the body that related to the dominant hand. Digitizing marks were attached both on theoutside and on the inside of the wrists.

Videotaped images were processed using a Pinnacle videocard, version 9.02, with at 30 Hz rate ofdata digitizing. To process the video images, we used Dvideow software, 4.0.

3.3. Data analysis

First, throughout the ‘‘A” and ‘‘B” trials, we coded the reached-for target (i.e., target ‘‘A” or target‘‘B”), which allowed us to calculate first, the frequency of reaches towards cued targets. To analyzethe history of reaches, and based on Diedrich et al. (2000), we calculated the cumulative memory in-dex (CMI) (Table 2). The more frequently a child reached toward a direction, the greater was the CMIin that direction.

Table 2Hypothetical example of the calculation of the relative cumulative memory index (CMI) of reaches toward ‘‘A” for a participantwho reached to ‘‘B” during ‘‘B” trials.

‘‘A” trials ‘‘B” trials

PerformanceA A B A A A B B

Memory for ‘‘A” reaches1/8 2/8 2/8 3/8 4/8 5/8 5/8 5/8

Memory for ‘‘B” reaches0/8 0/8 1/8 1/8 1/8 1/8 2/8 3/8

Memory for ‘‘A” reaches – Memory for ‘‘B” reaches = CMI1/8 2/8 1/8 2/8 3/8 4/8 3/8 2/8

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We also coded location of looking, at the moment the child touched a target—whether they reachedtoward ‘‘A” or ‘‘B”—for both target ‘‘A” input and target ‘‘B” input. The possible locations for lookingwere ‘‘looking at ‘‘A” target (A)”, ‘‘looking at ‘‘B” target (B),” and ‘‘looking at other places (O).” Thesecombinations represent looking-reaching coupling. We computed the frequency of these locationsfor the group in both ‘‘A” and ‘‘B” trials. This analysis was behaviorally coded, based on videotapedimages from a frontal view of the participant relative to the targets. Trained coders were assessedfor reliability within trials and between coders (reliability values were higher than 80% agreement).

3.4. Statistical analyses

The frequency of the reaching behavior, and the looking-reaching coupling location were subjectedto a non-parametric statistical analysis using the Wilcoxon Signed Ranks Test for a 2-tailed pair in or-der to identify the presence of perseveration within each assessment and differences in perseverationamong periods of assessment. Regression analyses were performed individually for each assessmentperiod, with the CMI as the dependent variable, and trials (i.e., A1 through B2 targets) as the indepen-dent variable. The slope of the regression analyses was used as the dependent variable in a one-wayANOVA (periods of assessment) with repeated-measures. Also, using the CMI standard deviation asthe dependent variable, CMI variability was analyzed through a one-way ANOVA (periods of assess-ment) with repeated-measures. When appropriate, Bonferroni’s post-hoc analysis was carried out inorder to detect where the differences resided. When effects showed significant differences, we mea-sured the relative magnitude of our sample size using Cohen’s effect sizes method (Thalheimer & Cook,2002). According to Cohen’s definition, an effect size of .8 is large, .5 is medium or moderate, and .2 issmall.

4. Results

The results of this study indicated that a majority of the children made the ‘‘A not B” error through-out the process of yearly data gathering, particularly for the first assessment period. For all four assess-ment episodes, all but one child remained in the group. Participant LEB dropped out after the secondassessment.

4.1. Frequency of reaches

Table 3 summarizes the absolute number and frequency of reaches by the children towards target‘‘A1” through ‘‘A6” and ‘‘B1” and ‘‘B2” during the four assessment periods. In the first assessment, mostchildren reached towards the ‘‘A” target during the ‘‘A” trials. When cued toward ‘‘B” target, one childreached once towards ‘‘B” (i.e., B1). Three months later, during the second assessment, although mostchildren reached to ‘‘A” target when cued to ‘‘A,” some reached to ‘‘B” targets. Then, when cued to ‘‘B”target, three children reached once towards ‘‘B,” and one reached correctly towards ‘‘B” target in bothtrials. By the third assessment, most children continued to correctly reach toward ‘‘A” when cued to-ward ‘‘A.” Four reached once toward ‘‘B” when cued toward ‘‘B,” and two reached toward ‘‘B” in bothtrials. By the fourth assessment, some children reached toward ‘‘B” when cued toward ‘‘A” target insome of the A trials. During the ‘‘B” trials, also, four reached once toward ‘‘B” when cued toward‘‘B,” and two reached toward ‘‘B” in both trials. The correct reaches towards ‘‘B” either in B1, B2, orboth were not necessarily performed by the same children.

