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Baboons (Papio papio) spontaneously process the first-order but not second-order configural properties of faces.

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American Journal of Primatology 70:415–422 (2008)
RESEARCH ARTICLE
Baboons (Papio papio) Spontaneously Process the First-Order but not
Second-Order Configural Properties of Faces
CAROLE PARRON AND JOËL FAGOT
CNRS, Mediterranean Institute of Cognitive Neurosciences, UMR 6193 University of Méditerranée, Marseille, France
A two-alternative forced-choice discrimination task was used to assess whether baboons (N 5 7)
spontaneously process qualitative (i.e., first-order) or quantitative (i.e., second-order) variations in the
configural arrangement of facial features. Experiment 1 used as test stimuli second-order pictorial faces
of humans or baboons in which the mouth and the eyes were rotated upside down relative to the normal
face. Baboons readily discriminated two different normal faces but did not discriminate a normal face
from its second-order modified version. Experiment 2 used human or baboon faces for which the firstorder configural properties had been distorted by reversing the location of the eyes and mouth within
the face. Discrimination was prompt with these stimuli. Experiment 3 replicated some of the conditions
and the results of experiment 1, thus ruling out possible effects of learning. It is concluded that baboons
are more adept at spontaneously processing first- than second-order configural facial properties, similar
to what is known in the human developmental literature. Am. J. Primatol. 70:415–422, 2008. c 2007
Wiley-Liss, Inc.
Key words: face; configural processing; nonhuman primate; Thatcher illusion
INTRODUCTION
Faces are complex stimuli that can be discriminated by a series of ‘‘featural’’ as well as ‘‘configural’’
information [Carey & Diamond, 1977]. Facial features
consist of facial parts that can be identified in
isolation, such as the nose or the mouth. By contrast,
the configural properties of the face refer to the
spatial layout of these features; for instance, the
distance between the nose and the mouth may differ
from one individual to another or may vary with facial
expressions. Numerous studies have demonstrated
that humans strongly rely on configural information
to recognize faces [Bruce et al., 1991; Rhodes et al.,
1993], although consideration of facial features may
serve the processing of other aspects of the faces, such
as their expressed emotions [Deruelle & Fagot, 2005].
Whether or not configural information prevails
over featural information in face recognition in
animals remains unknown, especially for nonhuman
primates. The main source of uncertainty derives
from studies in which face recognition was assessed
with upright and inverted stimuli. Because features
remain unchanged with inversion, effects of inversion in humans are usually interpreted as demonstrating the prevalence of configural over featural
cues during face processing [Maurer et al., 2002].
Several studies on nonhuman primates have
reported that inversion of facial stimuli has few
effects on recognition performance. For instance,
Rosenfeld and Van Hoesen [1979] found that inver-
r 2007 Wiley-Liss, Inc.
sion of the face of conspecifics did not disrupt the
recognition performance of rhesus macaques in
a forced-choice discrimination task. Also using
pictures of conspecifics, Bruce [1982] reported
similar results in cynomolgus macaques. Tomonaga
et al. [1993] found no effect of stimulus inversion in a
chimpanzee (Pan troglodytes) tested on a face
discrimination task with familiar faces of humans
and chimpanzees. Such findings seem to contradict
the vast literature suggesting the prevalence of
configural properties in face recognition by humans
[e.g., Rhodes et al., 1993]. However, some other
primate studies converge with the human literature,
finding that upright faces are processed in a different
way than upside-down faces. Thus, using a sensory
reinforcement procedure, Tomonaga [1994] found
that macaques looked longer at upright than inverted photographs. Also, Parr et al. [1998] reported
Contract grant sponsor: OMLL Eurocores; Contract grant
sponsor: EC NEST SEDSU; Contract grant number: 012984;
Contract grant sponsor: CNRS OHLL; Contract grant number:
2004-004.
Correspondence to: Joël Fagot, INCM, CNRS, 31 Chemin
Joseph Aiguier, 13402 Marseille cedex 20, France.
E-mail: fagot@incm.cnrs-mrs.fr
Received 2 July 2007; revised 24 October 2007; revision accepted
26 October 2007
DOI 10.1002/ajp.20503
Published online 20 November 2007 in Wiley InterScience
(www.interscience.wiley.com).
