Differential modulation of the default mode network
via serotonin-1A receptors
Andreas Hahna, Wolfgang Wadsakb, Christian Windischbergerc, Pia Baldingera, Anna S. Höflicha, Jan Losaka,
Lukas Nicsb, Cécile Philippeb, Georg S. Kranza, Christoph Krausa, Markus Mitterhauserb, Georgios Karanikasb,
Siegfried Kaspera, and Rupert Lanzenbergera,1
aDepartment of Psychiatry and Psychotherapy,bDepartment of Nuclear Medicine, andcMagnetic Resonance Center of Excellence, Center for Medical Physics
and Biomedical Engineering, Medical University of Vienna, 1090 Vienna, Austria
Edited by Marcus E. Raichle, Washington University in St. Louis, St. Louis, MO, and approved January 4, 2012 (received for review October 19, 2011)
Reflecting one’s mental self is a fundamental process for evaluat-
ing the personal relevance of life events and for moral decision
making and future envisioning. Although the corresponding net-
work has been receiving growing attention, the driving neuro-
chemical mechanisms of the default mode network (DMN)
remain unknown. Here we combined positron emission tomogra-
phy and functional magnetic resonance imaging to investigate
modulations of the DMN via serotonin-1A receptors (5-HT1A), sep-
arated for 5-HT autoinhibition (dorsal raphe nucleus) and local
inhibition (heteroreceptors in projection areas). Using two inde-
pendent approaches, regional 5-HT1A binding consistently pre-
dicted DMN activity in the retrosplenial cortex for resting-state
functional magnetic resonance imaging and the Tower of London
task. On the other hand, both local and autoinhibitory 5-HT1A
binding inversely modulated the posterior cingulate cortex, the
strongest hub in the resting human brain. In the frontal part of
the DMN, a negative association was found between the dorsal
medial prefrontal cortex and local 5-HT1Ainhibition. Our results
indicate a modulation of key areas involved in self-referential pro-
cessing by serotonergic neurotransmission, whereas variations in
5-HT1Abinding explained a considerable amount of the individual
variability in the DMN. Moreover, the brain regions associated
with distinct introspective functions seem to be specifically regu-
lated by the different 5-HT1Abinding sites. Together with previ-
ously reported modulations of dopamine and GABA, this regional
specialization suggests complex interactions of several neuro-
transmitters driving the default mode network.
functional connectivity|resting-state networks|neurotransmitter modulation
activation of a set of regions known as the default mode network
(DMN) (2). This intrinsically organized (3) and individually
adaptive (4, 5) network has been implicated in introspective
processes including, among others, episodic memory and theory
of mind (6, 7). Accordingly, the DMN can be activated by direct
confrontation with moral dilemma (8) and self-referential judg-
ments of mental states and personal future (9). The DMN has
also been identified in the resting state, which encourages the
processing of introspective thoughts or “mind wandering” (10),
within both positron emission tomography (PET) (2) as well as
spontaneous fluctuations in blood oxygenation level-dependent
(BOLD) signal (11). Importantly, the spontaneous BOLD ac-
tivity acquired at rest shows similar connectivity patterns com-
pared with task-evoked responses and anatomical connections
(12–14). On the other hand, the DMN was initially discovered by
its consistent deactivation throughout various cognitively de-
manding tasks (15) and during externally focused attention (1),
from which the name “task-negative network” emerged (3). Al-
though this network has been robustly identified across imaging
modalities and tasks, there is still considerable variability between
subjects (11, 16). For instance, individual DMN activity seems to
be associated with task performance (17), learning (4, 5), and
ithout performing particular tasks, the human brain
maintains a baseline state (1) that is characterized by the
creativity (18) as well as personal emotionality (19). Further-
more, DMN coupling increases with maturation (20) and
declines again in normal aging (21). Hence, it is hardly surprising
that alterations in the functional connectivity networks have
been found in almost every mental disorder, including de-
pression (6, 22, 23), anxiety disorders (24), schizophrenia (25),
attention deficit hyperactivity disorder (26), and Alzheimer’s
disease (21, 27). This emphasizes the importance of individual
variations in the default mode network across subjects and
Despite the growing interest in the DMN and its robust
identification with PET and spontaneous BOLD fluctuations, its
underlying neurochemical modulators substantially remain un-
known. Cerebral blood flow, and hence the BOLD contrast, is
locally controlled via glutamate signaling (28) but globally reg-
ulated by monoamine neurotransmitters including dopamine
(29), noradrenaline (30), and serotonin (5-HT) (31). Within the
5-HT system, several receptor subtypes show considerable spatial
overlap with the DMN (32), whereas the main inhibitory re-
ceptor (5-HT1A) takes an exceptional role due to its dual ex-
pression. Located on cell bodies of serotonergic neurons in the
midbrain raphe nuclei, 5-HT1Areceptors have an autoinhibitory
function on tonic serotonergic neurotransmission and, hence,
represent a putative indicator of 5-HT cell firing (33, 34) and
serotonin synthesis (35). On the other hand, within projection
areas, 5-HT1Aheteroreceptors are expressed on glutamate and
GABA neurons (36, 37), which in turn drive the hemodynamic
response to neural processing (28).
