Developmental Science 11:4 (2008), pp 563 –574 DOI: 10.1111/j.1467-7687.2008.00702.x

© 2008 The Authors. Journal compilation © 2008 Blackwell Publishing Ltd, 9600 Garsington Road, Oxford OX4 2DQ, UK and 350 Main Street, Malden, MA 02148, USA.

Blackwell Publishing LtdPAPER

Effect of partial occlusion on newborns’ face preference and recognition

Lucia Gava,1 Eloisa Valenza,1 Chiara Turati1 and Scania de Schonen2

1. Department of Developmental and Social Psychology, University of Padua, Italy2. Laboratory of Cognitive Development, CNRS, Université René-Descartes, Paris, France

Abstract

Many studies have shown that newborns prefer (e.g. Goren, Sarty & Wu, 1975; Valenza, Simion, Macchi Cassia & Umiltà,1996) and recognize (e.g. Bushnell, Say & Mullin, 1989; Pascalis & de Schonen, 1994) faces. However, it is not known whether,at birth, faces are still preferred and recognized when some of their parts are not visible because hindered by other configurations,that is when faces are partly occluded. Also, it is not known whether newborns’ preference for an upright over an inverted faceand newborns’ face recognition are differentially affected depending on the salience of the occluded face features. Seventy-sevennewborns (mean age of 43.5 hrs) were tested using the preferential looking (Experiment 1) and the habituation techniques(Experiment 2). Results demonstrated that newborns prefer and recognize occluded faces even if some portions of them arenot available, at least when the hindered features are not salient. On the contrary, these abilities are affected by obscuring highsalience facial features (i.e. eyes). However, while in the case of face detection, eyes occlusion completely prevented newborns’face detection, in the case of face recognition an analogous stimulus manipulation heavily impaired, but did not totally preclude,newborns’ recognition performance. The data collected improve our comprehension of newborns’ way of processing and encodinginformation to detect and recognize faces.

Introduction

The human face is one of the most complex visualpatterns to which human beings are exposed in eitheradulthood or infancy. Nevertheless, when presented witha visual scene, we are capable of locating a face withouteffort and, often, in well under a second (Lewis &Edmonds, 2003). This ability, termed face detection,involves a decision as to whether a given stimulus is aface, and it is usually investigated testing the capacity todiscriminate between a face and a jumbled face (Valentine& Bruce, 1986) or a face that had one feature replacedby another (e.g. a mouth replaced by an eye; Cooper &Wojan, 2000). Other studies investigated face detectionemploying visual-search paradigms in which subjectswere asked to indicate whether there was an intact facewithin an array of distracters (Nothdurft, 1993; Brown,Huey & Findlay, 1997; Elgavi-Hershler & Hochstein,2002; Lewis & Edmonds, 2002, 2003, 2005). Adults havea remarkable ability to detect faces, even in the absenceof normal facial features. They readily detect a facewhen presented with a painting by Arcimbaldo in whichan arrangement of fruit or vegetables forms a face(Moscovitch, Winocur & Behrmann, 1997), or whenpresented with a two-tone mooney face, at least whenthe stimuli are upright (Kanwisher, Tong & Nakayama,

1998). In addition, faces are detected more easily thannon-facial objects – an effect that was referred to as theface-detection effect and is maintained when faces areinverted, reduced to grey-scale, or blurred.

Furthermore, although in comparison to most othertypes of stimuli, faces are more alike than dissimilar, weare able to recognize thousands of different individualfaces even under unfavourable circumstances, for instancewhen the eyes and the mouth of a familiar face arereplaced with those of a different face (Hines, Jordan-Brown & Juzwin, 1987; Lee & Perrett, 2000; Yamaguchi,Hirukawa & Kanazawa, 1995) or when some internalfeatures (e.g. eyes) are removed (Ellis, Shepherd & Davies,1979; Young, Hay, McWeeny, Flude & Ellis, 1985;Nachson, Moscovitch & Umiltà, 1995; Leder, Candrian,Huber & Bruce, 2001; Moscovitch et al., 1997). Thisability, termed face recognition, results from recognitionmemory competencies and refers to the ability to dis-criminate among different exemplars of the face category,recognizing a face as familiar.

Many developmental studies aimed at examining theorigins of face processing have demonstrated that facedetection and recognition appear very early duringpostnatal life. From birth, newborns prefer looking atmoving (i.e. Goren et al., 1975; Maurer & Young, 1983;Johnson, Dziurawiec, Ellis & Morton, 1991) or static

Address for correspondence: Lucia Gava, Dipartimento di Psicologia dello Sviluppo e della Socializzazione, via Venezia 8, 35131 Padova, Italy;e-mail: lucia.gava@unipd.it

