Development of Preterm and Full-Term Infant Ability on AB, Recall Memory, Transparent Barrier Detour, and Means-End Tasks

Development of Preterm and Full-Term InfantAbility on AB, Recall Memory, TransparentBarrier Detour, and Means-End Tasks

Alexandra Matthews, Ann E. Ellis, and Charles A. NelsonUniversity of Minnesota

MATTHEWS, ALEXANDBA; ELLIS, ANN E.; and NELSON, CHARLES A. Development ofPreterm andFull-Term Infant Ability on AB, Recall Memory, Transparent Barrier Detour, and Means-EndTasks. CHILD DEVELOPMENT, 1996, 67, 2658-2676. 10 preterm and 10 full-term infants weretested longitudinally from 28 to 60 weeks of age on a modified version of the AB task, a nonreach-ing AB task, a Barrier Detour task, a Means-End task, and Perseveration in the Means-End task.Results show that age-corrected (age since conception) premature infants tolerated longer delaysthan full-term infants on the modified and nonreaching AB tasks. However, when compared bychronological age (age since birth), there were no group differences on either the reachingor nonreaching AB task. No group differences were found on Barrier Detour, Means-End, orPerseveration in either the age-corrected or chronological age comparisons. The results suggestthat the function that mediates modified AB performance is one of memory and not of persevera-tion or means-end ability. Further, these findings suggest that current proposals about braindevelopment based on single samples of infants may be tenuous. Finally, the results of thisstudy suggest that development of the brain structure(s) that mediate modified AB performanceis strongly influenced by experience in the postnatal environment.

Recent neuropsychological accounts of ine A-not-B performance in children whothe Piagetian A-not-B error are based on may be on a different developmental tra-the assumption that maturational and ex- jectory, namely, healthy frail-term andperiential events trigger modifications in healthy preterm human infants. The currentthe brain, resulting in bebavioral changes research was conducted with this goal in(Diamond, 1985; Duffy, Als, & McAnulty, mind.1990; Goldman-Rakic, 1985; Greenough &Black, 1992; Ross, Tesman, Auld, & Nass, Diamond (Diamond, 1990a, 1990b; Dia-1992). Indeed, the pervasiveness of the A- mond & Goldman-Rakic, 1989) and Bell andnot-B error suggests that there may be a Fox (1992) have argued that successfril per-strong maturational component to this skill, formance by human infants on a modifiedlending support to neuropsychological version of the AB task is tied to the develop-models. Advocates of these models believe ment of the dorsolateral prefrontal cortex,that the development of performance and This hypothesis is based, in part, on findingsthe achievement of errorless performance from two areas of investigation: longitudinalon the AB task provide a window into the studies of human infants' performance on adevelopment and maturation of neuro- modified version of the AB task and compar-logical substrates underlying this skill, ative research with rhesus monkeys. ThisOne way to explore these claims is to exam- empirical work provides a compelling case

Portions of the data reported in this article were presented at the biennial meeting of theSociety for Research in Child Development, Seattle, April 1991; New Orleans, March 1993; andat the eighth International Conference on Infant Studies, Miami, May 1992. Portions of thesedata also appeared in an unpublished doctoral dissertation by the first author, at the Universityof Minnesota. The research presented in this article was supported in part by grants from NIHto the Center for Research in Learning, Perception, and Cognition (HD07151); from the NIMHto Charles A. Nelson (MH46860); and an NIH First Award to Patricia Bauer (HD28425). Theauthors would like to thank Patricia Bauer for consultation on all aspects of this research andAdele Diamond for commenting on earlier drafts of this article. The authors would also like tothank the children, parents, and research assistants who contributed to this study. Correspon-dence concerning this article should be addressed to Ann E. Ellis, Department of Psychology,Grinnell College, Grinnell, IA 50112. Electronic mail may be sent via Internet to [email protected]. Alexandra Matthews is now at Sunset Mental Health Services, Department ofPublic Health, San Francisco, California; Ann E. Ellis is now at Grinnell College.

[Child Development, 1996,67,2658-2676. © 1996 by the Society for Research in Child Development, Inc.All rights reserved. 0009-3920/96/6706-0032$01.00]

Matthews, Ellis, and Nelson 2659

for the neurological explanation of modifiedAB performance.̂

Diamond (1985) longitudinally testedinfants every 2 weeks from 7 to 12 monthsof age using the modified AB task. She foundthat 7.5-month-old infants committed the A-not-B error at an average delay of less than2 sec, whereas 12-month-olds committed theerror at an average delay of 10 sec. Bell andFox (1992) found that infants tolerated an av-erage delay of 0 sec at 7 months and 7.5 secat 12 months before committing the A-not-Berror. Differences in delays reported by Belland Fox and Diamond may be attributed todifferent reporting techniques. Diamond re-ported the delay required to produce the A-not-B error, whereas Bell and Fox reportedthe delay tolerated before producing the A-not-B error. Taken together, these results in-dicate that the delay needed to produce theA-not-B error increases continuously at anaverage rate of 1.5—2 sec per month betweenthe ages of 7.5 and 12 months.

The delayed response (DR) task is con-sidered to be the nonhuman primate equiva-lent of the modified AB task (Diamond &Goldman-Rakic, 1985; Williams, 1979). Inboth tasks, the subject watches while the ex-perimenter hides a desirable object in oneof two hiding wells. The wells are then cov-ered simultaneously, and a short delay is im-posed. During the delay, the subject's atten-tion is directed away from the hiding wells.Following the delay, the subject is permit-ted to search for the object. If the subjectreaches correctly, she or he is rewarded withreceipt of the desired object. If the subjectreaches incorrectly, the experimenter showsthe subject where tlie object was hidden butdoes not give the object to tbe subject.

The modified AB task used by Diamond(1985) and the DR task differ in severalways. For example, animals performing theDR task are deliberately food deprived,whereas this is not the case with human in-fants in the modified AB task. In addition, ascreen blocks the animal's view during thedelay period on the DR task, whereas par-ents hold human infants' hands to restrainthem from reaching, and a distraction is usedto prevent infants from looking at the appara-

tus during the delay. Finally, the two tasksdiffer on how a decision is made as to wherethe object is hidden. In the DR task, objecthiding is varied randomly by a predeter-mined schedule, whereas in the modifiedAB task, the object is hidden in the samewell until the subject makes a correct re-sponse for a predetermined number of trials.However, once the decision on where tohide the object has been made, the trial onboth tasks is identical (Diamond, 1990a).Thus, in both tasks, the subject is presentedwith two hiding wells that differ only inspatial location. In both tasks, to performsuccessfully, the subject must encode thelocation of the desired object, hold this infor-mation in working memory during a delay,and make a response based on this storedinformation while simultaneously inhibitinga previously rewarded response, if it is notcorrect on the current tried. Diamond andDoar (1989) demonstrated that human in-fants performed similarly on both the modi-fied AB task and the DR task.

