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Unique morphology of the human eye and its adaptive meaning: Comparative studies on external morphology of the primate eye


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In order to clarify the morphological uniqueness of the human eye and to obtain cues to understanding its adaptive significance, we compared the external morphology of the primate eye by measuring nearly half of all extant primate species. The results clearly showed exceptional features of the human eye: (1) the exposed white sclera is void of any pigmentation, (2) humans possess the largest ratio of exposed sclera in the eye outline, and (3) the eye outline is extraordinarily elongated in the horizontal direction. The close correlation of the parameters reflecting (2) and (3) with habitat type or body size of the species examined suggested that these two features are adaptations for extending the visual field by eyeball movement, especially in the horizontal direction. Comparison of eye coloration and facial coloration around the eye suggested that the dark coloration of exposed sclera of nonhuman primates is an adaptation to camouflage the gaze direction against other individuals and/or predators, and that the white sclera of the human eye is an adaptation to enhance the gaze signal. The uniqueness of human eye morphology among primates illustrates the remarkable difference between human and other primates in the ability to communicate using gaze signals.
Content may be subject to copyright.
Hiromi Kobayashi &
Shiro Kohshima
Biological Laboratory, Faculty
of Bioscience and
Biotechnology, Tokyo Institute
of Technology (c/o Faculty of
Science), 12-1, O-okayama
2-chome, Meguro-ku, Tokyo
152-8551, Japan. E-mail:
Received 30 October 1998
Revision received
29 January 2001
and accepted 5 February
Keywords:primates, eye
morphology, sclera colour,
communication, adaptation,
human evolution, theory of
Unique morphology of the human eye and
its adaptive meaning: comparative studies
on external morphology of the primate eye
In order to clarify the morphological uniqueness of the human eye
and to obtain cues to understanding its adaptive significance, we
compared the external morphology of the primate eye by measuring
nearly half of all extant primate species. The results clearly showed
exceptional features of the human eye: (1) the exposed white sclera is
void of any pigmentation, (2) humans possess the largest ratio of
exposed sclera in the eye outline, and (3) the eye outline is extraor-
dinarily elongated in the horizontal direction. The close correlation of
the parameters reflecting (2) and (3) with habitat type or body size of
the species examined suggested that these two features are adapta-
tions for extending the visual field by eyeball movement, especially in
the horizontal direction. Comparison of eye coloration and facial
coloration around the eye suggested that the dark coloration of
exposed sclera of nonhuman primates is an adaptation to camouflage
the gaze direction against other individuals and/or predators, and that
the white sclera of the human eye is an adaptation to enhance the gaze
signal. The uniqueness of human eye morphology among primates
illustrates the remarkable dierence between human and other
primates in the ability to communicate using gaze signals.
2001 Academic Press
Journal of Human Evolution (2001) 40, 419–435
Available online at on
Recognizing others’ gaze direction is one of
the important cognitive bases for communi-
cation in humans (Gibson & Pick, 1963;
Kendon, 1967). To clarify the biological
basis of this ability, especially in relation to
the evolution of social intelligence,
researchers have experimentally examined
the cognitive ability to detect gaze direction
of others in nonhuman primates (Gomez,
1991;Itakura & Anderson, 1996;Tomasello
et al., 1998). However, little attention has
been given to external morphology of the
eye, although this ability of humans might
be supported by a unique morphology of
the human eye. For example, in humans, the
widely exposed white sclera (the white of the
eye) surrounding the darker coloured iris
makes it easy for others to discern the gaze
direction and has been said to be a charac-
teristic of humans not found in other pri-
mate species (Morris, 1985). However, this
has not been examined in detail, partly
because of the diculty in measuring the
soft parts of living animals.
In this study, we measured the external
eye morphologies of nearly half of all extant
primate species with video camera and
computer-aided image analysing techniques
to clarify the morphological uniqueness of
the human eye and to understand adaptive
meanings of external eye morphology in
primates. The results clearly showed excep-
tional features of the human eye in both
Address correspondence to: Hiromi Kobayashi,
Ph.D., 6–8, Nakanoshima-cho, Fukakusa, Fushimi-ku,
Kyoto-city, Kyoto, 612-0049, Japan. Tel.: + 81 75 644
1402; Fax: + 81 75 644 1402.
0047–2484/01/050419+ 17$35.00/0 2001 Academic Press
shape and coloration. In our preceding
paper (Kobayashi & Kohshima, 1997), we
briey reported the morphological unique-
ness of the human eye and discussed its
adaptive meanings. In the present paper we
fully analysed the results and examined the
following hypotheses on adaptive meanings
of primate eye morphology.
We measured width/height ratio of the eye
outline (WHR) and an index of exposed
sclera size in the eye outline (SSI) to analyse
eye shape. These eye-shape parameters
closely correlated with habitat type or body
size of the species examined. To explain the
correlation, we postulated a hypothesis that
these two features are adaptations for
extending the visual eld by eyeball move-
ment, especially in the horizontal direction.
This hypothesis was examined and sup-
ported by analysing the eye movement of
video-recorded primates and comparing the
way that gaze direction changes among
species with various body sizes and habitat
To explain the unique coloration of the
human eye with its exposed white sclera void
of any pigmentation, we postulated a
hypothesis that only coloration of the human
eye is adapted to enhance the gaze signal
while eye coloration of other primates is
adapted to camouage the gaze direction
against other individuals and/or predators.
This hypothesis was examined and sup-
ported by analysing relationships among iris
coloration, sclera coloration and facial col-
oration around the eye. Our results sug-
gested that unique features of the human eye
started to evolve as adaptations to large body
size and terrestrial life and were completed
as a device for communication using gaze
Eye shape measurements
A total of 874 adult animals (88 species:
Prosimii; 10, Ceboidea; 26, Cercopithe-
coidea; 43, Hominoidea; 9) were studied
(Table 1). Facial images of 80 species were
recorded by video camera at the Japan
Monkey Centre. Facial images of eight
species (Microcebus (1), Loris tardigradus (2),
Perodicticus potto (1), Tarsius (1), Saguinus
imperator (1), Pithecia monachus (1), Cacajao
rubicundus (1), Cercopithecus hamlyni (1))
were collected from books (Itani & Uehara,
1986;Yoshino, 1994). For humans, facial
images of 244 Japanese, 347 Caucasian and
68 Afro-Caribbean adults were studied. 244
Japanese, 280 Caucasian and 2 Afro-
Caribbean images were recorded by video
camera, and 67 Caucasian and 66 Afro-
Caribbean images were collected from
books (Ohara, 1970;Gomi, 1994).
