Faces are special for newly hatched chicks: evidence for inborn domain-specific mechanisms...

PAPER

Faces are special for newly hatched chicks: evidence for inborndomain-specific mechanisms underlying spontaneous preferencesfor face-like stimuli

Orsola Rosa-Salva,1 Lucia Regolin1 and Giorgio Vallortigara2

1. Department of General Psychology, University of Padova, Italy2. Center for Mind ⁄ Brain Sciences, University of Trento, Italy

Abstract

It is currently being debated whether human newborns’ preference for faces is due to an unlearned, domain-specific and configuralrepresentation of the appearance of a face, or to general mechanisms, such as an up-down bias (favouring top-heavy stimuli, whichhave more elements in their upper part). Here we show that 2-day-old domestic chicks, visually na�ve for the arrangement of innerfacial features, spontaneously prefer face-like, schematic, stimuli. This preference is maintained when the up-down bias is controlledfor (Experiment1) or when put in direct conflict with facedness (Experiment 4). In contrast, we found no evidence for the presenceof an up-down bias in chicks (Experiment 2). Moreover, our results indicate that the eye region of stimuli is crucial in determiningthe expression of spontaneous preferences for faces (Experiments 3 and 4).

Introduction

Domain-specific knowledge constitutes an importantissue in cognitive science. Existing evidence indicatesthat faces are processed by dedicated domain-specificcircuits in the human brain (e.g. Farah, Rabinowitz,Quinn & Liu, 2000; Kanwisher, 2000) and in animals(adult and infant monkeys: Perrett, Hietanen, Oram &Benson, 1992; Rodman, Skelly & Gross, 1991; sheep:Kendrick, Da Costa, Leigh, Hinton & Pierce, 2001; seeTate, Fischer, Leigh & Kendrick, 2006, for a review). Thisidea is also consistent with the striking face processingabilities of human beings (and animals, see Tate et al.,2006). It has been hypothesized that, due to the relevanceof faces in social interactions, innate face-specificprocessing devices would have been evolved by naturalselection. Nevertheless, some findings suggest that boththe specificity of brain areas involved and the high level ofperformance usually found in face processing couldsimply be due to the large amount of experience thatsocial animals have with faces (see Kanwisher, 2000;Nelson, 2001; Tov�e, 1998, for reviews on this debate).

A number of studies have demonstrated that, even froma few hours or minutes after birth, a schematic patternrepresenting an upright face elicits greater visual attentionin human newborns than do similar stimuli presenting thesame internal face features in scrambled positions orarranged upside down (Goren, Sarty & Wu, 1975;Johnson, Dziurawiec, Ellis & Morton, 1991; Macchi

Cassia, Simion & Umilt�, 2001; Macchi Cassia, Turati &Simion, 2004; Morton & Johnson, 1991; Simion et al.,1998; Valenza, Simion, Macchi Cassia & Umilt�, 1996).According to Morton and Johnson (1991) this evidencewould indicate that human newborns respond to theunique structural configuration of the face (i.e. tofacedness) due to a subcortical (Johnson, 2005; Simionet al., 1998) face-detecting mechanism (CONSPEC) thatcontains a configural representation of the structure ofthe face’s inner features (three darker areas in an upside-down triangular configuration). During the first monthsof life the CONSPEC mechanism would orient theindividual’s attention toward stimuli that match thestructure of a face (thereby providing newborns withextensive experience of faces). CONSPEC is contrastedwith a mechanism termed CONLERN, which isresponsible for learning about the visual characteristicsof individual faces.

The origins of the CONSPEC ⁄ CONLERN hypothesisare rooted in research with animals, most notablywith domestic chicks (Gallus gallus domesticus). Theexperiments of Johnson, Bolhuis and Horn (1985) andJohnson and Horn (1988) showed that, contrary to widelyheld beliefs, filial imprinting seems to consist of twoseparate processes: an inborn predisposition of the youngbird to attend to visual stimuli that resemble a conspecific,such as a stuffed jungle fowl (or even another animal) and alearning mechanism whereby the chick learns by exposureto recognize the specific characteristics of its own mother

Address for correspondence: Orsola Rosa-Salva, Department of General Psychology, University of Padova, Via Venezia 8, 35131 Padova, Italy; e-mail:[email protected]

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Developmental Science 13:4 (2010), pp 565–577 DOI: 10.1111/j.1467-7687.2009.00914.x

hen (guided by the innate predisposition to attend to hen-like stimuli).

Subsequently, research turned to studies on humaninfants’ face preferences and a series of papers werepublished (for a review see Johnson & Morton, 1991) inwhich it was argued that mechanisms such as CONSPECand CONLERN would be present in human neonates.

Subsequent work, however, has cast some doubt on theprecise nature of the CONSPEC mechanism. In particularit has been argued that newborn infants’ preferences forfaces could be a secondary effect determined by non-specific biases due to constraints imposed by theimmature visual system of the child. In particular,Turati, Simion, Milani and Umilt� (2002) providedevidence that the preference for face-like stimuli couldbe determined by an ‘up-down bias’ that directs the babies’attention toward any configuration presenting moreelements in the upper part (a ‘top-heavy configuration’).This could be due to a greater sensitivity of the upper-visual field, making top-heavy patterns more easilydetectable (see also Simion, Macchi Cassia, Turati &Valenza, 2001; Turati, 2004; for similar evidence in chickssee Hodos & Erichsen, 1990). However, the up-down biastheory has recently been challenged by studies on the roleof contrast polarity, demonstrating that newborns’preferences are shaped so as to ensure that babies attendto human faces as they appear under natural toplit illumination (Farroni, Johnson, Menon, Zulian,Faraguna & Csibra, 2005).

