Repetition suppression of ventromedial prefrontal
activity during judgments of self and others
Adrianna C. Jenkins*†, C. Neil Macrae‡, and Jason P. Mitchell*
*Department of Psychology, Harvard University, Cambridge, MA 02138; and‡School of Psychology, University of Aberdeen, Aberdeen AB24 2UB,
Edited by Edward E. Smith, Columbia University, New York, NY, and approved January 31, 2008 (received for review September 15, 2007)
One useful strategy for inferring others’ mental states (i.e., men-
talizing) may be to use one’s own thoughts, feelings, and desires
as a proxy for those of other people. Such self-referential accounts
of social cognition are supported by recent neuroimaging obser-
vations that a single brain region, ventromedial prefrontal cortex
(vMPFC), is engaged both by tasks that require introspections
about self and by tasks that require inferences about the minds of
others perceived to be similar to self. To test whether people
automatically refer to their own mental states when considering
those of a similar other, we examined repetition-related suppres-
sion of vMPFC response during self-reflections that followed either
an initial reflection about self or a judgment of another person.
Consistent with the hypothesis that perceivers spontaneously
engage in self-referential processing when mentalizing about
particular individuals, vMPFC response was suppressed when self-
reflections followed either an initial reflection about self or a
judgment of a similar, but not a dissimilar, other. These results
suggest that thinking about the mind of another person may rely
importantly on reference to one’s own mental characteristics.
functional neuroimaging ? mentalizing ? self-reference ? social cognition
thoughts and feelings, likes and dislikes, current goals and
intentions, and enduring dispositions and personality traits (1).
Although this understanding of others depends critically on a
capacity for rapidly inferring the internal states of those around
us (2–4), little is known about how exactly one successfully gains
insight into the inner workings of another’s mind. After all, no
individual, yet we routinely infer such mental content quickly
and easily (5).
One possible solution to the problem of mentalizing may be
found in the use of one’s own thoughts and feelings as a basis for
understanding those of others (6–9). Although the mental states
of other people are inherently imperceptible, perceivers do enjoy
immediate access to a highly similar system: their own minds. As
such, one may infer another person’s internal states by sponta-
neously imagining one’s own thoughts, feelings, or desires under
similar circumstances and then assuming that the other person
would experience comparable mental states, a view alternately
described as ‘‘simulationist,’’ ‘‘projectionist,’’ or ‘‘self-
referential’’ accounts of social cognition.
Importantly, introspection can only provide insight about
another’s feelings, beliefs, and preferences to the extent that
one’s own mind serves as a reasonable proxy for that of the other
person. If two people tend to experience very different mental
states in the same situations, neither would be well advised to
attempt to mentalize about the other on the basis of her own
as a basis for understanding those of others should be limited to
situations in which one can assume that another person generally
thinks and feels similarly to oneself. Perceivers may less readily
use their own mental states as a guide to the thoughts and
umans consistently explain the behavior of those around
them by appealing to others’ mental states; that is, their
feelings of people perceived to be substantially dissimilar from
Recently, researchers have used functional neuroimaging to
illuminate a specific link between introspection about self and
mentalizing about those people perceived to be similar (10, 11).
Across several studies, mentalizing about similar versus dissim-
ilar others has been associated with a distinct division of labor
in the medial prefrontal cortex, a region ubiquitously identified
in neuroimaging studies of mental state inference (12–14).
Specifically, a dorsal aspect of the medial prefrontal cortex has
been associated with mentalizing about people perceived to be
dissimilar from oneself, whereas a more ventral aspect of medial
prefrontal cortex (vMPFC) has been linked to mentalizing about
those perceived to be similar. Critically, this vMPFC region also
has been observed repeatedly during tasks that require partic-
ipants to introspect about their own mental experiences (15–18),
suggesting a connection between tasks that require self-
referential thought and those that require inferences about the
mental states of similar others.
