Sex differences in the cerebral BOLD signal response
to painful heat stimuli
Eric A. Moulton, Michael L. Keaser, Rao P. Gullapalli, Ranjan Maitra and Joel D.
Greenspan
Am J Physiol Regulatory Integrative Comp Physiol 291:257-267, 2006. First published Apr 6, 2006;
doi:10.1152/ajpregu.00084.2006
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Am J Physiol Regul Integr Comp Physiol 291: R257–R267, 2006.
First published April 6, 2006; doi:10.1152/ajpregu.00084.2006.
CALL FOR PAPERS
Sex and Gender Differences in Pain and Inflammation
Sex differences in the cerebral BOLD signal response to painful heat stimuli
Eric A. Moulton,1,2 Michael L. Keaser,1,3 Rao P. Gullapalli,2,4 Ranjan Maitra,5 and Joel D. Greenspan1,2,3
1
Department of Biomedical Sciences, Dental School, 2Program in Neuroscience, 3Research Center for
Neuroendocrine Influences on Pain, and 4Department of Diagnostic Radiology, University of Maryland
Medical School, Baltimore, Maryland; and 5Department of Statistics, Iowa State University, Ames, Iowa
Submitted 31 January 2006; accepted in final form 21 March 2006
nociception; fMRI; somatosensory cortex; neuroimaging; prefrontal
cortex; negative blood oxygenation level-dependent signal
A NUMBER OF STUDIES HAVE FOUND that women are more sensitive
to experimental painful stimulation than men. Yet, this is
neither a universal nor large effect, given that no differences
are found in approximately one-third of the published studies,
and statistically significant differences are often in the smallto-moderate range (9, 27). Nonetheless, enough of a difference
is found with enough regularity to suggest that women and men
may perceive painful stimuli differently. Related to this are
observations from both animal and human studies that nociceptive processing may be different between men and women,
Address for reprint requests and other correspondence: J. D. Greenspan,
Dept. of Biomedical Sciences, Univ. of Maryland Dental School, 666 W.
Baltimore St., Rm. 5-A-14, Baltimore, MD 21201 (e-mail: jgreenspan
@umaryland.edu).
http://www.ajpregu.org
thereby providing a basis for sex differences in perception and
behavior (7, 19).
Only a few neuroimaging studies have reported sex differences in cerebral responses to noxious stimuli. A 15O-positron
emission tomography (PET) study reported significantly
greater activation in women vs. men in several regions of
interest (ROIs) in response to noxious heat (24). In that report,
sex differences in the PET results may have been related to sex
differences in perceived pain intensity. In contrast, a subsequent PET study reported significantly greater activation in
men vs. women in some of the same ROIs in response to
painful laser stimuli (6), in this case, using stimuli that were
matched for perceived intensity. Other PET studies have described sex differences in responses to visceral pain, although
principally with chronic visceral pain patients (3, 22). The
differences reported were mostly in the direction of greater
activation in men, although some brain regions showed the
opposite difference in the later study. An intriguing functional
magnetic resonance imaging (fMRI) study reported that sex
differences can be found in the pain-related activation of
several cortical areas, but the observation of such differences
depended on the menstrual cycle phases of the women (1).
There are clearly too few data at this point to conclude
anything about sex differences in cortical responses to pain.
Differences in results among the aforementioned studies can be
attributable to protocol differences, but also to differences in
the choice of dependent variables and analysis approaches. We
sought to address this issue by looking for sex differences in
several parameters of the fMRI-derived blood oxygenation
level-dependent (BOLD) response to painful heat stimuli
within multiple ROIs of the pain neuromatrix. Portions of these
results have been reported in abstract form (11, 21).
MATERIALS AND METHODS
The data for this study were derived from a larger project comprising a dissertation thesis. The data collection methods have been
described elsewhere (20) but are also provided below.