Based on this regulatory cascade of the BOLD signal and their
intense spatial overlap (Fig. 1), we expected to find widespread
modulations of 5-HT1Areceptors onto the DMN. Specifically,
this study aimed to evaluate whether both local and auto-
inhibitory 5-HT1Abinding may predict the individual variability
in DMN activity. To test this hypothesis, we combined PET and
BOLD functional magnetic resonance imaging (fMRI) data
obtained from 28 healthy subjects. For a thorough evaluation of
5-HT1Aassociations across tasks, the DMN was calculated from
Author contributions: W.W., C.W., M.M., G.K., S.K., and R.L. designed research; A.H., W.W.,
C.W., P.B., A.S.H., J.L., L.N., C.P., C.K., M.M., G.K., S.K., and R.L. performed research; A.H.,
J.L., L.N., C.P., and G.S.K. analyzed data; and A.H., C.W., P.B., A.S.H., G.S.K., C.K., and
R.L. wrote the paper.
Conflict of interest statement: Without any relevance to this work, S.K. declares he has
received grant/research support from Eli Lilly, Lundbeck A/S, Bristol-Myers Squibb, Servier,
Sepracor, GlaxoSmithKline, and Organon; has served as a consultant or on advisory
boards for AstraZeneca, Austrian Science Fund, Bristol-Myers Squibb, GlaxoSmithKline,
Eli Lilly, Lundbeck A/S, Pfizer, Organon, Sepracor, Janssen, and Novartis; and has served on
speakers’ bureaus for AstraZeneca, Eli Lilly, Lundbeck A/S, Servier, Sepracor, and Janssen.
R.L. has received travel grants and conference speaker honoraria from AstraZeneca and
Lundbeck A/S. M.M. and W.W. have received speaker honoraria from Bayer.
This article is a PNAS Direct Submission.
1Towhom correspondenceshouldbeaddressed. E-mail:rupert.lanzenberger@meduniwien.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.
| February 14, 2012
| vol. 109
| no. 7
two independent approaches using fMRI, namely (i) the spon-
taneous fluctuations in BOLD signals acquired at rest and (ii)
the Tower of London (TOL) paradigm, a cognitively demanding
task that involves planning and working memory (38).
Resting-State fMRI and 5-HT1ABinding. The default mode network
was assessed with functional connectivity analysis from sponta-
neous BOLD activity at rest. To avoid seed selection bias (39),
we computed the DMN from the average fMRI time course of
the entire network instead of restricting the seed to a single re-
gion. This allows an interpretation beyond simple interregional
connectivities but more generally as each voxel’s functional
contribution to the network.
Similar to previous reports, functional connectivity analysis
showed significant involvement in the DMN for the posterior cin-
gulate, retrosplenial, medial prefrontal, and lateral parietal cortices
(P < 0.05, family-wise error (FWE)-corrected; Fig. 1B). In contrast,
we found no significant (para)hippocampal contribution (3), which
is, however, in line with previous work (16). The task-positive
counterpart of the DMN comprised the intraparietal and inferior
precentral sulci, inferior precentral gyrus, and supplementary
motor area as well as dorsal lateral prefrontal cortex and insula.