In summary, the existing literature on human infantsleaves unresolved the question of whether there is a specificor non-specific mechanism, which could explain thespontaneous preference for face-like stimuli seen innewborns. Part of the controversy may depend upon theeffect of early learning (for evidence on fast learning aboutthe appearance of faces shortly after birth, see Bushnel, Sai& Mullin, 1989; Simion, Farroni, Macchi Cassia, Turati &Dalla Barba, 2002; Turati, Macchi Cassia, Simion &Leo, 2006; Walton & Bower, 1993). Carefully controlledstudies with animals could help resolve this issue, butsurprisingly little research has been aimed at investigatingwhat specific features of the head region stimulateCONSPEC in chicks. A relevant exception is a study(Bolhuis, 1996) demonstrating that eyes are an importantpart of the stimulus in this regard, but that other aspects ofthe stimulus are also sufficient for the expression of thepredisposition to approach conspecific-like objects. Forthis reason we decided to use the domestic chick as a usefulmodel for the study of na�ve subjects’ preferences: it isvirtually impossible to obtain a complete absence ofexposure to faces in human and even in atricial non-human animal neonates (Sugita, 2008).

Previous studies on the domestic chick suggest thepresence of a representation for the appearance of asocial object, on the basis of a predisposition forimprinting on naturalistic (hen-like) objects withrespect to artificial stimuli (Bolhuis & Trooster, 1988;Cherfas & Scott, 1981; Kent, 1987; Salzen & Meyer,

1967) and of a spontaneous preference to approach astuffed hen with respect to an artificial stimulus (Johnsonet al., 1985; Johnson & Horn, 1988). Further studies havesuggested that the predisposition that chicks possesscould be extremely general and so broad as not to bespecies specific (Johnson & Horn, 1988). The crucialfactor required to elicit chicks’ preferences seemed to bethat a stimulus possesses a neck and a head (containing,of course, internal face features) (Johnson & Horn,1988). This result presents a striking resemblance to thatpreviously described for human newborns. Thus, weinvestigated whether domestic chicks spontaneouslyprefer schematic faces or other top-heavy stimuli andwhether such a preference is determined by an up-downbias, as suggested by some literature on newborn babies(Turati et al., 2002).

Experiment 1

The aim of the first experiment was to investigatewhether a spontaneous preference for schematic face-likeconfigurations is present (prior to any visual experienceregarding the structure of faces’ inner features) in newlyhatched domestic chicks, when the role of the up-downbias is controlled for. Chicks’ preferences were testedbetween two top-heavy stimuli, only one of whichrepresented a face.

Subjects

Subjects were 138 (69 male and 69 female) domesticchicks (Gallus gallus domesticus) of a strain (‘Hybro’)derived from the White Leghorn breed. Eggs wereobtained weekly from a commercial hatchery (AgricolaBerica, Montegalda (VI), Italy) on the 14th day ofincubation. Eggs were incubated (MG 70 ⁄ 100 Ruraleincubator) from Days 14 to 17. On the 17th day ofincubation eggs were placed in a hatchery (MG 100).During incubation and hatching, eggs and chicks weremaintained in complete darkness.

Rearing conditions

After hatching in darkness, chicks were immediatelyplaced singly in metal home-cages (28 cm · 16 cm ·40 cm), lit (24 h ⁄ day) by 36 W fluorescent lamps (placed15 cm above the cages). The cage walls and floor werelined with white opaque paper. Chicks were maintainedat a controlled temperature (c. 28–31�C) and humidity(c. 70%), with food and water available ad libitum.

For a subsample of 34 chicks (17 male and 17 female),an artificial ‘imprinting’ object (rearing object) waspresent in each cage. It consisted of an orangecardboard cut-out shape, representing a featureless face(Figure 1, left), identical in shape and outline to that ofthe experimental stimuli employed later at test. Thefeatureless face was placed on one of the walls in each

566 Orsola Rosa-Salva et al.

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cage, with its lower boundary (the base of the ‘neck’)adjacent to the cage floor, so that the round-shaped upperpart of the stimulus (the ‘face’) was presented at about theheight of the line of sight of a standing newborn chick.This procedure did not provide chicks with anyinformation regarding the internal features of a face, butincreased the likelihood that subjects would perform theexperimental task: at test chicks had to approach one oftwo stimuli that were both equally similar to the objectthat they had been reared with. Quite likely imprintingtook place in our chicks, although no formal test wasperformed, demonstrating that a preference haddeveloped. We will therefore refer to the object chickswere exposed to as the ‘imprinting object’ (and to thechicks as ‘imprinted chicks’).

The remaining 104 chicks (52 males and 52 females)were reared in exactly the same conditions as describedabove, but in the absence of any conspicuous object withinthe cage.

Prior to the test, chicks did not receive any visualexperience concerning the structure of the internalfeatures of a face. In particular, chicks never saw theexperimenter’s face or the face of another chick.Whenever required, chicks were manipulated incomplete darkness or after covering their head or eyeregion. Chicks were kept inside a closed cardboard boxwhen transported.

Apparatus

The test apparatus was a white plywood choice runway (45cm long · 22.3 cm wide), with the two experimental stimulipresented at the two opposite ends. The choice runway wasdivided into three virtual sectors (each 15 cm long): acentral area equidistant from the two experimental stimuli,

and two side-areas, each ending with a translucent glassscreen upon which one of the two stimuli was placed. Thestimuli were placed so that their lower boundary (the baseof the ‘neck’) was at the same level as the runway floor, andthe round-shaped upper part of the stimulus (the ‘face’) wasin line with the eyes of a standing chick. The choice runwaywas lit by a 40 W lamp placed beyond each glass screen,while the rest of the room was maintained in darkness. Avideo camera, placed above the apparatus, recorded thechicks’ behaviour during the test and was connected to amonitor screen, enabling the experimenter to scorebehaviour during the test.