Two parameters were measured for each
species: the width/height ratio of the eye
outline (WHR) and an index of exposed
sclera size in the eye outline (SSI). Frontal
full-face images without obvious facial
expression of subjects were recorded by
video camera. These images were processed
and analysed on a Macintosh Quadra
840AV computer using the public domain
NIH Image program. For each image, (A)
the distance between the corners of the eye,
(B) the longest perpendicular line between
the upper and lower eyelid, (C) width of the
exposed eyeball, and (D) diameter of the iris
were measured (Figure 1). WHR means
(A)/(B) and SSI means (C)/(D). Data of
weight, crownrump length and habitat type
of primates were collected from books (Itani
& Uehara, 1986;Napier & Napier, 1985)
since we could not get permission for
physical contact with primates. Walking-
height and sitting-height of primates were
measured in the Japan Monkey Centre using
marks on the wall of the cages.
Eye-coloration measurements
Coloration of the exposed sclera (including
the conjunctiva to be precise), iris and face
around the eye was recorded for each of 91
species by direct observation of living
420 .  .
animals (82 species) and of eyeball speci-
mens (55 species, 124 animals) kept in the
Japan Monkey Centre (Table 1). The sclera
colour included in the term ‘‘Pale brown’’
was a paler one than yellow ochre: 10YR
6/7·5 of Munsell Colour System (see
Figure 9).
Eye coloration of 82 primate species were
classied into 4 types (see Type 14, Figure
11) by the dierences of colour or contrast
between sclera and iris/face. This classica-
tion was carried out by one person observing
living animals. To check reliability of this
classication, another person independently
classied the face pictures of 76 primate
species (see Figure 11). The results agreed
in 70 species (92%). Disagreement was only
observed between Type 1 and Type 2 in 6
species (8%).
Eyeball specimens of the Japanese
macaque (1 subject) and crab-eating
macaque (2 subjects) were supplied from a
co-operative program of the Primate
Research Institute, Kyoto University,
Inuyama, Aichi, Japan. Eyeballs were xed
with 4% paraformaldehyde in 0·1 M phos-
phate buer (pH 7·2) at 4C overnight. The
tissue including the conjunctiva and cornea
separated from eyeballs was washed several
times in cold phosphate-buered saline
(PBS), dehydrated in an ethanolic series
nishing xylene and embedded in paran.
Serial sections with a 4 m thickness were
cut with disposable blades, oated on water
and placed on slides. These sections were
deparanized in xylene, washed in ethanol
and PBS and studied by light microscopy
(see Figure 10).
Eye movement analysis
To analyse the movement of the eye and the
head when the animals change the direction
of their look, eating food by hand in cages
various primates were video-recorded. As
for humans, persons eating alone in a
hamburger shop with a hamburger in their
hands were video-recorded.
To calculate the ratio of scanning per-
formed only by eyeball movement, move-
ments of the eyeball and the head scanning
were counted (total observation time:
10,037 sec) for 29 individuals of 18 species
(Table 1).
To calculate the ratio of horizontal scan-
ning to vertical scanning frequency and
time duration of horizontal and vertical
scanning were measured (total observation
time=12,579 sec) for 40 individuals of 26
species (Table 1): arboreal species: Lemur
catta,Cebus apella,C. albifrons*, Pithecia
pithecia*, Ateles belzebuth*, A. georoyi,A.
paniscus,Cercocebus galeritus,Cercopithecus
cephus*, Colobus angolensis,Presbytis cristata,
P. vetulus,P. francoisi*, Nasalis larvatus,
Hylobates lar and H. pileatus; semi-arboreal
species: Macaca fuscata*, Cercocebus torqua-
tus,Mandrillus sphinx*, M. leucophaeus*,
Cercopithecus ascanius,Presbytis entellus and
Pan troglodytes; terrestrial species: Papio
hamadryas,Erythrocebus patas and Homo
sapiens (*: duration time only) (Table 1).
All these videotape analyses were carried
out by one person. To check reliability of the
analyses, a second person scored every
0·5 sec randomly sampled 20% of the video-
tapes independently. Agreement between
the persons was 82% on average. The
Cohens kappa (Bakeman & Gobbman,
1997) was 0·63 on average.
Results and discussion
Unique shape of the human eye
Figure 2(a) shows that human eyes have the
largest exposed sclera area and show
extraordinary horizontal elongation of the
eye outline among primates. SSI increased
in the following order: Prosimii (prim-
itive type) <Ceboidea<Cercopithecoidea<
Hominoidea. WHR increased in the follow-
ing order: Prosimii (primitive type)
<Ceboidea<Cercopithecoidea and Homi-
noidea [the dierence between phylogenic
groups was signicant with analysis of
421     
Table 1A
Eye shape
[Figure 2(a)]
Eye colour
(Figure 9) Eye movement
Living animals
(Figure 11)
specimens (Figure 5)
(Figure 7
(Figure 7
Microcebus 13
Lemur catta 33 111
L. macaco 222
L. fulvus 22
Varecia variegata 361
Loris tardigradus 25
Perodicticus potto 12
Galago senegalensis 321
Otolemur crassicaudatus 122
Tarsius 12
Callimico goeldii 121
Callithrix jacchus 332
C. argentata 22
C. humeralifer 22
G. georoyi 332
C. penicillata 21
Cebuella pygmaea 346
Saguinus midas 122
S. weddelli 11
S. imperator 111
S. labiatus 113
S. mystax 222
S. oedipus 143
Cebus capucinus 333
C. albifrons 1412
C. apella 23 111
C. nigrivittatus 1
Aotus trivirgatus 342
Callicebus moloch 252
Saimiri sciureus 2102
Pithecia pithecia 2211
P. monachus 1
Cacajao rubicundus 1
Alouatta caraya 116
Ateles paniscus 12 111
A. belzebuth 24 1
A. georoyi 352222
Lagothrix lagothricha 24
Macaca sylvanus 33
M. silenus 231
M. maurus 11
M. nemestrina 332
M. nigra 11
M. fascicularis 335
M. fuscata 671 1
M. fuscata yakui 332
M. mulatta 41817
422 .  .
Table 1B
Eye shape
[Figure 2(a)]
Eye colour
(Figure 9) Eye movement
Living animals
(Figure 11)
specimens (Figure 5)
(Figure 7
(Figure 7
M. cyclopis 510
M. sinica 341
M. assamensis 111
M. radiata 291
M. thibetana 34
M. arctoides 12
Cercocebus torquatus 22 222
C. torquatus lunulatus 23
C. atys 111
C. galeritus agilis 111111
C. galeritus chrysogaster 23
C. albigena 1
Papio papio 332
P. anubis 12 50 2
P. cynocephalus 13
P. hamadryas 391111
Mandrillus sphinx 34 1
M. leucophaeus 241 2
Theropithecus gelada 111
Cercopithecus hamlyni 11
C. neglectus 33
C. mitis 25
C. ascanius schmidti 22
C. cephus 22 2
C. ascanius 23 222
C. mona 221
C. petaurista 111
C. aethiops 25
Miopithecus talapoin 22
Allenopithecus nigroviridis 13
Erythrocebus patas 44 222
Colobus angolensis 56 111
C. guereza 22
Presbytis cristata 287444
P. francoisi 241 1
P. pileata 111
P. vetulus 22 111
P. entellus 472111
P. obscura 2
Nasalis larvatus 22 222
Hylobates lar 22 111
H. agilis 112
H. pileatus 22 111
H. syndactylus 22
H. klossii 1
Pongo pygmaeus 231
Pan troglodytes 9132333
P. paniscus 11
Gorilla gorilla 46
Homo sapiens 659 247 2 2 2
423     
variance (ANOVA) (SSI: F (3,90)=14·68,
P<0·01, WHR: F (3,90)=32·77, P<0·01).