5-HT1Areceptor binding potentials (BPND) were estimated
with PET and the radioligand [carbonyl-11C]WAY-100635. Re-
ceptor quantification was carried out for whole-brain maps (Fig.
1A) and the dorsal raphe nucleus, which reflect 5-HT1A in-
hibition via local (i.e., heteroreceptor) and autoreceptor binding,
respectively. Hence, entering both variables into a linear re-
gression model enables a separation of local and autoinhibitory
5-HT1Aeffects as independent predictors of the DMN (i.e., ad-
justing for regional receptor binding when interpreting 5-HT1A
autoinhibition, and vice versa).
For 5-HT1A heteroreceptors, local positive and negative
modulations of the DMN were found in the retrosplenial cortex
(RSC; R2= 0.64, P < 0.05, FWE-corrected; Fig. 2A) and the
dorsal medial prefrontal cortex (dmPFC; R2= 0.45, P < 0.001),
respectively (Table S1). On the other hand, both 5-HT1Aauto-
and heteroreceptor binding predicted individual DMN con-
tributions in the posterior cingulate cortex (PCC) but in opposite
100635. (B and C) Independent representations of the DMN (P < 0.05, FWE-corrected) were obtained from rsfMRI (B) and deactivations within the Tower of
London task (C). rsfMRI and TOL show inverse patterns because the seed region for rsfMRI was set to the task-negative part of the DMN (3). x = 0 mm MNI
Average maps across 28 healthy subjects computed for PET and fMRI. (A) 5-HT1Areceptor BPNDas measured with the radioligand [carbonyl-11C]WAY-
respectively]. (A) Local 5-HT1Areceptors influence individual contributions to the DMN in the RSC. (B) Similar to resting-state fMRI (z scores in A), RSC
deactivations (parameter estimates) during the TOL paradigm are modulated by local 5-HT1Abinding. The RSC effects in A and B show inverse patterns due to
choice of rsfMRI seed region (Fig. 1 B and C). (C) On the other hand, the PCC is regulated through both 5-HT1Alocal and autoinhibition in an inverse manner
(A and C), suggesting a well-balanced interaction between dorsal raphe nucleus autoreceptors (C) and local 5-HT1Ain the PCC expressed on downstream
glutamate and GABA neurons (A). Images are shown at P < 0.001, cluster-level k = 15 voxels = 120 mm3. Scatter plots illustrate the association for the entire
cluster. Note that variables are mean-centered after adjustment for nuisance covariates (including correction for regional 5-HT1Abinding when interpreting
5-HT1Aautoinhibition, and vice versa), which introduces negative values in the plots (see Table S2 for actual BPNDvalues). x = 2, 7, and 3 mm MNI stereotactic
space for A, B, and C, respectively.
Modulation of the DMN via 5-HT1ABPNDseparated for auto- and heteroreceptors [i.e., dorsal raphe nucleus (DRN) and local 5-HT1Abinding sites,
| www.pnas.org/cgi/doi/10.1073/pnas.1117104109 Hahn et al.
directions (R2= 0.55, P < 0.05, FWE-corrected and R2= 0.44,
P < 0.001, respectively; Fig. 2 A and C and Table S1). In the task-
positive part, a significant association between local 5-HT1A
BPNDand the DMN was found in the insula (R2= 0.48, P < 0.05,
FWE-corrected; Table S1).
Tower of London fMRI and 5-HT1ABinding. In a second approach, we
aimed to validate these findings with an independent represen-
tation of the DMN. Because this network exhibits a main char-
acteristic of deactivation during cognitively demanding tasks (3,
15), participants additionally completed the Tower of London
paradigm within the same fMRI session (38, 40).
Group analysis showed deactivations predominantly in the
posterior DMN and to a lesser extent in medial prefrontal areas
and the insula (P < 0.05, FWE-corrected; Fig. 1C). Task-specific
activations were found in inferior and superior parietal areas,
precentral gyrus, supplementary motor area, dorsal lateral pre-
frontal cortex, and anterior insula (Fig. 1C and Fig. S1A).