Test stimuli

Test stimuli (see Figure 2a, b) were identical in colour,shape and dimensions to the rearing object. The onlydifference was the presence, in each test stimulus, ofthree square black blobs representing internal features.Note that 2-day-old chicks have excellent visual acuity,allowing them to discriminate the internal elements oftest stimuli (Schmid & Wildsoet, 1998). Both stimuliwere top-heavy configurations (having two elements intheir upper part and one in their lower part), similar tothose used by Turati et al. (2002). The average height ofthe black blobs representing the internal features ineither the upper or lower halves of the two stimuli wasidentical. Both configurations were symmetrical on thevertical axis, but only one represented a face.

The blobs in the face-like configuration were arrangedin the form of an upside-down triangle. This generalconfiguration roughly corresponds to that of a hen’s or ofa chick’s face (as well as of a human face) in frontal viewwhen under natural (top lit) illumination (Figure 3). Thetwo upper black blobs corresponded to the eyes of thehen, whereas the lower blob could correspond to the billand to the shadowed area beneath it. All three blobspresent in the non-face-like configuration were alignedalong the vertical axis, so that the configuration could notpossibly resemble a face. Test stimuli were inspired byconfigurations previously used in studies on preferencesfor face-like displays in human newborns (see Morton &Johnson, 1991, for a review), but adapted to increase theirresemblance to the face of a hen (the new stimuli had arounder outline, a more pronounced neck and eyespositioned differently with respect to those usuallyemployed with infants).

Procedure

On the second day of the chicks’ life, the featureless face wasremoved from each cage 20 minutes before the start of thetest. Each subject was carried to the experimental room(kept at 29–30�C with a humidity of 68%), and placed in thecentral area of the test apparatus. The chick’s position at thestarting point with respect to the test stimuli, as well as theposition of the two stimuli within the apparatus, wascounterbalanced across animals.

Figure 1 Standard ‘long-neck’ (on the left) ‘imprinting’stimulus used in Experiments 1–4 and ‘short-neck’ (on the right)version used with a sub-sample of subjects in Experiment 2.Long-neck stimuli had the following dimensions: overall 10 cmhigh · 5.6 cm wide; neck length 3.5 cm; external boundarymarked by a black line 0.5 mm thick. Short-neck stimuli’sdimensions were: overall 7.5 cm high · 5.6 cm wide; necklength 1 cm.

Newly hatched chicks’ preferences for faces 567


Chicks’ behaviour was recorded for a total of 6consecutive minutes. If the chick remained in the midcompartment we determined no choice to have been made,whereas entrance and residence in one of the sidecompartments indicated a preference for the adjacentobject (Vallortigara & Andrew, 1991). A computer-drivenevent recorder allowed the experimenter to score the time(seconds) spent minute by minute by the chick in each ofthe three areas during the overall test period.

Behavioural measures considered were:

• first stimulus approached by each chick (first sidesector entered);

• proportion of time spent near the face-like stimulus(in the sector adjacent to the face-like configuration).

All measures were scored blind (the scorer was unawareof the aims of the research conducted).

Data analysis

To compare the number of chicks that approached first theface-like or the non-face-like configuration we used thechi-square test of independence. The number of chicks thatapproached the face-like and the non-face-likeconfiguration as first stimulus was compared betweenchicks exposed and not exposed to the ‘imprinting’stimulus, using chi-square applied on contingency tables.

To represent the proportion of time spent near theface-like stimulus, an index was calculated from the timespent in the two side sectors using the formula:

Time by the face-like stimulus=

ðTime by the face-like stimulus

þ Time by the non-face-like stimulusÞ

Significant departures from chance level (0.5), whichindicated a preference for the face-like (> 0.5) or non-face-like stimulus (< 0.5), were estimated by one-sampletwo-tailed t-test. The proportion of time spent near theface was also compared between imprinted and non-imprinted chicks and between the first and second half ofthe test with repeated measures ANOVA on the values ofthe corresponding index (see above) calculated for thefirst 3 minutes of the test (minutes 1–3) and for theremaining 3 minutes of test (minutes 4–6). Factorsincluded in the ANOVA were presence of an ‘imprinting’object (‘imprinted’ vs. ‘non-imprinted’ chicks, betweensubjects), and time (first 3 minutes vs. second 3 minutesof test, within subjects).

Results

The number of chicks that approached the face-like orthe non-face-like configuration as first stimulus did notsignificantly differ between imprinted and non-imprintedsubjects (v2

1 = 2.053; p = .152; 24 ‘imprinted’ chicksapproached the face and 10 the non-face; 59 ‘non-

Figure 2 Experimental stimuli used in Experiment 1 (a and b),Experiment 2 (long-neck stimuli: c and d; short-neck stimuli: eand f), Experiment 3 (g and h), Experiment 4 (j and k). Theconfigurations represented a top-heavy face-like stimulus (a), atop-heavy non-face-like stimulus (b, c, e, h and k), a bottom-heavy face-like stimulus (g and j) and a bottom-heavy non-face-like one (d and f). The dotted line shown in Figure 2g–kwas not present in the real stimuli and was added to the figurein order to illustrate stimuli’s properties in terms of the relativeposition of the eyes. The square black blobs constituting stimuliinternal features were 0.9 · 0.9 cm in size.



imprinted’ chicks approached the face and 45 the non-face). Overall, the number of chicks that approached theface-like configuration as the first stimulus wassignificantly higher than the number of chicks thatapproached the non-face-like configuration (v2

1 = 5.681;p = .017).