Subjects. Twenty-eight subjects (11 male, mean age 29, range:
22– 45 years old; 17 female, mean age 30, range: 23– 42 years old)
satisfactorily completed the study. Subjects provided written informed
consent to participate in the study and were paid $25/h. Additional
subjects were recruited, but excluded on the basis of psychophysical
screening (n ⫽ 20; criteria described below), scheduling problems
(n ⫽ 2), technical problems with fMRI scanning (n ⫽ 5), and signal
The costs of publication of this article were defrayed in part by the payment
of page charges. The article must therefore be hereby marked “advertisement”
in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
0363-6119/06 $8.00 Copyright © 2006 the American Physiological Society
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Moulton, Eric A., Michael L. Keaser, Rao P. Gullapalli, Ranjan
Maitra, and Joel D. Greenspan. Sex differences in the cerebral
BOLD signal response to painful heat stimuli. Am J Physiol Regul
Integr Comp Physiol 291: R257–R267, 2006. First published April 6,
2006; doi:10.1152/ajpregu.00084.2006.—There are limited data addressing the question of sex differences in pain-related cerebral
processing. This study examined whether pain-related blood oxygenation level-dependent (BOLD) signal change measured with functional magnetic resonance imaging (fMRI) demonstrated sex differences, under conditions of equivalent pain perception. Twenty-eight
healthy volunteers (17 women, 11 men) were subject to a fMRI scan
while noxious heat stimuli were applied to the dorsum of the left foot.
Significant BOLD signal modulation was observed in several nociceptive processing regions of interest (ROIs) in all subjects. There
were no sex differences in the spatial extent of BOLD signal change
for any ROI, but the signal amplitude was lower for women in most
ROIs and significantly so for the primary somatosensory cortex (S1),
the midanterior cingulate cortex, and the dorsolateral prefrontal cortex
(DLPFC). The BOLD signal response could be positive or negative,
and frequently, both polarities were observed within a single ROI. In
most ROIs, women show proportionately more voxels with negative
signal change than men, and this difference was statistically significant for the S1 and the DLPFC. The time course of the negative signal
change was very similar to that of the positive signal change, suggesting that the latter was not “driving” the former. The location of
negative and positive clusters formed distinct patterns in several of the
ROIs, and these patterns suggest something other than a local “steal”
phenomenon as an explanation for the negative signal changes. Sex
differences in baseline cerebral blood flow may contribute to the
BOLD signal differences observed in this study.
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SEX DIFFERENCES IN CEREBRAL RESPONSES TO HEAT PAIN
thermal intensity of the two temperatures within five points of each
other on a 100-point visual analog scale (VAS) (n ⫽ 1). Subjects that
had no sensation of heat with the 41°C stimulus were also excluded
(n ⫽ 2). Subjects were also excluded if their pain tolerance was below
45°C (n ⫽ 3) or above our range of testing (50°C) (n ⫽ 2).
Experimental protocol. During the fMRI session, subjects were
scanned while each of the 16-s target temperatures were applied three
times each in random sequence. Each thermode application period
was separated by 38 s. To avoid primary afferent habituation or
sensitization, each thermode placement was rotated sequentially according to the warmth maps, effectively lengthening the interstimulus
interval for any single stimulated area to 5 min and 8 s.
After each stimulus, subjects were presented with a computerized
VAS (DAPSYS; Brian Turnquist, Johns Hopkins University, http://
www.dapsys.net) via MRVision 2000 goggles (Resonance Technologies, Van Nuys, CA), at which time they rated their peak sensation
of thermal intensity using an MR-compatible trackball (Fellowes,
www.fellowes.com), which controlled a cursor moving along the
vertical VAS. The extremes of the VAS were labeled “no warmth”
and “most intense pain imaginable,” with a marker for “just painful”
located at the lower quarter of the scale. Subjects were instructed to
use the range from “no warmth” to “just painful” to rate nonpainful
heat and to use the range from “just painful” to “most intense pain
imaginable” to rate painful heat. Ratings were stored as numbers from
0 –100, with 25 corresponding to “just painful.”
Fig. 1. Measurement of pain-related blood oxygenation level-dependent (BOLD) signal amplitude change across all prescribed regions of interest (ROIs). Top:
extent of BOLD signal change measured as the number of significantly modulated 1-mm3 voxels, separately for two pain intensities and the two sexes. Bottom:
%signal change for the same significantly modulated voxels. S1 and S2, somatosensory cortexes; mid-ACC, midanterior cingulate cortex; rACC, rostal ACC;
pINS, posterior insula; aINS, anterior INS; DLPFC, dorsolateral prefrontal cortex; SMA, supplementary motor area; R, right or contralateral to stimulation; L,
left. Data are presented as means ⫾ SE. **Significant sex difference at P ⬍ 0.05; ^^significant interaction of sex and pain level at P ⬍ 0.05.
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artifacts identified during fMRI image analysis (n ⫽ 5). The University of Maryland Institutional Review Board for the Protection of
Human Subjects approved the procedures and protocols for this study.
