Cerebral Cortex December 2008;18:2729--2734
doi:10.1093/cercor/bhn034
Advance Access publication March 27, 2008
Human Amygdala Sensitivity to the Pupil
Size of Others
K.E. Demos, W.M. Kelley, S.L. Ryan, F.C. Davis and P.J. Whalen
Department of Psychological and Brain Sciences, Center for
Cognitive Neuroscience, Dartmouth College, Hanover, NH
03755, USA
Stimulation of the amygdala produces pupil dilation in animal and
human subjects. The present study examined whether the
amygdala is sensitive to variations in the pupil size of others.
Male subjects underwent event-related functional magnetic
resonance imaging while passively viewing unfamiliar female
faces whose pupils were either unaltered (natural variations in
large and small pupils) or altered to be larger or smaller than their
original size. Results revealed that the right amygdala and left
amygdala/substantia innominata were sensitive to the pupil size of
others, exhibiting increased activity for faces with relatively large
pupils. Upon debrief, no subject reported being aware that the
pupils had been manipulated. These results suggest a function for
the amygdala in the detection of changes in pupil size, an index of
arousal and/or interest on the part of a conspecific, even in the
absence of explicit knowledge.
Keywords: amygdala, arousal, attractiveness, fMRI, pupil size
Introduction
Humans are well adapted to detect and interpret the subtle yet
meaningful cues found within the human face. For example,
eye widening is a signal of heightened vigilance and arousal on
the part of the expresser, indicating that this individual has
detected a salient event in the immediate environment. The
human amygdala is sensitive to signals such as these, in part
because of the outcomes these expressions have predicted for
us in the past. For example, the widening eyes of a conspecific
suggest a significant change in that individual’s arousal state in
response to a proximal environmental event, and communicate
to the viewer that he or she may do well to adopt a similar state
of arousal. Consistent with this notion, multiple neuroimaging
studies have demonstrated that the amygdala is responsive to
fearful facial expressions (Breiter et al. 1996; Morris et al. 1996;
Whalen et al. 2001) and that subjective awareness of the
expression is not necessary to produce this response (Whalen
et al. 1998; Pessoa et al. 2006). Indeed, a particular sensitivity to
the telltale, widened eyes of a fearful face may be the basis for
this effect (Whalen et al. 2004).
Like eye widening, pupil dilation is considered another facial
signal indicating heightened vigilance on the part of a conspe-
cific. Indeed, data from both animal (Applegate et al. 1983;
Ursin and Kaada 1960) and human (Gloor 1997) subjects show
that electrical stimulation of the amygdala produces both eye-
widening and pupil dilation. Based upon these findings, the
present study was designed to investigate whether greater
amygdala activity would be observed in response to presented
faces with larger pupils compared with faces with smaller
pupils, and, if so, whether this response would occur
irrespective of participants’ subjective awareness of the
manipulation.
Interestingly, pupil dilation has long been thought to convey
interest in conspecifics, as women of the Victorian Era and the
Italian Renaissance purposefully dilated their pupils using
a poisonous extract from the Belladonna plant to appear more
attractive to male suitors. Because experimental research on
this subject has offered conflicting reports as to whether pupil
size affects the perceived attractiveness of presented faces
(Hess and Polt 1960; Janisse 1973) and attractiveness ratings
can be associated with amygdala activity (Winston et al. 2007),
we thought it important to assess a subject group where
attractiveness ratings could also be readily measured. To this
end, we studied male subjects viewing images of female faces
that depicted big or small pupils (Fig. 1). Such a design allows
for an assessment of the relationship between amygdala activity
and pupil size, where any impact of the perceived attractive-
ness of the faces can be determined.
Methods
Subjects
Twenty-seven right-handed males from the Dartmouth community
between the ages of 18 and 33 (mean age = 22 years) participated in
this experiment. No subjects reported abnormal neurological history
and all had normal or corrected-to-normal visual acuity. Each subject
provided informed consent in accordance with the guidelines set by
the Committee for the Protection of Human Subjects at Dartmouth
College, and received either course credit or monetary compensation
for participating in this study.