As regards the ANOVA on the proportion of time spentnear the face-like stimulus, a main effect of the presence ofan ‘imprinting’ object (F(1,87) = 6.078; p = .016, Figure 4)was observed. The ANOVA did not reveal any significantmain effect of time (F(1,87) = 1.406; p = .239, Figure 4).A marginally non-significant interaction presence ofan ‘imprinting’ object · time (F(1,87) = 3.419; p = .068,Figure 4) was, however, apparent: ‘non-imprinted’chicks’ behaviour seemed to differ between the first andsecond half of the test (Figure 4). A paired samples t-testrevealed that, for ‘non-imprinted’ chicks, the proportion oftime spent near the face-like stimulus was significantlydifferent between the first (minutes 1–3) and secondhalf of the test (minutes 4–6) (t(62) = 2.539; p = .014; seeFigure 4). This same comparison was not significant for‘imprinted’ chicks (t(25) = )0. 590; p = .561; see Figure 4).

As regards ‘imprinted’ chicks, the proportion of timespent near the face-like stimulus for the whole length of thetest was significantly different from chance level(mean = 0.734; SEM = 0.066; t(33) = 3.525; p = .001):chicks spent more time near the face-like configuration.For ‘non-imprinted’ chicks, the proportion of overall timespent near the face-like stimulus was not significantlydifferent from chance level (mean = 0.524; SEM = 0.041;t(102) = 0.598; p = .551). However, during the first half ofthe test (minutes 1–3) ‘non-imprinted’ chicks showed apreference for staying near the face-like stimulus (mean =0.608; SEM = 0.054; t(64) = 1.975; p = .053), whereasduring the second part of the test (minutes 4–6) chicksspent the same amount of time near the two stimuli(mean = 0.497; SEM = 0.041; t(101) = )0.059; p = .953).

In fact, an independent samples t-test revealed that theproportion of time spent near the face-like stimulus wassignificantly different between ‘imprinted’ and ‘non-imprinted’ subjects, if we consider the second half of thetest only (t(134) = 2.894; p = .004, Figure 4), whereas thiscomparison was not significant when only the first half ofthe test was considered (t(52) = 1.548; p = .128, Figure 4).

Discussion

Visually inexperienced chicks prefer a schematic stimulusrepresenting the structure of a face with respect to asimilar top-heavy configuration that lacked the facednessproperty. This finding is consistent with the modelproposed by Morton and Johnson (1991), claiming that

Figure 4 Mean proportion of time spent near the face-likestimulus in Experment 1, for ‘imprinted’ (darker columns) and‘non-imprinted’ chicks (lighter columns). Time spent near theface is represented separately for the first (minutes 1–3) andsecond half (minutes 4–6) of the test (group means with SEM areshown).

(a) (b) (c)

Figure 3 Images obtained in order to illustrate that a hen’s face (a) and a chick’s face (c) seen from a frontal view under top-downillumination present a triangular configuration of dark areas (the two eyes and the shadowed area under ⁄ around the bill). Notice thatboth the mother hen and sibling chicks can become the imprinting object of a newborn chick in the natural environment. To create theimage (a) the left-hemiface of the original image (b), which was illuminated from above, was mirror reflected in order to obtain aconfiguration presenting a symmetrical shading pattern. In addition the right-hemiface eye was superimposed upon this left-hemifaceconfiguration (since the left-hemiface eye was partially closed in the photograph). The resulting image was then placed against auniform background and filtered employing the contrast-increase and blurring functions of the Photoshop 6.0 software program (AdobeSystems, Inc., Mountain View, CA). The image (c) was obtained from the photograph of one chick of the same strain of those employedin the present work. The original image was simply processed with the blurring function of the Photoshop 6.0 software program.



such a preference would be guided by an unlearnedrepresentation (CONSPEC) attracting attention towardstimuli whose internal features are arranged according toa face-like structure.

On the other hand, our results are not consistentwith those obtained by Turati et al. (2002, secondexperiment), that showed that babies did not exhibit anypreference between a face-like and a non-face-likeconfiguration, when both of them were top-heavystimuli. One possible explanation for this could befound in differences in symmetry between the stimuliused here and those used by Turati et al. (2002). In thepresent experiment both the face-like and the non-face-like configurations were symmetrical along the verticalaxis. The stimuli used by Turati and colleagues differednot only according to the property of facedness, but alsoin their symmetry. Since symmetry is an importantstructural property of a stimulus, it is possible tohypothesize that this could have influenced the resultsobtained (e.g. asymmetrical stimuli could be moredifficult to process and thus require additionalattentional resources; Bornstein, Ferdinandsen &Gross, 1981; Fisher, Ferdinandsen & Bornstein, 1981).

A sample of our chicks was exposed during rearingto a stimulus representing a ‘featureless face’, the outlineof which was identical to that of the test stimuli, butlacking in any internal feature. Results from the sample ofchicks not exposed to any conspicuous object duringrearing demonstrated that, in contrast to the chicksexposed to the featureless face, ‘non-imprinted’ chicksseemed to lose interest in the face-like stimulus as testtime went on. It is likely that this reduction in interest forthe preferred stimulus, which was observed only in thissample of chicks, was due to the fact that these subjectswere lacking in experience of any conspicuous visualobject and could have had slightly altered social behaviour,including a less sustained interest in social stimuli. Thepresence of conspicuous objects is normally part of achick’s natural environment. Moreover, this result is alsoconsistent with previous literature showing the role ofnon-specific experience, such as ‘priming’ visual input,in the preference for conspecific-like objects (Bolhuis,Johnson & Horn, 1985). The exposure to the outlinelacking any features seems therefore to represent a morereliable and ecologically valid procedure and was main-tained in all subsequent experiments.

Experiment 2

Results of Experiment 1 do not allow any inferenceregarding the presence of an up-down bias per se (Simion,Valenza, Macchi Cassia, Turati & Umilt�, 2002; Turatiet al., 2002) in visually inexperienced chicks. It could bethat any preference for top-heavy configurations iscompletely absent in chicks. If this species has evolved arepresentation for the recognition of faces (the triangulararrangement of features detected by CONSPEC), any

other broad kind of bias (such as that for top-heavyconfigurations) would be useless for the detection ofconspecifics. On the other hand, it could also behypothesized that the up-down bias is present also inchicks, but that it coexists with a stronger preference for aCONSPEC-like kind of configuration, or that the up-down bias could be found in chicks as a by-product of thepreference for CONSPEC-like stimuli (due to ageneralization of the preference for configurationspresenting some face-like properties, such as top-heaviness). These issues were explored in Experiment 2.

