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Automatic and Intentional Brain Responses During Evaluation of Trustworthiness of Faces

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Successful social interaction partly depends on appraisal of others from their facial appearance. A critical aspect of this appraisal relates to whether we consider others to be trustworthy. We determined the neural basis for such trustworthiness judgments using event-related functional magnetic resonance imaging. Subjects viewed faces and assessed either trustworthiness or age. In a parametric factorial design, trustworthiness ratings were correlated with BOLD signal change to reveal task-independent increased activity in bilateral amygdala and right insula in response to faces judged untrustworthy. Right superior temporal sulcus (STS) showed enhanced signal change during explicit trustworthiness judgments alone. The findings extend a proposed model of social cognition by highlighting a functional dissociation between automatic engagement of amygdala versus intentional engagement of STS in social judgment.
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It is conjectured that human survival has depended to a large
extent on accurate social judgments and that, as an evolution-
ary consequence, modular cognitive processes are devoted to
such functions
1
. Neuropsychological studies and human func-
tional imaging provide partial support for this idea of a dedi-
cated ‘social intelligence, particularly studies that address
perception of facial expression
2–7
. However, facial emotional
expression is only one aspect of social judgment made about
others. In many situations, individuals must also decide whether
another person is someone to approach or avoid, trust or dis-
trust. Preliminary evidence regarding the neural underpinnings
of this sort of evaluative judgment comes from studies in which
patients with bilateral amygdala lesions make abnormal social
judgments about others based on facial appearance
8
. These
abnormalities are most pronounced in relation to faces that
received the most negative ratings by control subjects. Notably,
such deficits are not apparent in subjects with unilateral amyg-
dala lesions
8
. Patients with damage to ventromedial prefrontal
cortex also have difficulties with trustworthiness decisions
9,10
.
The most influential neurobiological model of social cogni-
tion
11
, based on inferences largely from neurophysiological record-
ings in non-human primates, postulates that the superior temporal
sulcus acts as association cortex for processing conspecifics’ behav-
ior and that socially relevant information is subsequently labeled by
the emotional systems, such as amygdala and orbitofrontal cortex.
More recent models of human social cognition also include sen-
sory regions such as the face-processing area in fusiform gyrus and
somatosensory cortex (including insula, SI and SII)
12–14
.
Here we used event-related functional magnetic resonance
imaging (fMRI) to ascertain the neural substrates mediating eval-
Automatic and intentional brain
responses during evaluation of
trustworthiness of faces
J.S. Winston
1
, B.A. Strange
2
, J. O’Doherty
1
and R.J. Dolan
1,3
1
Wellcome Department of Imaging Neuroscience, 12 Queen Square, London WC1N 3BG, UK
2
Institute of Cognitive Neuroscience, 17 Queen Square, London WC1N 3AR, UK
3
Royal Free and University College Medical School, Roland Hill Street, London NW3 2PF, UK
Correspondence should be addressed to J.S.W. (j.winston@fil.ion.ucl.ac.uk)
Published online: 19 February 2002, DOI: 10.1038/nn816
Successful social interaction partly depends on appraisal of others from their facial appearance. A
critical aspect of this appraisal relates to whether we consider others to be trustworthy. We
determined the neural basis for such trustworthiness judgments using event-related functional mag-
netic resonance imaging. Subjects viewed faces and assessed either trustworthiness or age. In a
parametric factorial design, trustworthiness ratings were correlated with BOLD signal change to
reveal task-independent increased activity in bilateral amygdala and right insula in response to faces
judged untrustworthy. Right superior temporal sulcus (STS) showed enhanced signal change during
explicit trustworthiness judgments alone. The findings extend a proposed model of social cognition
by highlighting a functional dissociation between automatic engagement of amygdala versus inten-
tional engagement of STS in social judgment.
uative social judgment. Processing of facial emotion can be
implicit, occurring when subjects make judgments about facial
attributes unrelated to emotion (for example, refs. 5–7, 15, 16). To
establish whether trustworthiness judgments might be similarly
processed, we used a task in which subjects viewed faces while
making either explicit judgments whether an individual was trust-
worthy or an unrelated age assessment. To account for individ-
ual differences in trustworthiness judgment, we acquired ratings
of trustworthiness for each stimulus from each subject after scan-
ning and used these ratings as parametric covariates in our sub-
sequent analysis. Based on the models of social cognition outlined
above
11–14
, along with the neuropsychological findings
8
, we pre-
dicted that discrete brain regions, the amygdala, orbitofrontal
cortex, fusiform gyrus and superior temporal sulcus, would be
implicated in trustworthiness assessments. Consequently, these
areas formed regions of interest in our statistical analysis. Our
data indicate that social judgments about faces involve such a
network and that this network is differentially modulated by
implicit and explicit evaluations.
