Attractiveness in the Eyes: A Possibility of
Positive Loop between Transient Pupil
Constriction and Facial Attraction
Hsin-I Liao1, Makio Kashino1,2, and Shinsuke Shimojo3
Abstrakt
■ Contrary to the long-held belief of a close linkage between
pupil dilation and attractiveness, we found an early and tran-
sient pupil constriction response when participants viewed an
attractive face (and the effect of luminance/contrast was con-
trolled). While human participants were making an attractive-
ness judgment on faces, their pupil constricted more for the
more attractive (as-to-be-rated) faces. Further experiments
showed that the effect of pupil constriction to attractiveness
judgment extended to intrinsically esthetic visual objects such
as natural scene images (as well as faces) but not to line-
drawing geometric figures. When participants were asked to
judge the roundness of faces, pupil constriction still correlated
with their attractiveness but not the roundness rating score, In-
dicating the automaticity of the pupil constriction to attractive-
ness. When pupillary responses were manipulated implicitly by
relative background luminance changes (from the prestimulus
screen), the facial attractiveness ratings were in accordance with
the amount of pupil constriction, which could not be explained
solely by simultaneous or sequential luminance contrast. Der
overall results suggest that pupil constriction not only reflects
Aber, as a part of self-monitoring and attribution mechanisms,
also possibly contributes to facial attractiveness implicitly. ■
EINFÜHRUNG
Pupillary response reflects not only the peripheral nervous
system’s activity in response to ambient luminance changes
(d.h., the pupillary light reflex) but also the central nervous
system’s activity underlying cognitive functions such as
attention (Eldar, Cohen, & Niv, 2013; Einhäuser, Stout,
Koch, & Fuhrmann, 2008; Aston-Jones & Cohen, 2005),
Erinnerung (Zokaei, Board, Manohar, & Nobre, 2019; Naber,
Frässle, Rutishauser, & Einhäuser, 2013; Goldinger &
Papesh, 2012), decision-making (de Gee, Knapen, &
Donner, 2014; Einhäuser, Koch, & Fuhrmann, 2010), emotion
(Bradley, Miccoli, Escrig, & Lang, 2008; Partala & Surakka,
2003), and interpersonal impressions and attitudes (Hess,
1965, 1975; Janisse, 1973; Hess & Polt, 1960). Im
Middle Ages, women ingested belladonna to dilate their
pupils, which was supposed to make them appear seduc-
tiv. Nowadays, people can use cosmetic contact lenses to
make the pupil appear larger (by changing the color and/or
appearance of the iris). These cosmetic techniques are
based on the long-held belief of a close link between pupil
dilation and positive attitudes such as (sexual) interests
and/or emotional arousal and thus of a mutual path
between the actor and the observer. Evidence in the early
1960s showed that actors’ faces with enlarged pupils were
perceived as more attractive to observers (Bull & Shead,
1NTT Corporation, Atsugi, Kanagawa, Japan, 2Tokyo Institute of
Technologie, 3California Institute of Technology
1979; Hess, 1965, 1975; Stass & Willis, 1967). On the observer
Seite, evidence indicated that individuals’ pupils dilated when
they were viewing emotionally toned stimuli, wie zum Beispiel
pictures of a baby for female participants and pictures of a
partially nude man or woman for female and male par-
ticipants, jeweils (Hess, 1965; Hess & Polt, 1960; vgl.
Janisse, 1973). This may be because of arousal and/or sexual
attraction (Rieger & Savin-Williams, 2012; Caryl et al., 2009;
Tombs & Silverman, 2004; Hess, Seltzer, & Shlien, 1965),
which activates the sympathetic nervous system to induce
pupil dilation. Together with activation of the mirror neuron
system that may be involved in a positive circulation between
the observer and the observed face (d.h., the actor), an intu-
itive prediction has been that the pupils of people who are
attracted to faces they see dilate as an automatic response.
Dann, im Gegenzug, they would appear attractive to observers.
Such interpersonal, positive feedback has been assumed
for a long time.
Jedoch, there is room for skepticism because the
dynamic of the pupillary response to attractiveness could
be more complicated than has been thought. Zum Beispiel,
the pupillary dilation (in observers) found in the early era
may have been confounded with stimulus luminance or
contrast to which the pupil responds most sensitively
and/or insufficient baseline conditions ( Janisse, 1973).
Recent studies, which have had finer control over stimulus
luminance and contrast with various tested conditions,
have found that the pupil dilates to not only positive but
also negative emotional stimuli (Burley, Gray, & Snowden,
© 2020 Massachusetts Institute of Technology. Published under a
Creative Commons Attribution 4.0 International (CC BY 4.0) Lizenz.
Zeitschrift für kognitive Neurowissenschaften 33:2, S. 315–340
https://doi.org/10.1162/jocn_a_01649
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2019; Bradley et al., 2008). This suggests that it dilates to
arousal stimuli in general, not particularly to a positive emo-
tion and/or evaluation such as attractiveness. Darüber hinaus,
most evidence from previous studies was based on pupil
size averaged over several seconds, while the participants
were asked to just passively view the stimuli (z.B., um
10 sec in Attard-Johnson, Óciardha, & Bindemann, 2019;
Rieger & Savin-Williams, 2012; Atwood & Howell, 1971;
Barlow, 1969; Koff & Hawkes, 1968; Nunnally, Knott, &
Duchnowski, 1967; Stass & Willis, 1967; Hess, 1965; 2–
6 sec in Bradley et al., 2008). The long-lasting, sustained pu-
pil dilation response reflecting arousal may be different
from the fast, transient component that presumably reflects
other cognitive states and thus affect the feeling of attrac-
tiveness. In der Tat, other studies showed that pupils in gen-
eral quickly constrict in response to the mere onset of visual
presentation (even when the mean luminance is equated,
z.B., Kimura, Abe, & Goryo, 2014) and that this early and
reflexive pupillary constriction response is modulated by
various cognitive factors such as memory (Naber et al.,
2013), attention (Binda, Pereverzeva, & Murray, 2013,
2014; Mathôt, Dalmaijer, Grainger, & Van der Stigchel,
2014; Mathôt, van der Linden, Grainger, & Vitu, 2013),
and perceptual brightness when the physical luminance is
kept the same (Suzuki, Minami, Laeng, & Nakauchi, 2019;
Laeng & Endestad, 2012). Zum Beispiel, in Naber et al.
(2013), participants were asked to memorize various natural
scene images presented one by one (memorization phase)
to recall later in the retrieval phase. The results showed that,
during the memorization phase, pupils constricted more
strongly to certain images, which upon retrieval, war
found to be better memorized. Taking into the evidence
that people tend to better memorize attractive faces than
they do moderately attractive ones (Shepherd & Ellis,
1973), we hypothesize that the pupil constricts more
strongly for more attractive faces, at least during the encod-
ing and/or memorization period, although the underlying
mechanism remains unclear.
Aside from the literature on pupil responses to attrac-
tiveness, the issues can be discussed in a different context,
nämlich, affective decision-making, which is a dynamic pro-
cess to which various factors contribute, such as physiolog-
ical arousal (z.B., the somatic marker hypothesis; Damasio,
1996), gaze (Shimojo, Simion, Shimojo, & Scheier, 2003),
and perceptual fluency via mere exposure (Zajonc, 1968).
Shimojo and colleagues demonstrated that active gaze
engagement not only reflects but also affects preference
decision-making (the “gaze cascade” effect), suggesting a
positive loop between seeing and liking (Shimojo et al.,
2003). They simply revealed a gaze bias toward a to-be-
chosen face to show that gaze “reflects” preference, Aber
they were also successful in biasing preference decisions
by manipulating gaze, thus demonstrating that gaze also
“affects” preference. Just as gaze allows foveal scrutiny,
pupil constriction improves visual acuity (Campbell &
Gregory, 1960). Daher, we hypothesize that pupil constric-
tion may also be actively involved in, or at least concur with,
the formation of preference via an enhancement of seeing
and thus liking.
Along that line, we further speculate that the more
implicit the information (causal factor) Ist, the stronger
the decision-making processing may be affected. Das
seemingly counterintuitive prediction is proved true at
least occasionally in the literature regarding the mere ex-
posure effect (Bornstein, 1989). Under certain conditions,
repetitively presented stimuli get more preferable (d.h., A
stronger mere exposure effect is produced) when they
are presented subliminally rather than suprathreshold.
This pattern of results has been interpreted to mean that,
when information is implicit, das ist, subliminal, partici-
pants often do not causally attribute their decision to the
repetitive presented stimuli per se and are thus more likely
to attribute it to their own internal preference. The misat-
tribution in affective decision-making was observed in the
gaze cascade effect mentioned above. When people’s
preferences were affected by their gaze manipulated
(Shimojo et al., 2003, Experiment 2), most of them were
not aware of the gaze bias to begin with, and those few
who were aware of it did not attribute their preference
to it. There was yet another study in which the participants
were fully aware of all the stimuli (again faces); Jedoch,
they confused their intended choice with the actual out-
kommen. Das ist, they thought they preferred a particular face
but in fact chose a different one beforehand, welches ist
known as choice blindness ( Johansson, Hall, Sikström, &
Olsson, 2005). In such cases, the retrospectively derived
reasons for why a choice is made are inevitably the result
of misattribution. The influence of pupil constriction on
attractiveness judgment, or the concurrence of them, Wenn
it occurs, could also be misattributed and implicit. Das
is because the pupillary response itself is implicit (mehr
so than gaze shifts) and thus cannot be voluntarily con-
trolled or attentively introspected.