Apparatus
Imaging was performed on a Philips Intera Achieva 3-Tesla scanner
(Phillips Medical Systems, Bothell, WA) with a SENSE (SENSEitivity
Encoding) head coil. During scanning, visual stimuli were generated
with an Apple G3 Laptop computer running Psyscope software (Cohen
et al. 1993). An Epson (model ELP-7000) LCD projector displayed
stimuli on a screen positioned at the head end of the scanner bore
which subjects were able to view through a mirror mounted on top of
the head coil. Following scanning subjects were behaviorally tested via
an Apple PowerBook G4 running Psyscope software.
Imaging
Anatomic images were acquired using a high-resolution 3D magneti-
zation-prepared rapid gradient echo sequence (60 sagittal slices, time
echo [TE] = 4.6 ms, time repetition [TR] = 9.9 ms, flip angle = 8
, voxel
size = 1 3 1 x 1 mm). Functional images were collected in a single run
using T2* fast field echo, echo planar functional images sensitive to
blood oxygenation level--dependent (BOLD) contrast (TR = 2500 ms,
TE= 35 ms, flip angle = 90
, 3 3 3 mm in-plane resolution). During
the functional run, 190 sets of axial images (36 slices, 3.5-mm slice
thickness, 0.5 mm skip between slices) were acquired parallel to the
 2008 The Authors
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which
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horizontal axis of the anterior and posterior commissures, allowing
complete brain coverage.
Procedure
A total of 73 unfamiliar female faces were compiled from a standardized
face database used in previous studies (Kelley et al. 1998; Wig et al.
2004). These forward-facing face images were originally sampled from
the media and cropped below the chin line and around the outer
hairline and were presented on a solid black background. Approxi-
mately half of the images (37) portrayed female faces with naturally
large pupils; the remaining half (36) portrayed female faces with
naturally small pupils as determined by 3 independent raters. Each
image was digitally edited using Adobe Photoshop 7.0 (San Jose, CA) to
create an altered big-pupil or small-pupil counterpart. In order to
preserve the natural variations in pupil size within a biologically
plausible range, naturally big pupils were reduced by 30% in area and
naturally small pupils were enlarged by 30% in area (Fig. 1). This
allowed each individual face to be presented in either its natural or
altered state and ensured that faces comprising the big and small pupil
conditions were made up of equal numbers of natural and altered faces.
During scanning, subjects viewed only 1 version of the face. Face
stimuli were further counterbalanced such that half of the subjects
viewed the big-pupil version (e.g., naturally big) and the other half
viewed the small-pupil version (e.g., altered small). The pupils of faces
comprising the BIG condition were, on average, 52.4% of the entire iris
size with sizes ranging from 2.54--4.60 mm, (mean = 3.44 mm, SD = 0.56
mm) whereas the pupils of faces comprising the SMALL condition
were, on average, 36.5% of the entire iris, with sizes ranging from 1.52
to 4.24 mm (mean = 2.40 mm, SD = 0.49 mm). Pupil size did not differ
between the left and right eyes of each face (BIG: mean right =
3.44 mm, mean left = 3.40 mm; t [72]=1.53, P = 0.131; SMALL: mean
right = 2.40 mm, mean left = 2.36 mm; t [72] = 0.86, P = 0.393).
All faces were normalized on explicit measures of valence and
arousal in a separate set of 15 male volunteers. Each subject viewed
a single version of each face (natural or altered) and the stimuli were
counterbalanced such that half of the subjects viewed the big-pupil
version of a given face while the remaining half viewed the small-pupil
version of that face. Explicit ratings of valence and arousal did not differ
as a function of pupil size (VALENCE: t [14] = 0.96, P = 0.355; AROUSAL:
t [14] = 1.44, P = 0.171).
During scanning, subjects passively viewed faces of each type (BIG
and SMALL pupils) presented 1 at a time for 2000 ms each in an
event-related functional run. Face trials were pseudorandomly inter-
mixed with jittered periods of fixation, creating a variable interstimulus
interval ranging from 0 to 7500 ms and allowing for computation of
unique estimates of the hemodynamic response associated with
viewing faces with BIG and SMALL pupils (Ollinger et al. 2001).