R
ESULTS
Behavioral
After scanning, on average subjects labeled more than half of the
120 faces as having ‘neutral’ emotional expressions (mean, 65).
Labeled emotional expression interacted significantly with trust-
worthiness score across the group of subjects (Kruskal-Wallis test,
p < 0.001). Mann-Whitney U tests showed that the trustworthi-
ness scores (from 1, least trustworthy, to 7) did not differ signif-
icantly between ‘disgusted,’ ‘fearful’ and ‘surprised’ faces and
‘neutral’ faces (p > 0.05 in all cases). ‘Happy’ faces (mean trust-
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nature neuroscience • volume 5 no 3 • march 2002 277
© 2002 Nature Publishing Group http://neurosci.nature.com
worthiness rating, 4.0) were rated as significantly more trust-
worthy than ‘neutral’ faces (mean rating, 3.9), and ‘angry’ (mean
rating, 2.7) and ‘sad’ (mean rating, 3.6) faces as significantly less
trustworthy (p < 0.01 in all cases). Mean trustworthiness scores
(Fig. 1a) were significantly correlated with mean scores for anger,
happiness and sadness from the second group of subjects (see
Methods) (p < 0.01 for each, two-tailed; Fig. 1b–e).
Neuroimaging
Linear contrasts were performed to produce statistical paramet-
ric maps (SPMs) of the main effect of task (explicit or implicit
processing of trustworthiness), the main effect of trustworthi-
ness and the interaction between these two factors. An addition-
al model in which the effects of facial emotion of the stimuli were
included as covariates of no interest was used to generate an SPM
related to the main effect of trustworthiness independent of
effects of facial emotional expression.
Fig. 2. Main effect of explicit social judgments. (a) Random-
effects SPM overlaid on a normalized structural scan from
a single subject showing activation in right superior tem-
poral sulcus region (x, y, z = 56, –44, 4; Z = 4.27; p < 0.05
small volume corrected) when making judgments about
trustworthiness compared to age. For illustration, using
threshold p < 0.001 uncorrected, extent threshold of
5 voxels. (b) BOLD signal measure categorized by task
and trustworthiness of faces. ‘Low,’ ‘med’ and ‘high’ refer
to the least trustworthy third, median third and most
trustworthy third of faces calculated in the second model
described in Methods. The y-axis represents mean (across
subjects) percentage signal change relative to whole brain
mean over scanning session for each event type. There is
no clear pattern of response to the faces according to
trustworthiness. Note that statistical inference is drawn
only from the parametric model described in Methods,
and not from the illustrative model in (b).
278 nature neuroscience • volume 5 no 3 • march 2002
A significant activation in the explicit compared to implicit
task, independent of trustworthiness, was found in the right pos-
terior superior temporal sulcus (x, y, z coordinates, 56, –44, 4;
Z = 4.27; p < 0.05, corrected for multiple comparisons across a
small volume of interest; Fig. 2; Table 1). Additionally, primary
visual cortex was significantly activated in this contrast. Attentional
and emotional manipulations are known to alter neural respons-
es in early visual cortex
17
, and we propose that similar processes
engendered by the explicit task account for this latter activation.