In multiple comparison (LSD), the dier-
ence between phylogenic groups, except
WHR between Cercopithecoidea and
Hominoidea, was signicant (SSI: MSe=
0·02, P<0·01, WHR: MSe=0·04, P<0·01)].
Even in Hominoidea species, SSI and WHR
of human were exceptionally high [Figure
2(b): the dierence between phylogenic
groups was signicant with ANOVA (SSI: F
(4,675)=79·82, P<0·01, WHR: F (4,676)=
33·92, P<0·01). In multiple comparison
(LSD), the SSI dierence between species
of Hominoidea, except between orang-utan
and gorilla and between orang-utan and
chimpanzee, was signicant (MSe=0·02,
P<0·01). The WHR dierence between
human and others was signicant (MSe=
0·19, P<0·01). We measured both sexes of
three human races: Mongoloid, Caucasian
and Afro-Caribbean, and sexual and racial
dierence were slight in these parameters
relative to interspecies dierence.
It is possible that the dierence in these
eye shape parameters reect some dierence
in visual function and/or adaptation to
some environmental or physiological factor
such as the habitat and body size of the
species. Thus, we analysed relationships
between the eye shape parameters and these
Relationship between eye shape and visual
Primates are mammals with well developed
visual function. Many primate species have
(1) cone cells for colour vision, (2) the fovea
(a dense concentration of cones in the retina
focused on the centre of gaze) for a high
resolution visual image, (3) forward-facing
eyes for wide stereoscopic vision by both
eyes, and (4) a well developed postorbital
plate behind the eyes. It is possible that the
dierence in eye shape parameters among
phylogenic groups has some relationship
with these anatomical structures for well
developed visual functions. However, since
most primate groups except the prosimians
have all these anatomical structures, the
dierence in eye shape parameters among
phylogenic groups cannot be explained by
the dierence in these visual functions. For
example, some nocturnal prosimian species
are the only primates which lack both cone
cell and a fovea in their eyes (Wolin &
Massopust, 1970;Aleri et al., 1976;
Webb & Kass, 1976;Debruyn et al., 1980;
Castenholtz, 1984). Figure 3 shows the re-
lationship of eye shape parameters with
orbital axis angle measured from the
cranium (Shigehara, 1996) which reects
the ability of stereoscopic vision. In this
gure Prosimians with high values of orbital
axis angle showed low values of SSI and
WHR. However, among simians with more
forward-facing eyes and developed post-
orbital closure, SSI and WHR spread over
various values and no signicant correlation
is observed. These facts suggest that the
variation in the eye shape parameters
of each phylogenic group does not reect
evolutionary trends in these visual functions.
Relationship between body size and SSI
SSI correlates well with various body size
parameters (weight: r=0·59, P<0·001,
crownrump length: r=0·59, P<0·001,
sitting height: r=0·65, P<0·001, walking
height: r=0·72, P<0·001). The best
Figure 1. (A): the distinace between the corners of the
eye, (B): the longest perpendicular line to (A) between
the upper eyelid and lower eyelid, (C): width of
the exposed eyeball, and (D): diameter of the iris
were measured. WHR means (A)/(B) and SSI means
424 .  .
Figure 2. Variation of WHR (width/height ratio of the eye outline) and SSI (index of exposed sclera size
in the eye outline) among the phylogenic groups of primates (a), and in Hominoidea (b).
425     
correlation was observed with walking
height (Figure 4). This means that species
with larger body size have a larger exposed
scleral area.
A larger SSI means a smaller iris relative
to the eye outline and probably a greater
ability for visual eld extension by eyeball
movement; in eyes with a large SSI the small
iris has a wider space to move within the
open eye outline. In mammalian animals,
only primates have central foveae necessary
for ne vision. Therefore, eyeball and/or
head movement are important, especially for
primates, to adjust the images to the central
If we suppose that a larger exposed sclera
is an adaptation for extending the visual eld
by eyeball movement, the correlation
between SSI and body size can be explained
by the theory of scaling. This is because, as
body size becomes larger, visual eld exten-
sion by eyeball movement becomes more
eective than that by head or body move-
ment. This is so because as body height
becomes greater, the weight of the head and
body increases proportionally to the cube of
Figure 3. Orbital axis angle and WHR/SSI. Orbital axis angle is the angle formed when right and left
orbital axes meet. Orbital axis is the line between the lowest point of the optic canal and the centre of the
orbital width (Shigehara, 1996). Circles, prosimians; squares, simians.
426 .  .
body height. In contrast, the force required
for movement increases only with the square
of body height as it depends on the size of
the muscle cross-section. Moreover, since
the relative growth of the eyes to body height
is smaller than that of head and body size,
comparative eyeball size becomes smaller in
larger animals (Schultz, 1940). Thus, to
save energy when changing the direction of
gaze, a large-sized species would move the
eyeball more often than a small-sized
species, and have a larger exposed scleral
area. Besides, in small species with com-
paratively large eyeballs in a small skull,
space for muscles moving the eyeball may be
seriously limited.
To examine this hypothesis, we video-
recorded various primates (18 species, 29
individuals) eating food by hand in cages,
and counted the movements of head and
eyeball when they changed the direction in
which they were looking. The results
showed that the proportion of scanning per-
formed only by eyeball movement was cor-
related with SSI (Figure 5,r=0·73,
P<0·001). It was exceptionally high in
humans (6128% of horizontal scan, n=5)
compared with other primates (4·324·4%,
mean=10·6%). The highest proportion
in nonhuman primates was observed in
chimpanzees (2035%, n=3), the largest
nonhuman species examined. These results
agreed with our hypothesis.
Relationship between habitat type and WHR
The mean value of WHR is greatest in
terrestrial species, moderate in semi-
arboreal species and lowest in arboreal
Figure 4. SSI and walking height.
427     
species [Figure 6:Dierence among habitat
types was signicant with ANOVA and with
multiple comparison (ANOVA: F (2,127)=
24·63, P<0·01, LSD: MSe=0·052,
P<0·01)]. This result suggests that a hori-
zontally elongated eye outline is adaptive to
terrestrial life in some way. We speculated
that horizontal elongation of the eye outline
is adaptive in extending the visual eld hori-
zontally by eyeball movement, and terres-
trial life needs more horizontal scanning
than vertical scanning.
To examine this hypothesis, we observed
various primates eating food by hand in
cages and measured the time and frequency
of horizontal scanning and vertical scanning.
Figure 5. SSI and eye movement. EyeMove/TotalMove= frequency of gaze direction change only by eye
movement/frequency of all gaze direction change by head movement or/and eye movement.
Figure 6. WHR and habitat type.
428 .  .
