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RESEARCH ARTICLE
PET and MRI show differences in cere
al
asymmetry and functional connectivity between
homo- and heterosexual subjects
Ivanka Savic and Per Lindström
+ See all authors and affiliations
PNAS July 8, XXXXXXXXXX9408; https:
doi.org/10.1073/pnas XXXXXXXXXX
Edited by Jan-Åke Gustafsson, Karolinska Institutet, Stockholm, Sweden, and approved April 30, 2008
(received for review Fe
uary 27, 2008)
Article Figures & SI Info & Metrics
Abstract
Cere
al responses to putative pheromones and objects of sexual attraction were
ecently found to differ between homo- and heterosexual subjects. Although this
observation may merely mi
or perceptional differences, it raises the intriguing
question as to whether certain sexually dimorphic features in the
ain may diffe
etween individuals of the same sex but different sexual orientation. We addressed
this issue by studying hemispheric asymmetry and functional connectivity, two
parameters that in previous publications have shown specific sex differences.
Ninety subjects [25 heterosexual men (HeM) and women (HeW), and 20
homosexual men (HoM) and women (HoW)] were investigated with magnetic
esonance volumetry of cere
al and cerebellar hemispheres. Fifty of them also
participated in PET measurements of cere
al blood flow, used for analyses of
functional connections from the right and left amygdalae. HeM and HoW showed a
ightward cere
al asymmetry, whereas volumes of the cere
al hemispheres were
symmetrical in HoM and HeW. No cerebellar asymmetries were found. Homosexual
subjects also showed sex-atypical amygdala connections. In HoM, as in HeW, the
connections were more widespread from the left amygdala; in HoW and HeM, on
the other hand, from the right amygdala. Furthermore, in HoM and HeW the
connections were primarily displayed with the contralateral amygdala and the
anterior cingulate, in HeM and HoW with the caudate, putamen, and the prefrontal
cortex. The present study shows sex-atypical cere
al asymmetry and functional
connections in homosexual subjects. The results cannot be primarily ascribed to
learned effects, and they suggest a linkage to neurobiological entities.
amygdala homosexuality cere
al lateralization cere
al connectivity magnetic
esonance volumetry
One of the more controversial questions in the neurobiology of human behavio
elates to the mechanisms of sexual orientation. This issue received increasing
interest over the last decade and was further substantiated by recent results from
imaging studies of cere
al activation in homo- and heterosexual subjects. During
judgment of face attractiveness and when viewing sexually arousing films the
cere
al response was found to be invariant to the prefe
ed sexual stimulus/face
[male in heterosexual women (HeW) and homosexual men (HoM), and female in
heterosexual men (HeM) and homosexual women (HoW)] and located in certain
core regions of the reward circuitry and the motor cortex (1, 2). Furthermore, in a
series of PET activation studies during smelling of putative pheromones we
detected a sex differentiated activation of the anterior hypothalamus in HeM and
HeW (3) and a sex-atypical (almost reciprocal) pattern of activation in HoM and
HoW (4, 5). While intriguing, none of these studies provided conclusions about the
underlying mechanisms because they imaged perceptional processes, which could
e innate, as well as learned. By indicating a link between sexually dimorphic
egions of the mid
ain and the sexual orientation, they fueled, however, the on-
going discussion about the neurobiology of sexual orientation and raised several
new questions. One is whether the sexual dimorphism reported in the literature (6)
could be sex-atypical in homosexual subjects even with respect to factors that are
not directly associated with behavior. Another is whether possible differences
etween homo- and heterosexual subjects could be present also in the cere
al
circuits outside those strictly involved in reproduction.
In the present study we addressed these issues by investigating age-matching
groups of homo- and heterosexual men and women with respect to two separate
parameters, both unlikely to be directly affected by learned patterns and behavior,
and both showing sex-dimorphic characteristics in previous studies. The first
parameter was cere
al and cerebellar asymmetry measured with magnetic
esonance (MR) volumetry. The second parameter was the functional connectivity
from the right and left amygdalae, analyzed on the basis of PET measurements of
egional cere
al blood flow (rCBF) during rest and passive smelling of unscented
air. Our choice to measure hemispheric volumes was based on survey of the
cu
ent literature providing some indications of a sex-atypical lateralization in HoM,
and to a certain extent also in HoW during certain neuropsychological tests. One
example is studies of dichotic listening showing a more pronounced right–ea
preference in HeM compared with HoM (and also HeW), in whom no significant
lateralization was detected (7, 8). Another is the report that HoM, like HeW, have a
elatively larger anterior commissure compared with HeM (9), providing a possible
anatomical substrate for higher interhemispheric connections (10). Furthermore,
HoM are, like HeW, reported to outperform HeM in certain ve
al tests. Such tests
involve language circuits, which according to some studies are more symmetrical
in women (11, 12). HoM also show an inferior performance in visuospatial tasks,
and particularly in tests of mental rotation and navigation strategy (13–15). These
functions are processed primarily by the right parietal lobe, which is relatively large
in men than in women (16, 17). The data from HoW are sparse. HoW score like
HeW on most cognitive measures except for ve
al fluency and mental rotation,
two tests in which they perform more like HeM (13, 14). They are reported to have a
slightly stronger right-ear preference during dichotic listening than HeW, and some
studies show similarities with HeM also in click-evoked otoacoustic emissions
(CEOAEs) (13, 16, 18). Together, these data provide a rationale for possible global
hemispheric differences with respect to sex and sex orientation.
