Spatial Working Memory Deficits in Children at Ages 3–4 Who WereLow Birth Weight, Preterm Infants

Stefano Vicari and Barbara CaravaleIRCCS Ospedale Pediatrico Bambino Gesu and Libera

Universita Santa Maria Assunta

Giovanni Augusto CarlesimoUniversita degli Studi di Roma “Tor Vergata” and IRCCS

Fondazione Santa Lucia

Anna Maria Casadei and Federico AllemandUniversita degli Studi di Roma “La Sapienza”

The aim of this study was to investigate attention and perceptual and spatial working memory abilitiesin preterm, low birth weight preschool children without evident brain disorders as determined by normalcerebral ultrasound findings and normal motor development. The authors evaluated 19 preterm and 19typically developing children who were matched for IQ and chronological age. Results indicated thatchildren born prematurely without major neurological deficits and with a normal cognitive level mayhave specific difficulty in sustained attention, visuospatial processing, and spatial working memory whenevaluated at ages 3–4. This finding is relevant for understanding the qualitative aspects of cognitivedevelopment in preterm children and the neurobiological substrate underlying this development.

Preterm, low birth weight (LBW) infants are more likely to haveneuromotor problems or to perform poorly on tests investigatingspecific neuropsychological abilities than are children born at term(Mattews, Ellis, & Nelson, 1996; Mutch, Leyland, & McGee,1993; Rose & Feldman, 1996). Often, cognitive and behavioralimpairments are also detected when these children become ado-lescents and young adults (Allin et al., 2001; Stewart et al., 1999).These deficits are considered to be the consequences of the med-ical complications associated with preterm birth, such as hypoxia,intraventricular hemorrhages, and metabolic and nutritional in-sults, which may cause cerebral abnormalities and impair normalbrain development. Although the association between cerebralabnormalities and cognitive impairments usually observed in pre-term children is not yet clearly defined (Curtis, Lindeke, Georgieff,& Nelson, 2002), recent neuroimaging studies have reported spe-cific brain abnormalities in children born preterm. For example,

Stewart et al. (1999) documented the increasing prevalence ofneurodevelopmental brain lesions on MRI scans in a cohort of 72adolescents who were born very preterm, compared with 21 typ-ically developing age-matched control participants. Namely, theabnormalities reported by the authors concerned ventricular dila-tion, thinning or atrophy of the corpus callosum, white-mattersignal change, and posterior trigonal dilation. Neuropsychologicalassessment of this sample showed impairment only in word pro-duction, with performances in the normal range on measures ofattention, memory, perceptual skills, and visuomotor and executivefunctions, regardless of the abnormality of the MRI scan (Rushe etal., 2001). Also, in a third study conducted on the same cohort,Allin et al. (2001) documented reduced cerebellar volume in 67adolescents who were preterm, compared with 50 age-matched,full-term control participants. It is interesting that no relationshipwas found between motor neurological variables and cerebellarvolume; however, significant associations were found betweencerebellar volume and IQ, digit span, and reading age.

Isaacs et al. (2000) provided more data on the relationshipbetween cerebral abnormalities and cognitive functioning in chil-dren born preterm. Consistent with data showing the particularvulnerability of the hippocampus to some specific events such ashypoxia–ischemia and hypoglycemia, which usually affect pretermnewborns, their findings showed a direct correlation between ev-eryday long-term memory deficits and reduced hippocampal vol-ume in 11 children born preterm compared with 8 age-matchedcontrol children. Another adverse event that commonly occurswith preterm birth is intraventricular brain hemorrhage, which inturn may cause a necrotic lesion of the periventricular white matterand head of the caudate nucleus because of the anatomic contiguityof these structures (Casey et al., 1997). Studies showing thenegative effects of preterm birth on cognitive development havefocused mostly on global measures of intelligence (Allin et al.,2001; Sykes et al., 1997) or on general neuropsychological abili-ties such as perceptual-motor or executive functions (Jongmans,Mercuri, de Vries, Dubowitz, & Henderson, 1997; Luoma, Herr-gard, & Martikainen, 1998).