To identify the consistency of reaching toward ‘‘A” when cued toward ‘‘A” target, we analyzed themaximum possible number of reaches in ‘‘A” (i.e., 6 reaches) against the actual number of reaches in‘‘A” for each assessment period separately. Wilcoxon Signed Ranks Test, comparing for a 2-tailed pairof comparisons, showed significant differences in assessments 2 (z = �2.271, p = .023) and 4 (z = 2.041,p = .041), but not for assessments 1 and 3. This indicates that, in assessment 1 and 3, children wereconsistently reaching towards ‘‘A” target when cued to ‘‘A,” however, in assessments 2 and 4 somechildren chose to reach toward the ‘‘B” target when cued to ‘‘A.”

Table 3Absolute number and percentage of reaches towards ‘‘A” and ‘‘B” targets for each assessment period.

A and Btrials

1st assessment(n = 9)a

2nd assessment(n = 9)a

3rd assessment(n = 8)a

4th assessment(n = 8)a

Absolute number of reaches inA during ‘‘A” trials 6A

A1 9 9 8 7A2 8 8 6 7A3 8 7 6 6A4 9 6 6 5A5 9 7 6 7A6 9 7 6 6

Mean percentage of reaches in Ab 96.3% 81.5% 79.2% 79.2%Absolute number of reaches in

B during ‘‘B” trials 2BB1 1 3 4 4

B2 0 1 2 2Mean percentage of reaches in Bc 5.5% 22.2% 33.3% 37.5%

a In parenthesis is the number of children in each assessment period.b In A trials, 100% possible reaches by the entire group corresponded to 54 reaches in assessments 1 and 2, and 48 reaches in

assessments 3 and 4.c In B trials, 100% possible reaches by the entire group corresponded to 18 reaches in assessments 1 and 2, and 16 reaches in

assessments 3 and 4.

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When we compared amongst assessment periods, Wilcoxon Signed Ranks Test showed that thenumber of reaches performed in ‘‘A” was significantly different only between assessments 1 and 2(z = 2.070, p = .038). Therefore, the number of reaches towards ‘‘A” performed during the first assess-ment was closer to the maximum (i.e., 6 reaches for ‘‘A” trials).

Results for the maximum possible number of reaches in ‘‘B” (i.e., 2 reaches), compared against theactual reaches in ‘‘B” for each assessment period, were analyzed using the Wilcoxon Signed Ranks Test.The 2-tailed pair of comparisons show significant differences in assessment 1 (z = �2.762, p = .006), 2(z = �2.460, p = .014), 3 (z = �2.271, p = .023) and 4 (z = 2.428, p = .015). For ‘‘B” target, reaches werecommonly directed towards ‘‘A” target.

The comparisons made amongst the assessment periods showed that the number of reaches to-ward ‘‘B” target during all four assessments was not significantly different for any of the comparisons.Although, the percentage of reaches in ‘‘B” during ‘‘B” input increased from assessment 1 (5.5%) toassessment 4 (37.5%), the number of reaches toward ‘‘A” was significantly higher than were those to-ward ‘‘B,” therefore confirming perseveration.

4.2. The cumulative memory index (CMI)

The cumulative memory index (CMI) is depicted in Fig. 3. The strength of reaches toward ‘‘A” andthen to ‘‘B”, represented by the CMI, increased linearly toward 1 for all four assessment periods. Par-ticularly evident is the larger variability of the three last periods of assessments as compared to thefirst one. The slope of the linear regression—calculated for each participant—was used in a univariateanalysis with periods of assessments as the factor. No significant effect was detected. Since the vari-ability in each trial (A1 through B2) seemed to increase across assessment periods, we employed aone-way ANOVA using the CMI standard deviation of each assessment period as the dependent vari-able. The results show a main effect for the assessment periods (F3,21 = 12.49, p 6 .0001). Cohen’s effectsize indicates a medium magnitude effect (d = .64). Bonferroni adjustments for pairwise comparisonsshow that, compared to the first assessment, variability was higher for assessment 3 (p = .037) and 4(p = .009), and compared to assessment 2, variability was higher for assessment 3 (p = .006) and 4(p = .029).