416 / Parron and Fagot
better discrimination of human and chimpanzee
faces by chimpanzees, when these faces were shown
right side up. These studies showing variable effects
of inversion remain difficult to reconcile and it is
unclear if different outcomes are due to subjectrelated factors (e.g., species, past exposure to
pictures) or variations in test procedures. Nevertheless, the data minimally suggest that nonhuman
primates can potentially process both featural
information, when recognition performance survives
inversion, and configural information, when performance is disrupted by face inversion. Additional
experimental studies are called for to clarify how
nonhuman animals process faces.
Diamond and Carey [1986] advised researchers
to distinguish first- from second-order configural
relational properties within a face. First-order
properties refer to the global qualitative spatial
relations among facial features (e.g., the nose is
above the mouth) and provide a means for recognizing faces from other visual stimuli. Second-order
relational properties refer to the fine spatial relations
among features (e.g., the distance between the nose
and the mouth relative to the prototypical face
arrangement). Subtle spatial second-order information might be crucial to individuate faces, at least in
humans. This study considered this distinction to
further examine how a nonhuman primate—the
baboon—processes facial stimuli. To our knowledge,
the distinction between first- and second-order cues
in nonhuman primates’ face processing has been
considered so far only by Parr et al. [2006]. In a first
experiment, those authors used composite faces as
stimuli in which the upper and lower half parts of the
faces were aligned or misaligned, disorganizing
second-order relations. In a second experiment,
facial subparts were moved apart keeping the firstorder configural properties constant, or were both
moved apart and disorganized to simultaneously
disrupt first- and second-order cues. Results from
these two experiments converged to show that face
recognition was impaired in chimpanzees when
second-order cues had been manipulated.
Three original experiments are reported here.
The first experiment used a ‘‘Thatcher illusion’’
procedure [Thompson, 1980] to specifically test
the processing of second-order configural cues by
baboons. Examples of ‘‘Thatcherized’’ faces are
given in the first column of Figure 1, and the original
stimuli from which they derive are shown in the
second column of Figure 1. The ‘‘Thatcherized’’ faces
were made by rotating by 1801 the mouth and the
eyes within a normal face. Normal humans very
easily discriminate the normal face from its ‘‘Thatcherized’’ version when both are shown upright. By
contrast, discrimination is much harder when the
stimuli are both presented upside-down, as illustrated in Figure 1 Block 1a and b. Because the
original and ‘‘Thatcherized’’ faces differ only by
Am. J. Primatol.
second-order configural cues, ready discrimination
between the upright normal and ‘‘Thatcherized’’
faces demonstrates that our visual system processes
second-order configural properties of the face in that
viewing condition [Lewis & Johnston, 1997]. By
contrast, our relative blindness to facial differences
with upside-down ‘‘Thatcherized’’ faces suggests
that a featural processing mode is adopted when
viewing inverted faces. Before the current research,
we knew of only one published study on the Thatcher
illusion in animals [Jitsumori & Yoshihara, 1997]:
pigeons showed no Thatcher illusion, suggesting
possible human/animal differences in the processing
of second-order facial stimuli. The paucity of Thatcher illusion studies in nonhumans along with the
apparent human–pigeon differences in the processing of ‘‘Thatcherized faces’’ prompted our first
experiment with baboons, whose visual system is
more homologous to the human visual system than
that of pigeons [Fobes & King, 1982].
The second experiment assessed the processing
of first-order facial configural properties by the same
baboons. In humans, processing of first-order facial
properties emerges earlier in ontogeny than the
processing of second-order properties [e.g., Mondloch
et al., 2003; Valenza et al., 1996]. This suggests that
first-order properties might be relatively more
salient, and therefore potentially more accessible to
baboons. To assess this hypothesis, baboons were
required to discriminate a normal face from a ‘‘firstorder’’ modified face created by shifting the location
of the eyes and the mouth (see Fig. 1c).
To confirm that the results of experiments 1 and
2 were not due to a learning effect, baboons were
retested in experiment 3 using a subset of stimuli
used in experiment 1. In addition, familiar faces of
human or baboon models were systematically used as
stimuli in experiments 1–3. According to Diamond
and Carey [1986], the ability to exploit second-order
relational properties that individuate members of
classes is promoted by a stimulus familiarity or
expertise. Use of familiar models in our experiments
was aimed at promoting consideration of secondorder cues by the baboons.