Stefano Vicari and Barbara Caravale, Servizio di Neurologia e Riabili-tazione, Istituto di Recovero e Cura a Carattere Scientifico (IRCCS) Os-pedale Pediatrico Bambino Gesu, Rome, and Facolta di Psicologia, LiberaUniversita Santa Maria Assunta, Rome. Giovanni Augusto Carlesimo,Clinica Neurologica, Universita degli Studi di Roma “Tor Vergata,” andIRCCS Fondazione Santa Lucia, Rome. Anna Maria Casadei, Dipartimentodi Ginecologia, Perinatologia e Puericultura, Universita degli Studi diRoma “La Sapienza,” Rome. Federico Allemand, Dipartimento di ScienzeNeurologiche e Psichiatriche dell’Eta Evolutiva, Universita degli Studi diRoma “La Sapienza.”

The financial support of the Italian Ministry of Health Project 0AN/F,“Hypoxic-Ischemic Brain Damage in the Newborn: Epidemiological andExperimental Studies on Diagnosis, Therapy and Rehabilitation,” is grate-fully acknowledged. We thank Romina Partigianoni and Serafina Galiffafor their assistance in collecting data. We especially thank the children whoparticipated in the study and their parents.

Correspondence concerning this article should be addressed to StefanoVicari, Servizio di Neurologia e Riabilitazione, IRCCS Ospedale Pedi-atrico Bambino Gesu, Lungomare Guglielmo Marconi 36, I-00058, SantaMarinella, Rome, Italy. E-mail: vicari@opbg.net

Neuropsychology Copyright 2004 by the American Psychological Association2004, Vol. 18, No. 4, 673–678 0894-4105/04/$12.00 DOI: 10.1037/0894-4105.18.4.673

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In the present study, we investigated the hypothesis that pretermchildren have a specific impairment of spatial working memory.There are both practical and theoretical reasons for addressing theissue of particularly impaired working memory abilities in childrenborn preterm. From a practical point of view, it should be recalledthat working memory, a limited capacity system for the temporarystorage of information held for further manipulation (Baddeley,1986), is central to performance in many cognitive domains. Theseinclude learning, planning, and problem solving as well as lan-guage acquisition, comprehension, and production. A workingmemory dysfunction in preterm children could be a potentialpredictor of poor cognitive development, thus providing indica-tions for early rehabilitative interventions. From a neurobiologicalperspective, poor working memory abilities in children born pre-term can be predicted on the basis of the previously mentionedevidence of frequent hippocampal atrophy (Isaacs et al., 2000) andof necrotic lesions in the head of the caudate nucleus (Casey et al.,1997). In fact, more recent data from both patients with braindamage (Ploner et al., 1999) and healthy individuals (Lacquaniti etal., 1997) suggest that, in addition to its well-known role inlong-term memory functioning (Tranel & Damasio, 2002), thehippocampal formation has a role in visuospatial working memory.Spatial working memory has also been linked to functioningwithin the dorsolateral prefrontal cortex in adult monkeys and inhumans (Baldo & Shimamura, 2002; Goldman-Rakic & Rosvold,1972; Nelson, Lebessi, Sowinski, & Hodges, 1997). Damage to thecaudate nucleus could interfere with the temporary retention ofvisuospatial information because of its regulatory role in frontallobe cognitive functions via the striatal–frontal loops reciprocallyconnecting the frontal cortex with the basal ganglia formation(Alexander, DeLong, & Strick, 1986; Owen et al., 1992).

Deficits in episodic long-term memory and working memoryhave been frequently reported in children with histories of neonatalintensive care treatment or intraventricular hemorrhage followingpreterm birth (Curtis et al., 2002; Luciana, Lindeke, Georgieff,Mills, & Nelson, 1999; Ross, Boatright, Auld, & Nass, 1996; Ross,Tesman, Auld, & Nass, 1992). Moreover, Briscoe, Gathercole, andMarlow (2001) reported increased risk of everyday memory dif-ficulties in populations of preterm children. Of interest, Luciana etal. (1999) found that preterm children’s memory was impairedcompared with that of control children only when task difficultyincreased (more than three item searches). This finding suggeststhat only tasks with a high cognitive demand are reliable fordetecting memory impairments in these children. Further, a meth-odological bias—that is, use of tests that were too simple—mayhave been responsible for the failure of some studies to demon-strate differences in cognitive and, specifically, memory function-ing in preterm children.