4.3. Looking-reaching coupling

Results from the location toward which each child was looking at the time of reaching were com-puted for all trials that were related to input ‘‘A” (i.e., 6 trials) and to input ‘‘B” (i.e., 2 trials). To rep-

Fig. 3. The mean relative cumulative memory index strength for reaches toward ‘‘A” and ‘‘B” for the 1st (a), 2nd (b), 3rd (c), and4th (d) assessment periods.

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resent looking-reaching coupling, we coded looking toward ‘‘A” locations, ‘‘B” locations, and otherplaces (O). Fig. 4 represents the group mean frequency (and standard deviation) for looking toward‘‘A” and ‘‘B” locations for each assessment period. Frequency of ‘‘looking at other places” was quitelow, i.e., 10%. The percentage of time participants looked at ‘‘A” location during input ‘‘A” was approx-imately 80% during assessment 1, and dropped linearly, reaching 50%, by the 4th assessment period.The percentage of time participants looked at ‘‘B” location during input ‘‘B” was 20% during assess-ment period 1, and increased to 40% and 50%, respectively, during assessments 2 and 3, then droppedback to 20% by assessment 4.

To identify those children who consistently coupled looking-reaching toward target ‘‘A” when tar-get ‘‘A” was cued, for each assessment period we computed (from the moment the participant touchedthe target) whether they reached toward ‘‘A” or ‘‘B.” We also compared the maximum possiblenumber of times they could have looked toward target ‘‘A” (i.e., 6) against the number of times they

Fig. 4. Group mean and standard deviations for the frequency (%) of looking-reaching coupling during input ‘‘A” and input ‘‘B”for the four assessment periods.

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actually looked toward target ‘‘A.” Wilcoxon Signed Ranks Test for a 2-tailed pair of comparisons showsignificant differences for all of the assessment periods: assessment 1 (z = �2.041, p = .041), assess-ment 2 (z = �2.032, p = .042), assessment 3 (z = �2.232, p = .026), and assessment 4 (z = �2.264,p = .024). Therefore, since the number of times they actually looked toward target ‘‘A” was signifi-cantly less than expected (i.e., 6 times), we assumed that looking was decoupled from reaching. Sim-ilar results were found for input ‘‘B.” The number of times they actually looked at target ‘‘B” wassignificantly less than expected (i.e., 2 times) for assessment 1 (z = �2.646, p = .008), assessment 2(z = �2.428, p = .015), assessment 3 (z = �2.333, p = .020), and assessment 4 (z = �2.460, p = .014). Dur-ing the input in ‘‘A” target, the rate of looking-reaching coupling was different between assessments 3and 4 (z = �2.070, p = .038). During the input for ‘‘B” target, differences were significant betweenassessments 1 and 2 (z = �2.000, p = .046) and 1 and 3 (z = �2.236, p = .025).

4.4. Summary of results

The main findings and empirical implications are summarized in Table 4. Results of the compari-sons are presented relative to the perseveration phenomenon and perceptual-motor couplement.

5. Discussion

Utilizing the Piagetian ‘‘A not B” task modified by Thelen and collaborators, we designed our studyto identify, first, the presence of perseverative behavior in children with ID; and second, if persever-ative behavior was consistently identified, to observe whether or not this perseverative tendencywas resilient with regard to developmental changes (Smith et al., 1999). Therefore, we assessedthe participants longitudinally over a year-long period. Our premise was that ID children indeed per-severate for longer periods than do their normal developing peers (Munakata, 1998; Smith et al.,1999). Our results show that over a period of 1 year, the children with ID continued to make the‘‘A not B” error.

5.1. Do infants or children with ID perseverate at all, and, if so, for how long?

This error reflects basic perceptual-motor processes involved in visually-elicited reaching for ob-jects (Diedrich et al., 2000). Therefore, understanding the perseverative behavior in children with IDcan help us understand the process of achieving motor stability through perseveration, and the rolethat flexibility plays on the adaptation mechanism of action systems.

Perseveration was assumed to occur when the children reached toward ‘‘A” although they werecued toward ‘‘B.” It is a fact that most of these ID children exhibited perseveration, especially at thefirst period of assessment.