EXPERIMENT 1: PROCESSING OF SECONDORDER RELATIONAL PROPERTIES
Participants
The participants were seven 18-year-old Guinea
baboons (Papio papio), five males (B03, B07, B09,
B11, B15) and two females (B04, B08), living in social
groups of two to four individuals within the primate
facility of the CNRS (Marseille). They were already
familiar with the two-alternative forced-choice discrimination task procedures [e.g., Deruelle et al.,
2000]. They were fed normally after completion of
daily training or testing sessions.
Configural Processing by Baboons / 417
Fig. 1. Illustration of some of the face stimuli used in blocks 1–3 and their modified version. (a) ‘‘Thatcherized’’ version of the stimuli,
(b) normal faces, (c) first-order modified stimuli. All facial stimuli were presented either upright or inverted to two groups of baboons.
Human faces published with written consent.
Apparatus
The baboons were tested in an experimental
booth (68 50 72 cm) comprising a food dispenser,
a view port and two hand ports that provided free
access to an analogue joystick controlling isomorphic
displacements of a cursor on a 17-in color monitor
screen located 49 cm away from the view port. The
food dispenser delivered 190-mg banana-flavored
food pellets inside the test apparatus in accordance
with the prevailing reinforcement contingency.
Inversion of the eyes and mouth involved an average
of only 7.97% (SE 5 1.17) of the total number of
pixels composing the original faces. The four human
models were all familiar to the baboons, being the
caretakers and the first author. The two baboon
models were one familiar male and one familiar
female, both nonparticipants. All stimuli were shown
on a black background with a 1024 768 definition.
They subtended approximately 8 81 of visual angle.
Procedure
Stimuli
The stimuli were color digitized frontal views of
four human and two baboon faces, (see Fig. 1b)and
their ‘‘Thatcherized’’ versions created by turning the
eyes and the mouth upside-down (see Fig. 1a).
One group of baboons (i.e., B03, B04, B08 and
B09) was tested with upright faces and the other one
(i.e., B07, B11 and B15) with the same but inverted
faces. Each group received three consecutive blocks
of test sessions differing only in the models
Am. J. Primatol.
418 / Parron and Fagot
presented as stimuli. To control for any effect of
block order, and to investigate potential differences
in the processing of human and baboon faces, blocks
1 and 3 used two different pairs of human faces as
stimuli whereas block 2 used pictures of baboons’
faces (see Fig. 1). Using this design, any effect of
species should be revealed by comparing the performance on block 2 (pictures of baboons) and blocks 1
and 3 (pictures of humans). By contrast, learning
effects should be revealed by a performance increment from blocks 1 to 3, independent of the stimulus
species.
Each block was divided in two phases aimed at
identifying processing differences between normal
and ‘‘Thatcherized’’ faces. The first phase used only
normal faces. It consisted of six sessions of 20
randomly ordered trials, in which the baboons had
to select the face assigned as positive (S1) from
among two left–right counterbalanced normal faces.
Responses consisted of moving the joystick so as to
place the cursor on the S1. There was no time limit
for responding. The second phase used ‘‘Thatcherized’’ faces. It consisted of six additional sessions of
the 20 trials-test in which the previous S1 stimulus
(i.e., a normal face) continued as S1, but was now
presented against its ‘‘Thatcherized’’ (S) version.
For all the participants, the second phase followed
the first phase of the same block.
In all blocks and phases, correct responses were
food-reinforced, whereas wrong responses triggered
a 3-sec time-out green screen and no reward delivery.
A 6-sec inter-trial interval separated two consecutive
trials, irrespective of their outcome, and therefore
followed the time-out in case of an error. Both scores
and response times were retained for statistical
analyses. The participants completed 1–3 sessions
in a day.
Results
Scores were analyzed with a four-way group
(upright, inverted) by block (1, 2 and 3), by phase
(phase 1 vs. phase 2), by session (1–6) analysis
of variance (ANOVA). This analysis showed that
baboons tested with the upright faces behaved
indifferently from baboons tested with inverted faces
(mean upright 5 73.8%, SD 5 3; mean inverted 5
73.9%, SD 5 2.94, F(1,5) 5 .002, P 5 .96). In addition,
none of the possible two- or three-way interactions
involving the group factor was even close to
significance (all P4.18).
The effect of phase was significant, F(1,5) 5
74.32, Po.001. As shown in Figure 2, performance
was significantly greater on average in phase 1 using
normal faces (mean 5 88.26% correct, SD 5 4.18%)
than in phase 2 using ‘‘Thatcherized’’ faces
(mean 5 59.2% correct, SD 5 3.33%). This finding
indicates a poor sensitivity of the baboons to the
configural variations distinguishing a normal face
Am. J. Primatol.