METHODEN
We conducted seven experiments to address the issue
of pupillary response and attractiveness judgment. Exper-
iments 1 Und 2 examined how the pupil responded when
seeing an image that was evaluated as attractive. Experi-
ments 3–5 were conducted, together with Experiments 1
Und 2, to examine potential factors that might affect the
result and/or account for the inconsistency between our
finding (pupil constricts to attractiveness) and the literature,
which demonstrated pupil dilation to attractiveness. Der
factors included stimulus presentation time, task demand
(attractiveness judgment, roundness judgment, or passive
viewing), stimulus category (faces, natural scenes, oder
geometric figures), and baseline pupil response pattern
(constriction or dilation) caused by sequential luminance
contrast change. Experiment 6 examined whether implicit
pupil manipulation contributes to attractiveness judgment,
and Experiment 7 ruled out potential confounding in
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Experiment 6. Tisch 1 shows the overview of the critical
manipulations in each experiment.
In Experiment 1, participants looked at a face presented
at the center of a screen and rated how attractive the face
was on a scale from 1 (least attractive) Zu 9 (most attrac-
tiv) while an infrared camera recorded their pupillary re-
sponses. Data were sorted based on the attractiveness
judgment individually to examine how the pupil reacted
to attractive faces (the face attractiveness condition). Two
other conditions were added to examine whether the effect
of pupil constriction to attractive faces, if it occurs, was spe-
cific to faces or attractiveness judgment. In the geometric
figure attractiveness condition, participants evaluated the
attractiveness of a geometric figure, but not a face. Im
face roundness condition, they viewed the same set of
the faces to rate how round the faces were, ignoring their
attractiveness. The three conditions were conducted in
separate blocks in a counterbalanced order across partici-
Hose. Each trial started with a 3-sec gray fixation display,
followed by the target image (faces or geometric figures).
Participants were free to inspect the stimuli as long as they
wanted before making a decision.
Experiment 2 aimed to replicate the finding of pupil con-
striction to attractive faces with additional luminance con-
trols. Erste, the luminance among the faces was equated,
and the mean luminance of the faces, as well as that of the
fixation display presented before the faces, was the same as
the background (so that there was no mean luminance
change over time). Zweite, instead of using line-drawing
geometric figures, we used natural scenes. The photos
were image processed to equate their mean luminance
by following the same procedure as for the face images.
The rest of the experimental procedures were the same
as in Experiment 1.
Tisch 1. Overview of the Critical Manipulations in Each Experiment
Ziele
Verfahren
Task
Sti.
E1
To examine how the pupil
responds to attractiveness.
E2
To replicate the result of E1
by using equal luminance
Bilder.
E3
To examine whether stimulus
presentation duration affects
the result.
E4
E5
E6
E7
(1) To examine whether task
demand affects the result.
(2) To minimize the effect
of stimulus luminance on
pupils.
To examine whether baseline
pupil response pattern
affects the result.
To examine whether implicit
pupil size manipulation
contributes to attractiveness.
To rule out the confounding
of sequential luminance
contrast in E6.
Att., Rnd.
FC, GF
Participants made the
judgment of an
image while their
pupillary responses
were recorded.
Same as E1
Att., Rnd.
FC (eql),
NS (eql)
Att.
FC (eql)
Participants viewed the
image for 5 sec and
held their response
until the stimulus
was off.
Same as E3, or just
Att., Nicht.
FC (Linie)
viewing the stimulus
für 5 sec without
judgment.
Baseline Pupil Response
Pattern Caused by Sequential
Luminance Contrast Change
Pupil constriction to FC,
pupil dilation to GF.
Pupil constriction to both
FC and NS (even when
the mean luminance
change is equated).
Pupil constriction to FC
(even when the mean
luminance change is
equated).
Pupil constriction to FC
in both Att. and Pas.
(slightly) Bedingungen.
Same as E1
Att.
FC, GF
Pupil dilation to FC,
Same as E1
Same as E1
Att.
Att.
FC
FC
pupil constriction to GF.
The amount of pupil size
change was manipulated.
The amount of pupil size
change was equated
while the sequential
luminance contrast
change was manipulated.
Att. = attractiveness task; Rnd. = roundness task; Nicht. = passive viewing; Sti. = stimuli; FC = faces; GF = geometric figures; NS = natural scenes; eql =
equal luminance; line = line drawing.
Liao, Kashino, and Shimojo
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Experiment 3 aimed to examine the time course of the
effect of pupil constriction to attractiveness. In other
Wörter, does the effect occur only shortly after the stimulus
presentation, or does it last long for several seconds? Im
experimental procedure, the face image was presented for
5 sec and participants were asked to hold their judgment
response until the face disappeared. Their pupillary
responses were recorded during the whole 5-sec inspec-
tion period.
Experiment 4 had two purposes. Erste, we aimed to reex-
amine the finding of pupil constriction to attractive faces
over time as in Experiment 3 by adding another factor: Aufgabe
demand. This was to examine whether pupil dilation to
attractiveness could be observed in later time course and
when no task demand was involved, as in the similar con-
dition where pupil dilation was observed in the literature.
Zu diesem Zweck, in addition to the attractiveness judgment
condition as was done in Experiment 3, we added a passive
viewing condition, in which participants viewed the same
set of images presented in the attractiveness judgment
condition without any task demand. The two conditions
were conducted in a counterbalanced order across partic-
Ipants. Pupil data in the passive viewing condition were
sorted by attractiveness ratings obtained in the attractive-
ness judgment condition for each participant individually.
The second purpose of Experiment 4 was to minimize the
effect of stimulus luminance change on pupils. We there-
fore used line-drawing face images. The faces were pre-
sented with their luminance contrast slowly enhanced,
instead of sudden flash as in the previous experiments,
to minimize the transient change on the screen.
In Experiment 5, we examined whether the baseline pupil
response (constriction or dilation) is critical to the finding
of pupil constriction to attractiveness. The luminance of
the fixation display before the target display, sowie
the background of the target display, was manipulated
so that the pupil response baseline change was dilation
to faces and constriction to geometric figures, in the oppo-
site direction compared with Experiment 1.
Experiment 6 was to examine whether pupil constriction
contributes to facial attractiveness judgment. Zu diesem Zweck,
while keeping the target image the same, we manipulated
the luminance of the fixation display (before the target
display) so that it would change from black or gray to alter
the amount of pupil constriction when the page flipped.
Because of the nature of the pupillary light reflex (Ellis,
1981), the pupil should constrict more strongly when the
target image follows a black than a gray fixation display. Wir
also changed the luminance of the target background to
black or gray, to serve as fillers to make the critical manip-
ulation, das ist, the fixation display change, less noticeable.
With this manipulation, we also aimed to examine the rel-
ative contribution of pupil constriction and simultaneous
luminance contrast (induced by the target background)
to attractiveness judgment. Although the luminance of
the target background may also affect the pupillary re-
sponse, its influence is expected to be smaller than that
of the fixation display. If the attractiveness judgment is
affected more by the simultaneous contrast than the pupil
constriction, the target background luminance should
have a stronger influence on attractiveness judgment than
the prestimulus fixation display. Participants rated the
attractiveness of the faces presented at the center of the
target display as in Experiment 1 (Figure 6A).
In Experiment 7, we examined whether sequential lumi-
nance contrast alone, when not inducing a strong difference
in pupil response, causes differences in attractiveness
judgments. We divided visual fields into two halves (left/
Rechts) with luminance disparities (black and white) im
fixation display and then presented the target image
(against a gray background) to the left or right visual field
(see Figure 7A). In this case, there was sequential lumi-
nance contrast to the target image (different luminance
conditions depending on the spatial relationship between
the target image location and the fixation display’s lumi-
nance disparities, d.h., black on the left or right visual
fields), but the overall average luminance of the fixation
display remained the same to induce a similar pupillary
light reflex (to the face display with a gray background).
Participants were allowed to move their gaze to the face
Position (Experiment 7a) or were instructed to always
fixate the center even when the face was presented periph-
erally (Experiment 7b).
Teilnehmer
Sixty-eight adults (43 Frauen, age range of 20–48 years,
median age = 35 Jahre) participated in the current study:
13 in Experiment 1, 15 in Experiment 2, 17 in Experiment 3,
12 in Experiment 4, 10 in Experiment 5, 11 in Experiment 6
(the same group of participants as in Experiment 1 mit
the two excluded because of the data lost by program
error for the first two participants), 16 in Experiment 7a
(the same group of participants as in Experiment 2, Plus
one who was excluded from Experiment 2 because of an
accidental data recording loss for the last participant), Und
17 in Experiment 7b (the same group of participants as in
Experiment 3). All had normal or corrected-to-normal
vision and were naive about the purpose of the experi-
gen. The current study was performed in accordance
with the Declaration of Helsinki. All participants gave
written informed consent before the experiment and re-
ceived payment for their participation.
Apparatus and Stimuli
Visual stimuli were presented on an 18.1-in. monitor (Eizo
FlexScan L685Ex) with a 60-Hz frame rate, controlled by a
personal computer (Dell OptiPlex 755). In Experiment 1, In
the target display, a target image was presented at the cen-
ter of the screen against a gray background (21.04 cd/m2).