The children with ID in this study tended to become more variable in their choices of reaching afterthe first period of assessment. For instance, when they reached for a target ipsilaterally, some of themtended to reach toward the other target immediately afterward (see Section 3 for prevention of simul-taneous bimanual reaching). When reaching was performed contralaterally to the target (towardeither ‘‘A” or ‘‘B”), the children did not employ bimanual reaching. Because the options for reachingwere visually obvious (i.e., they could see the two lids), the children with ID simply were able to maketheir own rules and escape the task-goal set by the experimenter. Their attention deficiency could ex-plain why some of the ID children chose to reach toward the targets ‘‘by chance,” therefore, exhibitingdisorganized behavior strategies and making it difficult to determine the influence that prior reacheshad on the error. During assessment 1, perseveration was more prevalent and the attention was likelyfocused towards the first ‘‘A” trials, until a strong motor memory field was created. We have to con-sider that the first three ‘‘A” trials were more salient due to the strategy of placing ‘‘A” target closer tothe edge of the box. The further reaches were likely determined by automatism, including the ‘‘B” tri-als. As for the ‘‘B” trials, compared to further assessment periods, the larger number of children whoperseverated during the 1st assessment could imply, also, lack of attention. Herein, lack of attentionlikely failed to resolve the influence of the memory field created during the previous reaches. Since

Table 4Summary of main results for each investigated parameter and implications.

Variable Comparison Result Implication

Frequency ofactualreacheswithin eachassessmentperiod

Actual reaches comparedto maximum number ofreaches towards ‘‘A”target (i.e., 6 reaches)

Less reaches towards ‘‘A” targetwithin assessments 2 and 4

When cued in ‘‘A”, reachesoccurred either in ‘‘A” or ‘‘B”,demonstrating inconsistency inthose two periods

Actual reaches comparedto maximum number ofreaches towards ‘‘B”target (i.e., 2 reaches)

Less reaches towards ‘‘B” targetwithin all assessments periods

When cued in ‘‘B”, reachesoccurred mostly in ‘‘A”,demonstrating perseveration in allperiods

Frequency ofactualreachesamongassessmentperiods

Actual reaches comparedto maximum number ofreaches towards ‘‘A”target (i.e., 6 reaches)

Differences were observed onlybetween assessments 1 and 2

In the first assessment, reachestowards ‘‘A” target werepredominant; in the finalassessment periods, some reacheswere directed towards ‘‘B”

Actual reaches comparedto maximum number ofreaches towards ‘‘B”target (i.e., 2 reaches)

No differences were found amongassessment periods

In general, in all assessmentperiods, reaches towards ‘‘A” targetwere predominant when cued in‘‘B”

Cumulativememoryindex (CMI)

Multiple comparisons ofassessment periods forCMI

Variability increased progressivelywith the periods of assessment;reaches in ‘‘A” and ‘‘B” becamemore inconsistent along thesuccessive assessments

Towards the final assessmentperiods, children reached towards‘‘B” target during ‘‘A” trials, andstarted correctly reaching towards‘‘B” in some ‘‘B” trials, which isreflected in the increasedvariability

Looking-reachingcoupling

Number of times childrenlooked consistently at thecued target ‘‘A”

Eye was decoupled from reaching;they reached to ‘‘A” but lookedelsewhere; a decrease in thisdecoupling was observed towardsthe final assessment periods

Although variability remainedconstant, looking-reachingchanged towards the finalassessment period for the ‘‘A”(cued) target, when the childrenstarted looking more often to otherplaces; when cued towards ‘‘B”target, the children started lookingmore consistently to targets theyreached for, as periods ofassessment followed

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attention deficit is an important aspect of perceptual and cognitive processes in children with ID, wewould expect that variability in their reaching behavior would be similar throughout the assessmentperiods, and especially during the first one. This was not quite the case. Therefore, is difficult to deter-mine what exactly happened after the first assessment period. We could speculate that the variabilityfound during the further assessment periods could be due to an awareness of the presence of a secondlid, even though few bimanual attempts were made. For some children in our study, choosing optionsoutside of the task requirement (e.g., reaching at ‘‘B” while target ‘‘A” is cued, then immediately after-ward reaching toward ‘‘A” as well) might be a step in their process of decision making, although it isnot efficient relative to the rules of the task. In either case, it is difficult to justify their reaching ten-dencies within the context of the perseveration phenomenon. According to Marcovitch and Zelazo(1999), attention is a key factor in the occurrence of the error in older children without disabilities.