That the same brain region appears to subserve introspection
about oneself and mental state inferences about similar others
these two otherwise disparate tasks and is consistent with
suggestions that perceivers may spontaneously refer to their own
mental states to infer those of other people. However, although
colocalization of function provides positive evidence that two
tasks draw on the same set of mental operations, the limited
spatial resolution of hemodynamic imaging techniques, such as
fMRI, prevents researchers from using shared functional neu-
roanatomy as the basis for strong conclusions about the overlap
of cognitive process. Because such techniques integrate neural
activity across hundreds of thousands of neurons, activation of
the same brain voxel by different tasks might occur because each
activates distinct, but neighboring or interdigitated, neuronal
populations. In this way, two tasks could possibly coactivate the
same brain voxel despite engaging different sets of neurons that
subserve disparate cognitive processes.
Fortunately, such technical limits can now be circumvented by
recently developed paradigms that support stronger conclusions
regarding the coactivation of the same neurons by different
stimuli or different tasks. These techniques rely on an effect
known as ‘‘repetition suppression,’’ the observation that neural
activity in stimulus-sensitive brain regions is typically reduced
when a stimulus is repeated (19). Repetition suppression was
initially reported during single-cell recordings in monkeys (20–
Author contributions: A.C.J. and J.P.M. designed research; A.C.J. and J.P.M. performed
research; A.C.J. and J.P.M. analyzed data; and A.C.J., C.N.M., and J.P.M. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Freely available online through the PNAS open access option.
†To whom correspondence should be addressed. E-mail: email@example.com.
This article contains supporting information online at www.pnas.org/cgi/content/full/
© 2008 by The National Academy of Sciences of the USA
March 18, 2008 ?
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no. 11 ?
22) and has since been observed consistently across a number of
studies that measured the fMRI BOLD response in humans
(23–29), where it has been used to characterize the response
profiles of brain regions involved in a variety of cognitive
processes, such as those subserving visual processing, memory,
refs. 19, 30–33).
Although the precise physiological basis of repetition suppres-
sion has yet to be fully elucidated, researchers generally agree
that the suppressed fMRI BOLD response to repeated stimuli
must reflect changes in the firing properties of neurons that
subserve the processing of a stimulus, and that suppression
across two stimuli indicates that the same (or at least a largely
overlapping) population of neurons is engaged by both stimuli
(19, 30–33). For example, a demonstration of repetition sup-
pression for the number “3” when it follows “4” but not when it
neurons that code for the number “3” also participate in
representations of similar numerosities (such as “4”), but not in
representations of more distant numerosities.
These characteristics of repetition suppression render it well
suited for examining the hypothesis that mentalizing about
introspecting about one’s own mental characteristics. If (i)
repeatedly considering one’s own mental states produces repe-
tition suppression in self-sensitive regions such as vMPFC, and
(ii) one engages in self-referential processing when considering
the minds of similar others, then (iii) repetition suppression also
should be observed when perceivers first mentalize about a
similar other and then introspect about self. To test these
hypotheses, participants in the current study underwent fMRI
scanning while answering a series of questions that required
introspection about their opinions or preferences (e.g., ‘‘How
frustrated do you get sitting in traffic?’’; see Methods). Imme-
diately before each of these self-reflections, participants per-
formed one of three different types of judgments: (i) an initial
self-reflection; (ii) a judgment of the opinions/preferences of a
person manipulated to be perceived as similar to self; or (iii) a
be dissimilar from self (participants considered the identical
opinion question across phases on half the trials and two
different opinion questions for across phases on the other half of
trials). Of critical interest was the vMPFC response during
self-reflection as a function of the target of the immediately
preceding judgment. We expected to observe substantially re-
duced activity in this region for self-reflections immediately
preceded by a prior self-reflection (self-after-self), that is, a
significant suppression of the BOLD response when processing
to the extent that self-referential processing spontaneously ac-
companies mentalizing about similar others, we expected similar
by judgments of similar others (self-after-similar). In contrast,
because referring to one’s own mental states should not be an
appropriate strategy for mentalizing about dissimilar others, no
suppression should be observed when self-reflections follow
judgments of dissimilar others (self-after-dissimilar).