As predicted, significant bilateral amygdala activation was evi-
dent in the contrast of untrustworthy to trustworthy faces (right,
–18, 0, –24; Z = 4.29; left, –16, –4, –20; Z = 3.92; both p < 0.05,
corrected for multiple comparisons across a small volume of inter-
est; Fig. 3a). This examination of parametric data based on each
subject’s ratings of faces indicates that more untrustworthy faces
evoke greater BOLD responses in the amygdala (Fig. 3c and d).
articles
Fig. 1. Trustworthiness and emotion ratings for stimuli. (a) Means and standard deviations of trustworthiness scores of stimuli, rank-ordered by
trustworthiness score. (be) Mean emotion scores (from second cohort of sixteen subjects) and mean trustworthiness scores (from cohort of sub-
jects scanned with fMRI) for anger (b), fear (c), happiness (d) or sadness (e). Lines of best fit are derived by linear regression. Both rating scales
ranged from 1 (low degree of emotion or highly untrustworthy) to 7 (highly emotional or highly trustworthy).
a
b
d
c
e
a
b
© 2002 Nature Publishing Group http://neurosci.nature.com
Further areas showing increased response to untrustworthy
faces included left superior temporal sulcus (–50,–58,10; Z = 4.15)
and a region of the right superior middle insula (42, –4, 12; Z = 3.48;
Fig. 3a and b). Additionally, bilateral activation in the fusiform
gyrus was evident in this contrast (right, 44, –46, –24, Z = 3.58;
left, –48, –48, –24; Z = 3.60; both p < 0.05, corrected for multiple
comparisons across a small volume of interest; Fig. 4). Table 2 pre-
sents regions highlighted by this contrast as well as regions high-
lighted as more responsive to faces rated as trustworthy.
To ensure that the main effect of untrustworthiness was not dri-
ven by a highly significant activation in just one of the tasks alone,
a masked conjunction of simple effects of trustworthiness under
implicit and explicit task conditions was carried out (Methods).
This analysis confirmed that bilateral amygdala,
fusiform gyrus and right insula showed signifi-
cant responses to untrustworthy faces indepen-
dent of task. Notably, left STS activation was not
observed in this contrast, and a post-hoc test
revealed that the effects in this region were dri-
ven principally by trustworthiness judgments
under the explicit task.
The contrast pertaining to the interaction
of task and trustworthiness demonstrated an
area in the lateral orbitofrontal cortex (–28, 42,
10; Z = 3.73, p < 0.0001, uncorrected) respon-
sive to untrustworthy faces in the implicit task
and to trustworthy faces in the explicit task.
However, this activation failed to survive cor-
rection for multiple comparisons across the
entire volume of orbitofrontal cortex. No other
areas about which we had a prior hypothesis
were revealed in this contrast or in the reverse
interaction term.
Using an additional model that partialed
out effects from facial expression of basic emo-
tions in the stimulus set, we performed a ran-
dom effects analysis across the 14 subjects.
Even under these stringent criteria, right
amygdala activation was still evident in this
model at both uncorrected (p < 0.001) and
small-volume corrected (p < 0.05 correct-
ed for multiple comparisons across bilater-
al amygdala volume) thresholds (Fig. 5).
This activation (peak at 22, 2, –18; Z = 4.06)
overlapped with that reported in our pri-
mary model. At lower thresholds (p < 0.005,
uncorrected), there was additional activa-
tion in left amygdala.
D
ISCUSSION
The question addressed in this study was whether the dimension
of trustworthiness in faces and the process of making social judg-
ments are associated with distinct patterns of brain activation.
By implication, the study is an explicit test of a proposed neuro-
biological model
11
. The principal findings of activation in amyg-
dala, orbitofrontal cortex and STS are highly consistent with this
model. We also extend previous lesion data
8
by showing amyg-
dala activity in response to untrustworthy faces regardless of
whether subjects were explicitly making trustworthiness judg-
ments. This finding echoes earlier studies of obligatory threat-
related processing in the amygdala
18–21
. In contrast to many
imaging studies of facial emotion (for example, ref. 5), the amyg-
Fig. 3. Main effect of trustworthiness in amyg-
dala and insula. (a) Significant increases in BOLD
signal to untrustworthy faces in the right and left
amygdalae and right insula (right amygdala, –18,
0, –24; Z = 4.29; p < 0.01 corrected; left amyg-
dala, –16, –4, –20; Z = 3.92; p < 0.025 corrected;
right insula, 42, –4, 12; Z = 3.48; p < 0.001
uncorrected). (bd) Responses to faces as a
function of degree of individually rated trust-
worthiness for right insula (b), left amygdala (c)
and right amygdala (d). Note greater responses
to less trustworthy faces across all these
regions. The y-axis is as in Fig. 2.