There were three conditions. In the face attractiveness
and face roundness conditions, the target image was a
Gesicht (6.42° width × 7.83° height) generated by FaceGen
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(Singular Inversions Inc.) Software. Faces consisted of eight
subcategories of the combination of two races (Asian or
European), Geschlecht, and age range (old or young). Dort
war 20 face images in each subcategory, daher 160 face im-
ages in total (mean luminance = 25.57 cd/m2; maximum of
43.13 cd/m2 and minimum of 12.82 cd/m2). In the geo-
metric figure attractiveness condition, a geometric figure,
in black lines, with 10.62° width × 7.83° height was pre-
sented at the center of the screen to serve as the target. Der
figures were Fourier descriptors generated by a MATLAB pro-
Gramm (MathWorks Inc.) with properties specified and var-
ied as a combination of symmetry (symmetric or asym-
metric) and simplicity (simple or complex). One hundred
sixty geometric figures were generated (mean luminance =
16.46 cd/m2; maximum of 23.22 cd/m2 and minimum of
4.27 cd/m2). All the target displays were interlaid with a
fixation display, which consisted of a black fixation cross
(0.5° × 0.5°, 0.35 cd/m2) against a gray background.
In Experiment 2, the stimuli and experimental structure
were the same as in Experiment 1, except that instead of
the geometric figures, we used natural scene images. Der
original images were color photos collected from public
websites. They consisted of eight subcategories: Tier,
food, flower, mountain, sky, lake, ocean, and desert. Dort
war 20 images in each subcategory, daher 160 images in
total. The size of the images was within 8° width or 9.75°
height, presented at the center of the screen. The original
color natural scenes and face images (used in Experiment 1)
were modified to be in the gray scale with the same mean
luminance as the background (21.04 cd/m2) by using the
SHINE toolbox ( Willenbockel et al., 2010).
In Experiment 3, the same set of face images as in Experi-
ment 2 was used. No natural scene images were used in
Experiment 3. In Experiment 4, 40 face images (five in each
subcategory as used in Experiment 1) were selected for
further processing. Each face was manually line-drawn
based on the original image with the pupil adjusted in five
different sizes (see example in Figure 8). In Experiment 5,
the stimuli were the same as in Experiment 1, except for the
following modifications. Erste, the faces were presented
against a white background (94.04 cd/m2). The interlaid fix-
ation display for faces consisted of a light gray fixation cross
(58.66 cd/m2) against the white background. Zweite, für
geometric figures, the background was black (0.35 cd/m2),
and the fixation cross was dark gray (2.44 cd/m2) in the inter-
laid fixation display.
In Experiment 6, we used the faces that were judged as
median attractive by individual participants in Experi-
ment 1. For each participant and in each race subcate-
gory (combined across gender and age), the rating scores
were rank ordered, und das 20 faces that corresponded to
the median attractive rank order were used. No geometric
figures were used in Experiment 6. Faces were presented
at the center of the screen against a gray (21.04 cd/m2) oder
black (0.35 cd/m2) background. The interlaid fixation
display consisted of a black fixation cross (0.5° × 0.5°,
0.35 cd/m2) against the gray background or a gray
fixation cross (0.5° × 0.5°, 21.04 cd/m2) against the
black background.
In Experiment 7a, following the same procedure as in
Experiment 6, für jeden Teilnehmer, we selected 40 medi-
an attractive faces (20 for each race) used in Experiment
2 based on individual judgments. In Experiment 7b, Die
faces were selected based on the individual judgments in
Experiment 3. In both Experiments 7a and 7b, no natural
scene images were used. Faces were presented to the left
or right visual field with 5.03° of eccentricity against the
gray background (21.04 cd/m2). In Experiment 7a, Die
interlaid fixation display consisted of a gray fixation cross
(21.04 cd/m2) against the background with luminance
disparity across the visual field: black (0.35 cd/m2) on the
left and white (94.04 cd/m2) on the right or vice versa. In
Experiment 7b, the fixation cross was red and remained
visible during the face target presentation.
Design
In all experiments, each trial consisted of the target display
after the fixation display presented for 3 Sek. In Experi-
gen 1 Und 2, the three conditions were considered as
within-participant factors in different blocks with a counter-
balanced order among the participants. In the face attrac-
tiveness and face roundness conditions, the faces of
different races (Asian and European) were presented in
separated subblocks. Each subblock consisted of 80 Gesicht
images presented in a randomly assigned order. Dort
was no break between the subblocks. In the geometric
figure attractiveness condition (in Experiment 1), alle 160
geometric figures were presented in a randomly assigned
Befehl. In the natural scene attractiveness condition (In
Experiment 2), the images of different subcategories were
presented in separate subblocks without a break between
ihnen. The order of the images within subblocks and the
order of the subcategories were randomized.
In Experiment 3, only the face attractiveness condition
was conducted. The face was presented on the screen
für 5 sec and then replaced by the fixation display. In
Experiment 4, we applied the nested design in which each
participant viewed 40 face images only, all with different
Identitäten. Each pupil size level was presented eight times
with different face identities. There were two conditions:
attractiveness rating and passive viewing. The two condi-
tions were considered as within-participant factors in
separate blocks with the order counterbalanced across
the participants. The same set of stimuli was used in the
two conditions, with the trial order randomly assigned.
The face image was presented with its luminance contrast
gradually increased for 1 Sek, stayed at the center of the
screen for 3 Sek, and then gradually disappeared for
1 Sek. In Experiment 5, the design was the same as in
Experiment 1 except that there were only two conditions:
face attractiveness and geometric figure attractiveness
Bedingungen (no face roundness condition).
Liao, Kashino, and Shimojo
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In Experiments 6, 7A, and 7b, the two types of fixation
display and the two types of target display were considered
as within-participant factors. In each race subcategory, Die
20 median attractive faces were presented for four times, In
each of the 2 (fixation display) × 2 (target display) condi-
tionen. There were thus 80 trials in each race subcategory
Und 160 trials in total. As in Experiment 1, the faces of
different races were presented in different subblocks in a
randomly assigned order. In each subblock, Die 80 Versuche
with different manipulation conditions were presented in
a randomly assigned order.
Verfahren
Participants sat in front of the monitor at an 80-cm distance
with their head supported on a chin rest. In each
session/experimental block, participants went through the
5-point Eyelink calibration program to calibrate and validate
their eye data. After the calibration procedure, the experi-
ment started without practice trials. Participants were in-
structed to fixate the central fixation cross during the
fixation display. Once the target display was shown, Sie
were asked to make a judgment (attractiveness or round-
ness) on the target image (faces, geometric figures, oder
natural scenes). In Experiments 1, 2, 5, 6, 7A, and 7b, Sie
were free to take their own pace in making the decision. In
Experimente 3 Und 4, they were asked to give the rating
score after the face disappeared. In the passive viewing
condition of Experiment 4, they were asked to look at the
faces without any task involved. In all experiments except
Experiment 7b, they were allowed to move their gaze to
the stimulus position. In Experiment 7b, they were in-
structed to fixate the central fixation cross even when the
face was presented peripherally. In the attractiveness judg-
ment condition, participants rated how attractive the face,
geometric figure, or natural scene was, das ist, how much
they liked the given image. In the roundness judgment
condition, participants rated how round the face was.
They indicated their answer by pressing the number pad
on a keyboard from 1 (least attractive/round) Zu 9 (most
attractive/round). They were encouraged to use all nine
numbers if possible but not necessarily equate the distribu-
tion so that they would make their judgment naturally. Nach
they gave their answer or 0.5 sec after the stimulus disap-
peared in the passive condition of Experiment 4, the next
trial started with the 3-sec fixation display. Teilnehmer
made judgments for all trials straight without break. Jede
session took about 20 min, and there was more than a
20-min break between the sessions.
Eye Metrics Data Analysis
Eye movements (including pupillary responses and gaze
location) were recorded binocularly with an infrared eye-
tracker camera (Eyelink 1000 Desktop Mount, SR Research
Ltd.). The camera was positioned below the monitor. Der
sampling rate of the recording was 1000 Hz. Because pupil-
lary responses are consensual, only data from the right eye
wurden benutzt. Data during blinks were interpolated using
shape-preserving piecewise cubic function. During the time
window of −1- to 2-sec reference to stimulus onset, blinks
accounted for 15.1% of data points in Experiment 1, 8.4% In
Experiment 2, 19.1% in Experiment 5, 18.3% in Experiment
6, 11.6% in Experiment 7a, Und 16.5% in Experiment 7b. Der
blink rate during the −1- to 5-sec time window reference to
the target onset was 11.7% Und 21.4% for Experiments 3 Und
4, jeweils. The blink rate was in the normal range when
natural blinking was allowed (z.B., Goldinger & Papesh,
2012). To compare the pupillary response results across par-
ticipants and conditions, pupil diameter data were normal-
ized using all the data recorded in each session and
baseline corrected by subtracting the mean of the data
during the 1-sec period before the stimulus onset. Für
the gaze-contingent pupillary response analysis, in each
trial, all the gazes, das ist, the fixations that were detected
from the Eyelink system, were first identified (mean dura-
tion = 421 msec in Experiments 1–5). The normalized
pupil diameter data were then baseline corrected by sub-
tracting the mean of the data during the 100-msec period
before the gaze onset. For the gaze location analysis, Die
gaze location data were averaged during target presenta-
tion, das ist, 0–2 sec after the target onset in Experiments
1, 2, 5, Und 7 as well as 0–5 sec in Experiments 3 Und 4. Der
gazed local luminance was calculated by averaging the
luminance of the image regions within 1° of visual angle
of the gazing point across time.