Behavioral Data. Postscanning questionnaires confirmed that par-
ticipants generally held liberal attitudes and perceived them-
selves to be more similar to the liberal than to the conservative
target. On average, participants reported their sociopolitical
attitudes as 3.03 on a 7-point scale (1 ? very liberal, 4 ? neither
liberal nor conservative, and 7 ? very conservative). Likewise,
participants rated the liberal target to be more similar to self
(M ? 4.80 on a 7-point scale) than the conservative target (M ?
3.00; P ? 0.02). Moreover, no participant rated the conservative
target to be more similar to self than the liberal target. Accord-
ingly, the liberal target was treated as the similar other and the
conservative target as the dissimilar other for subsequent fMRI
Confirming the appropriateness of these target assignments,
participants judged the preferences of similar targets to be more
closely in line with their own self-reflections than were the
preferences of dissimilar targets. For each opinion question, we
calculated the mean absolute difference between the partici-
pant’s self-reported opinion and (i) that of the similar other and
(ii) that of the dissimilar other. Consistent with the notion that
one’s own preferences may more strongly inform judgments of
those perceived to be similar to oneself, judgments of similar
others were significantly closer to one’s self-reported opinions
than were judgments of dissimilar others [Ms ? 0.73 vs. 1.12,
respectively; t (12) ? 3.25, P ? 0.01].
fMRI Data. Regions of interest (ROIs) in vMPFC were identified
through two independent analyses. We first examined results
from the explicit self-reference task, during which participants
judged how well an adjective described either their own person-
ality or that of a familiar, but not personally known, other.
Consistent with earlier studies (15–18), a single region was
obtained from the random effects contrast of self ? other trials
located in vMPFC (Fig. 1). The response in this region both to
self-singletons (M ? 0.06) and to similar singletons (M ? ?0.05)
was significantly greater than the vMPFC response to dissimilar
singletons (M ? ?0.11; both Ps ? 0.04). Consistent with extant
literature suggesting that the vMPFC responds preferentially
during judgments about self, the response to similar singletons
was intermediate between dissimilar singletons and self-
trials on the opinion-judging task, during which participants
reported their own opinions/preferences immediately after an
initial judgment of self or one of the two other targets. We
expected to observe suppression of activity in this vMPFC region
for trials on which participants introspected about self immedi-
ately after introspecting a first time (i.e., self-after-self) because
on these trials participants would be performing the very same
task (i.e., reflecting on their own opinion/preference) twice
consecutively. Consistent with this prediction, self-after-self
judgments were associated with a robust suppression of activity
in vMPFC. Indeed, as displayed in Fig. 1, although vMPFC
response to self-reflections was typically greater than baseline
when participants self-reflected in isolation (i.e., for self-
singleton trials), activity in this region was significantly reduced
during self-reflections that followed an initial self report. That is,
although this vMPFC ROI responded preferentially during
self-reflection, its response was substantially suppressed when
participants self-reported their opinions twice in a row. Impor-
tantly, no difference was observed in the level of repetition
suppression during self-after-self trials as a function of whether
participants responded to the identical or a different opinion
question on the second phase of the trial [t (12) ? 0.50, P ? 0.62].
Likewise, identical and different judgment pairs both differed
significantly from self-singletons (both Ps ? 0.05).
In contrast, we expected to observe robust vMPFC activation
for self-reflections that immediately followed judgments of
dissimilar others (self-after-dissimilar) because perceivers
should not spontaneously engage in self-referential thought
when mentalizing about those perceived to be different from
self, and thus less initial vMPFC processing should occur before
the initial self-reflection. Indeed, self-after-dissimilar judgments
were associated with a significant increase in vMPFC response
over baseline [t (12) ? 4.75, P ? 0.0005]. This response also was
significantly greater than for self-after-self and self-after-similar
trials for both identical and different trials (all P values ? 0.008),
www.pnas.org?cgi?doi?10.1073?pnas.0708785105 Jenkins et al.
indicating that the response of vMPFC during self-reflection was
not suppressed when participants first mentalized about a dis-
Of critical interest was whether activity in vMPFC would
demonstrate repetition suppression for trials on which partici-
pants introspected about self immediately after making a judg-
ment about a person perceived to be similar (self-after-similar).