Work aimed at understanding brain de-velopment and performance on the DR taskin rhesus monkeys has implications for hu-man infant AB performance. Many similari-ties between human and rhesus monkeyshave been noted in brain structure andgrowth. For example, the brains of both spe-cies are similar in cortical expanse, neuronalorganization, development, and capacity forexpressing complex behavior (Goldman-Rakic, 1985). Additionally, numerous re-searchers have demonstrated close parallelsbetween human infant and rhesus infantcognitive development (Diamond & Gold-man-Rakic, 1985; Goldman-Rakic, 1987;Goldman-Rakic, Isseroff, Schwartz, & Bug-bee, 1983; Williams, 1979). It has been ob-served that rhesus infants and human infantspass through the same developmental stagesin the same order at an age ratio of roughly4:1. On a task such as AB, rhesus monkeyscomplete development by 2—4 months ofage (Williams, 1979), whereas human infantsreach the same level of performance byabout four times that age, roughly 9—12months of age (Diamond, 1985). These simi-larities suggest that rhesus monkeys providean excellent animal model for the study ofhuman cognitive development.

^ It is worth noting that the AB task used by Diamond (1985) and Bell and Fox (1992) differsfrom that used by Piaget (Piaget, 1954) in two ways. First, infants are presented with several Bhiding trials within a single testing session, and second, distraction is used to direct the infants'attention away from the hiding wells during the delay period. Thus, Caution is warranted inlinking performance on this task to Piaget's notion of object permanence. As such, henceforth.Diamond's and Bell and Fox's technique will be referred to as the modified AB task.

2660 Ghild Development

Diamond and Goldman-Rakic (Dia-mond, 1990a; Diamond & Goldman-Rakic,1989) capitalized on these species similari-ties and have drawn a parallel between thebehavior of human infants on the modifiedAB task and the behavior of adult rhesusmonkeys with cortical lesions on DR. It hasbeen widely reported that the prefrontal cor-tex underlies adult monkey performance onthe DR task (Alexander, 1982; Alexander &Goldman, 1978; Goldman-Rakic, 1987; Pass-ingham, 1985; Rosvold, Szwarcbart, Mirsky,& Mishkin, 1961). For example, when com-pared to the performance of intact monkeysor monkeys with lesions to the parietal cor-tex, only the performance of monkeys withlesions to the dorsolateral prefrontal cortexwas impaired on the DR task when a delaywas imposed (Diamond & Goldman-Rakic,1989).

Diamond and Goldman-Rakic (1989)concluded that performance of prefrontallylesioned monkeys on DR is comparable, inall respects, to performance of 7.5-9-month-old human infants on modified AB. Theynoted that both human infants and monkeyswith lesions to dorsolateral prefrontal cortexperform successfully on modified AB or DR,respectively, when no delay is imposed. Incontrast, both human infants and ablatedadult monkeys fail on their respective taskswhen a delay is imposed. Thus, drawingcomparisons across both species and devel-opmental periods, these, researchers ad-vanced the notion that the dorsolateral pre-frontal cortex is the primary structureunderlying modified AB performance andthat maturation of this structure is necessaryfor successful performance on this task.

Diamond (1985,1990a, 1990b) proposesthat immaturity of the prefrontal cortex man-ifests itself in perseverative errors, whichare ultimately responsible for the A-not-B er-ror. She suggests that the adult analog of in-fant behavior in the A-not-B error can beseen in patients with frontal lobe damage.Indeed, perseverative behavior is a promi-nent feature of adult patients with dys-function of the prefrontal cortex (Diamond,1985; Freedman & Oscar-Berman, 1986;Goldman-Rakic, 1987; Oscar-Berman, Mc-Namara, & Freedman, 1991).

To substantiate her perseveration hy-pothesis with human infants. Diamond re-lates performance on the modified AB taskto performance on the Barrier Detour/Objectretrieval task, a task that also seems to re-quire inhibition of a previously rewarded re-

sponse (Diamond, 1990b; Diamond & Gold-man-Rakic, 1985). The Barrier Detour taskrequires subjects to detour around a trans-parent barrier in order to retrieve a desiredobject. Subjects must inhibit the urge toreach straight through the transparency forthe bait, along their direct line of sight. Dia-mond found that human infants 6.5-8months of age and adult monkeys with fron-tal lobe lesions were unable to retrieve thereward if they saw it through a transparentbarrier (Diamond, 1990b). Rather than de-touring around the transparency, human in-fants and prefrontally lesioned monkeys hitand scratch at the transparency. Diamondfound that human infants pass through anage-related sequence of phases in perfor-mance on Barrier Detour and that the ageperiod during which infants improve on Bar-rier Detour coincides with the age periodduring which they improve on the modifiedAB task.

However, a recent study failed to repli-cate Diamond's findings. Bell and Fox(1992) reported that there was an associationbetween frontal EEG power and perfor-mance on modified AB. However, these au-thors found that group EEG differences onthe modified AB task did not carry over tothe Barrier Detour task. In attempting to ac-count for their failure to find relations be-tween EEG measures and Barrier Detourperformance when they did find relationsbetween EEG and modified AB, Bell andFox suggest that the memory component re-quired in modified AB may be the criticaldifferentiating factor. Bell and Fox note thatwhile both modified AB and Barrier Detourrequire the infant to sequence a series of ac-tions, only the modified AB task requiresmemory.

Diamond (1985) believes that memorycannot fully explain the A-not-B error, not-ing that the error occurs when there is nodelay and, therefore, no memory require-ment. This belief that the A-not-B error maynot be due to limitations in memory is onewith which many researchers would agree(Baillargeon, DeVos, & Graber, 1989; Nel-son, 1995; Wellman, Cross, & Bartsch, 1986).For example, Baillargeon et al. tested loca-tion memory in 8-month-old infants andfound that infants at this age could remem-ber the location of a hidden object for delaysof up to 70 sec. Baillargeon et al. believe thattheir results call into question explanationsthat account for infants' search errors interms of faulty or immature memory mecha-nisms. They suggest that infants' persevera-


tive search errors reflect limitations of prob-lem-solving ability.

Diamond's suggestion that brain matu-ration, rather than experience, is the primarymechanism in developing mastery of the ABtask is intriguing. It invites investigation ofpopulations of individuals that are suspectedof progressing through brain development indifferent ways than healthy friU-term humaninfants. Healthy preterm infants providesuch a population. According to Duffy et al.(1990), preterm infants are different fromfull-term infants on a variety of behavioraland electrophysiological measures. Duffy etal. theorize that environmental alterationsmay have profound effects on brain develop-ment. Gertainly there are profound differ-ences between the prenatal and postnatalenvironments and in the requirementsplaced upon infants by these respective en-vironments. Might it be the case that thetime elapsing from birth and the environ-mental stimuli and demands experienced bythe infant during this time may be the pri-mary factors in developing mastery of themodified AB task?

