individual differences in infant visual attention: recognition of degraded visual forms by...

Individual Differences in Infant VisualAttention: Recognition of Degraded VisualForms by Four-Month-Olds

Janet E. Frick and John ColomboUniversity of Kansas

FRICK, JANET E., and COLOMBO, JOHN. Individual Differences in Infant Visual Attention: Recog-nition of Degraded Visual Forms by Four-Month-Olds. CHILD DEVELOPMENT, 1996, 67,188-204.In 5 experiments, 4-month-old infants were tested for their ability to recognize degraded visualtargets as a function of individual differences in fixation duration. Targets were degraded byremoving 10% of the total contour either from vertices (vertex-absent) or from midsegments(vertex-present). Both qualitative and quantitative differences were found in long and shortlookers' ability to recognize the degraded forms. Short-looking infants were able to recognizedegraded forms in both vertex-absent and vertex-present conditions, but the vertex-absent dis-crimination was more difficult. Long-looking infants required longer familiarization times beforeshowing evidence of recognition in the vertex-present condition, and were unable to recognizetargets in which contour was removed at vertices. The findings are discussed within the frame-work ofthe persistence of early visual processing strategies, and reliance of long-looking infantson particular local elements in visual analysis.

One of the important themes emerging Fantz, 1973, 1974; Rose, 1983) populations,from recent work on individual differences The predictive validity of fixation durationin early development is the relevance of from infancy for later cognitive status is fur-measures of infant visual attention for the ther supported by research showing correla-prediction of cognitive status in later child- tions between individual differences in fix-hood (Bornstein & Sigman, 1986; Lewis & ation duration and recognition memoryBrooks-Gunn, 1981; Miller et al., 1979; performance during infancy. For example.Miller, Spiridigliozzi, Ryan, Callan, & long-looking infants have been shown to re-McLaughlin, 1980; Rose, Feldman, & Wai- quire additional familiarization before beinglace, 1988,1992; Rose, Slater, & Perry, 1986; able to match the visual recognition perfor-Sigman, Cohen, Beckwith, & Parmelee, mance of short-looking infants (Colombo,1986). Longitudinal research in this area has Mitchell, Coldren, & Freeseman, 1991;shown that infants who engage in extended Freeseman, Colombo, & Coldren, 1993).or prolonged fixation durations ("long-looking" infants) may be at some risk for The nature ofthe mechanisms underly-later cognitive deficit (Colombo, 1993). This ing differences between long- and short-is supported by data from both normal (Rose looking infants with respect to their perfor-et al., 1986; Sigman et al., 1986) and clinical mance on visual discrimination tasks and(Gunn, Berry, & Andrews, 1982; Miranda & their long-term cognitive outcome has been

Portions of this research were submitted by the first author in partial fulfillment ofthe M.A.degree awarded by the Department of Human Development at the University of Kansas. Theaudiors thank Greg Austin, Sara Coleman, Ann Egli, Heidi EUenberger, Mark Epstein, KelleyFuhrman, Norman Harrigan, Mark Mcllroy, Kristi Ortiz, Jennifer Ryther, and Eric Syphers fortheir assistance in data collection and preparation. We are especially grateful to the staff of theUniversity of Kansas Regents Center for their cooperation in our research, and as always, wethank the families who participated in these studies. Preliminary versions of this paper werepresented at the meeting of the Society for Research in Child Development, March 1993, inNew Orleans. This work was supported by NIH predoctoral training grant HD07173-13 whichsupported J.F., a General Research Fund Award from the University of Kansas, and by NIHgrant HD29960-01. This program of research is also supported by the University of Kansas MentalRetardation Research Center. Address correspondence and requests for reprints to the authorsat the Department of Human Development, 4001 Dole Human Development Center, Universityof Kansas, Lawrence, KS 66045-2133. E-mail: [email protected] or Colombo®kuhub.cc.ukans.edu.

[Child Development, 1996,67,188.-204. © 1996 by the Society for Research in Child Development, Inc.All rights reserved. 0009-3920/96/6701-0016$01.00]

Frick and Colombo 189

the subject of recent theorizing (Colombo,1993; McCall & Carriger, 1993). The factthat short lookers are able to solve visual dis-crimination tasks with less exposure thanlong lookers led Colombo and Mitchell(1990) to speculate fhat fixation duration mayreflect some aspect of speed of informationprocessing. However, differences in speedmay be attributable to a number of factors.For example, such differences could be pro-duced by purely quantitative diflFerences inneural organization or integrity. Alterna-tively, differences in speed of processingcould arise from the use of different"modes" or "strategies" of visual intake andprocessing, which would predict qualitativedifferences in how the two groups of infantsprocess visual information.

One specific area in which potentialqualitative differences between long- andshort-looking infants have been investigatedis in the processing of global and local stimu-lus properties (e.g., Colombo et al., 1991;Colombo, Freeseman, Coldren, & Frick,1995; Freeseman et al., 1993). Visual per-ception research has revealed that adultstypically process visual stimuli in a global-to-Iocal sequence (Hughes, Layton, Baird, &Lester, 1984; Kimchi, 1992; Lamb & Robert-son, 1990; Navon, 1977, 1991; Pomerantz,Sager, & Stoever, 1977; Weisstein & Harris,1974). That is, the overall configuration of avisual stimulus is processed first, followedby analysis of the smaller, local features. Ithas been hypothesized (Colombo et al., 1991,1995; Freeseman et al., 1993) that whileshort-looking infants might follow the global-to-local sequence of visual processing typi-cal of adults, long-looking infants might gen-erally engage in a more laborious feature-by-feature visual analysis.

Recent studies have revealed that, in in-fants older than 4 months of age, the globalor eonfigural properties of visual stimuli takeprecedence in processing, as they do inadults (Colombo, Laurie, Martelli, & Hartig,1984; Ghim & Eimas, 1988; Quinn & Eimas,1986; Van Giffen & Haith, 1984). Infants areable to solve discrimination tasks involvingglobal stimulus properties with less familiar-ization than is required to discriminate localstimulus properties (Colombo et al., 1991;Freeseman et al., 1993), a finding consistentwith the adult literature (Miller, 1981; Na-von & Norman, 1983; Pomerantz et al., 1977;Weisstein & Harris, 1974).