If the same neural processing accompanies both introspection
and mentalizing about similar others, the response of vMPFC
should fail to distinguish between introspection about self and
judgments of similar others. Consistent with this prediction, just
as for self-after-self judgments, vMPFC response to self-
reflections was substantially suppressed when participants re-
ported their opinion immediately after judging a similar other.
For identical judgments, self-after-similar trials were associated
with nearly indistinguishable levels of repetition suppression as
for self-after-self judgments [t (12) ? 0.06, P ? 0.95; for different
judgments, P ? 0.11]. As for self-after-self trials, the response to
self-after-similar judgments was negative-going and did not
differ significantly from baseline activity (repeated, P ? 0.19;
novel, P ? 0.93). Together these results suggest that activity in
vMPFC, a brain region widely acknowledged to subserve self-
referential thought, can be suppressed either by repeatedly
introspecting about the self or by introspecting about self
immediately after judging a similar, but not a dissimilar, other.
In addition, a second vMPFC ROI was defined from trials
within the opinion-judging task by contrasting self-singleton
trials to both similar- and dissimilar-singleton trials (i.e., self ?
other). Random effects analysis identified a region of vMPFC
that was preferentially engaged by judgments of self [see sup-
porting information (SI) Fig. 2]. Importantly, the pattern of
repetition suppression within this alternative vMPFC ROI was
indistinguishable from that in the region defined by the explicit
self-reference task (with the exception of self-after-similar-
the pattern of findings observed in the vMPFC ROI defined
independently by the explicit self-reference task and confirm
that vMPFC activity was suppressed for self-reflections that
either followed an initial self-reflection or a judgment of a
similar, but not dissimilar, other.
Secondary Data and Analyses. In both vMPFC regions, the pre-
dicted pattern of repetition suppression was observed across
both identical judgments (when participants judged the same
opinion question twice within the same trial) and different
judgments (when participants judged a different question in the
second phase than in the first phase of the trial). The 3 ? 2
interaction of pair type (self-after-self, self-after-similar, and
self-after-dissimilar) ? question repetition (identical or differ-
ent) did not approach significance in either vMPFC region (both
Ps ? 0.18), suggesting that the pattern of repetition suppression
was similar across identical and different trials. Likewise, when
restricted to self-after-similar and self-after-dissimilar trials, the
2 ? 2 interaction of trial type ? question repetition failed to
approach significance in either region (both Ps ? 0.55), suggest-
ing that vMPFC activity was similarly suppressed for self-
reflections after judgments of similar others, relative to judg-
ments of dissimilar others. Most critically, the same pattern of
differences was observed between self-after-similar trials in both
regions regardless of whether questions were identical or differ-
ent. Specifically, vMPFC activity did not differentiate between
self-after-similar and self-after-self trials for either identical
(both Ps ? 0.56) or for different (both Ps ? 0.20) trials, but was
consistently lower for self-after-similar than self-after-dissimilar
(all Ps ? 0.05). Finally, pairwise t tests conducted between all
self-different) revealed a significant difference only for self-
after-similar trials in the vMPFC region defined from the explicit
characteristics were compared with judgments of another person (i.e., self ? other). On a separate task, participants completed a series of paired judgments,
in which they introspected about their own preferences and opinions immediately after one of three types of judgments: (i) an initial report about self
(self-after-self), (ii) a judgment of a person with the same sociopolitical attitudes as oneself (self-after-similar), or (iii) a judgment of a person with opposing
attitudes (self-after-dissimilar). On an equal number of trials, participants considered the identical question for prime and self or a different question across the
two phases. The bar graph depicts the BOLD response associated with these self-reports after subtracting out the response associated with the initial judgment
figure includes the response in this region to self-reports made in isolation (gray bar). Significant repetition suppression was observed for self-reports that
followed either an initial self-report (blue bars) or a judgment of a similar other (red bars), but not judgments of a dissimilar other (green bars). Error bars
represent the 95% confidence interval for within-subject designs (43).