Although pupil dilation has been anecdotally linked to increases in
perceived attractiveness, the extant experimental literature has been
mixed (Hess and Polt 1960; Janisse 1973). Nonetheless, attractiveness
ratings have been recently associated with changes in amygdala activity
(Winston et al. 2007). To ensure that putative differences in amygdala
activity in response to changes in pupil size were not confounded with
changes in perceived attractiveness, each subject participated in
a postscan behavioral session. Following scanning, subjects were asked
to rate each face on a 9-point Likert scale of attractiveness (1 =
extremely unattractive; 5 = average; 9 = extremely attractive). Faces
were again presented in random order for 2000 ms followed by a 1500-
ms fixation crosshair with the labeled scale appearing at the bottom of
the screen below each face. Subjects were given 3500 ms to respond.
Each participant 1st completed a practice session with a set of 15
novel faces in order to assimilate to the task, and then performed the
task on the same faces they had viewed during scanning.
Assessment of Explicit Knowledge
To determine whether subjects were aware of the experimental
manipulation, after viewing all faces, subjects were given a comprehen-
sive list of 16 facial features (e.g., forehead, eyes, pupils, nose, nostrils)
and were asked to recall if they noticed anything about the faces with
respect to any of the listed features. After providing their responses,
subjects were informed that 1 facial feature was in fact systematically
varied. Subjects were then instructed to select 1 of the 16 facial
features even if they felt that they were guessing.
Functional Magnetic Resonance Imaging Analysis
Functional magnetic resonance imaging (fMRI) data were analyzed
using the general linear model for event-related designs in SPM2
(Wellcome Department of Cognitive Neurology, London, UK). Data
were preprocessed to remove sources of noise and artifact, corrected
for differences in acquisition time between slices for each whole-brain
volume, realigned within the run to correct for head movement, and
coregistered with each participant’s anatomical data. Functional data
were then transformed into a standard anatomical space (3-mm
isotropic voxels) based on the ICBM 152 brain template (Montreal
Figure 1. Examples of big- and small-pupil faces. Face images depicted females with naturally big pupils and naturally small pupils. An altered big- or small-pupil counterpart of
each face was created such that the same individual could be represented in either the big- or small-pupil condition. Shown here is a photograph depicting naturally big pupils (a,
c) and its altered small-pupil counterpart (b, d).
2730 Human Amygdala Sensitivity to the Pupil Size of Others d Demos et al.

Neurological Institute) which approximates Talairach and Tournoux’s
(Talairach and Tournoux 1988) atlas space. Normalized data were then
spatially smoothed (6-mm full-width-at-half-maximum) using a Gaussian
kernel. Analyses took place at 2 levels: formation of statistical images
and regional analysis of hemodynamic responses.
For each subject, a general linear model incorporating condition
effects (modeled as events convolved with the canonical hemodynamic
response function), and covariates of no interest (a session mean, a linear
trend to account for low-frequency noise, and 6 movement parameters
obtained from realignment) were used to compute parameter estimates
(b) and t-contrast images (containing weighted parameter estimates) for
each comparison at each voxel. These individual contrast images were
then submitted to a 2nd-level, random-effects analysis to create mean
t-images images (thresholded at P < 0.01, uncorrected, minimum clus-
ter size = 5 voxels). Monte Carlo simulations indicate this threshold
corresponds to a small-volume corrected alpha (P = 0.05) based upon the
size of the amygdala bilaterally (Kim et al. 2003, 2004; Whalen et al. 2004;
Johnstone et al. 2005).
In order to more closely explore the patterns of activity in the
amygdala, region of interest (ROI) analyses were conducted. ROIs were
defined functionally from a set of peak activations observed in the
direct contrast of big- versus small-pupil faces (all contiguous voxels
within 6 mm of the peak that exceeded P < 0.01). For each participant,
signal intensities for face conditions relative to fixation baseline from
ROIs were submitted to offline statistical analyses.
Results
fMRI Results
When compared directly, big-pupil faces yielded greater
activity than small-pupil faces within the amygdaloid complex
(Fig. 2). This effect was observed bilaterally, in the amygdala
proper (right amygdala ROI: 27 –7 –15; t [26] = 3.32, P < 0.005)
and extended into the substantia innominata within the ventral
basal forebrain in the left hemisphere (Left Amygdala/SI ROI: –
30 –1 –10; t [26] = 3.96, P < 0.005) (Fig. 3). ROI analyses further
revealed that when each face condition was considered relative
to the fixation baseline, activity significantly increased in
response to big pupils (R Amygdala: t [26] = 4.67, P < 0.0001;
L Amygdala/SI: t [26] = 5.34, P < 0.0001) and did not differ from
baseline in response to small pupils (R Amygdala: t [26] = 0.85, P
= 0.40; L Amygdala/SI: t [26] = –1.43, P = 0.17).