The result shows that the ratio of horizontal
scanning to vertical scanning is higher in
terrestrial species than in arboreal species
[Figure 7(a): the dierence among habitat
types was signicant with ANOVA (time: F
(2,23)=14·5, P<0·01, frequency: F (2,15)=
4·53, P<0·05). The dierence between
terrestrial species and arboreal ones was
signicant in multiple comparison (LSD)
(time: MSe=20·76, P<0·01, frequency:
MSe=20·05, P<0·01)]. These ratios were
also correlated with WHR [Figure 7(b),
time: r=0·74, P<0·001, frequency: r=0·88,
P<0·001]. The results support our hypoth-
esis. These investigations suggest that the
shape of the eye outline and relative size of
the exposed scleral area are the result of an
adaptation for visual eld extension by
eyeball movement.
Unique coloration of the human eye
The colour of exposed sclera. The following
four colorations of exposed sclera were
observed (Figures 8 and 9); (a) in almost all
nonhuman primates (85 species out of 92
species, or 92%) the exposed part of the
sclera is uniformly brown or dark brown, (b)
Macaca sylvanus and M. nemestrina with a
pale brown body colour had sclera coloured
pale brown, (c) Saguinus midas,S. labiatus,
Callithrix argentata and Callimico goeldii had
brown sclera with a white part in corner of
the eye, (d) humans were the only primates
Figure 7. (a) Habitat type and the ratio of horizontal scanning to vertical scanning. (b) Ratio of horizontal
scanning to vertical scanning and WHR.
429     
having white sclera without any pigmenta-
tion. Microscopic analysis of the eyeball
section specimens from the Japanese
macaque and crab-eating macaque revealed
Figure 8. Three types of sclera coloration in nonhuman
Figure 9. Variation of scleral colour. The shaded portion of the nonhuman primate eyeballs shows the
general area where colour was noted. The solid line surrounding the cornea represents the eye outline.
Figure 10. Eyeball section of crab-eating macaque.
430 .  .
that the brown coloration of the exposed
sclera was due to pigmentation in the epi-
thelium cornea, conjunctiva and sclera
(Figure 10). External observations of other
nonhuman primate eyes suggest that their
dark coloration is also due to similar pig-
mentation. Humans have transparent
conjunctiva and white sclera without pig-
mentation. The inner part of the sclera in
nonhuman primates was also white like that
of humans.
Adaptive meaning of dark-coloured sclera.
Nonhuman primates have sclera coloured
brown. As pigmentation costs some energy,
the dark coloration of the exposed sclera
probably has some adaptive function. Brown
coloration of the exposed sclera was
observed in many other mammal species
too, and the following two hypotheses have
been proposed on the adaptive meanings of
sclera colour. (1) Anti-glare theory: it was
pointed out that the pigmentation may be an
anti-glare device because it seemed to be
absent in nocturnal or crepuscular species
(Duke-Elder, 1985). However, our results
on primate species were contrary to this
expectation: nocturnal species (Galago sen-
egalensis,Tarsius syrichta,Perodicticus potto,
Nycticebus coucang and Aotus trivirgatus) also
had coloured sclera and diurnal humans had
no pigmentation. Therefore, our results
cannot be explained by the ‘‘anti-glare
theory’’. (2) Gaze camouage theory: in
many nonhuman primates, gaze direction is
important in intraspecic communication
(Chance, 1962;van Hoo, 1962;Andrew,
1964). For example, direct eye contact is
associated with gestures predominantly
showing a tendency to attack in many
monkeys. In macaques, it is reported
that sclera pigmentation obscures gaze
direction and may be adaptive for escaping
the attacks of other individuals (Perrett &
Mistlin, 1990). Coloured sclera obscuring
gaze direction may serve to deceive
natural predators too, by making it dicult
for predators to know if the prey has
them in their gaze. If prey animals can
make it known to the predator that
they already know of its presence, their
chances of survival may increase (Sherman,
To examine the gaze camouage theory,
we analysed the relationship between sclera
colour, iris colour and face colour around
the eye. If the dark coloration of exposed
sclera is adaptation for gaze camouage, the
colour of exposed sclera should be similar to
the colour of iris and/or face around the
eyes, to make it dicult to detect the pos-
ition of iris in the eye outline and/or the eye
outline in the face.
Relationship between sclera colour, iris colour
and face colour
Figure 11 shows the relationship between
sclera colour, iris colour and face colour
around the eye. 82 primate species observed
were classied into the following four types
by the dierence of colour or contrast
between sclera and iris/face.
Type (1) faceYscleraYiris (43 species):
darkness of sclera colour is similar to that
of face and iris; both eye outline in the face
and iris position in the eye outline are
Type (2) face<scleraYiris (37 species):
sclera colour is darker than face colour but
sclera colour is similar to iris colour; eye
outline in the face is clear but iris position in
the eye outline is unclear.
Type (3) faceYsclera>iris (1 species,
rued lemur): sclera colour is darker than
iris colour but sclera colour is similar
to face colour; eye outline in the face is
unclear but iris position in the eye outline is
Type (4) face>sclera<iris (1 species, Homo
sapiens): sclera colour is paler than face
colour and iris colour; both eye outline in
the face and iris position in the eye outline
are clear.
431     
Figure 11. Colour patterns of sclera, iris and face skin around the eye.
Figure 12. Dierence of gaze stimulus between human and orang-utan.
Almost all nonhuman primate species
observed (80 out of 81 species) belonged to
Type 1 or Type 2 coloration. In these
coloration types, the position of the iris
in the eye outline was unclear because of
similarity between sclera colour and
iris colour (‘‘Gaze camouage type’’). In
Type 1 coloration (43 species), the position
of the eye outline in the face was also
unclear because of similarity between
sclera colour and face colour around the
eyes. In addition, the rued lemur (Varecia
variegata), the only species that had
Type 3 coloration, has a very small exposed
sclera area (SSI=1·08) and almost all the
area of its eye outline is occupied by
the iris. Thus, the rued lemurs eye also
can be seen as a ‘‘gaze camouage
type’’. The results thus support the ‘‘gaze
camouage theory’’.
In contrast, humans were the only species
that had Type 4 coloration, in which both
eye outline in the face and iris position in
the eye outline were very clear because the
colour of the exposed sclera is paler than
that of the facial skin and iris (‘‘gaze
signalling type’’). The human was the
only species with sclera much paler than
the facial skin. Because of this coloration,
it is very easy to discern the gaze direction
in human, in contrast to the gaze-
camouaging eyes of the other primates.
Figure 12 shows the contrast between sclera
colour and face/iris colour of human and
orang-utan (Pongo pygmaeus). In this gure,
darkness of colours was shown by 256 steps
grey scale number (white=0, black=255,
Figure 12). Even great apes that have SSI
and WHR near that of humans [Figure
2(b)] had ‘‘gaze camouage eye’’ type with
brown sclera, brown facial skin and brown
iris, in which the eye position and iris pos-
ition is unclear. In contrast to the gaze-
camouaging eyes of orang-utan, the human
sclera is remarkably paler than the facial skin
and the iris, and it is very easy to discern the
gaze direction.