A region of vMPFC (?6, 45, 3; 47 voxels in extent) was defined from an explicit self-reference task in which judgments of one’s own personality
Jenkins et al.PNAS ?
March 18, 2008 ?
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replicated in the alternate vMPFC region identified from self ?
other from within the judgment task (SI Fig. 2), it remains
unclear whether less repetition suppression occurs when a
different question is asked about self and a similar other.
Unsurprisingly, participants gave the identical behavioral re-
sponse for self and other (e.g., responding 3 to both other and
self) more often for self-after-similar than self-after-dissimilar
targets (37% vs. 29% of trials, respectively; P ? 0.05). That
participants more often made the same behavioral response
consecutively for self-after-similar than self-after-dissimilar tri-
als opens the possibility that vMPFC suppression associated with
self-after-similar trials could result from repeated motor output,
rather than shared cognitive operations between thinking about
self and similar others. However, further analysis of the fMRI
data belied this possibility. In a secondary analysis of the fMRI
data, trials were subconditionalized as a function of whether the
same behavioral response was made twice in a row, resulting in
four trial types: self-after-similar, same response; self-after-
similar, different response; self-after-dissimilar, same response;
and self-after-dissimilar, different response (the creation of
subconditions was not possible for self-after-self trials, for which
prohibitively few different responses were obtained). When
analysis was restricted to those trials on which participants made
the same behavioral response twice in a row (e.g., pressing 3 for
both other and self), we continued to observe repetition sup-
pression for self-after-similar, but not for self-after-dissimilar,
judgments, although the difference between the two trial types
only reached marginal significance (P ? 0.07), most likely owing
to the reduced power inherent in reducing the number of trials
per condition (e.g., as few as 10 trials in a condition for some
In addition, we conducted a secondary analysis restricted only
to identical trials, segregating trials on which participants made
the same response twice in a row (e.g., 3 to the prime and to self)
from those on which participants made two different responses
across the two phases of the trial. Critically, the pattern of
repetition suppression did not differ as a function of whether
participants made the same behavioral response twice in a row
to the identical question. First, the difference in the amount of
repetition suppression for self-after-similar versus self-after-
dissimilar trials did not differ as a function of whether partici-
pants made the same behavioral response: the pair type (self-
after-similar, self-after-dissimilar) ? response overlap (same
response, different response) interaction did not approach sig-
nificance (F ? 1.07, P ? 0.32). Second, no simple effect was
observed between same versus different responses for either
self-after-similar or self-after-dissimilar trials (both Ps ? 0.26),
suggesting that repetition suppression was not significantly af-
fected by whether participants made the same behavioral re-
sponse twice in a row. Although these findings are consistent
with the interpretation that the pattern of repetition suppression
did not differ as a function of making the same behavioral
response, results must be interpreted cautiously because these
analyses are based on a small subset of the data with reduced
power to detect any differences inherent in making the same
response or not.
Finally, an accompanying behavioral study reinforced the
observation that the facilitation of self-after-similar trials did not
result from making a repeated behavioral response. In this data
collection, we made use of a behavioral analogue of repetition
suppression, wherein repeated processing results in speeded
performance on subsequent trials of the same kind (i.e., repe-
tition priming) (33). Specifically, the primary dependent mea-
sure in this supporting study was the speed with which a separate
group of participants reported their introspection about self
after judgments of a similar or dissimilar other. Consistent with
the fMRI data, participants (n ? 14) were significantly faster to
self-reflect after a judgment of a similar (M ? 1,990 ms) than a
dissimilar (M ? 2,079 ms) target [t (13) ? 2.84, P ? 0.02]. (The
parallel analysis of response time in the fMRI experiment was
precluded by the abbreviated length of trials necessitated by
rapid event-related scanning, such that participants were typi-
cally near the ceiling allowed by the response window.) Consis-
tent with the secondary analysis of fMRI data, the significant
difference in reaction time between self-after-similar versus
self-after-dissimilar was observed even when analysis was re-
stricted to those trials on which participants made the same
behavioral response for both self and the other person (M diff ?