The choice to measure amygdala connectivity was based on several reports about
sex differentiated amygdala lateralization in processing of emotional memories
(with an activation of the right amygdala in men, and the left amygdala in women)
(19, 20). Furthermore, Kilpatrick et al. (21) recently found that the amygdala exhibits
sex-differentiated functional connections during rest: in men, the connections were
mainly displayed from the right amygdala and targeted to the sensorimotor cortex,
striatum, and pulvinar, whereas in women they were more pronounced from the left
amygdala and targeted to the subgenual cortex and the hypothalamus. Because
measurements of the resting state functional connectivity are independent of user,
perceptive, cognitive, or behavior-related tasks, they lend themselves to studies of
more crude potential neurobiological co
elates to sex and sexual orientation. In
addition, the amygdala is a key structure in the limbic networks and exhibits high
density of estrogen and androgen receptors (22, 23).
The present study was ca
ied out under the hypothesis that:
i. The hemispheric volumes are symmetrical in HeW but not HeM;
ii. in HeW the amygdala is functionally connected primarily with the subgenual cortex
and the hypothalamus, in HeM with the sensorimotor cortex and the striatum;
iii. the side difference in hemispheric volumes, as well as the pattern of amygdala
connectivity, could be sex-atypical in homosexual subjects.
Results
Hemispheric Volumes.
HeM and HoW had significantly asymmetrical hemispheric volumes, with large
ight hemisphere (P = XXXXXXXXXXfor HeM and P < 0.001 for HoW; paired t tests). In
contrast, no asymmetry was detected in HeW (P = XXXXXXXXXXor in HoM (P = 0.8749)
[Table 1 and supporting information (SI) Fig. S1]. The ANOVA showed a significant
overall group difference (P = 0.0008; F = 6.168, df = 3). Fisher's post hoc test
evealed that the asymmetry in HeM was significant in relation to HeW (P = 0.0005)
and HoM (P = XXXXXXXXXXLikewise, the asymmetry index in HoW was significant in
elation to HeW and HoM (P = XXXXXXXXXXand P = 0.0344, respectively). No difference
was found between the HeM and HoW, or between HeW and HoM. The observed
differences in asymmetry were not related to a particular hemisphere (Table 1). A
post hoc evaluation detected asymmetry exceeding two standard deviations of that
in HeW (who were hypothesized to have symmetrical volumes) in 1 HeW, 4 HoM, 7
HoW, and 12 HeM.
Table 1.
Volumes of interest
The cerebellar hemispheres were symmetrical in all four populations (P = 0.871, F =
0.236, df = 3; one-way ANOVA), without any group difference (Table 1). The inter-
ater co
elations was 0.85, P < 0.001, for cere
al hemispheres, and 0.93, P
0.001, for the cerebellar hemispheres. The co
esponding intra-rater co
elations
were 0.88 and 0.95 (P < 0.001).
Functional Connectivity.
Only positive covariations are reported because only one significant negative
covariation was observed [detected in HeM, with respect to the right amygdala,
and located in the occipital cortex, (z = 4.5; size 5.3, Talairach coordinates 6, −80,
16)].
HeW had more widespread connections from the left amygdala, HeM from the right
amygdala (Table 2). Furthermore, in HeW connections were displayed with the
contralateral amygdala, the anterior cingulate and subcallosum, and the
hypothalamus, whereas in HeM the connectivity clusters covered the putamen and
caudate, and portions of the agranular insular cortex. Both HeW and HeM showed
covariation with portions of the temporal neocortex (Tables 2 and 3 and Fig. 1).
Table 2.
Significant covariations from the left amygdala
Table 3.
Significant covariations from the right amygdala
Fig. 1.
Covariations with the respective amygdala seed region in hetero- and homosexual subjects. The
Sokoloff scale indicates T values. Clusters detected at T = 3.0 are superimposed on the standard MR
image of the
ain.
The connectivity pattern in homosexual subjects was almost reciprocal in relation
to the same-sex controls. First, in HoM the connections were more widespread
from the left amygdala, in HoW from the right amygdala (Fig. 1). Second, HoM, just
as HeW, displayed connections with the contralateral amygdala, the anterio
cingulate, the subcallosum, and the hypothalamus. In HoW, on the other hand,
connections were displayed with the putamen and the o
itofrontal and prefrontal
cortex, but not with the contralateral amygdala or the cingulate cortex (Tables 2
and 3 and Fig. 1).
All four groups also showed functional connections with the temporal neocortex
ipsilateral to the amygdala seed region