Importantly, the greater amygdala activity for BIG versus
SMALL pupils did not depend on whether pupils were viewed
in their natural or altered state. When amygdala activity was
considered as a function of both pupil size (BIG vs. SMALL) and
image type (NATURAL vs. ALTERED), a 2 3 2 repeated
measures ANOVA revealed a main effect of pupil size (R
Amygdala: F1,26 = 10.35, P < 0.005; L Amygdala/SI: F1,26 = 30.06,
P < 0.0001), no main effect of image type (both F’s < 1), and no
interaction (both F’s < 1).
Although the amygdala was the sole a priori ROI herein, other
brain regions identified exhibiting greater activity for big- versus
small-pupil faces included the left superior frontal gyrus
(Broadmann’s area [BA] 8 and 11), the medial frontal gyrus (BA
10), the lingual gyrus (BA 19), a region of the left inferior parietal
lobe (BA 40), and the left ventral lateral nucleus of the thalamus
(Supplementary Table 1). The response in regions including the
precuneus (BA 7), right inferior frontal gyrus (BA 45), and the
inferior occipital gyrus (BA 18) was greater for viewing SMALL
compared with BIG pupils (Supplementary Table 1).
Effect of Attractiveness
Behaviorally there were no differences in subsequent ratings of
attractiveness between faces with big pupils and faces with
small pupils (mean BIG = 4.65, SD BIG = 0.63, mean SMALL =
4.56, SD SMALL = 0.65; t [26] = 1.50, P = 0.15). Importantly, the
difference in amygdala activity in response to BIG versus
SMALL pupils did not reflect the perceived attractiveness of the
faces. When attractiveness ratings were included as a separate
covariate in the model, amygdala sensitivity to pupil size was
preserved (R Amygdala ROI: 27 –7 –15; t [24] = 2.83, P < 0.01; L
Amygdala/SI ROI: –30 –1 –10; t [24] = 4.84, P < 0.001).
Effect of Expression
A final potential factor we considered post hoc was whether
facial expression influenced amygdala response to pupil size.
Although facial expressions were not explicitly manipulated in
the current study, the face stimuli employed here included
both neutral (i.e., expressionless) and happy (i.e., smiling) faces.
To explore this possibility, we conducted a 2nd 2 3 2 ANOVA
examining the effects of pupil size and facial expression.
Results of this analysis revealed a main effect of pupil size in
both the left and right amygdala (R Amygdala: F1,26 = 9.33, P <
0.005; L Amygdala/SI: F1,26 = 25.67, P < 0.0001), no main effect
of facial expression (R Amygdala: F1,26 = 2.24, P = 0.15; L
Amygdala/SI: F < 1), and no interaction (R Amygdala:
F < 1; L Amygdala/SI: F1,26 = 1.84, P = 0.19).
Debriefing Results
An important facet of the present study design was that big-
and small-pupil images of the same facial identity were not
presented within subject. This was done to mitigate subjects’
Figure 2. Amygdala responses during passive viewing of big- and small-pupil faces.
Coronal sections show greater activity for big- versus small-pupil faces. Images are
coronal sections in Talairach and Tournoux (1988) atlas space. Colored pixels
exceeded the statistical threshold (P\ 0.01, uncorrected, minimum cluster size 5 5
voxels) and are superimposed on corresponding anatomy images. The left side of the
image corresponds to the left hemisphere at a y coordinate of
1 and the right side
of the image corresponds to a y coordinate of
7. Greater activity to BIG versus
SMALL pupils was observed in both the right amygdala (27
7
15) and the left
amygdala extending into the substantia innominata within the ventral basal forebrain
(
30
1
10).
Cerebral Cortex December 2008, V 18 N 12 2731

awareness of the experimental manipulation of pupil size.
When subjects were given a list of 16 face parts and asked to
comment on any face feature they wished, none spontaneously
reported that they had noticed anything about pupils. Further,
when subjects were then informed that 1 facial feature had
indeed been manipulated, none correctly guessed pupil size.