299 ms; P ? 0.02). In other words, introspections about one’s
own attitudes were significantly more facilitated by first men-
talizing about a similar than a dissimilar target, even when
controlling for participants’ tendency to make the same response
for self and similar others.
These results underscore the tight link between thinking about
oneself and thinking about other people, suggesting that self-
referential processing may be triggered spontaneously when
considering the mental states of others. Using two different sets
of contrasts, we identified ROIs in vMPFC that, consistent with
earlier studies of self-reference (15–18), responded preferen-
tially during trials that required introspection about one’s own
mental characteristics. We then examined the pattern of re-
sponse in these ROIs during an opinion-judging task in which
participants reported their preferences and opinions immedi-
ately after a prior self-reflection, a judgment of a similar other,
or a judgment of a dissimilar other. Extending earlier observa-
tions that repeatedly processing the same stimulus leads to the
suppression of the activity in task-sensitive brain regions, the
response of vMPFC was attenuated for self-reflections that
immediately followed a preceding introspection about self. Crit-
ically, this same repetition suppression was observed for self-
reflections that followed judgments of similar, but not dissimilar,
targets. That is, whereas self-reflections were associated with a
significant activation of vMPFC after judgments of a dissimilar
other, the response of this region was equivalently suppressed
during self-reflections that followed judgments of a similar other
as during those that followed initial self-reflections, suggesting
that vMPFC failed to discriminate between self-referential
thought and mentalizing about a similar other.
The use of repetition suppression provides particularly strong
support for the conclusion that mentalizing about similar others
draws on the same cognitive processes as introspecting about
oneself. Most proposals regarding the physiological basis of
repetition suppression conclude that the likely basis of the effect
is that the same, or largely overlapping, population of neurons
subserves the processing of two distinct stimuli. These changes
may include neurons ‘‘fatiguing’’ upon repeated firings, briefer
neuronal firing durations, or the recruitment of fewer total
neurons to process stimuli a second time (19, 33, 34). Regardless
of the precise nature of the neuronal change, observing repeti-
tion suppression in a brain region across two seemingly distinct
stimuli provides evidence that the neurons in that region are
insensitive to any difference between the stimuli. This interpre-
about self serve double-duty by also participating in representing
the minds of similar others. Accordingly, these results contribute
additional support for simulationist views of social cognition,
which have suggested that one important mechanism for under-
standing the thoughts and feelings of others is reference to one’s
own mental states.
The observed dissociation between the functional neuroanat-
omy associated with mentalizing about similar and dissimilar
others joins several other recent observations that likewise
www.pnas.org?cgi?doi?10.1073?pnas.0708785105Jenkins et al.
suggest that the cognitive processes deployed during mentalizing
will vary as a function of exactly whose mental states one is
attempting to infer (10, 11). When another person is assumed to
be sufficiently similar to self, perceivers appear to make use of
the same processes deployed for introspecting about their own
mental characteristics, but decline to do so for others assumed
to be dissimilar from self. Such findings reinforce the emerging
view that social cognition, rather than being composed of a
single, all-purpose module for mentalizing (35), relies on a
number of different strategies for mentalizing that vary with the
particulars of the social environment (36–39). Of course, exactly
how perceivers determine whether a particular individual is
sufficiently similar to justify the use of self-referential mental-
izing remains an open question, as does characterization of the
cognitive processes that subserve mentalizing about dissimilar
Putting introspection to use in mentalizing undoubtedly pro-
vides a rich starting point for contemplating the minds of others,
allowing perceivers to bring to bear the full complement of their
own attitudes, feelings, and beliefs in inferring those of another
person. Somewhat ironically, however, because one’s own men-
tal states may serve as an appropriate proxy only for those whom
we assume think and feel like we do, human social cognition may
possess an intrinsic bias to discriminate between those perceived
to be similar, like-minded members of one’s ingroup and those
perceived to be dissimilar, exotic others. Indeed, that our minds
naturally segregate dissimilar from similar others and then
mentalize in a distinct way about those perceived to be different
from self may be one of the factors that gives rise to aspects of
outgroup prejudice, such as stereotypes about members of
various racial, ethnic, or cultural backgrounds. Accordingly, one
strategy for successfully counteracting such biases may be to
augment the degree to which perceivers engage in self-
referential mentalizing about otherwise dissimilar others, for
example, by consciously taking the perspective of another person
(44). Like many of the cognitive heuristics that typically serve us
well, but periodically lead to undesirable or maladaptive behav-
ior (40), the use of self-reference in mentalizing may be a
double-edged sword: a useful strategy for providing rich and
accurate insights into the minds of similar individuals, but rife
with the potential to exclude those minds assumed at first glance
to be different from our own.