With respect to the global-local hypoth-esis as it relates to long- and short-looking

infants, the available evidence suggests thatboth groups of infants process global stimu-lus attributes more quickly than local ele-ments (Colombo et al., 1891; Freeseman etal., 1993). Thus, to date, no qualitative differ-ences have emerged between long and shortlookers in terms of sensitivity to global orlocal stimulus properties. However, despitethese findings, it is possible that qualitativedifferences in long and short lookers' pro-cessing of global and local properties mightbe reflected in ways other than through dif-ferences in sensitivity to different visualproperties (e.g., Colombo et al., 1995).

Recognition of Degraded Visual FormsOne method for investigating possible

qualitative differences in infants' reliance onglobal and local visual properties is throughtests for the recognition of degraded figures(i.e., visual images from which some part ofthe contour has been removed). Recognitionof degraded figures may generally be consid-ered to depend on attention to global visualproperties; under conditions where contourhas been removed, reliance on local compo-nents or features will be much less effectivein facilitating stimulus recognition. Thus, aninvestigation of how readily degraded fig-ures are recognized can be considered to bea convergent test of global processing, sinceglobal processing would presumably beneeded to recognize a target whose local fea-tures had been degraded.

The present studies were designed totest proposed differences between long andshort lookers' processing of global and localstimulus properties. If visual analysis is ini-tially based on a global scan, deletion of asmall portion of the contour should pose lit-tle problem for recognition. Thus, it mightbe predicted that short lookers (who havebeen hypothesized to use a global-to-localvisual intake strategy) should be able to rec-ognize degraded targets following relativelyshort stimulus exposures.

On the other hand, recognition of de-graded targets should be considerably im-paired (if not completely hindered) if infantsengage in a local visual analysis. If, as hasbeen hypothesized, long lookers rely on lo-cal stimulus attributes for their discrimina-tion of global changes in stimuli (see Co-lombo et al., 1995), then recognition ofdegraded patterns may not be possible forthem.

Recognition of degraded visusJ stimulihas been examined in a number of studiesin the adult literature (Biederman, 1987;

190 Child Development

Biederman, Beiring, Ju, & Blickle, 1985;Biederman & Blickle, 1985; Blickle, 1989).These studies have indicated that visual rec-ognition is differentially affected by the re-moval of contour from vertices or from mid-segment areas of visual stimuli. Whencontour is removed from vertex areas of adegraded visual stimulus, recognition maybe considerably slowed or even completelyhindered for adults {Biederman, 1987). Fromthis framework, vertex contour is critical indefining the global organization of a visualstimulus; when it is missing, adults may notbe able to determine how separate localstimulus attributes form a coherent whole.Midsegment contour, on the other hand,may be more easily "filled in" by the visualsystem than vertex contour, since the stimu-lus components that are missing contour arestill aligned. We therefore included manipu-lations of locus of contour deletion in thesestudies in order to further examine possibledifferences in long- and short-looking in-fants' recognition of degraded figures.

Recognition of degraded forms by 8-17-week-old infants has previously been exam-ined by Schwartz and Day (1979, Experi-ment VI). They systematically varied thelocus of "gaps" in contour while examininginfants' ability to recognize incomplete tri-angles. Their results suggested that the sizeof contour gaps more greatly impaired stimu-lus recognition than did the location (verti-ces or midsegments) of degraded contour.The relevance of Schwartz and Day's (1979)findings to the present questions is limited,however, for several reasons. First, targetswere constructed of discrete dots, ratherthan line segments, and degradation was ac-complished by removing individual dotsrather than interrupting continuous seg-ments of contour. Thus, none of their stimulicontained true "contour," either at verticesor midsegments. Second, the targets sub-tended 30° of the infants' visual field; sucha large visual angle generally hinders theperception of the overall stimulus configu-ration and forces attention on the presenceor absence of particular local features (seeLamb & Robertson, 1990). Thus, discrimina-tion of the targets may have been based onthe presence or absence of dots in particularspatial locations, rather than any sort of con-figural processing. Finally, Schwartz andDay (1979) fully habituated infants beforetesting for recognition of complete figures;in fact, infants were presented with the ha-bituation stimulus for eight trials totaling160 sec. Thus, recognition was assessed after

what might be considered a relatively com-plete encoding of the stimulus. Schwartzand Day's (1979) Experiment VI was not de-signed to address the possibility that vertexcontour facilitates form recognition at earlierstages of processing than does contour atmidsegments.

In summary, the aim of the current stud-ies was to examine the importance of vertexcontour for infants' recognition of degradedfigures, and to relate these findings to poten-tial differences in short- and long-looking in-fants' modes of visual intake. In the first ex-periment, this was accomplished byestablishing infants' characteristic fixationduration profiles in a pretest, and then as-sessing their recognition of degraded visualforms under vertex-present versus vertex-absent conditions.

Experiment 1The first study examined whether the

presence of vertex contour in degraded fig-ures facilitates stimulus recognition, andwhether short- and long-looking infantsdemonstrate similar patterns of recognitionfollowing equal amounts of familiarization.

METHOD

SubjectsSubjects in all experiments were re-

cruited by mail and telephone from the Kan-sas City metropolitan area. Infants werescheduled to participate within 1 week oftheir 4-month birthday (approximately 18weeks, or 124—131 days). The average agefor infants in Experiment 1 was 129 days(range 116—138 days). In all studies, infantswere screened for prematurity (gestation of<37 weeks) as well as eye problems thatwould hinder visual processing (such as stra-bismus or current use of eye drops). For in-fants in Experiment 1, the average length ofgestation was 40.0 weeks, and the averagebirth weight was 3601.0 grams. Mothers av-eraged 29.4 years of age and 15.6 years ofeducation, while fathers averaged 31.4 yearsof age and 16.4 years of education. Infantswere generally first- or second-born.

Forty-seven infants were tested for Ex-periment 1, and complete data sets were ob-tained from 40 of these infants. Of the addi-tional seven infants, three were excludeddue to fussiness, one because of equipmentfailure, one due to prematurity, and two be-cause of complete lateral side bias (seebelow).