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(i.e. Macchi Cassia, Simion & Umiltà, 2001; Mondloch,Lewis, Budureau, Maurer, Dannemiller, Stephens &Kleiner-Gathercoal, 1999; Simion, Valenza, Umiltà &Dalla Barba, 1998; Turati, Simion, Milani & Umiltà,2002; Valenza et al., 1996) highly schematic facelikepatterns rather than other equally complex visual stimuli.More recently, such evidence has been extended also toreal face images (Macchi Cassia, Turati & Simion, 2004).Concerning face recognition, it is well documented thatfrom the first days of life infants recognize their ownmother’s face, showing a visual preference for the faceof the mother over that of a female stranger (i.e.Bushnell, 2001; Pascalis, de Schonen, Morton, Deruelle& Fabre-Grenet, 1995; Walton, Bower & Bower, 1992).Furthermore, using the habituation technique, it hasbeen demonstrated that newborns are able to recognizefacelike configurations (Turati & Simion, 2002) andimages of unfamiliar real faces to which they have beenhabituated (de Heering, Turati, Rossion, Bulf, Goffaux& Simion, in press; Pascalis & de Schonen, 1994; Turati,Macchi Cassia, Simion & Leo, 2006). Newborns are ableto recognize an image of an unfamiliar face bothimmediately and after a 2-minute retention interval(Pascalis & de Schonen, 1994). Also, both the inner andthe outer features are sufficient cues for newborns’ facerecognition, although the outer part of the face enjoysan advantage over the inner part (Turati et al., 2006).Finally, within the range of spatial frequencies visible atbirth, only the extreme low SF range appears useful fornewborns’ face recognition process (de Heering et al., inpress).

Infants’ ability to detect and recognize faces has beenexplored using wide-ranging stimuli manipulations.However, these experimental manipulations were oftenartificial because generated modifying parts of a face(e.g. ‘Thatcher effect’; Bhatt, Bertin, Hayden & Reed,2005), removing the internal or external features (e.g.Turati & Simion, 2002; Turati et al., 2002, 2006), ordisplacing the features from the correct/natural positions(e.g. ‘scrambled faces’; Macchi Cassia et al., 2004). Here,we sought to investigate the origins of face processingovercoming the artificial experimental manipulationadopted in previous studies and exploring newborns’face processing in perceptual conditions that moreclosely resemble the circumstances that naturally occurin newborns’ visual world. Specifically, we examinedhow newborns process a face whose features are simplyoccluded, rather than removed or displaced.

In our daily life, most of the objects we see are partlyoccluded by other objects. This perceptual condition isvery frequent also in the case of face stimuli, that oftenappear partly hidden behind other objects in infants’visual environment. Nonetheless, no study has everattempted to explore how the partial occlusion of a faceaffects newborns’ face detection and only one study hasinvestigated whether young infants are able to discrimi-nate between partly occluded faces (Bushnell, 1982). Theauthor demonstrated that, from 5 weeks of age, infants

are able to discriminate the face of their mother fromthat of a female stranger when the eyes are occluded bya pair of pince-nez lightly smoked glass, or the mouth isoccluded by a flesh-colored rectangle of Elastoplast.Conversely, it is not until 19 weeks that occluding thehair/face outline with an identical wig allows infants todiscriminate between faces (however, see data showingthat mother face recognition becomes possible at about6 weeks when the hair face outline is occluded with ascarf; Bartrip, Morton & de Schonen, 2001). Thus, theevidence obtained revealed that, very early in life,mother recognition is prevented by the occlusion of thehair but not by the occlusion of the eyes.

However, the ability to detect and recognize a partlyoccluded face should be present very early in the humanspecies given that even young non-human animals havebeen found able to recognize their own conspecifics whenpartly hidden behind other objects. For instance, evennewborn chicks manifest the ability to recognize a partlyoccluded imprinting-object (Vallortigara, Regolin &Pagni, 1999). Moreover, recent studies have shown thata few days from birth human infants are able to recognizethe perceptual similarity between two stimuli that areidentical except for the presence versus absence of anoccluder (Valenza, Zulian & Leo, 2005). If this competenceis present at birth in the case of non-face objects, it islikely that it might be available also in the case of facesthat represent a highly salient visual stimuli category fornewborns.

Further, results obtained by Bushnell (1982), showingthat infants’ recognition performance does not varydepending upon whether the eyes are occluded, appearto be in contrast with what has been observed in adults.Adults’ ability to recognize an occluded face is affectedby the salience of the internal features occluded, the eyesgiving rise to the greatest disruption of recognitionperformance when masked (Roberts & Bruce, 1988;Bruce, Burton, Hanna, Healey, Mason, Coombes, Fright& Linney, 1993; O’Donnell & Bruce, 2000; Leder et al.,2001; Hood, Macrae, Cole-Davies & Dias, 2003). In detail,it has been shown that to recognize a face, eyes are moreimportant than mouth, which is in turn more importantthan nose (Bruce et al., 1993). These results clearly high-light the special role played by the eyes in adults’ facerecognition, but the presence of the eyes is very impor-tant also during adults’ face detection: while reactiontimes also increase when other features are obscured,these effects do not reach significance and are less thanthe effect of obscuring the eye region in visual searchtasks that require detecting a face among non-facedistracters (Lewis & Edmonds, 2003). The fact that theeyes have a privileged role as the primary focus of anobserver’s attention is also supported by the recording ofscanning patterns that reveal that both adults (Yarbus,1967) and 2–3-month-old infants (Maurer & Salapatek,1976; Turati, Valenza, Leo & Simion, 2005; Simion, Turati,Valenza & Leo, 2006) preferentially fixate the eye regionwhen they are visually exploring faces. Furthermore,

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some studies have shown that eyes are an importantsource of visual information from birth (e.g. Batik,Baron-Cohen, Wheelwright, Connellan & Ahluwalia,2000; Farroni, Massaccesi, Pividori & Johnson, 2004).For instance, Batik and colleagues (2000) demonstratedthat newborns prefer a real face with eyes open ratherthan one with eyes closed.