Statistical Analysis
Experiments 1–5
For behavioral responses, mean RTs for all experiments
(except for Experiments 3 Und 4 because the response
was held and postponed) sind in der Tabelle aufgeführt 2. On average,
participants made decisions at around 2 Sek. The histo-
grams of rating for Experiments 1 Und 2 are shown in
Figuren 9 Und 10, jeweils. In all the conditions we
tested, extreme rating scores (d.h., 1 Und 9) were least
gegeben, with only about half the frequency of their adjacent
scores (d.h., 2 Und 8, jeweils). We therefore combined
trials with rating scores of 1 Und 2 and those with scores
von 8 Und 9 together to reduce the noise because of few
trials and to balance the trial numbers across conditions
for further analysis.
We averaged the pupil diameter 0.5–1.5 or 3–5 sec (nur
for Experiments 3 Und 4) after stimulus onset to represent
the pupil constriction/dilation response. Mean pupil
diameter data were subjected to a repeated-measure
ANOVA with the Seven-level rating score as the within-
participant factor in each condition. We also examined
the correlation between the mean pupil diameter and
the attractiveness rating on a trial-by-trial basis. Pearson’s
320
Zeitschrift für kognitive Neurowissenschaften
Volumen 33, Nummer 2
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Tisch 2. Mean RTs (ms) under Each Condition in All
Experiments except Experiments 3 Und 4
Experiment 1
Face – attractiveness
Geometric figures – attractiveness
Face – roundness
Experiment 2
Face – attractiveness
Natural scenes – attractiveness
Face – roundness
Experiment 5
Face – attractiveness
Geometric figures – attractiveness
Experiment 6
Gray–gray
Gray–black
Black–gray
Black–black
Experiment 7a
Black hemifield
White hemifield
Experiment 7b
Black hemifield
White hemifield
2308 (120.4)
2392 (272.1)
2296 (145.1)
2016 (184.0)
2027 (169.5)
1938 (132.2)
2172 (183.2)
2031 (155.0)
2185 (178.9)
2081 (159.7)
2199 (179.7)
2228 (233.3)
1678 (69.36)
1683 (73.48)
2122 (154.61)
2151 (179.73)
Numbers in parentheses are standard errors among participants (Auch
see Figures 9 Und 10).
correlation coefficients between the mean pupil diameter
and rating score for individual participants were calculated
and subjected to a one-sample t test to examine if they
deviated from zero. Partial correlation coefficients
between the mean pupil diameter and rating score were
also calculated when the effect of mean gazed local lumi-
nance over the same time window as the mean pupil
diameter was removed. Results of the ANVOA, linear
trend analysis, and correlation analyses (with and without
the mean gazed local luminance being partialed out) Sind
shown in Table 3.
Effect of Task Order between Attractiveness and
Roundness Tasks on Pupil Constriction under
Explicit- and Implicit-attractiveness Conditions
(Experimente 1 Und 2 nur)
Mean pupil diameter data sorted by facial attractiveness
were subjected to a repeated-measure ANOVA with the
Seven-level rating score as the within-participant factor
and Task order (attractiveness judgment first or round-
ness judgment first) as the between-participant factor.
We report the detailed statistical results here and summa-
rize interpretations in the Results section. In Experiment 1,
in the explicit-attractiveness condition, the effect of rating
was significant, F(6, 66) = 2.99, P < .02, but not the effect
of task order, F(1, 11) = 1.38, p = .26, or the interaction
between rating and task order, F(6, 66) = 0.74, p = .62.
The linear trend of pupil size against rating was significant,
F(1, 11) = 7.48, p < .02, and did not interact with task
order, F(1, 11) = 0.34, p = .57. The same pattern of results
was found in the implicit-attractiveness condition: The ef-
fect of rating was significant, F(6, 66) = 2.49, p < .04, but
not the effect of task order, F(1, 11) = 1.52, p = .24, or the
interaction, F(6, 66) = 1.21, p = .31. In contrast, the linear
trend of pupil size against rating was significant, F(1, 11) =
7.06, p < .03, with a marginally significant interaction with
the task order, F(1, 11) = 4.73, p = .05. Further analyses
showed that the linear trend of pupil size against rating
was found when the participant performed the roundness
task earlier than the attractiveness task, F(1, 5) = 8.42, p <
.04, but not the other way around, F(1, 6) = 0.93, p = .37.
In Experiment 2, in the explicit-attractiveness condition,
the effect of rating was significant, F(6, 78) = 4.72, p <
.001, but not the effect of task order, F(1, 13) = 2.61,
p = .13, or the interaction, F(6, 78) = 1.70, p = .13. The
linear trend of pupil size against rating was significant, F(1,
13) = 18.54, p < .001, and did not interact with task order,
F(1, 13) = 0.25, p = .62. In the implicit-attractiveness con-
dition, none of the main effects (F(6, 78) = 1.93, p = .09
and F(1, 13) = 0.82, p = .38, for the effect of rating and
task order, respectively) or interaction were significant,
F(6, 78) = 1.01, p = .42. The linear trend of pupil size against
rating was significant, F(1, 13) = 9.54, p < .01, and did not
interact with task order, F(1, 13) = 0.08, p = .79.
Experiment 6
Mean pupil diameter data (i.e., average pupil diameter
data 0.5–1.5 sec after the stimulus onset) were subjected
to a two-way ANOVA with Prestimulus luminance (i.e.,
the fixation display) and Target background luminance as
within-participant factors. The same ANOVA test was
conducted using the mean facial attractiveness rating
scores. Partial correlation analyses were conducted among
the following three variables: prestimulus pupil baseline,
pupil constriction amplitude, and attractiveness rating.
Prestimulus pupil baseline was calculated by averaging
pupil diameter data 0–1 sec before the stimulus onset.
Pupil constriction amplitude, that is, the amount of pupil
Liao, Kashino, and Shimojo
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Table 3. Statistical Results for the Mean Pupil Size during Specified Time Window (sec) in Experiments 1–5
ANOVA
Linear Trend Analysis
Correlation Analysis
Partial Correlation Analysis
Task
Att.
Att.
Rnd.
Imp.
Att.
Att.
Rnd.
Imp.
Att.
Sti.
FC
GF
FC
FC
FC (eql)
NS (eql)
FC (eql)
FC (eql)
FC (eql)
E1
E2
E3
E4
Att.
FC (line)
Pas.
FC (line)
E5
Att.
Att.
FC
GF
Time
0.5–1.5
0.5–1.5
0.5–1.5
0.5–1.5
0.5–1.5
0.5–1.5
0.5–1.5
0.5–1.5
0.5–1.5
3–5
0.5–1.5
3–5
0.5–1.5
3–5
0.5–1.5
0.5–1.5
F
3.06
0.36
1.66
2.45
4.49
3.15
1.13
1.93
7.09
4.54
0.81
3.10
2.21
0.17
0.57
0.43
p
.01*
.90
.14
.03*
F
7.91
0.24
0.65
5.39
p
.02*
.64
.44
.04*
<.001***
20.36
<.001***
.01*
.35
.09
<.001***
<.001***
.57
<.01**
.05
.99
.75
.86
6.48
3.43
10.33
18.98
9.41
1.69
2.89
2.88
0.48
1.29
0.82
.02*
.09
<.01**
<.001***
<.01**
.22
.12
.12
.50
.29
.39
r
t
−.09
−2.50
.00
.03
−.12
−.09
−.10
−.06
−.07
−.14
−.08
−.07
.09
.07
.04
−.07
.06
0.02
0.70
−2.94
−3.13
−3.93
−2.32
−2.89
−4.77
−2.62
−1.13
1.57
1.43
0.77
−2.12
0.85
p
.03*
.98
.50
.01*
<.01**
<.01**
.04*
.01*
<.001***
.02*
.28
.15
.18
.46
.06
.42
r
−.03
.02
.02
−.08
−.07
−.09
−.05
−.04
−.07
−.02
−.06
.10
.06
.05
−.04
.03
t
−1.05
0.50
0.39
−2.29
−2.65
−3.45
−2.08
−1.72
−3.00
−0.82
−0.91
2.03
1.16
0.95
−1.17
0.37
p
.31
.62
.70
.04*
.02*
<.01**
.06
.11
<.01**
.43
.38
.07
.27
.36
.27
.72
ANOVA examined whether pupil size differed among ratings. Linear trend analysis examined whether pupil size linearly correlated with rating using average data. Correlation analysis and partial correlation
analysis examined the correlation between pupil size and rating on a trial-by-trial basis, without and with the effect of gazed local luminance being removed, respectively. Att. = attractiveness task; Rnd. =
roundness task; Imp. = implicit attractiveness; Pas. = passive viewing; Sti. = stimuli; FC = faces; GF = geometric figures; NS = natural scenes; eql = equal luminance; line = line drawing; F = F statistic;
p = p value; r = mean Pearson’s correlation coefficient; t = t statistic.