Thermal stimulation. An MR-compatible Peltier thermal probe
with a 2.6 cm2 contact surface (model TSA-II; Medoc) was used to
apply heat stimuli. During pretesting, a 41°C search stimulus was
applied to the dorsum of the subject’s foot to identify areas sensitive
to innocuous heat. Six probe-sized (2.6 cm2) warmth-sensitive areas
were identified and used as guides for placement of the thermal
stimuli to avoid warmth-insensitive regions during the experiment (10).
The probe was preheated to a target temperature before being
placed onto the subject’s foot for 16 s. A pneumatic handle attached
to the thermode allowed the experimenter to continuously and consistently apply the probe with 300 – 400 g of pressure. Three target
temperatures were selected: one innocuous (41°C) and two painfully
hot. The painfully hot temperatures were determined separately for
each subject based on pain tolerance. Pain tolerance was assessed
during a separate test session by using an ascending series of stimuli
presented in 1°C steps. Tolerance was defined as the highest temperature a subject accepted without withdrawing from the stimulus. The
hottest temperature used in the fMRI sessions was identified as 1°C
below pain tolerance (pain 2), and the other as 2°C below tolerance
(pain 1). Subjects who described either of these temperatures as
nonpainful were excluded (n ⫽ 12), as were subjects who rated the
SEX DIFFERENCES IN CEREBRAL RESPONSES TO HEAT PAIN
AJP-Regul Integr Comp Physiol • VOL
matosensory cortex (S2), the posterior insula (pINS), the anterior
insula (aINS), rostral ACC (rACC), midanterior cingulate cortex
(mid-ACC), the supplementary motor area (SMA), and the dorsolateral prefrontal cortex (DLPFC). The foot area of S1 was defined as the
paracentral lobule and the portion of the postcentral gyrus extending
from the midline of the brain laterally to the beginning of the curve of
the incus. S2 was marked as part of the parietal operculum located on
the upper bank of the Sylvian fissure and behind the postcentral gyrus.
The pINS was identified as the insula located within the depths of the
Sylvian fissure at the same rostral-caudal level as S2. The aINS
consisted of the rest of the insula rostral to the pINS. The ACC as a
whole was identified as the area between the corpus callosum and
cingulate sulcus, extending caudally from the anterior boundary of the
splenium of the corpus callosum, to the rostral termination of the
cingulate sulcus. The ACC was divided into rACC and mid-ACC by
a division in the coronal plane through the subcallosal striatum, which
approximately splits Brodmann area 24 (BA 24) and BA 24⬘. SMA
was marked as the area immediately above the ACC, superior to the
cingulate sulcus, and stretched anterior-posteriorly from the caudal
limit of the ACC to the beginning of the superior frontal gyrus. The
DLPFC was marked as the triangular and opercular parts of the
inferior frontal gyrus that includes BA 44, 45, and 46.
For the voxel clusters identified within each ROI, two measures
were derived: spatial extent and signal amplitude. With regard to
significantly activated voxel counts for each ROI, a 2 ⫻ 2 mixed
ANOVA was performed, consisting of a within-subjects factor of
stimulus intensity (pain 1 and pain 2) and a between-subjects factor of
sex (men and women). Voxel count data were subject to a logarithmic
transformation before statistical analysis to more closely approximate
a normal distribution.
Fig. 2. Percentage of significantly modulated voxels with a negative signal
change. Data are presented as means ⫾ SE. **Significant sex difference at P ⬍
0.05; ^^significant interaction of sex and pain level at P ⬍ 0.05.
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The temperatures used for the pain 1 and pain 2 stimulus levels in
the scanner environment were determined on an individual basis. Of
the 41°C stimulus, 89% of the ratings were nonpainful warmth,
whereas the prescribed pain 1 and pain 2 temperatures were both
consistently painful and rated significantly different from one another
[2-tailed paired Student’s t-test, t(27) ⫽ 5.57, P ⬍ 0.001]. The mean
pain 1 temperature applied was 46.4°C (⫾1.1 SD), while the mean
pain 2 temperature was 47.4°C (⫾1.1 SD).
Image acquisition. Functional MRI scans were carried out using the
rectilinear echo planar imaging (EPI) method with a 1.5T Philips
Eclipse scanner (formerly Marconi Medical Systems, Cleveland, OH).