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nature neuroscience • volume 5 no 3 • march 2002 279
a
b
c
d
Table 1. Cerebral foci of activation in main effect of task.
Coordinates of peak activation (mm) Z score
Brain region xyz
Explicit versus implicit
Primary visual cortex 2 –98 8 4.49
Right posterior STS* 56 –44 4 4.27
Right superior frontal gyrus 10 14 70 3.99
Left premotor cortex –48 –2 26 3.84
Left extrastriate cortex –24 –76 32 3.75
Right cuneus 12 –40 56 3.62
Left primary sensory cortex –30 –28 72 3.62
Supramarginal gyrus –62 –40 34 3.50
Right anterior insula 48 34 –6 3.37
Left superior frontal sulcus –42 12 40 3.34
Left pre-SMA –6 10 54 3.34
Implicit versus explicit
Left fusiform gyrus –36 –36 –18 3.59
Right cuneus 4 –64 12 3.49
All values, p < 0.001 uncorrected. *p < 0.05 corrected for multiple comparisons across a small
volume of interest.
© 2002 Nature Publishing Group http://neurosci.nature.com
but no differential activity according to trustworthi-
ness. In other words, the right STS was activated
when subjects made explicit judgments about trust-
worthiness. In this regard, the STS showed activity
when subjects were required to make inferences con-
cerning the likely intentionality of others. This region
has been implicated in functional imaging studies on
biological motion
35
and biological-like motion
36
.
More critically, activity in posterior STS and adja-
cent regions at the temporo-parietal junction is
observed when subjects make theory of mind infer-
ences
37–39
. This region is suggested to be involved in
intention detection
40,41
, rather than biological
motion processing per se. Intention detection is a crit-
ical component in determining whether or not to trust an indi-
vidual, which may explain the activity in this region in our study.
Evidence from human patients with discrete lesions of
orbitofrontal cortex indicate that this region is critical for complex
social judgment
9,10
. Unlike the amygdala, this region showed task-
dependent activation. When subjects made explicit judgments of
trustworthiness, this region responded more strongly to faces
deemed trustworthy. By contrast, when judging age, this region
showed greater responses to untrustworthy individuals. Other stud-
ies have reported similar task-dependent responses in lateral
orbitofrontal cortex. For example, responses in a region of lateral
280 nature neuroscience • volume 5 no 3 • march 2002
dala response to untrustworthy faces was bilateral, supporting
neuropsychological evidence that patients with unilateral amyg-
dala lesions can successfully make trustworthiness judgments
8
.
To our knowledge, no previous study has demonstrated an auto-
maticity of amygdala response during complex social judgments.
In addition to the amygdala, the right insula was also activat-
ed by faces that subjects considered untrustworthy regardless of
task. The insula is activated in a wide variety of functional imag-
ing studies of emotion (for example, refs. 22–25). One suggested
role for the insula is the mapping of autonomic changes as they
affect the body where such mappings form the basis of ‘gut feel-
ings’ about emotive stimuli
26,27
. Thus, a
possible explanation for the insula activa-
tion that we observed is that a conse-
quence of amygdala activation is the
generation of autonomically mediated
changes in bodily states, which are then
re-mapped to the insula.
Differential activation in face-respon-
sive regions of the fusiform gyrus was
observed in relation to trustworthiness in
face stimuli. Increased activity is found in
modality-specific cortical areas in
response to stimuli with emotional con-
tent relative to non-emotional stimuli (for
example, refs. 20, 21, 28–31). Enhanced
extrastriate activation in response to emo-
tional stimuli has been attributed to mod-
ulatory influences from the amygdala
32
,
possibly mediated by anatomical back-
projections
33
. Indeed, a human lesion
study highlights a possible role for the
amygdala in enhancing perceptual pro-
cessing of threat stimuli
34
. We suggest that
such processes extend to faces represent-
ing potential threat at the social level and
that a neural consequence is enhanced
fusiform activation.
The right STS showed task-related acti-
vation in the explicit judgment condition
articles
Fig. 4. Main effect of trustworthiness in fusiform gyrus.