Compared with resting-state fMRI, the activation pattern
obtained from TOL shows great overlap with the DMN task-
positive part but exhibits considerably less pronounced deacti-
vations, especially in the mPFC and precuneus (Fig. 1 B and C).
To assess the influence of 5-HT1Areceptors, the individual
activation maps from TOL served as dependent variables of the
linear regression model. Similar to resting-state analysis, local
5-HT1Abinding predicted task-induced deactivations in the RSC
(R2= 0.55, P < 0.05, FWE-corrected; Fig. 2B and Table S1). In
the task-positive counterpart of the DMN, local 5-HT1Aeffects
were present in the precentral gyrus bilaterally (R2= 0.5, P <
0.001; Fig. S1 B and C and Table S1).
See SI Results for evaluation of potential confounders.
This work demonstrates considerable modulatory effects of se-
rotonin-1A receptor binding on the default mode network.
Specifically, both 5-HT1Aauto- and heteroreceptors appear to
regulate the posterior part of the midline core system (PCC),
engaged when assessing the personal significance of events (9).
In contrast, only local 5-HT1Ainhibition influences DMN sub-
parts involved in memory-based future envisioning (RSC) (41)
and self-referential judgments (dmPFC) (7), but in an inverse
manner. Hence, it seems that the different brain areas, which
entail various functional aspects of self-referential processing,
experience markedly distinct regulations via different 5-HT1A
binding sites. This regional specificity via neurotransmitter mod-
ulations complements previous work showing that the DMN can
be further segregated by taking into account the functional
specialization of different brain regions (9, 42–44).
Serotonin-1A Modulations During Rest and Cognitive Tasks. Using
two independent approaches, we demonstrate a robust influence
of the 5-HT1Areceptor on the DMN’s retrosplenial cortex. The
RSC has been extensively implicated in the processing of epi-
sodic memory (44) and future imagination (45). More precisely,
it has been suggested to act as a relay station between allocentric
and egocentric viewpoints, translating memories and events
(indexed by the hippocampus and thalamus) into a personal
context (41). With regard to the 5-HT system, episodic memory
processing and the RSC experience a considerable influence by
tryptophan depletion (46, 47) and the administration of selective
serotonin reuptake inhibitors (SSRIs) (48), respectively. Hence,
the current study further specifies the modulation of this in-
trospective function by serotonergic neurotransmission to the
Comparing the resting state and the TOL task, consistent 5-
HT1Aeffects were only found in the RSC. On the other hand,
additional associations were present in different subparts of the
DMN for each of the paradigms. Again, although the basic
anatomy of the default mode network is similar across tasks
(rest, self-referential, and cognitive) and stimulus domains,
there is still a functional specialization for each of these (8, 49,
50). Hence, the additional task-related neuroreceptor modu-
lations found in cognitively demanding paradigms may emerge
at the cost of missing associations in task-negative parts of the
DMN. Such a functional specialization may similarly apply to
emotional tasks. For instance, in response to threat-related
stimuli, a significant influence of the serotonin-2A receptor (51)
and transporter polymorphism (52) on amygdala–prefrontal
coupling has been described. These findings demonstrate the
relevance of evaluating modulatory effects of different neuro-
transmitters also in specific paradigms of self-referential and
Posterior Cingulate Cortex. In addition to memory-coding regions
(hippocampus, thalamus), the RSC receives intensive input from
the PCC (41), which is in turn innervated by the midbrain raphe
nuclei (53). Accordingly, we found a modulation of dorsal raphe
nucleus 5-HT1Aautoreceptor binding onto the PCC and to a
lesser extent by local 5-HT1Aheteroreceptors. Importantly, this
region has been identified as the strongest hub in the resting
human brain, with the highest number of functional connections
(54). In line with previous reports and similar to the RSC, PCC
activity is influenced by tryptophan depletion (55) and SSRI
administration (56). Again, these findings indicate that a sub-
stantial part of the serotonergic influence on this DMN core
region (9) is mediated by the different 5-HT1Abinding sites. The
negative DMN association in the PCC with local 5-HT1A
receptors is, however, not contradictive to the positive modula-
tion via autoreceptors (Fig. 2 A and C). 5-HT1Aautoreceptors
attenuate raphe nuclei cell firing (33, 34), leading to reduced
5-HT release at projection sites (35). This decreases local 5-
HT1Ainhibition of downstream glutamate neurons, which in turn
increases BOLD signal (28). Hence, our results suggest a regu-
lation of the DMN’s major hub (54) through a well-balanced
interaction between 5-HT1A auto- and heteroreceptors. The
importance of such a relation between different receptors has
been shown in anxiety disorders for this subtype (57) and for the
balance between 5-HT1Aand 5-HT2Areceptors (58) as well as in
healthy subjects for 5-HT1Aand the serotonin transporter (59).