ApparatusInfants were tested in a 2.0 X 2.0 m

booth, darkened on three sides by blackcloth and on the fourth side by black ply-wood. A 1.0 X 0.7 m translucent rear-projection screen was centered in the frontwall, and infants were seated in a parent'slap 0.5 m away, facing the screen. The stim-uli were projected onto the screen from be-hind using Kodak Carousel projectors. Thepresentation of stimuli and calculation offixation times were controlled by a Zenithmicrocomputer.

Infants' fixations were coded via buttonpress by an observer blind to stimulus condi-tion and orientation. The observer wasseated outside the booth to the side of thescreen, and viewed the reflection of thestimulus in the infant's cornea through a10-mm peephole.

In addition to the live coding of fixa-tions, each session was videotaped using aVHS video camera mounted above the rear-projection screen, with the lens focused onthe infant's face. Observers recoded 70% ofall sessions from videotape, and interob-server reliabilities were calculated by corre-lating records of the live observer with thosemade from the videotape. The average reli-ability over all fixation duration data was+ .92, and reliability for novelty preferencescores was -\- .96.

StimuliThe stimuli for this study included a

color slide of a female face, and achromaticslides of geometric patterns. The face stimu-lus was used to assess individual differencesin fixation duration. It consisted of a colorphotograph of a smiling, brown-hairedwoman wearing a dark jacket and standingagainst a pale yellow background. This slidehad previously yielded good variability infixation duration with this age group (Co-lombo, Mitchell, O'Brien, & Horowitz,1987). Past accumulated-looking procedureswith 20-sec familiarization have shown thatthe median peak fixation for 4-month-oldswith this stimulus is 9 sec (Colombo, Freese-man, Mitchell, &c Coldren, 1990).

The remaining targets w ere used in thefamiliarization/paired-comparison tasks toassess recognition of degraded stimuli (seeFigs. 1 and 2). They consisted of arrays ofblack concentric squares or diamonds on awhite background. All diamond stimuli wereidentical to the squares, except for being ro-tated 45° on their axes. Young infants candiscriminate between otherwise identical


Familiarize to:

Test:

(novel) (familiar)

FIG. 1.—Example of a vertex-present recog-nition task. Infants are familiarized to the nonde-graded target (top), and then are tested with thetwo degraded targets (bottom). In this example,preference for the degraded square would signifyrecognition of the familiarized diamond in its de-graded form.

visual targets on the basis of differences inorientation alone (Cornell, 1975; Schwartz &Day, 1979, Exp. IV).

In addition to the nondegraded stimuli,four degraded stimuli (two squares, two dia-monds) also were constructed by systemati-cally deleting 10% of the total contour of thenondegraded stimulus. In creating the de-graded stimuli, contour was either removedfrom all four sides in the midsegment areas,such that the vertices remained intact ("ver-tex-present" stimuli), or from the vertices,suchi that line segments appeared not to bejoined at the intersections ("vertex-absent"stimuli). A control experiment establishedthat infants' discriminations were not af-fected by perception of the subjective con-tours apparent in the degraded stimuli; de-tails of this experiment are reported in theAppendix.

DesignThis experiment employed a com-

pletely between-subjects factorial design,with factors of attentional profile (2: short orlong looker), familiar stimulus (2: square or


Familiarize to:

•

Test:

(novel) (familiar)FIG. 2.—Example of a vertex-absent recogni-

tion task. Logic is the same as for Figure 1. In thisexample, preference for the degraded diamondwould signify recognition of the familiarizedsquare in its degraded form.

diamond), and deletion condition (2: vertexpresent or absent). These factors are ex-plained below.

Attentional profile.—This refiects theinfant's classification as a long or shortlooker, based on his or her peak fixation tothe slide of a female face (procedure is de-scribed below).

Familiar stimulus.—The nondegradedstimulus presented to the infant during theaccumulation phase was balanced betweensubjects, with half the infants viewing thesquare and the other infants the diamond.

Deletion condition.—The type of de-graded stimulus used during the paired-comparison phase also was balanced acrosssubjects, with each infant viewing eithervertex-present or vertex-absent figures.

ProcedureInfants were seated in a parent's lap in-

side the testing chamber. Parents werebriefed about the testing procedures, andwere instructed not to speak or interferewith the infant's looking behavior in anyway. All participants complied with these re-quests.

Attentional profile classification.—Thecolor slide of the female face was used toassess individual differences in fixation du-ration. When projected on the screen, thestimulus subtended a visual angle of 20° by25°. The infant accumulated 20 sec of fixa-tion to this stimulus. Using this fixed-accumulation procedure, each infant viewsthe stimulus for the same length of time, butlong-looking infants accumulate the re-quired fixation with fewer looks of longerduration than do short-looking infants. Clas-sification of the infant as a "long looker" or"short looker" was based on the infant'speak fixation to this face stimulus, based ona split at the previously established medianof 9 sec (Colombo et al., 1990). Infants werethen randomly assigned to one of the experi-mental conditions.

FamiliarizationIpaired comparison task.—Following the looking-profile classifi-cation, infants were familiarized to either thenondegraded concentric square pattern orthe nondegraded concentric diamond pat-tern. During familiarization, the single stim-ulus remained in the center of the screenuntil 20 sec of fixation had been accumu-lated.

Following completion of familiariza-tion, infants were immediately presentedwith a paired-comparison test of stimulusrecognition in which two degraded visualstimuli (one square and one diamond) weresimultaneously presented to the right andleft of the infant's midline, separated by avisual angle of 25°, with each target subtend-ing a visual angle of 20°. Depending onwhich stimulus was used for familiarization,one was a degraded form of the familiar tar-get, while the other was a degraded form ofa novel target. For example, to an infant fa-miliarized to the nondegraded concentricsquare pattern, the novel stimulus would bethe degraded concentric diamond pattern.Infants' recognition of the familiarized butdegraded target would be indicated by se-lective fixation of the novel target. Infants'novelty preferences were calculated by tak-ing the proportion of time spent fixating thenovel target to the total fixation time accu-mulated during the choice trials. Noveltypreferences that significantly exceed chanceresponding (i.e., .50) are interpreted as indi-cating stimulus recognition (Fantz, Fagan, &Miranda, 1975).

The paired-comparison test consisted oftwo choice trials, with 5 sec of fixation accu-mulated during each. Following the first


choice trial, the lateral positions of the twostimuli were reversed, and another 5-sectrial began immediately. The side on whichthe novel target was first presented was ran-domly assigned by the computer.