The present study was aimed at investigating new-borns’ processing of a partly occluded face. In particular,we examined whether newborns’ face detection (Experi-ment 1) and recognition (Experiment 2) are preservedwhen some portions of the face are not visible becausepartly occluded by other configurations. Moreover, wetested whether the effect of occlusion was affected bythe salience of the occluded face features. Specifically,Experiment 1 investigated whether, a few days afterbirth, an upright real face is still preferred over the sameface upside-down, when the stimuli are partly occludedby vertical bars. Newborns’ face preference was testedunder two experimental conditions which differed solelyin the degree of salience of the occluded features. Experi-ment 2 sought to determine whether newborns maintaintheir ability to recognize the photograph of a real facewhen some of its features are occluded. Also, we testedwhether newborns’ recognition of a partly occluded faceis affected by the salience or the amount of occludedinformation.

Experiment 1

The goal of Experiment 1 was to test whether facepreference is observable in newborns under two differentconditions of partial occlusion: a High Salience Occlusioncondition, in which the eyes and part of the externalcontour were occluded, and a Low Salience Occlusioncondition, in which part of the external contour, themouth and the nose were occluded. It was predicted thatthe occlusion of salient information (i.e. eyes) mightimpair newborns’ ability to perceptually discriminatebetween face and non-face visual stimuli. Thus, newborns’preference for an upright over an upside-down facemight emerge under the Low Salience Occlusion, but notthe High Salience Occlusion condition.

Method

Participants

Thirty-nine healthy, full-term newborn infants (meanage = 41.8 hr, SD = 20.5 hr) were recruited at the maternityward of the Paediatric Clinic of Abano (Padua). Allinfants met the screening criteria of normal delivery,a birth weight between 2550 and 4000 g, and a 5 minApgar score above 7. One infant did not completetesting due to fussiness, three were excluded from thedata analysis due to position preference and one wasexcluded due to technical problems. Therefore, the final

sample consisted of 34 infants (18 females, 16 males),randomly assigned to two different conditions: 17 infantsto the High Salience Occlusion condition, and 17 to theLow Salience Occlusion condition. Infants were testedonly if awake and in an alert state, after the parents gavetheir informed consent.

Stimuli

Stimuli were achromatic photographs of three Caucasianwomen’s faces (aged 23–32 years). Models were photo-graphed in a frontal pose with neutral expression underthe same lighting conditions. They were asked not towear glasses or jewellery, and other minor distinctivedetails (e.g. blemishes or pimples) were digitally removedusing the software Adobe Photoshop. Once presented onthe PC monitors, they were 25.5 cm high (about 49°).The width varied from a minimum of 25 cm (about 48°)to a maximum of 30 cm (about 57°), so that the naturalproportions of each face were preserved.

The three images were then differentially manipulateddepending upon the two experimental conditions (i.e.Low Salience Occlusion condition and High SalienceOcclusion condition). So, in one version of the stimulifour grey vertical bars occluded part of the outer facefeatures (i.e. the hair and ears), the nose and the mouth(Low Salience Occlusion condition; see Figure 1). In theother version of the stimuli, the four grey vertical barsoccluded part of the outer face features (i.e. the hair andears) and the eyes (High Salience Occlusion condition;see Figure 1). The grey vertical bars were applied usingthe software Paint Shop Pro 7.02. Each bar was 25.5 cmhigh. The bar width varied from a minimum of 1.7 cm(about 3°) to a maximum of 2 cm (about 4°), in order tobe adapted to the dimensions of the features of eachsingle face presented. For this same reason, the distancebetween the bars varied from 3.5 cm (about 7°) to 5.3 cm(about 10°). The upside-down face stimuli were createdturning the original upright images through 180°, so,

Figure 1 Example of stimuli used in Experiment 1.

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within a pair, the upright and upside-down faces wereidentical except for the stimulus orientation. Luminanceof the presented stimuli was measured using a MinoltaChroma Meter CS-100. Luminance of the grey back-ground and the white bars was, respectively, 43.15 cd/m2

and 90.3 cd/m2 in either the upright or upside-downfaces. Luminance of the upright faces and the upside-downfaces was, respectively, 34.41 cd/m2 and 34.35 cd/m2. Theratio between the luminance of the upright face/upside-down face was 0.95, 0.97, 1.05, respectively, for thecouple 1, 2 and 3, so the difference within each pair isminimal.

Apparatus

The infant was placed on an experimenter’s lap, in frontof a black panel, at a distance of about 30 cm. The panelhad two square holes where the black screens of twocomputer monitors appeared. Infants’ eyes were alignedwith a red flickering LED, located in the centre of thescreen. The LED was used to attract the infant’s gaze atthe start of the preference phases, subtended about 2° ofvisual angle and, when turned on, blinked at a rate of500 ms on and 500 ms off. Stimuli were projected at adistance of 4 cm from the central LED. To prevent inter-ference from irrelevant distracters, two white panelsplaced on either side of the infant limited peripheralvision.

Procedure

Infants were tested with only one pair of faces each. Assoon as the infant seemed to be at ease and her/his gazewas properly aligned with the central flickering LED,one of the experimenters, who watched the infant’s eyesby means of a video-monitor system, started the sequenceof the trial by pressing a key on the computer keyboard.This automatically turned off the central LED andactivated the stimuli on the screens. The two stimuli wereidentical except for their orientation: The upside-downface was the upright face rotated 180° (see Figure 1).Under the Low Salience Occlusion condition each infantwas presented with a single pair of faces in which theeyes were visible, and under the High Salience Occlusioncondition each infant was presented with a single pair offaces in which the nose and the mouth, but not the eyes,were visible. Within each condition, each of the threefemale faces was used equally often and counterbalancedbetween subjects.