(a) Significant increases in BOLD signal to untrustworthy
faces in the fusiform gyrus bilaterally (right, 44, –46, –22;
Z = 3.58; p < 0.05, small volume corrected; left, –48, –48,
–24; Z = 3.60; p < 0.05, small volume corrected). This
activation is independent of task in both the left (b) and
right (c) fusiform gyrus. The y-axis is as in Fig. 2.
a
b
c
Table 2. Cerebral foci of activation in differential effects of trustworthiness of
faces.
Coordinates of peak activation (mm) Z score
Brain region xy z
Untrustworthy versus trustworthy
Right amygdala* 18 0 –24 4.29
Left superior temporal sulcus –50 –58 10 4.15
Right intraparietal sulcus 22 –54 48 4.00
Left extrastriate cortex –34 –90 24 3.94
Left amygdala* –16 –4 –20 3.92
Right pre-SMA 8 8 62 3.83
Left parahippocampal gyrus –18 –30 –18 3.81
Right auditory cortex 66 –18 4 3.75
Left inferior temporal gyrus –60 –14 –30 3.64
Left fusiform gyrus* –48 –48 –24 3.60
Right fusiform gyrus* 44 –46 –22 3.58
Thalamus 4 –12 14 3.52
Right insula 42 –4 12 3.48
Left superior temporal gyrus –58 –32 14 3.42
Trustworthy versus untrustworthy
Left insula –36 4 –4 3.65
Right dorsolateral prefrontal/
frontopolar cortex 34 52 6 3.23
All values, p < 0.001 uncorrected. *p < 0.05 corrected for multiple comparisons across a small volume of
interest.
© 2002 Nature Publishing Group http://neurosci.nature.com
orbitofrontal cortex vary between preference and recognition judg-
ment tasks on the same stimuli
42
. A dissociation between implic-
it/automatic social judgment and explicit (laboratory-tested) social
judgment has also been reported in a patient with orbitofrontal
cortex damage
9
. This patient remained able to evaluate social sit-
uations under explicit task instructions but was impaired in day-to-
day (“automatic”
9
) social judgments. Note that activation in this
region did not survive correction for multiple comparisons in our
study, and we emphasize effects in this region based on its known
involvement in social judgments
9,10
.
Several regions of interest in this study (amygdala, orbitofrontal
cortex and insula) are activated in processing specific facial expres-
sions. Facial expressions of fear consistently activate the amyg-
dala
5,6,43
, whereas facial expressions of disgust activate the anterior
insula
6,7
. Additionally, we demonstrate correlations between the
trustworthiness scores and scores for facial emotions attributed
to our stimulus set (Fig. 1be). Consequently, one possibility is
that differential patterns of activation seen in this study reflect
influences from one or more emotional expressions alone. We
assessed this possibility by analyzing the fMRI data with addi-
tional nuisance covariates pertaining to the degree of emotional
expression of each of four basic emotions (anger, fear, happiness,
sadness). A significant right amygdala response to untrustworthy
faces persisted, even after this secondary analysis accounted for
the variance attributed to facial emotion. These results suggest
that facial expressions of emotion provide a constituent element in
making trustworthiness judgments but that amygdala responses
also were independent of these effects. Notably, patients with bilat-
eral amygdala lesions show deficits in making social judgments
in the context of maintained ability to use information about emo-
tional expression of face stimuli
8
.
It is interesting to speculate how the results of this study might
generalize to social judgments about stimuli in other modalities.
Patients with bilateral amygdala lesions are able to make accu-
rate trustworthiness judgments based on verbal reports
8
. It is
plausible therefore that amygdala involvement in trustworthi-
ness decisions may be modality-specific. This hypothesis could
be tested in follow-up experiments involving trustworthiness
judgments about vocal stimuli, or scenarios about individuals
based on written descriptions. Our prediction would be that
superior temporal sulcus activation would remain in these other
contexts and that fusiform modulation would be substituted by
modality-specific cortical responses, for example, auditory cortex
in the case of vocal stimuli.
In conclusion, we present functional brain imaging evidence
for a neural substrate of social cognition that conforms to a pre-
viously proposed neurobiological model
11
. Our data extends this
model by highlighting a dissociation between automatic and
intentional engagement within this proposed circuitry. Thus,
social judgments about faces reflect a combination of brain
responses that are stimulus driven, in the case of the amygdala,
and driven by processes relating to inferences concerning the
intentionality of others, in the case of STS.