Fig. 2. Mean percentage of correct responses for each session and
with the normal (phase 1) and ‘‘Thatcherized’’ faces (phase 2) in
experiment 1. Bars correspond to standard errors.
from its ‘‘Thatcherized’’ version. Also reliable was
the main effect of block, F(2,10) 5 7.2, Po.03. Tukey
honestly significance post hoc tests (Po.05) provided
the following pattern of results: block 1 (human
faces: 67.2%) oblock 2 (baboon faces: 76.5%) 5 block
3 (human faces: 77.8%). Clearly, the overall performance increment appearing from blocks 1–3 was
independent of presenting human or baboon faces as
stimuli.
Finally, a reliable main effect of the session
emerged, F(5,25) 5 24.30, Po.001, attributable to
the significant phase (phase 1, phase 2) by session
(1–6) interaction of a higher order, F(5,25) 5 16.91,
Po.001 (see Fig. 2). Monkeys scored 88% correct on
average (SD 5 0.79%) in the second session of phase
1 (normal faces), all blocks confounded. Interestingly, the performance in the final session of phase 2,
with ‘‘Thatcherized’’ faces, remained poor (i.e.,
62.1% correct) with no clear evidence of learning.
D primes (d0 ) were computed to evaluate the
discrimination performance of individual baboons.
D0 is a measure from the signal detection theory that
considers the rate of correct hits and false-alarm
responses to evaluate discriminability of the stimuli.
A significant d0 indicates that the signal is readily
detected, or, in our context, that the feature is
processed and that the baboons perceived the
difference between a normal and a Thatcherized
face. D0 s were calculated for each subject and session
(1–6) and for phases 1 and 2. A Bonferroni correction
was applied in view of the large number of
comparisons (N 5 84). No baboon had a reliable d0
in the first session of phase 1, when they were facing
a novel discrimination problem. However, all baboons demonstrated a reliable d0 in sessions 2–6 of
phase 1, except for baboon B07 that demonstrated a
reliable d0 in sessions 3–6. By contrast, no baboon
discriminated reliably in phase 2, in any session.
Response times for correct trials were also
analyzed, omitting session as a factor owing to the
large number of errors per session limiting the data
set size. A group (upright-inverted) by block (blocks
Configural Processing by Baboons / 419
1, 2, 3), by phase (phases 1 and 2) ANOVA revealed
only a significant effect of phase, F(1,5) 5 26.76,
Po.001: response times were longer on average in
phase 2 (mean 5 1056 ms, SD 5 300) than in phase 1
(mean 5 787 ms, SD 5 217 ms). This result confirms
that the discrimination of the Thatcherized faces
from their normal faces (phase 2) was more difficult
for the baboons than the discrimination of the faces
of two different individuals (phase 1).
Discussion
Two conclusions may be drawn from the first
experiment. First, baboons can readily learn to
discriminate the faces of two different humans or
baboons, but struggle to tell apart a normal face from
its ‘‘Thatcherized’’ version. Second, this difficulty
emerged independently of the orientation of the
stimuli, that is, upright or inverted. These findings
are in sharp contrast with what has been reported in
the human literature, which shows that the Thatcher illusion emerges without training and with
upright faces only [e.g., Thompson, 1980]. Lack of
discrimination between the normal and ‘‘Thatcherized’’ faces suggests that the baboons did not
spontaneously process the second-order relational
properties of the facial attributes.
EXPERIMENT 2: PROCESSING OF ‘‘FIRSTORDER’’ RELATIONAL PROPERTIES
Given that experiment 1 failed to demonstrate
the processing of second-order relational properties
by baboons, experiment 2 assessed their ability to
process first-order spatial relations, which, in humans, emerge earlier in development [in newborn
infants: e.g., Goren et al., 1975; Valenza et al., 1996]
than the processing of second-order relations.
Stimuli
Experiment 2 used the same original faces as in
experiment 1 (see Fig. 1b). However, these faces were
contrasted with their first-order modified versions,
created by shifting the position of the eyes and the
mouth within the face (see Fig. 1c). These shifts
involved an average of only 8.17% (SE 5 1) of the
total number of pixels comprising the original faces.
The similar magnitude of pixel change in this
experiment and in experiment 1 (i.e., 7.97%) validates direct comparisons between these experiments.