Stimuli were shown bilaterally, one on the left and oneon the right of the central LED. Bilateral rather thancentral presentation was chosen because when newbornslook at a centrally presented stimulus, it is difficult foran observer to decide if they are actually looking at thestimulus or if they simply do not move their eyes fromthe central position. Stimuli remained on as long as theinfant fixated one of them (i.e. infant control procedure).When the infant shifted his gaze from the display for

more than 10 s, the experimenter turned off the stimuliand the central LED automatically turned on. Allinfants were given two trials, in which the position of thestimuli was counterbalanced. The stimuli initial left–rightposition was counterbalanced across subjects. The experi-menter, unaware of the hypotheses being tested and of thestimuli presented, recorded the number of orientingresponses toward the stimuli and duration of each fixationby pressing a button connected to the computer.

Videotapes of eye movements throughout the trialwere subsequently analysed frame by frame by a secondcoder unaware of the stimuli presented. Inter-coderagreement was 1.00 (Cohen Kappa) for the number oforienting responses toward the stimuli and 0.89 (Pearsoncorrelation) for total fixation time.

Results

Mean looking times toward each stimulus are shown inTable 1. In order to test whether newborns preferred theupright over the upside-down face in partly occludedconditions, a preference score (percentage) was computed.Each infant’s looking time at the upright stimulus duringthe two presentation phases was divided by the totallooking time to both stimuli, and subsequently convertedinto a percentage score. Hence, only scores significantlyabove 50% indicated a preference for the upright face.

Because a preliminary ANOVA revealed that no maineffect or interactions involved the distinct face presented,for subsequent statistical analyses data were collapsedover this variable.

To determine whether the upright face preferencescore was significantly different from the chance level of50% in each of the two conditions, two different one-sample t-tests were applied, one for each condition (seeFigure 1). Preference scores for the upright face weresignificantly above chance for the Low Salience Occlu-sion condition (M = 57.00%, SD = 11.25), t(16) = 2.9,p < .01. On the contrary, in the High Salience Occlusioncondition, the preference scores were not significantlyabove the level of chance (M = 47.29%, SD = 18.08),t(16) = 0.617, p = .546. A t-test for independent sampleswas run comparing the percentage of total fixation timestoward the upright face in the two different conditions(Low Salience Occlusion, High Salience Occlusion). Thecomparison was significant, t(32) = 2.062, p < .05.

Overall, the evidence obtained demonstrated thatwhen salient information is occluded (i.e. eyes) face pre-ference disappears, whereas when hindered features are

Table 1 Mean fixation times (in seconds) during thepreference test trials in Experiment 1 and Experiment 2

HSO condition LSO condition

Experiment 1 upright face 68.06 s 75.73 supside-down face 70.21 s 56.48 s

Experiment 2 new face 16.98 s 35.21 sfamiliar face 30.48 s 21.1 s

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not salient (i.e. mouth, nose and some portions of thehairline) face preference is preserved. In other words,newborns show a preference for the upright face onlywhen the most salient features remain visible (LowSalience Occlusion condition). These results suggest thatthe newborn’s face preference is affected by the salienceof the inner occluded feature (i.e. eyes).

Nevertheless, the outcomes of Experiment 1 referexclusively to newborns’ preference for faces. Aninteresting question concerns what happens whennewborns’ ability to recognize, rather than detect, apartially occluded face is tested. Experiment 2 was aimedto address this issue.

Experiment 2

The aim of Experiment 2 was to verify whether theability to recognize images of partly occluded real facesis present at birth. Using a visual habituation technique,newborns’ ability to recognize a partly occluded face wastested, either under a High Salience Occlusion conditionor under a Low Salience Occlusion condition. Besidesthe salience of the occlusion, in Experiment 2 a secondattribute was added and manipulated: the amount ofocclusion, which is the amount of information that washidden. Amount and salience of occluded informationwere put one against the other in order to investigatewhich of these two attributes greatly affects the ability torecognize partly occluded faces at birth. Thus, threevertical bars (i.e. high amount of occlusion) were super-imposed onto each face image in order to occlude lesssalient information such as the mouth, the nose andparts of the outer contour of the face (i.e. Low SalienceOcclusion-High Amount condition), whereas only twovertical bars (i.e. low amount of occlusion) were used inorder to occlude the most salient portion of the face –the eyes, together with some portions of the outercontour (i.e. High Salience Occlusion-Low Amount con-dition). We reasoned that if newborns’ face recognitionis affected more by the salience of occluded informationthan the amount of the masked information, newbornsshould recognize a face under the Low Salience-HighAmount Occlusion condition, but not under the HighSalience-Low Amount Occlusion condition. Alterna-tively, if face recognition is predominantly affected bythe amount of occluded information, then it shouldbe easier for newborns to recognize a face in the HighSalience-Low Amount Occlusion than in the LowSalience-High Amount Occlusion condition.

Method

Participants

Participants were 49 healthy, full-term newborns (meanage = 45.15 hr, SD = 18.97). The infants were recruitedat the maternity ward of the Paediatric Clinic of the

University of Padova and met the screening criteria ofnormal delivery, a birth weight between 2550 and 4000 g,and a 5 min Apgar score above 7. Fourteen newbornswere excluded from the final sample: seven infants weretested but excluded because they did not complete testingdue to fussiness, and seven showed a position bias duringthe preference test phase. Thus, the final sample included35 newborns. Infants were tested only if awake and in analert state, after their parents gave their informed consent.Sixteen infants were tested under the Low SalienceOcclusion condition, and 19 infants were tested underthe High Salience Occlusion condition.