It appears that the ID children, although playful with their choices, tended to reach more towardthe ‘‘A” target, regardless of which target was cued. However, one interesting aspect of their choicestoward ‘‘B” appeared: Some children reached for ‘‘B” target after being cued to the B1 target, but re-turned to ‘‘A” target after being cued to the B2 target. It seems that the memory field created by suc-cessive reaches toward ‘‘A” was weakened by the salience of the ‘‘B” location, as contrasted to theprevious ‘‘A” location (i.e., the A6 trial). But the memory field rose again in the ‘‘B2” trial, as if thememory field for the ‘‘B1” location had little effect on decision making during the ‘‘B2” trial.

The fact that not all of the ID children reached all of the time toward ‘‘A” when cued toward ‘‘A”could indicate that their memory about this location was compromised, which later affected their

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reaching efforts in the ‘‘B” trials. Of the five children, the three who reached correctly toward ‘‘B” inboth ‘‘B” trials, made only 2, 3, and 4 reaches, respectively, toward target ‘‘A.” Because they made onlya few reaching efforts in ‘‘A” when cued toward ‘‘A,” when the input was cued in ‘‘B,” the memory fieldfrom the previous reaches in ‘‘A” was not strong enough to interfere with the memory field for ‘‘B”location.

Our first prediction was corroborated. It stated that with reference to studies using non-disabledinfants these children would be more resilient to breaking perseverative patterns in the ‘‘A” not ‘‘B”reaching task. The lack of salience between targets is likely to be the main cause of such a tendency,as is robustly evidenced in the literature (Corbetta, Thelen, & Johnson, 2000; Diedrich et al., 2001; Mar-covitch & Zelazo, 1999; Smith et al., 1999). This tendency was confirmed in the successive assess-ments, although with more variability, as was indicated by the frequency of reaches as well as inthe IMC parameter.

5.2. What can the reaching behaviors during the ‘‘A not B” task teach us about the development of thesechildren?

An important finding of the current study is that it supports the notion that the developmental le-vel expected in such tasks is not fundamental in children with ID. While normally-developing childrentend to stop perseverating while performing this same task by the ages of 10–12 months (Clearfield,Diedrich, Smith, & Thelen, 2006), the children with moderate ID are subject to effects caused by theambiguity of the stimuli. Therefore, it is not their developmental level, but their inability to focusattention continuously on the task, that plays the main role in their ability to overcome the error.(The reasons for not focusing attentively on the task were discussed in the previous section.) Althoughthis reaching behavior seems to occur naturally and simply, there appear to be at least three concur-rent informational factors involved: (1) a reason, or motivation, for reaching in a particular direction;(2) the continuous salience of the target object; (3) a motor memory created by previous actions (i.e.,perseveration) (Diedrich et al., 2001). Such factors are true even for these individuals with intellectualdisabilities.

Indeed, one control element that breaks perseveration later in an infant’s life is a developing con-sciousness about the environment and task requirements. Therefore, attention and memory areimperative to the success of this kind of task. In the case of children with ID, salience of task wouldhave to have more significant contrast than that used in the ‘‘A not B” task protocols.

Thelen and her collaborators demonstrated that when salience (i.e., color or pattern) is contrast-ing, perseveration drops (Smith & Thelen, 2003; Smith et al., 1999; Spencer et al., 2001; Thelen &Smith, 1994). Also, the greater the number of trials in ‘‘A”, as well as the longer the delay betweenthe presentation of the ‘‘A” object and reaching, the greater the increase in chances forperseveration.

Thelen et al. (2001) explained perseveration in terms of a dynamic competition between faster andslower memories. Faster traces of memory result immediately after an input. Slower traces are builtupon the history of previous experiences within the task context. In very young infants, behavior istriggered by active memory traces, therefore, perseveration often doesn’t occur, or is unreliable. Asthey get older, a slower time scale memory is strengthened, and competition between the two tracesends in perseveration. This competition is also affected by time. Larger delays between cued target andreaching behavior is a determining factor in perseveration in older children who performed the ‘‘A notB” task within the context of a hidden object in a sandbox (Spencer et al., 2001).

Although our study included successive assessments over a period of 1 year, it is difficult to deter-mine whether or not these children would continue to perseverate indefinitely, since we found thatthe task itself becomes a non-motivating context. An attempt for a fifth assessment was made withthis group and the remaining majority refused to participate. They appeared to be disinterested andbored. Also, it is well established in the literature that children with ID stabilize their behaviors asdo their normally-developing peers, although at a slower pace (Block, 1991; Rarick, 1980; Reid,1986). If they are slower in stabilizing their behavior than are their normally-developing peers, it isreasonable to predict that they also would be slow to break a strong attractor state or that they wouldneed stronger behavioral information to change their intrinsic dynamic. Our data indicate a tendency

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toward perseveration reduction within a 1-year period. It may be the case that they need extra time toexhibit non-perseverative behavior.