All stimuli were shown on a black background with a
1,024 768 definition. They subtended approximately 8 81 of visual angle.
conditions would prevent overtraining with upright
or inverted faces. All other aspects of the procedure
were identical to experiment 1. The only difference
was that phase 2 now used as negative stimuli the
first-order modified versions of the faces instead of
their ‘‘Thatcherized’’ versions.
Results
Scores were analyzed with a four-way group
(upright, inverted) by block (1, 2 and 3) by phase
(phase 1 vs. phase 2) by session (1–6) ANOVA.
Baboons in the upright group performed similarly to
those in the inverted group (mean upright: 84.3%,
SD 5 2.6; mean inverted: 79.6%, SD 5 2.6),
F(1,5) 5 .385, P 5 .56. A reliable main effect of
session, F(5,25) 5 11.38, Po.001, indicated a performance increment. There was also a reliable effect of
phase, F(1,5) 5 12.06, Po.05, indicating a better
performance with normal faces (phase 1:
mean 5 95.2% correct, SD 5 7.4%) than with firstorder modified faces (phase 2: mean phase 2 5 82%
correct, SD 5 42.33%). Finally, interactions were not
significant with the exception of phase (1 vs. 2) by
session (1–6) two-ways interaction, F(5,25) 5 4.21,
Po.05. As shown in Figure 3, repeated testing had
greater effects on performance in phase 2 than in
phase 1.
Calculation of d0 s indicated that all baboons
correctly discriminated the two stimuli in phase 1,
from as early as the first test session (all d0 s were
significant for that phase after a Bonferroni correction). Most d0 s (i.e., 34/42) were also significant in
phase 2, considering the group or each individual
baboon, indicating that the baboons readily discriminated the normal face from its first-order version.
In fact, most of the nonreliable d0 s concerned the first
sessions of phase 2, before the baboons had learned
the discrimination.
Response times on correct trials were analyzed
following the same procedure as in experiment 1.
The group (upright, inverted) by phase (1, 2), by
block (1, 2, 3) ANOVA revealed reliable effects of
Procedure
Baboons tested in experiment 1 with upright
faces were now tested with inverted faces, and vice
versa. Although no group effect was obtained in
experiment 1, it was considered that changing
Fig. 3. Mean percentage of correct responses for each session and
with the normal (phase 1) and ‘‘first-order modified’’ faces
(phase 2) in experiment 2. Bars correspond to standard errors.
Am. J. Primatol.
420 / Parron and Fagot
both phase blocks; the former indicated longer mean
response times in phase 2 (mean 5 786 ms,
SD 5 140 ms) than in phase 1 (mean 5 1,030 ms,
SD 5 159 ms; F(1,5) 5 18.06, Po.01). This finding
confirmed accuracy results, suggesting that the face
stimuli were more easily discriminated in phase 1.
The main effect of block indicated decreasing mean
response times with repeated testing (mean block
1 5 990 ms, SD 5 219 ms; mean block 2 5 885 ms,
SD 5 207 ms; mean block 3 5 849 ms, SD 5 120 ms;
F(1,5) 5 13.41, Po.001). None of the interactions
reached significance, but the phase by block interaction
approached
significance
(F(2,10) 5 3.83,
P 5 .063). Inspection of response times suggests that
the effect of block was more pronounced in phase 2
(mean block 1 5 1,136 ms; mean block 2 5 1,053 ms;
mean block 3 5 900 ms) than in phase 1 (mean block
1 5 842 ms; mean block 2 5 717 ms; mean block
3 5 798 ms). This result is taken as additional
evidence that the processing of first-order configural
cues by baboons benefited from repeated testing.
Discussion
This experiment has shown that baboons are
capable of discriminating first-order relational properties within a face, even after only a few trials.
Comparison of experiments 1 and 2 moreover
suggests that first-order spatial properties are more
easily processed by baboons than second-order ones.
However, it remains to be demonstrated that the
different results in experiments 1 and 2 are not due
to an effect of learning or test order. To assess this
possibility, in experiment 3 all baboons were retested
in some of the same conditions as in experiment 1.
EXPERIMENT 3: ASSESSMENT
OF LEARNING EFFECT
Experiment 3 replicated the test condition of
experiment 1, block 1. It was expected that performance in this test would reach that of experiment 2
using first-order pairs, if baboons have learned to
process configural properties of the stimuli between
experiments 1 and 2. By contrast, absence of learning
effects between these two experiments should be
indicated in experiment 3 by chance level performance with these stimuli, as already obtained in
experiment 1.