The aim of the current study was to ex-amine the relative roles of maturational pro-gramming and environmental input in thedevelopment of the ability to perform themodified AB task, a nonreaching AB task.Diamond's Barrier Detour task, and a two-location Means-End task. We predicted thatfull-term infant performance would be bet-ter than preterm infant performance on themodified AB task and on any task wheregroup differences were found. We also pre-dicted that development of performance onthe modified AB task and the Barrier Detourtask would look similar. In addition, it washoped that the results of this study mightoffer some preliminary insight into the rela-tive roles of memory, means-end ability, andperseveration in performance on the modi-fied AB task.

MethodSUBJECTS

Ten full-term and 10 preterm infantsserved as subjects. Infant names were ob-tained from birth records. A letter was sentto all families with an infant in the Vital Sta-tistics Bureau birth announcements, whichpublishes birth information on all healthyinfants. Any parents who wanted their in-fants to participate in research returned apostcard with infant birth information. Forthis study, infant cards were pulled from the

file, and a letter was sent to parents ex-plaining the method and purpose of thestudy. All parents who were contactedagreed to participate in the study.

Parents' SES was computed using theHollingshead scale (1975). There was no re-liable SES difference between full-term andpreterm infants {p > .32). Nineteen of 20infants fell into the top two tiers of HoUings-head's SES levels, with the remaining in-fant, a preterm infant, falling into the fourthtier. Regression analysis revealed no sig-nificant effect of SES on prematurity {p =.39). Therefore, infant prematurity in thisstudy cannot be attributed to factors relatedto low SES.

Full-term infants met the dual criteriaof being born within 7 days of their due dateand weighing at least 6 pounds at birth. Pre-mature infants met the dual criteria used byEllis (1990) of being at least 14 days prema-ture and weighing less than 5.5 pounds atbirth. The premature infants in this studywere also required to meet a third criterionof having been released from the hospital onor before their due date.

Due date (EDG, which stands for Ex-pected Date of Confinement) informationwas obtained through a questionnaire ad-ministered to mothers before the study. Inall cases, mothers reported that they andtheir doctors were in agreement about EDC.All infants were screened for major illnessand complications of birth or prematurity sothat the major difference between groupswas that of gestational age and birthweight.

The 10 full-term infants had an averagebirthweight of 7.63 pounds (range of6.06-9.25 pounds; SD = 1.04). Eight of thefull-term infants were Caucasian, and twowere of mixed Caucasian and Asian-American heritage.

The 10 preterm infants had an averagebirthweight of 4.96 pounds (range of3.63-5.5 pounds; SD = 0.62). Their averageprematurity was —31.9 days (range of —40to -26 days; SD = 5.5). Nine of the preterminfants were Caucasian, and one was Afri-can-American. The percentage of ethnic mi-nority infants in the study far exceeded theminority population of the city and statewhere the study took place.

The age of preterm infants was cor-rected for prematurity so that infants weretested at equivalent age since conception,rather than age since birth. For the remain-der of this article, unless otherwise noted.

2662 Child Development

when infant ages are given they are cor-rected for prematurity. For premature in-fants, their actual (chronological) age isgreater by the degree to which they werepremature.

TESTING SCHEDULE AND OVERVIEW

Testing took place in a cognitive devel-opment laboratory at the University of Min-nesota. All infants were tested every 4 weeksfrom 28 weeks to 60 weeks of age. At eachmonthly testing session, all infants weretested on four tasks. In the modified AB task,similar to that used by Bell and Fox (1992)and Diamond (1985), infants were requiredto retrieve a toy from one of two coveredlocations after a delay. In a two-location,nonreaching AB task, infants saw a toyplaced in one of two identical boxes andwere required to look at the correct locationsifter a delay. In a Barrier Detour task, infantsretrieved a toy from an opaque or transpar-ent box with one open side. And finally, ina Means-End task, infants were required topull one of two identical cloths in order tobring a toy to within arm's reach. All fourtasks were administered at each testing ses-sion in random order. Testing sessionslasted approximately 1 hour. If infants be-came fussy, upset, hungry, or in need of adiaper change during testing, a break wastaken. However, in 98% of the sessions nobreak was necessary as infants remainedalert, interested, and engaged in solving thetasks.

CODING

All sessions were videotaped for codingat a later time. All tasks were coded by un-dergraduate researcb assistants who weretrained by the senior author and receivedcollege course credit for their work. Trainingcontinued until coders achieved at least 90%reliability with the senior author. Thereaf-ter, a minimum of 20% of randomly selectedsessions were checked for reliability.

AB TaskThe apparatus and testing procedure for

this task conformed to methods published byDiamond (1985).

Apparatus.—The AB apparatus stood 27inches high, 35 inches wide, and 15 inchesdeep. Embedded in its top were three wells,each 3.5 inches in diameter and 3 inchesdeep. The wells were 11 inches apart, centerto center. To ensure that all wells wereequally accessible to infants, the wells werearranged in an isosceles triangle, with the

center well at the apex and the base of thetriangle closer to the baby. The apparatuswas tan, with each well bordered by redtape. Light blue cotton baby washclothsserved as covers.

Procedure.—-At each testing session,only two wells were used. The center wellwas always used and the second well waseither the left or right side. The use of theright and left wells was varied randomly be-tween sessions.

Infants were seated in the parent's lapfacing the experimenter across the testing ta-ble. A collection of toys (including plastickeys, rattles, squeaky toys) were available sothe experimenter could find a toy that wasattractive to each infant. If the infant lost in-terest in the current toy, a different toy wasused.

A single trial consisted of the parentholding the infant's hands. The experi-menter then hid the toy in one well and cov-ered both wells with the cloths, making surethe infant was watching as the toy was beingbidden.

When the wells were covered, the delaybegan. The experimenter held the infant'sattention away from the wells during the de-lay by shaking a rattle, talking, or countingout loud. Thus, for delays greater than zeroseconds, infants experienced both delay anddistraction. Previous researchers (Bell &Fox, 1992) have theorized that this distrac-tion, rather than the delay itself, causes theA-not-B error. This is an important issue thatwill, it is hoped, be addressed in future re-search. Parents were asked to prevent theirinfants from reaching or leaning toward thewells until the experimenter signaled theend of the delay by saying "okay." Atthe end of the delay, the infant was allowedto search for the toy. Parents were instructednot to give infants any cues as to the locationof the toy, and all parents complied.

Reaching was scored according to thefirst cloth the infant removed. A correctreach was rewarded by praise, applause, andthe opportunity to retrieve and play with thetoy. Reward was contingent on removal ofthe correct cloth to maximize motivation toreach correctly and to minimize the possibil-ity that the infant was playing a differentgame from the one intended by the experi-menter. When infants did not reach cor-rectly, the parent and the experimenter re-mained neutral, and the experimenterremoved the correct cloth and retrieved thetoy as the infant watched.