The within-subject counterbalancing oflateral position on choice trials is commonlyemployed to control for lateral side bias, often seen in infants of this age (Liederman &Kinsbourne, 1980a, 1980b; Turkewitz,1980). However, the infants who showedcomplete lateral bias on both choice trials(i.e., looking entirely left or entirely right,irrespective of the stimulus) were excludedfrom final analyses, because such completebias results in a chance-level (i.e., .50) "nov-elty preference" score that is likely attribut-able to factors beyond those of a strictly per-ceptual nature (see also Colombo, Frick, &Ryther, 1993). Exclusion of infants showingcomplete side dominance minimizes thecontribution of extraneous (e.g., motoric orstate) factors to infants' observed perfor-mance on such tasks.

RESULTS AND DISCUSSION

For long-looking infants, the averagepeak fixation to the face stimulus was 13.5sec (SD = 3.6), and to the familiarizationstimulus was 5.8 sec (2.4). Short lookers' av-erage peak fixation to the face was 6.3 sec(1.7), and their average longest look to thefamiliarization stimulus was 4.1 sec (1.4). Itis of interest that infants' peak fixation to theface during the pretest was significantly cor-related with their peak fixation during famil-iarization, r(38) = .48, p < .01.1 The magni-tude of this correlation is in concordancewith previously reported levels of reliabilityof fixation duration across tasks (e.g., Co-lombo et al., 1987).

Novelty preferences were examined us-ing a three-way ANOVA, with the between-subjects factors of attentional profile (2: longor short), familiar stimulus (2: square or dia-mond), and deletion condition (2: present orabsent). This analysis yielded a significantmain effect for familiar stimulus, F(l, 32) =15.01, p < .01, which was qualified by a sig-nificant attentional profile x familiar stimu-

lus X deletion condition interaction, F(l,32) = 4.22, p < .05. The interaction indi-cated that short and long lookers were differ-entially affected by the vertex manipulation.

This three-way interaction was decom-posed to render it more interpretable. Sepa-rate familiar stimulus x deletion conditionANOVAs were performed for short and longlookers. The ANOVA for long lookersyielded no significant terms, thus indicatingthat their performance was equivalent in allfamiliar stimulus and deletion conditionconditions. A t test was performed on theiroverall novelty preference against chance(.50) to determine whether they had discrim-inated the degraded targets. This test wasnot significant, indicating that their overallnovelty preference {M = .53, SD = .11) didnot differ from chance, t{lQ) = 1.33, N.S. Insummary, long lookers showed no evidenceof having recognized either type of degradedtarget (vertex-present or vertex-absent).

The ANOVA for short lookers did yieldsignificant main effects for both familiarstimulus, F(l, 16) = 19.13, p < .01, and dele-tion condition, F(l, 16) = 6.36, p < .05, butno significant interaction, F(l, 16) = .64,N.S. The familiar stimulus effect was due tohigher novelty preference scores when in-fants were familiarized with the square stim-ulus than with the diamond stimulus. Thiswas an unexpected result; because it did notinteract with the deletion condition effect,and because effects involving the familiarstimulus factor did not replicate in any of thesubsequent studies in this series (see Exper-iments 2-5 below), this result is not dis-cussed further.

The main effect for deletion conditionwas due to better performance by short look-ers on the vertex-present task than on thevertex-absent task. T tests of novelty prefer-ences against chance (.50) indicated that per-formance in the vertex-present condition (M= .60, SD = .10) was above chance, t{9) =3.11, p < .02, but performance in the vertex-absent task (M = .52, SD = .10) was not,t{9) = 0.62, N.S.2 Thus, after 20 sec of famil-iarization, short lookers could recognizeonly degraded stimuli that retained contourin vertex areas. Figure 3 shows long- and

' The results and interpretations of this experiment are unchanged if classification of aninfant as a long or short looker is based on a median split of infants' peak fixations to thefamiliarization stimulus (nondegraded square or diamond).

^ In addition, the novelty preferences of short lookers in the vertex-absent condition andlong lookers in both conditions did not differ from each other, but did differ from novelty prefer-ences of short lookers in the vertex-present condition, *(36) = 1.80, p < .05 (one-tailed).

194 Child

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—r-

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Short

1

1 T1

Lookers

T T

____HiHiliH

Vertex:

B Present

r

Long Lookers

J Absent

FIG. 3.—Results of Experiment 1. Novelty preferences are presented as a function of attentionalprofile and deletion condition. Long lookers' novelty preferences are shown for each deletion condition,even though they did not differ from one another or from chance. Only the novelty preferences of shortlookers in the vertex-present condition (far left bar) significantly exceeded chance. Error bars representl S E .

sbort-looking infants' novelty preferencescores as a function of deletion condition.

In summary, the results of Experiment1 suggested the following conclusions, andgenerated a set of programmatic manipula-tions to be carried out in subsequent experi-ments. First, short-looking infants showedrecognition of degraded stimuli when vertexcontour was present, but not when it wasabsent. For this group, the next logical stepswere (a) to establish the lower bound for rec-ognition in the vertex-present condition bydecreasing familiarization, and {b) to deter-mine whether recognition would be possi-ble in the vertex-absent condition underconditions of increased familiarization.These were the goals of Experiment 2 andExperiment 3, respectively. Second, long-looking infants were observed to recognizeneither the vertex-degraded nor midseg-ment-degraded figures at 20 sec of familiar-ization. This was not particularly surprising,as long lookers have been shown repeatedlyto require longer familiarization before dem-onstrating stimulus recognition (Colombo etal., 1991). Tbus, the logical step to take instudying the perceptual capacities of thisgroup was to further increase familiarizationfor both types of degraded stimulus condi-tions. This was the objective of Experiments4 and 5.

Experiment 2In Experiment 2, we sought to establish

whether short lookers could show recogni-tion of degraded figures in the vertex-present condition under less familiarization.Thus, the basic procedures of Experiment 1were repeated here for short lookers, withthe exception that familiarization was short-ened to 10 sec.

METHOD

SubjectsNineteen short-looking 4-month-old in-

fants (M = 132 days old, range 124-139days) were tested for Experiment 2. Datafrom 16 of these infants were used in finalanalyses. Of the three infants excluded, onefell asleep during the testing session, onebecame fussy, and a third was excluded dueto equipment failure. The average length ofgestation for infants in Experiment 2 was40.6 weeks, and the average birtb weightwas 3781.2 grams. The values for other de-mographic variables were similar to thosereported in Experiment 1.