Stimuli and Procedure
One block of trials was run for each individual in
this experiment. This block was strictly identical to
block 1 of experiment 1, and therefore used the
second-order faces shown in Figure 1a and b.
Results and Discussion
All the baboons were comfortably above chance
in phase 1, with the average performance ranging
Am. J. Primatol.
Fig. 4. Mean percentage of correct responses for each session,
with the ‘‘Thatcherized’’ faces (phases 2 of experiments 1 and 3).
Bars correspond to standard errors.
from 93 to 97% correct, pooling over all sessions.
Phase 2 data from the first block of experiment 2
(using a first-order modified face) were directly
compared with the data obtained in phase 2 of
experiment 3 (second-order modified face) by way of
an experiment (experiments 2, 3) session (1–6)
ANOVA. This analysis revealed a reliable effect of
the experiment, F(1,6) 5 11.07; Po.05, corresponding to a better performance in experiment 2
(mean 5 76.31% correct) than in experiment 3
(mean 5 62.5%). The ANOVA also revealed a significant effect of session, F(5,30) 5 2.62; Po.05, but
experiment by session interaction was also significant, as shown in Figure 4, (F(5,30) 5 3.49, Po.05).
Response times were not analyzed due to the large
number of errors limiting the data set size.
Calculation of d0 s indicated that all baboons
continued to fail to discriminate the original face
from its ‘‘Thatcherized’’ version in experiment 3 (all
d0 s below Bonferroni corrected significance level), in
spite of intervening training in experiment 2. This
final experiment therefore confirmed that baboons
remain more capable of discriminating first-order
facial properties than second-order properties, even
accounting for learning effects.
GENERAL DISCUSSION
Experiment 1 showed that baboons did not
experience the Thatcher illusion in a human-like
manner. First, unlike humans [Rhodes et al., 1993],
baboons behaved indistinctively when presented
with upright and inverted faces. Second, while
baboons readily discriminated the faces of two
different individuals, they had serious difficulties
discriminating normal faces from their ‘‘Thatcherized’’ versions, even when these faces were presented
upright. In experiment 1, the lack of accurate
discrimination in phase 2 cannot be due to a
Configural Processing by Baboons / 421
misunderstanding of the task, as the baboons were
successful in phase 1, which used the same general
procedure. Similarly, poor discrimination in phase 2
cannot be explained by a lack of expertise or
familiarity with the stimuli, as only pictures of
familiar human or baboon models were shown. In
humans at least, increased familiarity via prolonged
exposure to specific stimuli enhances the encoding of
second-order relational information [Diamond &
Carey, 1986]. Our use of familiar models should
have facilitated the processing of second-order relational properties, but this turned out to be very
difficult for the baboons. In experiment 1, poor
performance in phase 2 rather suggests that secondorder relational variations distinguishing the normal
face from its ‘‘Thatcherized’’ version were not
readily perceived by the baboons, which contrasts
with reports in the human literature [e.g., Lewis &
Johnston, 1997].
Experiment 2 provided additional information
on the processing of configural cues by baboons.
Accurate discrimination of normal faces from their
first-order transformations was impressive and rapid. The baboons’ failure again to discriminate
second-order pairs in experiment 3 rules out the
possible effects of learning as an explanation for
their discrimination of the first-order face pairs in
experiment 2. Furthermore, the similar amount of
pixels differing between the normal and the test
faces (Thatcherized in experiment 1 and first-order
transformed in experiment 2) rules out the possibility that discrimination of the first-order stimuli
reflects a quantitatively inequitable alteration of the
stimuli. Considered together, the results of experiments 1–3 strongly suggest that the baboons were
much more skillful at discriminating first-order
configural properties of the faces than their secondorder properties.
It could also be proposed that our procedure
facilitated a featural mode of processing in phase 1,
and that this processing strategy led to a poor
discrimination performance in phase 2 of experiment
1, when the two faces (i.e., the normal face and its
Thatcherized version) had all their features in common
but differed in configural second-order cues. Although
this cannot entirely be ruled out, also given the use of a
small set of very different images as stimuli, this does
not account for the high levels of performance in phase
2 of experiment 2. Indeed, use of a featural mode of
processing should have led to a poorer discrimination of
the faces in that phase, but this did not occur.