Stimuli

The same set of photographs used in Experiment 1 wasemployed in Experiment 2. During the habituationphase, the face images were presented non-occluded.The specific face pair shown – out of the three possible– was counterbalanced between subjects. In the testphase, the face images were occluded. Pictures weredifferentially manipulated depending on the two testconditions. Hence, for each face there were two versions:one image in which three grey vertical bars occludedpart of the outer features (i.e. the hair and ears), thenose and the mouth (Low Salience-High Amount Occlu-sion condition; see Figure 2), and one in which two greyvertical bars occluded part of outer features (i.e. the hairand ears) and the eyes (High Salience-Low AmountOcclusion condition; see Figure 2). The grey verticalbars applied to each photograph were identical to thebars employed in Experiment 1. As in the previousexperiment, luminance of the stimuli was measuredusing a Minolta Chroma Meter CS-100. Luminance ofthe grey background in the habituation phase was43.15 cd/m2. The ratio between the luminance of eachpair of faces shown in the habituation phase was 1.00,since the same stimulus was presented bilaterally. Withregard to the stimuli shown in the test phase, theluminance of the grey background and the white bars isidentical to that reported in Experiment 1. The ratiobetween the luminance of the novel and familiar faceswas 0.96, 1.02, and 0.99, respectively, for the couples 1,2 and 3 in the High Salience Occlusion condition, and0.98, 0.97, and 0.98 in the Low Salience Occlusioncondition. Therefore, the difference within each pair isminimal in both experimental conditions.

Apparatus and procedure

The apparatus was the same as that employed in Experi-ment 1, whereas the procedure was different given thatin Experiment 2 an infant-control habituation/testprocedure (Horowitz, Paden, Bhana & Self, 1972) ratherthan a preference technique was employed. Testingbegan with the central flickering LED. As soon as theinfant’s gaze was properly aligned with the LED, thehabituation was begun by an experimenter who watched

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the infant’s eyes by means of a video-monitor systemand pressed a key on the computer keyboard. Thisautomatically turned off the central LED and activatedthe stimuli. A non-occluded face image was projectedbilaterally, on each side (i.e. left and right) of the centralLED. As in Experiment 1, a bilateral rather than acentral presentation was chosen. In the present experi-ment we would add one more reason for choosingbilateral presentation, which is that we wanted to minimizedifferences in stimulus presentation conditions acrosshabituation and test phases.

During the habituation phase, the stimuli remained onthe screen until a look-away criterion was met. Habituationwas established by recording the duration of individualfixations. The duration of each fixation on the stimuluswas recorded by pressing one of two buttons connectedto the computer, depending on whether the infantlooked at the right or left side of the display. Each trialended when a continuous look away from the stimuluswas 2 s or more. Trials repeated in this manner until,from the fourth trial on, the sum of fixations on anythree consecutive trials was 50% or less than the total ofthe first three trials. When the habituation criterion wasreached, the habituation phase was terminated and thepreference test phase began. This phase consisted of twotrials in which two stimuli were presented. Infants weresimultaneously presented with two images, both partlyoccluded: a novel face and the face to which infants hadbeen habituated. Stimuli were always shown in eitherleft or right positions, the position being reversed fromtest trial 1 to test trial 2. The initial left–right order ofpresentation was counterbalanced across subjects.Presentation lasted until each stimulus had been fixatedat least once and a total of 20 s of looking had beenaccumulated. Total looking time could exceed 20 s in thefollowing cases: (1) If only one stimulus had been fixatedwhen the 20-s criterion was reached. In this case, thetrial continued until one single fixation toward the other

stimulus was coded, so that the infant had the chance tofixate both the stimuli presented. (2) If the infant wasstill looking at one of the stimuli when the 20-s criterionwas reached. In this case, the trial continued until thelast fixation was terminated; so that the end of the fixationcorresponds to the infant disengaging from the stimulus.Videotapes of eye movements recorded during the testphase were subsequently analysed frame by frame by asecond coder unaware of the stimuli presented. Inter-coderagreement was 0.96 (Cohen Kappa) for the number oforienting responses toward the stimuli and 0.89 (Pearsoncorrelation) for total fixation time.

Results

Two separate t-tests for independent samples wereapplied to compare total fixation times and number offixations to reach the habituation criterion in the HighSalience-Low Amount Occlusion condition and in theLow Salience-High Amount Occlusion condition. In thecomparison, neither variable attained statistical signifi-cance. The average total fixation time was 81.03 s (SD =38.1 s) for the Low Salience-High Amount Occlusioncondition and 74.09 (SD = 33.0 s) for the High Salience-Low Amount Occlusion condition, t(33) = 0.3, p = .77.The average number of fixations was 8.38 (SD = 3.0) forthe Low Salience-High Amount Occlusion conditionand 7.53 (SD = 1.8) for the High Salience-Low AmountOcclusion condition, t(33) = 1.03, p = .31.

Mean looking time to each stimulus in the test phaseis reported in Table 1. In order to test whether newbornswere able to recognize the face image to which they werehabituated, even if partly occluded, a novelty preferencescore (percentage) was computed as in Experiment 1. Apreliminary analysis of variance (ANOVA) revealedthat no main effects or interactions involved the factorface pair. Two t-tests were performed comparing thepreference scores to chance (50%). The mean scores for

Figure 2 Example of stimuli shown in the habituation and test phases of Experiment 2.