Subjects

Subjects were 62 (31 male and 31 female) domesticchicks. Incubation and hatching were the same as inExperiment 1.

Rearing conditions and apparatus

The initial setting up of the chicks, rearing conditionsand the precautions taken to avoid chicks receiving anyvisual experience regarding the internal structure of aface were identical to those of the previous experiment.However, two types of rearing stimuli were employed inthe present experiment. Thirty-seven (18 males and 19females) of the 62 chicks were exposed to a featurelessface identical to that described in Experiment 1 (‘long-neck’ rearing stimulus, see Figure 1, left). In addition, asubsample of 25 (13 males and 12 females) of the 62chicks were exposed to rearing stimuli that differed withrespect to the former ones in having a ‘shorter neck’ (seeFigure 1, right). All the other features remained identicalbetween the two kinds of configuration. The variation ofthe neck length of ‘imprinting’ and test stimuli (seebelow) was introduced in order to allow a more accurateinvestigation of the presence of the up-down bias, byusing stimuli resembling more those employed in theoriginal study of Turati et al. (2002).

The testing apparatus was that described forExperiment 1.

Test stimuli

The test stimuli used in Experiment 2 were identical tothose used in the previous experiment except for theposition of the three square black blobs. Theconfigurations used for this experiment resembled thoseemployed by Turati et al. (2002). Both stimuli were non-face-like configurations, but one was a top-heavystimulus (Figure 2c, e), whereas the other was abottom-heavy configuration (Figure 2d, f). Moreover,two variants of test stimuli were used: one couple ofstimuli with a ‘long neck’ (Figure 2c, d, identical indimensions and overall shape to stimuli described forExperiment 1) and the other (Figure 2e, f) that differedonly in the presence of a ‘shorter neck’ (as alreadydescribed for ‘imprinting’ stimuli). Except for the



presence of a ‘long neck’ or a ‘short neck’, all the otherfeatures remained identical between the two pairs of teststimuli. Those subjects that had been exposed duringrearing to a long-necked featureless face were then testedwith long-neck stimuli, whereas subjects that had beenexposed to a short-neck featureless face were then testedwith short-neck stimuli.

Procedure and data analysis

The general procedure was the same as in Experiment 1.The number of chicks that approached the top-heavy andthe bottom-heavy configuration as first stimulus wascompared between chicks tested with long- and short-neck stimuli, using the chi-square applied on contingencytables. As regards the index representing the proportion oftime spent near the top-heavy configuration, significantdepartures from chance level (0.5) were estimated by one-sample two-tailed t-test. The proportion of time spentnear the top-heavy configuration was also comparedbetween long- and short-neck chicks and between the firstand second half of the test with repeated measuresANOVA on the values of the corresponding indexcalculated for the first 3 minutes of test (minutes 1–3)and for the remaining 3 minutes of test (minutes 4–6).Factors included in the ANOVA were neck length (long-vs. short-neck chicks, between subjects), and time (first vs.second 3 minutes of the test, within subjects).

Results

The number of chicks that approached the top-heavy orthe bottom-heavy configuration as first stimulus did notsignificantly differ between chicks tested with short- andlong-neck stimuli (v2

1 = 1.676; p = .196, 21 ‘long-neck’chicks approached the top-heavy and 16 the bottom-heavyconfiguration, 10 ‘short-neck’ chicks approached the top-heavy and 15 the bottom-heavy stimulus). Considering theoverall sample, the number of chicks that approached thetop-heavy configuration as first stimulus did not differfrom the number of chicks that approached the bottom-heavy configuration (v2

1 = 0.000; p = 1.000).The ANOVA on the proportion of time spent near the

top-heavy stimulus did not reveal any significant effects(neck length F(1,49) = 0.538; p = .467; time F(1,49) = 0.059;p = .809; neck length · time F(1,49) = 0.474; p = .495,Figure 5). Considering the overall sample, the proportionof time spent near the top-heavy stimulus for thewhole length of the test did not differ significantly fromchance level (mean = 0.472; SEM = 0.046; t(61) = )0.586;p = .560).

Discussion

In this experiment visually inexperienced chicks did notdemonstrate a preference for non-face-like top-heavyconfigurations with respect to similar bottom-heavyconfigurations, in contrast to the results reported by

Turati et al. (2002) for human newborns. This suggeststhat, if no face-like configuration is present, an up-downbias cannot be found in chicks. A difference in visualacuity between the lower and upper visual fields has beenhypothesized to be the cause of the up-down bias innewborns (see Introduction). Thus, the lack of any up-down bias in chicks challenges the hypothesis that thepresence of a higher visual acuity corresponding to theupper visual field could explain the bias in humannewborns.

Variation of the stimuli’s neck length did notsignificantly affect chicks’ preference toward the top- orbottom-heavy configuration. Thus, in all furtherexperiments, the standard (and more realistic) long-neckstimuli were used.

Experiment 3

The aim of the present experiment was to check whether(in the absence of any visual experience with faces’ innerstructure) chicks would maintain their preference forschematic face-like stimuli (Experiment 1) when thispreference is put in conflict with the hypotheticalinfluence of the up-down bias. In order to do so, wetested chicks’ spontaneous preference for a bottom-heavyface-like stimulus with respect to a top-heavy stimulus notrepresenting a face (see Figure 2g, h).