M
ETHODS
Subjects. Informed consent to partake in a study approved by the Joint
National Hospital for Neurology and Neurosurgery/Institute of Neurol-
ogy Ethics Committee was obtained from 16 right-handed Caucasian
volunteers (8 male, 8 female; age range 18–30 years; mean age 23.3 years).
Two subjects (both females) were excluded from the analysis; one revealed
psychiatric history after scanning and another provided extreme trust-
worthiness ratings. (Spearman’s rho of correlation of ratings with mean
of all other subjects, –0.445; for all remaining subjects, Spearmans rho
values were over 0.3.) All remaining subjects were free from psychiatric or
neurological history. All subjects except one had completed more than
two years of post-16 education, and mean length of post-16 formal edu-
cation was 4.8 years.
Stimuli. Grayscale frontal images of 120 Caucasian male faces were select-
ed from a larger selection of images following a pilot study outside the scan-
ner. The images were selected to cover a range of trustworthiness scores
rated by the subjects in the pilot study (n = 30; 13 females, 17 males, ages
17–32, mean age 23.5), but to score as low as possible on ratings of ‘happi-
ness’ and ‘anger’. Gaze direction of all stimuli was directly forward. Stimuli
were adjusted to be of approximately equal size and luminance and manip-
ulated such that each face was centered on a gray background in a 400 ×
400 pixel image. Of the 120 stimuli used in the imaging study, 60 were high
school student photographs and 60 photographs of university students.
There was no significant difference in average trustworthiness score between
the two groups (Mann-Whitney U test, p > 0.90).
Psychological task. The scanning session for each participant was divid-
ed into two parts. In one half of the session, 60 faces were presented
sequentially, and participants made a judgment, indicating with a push-
button response, whether the face was a high school or university stu-
dent. In the other half of the session, they judged whether the face was
trustworthy or untrustworthy. The order of tasks was counterbalanced
between participants. At the start of each task, a word appeared on screen
informing the subject of the task requirement (“School/Uni” or “Trust-
worthiness”).
Stimuli were presented on a gray background once each in random
order, randomly interspersed with 60 null events. Each stimulus was pre-
sented for 1 s with an inter-trial interval of 2 s. Between faces, a fixation
cross was presented. Null events were of 3 s duration, during which time
a fixation cross remained on screen. Stimuli subtended visual angles of
approximately 10°vertically and 5° horizontally.
Image acquisition. Subjects were scanned during task performance using
a Siemens VISION system (Erlangen, Germany) at 2 Tesla to acquire gra-
dient-echo, echoplanar T2*-weighted images with BOLD (blood oxy-
genation level dependent) contrast. Each volume comprised 33 × 2.2 mm
axial scans with 3-mm in-plane resolution, and volumes were continu-
ously acquired every 2.5 s. Subjects were placed in light head restraint
within the scanner to limit head movement during acquisition. Each run
began with 5 ‘dummy’ volumes (subsequently discarded) to allow for T1
equilibration effects. Additionally, a T1-weighted structural image was
acquired in each subject.
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nature neuroscience • volume 5 no 3 • march 2002 281
Fig. 5. Main effect of trustworthiness in amygdala independent of facial
emotion. Significant increases in BOLD signal in response to untrust-
worthy faces in right amygdala even when scores for four basic facial
emotions are additionally used as parametric covariates in the analysis.
This activation is significant at p < 0.05, corrected for multiple compar-
isons across the volume of bilateral amygdala. Activation peak at 18, 2,
–22 (Z = 4.06), but overlaps with right amygdala activation focus shown
in Fig. 2. At lower threshold of p < 0.005 uncorrected, activation is evi-
dent in left amygdala.
© 2002 Nature Publishing Group http://neurosci.nature.com
282 nature neuroscience • volume 5 no 3 • march 2002
All functional volumes were realigned
44
and slice timing corrected
(R. Henson et al., Neuroimage 9, 125, 1999), normalized into a standard
space
45
to allow group analysis, and smoothed with an 8-mm FWHM
Gaussian kernel to account for residual intersubject differences.