Apparatus, Design, and ProcedureAll aspects of the experimental method

were identical to those in Experiment 1,with the exception that familiarization timewas set at 10 sec, and only vertex-present

Friek and Colombo 195

degraded stimuli were used in the paired-comparison task. All infants were given alooking profile classification test, consistingof 20 sec of infant-directed fixation to thefemale face. Only infants whose peak fixa-tion to this stimulus was 9 sec or less (shortlookers) were assigned to this experiment(long-looking infants with a peak fixation oflonger than 9 sec were assigned to Experi-ment 4; see below). All other aspects of theexperimental procedure followed as in Ex-periment 1.


Short lookers' average peak fixation tothe face in the looking profile classificationtest was 7.3 sec (SD = 1.3), and their peakfixation to the familiarization stimulus was4.9 sec (2.4). Short-looking infants were notable to recognize vertex-present degradedstimuli following 10 sec of familiarization.Their mean novelty preference score was.52, (SD = .15), which did not significantlydiffer from chance (.50) responding, t{l5) =0.60, N.S. Novelty preferences did not varyas a function ofthe stimulus used during fa-miliarization.

With the lower temporal bound for thevertex-present task determined to be greaterthan 10 sec, we now sought evidence forshort lookers' recognition of vertex-absentdegraded figures. This was done by increas-ing familiarization for this more difficult con-dition in Experiment 3.

Experiment 3

METHOD

SubjectsTen infants (M = 130 days old, range

123—137 days) classified as short lookersduring the fixation-duration assessment par-ticipated in this experiment. One additionalinfant was tested, but became fussy duringthe session and was excluded. The averagelength of gestation for infants in Experiment2 was 39.7 weeks, and the average birthweight was 3498.4 grams. The values forother demographic variables were similar tothose reported in Experiment 1.

Apparatus, Design, and ProcedureIn Experiment 3, short-looking infants

accumulated 30 sec fixation to the nonde-graded square or diamond stimulus. Follow-ing familiarization, they were tested for rec-ognition of vertex-absent degraded stimuli.Novelty preference scores were calculatedas in previous experiments.


In the looking profile classification pre-test, short lookers' average peak fixation tothe face was 6.0 sec (SD = 1.5), and to thefamiliarization stimulus was 5.5 sec (1.7).Short lookers' responding did indicate rec-ognition of vertex-absent figures following30 sec familiarization; their novelty prefer-ences (M = .58, SD = .10) were significantlygreater than chance, t{9) = 2.75, p < .05.Novelty preferences did not vary as a func-tion ofthe stimulus used during familiariza-tion. Figure 4 shows short-looking infants'novelty preference scores from Experiments2 and 3.

The results of Experiments 1—3 upheldtwo predictions. First, short-looking infantsrequired less familiarization than long-looking infants to show successful recogni-tion ofthe degraded stimuli. This supportsother findings of processing advantages forshort-looking infants (Colombo et al., 1991).

Second, the locus of degradation did in-fluence how readily the degraded figureswere recognized. For short-looking infants,degraded stimuli that retained vertex con-tour were discriminated after less familiar-ization (20 sec) than stimuli lacking vertexcontour (30 sec).

Although the pattern of short lookers'recognition of degraded figures had been es-tablished in the first three experiments,long-looking infants had not shown recogni-tion of any type of degraded target. Itseemed likely that, as in previous studies,long lookers might need more extensive fa-miliarization before demonstrating signifi-cant recognition. Therefore, in Experiment4, familiarization was increased to a levelthat had been sufficient in previous researchfor long lookers' encoding of achromatic vi-sual targets.

Experiment 4

Previous studies (e.g., Colombo et al.,1991; Freeseman et al., 1993) have sug-gested that, in order to discriminate or recog-nize targets, long lookers need 20—30 sec fa-miliarization above that necessary for shortlookers to perform the same tasks. Thus,based on short lookers' successful recogni-tion of vertex-present figures at 20 sec, famil-iarization was lengthened for long lookers inExperiment 4 to 50 sec; they were thentested for recognition of both vertex-presentand vertex-absent stimuli.


70

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15 50CD

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40

30Experiment 2 Experiment 3

FIG. 4.—Gombined results of Experiments 2 and 3. The left bar represents short lookers' recogni-tion of vertex-present stimuli in Experiment 2, and the right har short lookers' recognition of vertex-absent stimuli in Experiment 3. Only in Experiment 3 (right har) did novelty preferences significantlyexceed chance. Error bars represent 1 SE.

METHOD

SubjectsTwenty long-looking infants (M = 130

days, range 122-137 days) participated inExperiment 4. An additional seven infantswere tested; three were excluded due tofussiness during the session, one for prema-turity, two due to complete lateral side bias,and one due to experimenter error. The av-erage length of gestation for infants in Ex-periment 2 was 39.6 weeks, and the averagebirth weight was 3603.6 grams. The valuesfor other demographic variables were simi-lar to those reported in Experiment 1.

Apparatus, Design, and ProcedureAll experimental procedures were iden-

tical to those in the preceding experiments,except that infants accumulated 50 sec of fa-miliarization to the nondegraded stimulusduring the familiarization phase. As before,familiar stimulus (square or diamond) wascounterbalanced across subjects. Followingfamiliarization, infants were administered apaired-comparison test for recognition ofthedegraded stimuli, just as in Experiments1—3. Half the subjects in this experiment (n= 10) were tested for recognition of vertex-

present stimuli, and the rest were testedwith vertex-absent stimuli.