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constriction change responding to the stimulus, was calcu-
lated by subtracting the prestimulus pupil baseline from
the mean pupil diameter data 0.5–1.5 sec after the stimulus
onset. Linear partial correlation coefficients for individual
participants were calculated and subjected to a one-sample
t test to examine if they deviated from zero.
Experiment 7
Mean pupil diameter data (i.e., average pupil diameter data
0.5–1.5 sec after the stimulus onset) in the two conditions
where the face followed white and black hemifields, re-
spectively, were subjected to a paired two-sample t test.
Figure 1. Pupil response results
in Experiment 1. Sample
stimulus images are shown
corresponding to individual
conditions. (A–D) Mean pupil
diameter change as a function
of time reference to the target
onset during (A) attractiveness
judgment for faces, (B)
attractiveness judgment for
geometric figures, (C) roundness
judgment for faces, and (D)
roundness judgment for faces
when the data were sorted by the
attractiveness of the faces.
Curves are parameterized with
rating score depending on
individual participants’ choices
(1 = least attractive and 9 =
most attractive for A, B, and D;
1 = least round and 9 =
roundest for C). The color
shadows represent standard
errors among participants. The
gray shadow represents the time
window for averaging the pupil
size to present the amount of
pupil constriction for statistical
analysis (see Methods for details
and Table 3 for results). (E–H)
Box plots of the mean pupil size
over the specified time window.
The plus signs represent
individual data.
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Liao, Kashino, and Shimojo
323
The same statistical test was conducted using the mean
facial attractiveness rating scores.
RESULTS
Transient Pupil Constriction Reflects Attractiveness
In Experiment 1, the pupil in general constricted in re-
sponse to the presentation of the faces. Most importantly,
during the inspection, the degree of pupil constriction in
each participant was linearly correlated with the facial
attractiveness rated immediately after the inspection in
every single trial (see Figure 1A for overall pupillary
response change and Figure 1E for the box plot for statis-
tical analysis: F(1, 12) = 7.91, p < .03, for the linear trend
analysis). The more attractive the face was, the more the
pupils constricted. In contrast, the amount of pupil con-
striction did not correlate with attractiveness judgments
for geometric figures, F(1, 12) = 0.24, p = .64 (see
Figures 1B and 1F), or roundness judgments for faces,
F(1, 12) = 0.65, p = .44 (see Figures 1C and 1G).
Intriguingly, when the faces were sorted by their attractive-
ness (although the explicit task demand was to judge their
roundness), the degree of pupil constriction showed a
linear correlation with the implicit, or task-irrelevant,
attractiveness of the faces, F(1, 12) = 5.39, p < .05 (see
Figures 1D and 1H)—the same pattern of results as when
the task demand was to judge the attractiveness (i.e., the
face attractiveness condition). Note again that the order of
the three conditions (face attractiveness, geometric figure
attractiveness, and face roundness) was counterbalanced
across participants. A further analysis involving Condition
order as a factor (see Methods for details) showed that
the pattern of the pupillary responses to facial attractive-
ness (either explicit or implicit) remained the same re-
gardless of the condition order ( ps > .05); it did not
matter whether the faces were judged on attractiveness
earlier than roundness or vice versa. Zusammenfassend, Die
overall results of Experiment 1 suggest that the pupil con-
striction response to facial attractiveness is task specific (In
contrast to roundness judgment), automatic, and free of
Erinnerung. A potential problem in Experiment 1, Jedoch,
is that we controlled luminance across stimuli rather
crudely (see sample images in Figure 11), and it may be
criticized that the result could be explained by the low-
level factor because the pupil is very sensitive to lumi-
nance (Kontrast).
Daher, in Experiment 2, all the face and natural scene im-
ages were in equal luminance with each other as well as the
background. Results showed that the amount of pupil
constriction was linearly correlated with the attractiveness
rating not only for faces, F(1, 14) = 20.36, P < .001
(Figures 2A and 2E), but also for natural scenes, F(1, 14)
= 6.48, p < .03 (Figures 2B and 2F). When participants
performed the roundness task, pupil constriction was
still linearly correlated with the attractiveness of the faces,
F(1, 14) = 10.33, p < .01 (Figures 2D and 2H), but not with
the roundness judgment, F(1, 14) = 3.43, p = .09
(Figures 2C and 2G). In summary, Experiment 2 replicated
the main finding of pupil constriction to attractiveness
faces and extended it to natural scenes. We can therefore
conclude that pupil constriction certainly reflects attrac-
tiveness either explicitly or implicitly.
Potential Factors That May Affect the Effect of Pupil
Constriction to Attractiveness
Before we move on to the second focus of this study,
namely, whether pupil constriction also contributes to
attractiveness judgment, we need to mention several addi-
tional, yet critical, issues. First, the biggest question would
be why there is such a big inconsistency between our find-
ing (pupil constriction to attractiveness) and the pupil dila-
tion to attractiveness demonstrated in the literature.
Second, it is still unclear whether the attractiveness judg-
ment for geometric figures could indeed not induce corre-
sponding pupil constriction. Finally, to what extent can the
result be explained by gaze and/or eye accommodation
factors? To address the first and second issues, we con-
ducted three additional experiments (Experiments 3–5)
to examine the factors including stimulus presentation
time, task demand, stimulus category, and baseline pupil
response pattern (constriction or dilation) caused by se-
quential luminance contrast change. In Experiment 3,
when prolonging stimulus presentation time, pupil con-
striction to attractive faces was found not only during
0.5–1.5 sec, F(1, 16) = 18.98, p < .001, but also during
3–5 sec, F(1, 16) = 9.41, p < .01, after the stimulus onset
(Figure 3). In Experiment 4, when using line-drawing faces
with different task demands, the overall pattern of results
changed (Figure 4). In the attractiveness rating condition,
there was a tendency of early pupil constriction to attrac-
tiveness, whereas it did not reach statistical significance,
F(1, 11) = 1.69, p = .2. Most importantly, in contrast with
the lasting effect of pupil constriction to attractiveness
found in Experiment 3, the most attractive faces (rated
as 8 or 9) induced the strongest pupil dilation response,
F(6, 66) = 3.10, p < .01, during the later time course
(3–5 sec). In the passive-viewing condition, although none
of the effects was significant, there was a tendency of pupil
dilation to the most attractive faces during the early time
course, F(6, 66) = 2.21, p = .05. In Experiment 5, not only
faces but also geometric figures were used to examine
whether the baseline pupil response pattern (constriction
or dilation) affects the finding of the linear trend of pupil
size against attractiveness rating. Results are shown in
Figure 5. When the pupil in general dilated to faces, mean
pupil size negatively correlated with attractiveness rating
(i.e., the smaller the pupil, the more attractive the face
was), although the effect was not significant, F(1, 9) =
1.29, p = .3. When the pupil in general constricted to geo-
metric figures, there was still no tendency of correlation
between pupil constriction and attractiveness, F(1, 9) =
0.82, p = .4, consistent with the result in Experiment 1.
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Figure 2. Pupil response results
in Experiment 2. Sample
stimulus images (luminance
equated) are shown
corresponding to individual
conditions. (A–D) Mean pupil
diameter change as a function
of time reference to the target
onset during (A) attractiveness
judgment for faces, (B)
attractiveness judgment for
natural scenes, (C) roundness
judgment for faces, and (D)
roundness judgment faces
when the data were sorted by
the attractiveness of the faces.
Curves are parameterized with
rating score depending on
individual participants’ choices
(1 = least attractive and 9 =
most attractive for A, B, and D;
1 = least round and 9 =
roundest for C). The color
shadows represent standard
errors among participants. The
gray shadow represents the
time window for averaging
the pupil size to present the
amount of pupil constriction,
for statistical analysis (see
Methods for details and Table 3
for results). (E–H) Box plots of
the mean pupil size over the
specified time window. The
plus signs represent individual
data.
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In summary, the overall results in Experiments 3–5 indi-
cated that none of the factors that we examined alone can
explain the discrepancy between our finding and the liter-
ature, but in general, the effect of pupil constriction to
attractive faces (still not to geometric figures) was more
effectively observed during the early time course (within
2 sec after the stimulus presentation; see Experiments 3
and 4), when the pupil generally constricted in response
to the relative sequential luminance increase (compare
Experiments 1 and 5) and when a task demand was
Liao, Kashino, and Shimojo
325
period (3–5 sec after stimulus onset; see Figure 4A) or
passive viewing (Figure 4B). This is indirectly consistent
with the literature, where most of the evidence for pupil
dilation to attractiveness came from pupillary responses
accumulated for 10 sec while participants just passively
viewed the stimulus (e.g., Rieger & Savin-Williams, 2012;
Atwood & Howell, 1971; Barlow, 1969; Koff & Hawkes,
1968; Nunnally et al., 1967; Stass & Willis, 1967; Hess, 1965).
In the following sections, we address the issue whether
gaze and/or eye accommodation factors might explain
the finding of pupil constriction to attractiveness, by con-
ducting additional gaze-related analyses for Experiments
1–5. The results and figures are shown in Table 3 and
Figures 12–14. First of all, we checked the gaze location
data to examine where exactly the participant scrutinized
on the target image. The heat maps of gaze location distri-
bution during target presentation, superimposed on
the sampled target image, are shown in Figure 12. As illus-
trated, the participants mostly looked at the center of the
image in Experiments 1–5. There was 80% of the gaze that
was located within 3.7° of visual angle in eccentricity.