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the novel face were significantly greater than the chancelevel for the Low Salience-High Amount Occlusioncondition (M = 61.5%, SD = 21.4%), t(15) = 2.15, p <.05. On the contrary, the mean scores for the novel facein the High Salience-Low Amount Occlusion conditionwere significantly smaller than 50% (M = 35.95%, SD =20.65%), t(20) = 3.12, p < .01. Thereby, the t-test forindependent samples applied to compare the percentageof total fixation times toward the novel face in the twodifferent conditions (Low Salience-High Amount Occlu-sion, High Salience-Low Amount Occlusion) wassignificant, t(35) = 3.65, p < .05.

Results obtained in Experiment 2 revealed that theamount of occluded information did not appear to be acritical variable, since faces were recognized even when agreater amount of information was hidden. On thecontrary, the salience of the occluded features proved tobe a crucial factor in affecting newborns’ recognitionprocess. When eyes were visible, babies recognized aperceptual similarity between the non-occluded faceshown in the habituation phase and the same faceoccluded presented during the test phase. Therefore,newborns clearly recognized the familiar face despitesome low salient features (portions of the hair, ears, nose,mouth) being hidden behind occluders. Such evidenceshows that the capacity to perceive occluded facial visualinformation is not confined to the face preferencephenomenon, but can be extended to more complexprocesses, such as recognition. When eyes were covered,newborns’ performance significantly differed from thechance level, but a preference for the familiar rather thanfor the novel face emerged. Previous studies showedthat, in the early stages of developing recognition of afamiliar stimulus, infants show a preference for familiarity,which then switches to a preference for novelty as therepresentation of the familiar stimulus becomes betterestablished. Specifically, models on infants’ habituationprofile state that, at the beginning of the recognitionprocess, infants look longer at the familiar stimulus andthat, only later, when recognition is well established, ashift from a familiarity to a novelty preference is observed(Roder, Bushnell & Sasseville, 2000; Hunter, Ames &Koopman, 1983; Rose, Gottfried, Melloy-Carminar &Bridger, 1982; Fantz, 1964; Sirios & Mareschal, 2002,2004). That is, a familiarity preference demonstrates somerecognition of the familiar face, but weaker recognitionthan does a novelty preference. For instance, whennewborns’ ability to recognize a learned face over strongmodifications, such as rotation, or photonegative andsize transformations was tested, a familiarity preferencewas obtained (Walton et al., 1992; Walton, Amstrong &Bower, 1997), suggesting a difficulty in recognizing themodified familiar stimulus: newborns react to significantperceptual changes in the visual stimulus with a persistentand extensive visual exploration of the familiar configu-ration. Since in our experiment an important perceptualdiscrepancy (obscuring of the eyes) was introducedbetween the non-occluded face shown in the habituation

phase and the same face occluded presented during thetest phase, newborns might have had difficulty recognizingthe modified familiar face, fixating the familiar stimulusfor a prolonged period of time. In light of the models oninfants’ habituation profile, recognition was still notentirely established, although some aspects of the stimuluswere already retained in memory thus inducing afamiliarity preference. In other words, when eyes werecovered, newborns recognized the familiar face, butfairly poorly, thus spending a lot of time checking it outclosely because of the unusual format. Conversely, wheneyes were visible, the familiar face was quickly recognizeddespite the format change, and thus newborns spentmore time looking at the more interesting novel face.Therefore our results showed some recognition of theHigh Salience Occlusion face, and stronger recognitionof the Low Salience Occlusion face.

General discussion and conclusion

Evidence gathered in the present study demonstratesthat from birth the human cognitive system may manifestthe ability to detect and recognize partly occluded faces.Also, the present findings highlight the important roleplayed from birth by eyes in the detection and recognitionof faces.

A face was still detected as a face by newborns even ifsome low-informative face portions were hidden. Morespecifically, our study demonstrates that newborn infantsstill treat an occluded face as a face even if someportions, but not the eyes, are occluded. This finding isin line with results obtained with adults, showing thatface detection is affected by obscuring the eyes, while theocclusion of other features does not significantly impairadults’ face recognition performance (Lewis & Edmonds,2003).

Why should eyes be so important in the detection offace? The peculiarity of the eyes as visual stimuli issupported in the field of social developmental cognitiveneuroscience. It is suggested that eyes represent a specialkind of stimulus that human adults are able to processrapidly and to which they are particularly sensitive(Langton, Watt & Bruce, 2000). Eyes contain a greatdeal of variation in the changes from light to dark, so itis very easy to extract them quickly from an image as apair of darker regions arranged at a similar height(Lewis & Edmonds, 2003). Moreover eyes might bespecifically important for face detection because theyform part of some larger configuration that aids detec-tion. It may in fact be the relationship between eyes andthe other features that provides their special role in facedetection (Lewis & Edmonds, 2003). The ability todetect the direction of gaze is thought to be particularlyimportant in guiding social interactions, and alsoemerges early in human development (Hood, Willen &Driver, 1998; Farroni, Johnson, Brockbank & Simion,2000; Farroni, Mansfield, Lai & Johnson, 2003; Farroni,

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Johnson & Csibra, 2004; Farroni et al., 2004). In researchwith human newborns, it has been demonstrated thatfrom birth infants are sensitive to, and prefer to look at,faces that allow eye contact (Farroni et al., 2000; Batiket al., 2000). These results support Baron-Cohen’s model(i.e. ‘mindreading model’; Baron-Cohen, 1995), accordingto which a mechanism (EDD; Eye-Direction Detector)that functions specifically to detect gaze direction isinnately provided and automatically detects the presenceof eyes or eye-like stimuli in the visual environment.