Our demonstration that nonhuman primates
considered first-order relational properties in facial
stimuli converges with earlier reports. Dittrich
[1990] trained longtailed macaques to recognize
schematic line drawings of a macaque face depicting
different expressions. He then tested post-learning
recognition using jumbled faces in which the firstorder properties of the facial features were disorga-
nized. Results revealed that recognition strongly
depended on the integrity of the first-order relational
properties. In another study, Myowa-Yamakoshi and
Tomonaga [2001] tested an infant gibbon’s discrimination of face-like and nonface-like drawings. The
stimuli consisted of two kinds of schematic faces, one
representing a normal configuration of a face and
another showing a first-order modified version in
which the configuration of the facial features was
disordered (one eye above the mouth, and the nose
above the other eye). The gibbon clearly preferred
looking at normal schematic faces. Similar results—
suggesting processing of first-order configural relations—were obtained in another experiment in the
same study in which a real pictorial face and its
scrambled first-order version were presented.
Parr et al. [2006] conducted three experiments on
the contribution of first- and second-order configural
cues in the processing of faces by chimpanzees. Their
first and third experiments manipulated second-order
cues by disaligning the upper or lower facial parts of
the stimuli (experiment 1) or by blurring the facial
features (experiment 3). Results suggested that
second-order cues at least partly controlled face
discrimination in these two tasks. Their second
experiment was even more relevant for the current
research because it assessed the respective contribution of first- and second-order cues during face
discrimination. In that experiment, the inner features
of a chimpanzee’s face were extracted and split into
several subparts, with gaps between them, to disrupt
the second-order relational properties of the faces. In
another condition, these same facial features were
moved apart and spatially rearranged, thus simultaneously disrupting first- and second-order relational
properties of the faces. The overall performance of the
chimpanzees was similar under these two conditions
(second-order manipulation: 56% correct, first plus
second-order manipulation: 59.2% correct).1 Lack of a
significant difference along with the generally poor
performance of the chimpanzees leave it unclear as to
whether the chimpanzees prioritize first- or secondorder configural cues.
Another interesting finding of this study is that
baboons tested with upright or inverted stimuli
provided identical results. It is tempting to propose
that the orientation of the face is not as important in
baboons as it is in humans. However, such an
interpretation is not compelling, because baboons
are quadrupedal, terrestrial primates, and conspecifics (or humans in the laboratory) are mostly likely
to be perceived in a human-like canonical upright
orientation. In addition, facial expressions are
1
Note that these means are different from those of Parr et al.
[2006; Table 1]. This difference reflects a mistake in Parr et al.’s
[2006] Table 1 in which the means of the ‘‘split’’ (second-order)
and ‘‘rearranged’’ (first-order) conditions were inverted.
Am. J. Primatol.
422 / Parron and Fagot
commonly used by baboons to convey social information [Altmann, 1980] and faces are thus ecologically
relevant stimuli for them. We suggest caution in
interpreting the lack of an inversion effect in our
task. First, absence of an effect leaves open the
possibility of a type II error. Second, our results were
collected using a between- rather than a withinsubjects design, and moreover with a relatively small
number of subjects, and therefore may have been
influenced by intersubject variability. The possible
effects of inversion require further testing.
In closing, we would like to emphasize that our
results do not imply that baboons are completely blind
to second-order information available in faces. The
processing of both first- and second-order configural
properties may serve adaptive functions in natural
settings. An analysis of first-order relational cues may
permit the animal to quickly discriminate facial from
nonfacial stimuli, and therefore potentially animals
from nonanimals. Second-order cues may help to refine
this initial rough analysis, and to individuate different
conspecifics, or to infer their emotional state. Following
this reasoning, we presume that baboons are capable of
processing second-order relational cues, even though
the ability has not been demonstrated here. This
assumption is supported by previous research by our
group showing that baboons can process second-order
relational properties contained in nonfacial stimuli if
the task forces them to do so [e.g., evaluate the distance
between a line and a dot; Dépy et al., 1998]. The main
interest of our findings, in this context, is that the
baboons seem to prioritize first-order relational properties in absence of extended training. This conclusion is
in line with the human developmental literature
showing that infants or children process first-order
cues well before second-order cues [e.g., Mondloch
et al., 2003; Valenza et al., 1996].
ACKNOWLEDGMENTS
The use and care of animals used in this
research was fully approved by the ‘‘Comité d’Ethique Régional de la Région Provence Alpes Côtes
d’Azur’’. All pictures employed for stimuli were used
with the written consent of the people concerned.
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