Another study using the ‘‘A not B” paradigm in children with mild and moderate ID was conductedby Cozzani (2005). Her study included children with Down syndrome (mean age = 4.7 years) who per-formed the ‘‘A not B” task using the sandbox paradigm. The sandbox task was originally designed todemonstrate that perseveration would manifest itself regardless of a specific period of development,as originally indicated in the Piagetian task (i.e., 8–10 months old); therefore the sandbox task wasdeveloped for children around 2 years old (Spencer et al., 2001). Cozzani’s results show perseverativetendencies in ID children as well. Because in the sandbox context targets are buried in front of the chil-dren, the reaching task also includes digging in the sandbox and then searching for the (non-visible)target. Aside from a higher frequency of perseverative behavior as compared to normally-developing2 years old, the children with ID showed an invariant kinematic profile in their reaches. Individuals inthe normally-developing group varied their reaching profiles due to ‘‘explorations” while theysearched for the exact location of the target, especially when attempting to retrieve the ‘‘B” target.The children with ID exhibited straight-forward reaches towards the box, mostly towards the ‘‘A” loca-tion, regardless of cued targets. Cozzani’s study, which examined similar age groups of children withID as those in the current study, helps illustrate that task requirements in the ‘‘A not B” paradigm arenot contingent upon age. In children with ID, the association between perseveration and the develop-mental level of cognition rely on many factors, especially attention and motivation levels.

5.3. What role does vision play in reaching behaviors during the performance of the ‘‘A not B” task?

When a baby sees as an object she is interested in, she grabs at it. According to Diedrich et al.(2001), reaching behavior emerges when visually enticing stimuli lead to a repetitive perceptual-mo-tor activity.

Our prediction about the looking-reaching coupling being an inconsistent phenomenon (that is,that the location where the child looks at the time of reaching is other than the reached toward orcued targets) cannot be completely accepted. While looking-reaching coupling reveals the role of vi-sual attention in such tasks, our results showed that some of the children continued to look consis-tently at the ‘‘A” target during the moment of hand contact with the target ‘‘A.” However, ingeneral, looking was decoupled from reaching. This situation declined and became more variable inthe later assessments, dropping to about one-half of its frequency for the ‘‘A” trials. At this point,the children either reached toward a target while looking to either of the other targets, or looked else-where. During the ‘‘B” trial of the last assessment, some children coupled their eyes with the hand atthe instant of the touching of the target, but this result failed to reach significance. Therefore, for theobservation of the motor perseveration phenomenon—resilient within this group—, the use of visualprocesses was unreliable. Immediately after the input and during the delay period (approximately5 s), almost all the children became distracted and looked away from the experimental layout. Theyspent much of the delay period inspecting their hands, turning backwards in their seats, even lookingunder the table, or trying to engage in conversations with the experimenter.

According to Horobin and Acredolo (1986), normally-developing infants who persist at looking to-ward a hiding location during the ‘‘A not B” task are much less likely to make the error than those whodo not. Moreover, visual distractions during the hiding event increase the likelihood of an incorrectsearch (Marcovitch & Zelazo, 1999).

6. Final considerations

As mentioned previously, children with ID often form habits and stabilize behaviors as do their nor-mally-developing peers, although at a slower pace (Block, 1991; Rarick, 1980; Reid, 1986). Fundamen-tal behaviors that ensure the gathering of information from the world, such as reaching, are subjectedto constraints that are intrinsic (cognitive status, for example) and extrinsic (salience of stimuli, forexample). It appears that the competition between those memory fields utilized during the perfor-mance of the ‘‘A not B” task which lead to perseveration is pervasive across different ages, and is

E. Mauerberg-deCastro et al. / Human Movement Science 28 (2009) 818–832 831

resilient to a homogeneous process of development. That is, these 4 years old children with ID canspeak, establish social and meaningful interactions, understand rules and regulations, play withadults’ demands, and so forth. Yet, they also make mistakes during the performance of a trivial reach-ing choice-task that are common to 8- to 10-month-old infants.