ers with no history of neurological problems (mean age 20.7 years, range
19–23). One additional participant, who was being treated for depression at
the time of the study, was excluded from analysis. All participants were
provided informed consent in a manner approved by the Human Studies
Committee of the Massachusetts General Hospital.
Stimuli and Behavioral Procedure. Participants were told that the experiment
investigated the ability to make inferences about others on the basis of
minimal information. Before scanning, participants read a short paragraph
about each of two unfamiliar target individuals depicted by face photo-
graphs. Following Mitchell et al. (11), one target was described as a college
student in the Northeast who maintained liberal social and political attitudes
Targets were always the same sex as the participant, and both the pairing of
particular faces to descriptions and the order of presentation (liberal target
first, conservative target first) were randomized across participants. Partici-
pants were given as much time as needed to read about each of the two
phases. Each trial began with the presentation of one of three primes: (i) the
photograph of the liberal target, (ii) the photograph of the conservative
target, or (iii) a chalk outline of a head with the word ‘‘me’’ written inside,
used to represent the participant her or himself. This prime image appeared
above a four-point response scale (1 ? not at all and 4 ? definitely). Simul-
taneously, an opinion question appeared between the prime and the re-
sponse scale, and participants were asked to use the scale either to estimate
to report their own response to the question. Opinion questions referred to a
range of personal issues that were pretested to be unrelated to political
‘‘like to be the center of attention?’’; ‘‘generally see things from many
ist artwork?’’). The prime image, question, and scale remained onscreen
together for 3,600 ms.
The self phase of each trial began after a 400-ms interval and was identical
to the chalk outline prime described above, in which participants reported
their own response to an opinion question. Self-judgments were condition-
alized as a function of the preceding target, resulting in three trial types:
the validity of these target assignments was confirmed by postscan question-
naires that asked participants to rate how similar they perceived each of the
two targets to be relative to self.
For half the trials, the same opinion question was asked in both the prime
and self phases of the trial (identical trials). In the remaining half of the trials,
a different question was asked in the two phases (different trials). However,
participants were instructed to consider each question individually. Although
none of the primary analyses was qualified by whether the identical or
for these two trial types for the sake of completeness.
Participants completed 240 such paired prime–self trials. In addition, the
experimental design included 90 singleton trials, on which participants saw
only the prime phase (30 each of self, similar, and dissimilar) without a
subsequent self phase. These singletons were included as catch trials used to
facilitate deconvolution of the hemodynamic response specifically associated
with the self phase (see below).
After the opinion-judging task, participants completed an explicit self-
responds preferentially during self-referential judgments (15, 16). On each of
100 trials, participants saw a single trait adjective that could be used to
or neurotic). Each trait adjective was accompanied by the name of one of two
targets: self or Bush. For self-trials, participants were asked to use a 4-point
scale to indicate how well the trait adjective described themselves. For Bush
trials, participants were asked to use the scale to indicate how well the
adjective described the current U.S. president, George W. Bush. This choice of
other was guided by earlier studies of self-referential processing, which have
typically used the current head of state (a familiar, but not personally known,
other) as a comparison to self-judgments (15, 16, 41). To optimize estimation
of the event-related fMRI response, on both the opinion-judging and explicit
self-reference tasks, trials were intermixed in a pseudorandom order and
separated by a variable interstimulus interval (400–8,000 ms) (42), during
which participants passively viewed a fixation crosshair.