Neuropsychological deficits have also been described in pretermchildren who do not present neurological deficits or ultrasoundabnormalities (e.g., Isaacs et al., 2000; Torrioli et al., 2000). Also,adolescents with a history of prematurity and very LBW butwithout neurological disorders or global intellectual disabilitieshave been shown to have specific cognitive disorders and, partic-ularly, spatial memory deficits (Curtis et al., 2002; Isaacs et al.,2000). These findings suggest that peculiar neuropsychologicaldisorders may also be found in preterm children with a low risk ofmental or behavioral abnormalities, for example, in children withnormal neurological outcomes in the first years of life. The pos-

sibility of revealing cognitive disorders in these children mayindicate the need for a comprehensive cognitive (not only neuro-logical) follow-up starting in early childhood. This, in turn, couldlead to specific rehabilitative strategies to prevent learning andbehavioral problems.

The aim of our study was to investigate spatial working memoryabilities in preschool children who were born preterm withoutevident brain disorders, as defined by normal cerebral ultrasoundfindings and normal motor development. We evaluated childrenborn at a gestational age of 34 weeks or younger in the same year(1998) at the Department of Gynecology, Perinatology and ChildHealth of the Universita degli Studi di Roma “La Sapienza.” Thechildren were followed up with pediatric and neurological evalu-ations at the same department for at least 2 years. We investigatedwhether children at low risk for general intellectual disorderspresent spatial working memory difficulties early in life and, if so,whether memory deficits are a consequence of the attentional orperceptual disorders often reported in children with histories ofprematurity (Curtis et al., 2002; Isaacs et al., 2000; Jongmans etal., 1997; Luciana et al., 1999; Ross et al., 1992, 1996; Torrioli etal., 2000).

Method

Participants

The total sample included 19 preterm children and 19 control childrenborn at term. Parents provided informed consent as specified by theinstitutional review board of the Ospedale Pediatrico Bambino Gesu forstudies on human participants. The preterm cohort was selected from agroup of infants who were discharged from the Department of Obstetricsand Gynecology, Policlinico Umberto I, Rome, Italy, between January 1and December 31, 1998, and who were enrolled in the pediatric andneurological follow-up program of the same department. The followingcriteria had to be met for inclusion in the study: (a) birth at a gestational ageof 34 weeks or younger; (b) no congenital abnormalities; (c) no abnormalcerebral ultrasound findings, including minimal changes during the neo-natal period; (d) enrollment in the pediatric and neurological follow-upprogram for at least 2 years; and (e) no major neurological deficits such ascerebral palsy, blindness, deafness, or cognitive delay (IQ � 90).

The children selected were born between Gestational Weeks 29 and 34(M � 32, SD � 1.5) and had a birth weight ranging from 910 to 2,330 g(M � 1,597 g, SD � 436.4 g). Figure 1 reports the distribution ofgestational age by weight. All of the participants were followed up at leastonce every 6 months for their first 2 years of life; clinical checkupsincluded pediatric and neurological exams as well as an evaluation ofcognitive and motor development. A complete visual assessment was alsomade during the 1st year of life.

The control group included children born at term with no history ofperinatal problems. All of these children were born between GestationalWeeks 38 and 42 (M � 40, SD � 0.5) and had a birth weight in the normalrange of 2,800–3,900 g (M � 3,340 g, SD � 481.9 g). They were matchedwith the experimental sample for age, parents’ educational level andoccupational status (as determined by a parents’ questionnaire), and IQ (asassessed by the Italian version of the L–M form of the Stanford–BinetIntelligence Scale; Bozzo and Mansueto Zecca, 1993). Sample demo-graphic characteristics are summarized in Table 1.

Cranial ultrasounds were performed serially on all 19 preterm infantswith an ATL Ultramark scanner equipped with a 5- or 7.5-MHz transducer.At least two examinations were made, one within the first 48 hr and one atthe end of the 1st week of life.

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Instruments

Attention and perceptual and spatial working memory abilities wereassessed with the specific tests outlined below.

Sustained attention test: Leiter International Performance Scale—Re-vised (LIPS–R; Leiter, 1997). This is a barrage task with a time limit(30 s). Four sheets of paper with the target picture (a puppet) at the top andtwo different pictures (puppets and balls) at the bottom are presented to thechild one at a time. For each sheet, the child has 30 s to cross out thepictures that are identical to the target. The score is the total number ofcorrectly crossed out images, ranging from 0 to 64.