A gradient echo single shot EPI sequence was used to provide 3.2 ⫻
3.2-mm acquired resolution over a 24-cm field of view (FOV). These
images were zero padded to 128 ⫻ 128 pixels to provide a resolution
of 1.875 ⫻ 1.875 mm. T2* weighting from this sequence was
accomplished with a gradient echo time (TE) of 35 ms. The repetition
time (TR) was 2,000 ms, which allowed the cerebrum to be covered
using 23 slices, with a slice thickness of 6 mm and no gaps between
slices. High-resolution anatomical volumetric scans (4.5 ms TE, 29
ms TR, 92 slices, with a slice thickness of 1.5 mm and a 0.938 ⫻
0.938 mm in-plane resolution over a 24-cm FOV) were acquired for
anatomical reference of the functional slices.
Image processing and analysis. Image processing and statistical
analysis were performed using AFNI (http://afni.nimh.nih.gov). The
first four volumes were removed from the functional scan series to
allow for signal equilibration. For motion correction, functional time
series images were spatially registered to the first of the remaining
volumes using the AFNI script 3dvolreg. Spiking artifacts in any time
series that exceeded 2.5 SDs of the overall signal were reduced using
the AFNI routine 3dDespike. Time series were temporally smoothed
using a moving three-point weighted (0.15– 0.70 – 0.15) average. To
increase the signal-to-noise ratio and to accommodate interindividual
differences in brain morphology, spatial blurring was applied to the
images for all time points using a 5-mm full-width, half-maximum
Gaussian blur. Linear, second-order, and third-order trends within the
time series were removed, and voxelwise normalization was achieved
by dividing the signal intensity at each time point by the voxel’s mean
intensity. If required, the high-resolution anatomical images were
manually registered to the functional images to achieve maximum
alignment using the AFNI software routine Nudger.
A general linear model (GLM) was used to identify regions in the
brain of individual subjects whose signal variation was significantly
related to the experimental protocol. Through voxelwise regression of
fMRI signal time courses, the GLM was used to model temporally
discrete responses, or regressors, to each of four conditions: the three
levels of thermal stimulation, and the rating task. The timing of the
experimental paradigm dictated which intervals of the signal corresponded to each condition. The responses were modeled using a
boxcar with a 4-s lag. Previous analysis with multiple lags indicated
that 4 s was at least as appropriate as other likely alternatives (20).
To create statistical parametric maps, Monte Carlo simulations
were run to estimate the likelihood of detecting false positives over
multiple comparisons (3dAlphaSim in AFNI). It was determined that
an individual voxel threshold of P ⬍ 0.01 and a cluster threshold of
four voxels (in original coordinate space) provided a corrected overall
alpha of P ⬍ 0.05. Given this threshold criteria, a BOLD signal
change map was first created on the basis of the full F value of the
GLM model for each voxel. Subsequently, partial FBOLD signal
change maps for both the pain 2 and pain 1 regressors were created as
subsets of the full F-map under this same threshold criteria. The pain
2 and pain 1 partial F-maps and the anatomical scans were then
spatially normalized to fit the human brain atlas of Talairach-Tournoux (33), and the voxels were resampled to 1 ⫻ 1 ⫻ 1 mm.
Anatomical reference maps. The locations of active voxel clusters
within both partial F-masks were categorized into different ROIs
based on structural landmarks (20). A priori ROIs included the foot
representation in primary somatosensory cortex (S1), secondary so-
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SEX DIFFERENCES IN CEREBRAL RESPONSES TO HEAT PAIN
and the largest voxel cluster containing only significantly activated
negative voxels were separately mapped. Using the AFNI routine
3dclust, the center of mass coordinate in Talairach coordinate space
was derived for each positive and negative voxel cluster. A 2 ⫻ 2,
repeated-measures ANOVA (signal change direction and temperature) was used to test for significant differences in location of the x-,
y-, and z-axes, separately for each ROI.
RESULTS
Pain ratings. The pain ratings were significantly different
for the two stimulus intensities, but not significantly different
between men and women (pain 1 mean ratings: men ⫽ 55,
women ⫽ 51; pain 2 mean ratings: men ⫽ 70, women ⫽ 72;
sex factor P ⫽ not significant). Additionally, although women
received slightly weaker stimuli on average, there was no
significant group difference in the temperatures used for men
vs. women (pain 1 average temperature: men ⫽ 46.5°C,
women ⫽ 46.3°C; pain 2 average temperature: men ⫽ 47.5°C,
women ⫽ 47.3°C; sex factor P ⫽ not significant).