Subjects and rearing conditions

Subjects were 44 (23 male and 21 female) domestic chicks.Incubation, hatching and chicks’ rearing and handlingprocedures were the same as described for the previousexperiments.

Figure 5 Mean proportion of time spent near the top-heavyconfiguration in Experiment 2 for long- (darker columns) andshort-neck chicks (lighter columns). Time spent near the faceis represented separately for the first (minutes 1–3) andsecond half (minutes 4–6) of the test (group means with SEMare shown).



Test stimuli

The test stimuli were identical to those used inExperiment 1, except for the position of the threesquare black blobs (see Figure 2g, h). Stimuli consistedof one top-heavy configuration and one bottom-heavyconfiguration. The stimuli’s internal features werearranged so that the bottom-heavy configurationrepresented a face-like display with its features locatedin the lower part of the face, whereas the top-heavyconfiguration represented a non-face-like display with itsfeatures located in the upper part of the face (seeFigure 2g, h). These two stimuli were inspired by theconfigurations used by Turati et al. (2002), but they wereslightly altered according to the schema, recentlyproposed by Johnson (2005), of an optimal pair ofstimuli to contrast the up-down bias theory withrespect to the CONSPEC theory. The main differencewith Turati and colleagues’ stimuli is that in our stimulithe two blobs representing the ‘eyes’ of the face-likeconfiguration were placed at the same height as thecorresponding blobs in the non-face-like configuration(i.e. the two configurations differed only for the positionof the third blob, either in the upper or lower part of thestimulus).

Procedure and data analysis

Apparatus, procedures and data analyses were the sameas in the previous experiments.

Results

The number of chicks that approached the face-likeconfiguration as first stimulus did not significantly differfrom the number of chicks that approached the non-face-like configuration (v2

1 = 0.364; p = .546; 20 chicksapproached the face, 24 the non-face).

The proportion of time spent near the face-likestimulus for the whole length of the test did not differsignificantly from chance level (mean = 0.426; SEM =0.056; t(43) = )1.316; p = .195). Moreover, a pairedsamples t-test revealed that the proportion of timespent near the face did not differ between the first andsecond half of the test (t(35) = )1.040; p = .305, Figure 6).

Discussion

Results of the present experiment seem to contrast withthose obtained in Experiment 1: visually inexperiencedchicks did not show the expected preference for theschematic bottom-heavy face-like configuration, whenconfronted with a top-heavy non-face-like one. Twopossible interpretations for this result would be thateither the expression of the preference for faces requiresthe face-like configuration to be top-heavy, or that in thepresent experiment chicks’ preference for faces wascontrasted with that for top-heavy configurations, and

could be weakened by this contrast. However, both theseexplanations are not completely convincing. Previousexperiments did not reveal any direct (Experiment 2) orindirect (Experiment 1) evidence for the presence of anup-down bias in chicks. Moreover, the results ofExperiment 3 are not completely consistent withthose obtained with newborns by Turati et al.(2002), even though it employed comparable stimuli. Infact, unlike chicks, human newborns show a preferencefor a top-heavy non-face-like stimulus compared to abottom-heavy face. This discrepancy could be due tomany factors (see General Discussion), includingstructural differences in the stimuli, such as theposition of the blobs representing the eyes in Turatiet al. (2002).

A further possibility is that the results of the presentexperiment actually reflected the presence of a weakerpreference for the face-like stimulus with respect toExperiment 1, and this in turn could be due to apeculiarity in the experimental stimuli used. In fact, boththe face-like and the non-face-like stimuli used in thepresent experiment were identical with regard to the ‘eyeregion’, i.e. the positions of the two blobs representing the‘eyes’ of the face stimulus with respect to thecorresponding blobs in the other stimulus. It is likelythat the eye region plays a major role in determiningpreferences for face-like configurations in chicks, morethan other ‘face traits’ do (consistent with evidenceavailable in the literature on domestic chickens, e.g.Bolhuis, 1996; but also on humans and other animalspecies, e.g. Easterbrook, Kisilevsky, Hains & Muir, 1999;Farroni et al., 2005; Keating & Keating, 1982; Kendrick,1991; Kendrick, Atkins, Hinton, Broad, Fabre-Nys &Keverne, 1995; Myowa-Yamakoshi & Tomonaga, 2001;Tate et al., 2006; Turati, Valenza, Leo & Simion, 2005).This could explain why the preference for a face-likeconfiguration is weakened when this configuration isconfrontedwith a non-face-like stimulus that is identical tothe face-like one with regard to the ‘eye region’.

Figure 6 Mean proportion of time spent near the face-likestimulus in Experiment 3 for the first (minutes 1–3) and secondhalf (minutes 4–6) of the test (group means with SEM areshown).



Experiment 4

The aim of this experiment was to confirm the finding,that emerged from Experiment 1, that chicks’ preferencefor schematic faces is maintained even when the role ofthe up-down bias is controlled for. We especially wantedto check whether it was possible to obtain direct evidenceof a preference for face-like stimuli when it is put in directconflict with the hypothetical influence of the up-downbias (i.e. that chicks would prefer a bottom-heavy face-likestimulus compared to a top-heavy non-face-like one). Wewanted to do this by investigating the role of the ‘eyeregion’ of stimuli in determining preferences for faces.Our hypothesis was that, by using a pair of stimuli(Figure 2j, k) that (unlike in Experiment 3) differed in the‘eye region’ (i.e. similar to the configurations used byTurati et al., 2002), the preference for the bottom-heavyface-like configuration would emerge. Two kinds ofevidence are in favour of such a hypothesis. First of all,there are data in the literature suggesting a fundamentalrole of eyes in determining both a preference for, andresponses to, faces (see Discussion of Experiment 3; recentdata imply a greater importance of eyes – as relevant tosocial communication – compared to the other facialfeatures, e.g. Farroni et al., 2005). Moreover, in thepresent experiment an increased perceptual differencebetween the two test stimuli could facilitate theindependent processing and differentiation of the face-like stimulus with respect to the non-face-like one.According to Morton and Johnson’s (1991) hypothesis,the configurations used in the present experiment couldbe more likely to produce a significant preference for thebottom-heavy face-like stimulus. On the other hand, afurther aim of the present experiment was also to teststimuli in which the top-heaviness and bottom-heavinesswere more pronounced than in Experiment 3. As aconsequence, the up-down bias theory would predict thatthe pair of stimuli employed in Experiment 4 should bemore effective in eliciting a preference for the top-heavynon-face-like configuration than those of the previousexperiment.