Medial Prefrontal Cortex and Serotonin-1A in Depression. The
associations found here could provide important implications
for major depressive disorder (MDD), considering alterations
in both 5-HT1Alevels (60–62) and DMN function (6, 22, 23).
Specifically, the negativity bias in MDD patients has been sug-
gested to emerge from a lack of DMN inhibition in the ventral
and dorsal mPFC (63), responsible for coding and reappraisal of
self-related stimuli, respectively (49). Moreover, SSRI adminis-
tration attenuates negative introspective processing of the mPFC
in subjects at risk for depression (64). Integrating the negative
dmPFC modulation found here leads us to speculate that the
deficient inhibition of the DMN in depression could be mediated
via the 5-HT1Areceptor subtype.
Regional Specificity of Neuroreceptor Modulations. In contrast to
our hypothesis of widespread interactions between the DMN via
5-HT1Areceptors, the observed effects were regionally specific to
certain subparts of the network. This is, however, consistent with
previous findings of local DMN modulations by other neuro-
transmitters. For instance, associations with the DMN have also
been found for dopamine and one of its degradation enzymes,
the catechol-O-methyltransferase (COMT). Here, the COMT
val158met polymorphism has been associated with differences
in mPFC–PCC connectivity at rest (65) and PCC deactivations
during two cognitively demanding paradigms (66). Furthermore,
task-related deactivations in ventral mPFC and precuneus are
Hahn et al. PNAS
| February 14, 2012
| vol. 109
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modulated by a dopamine D1/D2 receptor agonist (67) and
striatal dopamine transporter binding (68), respectively. Finally,
a significant relation has been reported for GABA concen-
trations in the anterior cingulate cortex and the local BOLD
response in emotional processing (69). Together, these results
point toward complex and regionally specific neurotransmitter
interactions driving brain networks at rest as well as during
cognitive and emotional processing.
To summarize, our results indicate that individual variations in
the execution of essential introspective functions experience
a considerable regulation by the major inhibitory serotonin re-
ceptor. Importantly, local and autoinhibitory 5-HT1A effects
exhibited a rather differential influence on areas with distinct
functionality. The regional specificity of our findings further
suggests that the DMN as a whole and hence the various aspects
of self-referential processing are driven by complex distribution
patterns of neuronal receptors, including their expression sites
on downstream glutamate and GABA neurons.
Materials and Methods
Subjects. Twenty-eight healthy subjects participatedinthis study(mean age ±
SD = 26.5 ± 6.8 y, 17 female), who in part also served as control subjects in
previous studies (57, 70–73) (SI Materials and Methods). This project was
approved by the Ethics Committee of the Medical University of Vienna and
the General Hospital of Vienna.
Positron Emission Tomography. PET measurements were carried out at the
Department of Nuclear Medicine, Medical University of Vienna, and are
essentially described elsewhere (57, 71) (SI Materials and Methods). Radio-
chemical variables for [carbonyl-11C]WAY-100635 at the time of injection
were injected dose = 381 ± 31.9 MBq, specific activity = 183.2 ± 147.1 MBq/
nmol, and mass of unlabeled compound = 3 ± 4.4 μg (mean ± SD); for syn-
thesis, see Wadsak et al. (74).