Although the gaze location was mainly distributed at the
center of the target image and the average luminance of
the images was equated in Experiments 2 and 3, there
might be still subtle luminance differences of the image
regions being gazed at, and this might explain why pupil
constricted stronger to attractive images. To clarify this con-
cern, we calculated the luminance of the image regions be-
ing gazed at, parameterized by rating score (see Figure 13).
Results showed that participants tended to prefer the face
image (but not the natural scene images or line-drawing
faces), with higher local luminance contrast in the center.
To further examine whether the local luminance (of the
image regions being gazed at) might explain the finding
of pupil constriction reflecting attractiveness rating, we
conducted a partial correlation analysis in which the corre-
lation between mean pupil size and attractiveness rating
was estimated when the effect of mean gazed local lumi-
nance was removed. Results are shown in Table 3. As
expected, the correlation between pupil size and attractive-
ness rating was indeed reduced in certain conditions.
Importantly, the correlation was still significant in most crit-
ical conditions: the face-implicit attractiveness condition in
Experiment 1 ( p < .05), the face-explicit attractiveness and
natural scene-attractiveness conditions in Experiment 2
( ps < .03), and the face-explicit attractiveness condition
in Experiment 3 ( p < .01). The overall results are consis-
tent with our conclusion in Experiment 2 that early tran-
sient pupil constriction reflects attractiveness when the
image luminance is controlled.
Accommodation to focus on near objects also induces
pupil constriction, that is, pupil near response (McDougal
& Gamlin, 2008; Mays & Gamlin, 1995). A parsimonious
hypothesis why pupil constriction reflects attractiveness
would be because of a co-occurrence of eye accommoda-
tion and pupil constriction, assuming that gazing and/or
focusing alone explains the effect. To test this hypothesis,
Figure 3. Pupil response results in Experiment 3 (delayed response). (A)
Mean pupil diameter change as a function of time reference to the target
onset during the delayed attractiveness judgment for faces (as shown in
a sample image). Curves are parameterized with rating score depending
on individual participants’ choices (1 = least attractive and 9 = most
attractive). The color shadows represent standard errors among
participants. The gray shadow represents the time window for averaging
the pupil size to present the amount of pupil response for statistical
analysis (see Methods for details and Table 3 for results). (B–C) Box plots
of the mean pupil size over the specified time window (0.5–1.5 sec for B
and 3–5 sec for C). The plus signs represent individual data.
required in contrast with passive viewing (see Experiment 4).
On the other hand, pupil dilation to attractiveness was
occasionally observed only for faces that were rated as most
attractive (an 8 or 9 rating score), either during a later time
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Volume 33, Number 2
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Figure 4. Pupil response results
in Experiment 4 (delayed
response or passive viewing
with line-drawing faces, as
shown in sample images). (A–B)
Mean pupil diameter change as
a function of time reference to
the target onset during the
delayed attractiveness judgment
for faces (A) or passive viewing
(B). Curves are parameterized
with rating score depending on
individual participants’ choices
(1 = least attractive and 9 =
most attractive). The color
shadows represent standard
errors among participants. The
gray shadow represents the
time window for averaging
the pupil size to present the
amount of pupil response for
statistical analysis (see Methods
for details and Table 3 for
results). (C–F) Box plots of
the mean pupil size over the
specified time window in
the specified task demand
conditions (0.5–1.5 sec in the
attractiveness rating condition
for C, 0.5–1.5 sec in the passive-
viewing condition for D, 3–5 sec
in the attractiveness rating
condition for E, and 3–5 sec in
the passive-viewing condition
for F). The plus signs represent
individual data.
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we examined how the pupil responded during accommo-
dation when viewing attractive visual objects. If pupil con-
stricted near focus time and this pupil constriction after the
focus to the visual object explained pupil constriction to
attractiveness, we expected to observe this focus-evoked
pupil constriction more vigorously when seeing attractive
visual objects.
We conducted the gaze-contingent pupillary response
analysis in which we aligned the pupil size data with the
gaze onset. Results are shown in Figure 14. In accordance
with the pupil near response, pupils on average constricted
after the gaze (i.e., during accommodation) when viewing
fixation display (the blue lines in Figure 14, except for the
geometric figure condition in Experiment 5 because the
luminance of the background was black whereas it was
white or gray in all the other conditions). Most importantly,
however, pupils in general dilated, instead of constricted,
after the gaze when viewing target images (the red lines in
Figure 14). This result surprised us in the sense that pupils
were expected to constrict near focus time, in particular,
when the face image was brighter than the background,
for example, in Experiment 1. We suspect that the pupil
dilation could be triggered by task demand, motor com-
mend, and/or emotional arousal to the faces as the dilation
was less observed when the rating response was post-
poned (Experiments 3 and 4), when the participant viewed
the images without any task (the passive-viewing condition
in Experiment 4), or when the to-be-judged stimuli were
natural scenes (Experiment 2) or geometric figures
(Experiment 1). In any case, the accommodation to focus
Liao, Kashino, and Shimojo
327
Figure 5. Pupil response results
in Experiment 5 (baseline
pupil response pattern was
manipulated by luminance
contrast change: pupil dilation
to faces and pupil constriction
to geometric figures). (A–B)
Mean pupil diameter change
as a function of time reference
to the target onset during
attractiveness judgment for
faces (A) or geometric figures
(B). Curves are parameterized
with rating score depending on
individual participants’ choices
(1 = least attractive and 9 =
most attractive). The color
shadows represent standard
errors among participants. The
gray shadow represents the
time window for averaging
the pupil size to present the
amount of pupil response for
statistical analysis (see Methods
for details and Table 3 for
results). (C–D) Box plots of
the mean pupil size over the
specified time window in the
face condition (C) and the
geometric figure condition
(D). The plus signs represent
individual data.
on the visual object did not necessarily lead to pupil
constriction, at least under the circumstance in our exper-
imental setup.
Together with the findings that the amount of pupil
constriction responding to visual images reflected attrac-
tiveness judgment regardless of whether the rating
response was asked immediately (Experiments 1 and 2)
or postponed (Experiment 3), and whether the visual
objects were faces or natural scenes, the overall results
indicate a clear dissociation between pupillary responses
to accommodation and to attractiveness. In other words,
the gaze-contingent pupillary response result indicates
that the eye accommodation alone cannot explain the
finding of pupil constriction to attractiveness.
Positive Loop between Pupil Constriction
and Attractiveness
Now, the second main objective of the current study was to
examine whether pupil constriction contributes to facial
attractiveness judgment. In Experiment 6, the amount of
pupil constriction was manipulated by relative background
luminance changes (from the prestimulus screen) to exam-
ine whether the attractiveness judgment for faces changed
accordingly. As shown in Figure 6B, displaying the face
after the black fixation display caused stronger pupil con-
striction than displaying it after the gray fixation display did,
F(1, 10) = 278.43, p < .001. The target background also
affected the pupillary response in that the pupil constricted
less for the black target background than it did for the gray
one, F(1, 10) = 153.51, p < .001, whereas the influence
of the pupil constriction was affected more strongly by
the luminance of the fixation display than that of the target
background (interaction: F(1, 10) = 55.26, p < .001).
In a casual survey after the experiment, most participants
reported that they were aware of the luminance change
in the target background but mentioned little about the
prestimulus display.
Critically, the attractiveness rating results are consistent
with our hypothesis that, when the pupil constricts more,
the face is evaluated as more attractive (see Figures 6B
and 6C for individual data). Specifically, parallel to the
amounts of pupil constriction, faces were rated more attrac-
tive after the black fixation display (mean rating score =
4.63 vs. 4.40), F(1, 10) = 7.22, p < .03. Note that the face
images were the same in their identities as well as in their
luminance in both the black and gray fixation display condi-
tions (see Methods for details). The results can only be at-
tributed to the prestimulus background luminance
changes. In contrast, the target background by itself did
not affect the rating (mean rating scores of 4.53 and 4.50
for the black and gray target backgrounds, respectively),
F(1, 10) = 0.18, p = .68. The nonsignificant difference in
the rating between the two types of target background indi-
cated that the simultaneous contrast alone could not affect
facial attractiveness judgments. Although the target
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background also induced significant changes in pupil size,
the effect may interact with target background luminance it-
self to obscure its influence on attractiveness judgment. This
is consistent with the casual survey in that some participants
claimed that the target background might have affected
their attractiveness judgment, but how it might have done
so was not consistent among their reports. Alternatively,
the lack of the effect of target background to attractiveness
judgment may be because of the weaker modulation of the
pupil constriction compared to the effect induced by the
prestimulus display. Either way, the results are consistent
overall with the interpretation that pupil constriction be-
cause of the sequential luminance contrast shift (from the
prestimulus background to the stimulus) leads to higher rat-
ings of attractiveness and that, in most cases, people are not
aware of the causal relationship there.
We would like to emphasize that, in our hypothesis, it is
the amount of pupil constriction (i.e., pupil constriction
amplitude), but not the prestimulus luminance, that con-
tributes to attractedness. To directly investigate the causal
relationship among pupil constriction amplitude, prestimu-
lus luminance (associated with prestimulus pupil base-
line), and attractiveness rating, we conducted partial
correlation analyses to examine whether it was the pupil
constriction amplitude or prestimulus pupil baseline that
predicted attractiveness rating when the effect of the other
pupil-related factor was removed. Results showed that,
consistent with our hypothesis, there was a significant
correlation between pupil constriction amplitude and
attractiveness rating when the effect of prestimulus pupil
baseline was removed (mean r = −.07), t(10) = −5.61,
p < .0001. By contrast, the correlation between prestimu-
lus pupil baseline and attractiveness rating was not signif-
icant when the effect of pupil constriction amplitude was
removed (mean r = −.02), t(10) = −0.77, p = .46. The
overall results indicated a direct link between pupil con-
striction amplitude, but not prestimulus pupil baseline
(or prestimulus luminance), and attractiveness rating.