Evolution of unique morphology of the human
Our results suggest that the unique shape of
human eye (largest SSI and WHR in pri-
mates) is a result of adaptations to extend
the visual eld by eyeball movement,
especially in the horizontal direction. In
human evolution, the ratio of exposed sclera
in the eye outline might increase because the
visual eld extension by eyeball movement
became more eective as body size
increased. And the eye outline might be
horizontally elongated because the terrestrial
life of humans needed more horizontal
scanning than vertical scanning by eyeball
Why have only humans discarded sclera
pigmentation? It may be because the neces-
sity for gaze camouage decreased and that
of gaze-signal enhancement increased in
human evolution. The predation risk might
decrease because of the enlarged body size
and the use of tools and re. Gaze-signal
enhancement might aid the conspecic
communication required for increased
co-operative and mutualistic behaviours to
allow group hunting and scavenging.
Co-operative and mutualistic behaviours
might need rened communication systems,
such as language, to inform ones intention
to other members of the group. The human
eye, with a large scleral area surrounding the
iris and a great ability of eyeball movement,
would have provided a chance for a drastic
change from ‘‘the gaze-camouaging eyes’’
into ‘‘the gaze-signalling eyes’’ through a
small change in scleral coloration. The SSI
and WHR of human eyes are even greater
than those of gorillas, the largest primate,
which suggests adaptation for gaze-signal
Eye morphology and gaze-signal
Baron-Cohen (1995) postulated a neural
mechanism (eye direction detector) in
human brains specialized to detect others
433     
eye direction, and discussed the possibility
that such a mechanism might be related with
the evolution of a ‘‘theory of mind’’.In
recent years, many studies have been carried
out to examine the cognitive ability to detect
othersgaze direction in various nonhuman
primates. The studies have suggested the
limited ability to detect othersgaze direc-
tion in nonhuman primates (Itakura &
Anderson, 1996;Tomasello et al., 1998).
However, there seems to be some confusion
in dening ‘‘gaze direction’’ in these studies.
Since gaze direction can be changed by
eyeball movement, by head movement and
by body movement, it should be dened by
considering all those factors: eyeball direc-
tion, head direction and body direction.
Most of these studies, however, dened
‘‘gaze direction’’ mainly by eyeball direction.
Our results (Figure 5) suggest that the con-
tribution of eyeball movement to the change
in gaze direction is extremely high in
humans compared with other primate
species. The gaze signalling eye coloration of
humans also suggests that the contribution
of eyeball direction to the gaze signal is
exceptionally high in humans. Therefore, for
nonhuman primates, head direction and
body direction might be more important to
detect othersgaze direction than eyeball
direction. We should pay more attention to
head direction and body direction in future
analyses of gaze-signals in nonhuman
Comparative analysis of the external mor-
phology of the primate eye revealed excep-
tional features of the human eye: (1) the
exposed white sclera is void of any pigmen-
tation, (2) humans possess the largest ratio
of exposed sclera in the eye outline, and (3)
the eye outline is extraordinarily elongated
in the horizontal direction. The close corre-
lation of the parameters reecting (2) and
(3) with habitat type or body size of the
species examined suggested that these two
features are adaptations for extending
the visual eld by eyeball movement,
especially in the horizontal direction.
Comparison of eye coloration and facial
coloration around the eye suggested that
the dark coloration of exposed sclera of
nonhuman primates is an adaptation to
camouage the gaze direction against other
individuals and/or predators, and that the
white sclera of humans is an adaptation to
enhance the gaze signal.
We would like to thank Shigetaka Kodera
for his interest in our study and for
allowing us to observe the animals of the
Japan Monkey Centre. It is a pleasure
to acknowledge the hospitality and encour-
agement of the members of JMC. We
wish to express our gratitude to Tetsuro
Matsuzawa, Takashi Kageyama and
Nobuo Shigehara for stimulating discussion
under the co-operative research programme
in Primate Research Institute, Kyoto
University, and to Manabu Ogiso,
Nobuyuki Saitoh and Teruhiko Hamanaka
for providing eyeball samples. We are
indebted to a number of our colleagues at
the Tokyo Institute of Technology and
the Primate Research Institute, Kyoto
University, especially to Mitsue Nomura,
Michael A. Human, Sou Kanazawa,
Masami Yamaguchi and Kazuhide
Hashiya, for their constructive criticism on
this paper.
Aleri, R., Pariente, G. & Sole, P. (1976). Dynamic
electroretinography in monochromatic lights and
uorescence electroretinography in lemurs. Doc.
Ophthal. Proc. Ser. 10, 169178.
Andrew, R. J. (1964). The displays in the Primates. In
(J. Buiettner-Janusch, Ed.) Evolutionary and Genetic
Biology of the Primates, vol. 2. New York: Academic
434 .  .
Bakeman, R. & Gottman, M. (1997). Observing inter-
action: an introduction to sequential analysis (2nd ed.).
Cambridge: Cambridge University Press.
Baron-Cohen, S. (1995). Mindblindness: An essay on
autism and theory of mind. Cambridge: MIT Press.
Castenholtz, A. (1984). The eye of Tarsius.In
(C. Niemitz, Ed.) Biology of Tarsiers, pp. 303318.
Stuttgart: Gustav Fischer.
Chance, M. R. A. (1962). An interpretation of some
agonistic postures: the role of ‘‘cut-o’’ acts and
postures. Symp. Zool. Soc. Lond. 8, 8189.
Debruyn, E. J., Wise, V. L. & Casagrande, V. A.
(1980). The size and topographic arrangement of
retinal ganglion cells in the Galago. Vision Res. 20,
Duke-Elder, S. S. (1985). The eye in evolution. In
(S. S. Duke-Elder, Ed.) System of Ophthalmology,p.
453. London: Henry Kimpton.
Gibson, J. J. & Pick, A. D. (1963). Perception of
another persons looking behaviour. Am. J. Psychol.
76, 386394.
Gomez, J. C. (1991). Visual behaviour as a window for
reading the mind of others in primates. In (A.
Whiten, Ed.) Natural Theories of Mind, pp. 195207.
Oxford: Blackwell.
Gomi, A. (1994). Americans 1.0 1994 Los Angeles.
Tokyo: Fuga Shobo.
Itakura, S. & Anderson, L. R. (1996). Learning to use
experimenter-given cues during an object-choice task
by a capuchin monkey. Cahiers De Psychologie
Cognitive-Current Psychology of Cognition 15, 103
Itani, J. & Uehara, S. (1986). Primates. In (D. W.
Macdonald, Ed.) The Encyclopaedia of Animals,vol.
3. Tokyo: Heibonsha Limited.
Kobayashi, H. & Kohshima, S. (1997). Unique mor-
phology of the human eye. Nature 387, 767768.
Morris, D. (1985). Body Watching. Oxford: Equinox
Napier, J. R. & Napier, P. H. (1985). The Natural
History of the Primates. Cambridge, MA: MIT Press.