Serotonin-1A (5-HT1A) autoreceptor binding of the dorsal raphe nucleus
was obtained as described previously (57). A spherical region of interest of
4-mm diameter was defined manually within two slices of the original
summed PET image (57, 71, 75, 76). This yields a representative estimate of
autoreceptor binding because in the human raphe nuclei, the majority
(80–100%) of 5-HT1Areceptors is expressed on cell bodies of serotonergic
neurons (77). PET scans were then motion-corrected and normalized to
Montreal Neurological Institute (MNI) space using SPM8 (Wellcome Trust
Centre for Neuroimaging; http://www.fil.ion.ucl.ac.uk/spm) via the corre-
sponding T1-weighted MRI (SI Materials and Methods). To assess local in-
hibitory effects, whole-brain 5-HT1Amaps were computed from the final
normalized and resliced images comprising a voxel size of 2 × 2 × 2 mm.
Quantification of the 5-HT1Areceptor binding potential (78) was done in
PMOD 3.3 (PMOD Technologies) with an improved version (79) of the non-
invasive Logan plot (80) for both the dorsal raphe nucleus region of interest
and voxel-wise 5-HT1Amaps. The model includes two major advantages,
namely no assumption of compartmental model configuration (80) and
stable results in the presence of noise (79), which makes it particularly suit-
able for voxel-wise applications. The cerebellar gray matter (excluding ver-
mis) served as a reference region due to negligible specific receptor binding
(81). The time from which the Logan plot describes a straight line (t*) was
automatically estimated from the insula with PMOD 3.3 and was 19.6 ± 4.5
min. 5-HT1Areceptor binding potentials are shown in Fig. 1A and Table S2.
Functional Magnetic Resonance Imaging. In addition to PET, each subject un-
derwent MRI measurements in a 3-T Medspec S300 (Bruker Biospin) while
performing a resting-state scan and the TOL paradigm. The two paradigms
parameters as described previously (40, 73, 82) (SI Materials and Methods).
Standard preprocessing for both paradigms was carried out in SPM8 using
default parameters if not specified differently. This included correction for
slice-timing differences and head motion (quality = 1), normalization to MNI
with a Gaussian kernel of 9 mm.
Resting-State Functional Connectivity Analysis. Resting-state fMRI (rsfMRI)
data were further processed in MATLAB R2010a (MathWorks) according to
an optimized procedure (82). Potential confounders of ventricular, white
matter, and global signal as well as motion parameters were corrected for
using linear regression analysis, and a band-pass filter was applied (12-term
finite impulse response (FIR) filter, 0.007 < f < 0.08 Hz). To avoid seed se-
lection bias (39), functional connectivity analysis of the DMN was computed
in several steps. First, a seed region (83) was defined within the posterior
cingulate cortex (cubic volume of 3 × 3 × 3 voxels = 216 mm3centered at x/y/
z = 0/−52/30 mm), because this represents the main functional connectivity
hub of the human brain (54). The averaged BOLD signal time course was
then cross-correlated voxel-wise with the entire brain, and correlation maps
were converted to z values by Fisher’s r-to-z transformation. A binary map of
the DMN was obtained by applying a one-sample t test across subjects and
thresholding at P < 0.05, FWE-corrected for multiple comparisons at the
voxel level. Similarly, a second DMN mask was created from a seed within
the mPFC [x/y/z = 0/50/22 mm (54)], which represents the frontal part of the
DMN core network (9). The final seed region was then calculated as the
intersection of the two binary DMN masks, covering a volume of 44.8 cm3
(=5,604 voxels). Hence, the obtained seed is not restricted to a particular
region, but comprises a conjunction of several network nodes (84). Again,
cross-correlation was then computed between the seed’s averaged BOLD
signal time course and the brain, followed by z transformation. This ap-
proach enables an interpretation of the resulting z values not simply as
connectivity between two regions but more generally as how strong each
voxel is functionally involved in the network (i.e., each voxel’s functional
connectivity with the network mean).
Tower of London Task. The TOL paradigm is a cognitively demanding task that
requires planning, spatial working memory, and problem solving (38, 40) (SI
Materials and Methods). Following previous associations between TOL and
the dopamine system (67), this paradigm provides a reasonable choice for
assessing neurotransmitter modulations of the DMN.