Spatial perception test: Developmental Test of Visual Perception—Second Edition (DTVP–2; Hammill, Pearson, & Voress, 1993). This testhas 18 trials. Each trial contains a target. In each successive trial, the targetis an increasingly complex geometric shape (an arrow in the 1st trial anda series of four overlapping triangles in the 18th trial). The child isrequested to match the target with an identical picture chosen from three,four, or five alternatives. Alternatives are similar to the target but may berotated 45°, 90°, or 180° or may present some components that arearranged differently in space. To solve the task correctly, the child mustmanipulate spatial information from the target or from parts of it. The scoreis the total number of alternatives correctly matched with their target,ranging from 0 to 18.

Spatial working memory test: Memory for Location 2, Revised (Cossu,Antonucci, & Nava, 1999; Kagan, 1981). This test was designed to assessworking memory for spatial stimuli in very young children. The participant

is presented with an array of six identical small plastic cups and one toy.Depending on the trial, the cups are displayed in three different spatialconfigurations: horizontal, diagonal, or L-shaped. In each trial, the exam-iner places the toy under a cup and quickly hides all the cups with a screenfor 1, 5, or 10 s. Immediately after the screen is removed, the participantmust point to the cup hiding the toy. For each spatial configuration, whichdoes not change during the delay, there are two trials of 1, 5, and 10 s. Weadopted this procedure to investigate the impact of the spatial codingvariable on the spatial working memory disorders exhibited by pretermchildren. The number of correct responses given in each session is scored.

Two different scores are provided by the task—delayed recall, thenumber of correct responses given after a delay of 1, 5, or 10 s (scoreranges from 0 to 10 for each delay); and spatial recall, the number ofcorrect responses for the three different spatial configurations: horizontal,diagonal, or L-shaped (score ranges from 0 to 6 for each spatialconfiguration).

Results

Data were analyzed with the Statistical Package for Windows,Version 5.0 (StatSoft, 1995, Tulsa, OK).

Attention

The number of elements correctly crossed out by the two groupson the sustained attention test was analyzed by a one-way analysisof variance (ANOVA), with group (preterm vs. control) as theindependent variable. The preterm children obtained a lower score(M � 41.2, SD � 13.0) than did the control children (M � 50.3,SD � 6.1), and the difference was significant, F(1, 36) � 7.6, p � .01.

Spatial Perception

The number of correct responses on the DTVP was analyzed bya one-way ANOVA, with group (preterm vs. control) as theindependent variable. The preterm children obtained a lower score(M � 4.9, SD � 3.2) than did the control children (M � 7.1,SD � 2.8), and the difference was significant, F(1, 36) � 4.7, p �.05.

Spatial Working Memory: Delayed Recall

Delayed recall scores of the two groups (see Figure 2) wereanalyzed by a two-way Group � Trial (recall after 1, 5, and 10 s)

Figure 2. Performances attained by the two groups in the delayed recalltask of the spatial working memory test.

Figure 1. Distribution of gestational age (in weeks) by weight for thepreterm children in our study.

Table 1Demographic Characteristics of Preterm Children and Full-Term Control Participants

CharacteristicPreterm(n � 19)

Control(n � 19)

Mean age (years, months) 3, 7 3, 7Age range (years, months) 3, 1–4, 3 3, 3–4, 3Mean IQ 113 115IQ range 97–131 105–126Gender

Male 10 12Female 9 7

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mixed ANOVA. The group effect was significant, F(1, 36) � 22.7,p � .0001, as was the trial effect, F(2, 74) � 34.4, p � .00001.Overall, the premature children’s performances were poorer thanthose of the control children (M � 6.8, SD � 1.7, vs. M � 8.7,SD � 1.2). Moreover, in the whole sample the number of itemscorrectly recalled decreased from the 1-s (M � 9.0, SD � 1.15) tothe 5-s (M � 7.5, SD � 2.1) to the 10-s (M � 6.8, SD � 2.0) delayinterval trials (all ps � .05). The Group � Trial interaction wasalso significant, F(2, 74) � 3.6, p � .05. Post hoc analyses(Tukey’s honestly significant difference test) performed to qualifythis interaction revealed that at the shorter delay (1 s), no signif-icant difference emerged between control children and pretermchildren (M � 9.5, SD � 0.7, and M � 8.5, SD � 1.3, respec-tively; p � .1). However, at longer delays the preterm childrenperformed significantly worse than did the control children. On the5-s delay recall trials, a significant difference emerged betweencontrol children (M � 8.6, SD � 1.3) and preterm children(M � 6.4, SD � 2.1; p � .0001). Also on the 10-s delay recalltrials, performances of the two groups were significantly different(control: M � 7.9, SD � 1.5; preterm: M � 5.7, SD � 1.7; p �.0001).