Subjects and rearing conditions

Subjects were 58 (28 male and 30 female) domestic chicks.Incubation, hatching and chicks’ rearing and handlingprocedures were the same as described for the previousexperiments.

Test stimuli

The test stimuli used in Experiment 4 differed from those ofExperiments 1 and 3 in the position of the three squareblack blobs. As in Experiment 3, stimuli consistedof one top-heavy non-face-like configuration and onebottom-heavy face-like one. In the new pair of stimuli,however, the two blobs representing the ‘eyes’ of the face-

like configuration were not placed at the same height as thecorresponding blobs in the non-face-like configuration, butat a relatively lower position (i.e. they were misaligned, seeFigure 2i, j). So, this new pair of stimuli was more similar tothe original configurations used by Turati et al. (2002), andwas used in order to obtain stimuli which could possibly bemore powerful in eliciting a preference for one of the twoconfigurations with respect to those used in the previousexperiment (see above).

Procedure and data analyses

Apparatus, experimental procedure and data analyseswere the same as in the previous experiments.

Results

The number of chicks that approached the face-likeconfiguration as first stimulus was not significantlydifferent from the number of chicks that approached thenon-face-like configuration (v2

1 = 2.483; p = .115; 35chicks approached the face-like, 23 the non-face-like),though there was a clear trend towards a preference forthe face-like stimulus.

The proportion of time spent near the face-like stimulusfor the whole length of the test was significantly differentfrom chance level (mean = 0.605; SEM = 0.050; t(57) =2.089; p = .041): chicks spent more time near the face-likeconfiguration. The proportion of time spent near the facedid not differ between the first and second half of the test(t(50) = )0.268; p = .790, Figure 7).

Discussion

The results of Experiment 4 confirmed and extendedthose of Experiment 1. Chicks preferred a face-likebottom-heavy object over a non-face-like top-heavy oneand this preference was maintained even when using apair of stimuli that, according to the up-down biashypothesis, should be particularly effective in eliciting apreference for the non-face-like top-heavy stimulus.

The results of Experiment 4 differed from those ofExperiment 1 in that no significant preference for theface-like stimulus emerged for the dependent variable‘first stimulus approached’ (though there was a clear trendin that direction). Again, a possible explanation could bethat in the present experiment the preference for faces waspartially counterbalanced by a preference for top-heavystimuli. Results obtained in Experiment 2, however, makethis explanation quite unlikely due to the absence ofevidence for a preference for top-heavy stimuli when noface-like configuration is present.

The present result is consistent with that of Experiment1 and is particularly remarkable in light of the fact that itreflects the presence of a stronger preference for faceswhen the test stimuli used differed in their eye region (asin the present experiment), with respect to a situation in



which both test stimuli were identical in the eye region(as in Experiment 3). This result can be explained bysuggesting that there is a crucial role of the stimuli’s eyeregion in eliciting preferences for faces (see Discussion ofExperiment 3).

General discussion

The results obtained demonstrate that domestic chicks,visually inexperienced with respect to faces,spontaneously prefer schematic stimuli presenting aface-like arrangement of internal features, even whenthe role of the up-down bias is controlled for. Thisfinding is relevant from several points of view.

First of all, this is, to the best of our knowledge, thefirst demonstration that chicks are sensitive to facednesswhen this property is embedded in a highly schematicstimulus, such as those usually employed for testing facepreferences in human babies. Our results imply thatfurther studies could make use of this peculiarity of thesocial behaviour of chicks in order to compare the resultsobtained in human newborns with respect to chicks’responses to similar stimuli, exploiting the possibilitiesgiven by this flexible animal model for the investigationof the neural bases of a predisposition for faces.

Moreover, the results seem to be in agreement with theexistence, put forward by Morton and Johnson (1991), ofan unlearned representation (CONSPEC), shared amongvertebrates, that directs the animal’s attention towardstimuli whose internal features are arranged according toa triangular face-like configuration. The presence of sucha representation in domestic chicks is not completelyunexpected. It was already known that chicks prefer toapproach naturalistic stimuli that resemble a hen, andthat the presence in the stimulus of a neck and a head(containing, of course, internal face features maintainingtheir reciprocal spatial positions) was crucial for elicitingthat preference (Johnson et al., 1985; Johnson & Horn,1988). Data also suggested that the representation of a

social object that could underlie this preference was quitegeneric and broad (probably, broad enough to beactivated when schematic stimuli are employed). Thisrepresentation was so broad, in fact, as not to be speciesspecific: na�ve chicks approach a potential predator, suchas a polecat, to the same degree as a hen (Johnson &Horn, 1988). Similarly, visually na�ve chicks do not showany preference between a point-light-display representingthe motion pattern of a walking hen or that of a cat(Vallortigara, Regolin & Marconato, 2005).