When an infant retrieved the toy cor-rectly from the same well three trials in arow, the toy was hidden in the other well.This criterion is more stringent than thatused by Diamond (1985), who required onlyone correct retrieval trial before reversal.When an infant retrieved the toy correctlyon two out of three change-of-side trials at agiven delay, the delay was incremented.

The delay between covering of thewells and release of the infant's hands wasvaried from 0 to 15 sec. The delay was tai-lored to each infant for each session. All in-fants received a delay of 0 sec in the firsttesting session. As in previous research (Bell& Fox, 1992; Diamond, 1985), after the firstsession the initial delay was based on thedelay tolerated by the infant in the previoussession. When an infant performed perfectlyat a given delay, the delay was incrementedby 2 sec until the delay was reached thatcaused the infant to make the A-not-B error.During the delay, infants were distracted toprevent them from looking, leaning, orstraining toward the correct location. Any re-sponse other tlian a correct retrieval wasscored as incorrect. Infants were scored asmaking an error if they reached to the empty,covered well; if they reached simulta-neously to both wells; if they did not reachat all; or if they reached to the third, uncov-ered well. By far the most common type oferror was reaching to the empty, coveredwell, constituting over 95% of errors. Thelatter two errors were extremely rare, oc-curring in less than 1% of trials. These arethe same scoring criteria used by Diamondand Bell and Fox.

The score given to each infant for eachsession was the delay needed to cause theA-not-B error in two out of three change-of-side trials. In some sessions, infant perfor-mance was nearly error free. In these cases,infant performance was better than A-not-Bbecause they made few, if any, search errors(Bell & Fox, 1992; Diamond, 1985). Thesecases would include sessions where infantsfailed to make any search errors or wherethey might err on the first change-of-sidetrial at a new delay, but not on the two sub-sequent change-of-side trials at that delay.To refiect infant competence accurately,when performance was judged to be betterthan A-not-B, one second was added to theinfant's best time. In some cases, infantsearch was random, or infants continued tosearch the first location even after numerous,repeated hidings at the second location. Thiswas judged to be worse than A-not-B perfor-

mance, and one second was subfracted fromthe infant's time, to accurately refiect infantcompetence.

Nonreaching AB TaskApparatus.—The apparatus consisted of

a puppet stage. The stage was a table wbichstood 36 inches high, 36 inches wide, and24 inches deep. The stage had light beigecurtains, that opened to either side, andskirting that occluded peripheral vision. Onthe stage, side by side 8 inches apart, stoodtwo boxes measuring 18 inches high X 12inches wide x 12 inches deep. The boxeswere plywood, painted tan, with smokedPlexiglas fronts. The backs of the boxes wereopen but covered with thick, black fabricthat could be sealed down with strips of Vel-cro. Mounted to the inside top of the boxeswere 8-inch-Iong, tubular, clear 50 watt lightbulbs. When the lights were illuminated, thecontents of the boxes were clearly visiblethrough their smoked Plexiglas fronts. Whenthe lights were turned off, the smoked Plexi-glas was, for all intents and purposes,opaque.

A variety of attractive, battery-operatedtoys were used for hiding, including a yap-ping, white "Husky" dog; a whistle-blowingclown with a red light bulb nose and wear-ing a red fire fighter's outfit; a brown bunnythat hopped, squeaked, and waved its earsback and forth; a yellow bunny that blew awhistle, banged cymbals together, and swiv-eled back and forth.

Procedure.—The testing procedure forthis task was designed to parallel the proce-dure for the standard AB task as closely aspossible.

Infants were seated in the parent's lapin front of the puppet stage, at a distance ofapproximately 2 feet. Parents wore opaqueglasses to reduce the chances that they couldcue the infants to the toy's location. The ex-perimenter stood behind the puppet stage.

At the beginning of a trial, the experi-menter activated the toy and the lights in-side both boxes. She showed the toy to theinfant, between the two boxes. She thenplaced the toy inside one box (A), and theinfant was allowed to watch the toy insidethe box for approximately 5 sec. The experi-menter then closed the curtains for a delay,making sure that the infant was watching asshe closed the curtains. During the delay,infants were distracted to prevent them fromlooking, leaning, or straining toward the cor-rect location. The delay was varied from 1


sec to 20 sec. The delay was tailored to eachinfant for each session. The minimum delaypossible was 1-2 sec because the curtainscould not be closed and opened morequickly. All infants received a delay of 1—2sec in the first testing session. At the end ofthe delay, the curtains were opened, and theinfant was allowed to look at the boxes.

Two experimenters independentlyjudged the direction of infant looking. Al-most 100% of the time infant looking wasscored for the first box at which infantslooked. Less than 1% of the time, an infantwould glance quickly at one box, then settlehis or her gaze on the other box. In theseinstances, infant looking was scored for thesecond box. Tbese trials occurred equally of-ten for correct and incorrect looking. Whenan infant looked at the correct box, the lightin that box was illuminated, the toy was acti-vated, and the infant was praised by the ex-perimenter. Infants were allowed to watchthe toy for up to 10 sec, or until they lookedaway, whichever occurred first. When an in-fant looked incorrectly, the experimenter re-mained neutral, and nothing happened forseveral seconds. This was essentially to givethe infant a time-out from reinforcement.The experimenter then removed the toyfrom the box and repeated the trial. Rewardwas contingent on correct looking to max-imize infant motivation for look correctly.When an infant had looked correctly on twoto three trials in a row at one location, theside-of-hiding was changed. When an infanthad looked correctly on two out of threechange-of-side trials at a given delay, the de-lay was increased.

When an infant performed perfectly at agiven delay, the delay was increased by 3sec until a delay was reached that caused theinfant to make the A-not-B error. The in-crease in delay was held as closely as possi-ble to 3 sec but varied somewhat from 2—5sec due to human and equipment factors. Asin the modified AB task, any response otherthan a correct look was scored as incorrect.Infants were scored as making an error ifthey looked to the empty box, if they lookedback and forth between the two boxes, or ifthey did not look at the boxes at all.

The score given to each infant for eachsession was the delay needed to cause theA-not-B error in two out of three change-of-side trials and was scored in the same wayas for the modified AB task.

Barrier Detour TaskThe apparatus and testing procedure for

this task conformed as closely as possible to

methods published by Diamond (1990b) andBell and Fox (1992).

Apparatus.—Two Plexiglas boxes wereused for this task. Each had a top and threesides, leaving the bottom and one side open.Their specifications conformed to methodspublished by Diamond (1990b) and Bell andFox (1992). The transparent box measured 6X 6 incbes, with sides 2 inches high. Theopaque box measured 4.5 x 4.5 inches withsides 2.5 inches high. Although the twoboxes were different sizes, their specifica-tions were not altered in order to maintaincontinuity with previously published re-search.