Imaging responses. The spatial extent of significantly activated voxels was not significantly different between men and
women for any of the ROIs, despite a large variation across
ROIs (Fig. 1, top). In contrast, the amplitude of BOLD signal
change (%signal change) was significantly greater for men in
Fig. 3. Measurement of pain-related BOLD signal amplitude change considering only those voxels with a significant positive signal change. Data are presented
as means ⫾ SE. **Significant sex difference at P ⬍ 0.05; ^^significant interaction of sex and pain level at P ⬍ 0.05.
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For signal amplitude analysis of the ROI single trial averages, the
average of the first four time points (0 – 8 s into the cycle or the
“baseline” of the signal response) was subtracted from the average of
time points 7–14 (14- to 28-s cycle or the estimated “peak” of the
signal response) for each subject. To characterize the signal amplitude
responses of each ROI, a 2 ⫻ 2 mixed ANOVA was performed on this
signal intensity change data, as described above for the spatial extent
data. On the basis of analysis of the data distribution, no logarithmic
transformation of these data was deemed necessary.
Analysis of direction of voxel signal change. Subsequently, positive
and negative BOLD signal changes were separately analyzed. Any
significantly activated voxel was deemed a negative BOLD signal
change if it had a negative value for its regressor coefficient in the
original GLM model, whereas any significantly activated voxel was
deemed a positive BOLD signal change if it had a positive value for
its regressor coefficient. One analysis consisted of testing whether the
percentage of negative signal change voxels (percentage of significantly modulated voxels, not percentage of all voxels in the anatomically defined ROI) differed significantly between men and women,
separately for each ROI and for the two painful levels of stimulation.
Subsequently, two 2 ⫻ 2 mixed ANOVAs were conducted for voxel
counts and signal amplitude change, separately for voxels only showing positive BOLD signal change and voxels only showing negative
BOLD signal change.
Locations of positive and negative voxel clusters. Within a given
ROI for each subject and for each painful temperature, the largest
voxel cluster containing only significantly activated positive voxels
SEX DIFFERENCES IN CEREBRAL RESPONSES TO HEAT PAIN
Despite this sex difference in the spatial extent of negative
BOLD signal change, there was no apparent or statistical sex
difference in the signal amplitude (Fig. 4, bottom).
To examine the time course of these responses, single trial
averages were generated for each ROI, separately for the two
levels of painful stimulation and averaged across subjects. This
was done for all significantly activated clusters and then
separately for positive and negative signal change clusters (Fig.
5). In general, the negative BOLD signal resembled a mirror
image of the positive BOLD signal with no apparent lag in the
response. Furthermore, there is no apparent sex difference in
the response waveform when reviewing positive or negative
responses separately but only when averaging across both
positive and negative significantly activated voxels.
The locations of the positive vs. negative clusters were
found to be nonrandomly organized in some ROIs. Those ROIs
along the midline (rACC, mid-ACC, and SMA) showed a very
distinct pattern of positive signal clusters lying near the midline, with negative clusters flanking laterally. This pattern can
be seen to some extent in individual subject’s results (Fig. 6),
but is more evident in group maps (Fig. 7). Other ROIs had a
less obvious segregated organization. On the basis of statistical
analyses, several ROIs demonstrated significant nonrandom
cluster organizations (Table 1). In the mediolateral (x) axis, the
right DLPFC showed positive clusters significantly more lat-
Fig. 4. Measurement of pain-related BOLD signal change considering only those voxels with a significant negative signal change. Data are presented as means ⫾
SE. **Significant sex difference at P ⬍ 0.05; ^^significant interaction of sex and pain level at P ⬍ 0.05.
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S1 cortex (bilaterally) and mid-ACC, whereas the DLPFC
showed a significant interaction between sex and stimulus
intensity (Fig. 1, bottom). Other regions showed a nonsignificant sex difference in the same direction, including the aINS
and the rACC.
Because significant BOLD signal change was based on the F
statistic of the regression, significant BOLD signal change
could reflect either positive or negative BOLD signal change.
When the positive and negative signal changes (based on the
sign of the coefficient produced by the regression) were examined separately, we observed that whereas most of the activated
clusters were positive, there was a fraction of clusters that had
negative signal change in most subjects in almost every ROI.
The percentage of negative BOLD voxels varied greatly across
ROIs, but almost all ROIs showed the same trend of a larger
percentage of negative BOLD signals for women, at least for
the pain 2 stimuli (Fig. 2).
When considering only significant positive voxels, there was
no significant sex difference in spatial extent of activation for
any ROI (Fig. 3, top). However, when significant negative
voxels were considered, there was often a trend for greater
extent in women than in men, which was found to be statistically significant for contralateral S1 and DLPFC, and for the
pain 2 level of stimulation for the ipsilateral aINS (Fig. 4, top).
