Gender differences in regional brain response to visceral pressure in IBS patients

European Journal of Pain (2000) 4: 157–172 doi:10.1053/eujp.2000.0167, available online at http://www.idealibrary.com on

Gender differences in regional brain response to visceral pressure in IBS patients Steven Berman, Julie Munakata, Bruce D. Naliboff, Lin Chang, Mark Mandelkern, Dan Silverman, Edward Kovalik and Emeran A. Mayer UCLA/CURE Neuroenteric Disease Program, UCLA Division of Digestive Diseases, Departments of Medicine, Physiology and Psychology, UCLA, and PET Center, Division of Nuclear Medicine, West LA VA Medical Center, Los Angeles, CA 90073, USA

In two experiments including a total of 30 irritable bowel syndrome patients, symptom-mimicking rectal pressure stimuli elicited changes in regional neural activation as measured by positron electron tomography (PET) cerebral blood flow images. Although most stimuli were not rated as painful, rectal pressure increased regional cerebral blood flow (rCBF) in areas commonly associated with somatic pain, including the anterior cingulate, insula, prefrontal cortex, thalamus, and cerebellum. Despite similar stimulus ratings in male and female patients, regional activations were much stronger for males. In both experiments, rectal pressure activated the insula bilaterally in males but not in females. Insula activation was associated most strongly with objective visceral pressure, whereas anterior cingulate activation was associated more with correlated ratings of subjective discomfort. The insula is discussed as a visceral sensory cortex. Several possible reasons for the insula gender effect are proposed. © 2000 European Federation of Chapters of the International Association for the Study of Pain

INTRODUCTION Functional bowel disorders are estimated to afflict more than 20 million people in the United States. Irritable bowel syndrome (IBS) is the best characterized of these and shares many features with a number of other ‘functional’ disorders, which are often intransigent to currently available therapies. These include non-coronary chest pain, non-ulcer dyspepsia, chronic urinary urgency, and chronic pelvic pain. There is significant overlap amongst all these syndromes, and between fibromyalgia and Paper received 22 June 1999 and accepted in revised form 24 February 2000. Correspondence to: Emeran A. Mayer, MD, UCLA/CURE Neuroenteric Disease Program, WLA VA Medical Center, Bldg. 115, Rm. 223, 11301 Wilshire Blvd., Los Angeles, CA 90073, USA. Tel: + 1 310-3129276; Fax: + 1 310-794-2864; E-mail: [email protected]

these conditions in the same patient (Mayer et al., 1996). The conditions are characterized by altered perception of visceral events and disturbances in autonomic activity (Malagelada, 1993; Richter & Bradley, 1993; Mayer & Gebhart, 1994). Patients with IBS present with symptoms of abdominal discomfort (chronically recurring symptoms of fullness, bloating, incomplete rectal evacuation) and/or crampy abdominal pain. Experimental studies using rectosigmoid balloon distension have provided evidence for both hypervigilance and for inducible visceral hyperalgesia in these patients (Naliboff et al., 1997; Naliboff et al., 1998). In addition to the perceptual alterations, IBS patients report alterations in bowel habits, with subgroups characterized as constipation-predominant, diarrhoea-predominant and alternating pattern. Circumstantial evidence suggests that these symptoms are related to altered

1090-3801/00/020157 + 16 $35.00/0 © 2000 European Federation of Chapters of the International Association for the Study of Pain

158

autonomic regulation of the enteric nervous system (Bassotti & Gaburri, 1988; Bassotti et al., 1990; Aggarwal et al., 1994; Furukawa et al., 1994). Such alterations in central autonomic control could manifest as altered gastrointestinal transit times and altered reflex responses to food intake or environmental stressors. IBS and related disorders such as pelvic pain and interstitial cystitis are more common in women than in men with prevalence ratios varying from 2:1 (IBS) to 9:1 (interstitial cystitis) (Drossman et al., 1993; Unruh, 1996; Berkley, 1997a&b). Several studies have demonstrated that the prevalence differences for IBS are not secondary to differences in health care utilization, since similar gender differences are observed in community based surveys (Drossman et al., 1993). Recently, preliminary evidence has been provided for a gender-specific response to therapy in IBS patients not suffering from constipation (Mangel & Northcutt, 1999). In general, women are more likely than men to complain of persistent, episodic or chronic visceral and musculoskeletal pains (Unruh, 1996; Berkley, 1997a&b). This is particularly true for recurrent pains of the abdomen and pelvis (Von Korff et al., 1988; Von Korff et al., 1990; James et al., 1991; Andersson et al., 1993; Eggen, 1993; Ektor-Anderson et al., 1993; Klonoff et al., 1993; Unruh, 1996), and for pain reports during the reproductive years. It has been suggested that these body region-specific differences may be related to differences in central nervous system (CNS) responses (in form of vigilance, autonomic and antinociceptive responses) to stimuli arising from the reproductive organs (Berkley, 1997a&b). According to this concept, women in the reproductive age may have more effective mechanisms of antinociception related to potentially noxious gender-specific events such as intercourse, pregnancy and delivery. At the same time, women are likely to be more vigilant and able to better discriminate between potentially harmful (urinary tract infections) and physiologic events (perimenstrual symptoms). Experimental studies using somatic pain stimuli applied under controlled conditions in