Procedure.—During this segment of thetesting session, infants were presented withtwo vertical barriers, as well as the boxesdescribed in the "Apparatus" section. How-ever, the data from the barriers will not bepresented here as they are not central to thehypotheses of the current study or for repli-cation of previous research.

For testing, infants were seated in theparent's lap facing the testing table. The ex-perimenter was seated across the table, fac-ing parent and child. A collection of toys (in-cluding plastic keys, rattles, squeaky toys)and Cheerios (breakfast cereal) was avail-able so the experimenter could find an ob-ject that was attractive to each infant. A toywas placed on the table in front of the infantand the box was placed over the toy. Thiswas done so as not to give the infant cluesabout the location of the open side.

In early testing sessions, infants werepresented with the boxes in the order of easeof solution. This was done to prevent infantsfrom becoming so frustrated that they wereunable to continue with the testing session.The opaque box was easier to solve than thetransparent box. The opening to the frontwas easier to solve than the opening to theside. Infants were first presented with theopaque box open to the front. Until infantswere able to retrieve the toy with the boxopening facing them, we did not administertrials with the opening to either side. Onceinfants were able to retrieve the toy from theside opening, the location of the openingwas varied at random. Once infants wereable to retrieve the toy from the transparentbox with the opening to the front, they werethereafter presented with either the opaqueor the transparent box first.

Infants were given as much time as theyneeded to retrieve the toy. If an infant's at-tention strayed from the task, the experi-


menter brought the infant's attention backby tapping or drumming on the box with herfingers. If, after approximately 1 min with abox, an infant had failed to retrieve the toy,the experimenter gave the infant "clues"about the location of the opening. Therewere three types of clues the experimentercould give: (a) putting her fingers throughthe opening to manipulate the toy, (h) trail-ing the toy out from the box and back in, or(c) trailing the box back and forth over thetoy. All three of these "clues" gave infantsinformation abut the location of the opening.

The videotaped Barrier Detour sessionswere coded for the strategies infants used toretrieve the toy. Infant strategy was madeup of a combination of looking and reachingbehavior and was categorized into five de-velopmental phases by criteria set forth byDiamond (1990b) and used by Bell and Fox(1992). In keeping with previous research,only infant behavior on the transparent boxwas used for purposes of phase classifica-tion. In general, during early sessions, be-havior on the opaque box was one phaseahead of behavior on the transparent box.However, infants rarely achieved the high-est phase classification on the opaque boxbecause they continued to use earlier phaselooking behaviors to retrieve toys from theopaque box.

Infants could either succeed or fail inretrieving the toy. Failure to retrieve the toycould be due to a failure to reach at all or afailure to reach through the opening. Failureto reach through the opening occurred mostoften with the transparent box and was dueto infants reaching straight for the toy alongtheir line of sight, which caused them to hitthe transparent Plexiglas with their hand.

Success in retrieving the toy could occurthrough combinations of looking and reach-ing that varied in sophistication. The leastsophisticated type of reaching behavior oc-curred when infants needed to fixate the toyvisually through the opening in the boxwhile reaching for the toy (Phase 1, PhaseIB). At the next level of sophistication, in-fants needed to look through the front open-ing before, but not during, reaching, butthey needed to fixate the toy through theside opening throughout reaching (Phase 2).At the next level, infants did not need to lookthrough the front opening at all and onlyneeded to look through the side opening be-fore, but not during, reaching (Phase 3). Atthe highest level of sophistication (Phase 4),infants did not need to look through theopening at all before retrieving the toy. At

this level, it was typical to see infants con-ducting a manual search of the box for theopening. For a more extended explanationof̂ the Barrier Detour coding, see Diamond(1990b) or Bell and Fox (1992).

Two coders classified all Barrier Detoursessions. The initial intercoder reliabilitywas over 90%, with any disagreements eas-ily resolved through discussion.

Two Location Means-End TaskApparatus.—Two light blue baby wash-

cloths were used for this task. The babywashcloths were the same ones used tocover the wells in the AB task.

Procedure.—The infant was seated inthe parent's lap facing the testing table. Theexperimenter sat across the table, facing par-ent and child. A collection of toys (includingplastic keys, rattles, squeaky toys) andCheerios breakfast cereal was available sothe experimenter could find an object thatwas attractive to each infant. If the infant lostinterest in the current object, a different ob-ject was used.

The two cloths were placed side byside, separated by approximately 3 inches,on the table in front of the infant The frontedges of the cloths were within easy reachof the infant while the back edges were outof arm's reach.

For a given trial, an attractive object wasplaced on top of one of the cloths, out ofarm's reach for the infant. Successful re-trieval required infants to grasp the frontedge of the cloth and pull it toward them,thereby bringing the toy within arm's reach.Successful retrieval was rewarded by praise,applause, and the opportunity to play withthe toy. Infants were scored as making anerror if they pulled the incorrect, vacantcloth; if they did not reach at all; if they si-multaneously pulled both cloths; or if theyclimbed up onto the table to grasp tbe toydirectly, without pulling the cloth. In thesecases, the parent and the experimenter re-mained neutral, and that trial was repeated.

When an infant had retrieved the toycorrecdy on two to three trials in a row onone side, the toy was switched to the otherside. The location of the toy was reversedat least twice in every session so that thecriterion of two out of three correct retrievalson change-of-side trials could be met.

Eacb infant received two scores on theMeans-End task for each session. The firstscore was a Means-End score. The Means-End score was devised by tallying the num-


ber of trials on which the infant erred anddividing it by the total number of trials theinfant was given. This yielded a score be-tween 0 and 1.

The second score was a Perseverationscore. The Perseveration score took into ac-count only the trials on which a repeatedbehavior led to a reaching error. When aninfant pulled the same cloth on two consecu-tive trials, the second trial would be countedas incorrect if the first trial had been incor-rect, or if the first trial had been correct butthe toy had been switched to the other cloth.The latter case corresponds to the A-not-Berror. The Perseveration score was com-puted by tallying the number of trials onwhich a repeated behavior led to an incor-rect response and dividing this number bythe total number of trials minus one. (Thefirst trial in any given session was notcounted into the denominator because therewas nothing for the infant to repeat).

ResultsData Analysis

A repeated-measures MANOVA wascomputed for each of the five major tasks:modified AB, nonreaching AB, Barrier De-tour, Means-End, and Means-End/Persever-ation. For each task, term (between-subjectsfactor), age at testing (within-subjects factor),and the interaction of term x age at testingwere entered as independent variables, withperformance on each task as the dependentvariable.

In addition, in order to examinewhether performance of the two infantgroups was equivalent when the groupswere equated for chronological age, ratherthan conceptional age, separate analyseswere conducted based on chronological ageof the infants. For each task, infant perfor-mance at each test session was entered intoa 2 (term: premature or full-term) x 9 (testsession) repeated-measures ANCOVA, withthe number of days premature entered as acovariate.