The differences that emerged between the two groups on thespatial working memory task may have been due to attentionaland/or perceptual difficulties of the preterm children. To controlfor these effects, we covaried results obtained on the spatialworking memory test (delayed recall) with performance on theattention and spatial perception tasks. The results of an analysis ofcovariance confirmed the significance of group, F(1, 36) � 10.5, p� .01, and of trial, F(2, 74) � 31.6, p � .0001. The Group � Trialinteraction approached significance, F(2, 74) � 2.9, p � .06.

Spatial Working Memory: Location Recall

Location recall scores of the two groups (see Figure 3) wereanalyzed by a two-way Group � Trial (recall from horizontal,diagonal, and L-shaped cup dislocation) mixed ANOVA. Thegroup effect was significant, F(1, 36) � 15.0, p � .01, as was thetrial effect, F(2, 74) � 28.2, p � .0001. Overall, the prematurechildren’s performances were poorer than those of the controlchildren (M � 4.4, SD � 1.2, vs. M � 5.2, SD � 0.6). Also,

significantly fewer items overall were recalled correctly as passingfrom the horizontal (M � 5.6, SD � 0.8) to the L-shaped (M � 4.7,SD � 1.1) to the diagonal (M � 4.2, SD � 1.2) cup dislocationcondition (all ps � .05). The Group � Trial interaction was notsignificant, F(2, 74) � 1.6, p � .2, suggesting a similar effect ofmanipulation of cup dislocation on the performance accuracy ofthe two groups of participants. Also in this case, differencesbetween the two groups may have been due to attentional and/orperceptual deficits in the preterm group. To control for theseeffects, we covaried results obtained on the spatial working mem-ory test (location recall) with the performances obtained on theattention and spatial perception tasks. Once again, the results of theanalysis of covariance confirmed the significance of group, F(1,36) � 6.0, p � .02, and of trial, F(2, 74) � 27.4, p � .00001.Again, the Group � Trial interaction was not significant, F(2,74) � 1.2, p � .2.

Relationship Between Gestational Age, Birth Weight,General Intelligence, and Spatial Working MemoryAbilities

Our sample of preterm children was somewhat heterogeneousfor both conditions at birth (gestational age and weight) andcurrent overall cognitive efficiency (see Table 1 and Figure 1). Tocontrol for the possible influence of these variables on the matu-ration of spatial working memory abilities, we calculated Pearsonproduct–moment correlations between gestational age, birthweight, IQ, and performance scores on the working memory tasks(both delayed and location). In no case did the correlation coeffi-cients approach significance (all rs � .35; p � .1).

Discussion

The results of the present study show that children born prema-turely, without major neurological deficits and with a normalcognitive level, may have specific difficulties in sustained atten-tion, visuospatial processing, and spatial working memory whenevaluated at 3–4 years of age and when compared with childrenmatched in chronological age who were born at term. The 19children involved in our study were all discharged from the neo-natal intensive care unit as being at low risk for neurodevelop-mental problems (negative cranial ultrasounds). During routineevaluations of the neuropediatric follow-up, they did not presentany motor or cognitive problems. Only specific assessments madein early childhood revealed specific neuropsychological difficul-ties. In our sample of preterm children, performance scores onworking memory tests did not correlate with either weight orgestational age at birth. This finding suggests that at the time thisinvestigation was made (when the children were 3–4 years old),the gestational lag linked to preterm birth was irrelevant in pre-dicting cognitive maturation. For this reason, the poor perfor-mances of these children compared with the performances of theirmatches born at term should be attributed to the cerebral insultsthat accompany preterm birth.

It can be argued that the ultrasound exam is not sufficient todetect subtle brain lesions and that MRI may be more informativewith these children (Stewart et al., 1999). This is certainly true, butbecause the preterm children included in our study were veryyoung, the MRI exam could have been performed only under

Figure 3. Performances attained by the two groups in the location recalltask of the spatial working memory test.