However, one may still argue that either adaptation to
the prestimulus luminance (i.e., the fixation display’s lumi-
nance) or sequential contrast may lead to brightness dif-
ferences in faces, which may affect the attractiveness
judgment. In Experiment 7, we examined whether sequen-
tial luminance contrast alone, when not inducing a strong
difference in pupil response, causes differences in attrac-
tiveness judgments. Here, the sequential luminance con-
trast is defined as Weber contrast, that is, (I − Ib)/Ib with I
and Ib representing the luminance of the target images and
the background, where only the local region surrounding
the target image is taken into account. As shown in the
experimental procedure (Figure 7A), the (local) sequential
luminance contrast was changed between the conditions
when the face followed the black and white hemifields.
By contrast, the pupil response was expected to be similar
between the two conditions assuming that the pupil re-
sponded to overall global luminance change in general.
Liao, Kashino, and Shimojo
329
Figure 6. Procedure and results in Experiment 6. (A) Illustration of
experimental procedure (not to scale). (B) Pupil response results:
mean pupil diameter change as a function of time reference to the
target onset during the facial attractiveness judgment. Curves are
parameterized with prestimulus and target background luminance
conditions. Dotted lines represent the gray prestimulus condition.
Solid lines represent the black prestimulus conditions. Gray lines
represent the gray target background conditions. Black lines represent
the black target background conditions. The numbers on the right
are mean attractiveness rating scores corresponding to the prestimulus
and target background luminance conditions. (C) Mean attractiveness
rating as a function of prestimulus and target background luminance.
Each black line represents individual participants’ mean rating score.
Error bars represent standard errors among participants.
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Figure 7. Procedure and results
in Experiment 7. (A) Illustration
of experimental procedure (not
to scale). (B–C) Pupil response
results in Experiment 7a (B) and
7b (C): mean pupil diameter
change as a function of time
reference to the target onset
during the facial attractiveness
judgment. Curves are
parameterized with the
relationship between the target
face and prestimulus hemifield’s
luminance conditions. The
numbers in the squares are the
mean attractiveness rating
scores corresponding to the
prestimulus hemifield’s
luminance conditions. (D–E)
Mean attractiveness rating as a
function of prestimulus
hemifield’s luminance in
Experiment 7 when (D) eye
movement was allowed or (E)
eye movement was not allowed
(participants fixated the central
fixation cross throughout the
trial). Each black line represents
individual participants’ mean
rating score. Error bars
represent standard errors
among participants.
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Results are shown in Figures 7B and 7C. As predicted, the
pupil constricted similarly regardless of whether the face im-
age followed the black or white hemifield in the prestimulus
fixation display, although the pupil constriction difference
was (marginally) significant in opposite patterns depending
on the eye movement condition (the overall gaze location
was confirmed to be on the face image in the peripheral in
Experiment 7a and to be at the central fixation point in
Experiment 7b; see Figure 15; the pupil constricted more
strongly when the face followed the white hemifield than it
did when the face followed the black one in Experiment 7a,
t(15) = 2.90, p = .01, and vice versa in Experiment 7b,
t(16) = 2.12, p = .05). Accordingly, facial attractiveness judg-
ments showed similar scores between the two prestimulus
local luminance conditions, t(15) = 0.59, p = .56, in
Experiment 7a, and t(16) = 1.63, p = .12, in Experiment
7b (see Figures 7D and 7E for individual data), whereas the
slight rating difference tendency was in accordance with
the amount of pupil constriction rather than the local
sequential luminance contrast. The overall results suggest
the rating differences found in Experiment 6 cannot be ex-
plained solely by the local sequential luminance contrast.
Instead, it is more in accordance with the causal contribu-
tion of pupil constriction to attractiveness judgment.
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Volume 33, Number 2
Figure 8. Sample line-drawing
faces used in Experiment 4. The
five different faces have five
different pupil sizes.
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Figure 9. Behavioral results in Experiment 1. (A) Mean RTs as a function of rating scores. Error bars represent standard errors among participants.
(B) Histograms of rating scores.
Liao, Kashino, and Shimojo
331
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Figure 10. Behavioral results in Experiment 2. (A) Mean RTs as a function of rating scores. Error bars represent standard errors among participants.
(B) Histograms of rating scores.
DISCUSSION
We found an early and transient pupil constriction response
in proportion to attractiveness judgment. The constriction
response was found to be specific to esthetic object catego-
ries such as faces or natural scenes, as opposed to relative
emotionally neutral objects such as line-drawing geometric
figures. When participants were asked to judge the round-
ness of faces, pupil constriction still correlated with their
attractiveness but not the roundness rating score, indicating
the automaticity of the pupil constriction to attractiveness.
The result of pupil constriction to attractive faces was repli-
cated in three experiments by using various face stimuli:
natural color images (Experiment 1) and equal luminance
images (Experiments 2 and 3). Potential confounding fac-
tors such as gaze location and eye accommodation were
ruled out. Moreover, when we manipulated pupillary
responses implicitly by manipulating relative background
luminance changes (from the prestimulus screen), the
facial attractiveness ratings were in accordance with the
amount of pupil constriction (Experiment 6), and the result
could not be explained solely by simultaneous or sequential
luminance contrast (Experiments 7a and 7b). In summary,
we found a tight link between pupil constriction and facial
attractiveness, at least under certain conditions. The finding
could have profound implications with respect to the
well-known theories in the James–Lange tradition concern-
ing mind–body interaction. It could also provide new clues
to the neurophysiological mechanisms underlying attrac-
tiveness decisions and to implementing various real-world
applications such as brain–machine interfaces and market-
ing strategies.
Pupil Constriction vs. Dilation to Attractiveness
Our counterintuitive finding of pupil constriction, rather
than dilation, to facial attractiveness reveals a heretofore
unknown relationship between the pupillary response
and affective decision-making. The discrepancy between
our results and those in the literature can be understood
by considering three potential factors. First, the pupil is
highly sensitive to subtle luminance (contrast) differences,
and early studies (especially in the 1960s and 1970s) might
be just not technically capable of controlling it. Second, the
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Volume 33, Number 2
Figure 11. Sample images that
were rated as least attractive
(rating score = 1) or most
attractive (rating score = 9)
randomly selected from all
participants in Experiments 1
and 2. As shown in the
examples, the same image (e.g.,
the cherry photo in the natural
scene category) could be rated
as least or most attractive
depending on individual choice.
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temporal scale of pupil size measurement may have been
different in those classical studies relative to ours. Indeed,
we found two phases of pupil response over time: early
constriction (approximately 0–2 sec from the stimulus on-
set) and late dilation (after 3 sec) to attractive faces
(Experiment 4). In contrast to our approach that analyzed
the dynamic changes in the pupillary response on a finer
scale, previous studies only took the average pupil size over
time, typically 10 sec after the stimulus presentation (e.g.,
Rieger & Savin-Williams, 2012; Atwood & Howell, 1971;
Barlow, 1969; Koff & Hawkes, 1968; Nunnally et al., 1967;
Stass & Willis, 1967; Hess, 1965). They may have glossed
over the two dynamic phases of the pupil response (constric-
tion and then dilation) and thus failed to reveal the early,
transient component of the pupil constriction response to
attractiveness judgment. Third, the cognitive state (mental
set) has an influence on pupil response (e.g., Binda et al.,
2013, 2014; Mathôt et al., 2013, 2014; Naber et al., 2013).
The noncontrolled cognitive state (i.e., passive viewing) in
those classical studies (e.g., Rieger & Savin-Williams, 2012;
Atwood & Howell, 1971; Barlow, 1969; Koff & Hawkes,
1968; Nunnally et al., 1967; Stass & Willis, 1967; Hess,
1965) made it difficult to uncover the effects of cognitive
processes for decision and/or attractiveness evaluation
per se. The involvement of cognitive processes may be
deeper than just serving to reveal a different aspect of the
relationship between the pupil response and attractiveness.
Putting the above factors aside, reviews of recent stud-
ies that demonstrate the correlation between pupil dila-
tion and attractiveness judgment have revealed that the
correlation really depends on the observer’s and the
observed face’s gender (Rieger & Savin-Williams, 2012;
Simms, 1967) and emotion (Harrison, Gray, & Critchley,
2009; Harrison, Singer, Rotshtein, Dolan, & Critchley,
2006), suggesting a more complicated mechanism than
a straightforward linkage between pupil dilation and
attractiveness. One must conclude the traditional belief
that pupil dilation reflects attractiveness simply does not
Liao, Kashino, and Shimojo
333
Figure 12. Heat maps of gaze
distribution during target
presentation in Experiments 1
(A), 2 (B), 3 (C), 4 (D), and 5
(E), superimposed on the
sampled target image. The
target images were scaled to
the visual display viewed in the
real experiments. The luminance
of the background represents
approximate luminance in the
real experiments: gray in A–D,
white in the face condition, and
black in the geometric figure
condition in E. The gaze data
were accumulated from all trials
and all participants.