Long lookers' average peak fixation tothe face during the pretest was 12.7 sec (SD= 2.9), and to the stimulus during familiar-ization was 10.4 sec (5.4). Long lookers wereable to recognize degraded figures onlywhen vertex contour was present. Longlookers' overall novelty preference for bothdeletion conditions was .55 (SD = .10),which was significantly greater than chance,t(19) = 2.35, p < .05. This effect, however,could be attributed exclusively to their rec-ognition of vertex-present degraded figures.The mean novelty preference score for longlookers in the vertex-present condition was.58 (SD = .08), which significantly exceededchance, t{Q) = 3.31, p < .01. Their recogni-tion of vertex-absent figures (M = .52, SD= .12), on the other hand, did not differ sig-nificantly from chance, *(9) = 0.63, N.S.None of these effects veiried as a function offamiliarization stimulus. Figure 5 showslong lookers' novelty preferences in Experi-ment 4 as a function of deletion condition.^

^ The difference between long lookers' novelty preferences for the two deletion conditionsfell short of significance, t(18) = 1.41, .05 < p < .10, one-tailed. However, as the tests againstchance indicate, only long lookers in the vertex-present condition behaved systematically inexhibiting recognition of the nondegraded forms.


oc

QL

70

60

"S 50oc

d)

a> 40

30Vertex Present Vertex Absent

FIG. 5.—Results of Experiment 4. Novelty preferences for long lookers are shown as a functionof deletion condition. Only in the vertex-present condition (left bar) did novelty preferences signii-cantly exceed chance. Error bars represent 1 SE.

The emergence of recognition undervertex-present, but not vertex-absent, condi-tions was in accord with the prediction thatvertex contour facilitates recognition of de-graded visual stimuli. However, while rec-ognition of degraded figures had finally beendemonstrated in long lookers, it had notbeen demonstrated for targets lacking con-tour at vertices. A final experiment was con-ducted to determine whether recognition ofvertex-absent degraded targets would bepossible for long lookers. This was done byincreasing familiarization even further, to amaximum level determined by the attain-ment of a habituation criterion.

Experiment 5In previous experiments, long-looking

infants had demonstrated recognition of ver-tex-present figures following a substantialincrease in familiarization time with respectto short lookers. However, even 50 sec famil-iarization was not sufficient for recognitionof vertex-absent targets. This meant that oneimportant question was still not answeredsatisfactorily by the present data, that is,whether long lookers could recognize de-graded visual targets when vertex contourhad been removed. If they could not performsuch a task, it would suggest that long look-ers were dependent on vertex contour forrecognition of these stimuli. This would rep-

resent a very different visual intake strategyfrom that used by short lookers. The alter-nate possibility was that the 50 sec of famil-iarization used in Experiment 4 was simplyinsufficient exposure time for this difficultdiscrimination. Thus, to address this ques-tion, long lookers were tested in Experiment5 under conditions of maximum familiar-ization.

To insure that familiarization would beraised to a sufficient level for each infant, aninfant-control habituation paradigm (Horo-witz, Paden, Bhana, & Self', 1972) was em-ployed in Experiment 5. With this paradigm,each infant determines his or her ownamount of familiarization and accumulatesfixation until a habituation criterion hasbeen reached. Following habituation, the in-fant may be tested for recognition in paired-comparison choice trials as before.

Although only long lookers wereneeded for this experiment (as they were theonly group that had not yet shown recogni-tion of vertex-absent targets), both long- andshort-looking infants were tested in Experi-ment 5. This allowed for an additional checkof short lookers' discrimination abilities afterhabituation, and controlled for the possibil-ity that potential deficits in long lookers' per-formance might be attributable to the use ofthe habituate-to-criterion paradigm.


METHOD

SubjectsThirty-five 4-month-old infants (M =

130 days old, range 124-138 days) weretested in Experiment 5. Of this number,complete data sets were obtained from 21.Of the other 14, five infants were excludedfor showing complete lateral side bias, threedue to fussiness, two because of inattentionto stimuli during the habituation session (notlooking at the experimental display for 3consecutive minutes), one due to prematu-rity, one because of maternal interference,one due to experimenter error, and onewhose peak fixation to the face fell exactlyon the median between long and short look-ers (i.e., 9.0 sec).

Of the original group of 35 infants, 17were short lookers, 17 were long lookers,and one became too fussy during the lookingprofile pretest to finish the session. The finalsample of 21 included 12 long lookers and 9short lookers. The average length of gesta-tion for infants in Experiment 2 was 40.2weeks, and the average birth weight was3609.6 grams. The values for other demo-graphic variables were similar to those re-ported in Experiment 1.

Apparatus, Design, and ProcedureAs in all of the previous experiments,

infants were administered the 20-sec accu-mulated-fixation pretest to the face. Follow-ing the pretest, infants were tested for recog-nition of vertex-absent targets. InExperiments 1 through 4, infants were famil-iarized with the nondegraded target throughan accumulated fixation procedure, whichresults in each infant receiving the same pre-determined amount of fixation to the famil-iarization target. In Experiment 5, however,infants were familiarized with the nonde-graded target through an infant-controlledhabituation procedure using 1-sec fixationonset and offset criteria (Colombo & Horo-witz, 1985). The criterion for habituationwas two consecutive looks at 50% of the av-erage of the longest two looks from the se-quence (see Colombo et al., 1987). Follow-ing attainment of habituation, infants werepresented with two 5-sec choice trials, re-versed for lateral positions, as in previousexperiments.

Only vertex-absent targets were used inthe choice trials, as this was the only condi-tion still unrecognized by long lookers (seeFig. 2). As before, familiarization stimulusand initial side of novel stimulus were coun-terbalanced across subjects.


Since the purpose of Experiment 5 wasto determine long lookers' ability to discrim-inate vertex-absent targets, and becauseshort lookers were included only as proce-dural controls, data for the two groups willbe discussed separately.

Short LookersShort lookers' average peak fixation to

the face stimulus during the pretest was 6.5sec (SD = 1.2), and their looks were consis-tently short through the subsequent habitua-tion session. Short lookers' average amountof total fixation accumulated during the ha-bituation phase was 40.1 sec (26.1), and theiraverage peak fixation during this phase was15.2 sec (8.9).

Consistent with the findings from Ex-periment 3, following an average of 40 secexposure to the familiar target, short lookersshowed significant recognition of the vertex-absent figures. Their average novelty prefer-ence was .62 (.14), which was significantlygreater than chance, t{8) = 2.67, p < .05.Thus, using a different experimental proce-dure from Experiment 3, short-looking in-fants again demonstrated the ability to dis-criminate degraded targets lacking vertexcontour.