The general processes, for different age groups, that produce goal-directed actions to rememberedlocations in the ‘‘A not B” task context, confirm the important role that visual and attentional pro-cesses play in the production of the error (Spencer et al., 2001). Our results confirm that the erroris not specific to a particular point in development. Our conclusion, then, is that developmental status,or integrity, can become a key factor in violating the premise that the ‘‘A not B” error occurs and dis-appears relative to a progression of age. In other words, evolved, complex, and diverse behaviors inchildren with ID do not preclude possibilities of confinement in stable, rigid modes of elementaryforms of behavior, such as a dual choice-task. Their biases, it seems, are contingent upon short-termmemory and attention deficits, and perhaps to a vulnerability to low-order system behavior (i.e., mo-tor memory during a reaching gesture) and repeated exposure to repetition.

7. Limitations of the study

Generalizations about individuals with ID and perseverative behavior in an ‘‘A not B” task should bemade with caution. The present study did not utilize a control group, and, therefore, its outcomes canonly be compared indirectly with previous studies. Another limitation concerns the small number ofparticipants, varied chronological ages, varied age at the enrollment in the study, and that the periodof assessments were limited to 1 year. The follow-up beyond 1 year, in our case, posed the problem ofmotivation and the children’s willingness to continue to participate in the study. An increasing num-ber of them refused to continue to participate after the last assessment period.

8. Future directions

This study expanded the documentation of the well-researched ‘‘A not B” error by investigating agroup of children with intellectual disability. Since the results showed a tendency for a long-lastingperiod of perseverative error, our next study with children with ID (in progress) will focus on compar-isons of ‘‘A not B” tasks that are typically constructed for different age groups, as well as expanding thetime period of longitudinal observations. Because attention was such an important issue in this task,reducing the delay between task input and permission to reach—by placing targets within touchingdistance—would allow us to make inferences about attention span and short-term memory regardingnew target locations. Finally, future studies using this paradigm with children with developmentaldisabilities should identify longitudinal changes in control mechanisms of reaching during the processof ‘‘breaking” the motor memory created by successive movements in the same direction. This wouldhelp to verify, in these children, the extent of flexibility and adaptation in their perceptual-motordevelopment.

Acknowledgments

This study was supported by the Brazilian research agencies, CNPq/PIBIC program, FAPESP, andFUNDUNESP/UNESP. We wish to thank the staff and directors of APAE, Rio Claro, Brazil, and the chil-dren with intellectual disabilities and their families who participated in this study. Finally, we wouldlike to thank Debra Frances Campbell for her review of the manuscript, as well as the journal HUMOVanonymous reviewers for their insightful comments.

References

Block, M. E. (1991). Motor development in children with Down syndrome: A review of the literature. Adapted Physical ActivityQuarterly, 8, 179–209.

Bushnell, E. W., & Boudreau, J. P. (1993). Motor development and the mind: The potential role of motor abilities as adeterminant of aspects of perceptual development. Child Development, 64, 1005–1021.

832 E. Mauerberg-deCastro et al. / Human Movement Science 28 (2009) 818–832

Charlton, J. L., Ibsen, E., & Lavelle, B. M. (2000). Control of manual skills in children with Down syndrome. In D. Weeks, R. Chua, &D. Elliott (Eds.), Perceptual-motor behavior in Down syndrome (pp. 25–48). Champaign, IL: Human Kinetics.

Charlton, J. L., Ihsen, E., & Oxley, J. (1996). Kinematic characteristics of reaching in children with Down syndrome. HumanMovement Science, 15, 727–743.

Clearfield, M. W., Diedrich, F. J., Smith, L. B., & Thelen, E. (2006). Young infants reach correctly in A-not-B tasks: On thedevelopment of stability and perseveration. Infant Behavior and Development, 29, 435–444.

Corbetta, D., & Thelen, E. (1996). The developmental origins of bimanual coordination: A dynamic perspective. Journal ofExperimental Psychology: Human Perception and Performance, 22, 502–522.

Corbetta, D., Thelen, E., & Johnson, K. (2000). Motor constrains on the development of perception–action matching in infantreaching. Infant Behavior and Development, 23, 351–374.

Cozzani, M. V. (2005). Perseveração motora em crianças. Impacto da condição de deficiência mental. Rio Claro: Tese de doutorado.Universidade Estadual Paulista (p. 138).

Cozzani, M. V., & Mauerberg-deCastro, E. (2005). O comportamento perseverativo e a tarefa A não B de Piaget. Restrição ao ciclopercepção-ação? Revista da Sobama, 10, 37–46.