Discussion
In the present study, greater amygdala activity was observed in
response to faces with big versus small pupils. This sensitivity
to the larger pupil size of others was observed in bilateral
regions across the amygdaloid complex and emerged even
though subjects did not report explicit knowledge of the
varying pupil sizes. Moreover, changes in pupil size were
unrelated to changes in perceived attractiveness. Subjective
ratings of attractiveness did not differ between big- and small-
pupil faces, a finding that discounts attractiveness as a basis for
the observed effect. Further, the amygdala’s response to big-
versus small-pupil faces did not differ as a function of the facial
expressions considered here (i.e., neutral and happy faces).
Pupil dilation has been interpreted as a general indicator of
increased vigilance, arousal, and/or interest (Hess 1965;
Steinhauer et al. 2004), indexing behavioral responses as
disparate as political interests, hunger, cognitive load, and
attraction (Hess 1965; Steinhauer et al. 2004). The nonspecific
nature of pupil dilation is relevant when considered in light of
data showing that the amygdala is particularly responsive when
cues predict multiple outcomes (Kapp et al. 1992; Whalen
1998; Holland and Gallagher 1999). In this way, amygdala
activity potentiates neuronal responses in sensory cortical
regions that may, in turn, facilitate subsequent information
processing (Amaral et al. 1992; Kapp et al. 1992; Whalen 1998;
Morris et al. 2001; Pessoa et al. 2006; Phelps 2006). Put simply,
amygdala sensitivity to pupil dilation is usefully conceptualized
as an alerting response related to the wide range of biologically
relevant outcomes that this signal can predict.
Pupil dilation is produced through the sympathetic nervous
system, whereas pupil constriction is produced by the para-
sympathetic nervous system. Specifically, dilation occurs via
sympathetic nerve fibers arising from the 1st, 2nd, and
3rd thoracic nerves of the spinal cord, which innervate the
radial muscles of the iris through connections in the superior
cervical sympathetic trunk ganglia. By contrast, the para-
sympathetic nerve fibers responsible for pupil constriction
arise from the Edinger--Westphal nucleus within the brain stem
and innervate the circular muscles of the iris. Although it is
anatomically plausible then that the amygdala facilitates pupil
dilation by either stimulating sympathetic inputs or inhibiting
parasympathetic inputs, research to date suggests that the
amygdala’s effect on dilation is likely an inhibition of para-
sympathetic input (Bitsios et al. 1996, 1998, 1999; Hourdaki
et al. 2005). The existing data further suggest that the amygdala
influences pupil dilation through an indirect route via the
hypothalamus (Koss and Wang 1972; Saper et al. 1976; Holstege
1987) and/or locus coeruleus (Loewy et al. 1973; Breen et al.
1983; Koss et al. 1984). Still, direct modulation of pupillary
kinetics is plausible as the amygdala is known to send extensive
projections to these regions of the brain stem and spinal cord
(Cedarbaum and Aghajanian 1978) that directly modulate pupil
size.
Upon excitation of the sympathetic nervous system, the
pupils obligatorily dilate, thus a conspecific’s pupil dilation is
often an indicator of his or her increased arousal. This notion
holds similarly for eye-widening, another indicator of height-
ened arousal, and accordingly, these data support and extend
the finding that the amygdala is also sensitive to the eye-region
of the face (Adolphs et al. 2005) and more specifically, eye-
widening (Morris et al. 2002; Whalen et al. 2004). Because
stimulation of the amygdala produces an increase in non-
specific arousal that is accompanied by peripheral responses
such as eye-widening and pupil dilation (Ursin and Kaada 1960;
Applegate et al. 1983; Kapp et al. 1994), it will be important to
investigate in a future study using a similar experimental design
whether the pupil size of participants varies as a function of the
size of observed pupils.
Indeed, a recent study demonstrated that amygdala activity
and the size of an observer’s pupil may be associated with the
size of perceived pupils (Harrison et al. 2006). This study
investigated whether amygdala activity in response to pupil
size differences interacted with the emotional expression of
the presented face (e.g., fearful, angry, sad and neutral). The
results showed that the amygdala was more responsive to
smaller pupils sizes, but only for sad faces, suggesting that this
Figure 3. Signal change in left- (a) and right-amygdaloid (b) regions for big- and small-pupil faces. Signal intensities (arbitrary units) for each condition are plotted relative to
a baseline control condition (fixating a crosshair). For both ROIs, activity was significantly greater than baseline when subjects viewed big-pupil faces but was no different from
baseline when viewing small-pupil faces. Error bars indicate standard error of the mean.