Ohara, K. (1970). One. Tokyo: Tsukui Shokan.
Perrett, D. I. & Mistlin, A. J. (1990). Perception of
facial characteristics by monkeys. In (W. C. Stebbins
& M. A. Berkley, Eds) Comparative Perception–
complex signals, pp. 187215. New York: John Wiley
& Sons, Inc.
Schultz, A. H. (1940). The size of the orbit and of the
eye in primates. AJPA Old Series 26, 389408.
Sherman, P. W. (1977). Nepotism and the evolution of
alarm call. Science 197, 12461253.
Shigehara, N. (1996). Metrical study of the direction of
the orbits in primates. Primates Res. 12, 165178.
Tomasello, M., Call, J. & Hare, B. (1998). Five pri-
mate species follow the visual gaze of conspecics.
Animal Behaviour 55, 10631069.
Van Hoo, J. A. R. A. M. (1962). Facial expressions in
the higher primates. Symposia of the Zool. Soc. Lond.
8, 97125.
Webb, S. V. & Kaas, J. H. (1976). The size and
distribution of ganglion cells in the retina of the owl
monkey, Aotus trivirgatus. Vision Res. 16, 12471254.
Wolin, L. R. & Massopust, L. C. Jr (1970). Mor-
phology of the primate retina. In (C. R. Noback & W.
Montagna, Eds) The Primate Brain, pp. 127. New
York: Appleton Century Crofts.
Yoshino, S. (1994). Animal Face. Tokyo: Nikkei
435     
... Both our white sclera and complex social cognition have long been considered unique among mammals, strengthening the hypotheses that these traits are linkeddi.e. cooperative eye hypothesis (Tomasello et al., 2007), gaze signaling, and gaze camouflage hypotheses (Kobayashi and Kohshima, 2001). However, the last two decades have revealed that our closest ape relatives, especially chimpanzees, exhibit complex perspectivetaking abilities (Hare et al., 2001(Hare et al., , 2006Krupenye et al., 2016) and that there may be greater variability in the eye color of apes and other primates than previously thought (Perea-García et al., 2022). ...
... In a preliminary, broader comparison across species, we found that in 19 of 70 mammal species, at least one individual exhibited white sclera (SOM Table S2). Contrary to the findings of Kobayashi and Kohshima (2001) that of 88 primate species, only humans exhibited white rather than pigmented 'brown' sclera, we found that several primate species exhibited white sclera, including callitrichine monkeys, Plecturocebus donacophilus, T. gelada, P. hamadryas, Trachypithecus sp., and E. collaris (Fig. 4), which is further supported by two recent studies (Kano, 2022;Perea-García et al., 2022). The discrepancy in the presence of white sclera in these species by Kobayashi and Kohshima and our study may be attributed to random sampling error due to small samples sizes. ...
... The iris may be conspicuous against dark as well as white sclera (Pereia-García et al., 2019;Caspar et al., 2021), but uniformly white sclera may enhance gaze detection, particularly at greater distances and low light (Yorzinski et al., 2021;Kano et al., 2021Kano et al., , 2022. Additional comparative studies of eye coloration across species are key for assessing hypotheses about the evolutionary processes influencing sclera and iris pigmentation and contrast (Emery, 2000;Kobayashi and Kohshima, 2001;Tomasello et al., 2007). The additional studies of gaze signaling, including the influence of eye color and morphology (Yorzinski et al., 2021) and whether conspecifics or other species vary in their ability to perceive gaze (Whitham et al., 2022a,b), will enable testing these hypotheses. ...
... It is also important to point out that the evolution of scleral appearance may be guided by various other factors aside from self-domestication, including communicative demands that may relate to irido-scleral contrast as well as the need for efficient photoprotection of the external eye [3,4,6,12]. Due to scaling effects and energetic constraints, larger-bodied mammals expose larger portions of the bulbar conjunctiva during glancing and rely on movements of the eyeball rather than the head to visually scan their surroundings [10]. Therefore, large species with typically bigger eyes [28] might be expected to show stronger pigmentation than small-bodied ones to more effectively protect their ocular epithelia from UV radiation, regardless of their domestication status. ...
... Corneal stem cells located at the limbus are particularly vulnerable to UV radiation and are likely to benefit from melanin shielding [3], as is the conjunctival epithelium itself. Data on primates suggest that the degree of habitual eye ball rotation, and thus radiation exposure of the conjunctiva, correlates positively with body size ( [10]; and thereby also eye size - [38]). If this pattern is applicable to mammals in general (as anecdotal observations might suggest, compare [47]), it would fit well with our preliminary finding that darker conjunctivae are characteristic of large-eyed species. ...
Full-text available
Numerous hypotheses try to explain the unusual appearance of the human eye with its bright sclera and transparent conjunctiva and how it could have evolved from a dark-eyed phenotype, as is present in many non-human primates. Recently, it has been argued that pigmentation defects induced by self-domestication may have led to bright-eyed ocular phenotypes in humans and some other primate lineages, such as marmosets. However, it has never been systematically studied whether actual domesticated mammals consistently deviate from wild mammals in regard to their conjunctival pigmentation and if this trait might therefore be part of a domestication syndrome. Here, we test this idea by drawing phylogenetically informed comparisons from a photographic dataset spanning 13 domesticated mammal species and their closest living wild relatives. We did not recover significant differences in scleral appearance or irido-scleral contrast between domesticated and wild forms, suggesting that conjunctival depigmentation, unlike cutaneous pigmentation disorders, is not a general correlate of domestication. Regardless of their domestication status, macroscopically depigmented conjunctivae were observed in carnivorans and lagomorphs, whereas ungulates generally displayed darker eyes. Based on our dataset, we also present preliminary evidence for a general increase of conjunctival pigmentation with eye size in mammals. Our findings suggest that conjunctival depigmentation in humans is not a byproduct of self-domestication, even if we assume that our species has undergone such a process in its recent evolutionary history.
... Indeed, the occlusion of the eyelid varies naturally according to the eccentricity of the gaze, in particular for gazes directed downward (which explains why Figure 3C has more wide-open eyes than Figure 3D). We intentionally kept this variable to make the stimuli more realistic, but since the visibility of the sclera is supposed to be a social cue indicating the position of the eye (Kobayashi and Kohshima, 2001), this factor could have affected participants' social interpretation ability. In addition, how wide the eyes are open could be an indicator of vigilance, adding to the importance of studying this factor in future studies. ...
... The observations could also be explained by the fact that high eccentricity gaze aversions would be used as an implicit, but conspicuous, signal of communication. Interestingly, the morphology of the human eye presents one of the proportionally largest areas of white sclera among all animals, which emphasizes the relevance of the sclera as a social signal since it indicates distinctly the position of the gaze (Kobayashi and Kohshima, 2001). Looking up is especially social since it makes the sclera clearly visible. ...