Contrary to the hypothesis formulated by Baron-Cohen,it might be that eyes are not the sufficient and necessaryelement able to trigger newborns’ face preference. In linewith Morton and Johnson’s structural hypothesis (1991;Johnson & Morton, 1991) – that maintains that faces arespecial for newborns because human infants possess adevice (i.e. Conspec) that contains structural informationconcerning the visual characteristics of conspecifics –hiding the eyes implies that the typical face pattern(three high contrast blobs in the correct positions of theeyes and the mouth) would be disrupted.

Finally, our results may be consistent with a recentline of evidence which, reconsidering and adjusting theproposals of the so-called sensory hypothesis, suggeststhat, at birth, the salience of the eyes depends on a spon-taneous preference for an up–down asymmetry in thedistribution of highly contrasted elements within a con-gruent contour. Specifically, it was proposed that newborns’attention is triggered by visual stimuli that present moreelements in the upper vs. the lower part, in a spatialdisposition congruent with the form of the surroundingcontour (Acerra, Burnod & de Schonen, 2002; Simion,Valenza, Macchi Cassia, Turati & Umiltà, 2002; Turati& Simion, 2002; Turati et al., 2002; Macchi Cassia et al.,2004; Simion, Macchi Cassia, Turati & Valenza, 2003).Furthermore, eyes as visual stimuli possess a number ofsimple and potentially powerful characteristics, suchas high contrast (i.e. between the sclera and iris) (e.g.Morison & Slater, 1985), concentric circles (i.e. pupils,iris, sclera) (e.g. Fantz, Fagan & Miranda, 1975),movement (e.g. Slater, Morison, Town & Rose, 1983)and being complex visual stimuli (e.g. Fantz, Ordy &Udelf, 1962). Further research might aim to disentanglewhich of the above hypotheses about the possible role ofthe eyes in newborns’ face detection is more forceful.

A clearer picture emerges when newborns are expectedto recognize, rather than detect, a partly occluded face.When eyes are visible, newborns clearly recognize a faceas more similar to the same face occluded than to anovel face occluded as well. These findings confirm andextend to faces the evidence obtained by Valenza andcolleagues (Valenza et al., 2005), who demonstrated thatnewborns are able to recognize the correspondencebetween two geometrical patterns that are identicalexcept for the presence or absence of an occluder. Also,by showing that newborns recognize a perceptualsimilarity between two stimuli, the results obtained arein line with existing evidence about infants’ ability to

organize different objects and events into coherentpatterns and treat them as equivalent (Quinn, Slater,Brown & Hayes, 2001; Turati, Simion & Zanon, 2003;Turati & Simion, 2002).

More intriguingly, the results of Experiment 2 revealthat the newborn’s ability to recognize the perceptualsimilarity between two faces that differ in the presence/absence of occluders appears much more difficult andfairly poor when eyes are not visible. This is exactly whatis observed when an adult is requested to perceive apartly occluded face, since adults’ ability to recognize anoccluded face is considerably reduced when the eyes aremasked (Leder et al., 2001; Hood et al., 2003; Roberts &Bruce, 1988; Bruce et al., 1993). Recently, it has beendemonstrated that even the eye gaze direction affectsadults’ face recognition, that is enhanced for faces withdirect rather then averted gaze (Hood et al., 2003).

Overall, the evidence on newborns’ face recognition(Experiment 2) appears to be in line with, although notentirely overlapping, that obtained on newborns’ facedetection (Experiment 1). Specifically, at birth both facedetection and face recognition are not minimally disruptedby obscuring low salience facial cues (i.e. nose, mouthand some portions of the hair), and are affected byobscuring high salience facial features (i.e. eyes). However,while in the case of face detection eye occlusion com-pletely prevented newborns’ face detection, in the case offace recognition an analogous stimulus manipulationheavily, although not entirely, impaired newborns’ recog-nition performance. Based on these findings, one mayconclude that, at birth, eyes are a necessary cue in orderto achieve face detection, and play an important, but notexclusive, role in allowing face recognition. This mightbe explained by the fact that, even at birth, face detectionand recognition require different processes that mightbenefit from different facial perceptual information.Specifically, it could be argued that, in the occluded eyestest condition of Experiment 2, newborns were requiredto recognize the non-occluded face presented in thehabituation phase within a stimulus that missed itsfacedness characteristics, as demonstrated in thepreference study (Experiment 1). The familiar stimulusdisplayed some visual features identical to those of theface to which newborns were habituated, but, at thesame time, it was no longer a face because eyes wereoccluded. Therefore, newborns might have needed toexplore the familiar stimulus for a protracted periodof time.