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Fig. 5. Single trial average waveforms for four ROIs. The time course is marked to identify baseline (Bsl; used to normalize signal to 100), stimulus period
(Stimulus), rating period (VAS), and an intertrial period (ITI). These waveforms were generated by first averaging across voxels for a given subject, and then
averaging across subjects, separately for men (dashed lines) and women (solid lines). ALL, all voxels found significantly modulated with respect to painful stimuli
(pain 1 and pain 2 graphed separately); POS, data derived only from those voxels with significantly positive signal change; NEG, data derived only from those
voxels with significantly negative signal change.
eral than negative clusters. In the anterior-posterior (y) axis, the
SMA showed positive clusters significantly more anterior than
were negative clusters. In the dorsoventral (z) axis, the midACC, the rACC, and right S2 showed positive clusters significantly more superior than negative clusters. One should note
that when these statistics are subject to Bonferroni correction;
only the y-coordinate for SMA still retains significance at P ⬍
0.05.
AJP-Regul Integr Comp Physiol • VOL
DISCUSSION
By using heat stimuli that evoked the same pain intensity in
men and women, we found significant differences in BOLD
signal amplitude in some ROIs, such that significantly greater
BOLD signal increases were found for men in S1 cortex,
mid-ACC, and DLPFC. This same trend was observed in most
other ROIs, as well. More detailed examination indicated that
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SEX DIFFERENCES IN CEREBRAL RESPONSES TO HEAT PAIN
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this difference is largely attributable to a greater proportion of
voxels demonstrating significant negative BOLD signal
changes in women vs. men.
It should be pointed out that the basis for identifying significance of BOLD signal change was the F statistic of the GLM
regression (a combination of full and partial F values). Relying
on the F statistic ignores the distinction between positive and
negative signal changes. If instead, a t statistic was used
(noting only the positive t values, as is typically done) those
results would look very much like the data presented here for
positive signal change only, which portrayed a very different
picture with respect to sex differences. Thus it is clearly quite
important to note which test statistic is used to define significant signal change when evaluating fMRI data.
Negative BOLD signal change. Although the negative
BOLD signal is often ignored in fMRI studies, another pain
imaging study specifically evaluated negative BOLD responses
to painful stimuli (25). This study described both positive and
negative BOLD signal changes in response to painful stimulation in many of the same ROIs found in the present study,
including S1, cingulate cortex, and SMA. Also in agreement
with the present study, the Porro et al. study (25) described 1)
a very similar time course for the positive and negative signals,
and 2) a wide variation in the proportion of negative signals
across ROIs.
The meaning of negative BOLD signal change is a topic of
considerable discussion in the neuroimaging field. In principle,
a negative BOLD response should reflect decreased blood flow
prompted by a decrease in brain activity in that region. One
explanation for this type of phenomenon is that with stimulation (or onset of any particular event), some brain regions will
AJP-Regul Integr Comp Physiol • VOL
become silenced in order for other brain regions to function
properly. The concept of the default network has been offered,
in which certain brain regions are often found to decrease the
BOLD signal in concert with attention-demanding event onset
of various types (12). This network appears to operate at a
large scale and includes brain regions, such as inferior parietal,
posterior cingulate, medial temporal, and medial prefrontal
cortices. For the most part, these brain regions do not overlap
with the pain-related ROIs evaluated in this study. The one
exception is the proximity of the inferior parietal region identified as a part of the default network with the parietal region
associated with S2. Yet, in the present study, the BOLD signals
derived from S2 did not behave remarkably differently from
those derived from many other ROIs and did not show a greater
negative signal change than other ROIs. Thus, at least as it is
currently envisioned, this concept of the default network is not
apparently applicable to the results described here.
Another possibility is that the negative signal change reflects
a steal phenomenon, in which the brain regions experiencing
an increase in blood flow (thus positive BOLD signal change)
do so at the expense of surrounding brain regions. This idea
cannot be discounted altogether, but several aspects of the
signal responses make this explanation less likely. First, although the positive and negative clusters are relatively close to
one another, they are usually not adjacent, nor do the negative
clusters surround the positive clusters. Second, in many instances, the positive clusters are found without any negative
clusters, and in a few cases negative clusters appear without
any positive clusters. Third, the waveform of the negative
signal changes simultaneously with the positive signal change
rather than lagging behind. Some of these properties of the
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Fig. 6. Locations of significantly modulated voxels with positive (yellow) and negative (blue) signal change in the cingulate cortex of two representative subjects.