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account for what is really going on between the brain and
the eyes. It is important for future studies to closely exam-
ine the dynamic changes in pupillary response over time
to isolate the effects of cognitive processes and emotional
arousal.
Possible Neural Mechanism of Pupil Response
to Attractiveness
Pupil size is controlled by two sets of antagonistic muscles,
namely, the iris sphincter muscle and the iris dilator muscle,
innervated by parasympathetic and sympathetic nerves,
respectively. It is thus naturally presumed that one possible
underlying neural mechanism of the correlation between
facial attractiveness and pupil constriction is based on the
activation of the parasympathetic nervous system. Usui and
Hirata (1995) proposed a nonlinear dynamical model for the
human pupillary muscle plant. The model states that the
human pupil response to a flash visual stimulus can be ex-
plained by a combination of an early, transient parasympa-
thetic activation (within 2 sec) and a slow, sustained
deactivation of the sympathetic activation, and this was con-
firmed by pharmaceutical manipulation (Yamaji, Hirata, &
Usui, 2000). This is consistent with the hypothesis that early,
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Volume 33, Number 2
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Figure 13. Mean local luminance contrast change as a function of time reference to the stimulus onset in Experiments 1 (A), 2 (B), 3 (C), 4 (D), and 5
(E). Local luminance contrast was calculated by averaging the luminance of the image region being gazed at, that is, within1° of visual angle of the
gazing point. Curves are parameterized with rating score depending on individual participants’ choices.
transient pupil constriction to attractiveness is driven by the
parasympathetic nervous system. The cause of pupil dilation
to attractiveness, in contrast, is more complicated. It could
be because of emotional arousal activating the sympathetic
nervous system (Bradley et al., 2008) during the longer time
course and/or under a passive-viewing situation, in particular,
when the pupil response is less affected by a flash visual stim-
ulus. Alternatively, it could be because of a deactivation
Liao, Kashino, and Shimojo
335
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Figure 14. Gaze-contingent pupillary response results in Experiments 1 (A), 2 (B), 3 (C), 4 (D), and 5 (E). Mean pupil diameter change as a function
of time reference to the gaze onset, parameterized by the gaze being detected during the period of target presentation or fixation display.
and/or rebound of the parasympathetic nervous system
after the early 2-sec transient activation. In addition, other
factors, such as stimulus properties and/or task demands,
may activate the autonomic nervous system interactively.
For instance, it is possible that the face and natural scene
images used in the current study are more likely to induce
a joyful, relaxing, and/or soothing experience that activates
the parasympathetic nervous system, in contrast to inducing
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Volume 33, Number 2
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Figure 15. Heat maps of gaze distribution during 0–2 sec reference to the target onset in Experiment 7 when (A) eye movement was allowed or (B)
eye movement was not allowed (participants fixated the central fixation cross throughout the trial).
excitement, which may activate the sympathetic nervous sys-
tem dominantly. In the same vein, the failure of observing
any effect with geometric figures may be because of the lack
of attractiveness of the images we used in general or the
high similarity of the images, which made the attractiveness
judgment less differentiable, compared with faces or natu-
ral scenes. It is conceivable that attractiveness has multiple
meanings, and the judgment may change depending on the
context (e.g., when choosing a life partner vs. a queen in a
beauty contest). The relationship between pupil response
and attractiveness is not as simple as conventionally
believed. To better understand the neural mechanism of
attractiveness formation, further studies should investigate
how different factors such as a stimulus’s emotional valance
and strength affect its attractiveness, together with other
physiological measurements under different time courses.
This approach may have potential impact on decoding
complicated emotions instigated by the interaction
between the sympathetic and parasympathetic nervous
systems from eye metrics. In any case, our finding of pupil
constriction to attractiveness, after eliminating various
artifacts/side factors, is sufficient to raise a warning, in the
least.
That said, this parasympathetic nervous system hypoth-
esis alone does not directly account for the causal contribu-
tion of pupil constriction to facial attractiveness. Instead,
one may need to assume some sort of positive loop be-
tween liking and seeing to understand all the results that
we report here. According to the positive loop account,
the longer we see, the more we like, and vice versa, which
is supported further by Shimojo et al.’s earlier findings of
the gaze cascade (Shimojo et al., 2003). The pupil con-
stricts to increase visual acuity/clarity to obtain a sharper
facial image, to make it more attractive, or to facilitate
Liao, Kashino, and Shimojo
337
prolonged inspection time. Moreover, prolonged inspec-
tion may further activate the parasympathetic nervous sys-
tem, by accompanying the feeling of calming and soothing.
These may together participate in the decision-making for-
mation of liking. Although highly speculative, this scenario
is not only feasible physiologically but nicely incorporates
the parasympathetic account as a part of an entire dynamic
loop as well and is thus consistent with both the correlation
results (Experiments 1–and 3) and the causal results
(Experiment 6). Further studies using different approaches
to manipulate the pupils are required to examine our hy-
pothesis and/or investigate the underlying neural mecha-
nisms. For instance, one may use eye-drop-administered
cholinergic drug to manipulate the pupil size at different
timing references to the stimulus presentation to investigate
how quickly or slowly the autonomic nervous system inter-
acts with the eye-controlled system. Alternatively, one may
use artificial pupils (e.g., a piece of paper with a tiny hole)
to adjust the amount of image information into the retina
to investigate the relative contribution of the retinal input
and physiological/muscular control of the pupil size to
attractiveness.
Mind–Body Interaction and Implications
The finding that the pupil manipulation contributes to
facial attractiveness judgments should be added to the long
list of evidence for the James–Lange tradition of body–
mind causality, regardless of whether the above parasym-
pathetic account and positive-loop interpretations are
valid. In addition to the classical association between phys-
iological arousal and experienced emotion such as eupho-
ria and anger (Schachter, 1964), our findings reveal an
until-now-unknown physiological cause, that is, pupil
constriction, to mind (facial attractiveness judgment).
Although the physiological status is altered for unknown
reasons, a reason has to be given at the conscious level.
This is not that surprising as shown in the suspension
bridge effect (Dutton & Aron, 1974), where people tend
to misattribute unknown physiological arousal, that is,
the anxiety induced by walking on a suspension bridge,
to romantic attraction. In our case, the physiological
change, that is, the pupil constriction, is (mis)attributed
to evaluative attitudes toward facial attractiveness. The
possible prolonged looking behavior because of the pupil
constriction response is (mis)attributed to the preference
for the seen image.
In the same vein, our finding can be also interpreted as a
new example of “cognitive dissonance” and its solution, that
is, pupil constriction, at the implicit level. Cognitive disso-
nance refers to a mental state where a person holds two or
more contradictory beliefs, ideas, or attitudes at the same
time and experiences uncomfortable stress because of
that. In relation to affective decision-making, it has been
shown that choice per se creates preference for the cho-
sen object (Brehm, 1956) to reduce cognitive dissonance
(i.e., if something is disliked, then why would one choose
it). The same logic also applies to how inspection per se
affects preference during which the brain and eyes, includ-
ing gaze and pupil response, are involved. People prefer
the object that they look at longer (Shimojo et al., 2003;
Zajonc, 1968). In contrast to gaze, the contribution of
the pupil response to decision-making is implicit in two
senses. First, it is an automatic response that is nearly im-
possible to voluntarily control. Second, the process of fa-
cial attractiveness formation via pupil constriction is hardly
identifiable by attentive causal introspection.
After decades of neglect, pupillometry has been recently
been revived by studies showing that pupil response
reflects various cognitive processes, including attention
(Eldar et al., 2013; Einhäuser et al., 2008; Aston-Jones &
Cohen, 2005), memory (Zokaei et al., 2019; Naber et al.,
2013; Goldinger & Papesh, 2012), decision-making (de
Gee et al., 2014; Einhäuser et al., 2010), and linguistic
(Schmidtke, 2018) and auditory (Zhao et al., 2019; Liao,
Kidani, Yoneya, Kashino, & Furukawa, 2016; Liao,
Yoneya, Kidani, Kashino, & Furukawa, 2016) processing.
However, reexamining its relationship with attractiveness
judgment has attracted little interest, because of the belief
in the correlation between pupil dilation and attractive-
ness. The current study uncovers a heretofore unknown
tight link between pupil constriction and attractiveness.
In addition, it also indicates that pupil response likely
participates in the mechanism underlying attractiveness
judgment formation. Our finding goes beyond the scope
of reading the mind from the eyes, to further imply that
the neural mechanism that controls pupil responses also
massively interacts with higher-level cognitive processes
such as preference formation.
Acknowledgments
This study was partly supported by Core Research for Evolutional
Science & Technology (CREST) from the Japan Science and
Technology Agency (JST). We would like to thank Ms. Ying-Chun
(Ivy) Chen and Prof. Regina W.-Y. Wang for providing the line
drawing face stimuli.
Reprint requests should be sent to Hsin-I Liao, Communication
Science Laboratories, NTT Corporation, 3-1 Morinosato
Wakamiya, Atsugi, Kanagawa 243-0198, Japan, or via e-mail: hsini
.liao.pb@hco.ntt.co.jp.
Funding Information
Japan Science and Technology Agency (http://dx.doi.org/
10.13039/501100002241).
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