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general anesthesia. The absence of any signs of neurological deficitdid not justify the use of such an intrusive exam, and for ethicalreasons it could not be performed only for research purposes.

The deficit in spatial working memory functioning is particu-larly relevant because it did not depend on perceptual or attentionaldisturbances. In fact, between-groups differences in working mem-ory performance also held when attentional and spatial processingabilities were taken into account. Moreover, the different spatialarrangement of the cups in the working memory tasks affectedperformance of control children and preterm children to the samedegree. This result supports the view that the preterm children’spoor performances on this task did not depend on difficulty in themental representation of the spatial configurations. Our data oncontrol children and preterm children are consistent with those ofCossu et al. (1999). These authors reported children’s increasingdifficulty in solving the spatial working memory task that we usedin the present study, in which participants had to pass from thehorizontal to the diagonal to the L-shaped configuration.

Additional support for the specificity of the working memorydeficit in preterm individuals comes from evidence that theirperformance progressively worsened in passing from the 1- to 5- to10-s delay interval trial. In fact, a performance level indistinguish-able from that of control children at the 1-s delay suggests that thepreterm children stored the spatial configuration pattern normally.However, their poor performance following the delay suggests thatmechanisms responsible for rehearsing the spatial representation toprevent its decay from memory were deficient in the group ofpreterm children (Logie, 1995). The results of this study confirmand extend conclusions of previous studies describing reducedspatial memory span in more severe preterm children, such asneonatal intensive care unit survivors of very LBW (Luciana et al.,1999), and increased risk of everyday memory difficulties inpopulations of preterm children (Briscoe et al., 2001).

Because of the unavailability of detailed morphological andfunctional data on brain maturation, any conclusions about theneural substrates of the spatial working memory deficit observed inthe preterm children in our study should be regarded as specula-tive. Animal and adult human studies have suggested that theremay be a prefrontal contribution in solving these tasks. Also,cooperation between the hippocampus and the striatum duringspatial working memory performance has been suggested (Nelsonet al., 1997). Although our findings are not sufficient to demon-strate a localized lesion process in LBW preterm children, theysuggest that the neural networks devoted to working memoryfunctioned less efficiently in these children than in the controlchildren. It may be inferred that even if LBW preterm children donot present any ultrasound abnormalities or neurological deficits,they may have abnormal brain maturation, particularly in corticalnetworks that involve the hippocampus, striatum, and prefrontalcortex.

The children included in the present study were followed up inthe first 2 years of life, and no deficits in cognitive developmentwere observed. Therefore, the real question concerns the relation-ship between the neuropsychological abilities we considered inthis study and other measures of intellectual functioning. Unfor-tunately, because we selected control children to match the pretermchildren on the basis of IQ, we are not able to explain thisrelationship. However, in our sample of preterm children, IQ didnot correlate with performance scores on the working memory

tasks, thus suggesting the inadequacy of global intelligence mea-sures to detect specific neuropsychological deficits. In fact, it hasbeen shown that global intelligence measures are inadequate fordetecting specific neuropsychological deficits in some cognitivedisorders (Detterman, 1987; Vicari, Albertini, & Caltagirone,1992).

Spatial working memory and attentional impairments do notnecessarily persist throughout cognitive development (Rushe et al.,2001). Follow-up assessment is crucial to determine whether thesechildren have a delay in brain maturation that will be resolved withage or whether less subtle forms of working memory problems willremain, causing or contributing to more severe cognitive andlearning disabilities (Hack et al., 2002; Stewart et al., 1999). Earlyeducational and rehabilitative programs may be necessary to avoidthis outcome.

In conclusion, we have described previously unreported deficitsin sustained attention, visuospatial processing, and spatial workingmemory in a group of LBW preterm children. These findings arerelevant for understanding of the qualitative aspects of cognitivedevelopment in preterm children, as well as in children expected tohave a normal outcome, and of the neurobiological substrateunderlying this development. From a practical point of view, thefinding of reduced sustained attention, visuospatial processing, andspatial working memory in LBW preterm children is worthy ofconsideration in the development of early educational and reha-bilitative interventions to improve their cognitive and behavioraloutcome.

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Received January 14, 2003Revision received September 22, 2003

Accepted October 13, 2003 �

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