The finding that newborns’ recognition of an occludedface appears harder when eyes are occluded contrastswith recent observations according to which, at birth,face recognition was maintained when the inner featuresof the face were removed, but disappeared when theouter contour was eliminated (Turati et al., 2006). Morespecifically, Turati and colleagues demonstrated thatnewborns failed to use the inner features alone as a cueto recognize a female stranger’s face to which theyhad been previously habituated. On the contrary, the

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removal of the inner region of the face did not preventnewborns’ face recognition. Thus, one may claim thatthe occlusion of the inner features should not affectnewborns’ ability to recognize a face, while the occlusionof the outer contours should prevent face recognition.However, faces shown in the present study were notoccluded for the inner or the outer features in isolation,but both in some portions of the contour and in someinner features. It is likely that, since birth, the face isencoded as a complex configuration in which the relationbetween all the components of the stimulus plays animportant role. When different experimental conditionsare employed (i.e. removal vs. occluding condition),there are changes in the relative importance of one or theother source of information, one being more prominentthan the other to act as an effective cue for recognition.In this sense, we suppose that recognition of partlyoccluded faces is more affected by the occlusion of somesalient inner features (i.e. eyes). This conclusion does notrule out the possibility that, in other conditions, theouter part of the face enjoys an advantage over the innerpart. Indeed, literature converges to suggest that theocclusion of an object by another object is more compli-cated than the simple removal of information about thehidden object (e.g. Nakayama, Shimojo & Silverman,1989). First of all, an occluding object hides informationabout the object which is not visible, but it also addsextraneous information which might interfere with theprocess of pattern recognition: occlusion introducesspurious edges at the occlusion boundaries and a dis-tinction has to be made between the real edges of themore distant object and these spurious edges. In otherwords, when artificial edges are introduced at theocclusion boundaries, a distinction between the realcontours, intrinsic to the occluded object, and theseartificial, extrinsic contours formed accidentally by theinterposition of a different object in the line of sight hasto be made. In addition, occlusion often divides a singleobject into several image fragments and it becomesimportant to link parts of the same object into a coherentperception. Finally, the occluded object is perceivedbehind the occluder, thus implying the processing of thedepth of the occlusion (Nakayama et al., 1989). Thismight explain why newborns succeeded in recognizing aface when the inner face portion was completelyremoved, relying exclusively on the outer part (Turatiet al., 2006), but found it difficult to recognize a facewhen some salient inner features were masked by theocclusion of a vertical grating.

Also, our findings might seem in contrast withBushnell’s data, according to which infants’ competenceto recognize a face in the first months of life is preventedby the occlusion of outer features and not by theocclusion of inner features (Bushnell, 1982). Actually,for two main reasons it is difficult to compare outcomesobtained in the Bushnell study with our results. First,Bushnell’s study aimed to investigate infants’ ability torecognize the mother’s face in a preference task. Several

authors agree in suggesting that the capacity to recognizethe mother’s face has to be at least partially differentiatedfrom the ability to recognize a face which has becomefamiliar through a habituation process (Bushnell et al.,1989; Walton et al., 1997; Pascalis & de Schonen, 1994;Pascalis et al., 1995; Bushnell, 2001). Second, a closeobservation of the stimulus material used by Bushnell(1982) reveals that the outer face features were not reallyoccluded but were replaced by other features, becausethe mother and the female stranger wore an identicalwig to standardise the hair-face outline. As previouslyexplained, the replacement of a feature triggers processesdifferent from those implicated in perceiving a partialocclusion (Nakayama et al., 1989).

Finally, two contrasting accounts might be offered toexplain how newborns recognized the correspondencebetween the non-occluded and the occluded faces:newborns might have filled in the partly hidden surface,thus perceiving the occluded stimulus as connected behindthe occluders, or might have simply perceived only whatis immediately visible of the occluded face. The firstexplanation might be supported by many studies that,using a rod-and-box display, have revealed that humaninfants are able to perceive object unity very early(Kellman & Spelke, 1983; Johnson & Aslin, 1995; Johnson& Náñez, 1995; Johnson & Aslin, 1996; Jusczyk, Johnson,Spelke & Kennedy, 1999; Slater, Morison, Somers,Mattock, Brown & Taylor, 1990; Valenza, Leo, Gava &Simion, 2006), although literature also suggests thatstarting from only about 7 months of age infants areable to recognize the unity of a partly occluded object inthe absence of motion cues (Craton, 1996). However itis difficult to compare our results with those reported inthe literature because the procedure adopted in ourstudy considerably differed from the standard procedureused to individuate the basic processes involved inperceiving object unity in infancy. In such a standardprocedure, infants are habituated to a moving partlyoccluded rod and then presented with a broken rod,whose portions correspond to the visible portions of therod presented during familiarization, together with acomplete rod. A novelty preference for the brokenstimuli is taken as evidence that infants succeeded in theunity completion task. Clearly, our study did not complywith these requirements, thus preventing any conclusionabout newborns’ ability to perceive the unity of anoccluded face. A more plausible and conservative inter-pretation suggests that newborns might have perceivedthe perceptual similarity between the non-occluded andthe occluded face, perceiving only what is immediatelyvisible of the occluded face. Thus, the results obtaineddo not shed light on the perceptual operations at thebasis of newborns’ ability to detect and recognize anoccluded face, but demonstrate that this competence ishighly affected by the degree of salience of the occludedinformation. More specifically, when eyes were occludedface detection did not occur and face recognitionbecame a much more difficult and demanding task.

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Acknowledgements

The authors are deeply indebted to B. Dalla Barba, M.E.Sotti and the nursing staff at the Paediatric Clinic of theUniversity of Padua and at the Casa di Cura of AbanoTerme for their collaboration. We also thank SandroBettella for his technical help.

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Received: 5 July 2006Accepted: 16 July 2007