One matter to be considered from thesedata is how the amount of familiarization ac-cumulated during habituation compared tothat needed by the infants previously toshow successful discrimination. In Experi-ment 1 and 3, short lookers successfully rec-ognized degraded figures following 20-30sec of familiarization to the nondegraded tar-get. Using the habituation procedure, how-ever, short lookers received an average of 40sec familiarization to the nondegraded tar-get. This discrepancy corroborates Fagan's(1974) assertion that habituation proceduresmay overestimate the amount of familiariza-tion needed for infants to encode visualstimuli successfully.

Long LookersLong lookers' average peak fixation to

the face during the pretest was 14.4 sec (SD= 4.4), and they also maintained consis-tently long fixations during the second ex-perimental phase. Their average peak fixa-tion during the habituation phase was 30.1sec (20.3), and their average amount of totalaccumulated fixation was 73 sec (45.8). Thiswas a substantially greater amount of fixationthan had been allotted in Experiment 4 (50sec), when long-looking infants had failed to


O

a

O

c

o

0a.

Short Lookers Long Lookers

FIG. 6.—Results of Experiment 5. Novelty preferences for long and short lookers are shown. Onlythe novelty preferences of short lookers (left liar) significantly exceeded chance. Error bars representlSE.

discriminate vertex-absent degraded figures,but had discriminated the vertex-presenttargets.

Even with this substantial increase infixation of the nondegraded target, however,long lookers did not demonstrate recogni-tion of the degraded, vertex-absent targets.Their group novelty preference was .49(.19), which did not differ significantly fromchance, t{ll) = -0.22, N.S. Figure 6 showsthe novelty preferences of long and shortlookers from Experiment 5.^

The picture that emerges from Experi-ment 5 is one of differences between longand short lookers in the stimulus informationnecessary for recognition of degraded forms.Specifically, the results suggest that longlookers are dependent on vertex contour forvisual analysis. Long lookers, despite havingbeen allowed essentially unlimited expo-sure to the nondegraded target, did not dem-onstrate recognition of that same targetwhen its vertex contour had been removed.Short lookers were again able to perform thistask, thus ruling out the possibility that longlookers' lack of recognition was somehow at-tributable to the change in familiarizationprocedure.

General Discussion

The results of the five experiments aresummarized in Table 1. The results clearlysupport the contention that vertex contourfacilitates stimulus recognition for infants; inaddition, the results are in accordance withthe postulation of both speed and strategydifferences between long- and short-lookinginfants. Each of these implications of thepresent findings will be discussed in turnbelow.

Locus of Contour and Infant VisualRecognition

These results suggest that the presenceof vertex contour in degraded figures eitherfacilitated the recognition of such figures(i.e., for short lookers) or simply made suchrecognition possible in the first place (i.e.,for long lookers). The current results indi-cate that stimulus vertices serve as importantreference points for visual processing fromas early as 4 months of age. Indeed, as Sala-patek (1968, 1975) found, the fixations ofvery young infants tend to cluster aroundvertex areas. The results from these experi-ments suggest that, without vertex contour,encoding is noticeably slowed, as it is foradults (Biederman, 1987).

* Short lookers' novelty preferences from Experiment 5 were greater than those of longlookers, t(19) = 1.77, p < .05 (one-tailed).


TABLE 1

OVERALL RESULTS OF EXPERIMENTS 1-5

FAMILIARIZATION (Sec)

10 20 30 50 HABITUATION

Short lookers;Vertex-present No Yes (Yes) (Yes) (Yes)Vertex-absent (No) No Yes (Yes) Yes

Long lookers;Vertex-present (No) No (No) Yes (Yes)Vertex-absent (No) No (No) No No

NOTE.—An entry of "Yes" in any cell indicates that the infants in that condition showednovelty preferences that significantly exceeded chance (.50). An entry of "No" indicates a nov-elty preference that did not differ from chance responding. Parenthetical entries denote as-sumed performance at familiarization levels not directly tested.

Global and local visual processing.—Asstated previously, tests of recognition of de-graded targets can be thought of as conver-gent tests of global visual processing, sincepresumably a visual analysis based on localstimulus properties would be gready hin-dered if some of those local properties wereremoved. The current results suggest twoconclusions regarding global and local vi-sual processing by infants. The first conclu-sion is that in some cases, global stimulusprocessing may be aided by the presence ofparticular local stimulus attributes. If bothinfants (Ghim & Eimas, 1988) and adults(Navon, 1977) engage in a global-to-local se-quence of visual analysis, as some previousresearch has suggested, then it may be thatcertain local properties (such as presence ofvertex contour) are integral to the definitionof global stimulus attributes. The secondconclusion is that long-looking infants ap-pear to be completely reliant on certain localstimulus properties for their visual pro-cessing, a matter to be taken up in the nextsection.

Implications for Individual Differences inInfant Visual Attention

These findings clearly replicate previ-ous reports (Colombo et al., 1991; Colombo& Mitchell, 1990) that short-looking infantsperform perceptual-cognitive tasks at brieferstimulus exposures than long-looking in-fants. As previously suggested, differencesin speed of processing might be attributedto different underlying causes; among theseare differences in neural speed or strategiesof visual intake. The current findings offeradditional evidence of a quantitative speeddifference between long and short lookersin how quickly they process visual stimuli.Short-looking infants showed successful dis-

crimination of the targets following 20 and30 sec of familiarization, while long lookersshowed no discrimination until receivingadditional familiarization.

The most intriguing finding from thesestudies, however, is that long-looking infantsfailed to discriminate vertex-absent figures,even after being given essentially unlimitedopportunity to view the nondegraded tar-gets. Despite nearly a 50% increase in famil-iarization from Experiment 4 to Experiment5, long lookers did not show recognition ofthe nondegraded targets when their vertexcontour was removed. This failure in recog-nition was apparently not attributable to theuse of the habituation paradigm in the lastexperiment, since short-looking infants so fa-miliarized did recognize the targets (as theyhad under conditions of briefer familiariza-tion in Experiment 3). As such, this findingrepresents a true qualitative difference inthe processing of visual stimuli betweenlong and short lookers (see also Colombo etal., 1995).

The question arises as to why the ab-sence of vertex contour apparently preventsthe recognition of degraded targets in longlookers, while the removal of other contourdoes not. One possible answer is that, intheir processing of visual patterns, long-looking 4-month-olds might persist in pat-terns of visual inspection that are character-istic of younger infants. Very young infantstend to cluster their fixations around corners,edges, and vertex areas, while older infantsscan more widely over stimulus contour(e.g., Salapatek, 1968). If long-looking in-fants' attention to visual stimuli paralleledthe scanning patterns that are characteristicof younger infants, it would be expected that


removal of such local features from visualpatterns would greatly hinder stimulus rec-ognition.