Full-text available
Gaze direction is a powerful social cue that indicates the direction of attention and can be used to decode others’ mental states. When an individual looks at an external object, inferring where their attention is focused from their gaze direction is easy. But when people are immersed in memories, their attention is oriented towards their inner world. Is there any specific gaze direction in this situation, and if so, which one? While trying to remember, a common behavior is gaze aversion, which has mostly been reported as an upward-directed gaze. Our primary aim was to evaluate whether gaze direction plays a role in the inference of the orientation of attention—i.e., external vs. internal—in particular, whether an upward direction is considered as an indicator of attention towards the internal world. Our secondary objective was to explore whether different gaze directions are consistently attributed to different types of internal mental states and, more specifically, memory states (autobiographical or semantic memory retrieval, or working memory). Gaze aversion is assumed to play a role in perceptual decoupling, which is supposed to support internal attention. We therefore also tested whether internal attention was associated with high gaze eccentricity because the mismatch between head and eye direction alters visual acuity. We conducted two large-sample (160–163 participants) online experiments. Participants were asked to choose which mental state—among different internal and external attentional states—they would attribute to faces with gazes oriented in different directions. Participants significantly associated internal attention with an upward-averted gaze across experiments, while external attention was mostly associated with a gaze remaining on the horizontal axis. This shows that gaze direction is robustly used by observers to infer others’ mental states. Unexpectedly, internal attentional states were not more associated with gaze eccentricity at high (30°) than low (10°) eccentricity and we found that autobiographical memory retrieval, but not the other memory states, was highly associated with 10° downward gaze. This reveals the possible existence of different types of gaze aversion for different types of memories and opens new perspectives.
... The curvilinear zone where the cornea and sclera fuse is the limbus 3 . Aside from being a zone of rupture of the curvature of the eyeshell, the limbus separates the very different tonalities of the glittering cornea 15 and of the colored iris seen through the cornea from the white, opaque sclera [15][16][17] . When SS and PLF occur on the shaded side of the eyeball, they bring the limbus out of the shadows, thereby reconstructing a rough, basic sense of eyeball modelling (Fig. 1, panel d). ...
... In humans, the very wide palpebral fissure 16,17 , the rearward-set temporal orbital margin 21,22 and the distinctly forward position of the eyeball in the orbit 23 leave the eyeshell (cornea and sclera) widely exposed to light, even when the eyeball is moved inwards or outwards. Direct or scattered daylight (albedo) 6,8 , results in almost infinite directions of light rays. ...
Full-text available
We aimed at searching in color art paintings through all art periods from Ancient Egyptian Art to Contemporary Art for sclerotic scatter (SS) and peripheral light focusing (PLF), using large-scale internet databases. These phenomena originate from the illuminated side of the eyeshell: limbal sclera for SS or cornea for PLF. They end at the darkness of the opposite limbal scleral side, where they are either scattered (SS) or concentrated (PLF) forming a clear arc or spot of light, respectively ("light through darkness"). We found SS and PLF in 83 and 2 paintings respectively out of 113,962 paintings analyzed. These forms of objective chiaroscuro help to reconstruct a basic sense of eyeball modelling by bringing the limbus out of the shadows. Among the paintings with SS, most were painted during the following periods: Renaissance (44), Baroque (15), Rococo (11). Sixty-one of the paintings with SS have a dark background compatible with the fact that SS is more easily observed under low ambient light illuminance ("light through darkness"). Acknowledgment of SS and PLF as subtle, rare characteristics of paintings should help see them in a new light, contribute to their proper treatment and restoration and is likely to enhance their economic value. [Supplemental materials:]
... Por otro lado, los ojos humanos tienen características únicas que los distinguen de otros animales como los chimpancés. A diferencia de estos, la esclerótica en los ojos humanos (membrana externa del globo ocular) es blanca, lo que permite a la especie conocer que capta la atención de otras personas a través de la dirección de la mirada y saber su estado emocional en situaciones sociales (Kobayashi & Kohshima, 2001;Morris, 2009). Entre las emociones que los ojos y las cejas expresan, se encuentran el miedo, la sorpresa y la felicidad (Ekman, 2003). ...
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Tradicionalmente, las teorías sociales han explicado el atractivo facial como una característica cultural aprendida a través de los procesos de socialización. Sin embargo, la Psicología Evolucionista ha propuesto que hay rasgos del atractivo físico que son universales y que han sido influenciados por la selección natural y sexual, ya que indican la calidad reproductiva y de salud de quien los posee. La investigación en torno a las características faciales y sus implicaciones sociales es amplia. Este texto tiene como objetivo resumir tales esfuerzos desde una perspectiva evolucionista y explorar sus implicaciones sociales. Además, se brinda una síntesis de la evolución facial, se examinan los mecanismos cerebrales implicados en la percepción del atractivo facial y se discuten las teorías de la Psicología Evolucionista sobre los rasgos que hacen a una persona atractiva y por qué. Por último, el texto concluye con una serie de predicciones sobre futuros estudios del atractivo facial y comparte una serie de bases de rostros que pueden ser utilizadas para la investigación y el desarrollo de tesis tanto de pregrado como de posgrado. Abstract Traditionally, social theories have explained facial attractiveness as a characteristic culturally learned through socialization processes. However, Evolutionary Psychology has proposed that there are universal traits of physical attractiveness influenced by natural and sexual selection, as they indicate the reproductive and health quality of those who possess them. A vast number of studies have analysed facial traits and their social implications. This paper aims to summarize these efforts from an evolutionary perspective and explore their social implications. Furthermore, a synthesis of facial evolution is provided, the brain mechanisms involved in facial attractiveness perception are examined, and some explanations of evolutionary psychology on the traits that make someone attractive and why are discussed. Finally, this review concludes with a series of predictions about the study of facial attractiveness and shares a series of face databases that can be used for research and for the development of both undergraduate and graduate theses.
... At the domain level (n = 28 domains, Bonferroni P value threshold < 1.7 x 10 -3 ; Figure 2), SNPs related to 'Ophthalmological' phenotypes presented the most recent evolutionary age (n = 823 SNPs, median evolutionary age = 30,484 years old), younger than expected by random chance (P = 2.1 x 10 -75 ; null-distribution of equally sized random sets out of all human-phenotypic SNPs, details in Methods; sensitivity analysis controlling for LD in SI Appendix). This domain of traits is widely noted to have been under strong adaptive changes in human evolution (17) and argued to include several distinctive features between Homo sapiens and Neanderthals (18). ...
Full-text available
Humans have undergone several anatomical adaptations throughout evolution. Paleontological records are a prime method of studying these adaptations, but they can unfortunately provide only a limited view of how modifications of 'soft traits' such as brain and cognition have contributed to the emergence of Homo sapiens. An additional approach includes the examination of when genetic variations associated with human phenotypes emerged in our history. Combining data from genome-wide association studies (GWAS) with dating data on the human genome, we systematically analysed the temporal emergence of single-nucleotide polymorphisms (SNPs) associated with modern-day human phenotypes over the last five million years. We show the genetic timeline of human-characteristic phenotypes to follow a distinct pattern with two bursts of genetic variation that co-emerge with milestones in the human lineage. Our findings suggest that SNPs associated with neocortical, neuropsychiatric, and ophthalmological traits appeared relatively recently in hominin evolution, with genes containing recently emerged SNPs linked to intelligence and neocortical area.