On the other hand, our results are not consistent withthe up-down bias theory that claims that the preferencefor face-like configurations in human newborns wouldemerge only as a secondary effect of a non-specific biasfavouring top-heavy configurations. Contrary to theresults obtained with human newborns by Turati et al.(2002), chicks seem to prefer face-like stimuli even whenthey are presented with other top-heavy configurations(Experiment 1), or when the face-like stimulus itself is abottom-heavy configuration that is compared to a top-heavy non-face-like stimulus (Experiment 4). Non-face-like top-heavy configurations do not elicit a spontaneouspreference per se (Experiment 2), however. Althoughthese non-significant results should be treated withcaution, it is still possible to argue that, if anypreference for top-heavy configurations were to bepresent in chicks, this preference should be weaker thanthat expressed by chicks for face-like stimuli andtherefore less effective than the up-down bias observedin newborns. In fact, in chicks any hypothetical up-downbias does not play a crucial role in face preference, and itdoes not seem to be strong enough to negate such apreference.

Let us consider some possible explanations for thepresence in chicks of a weaker (with respect to humannewborns), or absent, up-down bias. Some of thesearguments are based on differences between the twospecies, that increase the interest of the inter-speciescomparison. Nevertheless, it should not be forgotten thatremarkable similarities exist between the two species (e.g.bird-mammal cerebral and behavioural homologies;Jarvis et al., 2005; Vallortigara, 2006).

A first possibility is that the up-down bias, in order toemerge,woulddependuponthepresenceofacertaindegreeof visual experience with faces (which was completelyprevented in the chicks). Such a striking control of visualexperience cannot be obtained in experiments with humaninfants (and this is in fact one of the reasons for the interestin the use of animal model systems).

Another possible explanation was suggested to us byFrancesca Simion (personal communication, 2008), whonoticed that the experimental procedure used in thepresent work (unlike that used with human newborns bySimion et al., 2002b, and Turati et al., 2002) did notcontrol for the presentation of the stimuli in terms ofupper or lower hemi-visual field. In fact, chicks werecompletely free to move and to visually explore stimuliduring the test. However, it is important to note that this

Figure 7 Mean proportion of time spent near the face-likestimulus in Experiment 4 for the first (minutes 1–3) and secondhalf (minutes 4–6) of the test (group means with SEM areshown).



very same argument has been considered one of the maincriticisms with respect to some of the work on humannewborns on the up-down bias. In such studies, no eye-tracker evidence is reported, which would havedemonstrated that different parts of the stimuli fallupon the upper or lower visual field of the newborn.Moreover, this explanation would be relevant only for thenull results obtained in Experiment 2, becauseExperiments 1 and 4 provided direct evidence of apreference opposite to that predicted on the basis of theup-down bias theory. As regards Experiment 3, a morepowerful explanation of the results obtained was providedby the demonstration that a simple manipulation of theeye region of the stimuli (Experiment 4) determined thepresence of a preference for the face-like stimulus, againstthe predicted influence of the up-down bias. Finally, if theup-down bias were to be effective in newborns’ behaviouronly under such extremely controlled visual conditions(i.e. only if the stimulation provided to each part of thevisual field were rigidly determined), the role andrelevance of such a bias in an ecological situation wouldappear questionable. An organism is likely to evolvemechanisms enabling it to preferentially pay attention tosocial partners within its natural environment and naturalfree viewing conditions.

A further explanation could be that mechanismsunderlying face preferences evolved in the two speciesare different. Chicks could possess a more specificmechanism (a relatively ‘detailed’ representation of theappearance of faces) than human newborns (which maypossess a general preference for top-heavy stimuli only) inorder to direct attention to faces. The offspring ofprecocial species, such as the domestic chick, which areprimed to move independently and imprint on any salientobject, would need to accurately recognize biologicalobjects. In fact, the consequences of making a falsepositive response (i.e. responding to an inanimate top-heavy object by mistaking it for a face) would beextremely disadvantageous for a young animal that mayimprint on an inanimate feature of the surroundingenvironment. A newly hatched chick would be at risk ofmaking such an error due to its ability to actively followand respond to stimuli (i.e. walk away from the motherhen) and to the dramatic nature of the imprintingphenomenon occurring in this species. On the contrary,for a newborn human the same false positive responsewould be less dangerous.

Another interesting finding relates to the importanceof features present within the eye area of the stimuli(Experiments 3 and 4). This is consistent with otherevidence present in the literature suggesting that eyes arecrucial in determining face preferences in human babiesand animals, including the domestic chicken (seeDiscussion of Experiment 3, Bolhuis, 1996; Easterbrooket al., 1999; Kendrick, 1991; Kendrick et al., 1995; Myowa-Yamakoshi & Tomonaga, 2001; Turati et al., 2005).Evidence for the importance of eyes could fit well witheither the up-down bias hypothesis (eyes are important

because they are placed in the face-stimulus upper half) orthe CONSPEC hypothesis (eyes are crucial due to theirimportance for social communication; Farroni et al., 2005).The comparison between the results of Experiments 3 and 4led us to speculate that the predominant role of eyes inchicks’ face preferences could be difficult to account for interms of the up-down bias hypothesis. In fact, we foundevidence for a major role of eyes even if the face-likestimulus presented ‘eyes’ in its lower part.

It has long been debated whether face perception isserved by specific mechanisms (and the role of experiencein such mechanisms is still debated too; see Introduction).The present work supports the presence of a face-specificmechanism underlying spontaneous preferences inchicks. This domain-specific mechanism is activatedwithout any prior experience of the inner structure ofa face. The inborn representation of face structuredemonstrated here could constitute the basis for aninnate conspecific-detector or biological-object detectordevice, possibly shared among different classes ofvertebrates (Vallortigara et al., 2005; Vallortigara &Regolin, 2006; Simion, Regolin & Bulf, 2008; Troje &Westhoff, 2006; Johnson, 2006).

Acknowledgements

The authors wish to thank Camilla Romor and AdrianaBerti for helping with the chicks’ care and testing, DrJonathan N. Daisley for reading and correcting the paper,and Dr Chiara Turati for her comments on the manuscript.

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Received: 10 October 2008Accepted: 9 April 2009



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