Standard AR TaskUsing MANOVA, length of delay at

which the A-not-B error occurred for pre-term and full-term infants was analyzed. Theresults revealed a main effect of term, F(l,18) = 6.45, p = .02; a main effect of age,F(8, 11) = 4.23, p = .02; and no efifect ofthe term X age interaction, F(8, 11) = 1.32,p = .33. The results of the ANCOVA re-vealed that there was a main effect of testsession, F(8, 144) = 27.42, p < .001; no ef-

fect of term, F(l, 17) = .60, p = .45; and noeffect of the term x test session interaction,F(8, 144) = 1.46, p = .18.

Only when infants were compared byconceptional age was there a group difffer-ence on the modified AB task, with prema-ture infants tolerating longer delays thanfull-term infants. When infants were com-pared at either conceptional or chronologicalages, there was a main effect of infant ageon modified AB performance, indicating thatas infants got older they tolerated longer de-lays on the task (see Fig. 1).

Nonreaching AB TaskDelays needed to produce the A-not-B

error on the nonreaching AB task analyzedwith MANOVA revealed a significant maineffect of term, F(l, 18) = 10.62, p = .004;and a significant main effect of age, F(8, 11)= 5.78, p = .005; and no effect of the termX age interaction, F(8, 11) = 1.77, p = .19.Results of the ANCOVA revealed a main ef-fect of test session, F(8, 144) = 18.01, p <.001; no significant effect of term, F(l, 17) =.67, p = .44; and no significant term x testsession interaction, F(8, 144) = 1.65, p =.12.

When infants were compared by eitherconceptional or chronological age, there wasa main effect of infant age on nonreachingAB performance, so that as infants got olderthey were able to tolerate longer delays onthe task. However, only comparison by con-ceptional age yielded a group difference inperformance, with premature infants tolerat-ing longer delays than full-term infants (seeFig. 2).

Comparison of AB Looking and ReachingDirect evidence that looking and reach-

ing proceed from the same representationcomes from a study that compared lookingand reaching within trials of the standard ABtask (Janowski, 1993). Janowski comparedthe direction of infant looking and infantreaching, and recorded latency times for in-fant reaching. According to Janowski's re-sults, it takes infants 1—4 sec from the experi-menter's signal to initiate reaching. Infantreaching latency adds a few seconds to thedelay tolerated in the reaching version of theAB task.

In this study, the difference in the delayneeded to induce the A-not-B error in themodified AB task and the nonreaching ABtask is approximately 2 sec, which fallswithin the range of reaching latency re-ported by Janowski (1993). If 2 sec are added


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FIG. 1.—Delays tolerated in the standard AB task as a function of infant age and term. Infant agewas measured since conception (top panel) or since birth (bottom panel). When infants were comparedby conceptional age (top panel), preterm infants tolerated longer delays than full-term infants. Wheninfants were compared by chronological age, there was no overall group difference (bottom panel).Points represent mean delay tolerated; vertical lines depict standard errors of the means.

to the experimenter-delimited delay in thereaching task, delays tolerated in the reach-ing task coincide with the delays toleratedin the nonreaching version of the AB task(see Fig. 3). This suggests that infants do notcommit the A-not-B error despite correct re-call for the location of the toy, but preciselybecause they cannot recall the location.

Barrier Detour TaskBarrier Detour performance, as mea-

sured by reaching and looking behaviors

employed by infants to retrieve objects fromthe transparent box, was analyzed withMANOVA. Results revealed a main effect ofage, F(8, 11) = 15.07, p = .0001; and nosignificant effects of term, F(l, 18) = 0.69, p= .42; or the term X age interaction, F(8,11) = 1.21, p = .37. Results of the ANCOVArevealed a main effect of test session, F(8,144) = 35.87, p < .001; and no significanteffects of term, F(l, 17) = .20, p = .66; orthe term X test session interaction, F(8,144)= .98, p = .46.


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FIG. 2.—Delays tolerated in the nonreaching AB task as a function of infant age and term. Infantage was measured since conception (top panel) or since birth (bottom panel). When infants werecompared by conceptional age (top panel), preterm infants tolerated longer delays than full-term infants.When infants were compared by chronological age, there was no overall group difference (bottompanel). Points represent mean delay tolerated; vertical lines depict standard errors of the means.

The results of comparing infants bychronological or conceptional age on theBarrier Detour task revealed that both pre-mature and full-term infants improved onthe task with increased age/test sessions (seeFig. 4). This is in keeping with the resultsof previous research (Bell & Fox, 1992; Dia-mond & Coldman-Rakic, 1985). There wasno statistical difference between the perfor-mance of premature and full-term infants.

Means-End TaskThe dependent variable of the ratio of

Means-End errors to total Means-End trials

within a session was analyzed withMANOVA. Results revealed no significanteffects of term, F(l, 18) = 2.99, p = .10, age,F(8, 11) = 1.07, p = .44, or the term x ageinteraction, F(l, 11) = 1.67, p = .21. Resultsof the ANCOVA revealed a main effect oftest session, F(8, 144) = 8.34, p < .001; nosignificant effect of term, F(l, 17) = 1.19,p = .29; or the term X test session interac-tion, F(8, 144) = 2.24, p = .07.

When infants were compared at thesame chronological ages, Means-End errorrates dropped across testing sessions, but


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FIG. 3.—Comparison of the delays tolerated (with 2 sec added) in the standard AB task to delaystolerated in the nonreaching AB task, as a function of infant age and prematurity. Points representmean delays tolerated.

there were no group differences. In contrast,when infants were compared by concep-tional age, there was no effect of age onMeans-End performance, although the inter-action of prematurity and test session ap-proached significance (see Fig. 5).

Means-End/PerseverationThe dependent variable, the ratio of

perseverative errors committed in theMeans-End task to the total number of trialswhere there was opportunity for persevera-tion within a given session, was analyzed us-ing MANOVA. Results revealed no effectsof age, F(8, 11) = 2.35, p = .09, term, F(l,18) = .01, p = .94, or the term x age interac-tion, F(8, 11) = 2.08, p = .13. Results of theANCOVA revealed that there was no effectsof term, F(l, 17) = 1.77, p = .20, test ses-sion, F(8, 144) = 1.06, p = .39, or the termX test session interaction, F(8, 144) = 1.37,p = .25.

Whether compared by conceptional orchronological age, both infant groupsshowed similar performance on Means-End/Perseveration, and infant error rates wereconsistently low across all testing sessions(see Fig. 6).

DiscussionThe goal of this research was to examine

the development of performance on themodified AB task, a nonreaching AB task, theBarrier Detour task, and a Means-End task,in order to explore the relative roles of envi-

ronmental input and maturational program-ming in these tasks. It was predicted that theperformance of premature infants would beworse than the performance of their full-term counterparts on all tasks. However, theresults indicate that age-corrected prema-ture infants outperformed full-term infantson the modified AB task and the nonreach-ing AB task.