After scanning, participants answered two questions about their own
sociopolitical attitudes in random order (‘‘How politically liberal or conserva-
tive are you?’’ and ‘‘How socially liberal or conservative are you?’’) by using a
conservative). Finally, participants reported how similar they perceived each
Imaging Procedure. FMRI data were collected by using a 3 Tesla Siemens Trio
scanner. The opinion-judging task comprised five functional runs of 296
volume acquisitions, and the explicit self-reference task comprised two func-
tional runs of 130 volume acquisitions (26 axial slices, 5 mm thick; 1 mm skip).
Functional imaging used a gradient-echo echo-planar pulse sequence (TR ?
collected a high-resolution T1-weighted structural scan (MP-RAGE). PsyScope
software for Mac OS X (L. Bonatti, International School of Advanced Studies,
Trieste, Italy) was used to project stimuli onto a screen at the end of the
magnet bore, which participants viewed via a mirror mounted on the head
coil. A pillow and foam cushions were placed inside the coil to minimize head
FMRI data were preprocessed and analyzed by using SPM99 (Wellcome
Department of Cognitive Neurology, London, United Kingdom). First, func-
tional data were time-corrected for differences in acquisition time between
Jenkins et al. PNAS ?
March 18, 2008 ?
vol. 105 ?
no. 11 ?
slices for each whole-brain volume and realigned to correct for head move-
ment. Functional data were then transformed into a standard anatomical
full-width-at-half-maximum (FWHM)] by using a Gaussian kernel.
Statistical analyses were performed by using the general linear model in
which the event-related design was modeled by using a canonical hemody-
namic response function, its temporal derivative, and additional covariates of
no interest (a session mean and a linear trend). This analysis was performed
individually for each participant, and contrast images for each participant
were subsequently entered into a second-level analysis, treating participants
as a random effect.
(i.e., self trials vs. Bush trials). In addition, a second vMPFC ROI was defined
from within the opinion-judging task from the comparison of self ? other by
using singleton trials only (i.e., self-singletons ? similar singletons plus dis-
similar singletons). Peak coordinates were identified by using a statistical
criterion of 25 or more contiguous voxels at a voxel-wise threshold of P ?
0.001. This cluster size was selected on the basis of a Monte Carlo simulation
this cluster extent cutoff provided an experiment-wise threshold of P ? 0.05,
corrected for multiple comparisons.
whether they were paired or singleton, and (ii) the identity of the prime,
resulting in six trial types: self-after-self, self-after-similar (i.e., judgment of
the liberal target, then self), self-after-dissimilar, self-singleton, similar sin-
gleton, and dissimilar singleton. The parameter estimates associated with
after the above procedure. Of critical interest was the extent to which brain
activity associated with the self phase was suppressed as a function of the
identity of the preceding prime (self, similar, and dissimilar). Because the self
phase was always preceded by a prime (i.e., self and prime were intrinsically
confounded), we obtained such a measure of repetition suppression by sub-
self trials. For example, the response that was associated with the self phase
of self-after-similar trials was indexed as the difference of self-after-similar
this judgment was made immediately after an initial judgment of self, of a
similar other, or a dissimilar other.
ACKNOWLEDGMENTS. We thank D. L. Ames, M. Banaji, R. Buckner, Y. Jiang,
and L. Powell for advice and assistance. This work was supported by National
Science Foundation Grant BCS 0642448 (to J.P.M.), the Athinoula A. Martinos
Center for Biomedical Imaging, National Center for Research Resources Grant
P41RR14075, the Mental Illness and Neuroscience Discovery Institute, and a
Royal Society-Wolfson Fellowship (to C.N.M.).
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