Debriefing. After scanning, participants undertook a self-paced task in
which they rated all the faces on a scale of trustworthiness from 1 (high-
ly untrustworthy) to 7 (highly trustworthy). When all 120 faces had been
rated, a second task was performed, in which participants named emo-
tions that they perceived in the faces by means of a seven-way forced
choice procedure (neutral, happy, sad, angry, disgust, fear, surprise). To
assist subjects with this task, we gave them a printed sheet with pho-
tographs of one face from the Ekman and Friesen series
46
expressing each
of these seven emotions.
Emotion ratings for stimuli. An additional set of 16 subjects (10 males,
6 females; age range 19–34 years; mean age 23.7 years) undertook a task
in which they rated the degree of emotional expression within each face
on each of four basic emotions (anger, fear, happiness, sadness) in turn.
Ratings were from 1 (neutral for this particular emotion) to 7 (highest
degree of this particular emotion).
Data analysis. Imaging data were analysed with SPM99 using an event-
related model
47
. The experimental design allowed a parametric factori-
al analysis whereby trustworthiness was a parametric regressor and the
task (age or trustworthiness judgment) the second factor.
The presentation of each face was modeled by convolving a delta func-
tion at each event onset with a canonical hemodynamic response function
(HRF) and its temporal derivative to create regressors of interest. These
regressors were then parametrically modulated to model subject-specific
trustworthiness judgments: that is, the height of the HRF for stimuli was
modulated as a function of the trustworthiness score assigned to that stim-
ulus by the subject. Subject-specific parameter estimates pertaining to each
regressor were calculated for each voxel
48
. Contrast images were calculat-
ed by applying appropriate linear contrasts to the parameter estimates for
the parametric regressor of each event. These contrast images were then
entered into a one-sample t-test across the 14 subjects (that is, a random
effects analysis). In regions about which we had a prior hypothesis, we
applied a correction for multiple comparisons across a small volume of
interest to the p-values in this region
49
. We report predicted regions sur-
viving this correction at p < 0.05. Volumes of interest for amygdala,
orbitofrontal cortex and STS were defined by drawing a mask around the
regions bilaterally on a normalized T1 structural image with reference to an
atlas of human neuroanatomy
50
using the software package MRIcro
(http://www.psychology.nottingham.ac.uk/staff/cr1/mricro.html). Total
volume of the amygdala mask was approximately 10 cm
3
, volume of the
orbitofrontal mask approximately 50 cm
3
, and volume of the STS mask
approximately 20 cm
3
. In the case of the fusiform gyri, small-volume cor-
rection was based upon a sphere of 10 mm radius centered on coordinates
derived from a previous study
21
. We report descriptively activations outside
regions of interest surviving a threshold of p < 0.001 uncorrected with an
extent threshold of 5 contiguous voxels.
To ensure that the main effect of trustworthiness did not arise from a
highly significant activation in just one of the simple effects (i.e., that
activation was task independent), we created a mask from random-effects
SPMs for the simple effect of untrustworthiness under both tasks (each
thresholded at p < 0.05, uncorrected). This was used to mask the main
effect of trustworthiness. Activations surviving this masking procedure
reflect responses during both implicit and explicit judgments.
For the purposes of illustration, a second model was constructed by divid-
ing the events for each subject into three groups by rank score for individ-
ual stimuli (that is, the least trustworthy third of faces as one event type,
the median third as a second, and the most trustworthy third as a third).
This model is used in Figs. 1–3 to demonstrate the direction of BOLD sig-
nal change with respect to trustworthiness score. Note that statistical infer-
ences are drawn solely from the parametric model described above.
The mean ratings of facial emotion derived from a second set of 16
age-matched subjects (see above) were used to construct another
model for the data. In this model, subject-specific ratings for trust-
worthiness were entered as parametric covariates, as before. Addi-
tionally, mean ratings for each of the four emotions (anger, fear, hap-
piness, sadness) were entered as nuisance parametric covariates. The
parameter estimates for trustworthiness are therefore rendered inde-
pendent of the effects of the four facial expressions, and variance bet-
ter explained by the effects of a given facial expression will be attributed
to the regressor modeling that facial expression. Contrast images for
trustworthiness derived from this model were then entered into a ran-
dom-effects analysis.
Acknowledgements
This work was supported by a programme grant to R.J.D. from the Wellcome Trust.
Competing interest statement
The authors declare that they have no competing financial interests.
RECEIVED 5 DECEMBER; ACCEPTED
26 DECEMBER 2001
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