2732 Human Amygdala Sensitivity to the Pupil Size of Others d Demos et al.

result (opposite to that observed here) may be related to an
important interaction between facial expression, amygdala
activity and pupil size. The challenge then for future research
will be to disentangle the meaning of amygdala activity to the
pupil size of a conspecific across differing experimental
designs. For example, the findings of Harrison et al. (2006)
showed that the amygdala was only sensitive to pupil size for
faces with sad expressions; however, varying pupil sizes for sad
faces also produced significant differences in explicit ratings of
arousal and valence. Given the broad literature demonstrating
that the amygdala is sensitive to changes in valence and arousal
(Anderson et al. 2003; Kim et al. 2003, 2004; Small et al. 2003),
an alternative explanation of the Harrison et al. (2006) findings
is that the amygdala response observed in their work reflected
a more general sensitivity to valence and/or arousal. Further-
more, they report that subjects’ pupil size mirrored these
effects, demonstrating contagion such that subjects’ pupils
constricted in response to the viewing small-pupil faces.
Although we did not measure such changes in the present
study, 1 critical caveat to consider is that direct stimulation of
the amygdala is consistently associated with pupil dilation
rather than constriction (Ursin and Kaada 1960; Koss and Wang
1972; Loewy et al. 1973; Saper et al. 1976; Cedarbaum and
Aghajanian 1978; Applegate et al. 1983; Breen et al. 1983;
Koss et al. 1984; Kapp et al. 1994; Holstege 1987; Bitsios et al.
1996; Bitsios et al. 1998, 1999; Gloor 1997; Hourdaki et al.
2005). Thus, it is somewhat unclear under what circumstances
BOLD signal activation in the amygdala might index pupil
constriction.
In the present experimental design, pupil size was manip-
ulated within stimulus conditions that controlled for explicit
valence, arousal, and attractiveness ratings. Indeed, when
comparing the present results to other experimental designs,
it will be critical to note that these findings were observed in
male subjects while viewing female faces. Further, in the
present study, subjects did not have any explicit knowledge of
the differing pupil sizes between big- and small-pupil faces,
perhaps owing to the biologically plausible range of pupil sizes
presented and the slight overlap in size across conditions. Thus,
it is possible that amygdala response to these changes was
observed on an implicit basis in the present study. It is
interesting to note that Harrison et al. (2006) utilized a much
greater range of pupil sizes (i.e., 64--180% of natural size) which
was likely more noticeable to subjects, offering another
potential basis for the differential effects observed across these
2 initial studies. Clearly, then, both studies support the notion
that the amygdala is sensitive to subtle changes in pupil
size—findings that warrant future work aimed at determining
the precise functional relationship between amygdala activity,
pupil size, and implicit preference.
Conclusion
Though much is known about human amygdala responses to
stimuli that produce a strong state of fear (LeDoux 1996; LaBar
et al. 1998), more recent work has begun to elucidate a role for
the amygdala in detecting biologically relevant stimuli that call
for a more generalized (Hamann et al. 2002) or more subtle
level of state change (Whalen et al. 1998; Davis and Whalen
2001). Here we offer additional data showing that pupil dilation
is 1 such subtle signal of nonspecific arousal and vigilance to
which the amygdala is sensitive. Importantly, the big- and small-
pupil faces in our work did not differ in explicit ratings of
arousal, valence, or attractiveness, yet the amygdala responded
with increased activity to the big-pupil faces. Finally, this effect
occurred without explicit awareness on the part of subjects.
Future research could seek to define the effect of this implicit
environmental monitoring in terms of behavioral outcomes for
the organism of study.
Supplementary Material
Supplementary material can be found at: http://www.cercor.
oxfordjournals.org/
Funding
National Institute of Health (NIMH 069315 and NIMH 080716).
Dartmouth Brain Imaging Center.
Notes
We thank Jasmin Cloutier, Jed Dobson, Howard Hughes, Jay Hull, Joe
Moran, Tammy Moran, Leah Somerville, and Gagan Wig for their
assistance. Conflict of Interest : None declared.
Address correspondence to Kathryn Demos, BS, HB6207 Moore Hall,
Psychological and Brain Sciences, Dartmouth College, Hanover, NH
03755, USA. Email: kathryn.e.demos@dartmouth.edu.
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