For first-level data analysis, the boxcar function obtained from the design
was convolved with a standard hemodynamic response function in SPM8.
Similar to the resting state, additional nuisance regressors from realignment
parameters and ventricular and white matter signals were included in the
model. Notably, the activations obtained from the TOL task exhibit the in-
verse pattern compared with resting-state analysis (Fig. 1 B and C), because
the seed region of the rsfMRI connectivity calculations was set to the task-
negative part of the DMN (3).
Statistical Analysis. To investigate both local and autoinhibitory effects of
5-HT1Abinding on the DMN, a linear regression analysis was carried out
using the biological parametric mapping (BPM) toolbox (85) for SPM8.
The toolbox enables the calculation of voxel-by-voxel and region of in-
terest-by-voxel associations within the same model. Hence, independent
variables were defined as 5-HT1ABPNDwhole-brain maps and dorsal raphe
nucleus receptor binding, representing local inhibition and autoinhibition
via 5-HT1A, respectively. On the other hand, rsfMRI z maps reflecting the
DMN were used as dependent variables. The influence of 5-HT1Abinding
on de-/activations during the TOL paradigm were evaluated in the same
fashion, that is, using fMRI maps with parameter estimates as depen-
Because 5-HT1ABPNDof the dorsal raphe nucleus is highly correlated with
postsynaptic receptor binding (57, 86–88), we aimed to disentangle local
from autoinhibitory effects. Using the BPM toolbox, this was realized by
adjusting for regional 5-HT1A binding when interpreting 5-HT1A auto-
inhibition, and vice versa. This enables an evaluation of the two variables as
independent predictors of the DMN (i.e., assuming the same local 5-HT1A
binding across subjects for the assessment of autoinhibition, and vice versa).
Additional nuisance covariates included age for rsfMRI analysis as well as
age and reaction times for the TOL paradigm.
All calculations were spatially restricted to areas with a robust represen-
tation of the DMN and fMRI de-/activations, respectively. This was assessed by
one-sample t tests for each fMRI paradigm (P < 0.05, FWE-corrected at voxel
level; Fig. 1) with a separate evaluation of task-positive and -negative net-
works (Table S1). To provide a thorough assessment, we report results at an
uncorrected threshold of P < 0.001 voxel level with a minimum cluster size of
120 mm3(k = 15 voxels). However, considering the spatial constraints (339
and 192 cm3for resting state and TOL, respectively), correction for multiple
comparisons was applied at P < 0.05, FWE-corrected for peak voxels or
cluster extent (the latter at P < 0.001, uncorrected voxel level). All statistical
tests were carried out two-tailed.
To illustrate the association between 5-HT1Abinding and the DMN, scatter
plots were created in MATLAB R2010a. However, simply plotting the de-
pendent variable (Y) against the target explanatory (independent) variable
| www.pnas.org/cgi/doi/10.1073/pnas.1117104109Hahn et al.
(X) does not take into account the remaining independent variables (Z,
nuisance covariates), which are still included in the regression model. Hence,
partial regression plots were used to adjust the scatter plot for variables of
noninterest (89, 90). This is achieved by plotting the residuals of the re-
gression of Y on the remaining independent variables Z against the residuals
of the regression of X on the remaining independent variables Z. Notably,
this adjustment implies that plotted values can take positive and negative
values because residuals are mean-centered (i.e., mean = 0).
ACKNOWLEDGMENTS. We thank the medical and technical teams of the
U. Moser, E. Akimova, and M. Savli), the PET Center at the Department of
Nuclear Medicine (K. Kletter, R.Dudczak, L.-K. Mien, T. Zenz, and A. Krcal), and
the Magnetic Resonance Centre of Excellence (E. Moser and F. Gerstl). This
research was funded by Austrian National Bank Grant P11468 and Austrian
Science Fund Grant P 23021 (to R.L.). A.H. is a recipient of a DOC fellowship
of the Austrian Academy of Sciences at the Department of Psychiatry and
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