Other investigators have reported find-ing striking differences in brain activity andbehavioral measures between full-term andpremature infants shortly after birth (Duffyet al., 1990), with premature infants laggingbehind full-term infants. This raises thequestion of why the premature infants in thecurrent research demonstrated performancesuperior to their full-term counterparts inthe face of other reports to the contrary (Al-len & Alexander, 1990; Duffy et al., 1990;Ross et al., 1992). For example, Duffy et al.studied a group of full-term infants, a groupof moderately premature infants (3-7 weekspremature), and a group of very prematureinfants (8—14 weeks premature). The EECand behavioral data collected from infants atthe age of 2 weeks post-EDC revealed thatmeasures of the full-term infants differedfrom those obtained from both groups of pre-mature infants. Hence, moderately prema-ture infants' performance was simileir to thatof very premature infants rather than to thatof full-term infants. Duffy et al. concludedthat these EEG differences between groupsof infants indicated that the premature in-fants had suffered from subtle brain damage.

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FIG. 4.—Barrier Detour performance as a function of infant age and term. Infant age was measuredsince conception (top panel) or since birth (bottom panel). In both analyses, infant perfonnance in-creased with age, but there were no overall group differences. Points represent mean performance;vejtical lines depict standard errors of the means.

However, several factors about the pre-mature infants in the Duffy et al. study sug-gest that caution be applied in comparingthis group of infants to the infants in thepresent study. First, Duffy et al. found thatperinatal complications predicted some ofthe group differences in their data. There issome indication that the Duffy et al. infantssuffered more serious complications thanthe premature infants in this study. Second,Duffy et al. (1990) tested infants accordingto gestational age. This resulted in large dif-ferences between groups of infants in agesince birth. The premature infants were up

to 14 weeks older, chronologically, than thefull-term infants, a large difference in lightof the fact that testing took place 2 weekspost-EDC.

Ross et al. (1992) also examined AB per-formance of a group of age-corrected prema-ture infants and a group of full-term infants.In contrast to the findings of the presentstudy, Ross et ai.'s results indicated that full-term infants' performance exceeded the per-formance of the premature infants. How-ever, the premature infants in the Ross etal. study suffered from several serious risk

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factors, not present in the premature infantsin the current study, including lowerbirthweight, greater prematurify, and venti-latorj' assistance, any one of which could bedetrimental to optimal cognitive develop-ment. Thus, premature infants in both theDuffy et al. (1990) and Ross et al. studieswere less healthy than the infants in thepresent study, which may, in part, accountfor the reported differences in performance.

The superior performance of the pre-mature infants in this study suggests thathealthy premature infants have an advantage

over full-term infants in some areas of devel-opment, although because of the small sam-ple size of this study, this conclusion shouldbe taken as tentative. When time since birthwas statistically equated, there were no dif-ferences in performance by prematur̂ e orfull-term infants on any of the tasks. Thissuggests that some developmental processesmay proceed from the time of birth ratherthan the time of conception. Duffy et al.(1990) suggested the possibility that somecritical periods of brain development may betriggered by the environment, for example,by the transition that occurs at birth. When

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FIG. 6.—Perseveration in the Means-End task as a function of infant age and term. Infant age wasmeasured since conception (top panel) or since birth (bottom panel). In both analyses there were noeffects of infent age or birth group. Points represent mean ratios of infant errors,- vertical lines depictstandard errors of the means.

this notion is applied to findings from thepresent study, the suggestion arises that thedevelopment of the brain structure(s) thatmediates AB performance is affected by ex-perience in the postnatal environment, un-derscoring the notion that both experienceand maturation are important drivers in thedevelopment of performance on this andother cognitive tasks.

One possible explanation of environ-mentally mediated brain development iscaptured in Creenough and Black's (1992)discussion of experience-expectant and ex-

perience-dependent synapse production.Initially, there is a process of overproductionof synapses in widespread regions of thebrain that are selectively reduced to adultlevels with experience in the species-typicalenvironment. Creenough and Black holdthat synapses are overproduced with the ex-pectation that the developing organism willencounter the species-typical environment,hence experience-expectant synapse pro-duction. In contrast, experience-dependentsynapse processes generate synapses on de-mand, storing information that is unique tothe individual and that depends on individ-


ual experiences. According to these authors,repeated exposure to new information mayresult in gradual accretion of experience-dependent synapses.

Experience-expectant and experience-dependent synapse production may, in part,account for premature infants' superior per-formance on the two versions of the AB taskin the present study. Premature infants en-counter the species-typical postnatal envi-ronment earlier than full-term infants. Thisencounter may set into motion the processof experience-expectant synapse pruning.Additionally, the premature infants in thestudy had greater individual experience inthe environment (due to their greater chro-nological age) than the full-term infants and,therefore, more exposure to information thatmight contribute to experience-dependentsynapse development. When experience inthe postnatal environment was held constantby statistical correction for prematurity, per-formance of both groups was the same.

The results of this study also raise sev-eral general questions about the underlyingmechanism of modified AB performance.Previous reports of delays tolerated in themodified AB task are markedly differentfrom the results reported here (Bell & Fox,1992; Diamond, 1985). Based on the work of"Diamond (1985) and Bell and Fox (1992), itwas expected that infants* performance onthe modified AB task would increase from28 to 52 weeks of age, with an average in-crease in the delay tolerated of 1.5-2 sec permonth before infants would commit the ABerror. Infants at 52 weeks of age also wereexpected to commit the A-not-B error at anaverage delay of 10 sec. Finally, it was ex-pected that premature infants would lag be-hind full-term infants in performance. Re-sults indicated that while premature infants'performance was unexpectedly superior tothat of full-term infants, neither group at-tained predicted levels of'performance by 52weeks of age or showed the expectedmonthly rate of increase. Premature infantstolerated a 5.9-sec delay at 12 months, show-ing a 1.2-sec per month increase in delaytolerated; full-term infants tolerated a 1.8-sec delay at 12 months, showing a 0.5-secper month increase in delay tolerated.

The discrepancy between ttie data pre-sented here and data collected under similarcircumstances by other researchers is intri-guing. One possible explanation for thesedifferences may be that the infants in thepresent research were more similar to those

in Bell and Fox's (1992) short delay groupthan to the long delay group tested by Belland Fox, or to die group tested by Diamond(1985). A second alternative may be that,while every attempt was made to replicateDiamond's (1985) procedure, subtle differ-ences in testing may account for differencesin obtained results. Finally, it also is possi-ble that obtained results vary a great dealfrom one sample of infants to another (partic-ularly of concern when small sample sizesare used, such as in the present study), call-ing into question theoretical proposals basedon any one group of infants' modified ABperformance. At the present time, it is notpossible to determine which of these alter-natives is most plausible.