... la partie blanche) et sa surface globale. La différence de contraste entre sclère et iris permet dès lors très facilement une bonne discrimination de la direction du regard par autrui, même à distance (cf. Figure 5 ; Kobayashi & Kohshima, 2001 Kleinke, 1986). Parmi toutes les directions de regard possibles, celle qui est dirigée vers les yeux d'un autre individu et qui aboutit à une situation de regard mutuel ou contact par le regard, i.e. le regard direct, est un signal visuel saillant pour la plupart des mammifères. ...
La maladie d’Alzheimer (MA) est actuellement considérée comme un enjeu majeur de santé publique. Face à la stagnation des résultats issus des approches médicamenteuses, le développement et la validation de stratégies de prise en charge non médicamenteuses sont aujourd’hui particulièrement encouragées. Les odeurs et le regard direct (i.e. un regard dirigé vers soi qui aboutit à une situation de contact par le regard) sont deux indices contextuels connus pour avoir des influences bénéfiques communes sur la cognition normale. L’objectif de ces travaux de thèse est de déterminer si ces influences sont préservées dans le vieillissement normal et dans la MA débutante. Nous nous sommes attachées à déterminer notamment si, dans ces deux populations: i) le regard direct induit une évaluation plus positive d’autrui et améliore la mémoire des visages ainsi que des associations prénoms-visages, ii) les odeurs sont des indices pertinents pour stimuler la mémoire autobiographique, par rapport à d’autres indices sensoriels, iii) un effet cumulatif des odeurs et du regard direct sur l’évaluation d’autrui ainsi que sur la mémoire des visages peut être observé (données de ce dernier axe finalisées uniquement dans la population adulte jeune à ce jour). Nos travaux explorent ces questions à travers quatre études comportementales, dont une intégrant également des données d’oculométrie. Dans leur ensemble, nos résultats indiquent que les effets du regard direct sont préservés dans le vieillissement normale et la MA débutante: la perception d’un regard direct influence positivement l’évaluation d’autrui (étude 1), peut augmenter la mémoire des visages et la mémoire des prénoms (sans toutefois augmenter la mémoire de l’association visage-prénom – étude 2). Par ailleurs, dans ces populations, les stimuli olfactifs et visuels peuvent être considérés des outils de stimulation de la mémoire autobiographique plus pertinents que les stimuli auditifs (étude 3). Enfin, des données préliminaires de nature comportementale suggèrent une prédominance des effets des odeurs sur ceux du regard direct sur le plan de l’évaluation d’autrui chez les sujets jeunes (étude 4). La partie conclusive de cette thèse ouvre une réflexion sur les stratégies d’utilisation de ces indices dans un contexte clinique de stimulation cognitive des patients MA aux premiers stades de la maladie.
In this chapter, the nativist alternative to theory-theory is introduced and discussed. So-called modularity-theories appeal to innate domain-specific mechanisms, sometimes called modules, to explain cognitive development in certain specific domains. These modules contrast with domain-general learning mechanisms also endorsed by empiricist theory-theorists. We discuss this group of theories by introducing two representative accounts. First, Susan Carey and Elisabeth Spelke distinguish several basic mechanisms of core knowledge which process information about physical objects, numbers, agents and other domains. Second, Simon Baron-Cohen uses empirical evidence to support his distinction of several specific mechanisms supporting social cognition, each of them producing and processing representations of diverging complexity, for example, information about eye gaze, or about triadic relations between two agents and an object, culminating in a theory-of-mind module specialized for mindreading. Such nativists sometimes appeal to empirical evidence about early implicit false-belief understanding and about selected impairments in social understanding in autistic subjects.
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Homogeneously depigmented sclerae have long been proposed to be uniquely human—an adaptation to enable cooperative behaviour by facilitating interpersonal coordination through gaze following. However, recent evidence has shown that deeply pigmented sclerae also afford gaze following if surrounding a bright iris. Furthermore, while current scleral depigmentation is clearly adaptive in modern humans, it is less clear how the evolutionarily intermediate stages of scleral pigmentation may have been adaptive. In sum, it is unclear why scleral depigmentation became the norm in humans, while not so in sister species like chimpanzees, or why some extant species display intermediate degrees of pigmentation (as our ancestors presumably did at some point). We created realistic facial images of 20 individually distinct hominins with diverse facial morphologies, each face in the (i) humanlike bright sclera and (ii) generalised apelike dark sclera version. Participants in two online studies rated the bright-sclera hominins as younger, healthier, more attractive and trustworthy, but less aggressive than the dark-sclera hominins. Our results support the idea that the appearance of more depigmented sclerae promoted perceived traits that fostered trust, increasing fitness for those individuals and resulting in depigmentation as a fixed trait in extant humans.
We propose an anthropomorphization framework that determines an object’s body image. This framework directly intervenes and anthropomorphizes objects in ubiquitous-computing environments through robotic body parts shaped like those of human beings, which provide information through spoken directions and body language. Our purpose is to demonstrate that an object acquires subjective representations through anthropomorphization. Using this framework, people can more fully understand instructions given by an object. We designed an anthropomorphization framework that changes the body image by attaching body parts. We also conducted experiments to evaluate this framework. Results indicate that the site at which an anthropomorphization device is attached influences human perception of the object’s virtual body image, and participants in experiments understood several instructions given by the object more clearly. Results also indicate that participants better intuited their devices’ instructions and movement in ubiquitous-computing environments.
By means of electroretinographic techniques, using Lemur mongoz, we have been able to demonstrate: on the one hand, the existence of a dual system of cones and rods in the retina (we found an α-point during a dynamic electroretinogram made with a yellow monochromatic light); on the other hand, a possible fluorescent effect (we obtained a response with ultra-violet photostimulation) which can explain why these animals have an excellent vision under scotopic conditions. They may indeed possess a tapetum lucidum with fluorescent riboflavine.
The direction of the orbits in mammals, especially in primates, was examined to explain orbital convergence in primates. The orbital axes of old world monkeys are between 40°∼50°, while those of new world monkeys exceed 50°. The orbital axes tend. to be even larger in callithricids. In Prosimians, the axis angle ranges from 60°∼100°, and is clearly larger than those of the anthropoids. The orbital axis angle of carnivores is between those of prosimians and anthropoids. However, their orbital planes have not turned to the front, because the olfactory sense is also important for them. Ungulates have large orbital axis angles over 100°. It is clear that arboreality is possible even if the orbit has not turned to the front as it is in anthropoids, because tree shrews or squirrels do not have orbits rotated to the front as in anthropoids. Carnivores, although they are terrestrial mammals, have orbital axis angle as small as in primates. As a result, the frontal rotation of the orbit was not caused simply by the adaptation to arboreal life, supporting the visual predation hypothesis advocated by Cartmill (1972).