The polygons denote the anatomically prescribed boundaries of the rACC (red) and mid-ACC (rose).
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SEX DIFFERENCES IN CEREBRAL RESPONSES TO HEAT PAIN
negative signal change are similar to those reported in recent
visual system studies. Smith et al. (32) documented negative
BOLD signal changes in the V1, V2, and V3 cortexes, both
nearby and remote from positive BOLD signal changes in
response to localized grating stimuli. Similar results were
reported by Shmuel et al. (30), who also demonstrated a very
similar time course of the positive and negative signal changes.
Both studies argued that the spatial separation and the temporal
coincidence of the two signals make it unlikely that a local
blood steal phenomenon could be responsible for the negative
signal.
Another idea proposed to explain the negative signal change
is that a high baseline blood flow may not require a compensatory increase following neural activation. In that instance,
only the oxygenation changes associated with increased metabolism, not the compensatory increase in blood flow and
oxygenated blood volume, would occur, resulting in a signal
decrease (31). This concept may be relevant here, as noted
below.
Sex differences in BOLD signal. The few neuroimaging
studies that have examined sex differences in pain-evoked
responses are conflicting but methodologically disparate
enough to preclude precise comparisons. Paulson et al. (24)
AJP-Regul Integr Comp Physiol • VOL
reported greater regional cerebral blood flow (rCBF) in many
cortical ROIs of women vs. men in response to painful contact
heat but used comparable stimuli levels that were significantly
more painful for the women. Thus it is possible that this rCBF
difference is not reflecting sex differences in neural processing,
but rather is due to the perceived pain intensity difference.
Similar PET studies have shown significant correlations between rCBF measures and perceived pain intensity in many
ROIs (5). Derbyshire et al. (6) used heat pain (laser) stimuli
evoking comparable pain intensity for all subjects and described significantly less rCBF for women vs. men in several
ROIs (including S1, S2, thalamus, insula, and midcingulate
cortices), and the opposite difference in only the perigenual
cingulate cortex. Neuroimaging studies of visceral pain, while
often targeting patients with IBS, have also described significant sex differences in response (3, 22). (See also, in this issue
of AJP Regulatory, Integrative and Comparative Physiology,
Berman et al., Ref. 3a).
Sex differences in neuroimaging responses have been described for other neural systems but very few of other sensory
systems. Levin et al. (18) and Kaufmann et al. (17) described
greater extent and amplitude of BOLD responses in men vs.
women in striate cortex using flickering visual stimuli. Addi291 • AUGUST 2006 •
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Fig. 7. Centers of mass for each subject’s largest significant cluster, identified separately for positive and negative signal changes and for the two levels of
stimulation. x ⫽ mediolateral axis; y ⫽ rostrocaudal axis; z ⫽ dorsoventral axis; values are millimeters in Talairach coordinate space.
SEX DIFFERENCES IN CEREBRAL RESPONSES TO HEAT PAIN
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Fig. 7—Continued
tionally, Kaufmann et al. (17) reported nonsignificant sex
differences in the same direction in extrastriate cortex. However, Kastrup et al. (16) and Hedera et al. (15) reported lower
BOLD signal contrasts in men, using similar stimuli. Thus it is
still too early to draw any firm conclusions regarding the
generalization of sex differences in sensory-evoked BOLD
responses.
If, as noted above, baseline cerebral blood flow is an important factor in producing negative signal change, then it is
relevant that women often exhibit a higher baseline global CBF
AJP-Regul Integr Comp Physiol • VOL
than men (13, 14, 28). Part of this difference may be explained
by animal studies showing that estrogen increases cerebral
glucose metabolism in multiple brain regions (4, 23). In addition, human PET studies have demonstrated significant effects
of estrogen manipulations upon imaging results. Berman et al.
(2) manipulated the hormone levels of premenopausal women
and found that rCBF responses in cognitive tasks were reduced
in hypoestrogenic conditions despite comparable performance.
A comparison of postmenopausal women either taking or
refraining from estrogen replacement therapy (ERT) revealed
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SEX DIFFERENCES IN CEREBRAL RESPONSES TO HEAT PAIN
Table 1. Location of largest clusters’ centers of mass
separately for each axis
No.