The current findings also lead to severalpreliminary speculations about the mecha-nisms underlying modified AB performance.It must be noted that correlational analyseswould be needed to test fully these hypothe-ses, but the small sample size of this studyprecluded such analyses. Other authors haveproposed that looking and reaching versionsof the AB task proceed from different under-lying representations (Diamond, 1985), thatmodified AB and Barrier Detour perfor-mance should be related (Diamond, 1990a,1990b), or that AB and Means-End perfor-mance should be similar (Baillargeon et al.,1989).

Diamond (1990a, 1990b) proposes thatan immature dorsolateral prefrontal cortexleads infants to make perseverative errors onboth the modified AB task and the BarrierDetour task. Based on this proposal, similarresults would be expected from infantstested on the modified AB task, the BarrierDetour task, and the perseveration measureof Means-End abilify. However, for themodified AB and Barrier Detour tasks, thefindings from the current work appear tobe in keeping with those of Bell and Fox(1992), who found that EEC measures differ-entiated groups of infants on the AB task butnot on the Barrier Detour task. The patternof results reported here suggests that thereis no difference between premature and full-term infants on the Barrier Detour task,whereas there are significant group differ-ences on the modified AB task. Further, onthe perseveration measure of Means-Endability, perseverative errors rates occurredless than 10% of the time and remained rela-tively constant across all testing sessions,rather than decreasing as AB performanceincreased. Together, these findings suggestthat while perseveration may account for in-


fant performance on the modified AB task,there is little to indicate that this same per-severative tendency manifests itself in ei-ther the Barrier Detour or the Means-Endtask.

Alternatively, modified AB performancemay refiect some limitation in Means-Endproblem-solving ability, as suggested byBaillargeon et al. (1989), rather than a per-severative tendency. If this were the case,the development of infants' performance onthe modified AB task should resemble de-velopment of performance on the Means-End task. However, infant Means-End abil-ity increased sharply in the first 2 months ofthe study and remained high for the remain-der of the study, whereas AB abilify in-creased at a relatively constant rate acrossthe duration of the study. This suggests thatMeans-End problem solving captured by thetask used in this study is not related to per-formance on the modified AB task. Overall,then, the results of this study suggest littlerelation between AB performance and Bar-rier Detour, Means-End, or Means-End/Per-severation performance.

The results of the nonreaching AB taskdid, however, mirror the results of the modi-fied AB task. Premature infants toleratedgreater delays than full-term infants, andboth groups of infants tolerated greater de-lays with increasing age. These results sug-gest that both the modified AB task and thenonreaching AB task produce similar mem-ory demands and that these demands are dif-ferent from those produced in Baillargeon etal. (1989) AB looking task. For example, onthe nonreaching AB task, 8-month-oId in-fants in this study tolerated delays of 2-3sec, whereas infants in the Baillargeon et al.study of infant recognition memory tolerateddelays of up to 70 sec. This difference indelay tolerated by infants may be due to thefact that the two different tasks tap into twodifferent types of memory, recognition ver-sus recall (Hofstadter & Reznick, in press).Further, the parallel results for the modifiedand nonreaching versions of the AB task inthis study suggest that recall memory playsa critical role in infants' successful perfor-mance on the modified AB task.

The finding of parallel infant perfor-mance on the reaching and nonreaching ver-sions of the modified AB task may provideindirect information about the underlyingrepresentation from which AB performanceproceeds. It has been suggested that infantsmay tell us with correct looking that they

know where an object is hidden even thoughthey reach incorrectly (Diamond, 1985).This hypothesis has as its underpinning theimplicit assumption that looking and reach-ing in the AB task proceed from differentunderlying representations, that infant look-ing refiects correct knowledge about the cur-rent trial whereas infant reaching proceedsfrom a lingering representation of the previ-ous trial. Parallel infant performance on thetwo versions of AB in this study calls such ahypothesis into question.

Janowski (1993) and Hofstadter andReznick (in press) suggest that infant look-ing and reaching proceed from the same un-derlying representation. Hofstadter and Rez-nick found that, given a constant delay,infant reaching is more vulnerable to persev-eration than infant looking. They suggestthat the reaching requirement disrupts in-fant cognition in the modified AB task. Thisis an interesting proposal, but it does not ex-plain why infants are able to tolerate increas-ing delays with development. Janowski sug-gests that the difference in looking andreaching is due to infant reaching latencyadding seconds to the delay. It was found inthis study that delays in the AB looking taskwere parallel to, and approximately 2 seclonger than, delays tolerated in the modifiedAB task. If the A-not-B error in both thereaching and looking tasks results from fail-ure of recall memory, we are released fromthe necessity of having separate explana-tions for infant and adult search behavior.By this account infants, like adults, fall backon a previously rewarded search programonly when they do not, for whatever reason,have in mind (he current location of the ob-ject. Like adults, infants act on their memoryfor the location of the object as long as thatmemory persists.

As a whole, these findings suggest intri-guing differences between premature andfull-term infants in development of the abil-ity to solve the modified AB task, the non-reaching AB task, the Barrier Detour task,and the Means-Ends task, leading to severalspeculations about the relation betweenbrain development and behavior. First, thefindings presented here indicate that devel-opment of healthy, marginally premature in-fants is similar to that of full-term infants.Second, these findings indicate that thereare large differences between samples of in-fants in reported development of infants'performance on the modified AB task. Thissuggests that current proposals based on sin-gle samples of infants linking AB perfor-


mance to brain development may be tenu-ous. Third, it is suggested that whatevermediates AB performance, the function itmediates is probably one of memory, and notof perseveration or Means-End ability. Fi-nally, results of this study suggest that devel-opment of brain structure(s) that mediatemodified AB performance is strongly influ-enced by experience in the postnatal envi-ronment.

References

Alexander, G. E. (1982). Functional developmentof association cortex in monkeys: Behavioralstudies. In P. Rakic & P. S. Goldman-Rakic(Eds.), Research Program Bulletin, 20, 472-479.

Alexander, G. E., & Goldman, P. S. (1978). Func-tional development of the dorsolateral pre-frontal cortex; An analysis utilizing reversiblecryogenic depression. Brain Research, 143,233-249.

Allen, M. G., & Alexander, G. R. (1990). Grossmotor milestones in preterm infants: Correc-tion for degree of prematurity./ournai of Pe-diatrics, 116, 955-959.

Baillargeon, R., DeVos, J., & Graber, M. (1989).Location memory in 8-month-old infants in anon-search AB task: Further evidence. Cogni-tive Development, 4, 345-367.

Bell, M. A., & Fox, N. A. (1992). The relationsbetween frontal brain electrical activity andcognitive development during infancy. ChildDevelopment, 63, 1142-1163.

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Documents

Development of Preterm and Full-Term Infant Ability on AB, Recall Memory, Transparent Barrier Detour, and Means-End Tasks