Diamond, A. (1985). Development of the ability to use recall to guide action as indicated by infants’ performance on A not B.Child Development, 56, 868–883.

Diamond, A., Cruttenden, L., & Neiderman, D. (1994). AB with multiple wells: Why are multiple wells sometimes easier than twowells? Memory or memory + inhibition? Developmental Psychology, 30, 192–205.

Diedrich, F. J., Highlands, T. M., Spahr, K. A., Thelen, E., & Smith, L. (2001). The role of target distinctiveness in infant perseveratereaching. Journal of Experimental Child Psychology, 78, 263–290.

Diedrich, F. J., Thelen, E., Smith, L., & Corbetta, D. (2000). Motor memory is a factor in infant perseverative errors. DevelopmentalScience, 3, 479–494.

Hogg, J., & Moss, S. C. (1983). Prehensile development in Down’s syndrome and non-handicapped preschool children. BritishJournal of Developmental Psychology, 1, 189–204.

Hoover, J. H., & Horgan, J. S. (1990). Short-term memory for motor skills in mentally retarded persons: Training and researchissues. In G. Reid (Ed.), Problems in motor control (pp. 217–239). Amsterdam: North-Holland.

Horobin, K., & Acredolo, L. (1986). The role of attentiveness, mobility history, and separation of hiding sites on stage IV searchbehavior. Journal of Experimental Child Psychology, 41, 114–127.

Jobling, A., & Mon-Williams, M. (2000). Motor development in Down syndrome: A longitudinal perspective. In D. Elliott, R. Chua,& D. Weeks (Eds.), Perceptual-motor behavior in Down syndrome (pp. 225–248). Champaign, IL, USA: Human Kinetics.

Lew, A. R., Hopkins, B., Owen, L. H., & Green, M. (2007). Postural change effects on infants’ AB task performance. Visual, postural,or spatial? Journal of Experimental Child Psychology, 97, 1–13.

Marcovitch, S., & Zelazo, P. D. (1999). The A-not-B error: Results of a logistic meta-analysis. Child Development, 70, 1297–1313.Mauerberg-deCastro, E., & Angulo-Kinzler, R. (2000). Locomotor patterns of individuals with Down syndrome: Effects of

environmental and task constraints. In D. Elliott, R. Chua, & D. Weeks (Eds.), Perceptual-motor behavior in Down syndrome(pp. 71–98). Champaign, IL, USA: Human Kinetics.

Munakata, Y. (1998). Infant perseveration and implications for object permanence theories: A POP model of the AB task.Developmental Science, 1, 161–184.

Piaget, J. (1954). The construction of reality in the child. New York: Basic Books.Rarick, G. L. (1980). Cognitive-motor relationships in the growing years. Research Quarterly for Exercise and Sports, 51, 174–192.Reid, G. (1986). The trainability of motor processing strategies with developmentally delayed performers. In H. T. A. Whiting &

M. G. Wade (Eds.), Themes in motor development (pp. 93–107). Champaign, IL: Human Kinetics.Sherrill, C. (2004). Adapted physical activity, recreation and sport. Crossdisciplinary and lifespan (6th ed.). Dubuque: McGraw-Hill.Smith, L. B., & Thelen, E. (2003). Development as a dynamic system. Trends in Cognitive Sciences, 7, 343–348.Smith, L. B., Thelen, E., Titzer, R., & McLin, D. (1999). Knowing in the context of acting: The task dynamics of the A-not-B error.

Psychological Review, 106, 235–260.Spencer, J. P., Smith, L. B., & Thelen, E. (2001). Tests of a dynamics systems account of the A-not-B error: The influence of prior

experience on the spatial memory abilities of two-year-olds. Child Development, 72, 1327–1346.Streri, A., Lhote, M., & Dutilleul, S. (2000). Haptic perception in newborns. Developmental Science, 3, 319–327.Thalheimer, W., & Cook, S. (2002). How to calculate effect sizes from published research articles: A simplified methodology.

[Retrieved February 1, 2009 from http://work-learning.com/effect_sizes.htm].Thelen, E., Schöner, G., Scheiner, C., & Smith, L. B. (2001). The dynamics of embodiment: A field theory of infant perseverative

reaching. Behavioral and Brain Sciences, 24, 1–36.Thelen, E., & Smith, L. B. (1994). A dynamics systems approach to the development of cognition and action. London: MIT Press/

Bradford.