ROI
Pos-Mean
Neg-Mean
F Stat
P Value
x
x
x
x
x
x
x
x
x
x
x
x
x
2
4
16
15
20
21
14
9
16
14
23
22
24
R-S1
L-S1
R-S2
L-S2
Mid-ACC
rACC
R-pINS
L-pINS
R-aINS
L-aINS
R-DLPFCⴱⴱ
L-DLPFC
SMA
5.775
⫺7.738
56.978
⫺57.187
0.333
1.557
40.196
⫺40.211
39.669
⫺39.211
47.259
⫺45.332
⫺0.027
8.100
⫺9.700
56.256
⫺55.813
1.940
⫺1.455
40.432
⫺37.883
39.109
⫺37.621
42.772
⫺44.114
⫺2.917
0.303
0.534
0.121
0.380
0.887
3.584
0.019
2.293
0.195
1.150
5.765
0.366
1.185
0.680
0.518
0.733
0.547
0.358
0.073
0.892
0.168
0.665
0.303
0.025
0.552
0.288
y
y
y
y
y
y
y
y
y
y
y
y
y
2
4
16
15
20
21
14
9
16
14
23
22
24
R-S1
L-S1
R-S2
L-S2
Mid-ACC
rACC
R-pINS
L-pINS
R-aINS
L-aINS
R-DLPFC
L-DLPFC
SMAⴱⴱ
⫺38.775
⫺41.325
⫺24.300
⫺29.727
⫺0.845
27.531
⫺20.661
⫺21.167
7.481
6.329
21.487
22.359
ⴚ8.990
⫺43.900
⫺41.463
⫺22.688
⫺29.767
⫺3.320
30.236
⫺22.004
⫺24.850
2.406
4.604
23.548
24.243
ⴚ13.525
57.648
0.008
1.040
0.001
1.047
1.919
0.310
1.004
3.024
0.503
0.978
1.010
17.678
0.083
0.935
0.324
0.980
0.319
0.181
0.587
0.346
0.103
0.491
0.333
0.326
0.000
z
z
z
z
z
z
z
z
z
z
z
z
z
2
4
16
15
20
21
14
9
16
14
23
22
24
R_S1
L_S1
R-S2ⴱⴱ
L-S2
mid-ACCⴱⴱ
rACCⴱⴱ
R-pINS
L-pINS
R-aINS
L-aINS
R-DLPFC
L-DLPFC
SMA
67.700
65.275
19.906
18.623
38.458
24.969
13.350
10.561
4.047
3.950
5.900
4.896
59.715
64.950
64.500
15.934
15.523
34.738
19.407
12.368
13.578
3.853
4.432
⫺0.720
3.132
58.477
0.309
0.142
8.034
3.193
8.912
8.603
0.145
1.327
0.011
0.047
3.998
0.452
0.608
0.677
0.732
0.013
0.096
0.008
0.008
0.710
0.283
0.919
0.833
0.058
0.509
0.444
No. is the number of subjects that had significant clusters of both positive
and negative activation for both stimulus levels at any given region of interest
(ROI). These were the subjects analyzed statistically. The F value represents
the statistic for comparing positive vs. negative cluster locations, regardless of
stimulus level. S1 and S2, somatosensory cortexes; mid-ACC, midcingulate
ACC; rACC, rostal ACC; pINS, posterior insula; aINS, anterior INS; DLPFC,
dorsolateral prefrontal cortex; SMA, supplementary motor area; R, right or
contralateral to stimulation; L, left. **Data in bold represent statistically
significant results.
several PET response differences in memory tasks, including
greater deactivations for ERT users in several ROIs (26). In
addition and potentially most relevant to the present study,
Becerra et al. (1) reported significant differences in painrelated fMRI activation for women in different stages of the
menstrual cycle. Clearly, this area of investigation is still very
young and will require much more concerted research effort to
reveal general principles.
Sex differences in experimental pain perception—sometimes. Despite the considerable literature demonstrating sex
differences in experimental pain sensitivity, in the present
study, we did not find significant differences. The design of this
study was to use stimulus levels that evoked the same perAJP-Regul Integr Comp Physiol • VOL
ACKNOWLEDGMENTS
We are grateful to Drs. Steve Berman and Emeran Mayer of the Center for
Neurovisceral Sciences and Women’s Health at University of California, Los
Angeles for beneficial discussions of these results and collaborative meetings
between our centers facilitated by the National Institutes of Health Office of
Research on Women’s Health SCOR program.
Present address of E. A. Moulton: P.A.I.N. Group, Brain Imaging Center,
McLean Hospital, 115 Mill Street, Belmont, MA 02478.
GRANTS
This research was supported by National Institutes of Health Grants
R01-NS-39337 and P50-AR-49555.
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