The results of several studies supportthis contention. Long-looking infants areconsistently slower in their visual pro-cessing (e.g., Colombo et al., 1991; Freese-man et al., 1993). They tend to find localstimulus properties more salient duringstimulus encoding (Colomho et al., 1995),and tend to cluster their fixations around asmall area of stimulus contour (Bronson,1991). These findings, considered alongwith long lookers' inability to recognize de-graded targets lacking vertex contour, sug-gest some sensory or perceptual basis for thediscrepancy in perfonnance between longand short lookers. This might be attributableto differences in acuity, or other low-levelsensory factors. Alternately, it might be at-tributable to long lookers' persistence in pat-terns of scanning and encoding that are char-acteristic of younger infants, rather thandeveloping faster and more thorough pro-cessing strategies.®

Aslin (1985) has suggested that many is-sues of infant visual perception "will remainunanswered until measures of scanning aregathered with other measures of visual pro-cessing, such as preference, habituation, orconditioning" (p. 410). Further convergingevidence drawn from scanning studies mayserve to support the conclusions drawn fromthese experiments. In addition, develop-mental examinations of the various neuralsystems underlying vision and attention willundoubtedly add to our understanding ofthedevelopment of, and individual differencesin, infant visual attention.

AppendixThe manner in which contour was removed

to produce the degraded targets used in thesestudies caused the stimuH to connote subjectivecontours to adults: an "X" and a "plus" appear tooverlay the degraded targets (see Figs. 1 and 2). Ifinfants were capahle of perceiving the subjectivecontours connoted hy these degraded stimuH,then the subjective contours might have inter-acted with the actual remaining contours to inHu-ence the ease or difficulty of various discrimina-tions. To address these concerns, data werecollected to determine whether 4-month-old in-fants were able to extract the subjective contoursconnoted by these degraded targets within the

temporal constraints imposed by the experimentaldesign.

METHOD

SubjectsSixteen infants (M = 130 days, range 122-

137 days) participated in this control experiment.An additional two infants were tested; one wasexcluded due to fussiness, and one due to com-plete lateral side bias. The final sample of 16 in-cluded eight long lookers and eight short lookers.The values for demographic variables were simi-lar to those reported in Experiment 1.

StimuliSix stimuli were employed in this control ex-

periment: the four degraded targets used in Ex-periments 1-5 (vertex-present square and dia-mond, vertex-absent square and diamond), alongwith stimuli constructed in the form of an outlined"X" and an outlined "plus" (see Fig. 7). The "X"and the "plus" stimuli were constructed so as tocorrespond exactly to the size and shape of thesubjective contours connoted by the degraded ar-eas ofthe four degraded targets. Specifically, boththe vertex-present square and the vertex-absentdiamond appear to have a "plus" overlaying them,while both the vertex-present diamond and thevertex-absent square appear to have an "X" over-laying them. The "X" and the "plus" stimuli usedin the control experiment were identicallyformed; one stimulus could be obtained by rotat-ing the other stimulus 45°. Each subtended 22° ofvisual angle.

Apparatus, Design, and ProcedureThe experimental apparatus was identical to

that used in Experiment 1 (and all experimentsfollowing). Infants were familiarized to one ofthefour degraded targets (see Figs. 1 and 2) used inExperiments 1—5 (familiarization stimulus wascounterbalanced across subjects). The length ofthe familiarization period was set at 10 sec, be-cause in Experiment 1 (and all those that follow),the degraded targets (and thus, the subjective con-tours) were only shown to infants during two 5-secchoice trials. That is, the maximum time the in-fants were exposed to the subjective contours was10 sec. Thus, if perception of the subjective con-tours present in the degraded stimuli affected in-fants' discriminations, such perception wouldhave to have taken place within a 10-sec period.

Following this 10-sec familiarization to thedegraded target, infants were then tested for theirrecognition ofthe subjective contour connoted bythe target. This was done by pairing the "X" stim-ulus with the "plus" stimulus (see Fig. 7). Thesestimuli were paired in two 5-sec choice trials, withlateral position ofthe two stimuli counterbalancedacross the two trials. If infants were capable ofextracting the subjective contours under these

^ It is of some interest that this overall pattern of behavior in long-looking infants is concor-dant with a delay in the development of certain substrates of the geniculocortical system (Co-lombo, 1995; Lewis, Maurer, & Brent, 1989).


Familiarize to:

Test:

(novel) (familiar)

FIG. 7.—Example of task used in the controlexperiment to test for infants' perception of thesubjective contours present in the degraded tar-gets. Infants are familiarized to the degraded tar-get (top), and then are tested with the two targetsshown at the bottom. In this example, a preferencefor the "X" shown in the bottom left would signifyrecognition of the subjective "plus" shown in thevertex-present degraded square at top.

conditions, those exposed to the subjective con-tour connoting an "X" (such as is found in a ver-tex-absent square) would be expected to show apreference for the (novel) "plus," while those ex-posed to subjective contour connoting a "plus"(such as is found in a vertex-present square) wouldbe expected to show a preference for the (novel)"X."


Infants' overall preferences on choice trials(M = .47, SD = .15) did not differ significantlyfrom chance, t(15) = 0.77, N.S. The flndings wereentirely comparable for short lookers (M = .49,SD = .14) and long lookers (M = .46, SD = .17);neither differed significantly from chance (.50) re-sponding, ts(7) = — 0.24 and — 0.77, respectively,nor did they differ significantly from one another.Thus, at the level of exposure to the degraded tar-gets employed in the experiments described inthis report, infants showed no evidence of recog-nizing the subjective contours connoted by thedeletion of contour in those stimuli.

These data do not refute previous demonstra-tions of infants' perception of subjective contours

(e.g., Ghim, 1990). It should be noted, however,that these demonstrations have involved more ex-tensive familiarization than was provided here(e.g., 30 sec or more; see Ghim, 1990). Further,they have not consistently been successful withInfants younger than 7 months of age (see Berten-thal, Campos, & Haith, 1980).

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