European Journal of Pain (2000), 4

S. B ERMAN

ET AL .

the laboratory demonstrate that women generally exhibit lower thresholds, higher intensity ratings and less tolerance (Berkley, 1997a&b). These gender differences appear to be stimulus dependent, most consistently observed for pressure and electrical stimuli (Giamberardino et al., 1997). The presence, type and direction of some of these differences are influenced by menstrual cycle phase, health status, situational, motivational and nutritional factors. Two recent positron emission tomography (PET) studies in healthy volunteers reported evidence for gender differences in regional brain activation to somatic heat pain despite extensive similarities in brain activation by this stimulus between males and females (Paulson et al., 1998; Derbyshire et al., 1998). Derbyshire and associates (Derbyshire et al., 1998) reported a greater response in the perigenual anterior cingulate and inferior parietal cortex in female patients. Male patients showed a greater response in prefrontal and midcingulate cortices. Paulson and associates (Paulson et al., 1998) reported regional differences in the insula and thalamus. In contrast to an extensive literature on somatic pain (Unruh, 1996; Berkley, 1997a&b), there are few reported studies of gender differences in visceral pain perception (Jackson et al., 1994; Jackson et al., 1997). In a large sample of IBS patients, we recently found that female IBS patients did not differ from their male counterparts in self-reported abdominal pain, but significantly more often reported non-painful visceral sensations, such as nausea, alterations of taste and smell, greater sensitivity to food and medications, and more urinary urgency (Lee et al., 1999). In the current study, we test the hypothesis that the CNS response to a tonic, largely non-noxious stimulus differs between male and female patients with a diagnosis of IBS. We chose this type of stimulus because spontaneous IBS symptoms are presumably triggered by non-noxious physiological stimuli. The majority of patients complain of non-painful discomfort, such as sensations of fullness, bloating or urgency, rather than pain (Lembo et al., 1999; Lee et al., 1999).

G ENDER

SPECIFIC BRAIN ACTIVATION IN

IBS

METHODS Participants

Sample 1 In the initial sample, 13 adult patients with irritable bowel syndrome (6 men, 7 women) were recruited from UCLA Digestive Diseases Clinic and Harbor-UCLA Medical Center Gastroenterology Clinic. Patients were required to meet established symptom criteria for irritable bowel syndrome (Thompson et al., 1988), and be clinically and endoscopically without inflammatory or other structural intestinal disease. Data derived from six of the irritable bowel syndrome patients were included in a previous report (Silverman et al., 1997). Written and verbal informed consents were obtained from all subjects. The protocol was approved by the Human Subject Protection Committee at UCLA and the Human Research Committee at West LA VA Medical Center. All subjects were evaluated for levels of depression and anxiety by the SCL-90 symptom checklist (Derogatis & Lazarus, 1994), and were free from centrally neuroactive medications for at least 48 h preceding their PET scans.

Sample 2 (confirmatory sample) To determine whether any sex differences we found were sample-specific or representative of IBS patients in general, we examined a second sample who were being studied in an ongoing experiment characterizing the sensitizing effect of repetitive noxious sigmoid pressure conditioning. Identical recruitment procedures and inclusionary criteria as for sample 1 were applied. Seventeen adult patients with irritable bowel syndrome (11 men, 6 women) were studied.

Experimental design and protocol

Positron emission tomography (PET) scanning protocol Positron emission tomography was performed by dynamic imaging with a Siemens/CTI 953

159

tomograph (Siemens-Computer Technology, Inc., Knoxville, TN), collecting 31 contiguous data planes corresponding to an axial depth of 3.375 mm each in a 128 × 128 image matrix. Emission scans were reconstructed from projection data with calculated attenuation correction. PET data were obtained after intravenous bolus administration over approximately 5 s of 25 mCi (925 mBq) 15O-water. Four 30-s frames were collected, and the first frame, representing the approximate time of bolus transit to the brain, was discarded. Frames two to four were summed to construct an image integrating activity from 30–120 s post injection.

Stimulus protocol Rectal pressure stimuli were delivered by a programmable pump for 60 s each, with a 15 min interim balloon deflation. Eyes were closed during all scans, and extraneous auditory stimuli were excluded by headphones, through which a recorded message provided a general explanation of the distension protocol.

Sample 1 A latex balloon catheter was inserted into the rectum of each subject approximately 1 h prior to imaging. A baseline scan was first obtained without balloon inflation. In the second condition, low (20 mmHg) balloon inflation was accompanied by simulated tracer injection (i.e. saline only) and ‘scanning’, thereby minimizing novelty effects relating to the balloon inflation procedure that might otherwise be captured in subsequent scans. In the next condition, moderate (45 mmHg) rectal pressure was delivered during the scan. A taped message subsequently announced that pressure of much greater intensity would be delivered during the remaining scans. During the first of these, delivery of another pressure pulse from the pump was simulated without actual balloon inflation. We refer to this as an anticipation scan. In the initial sample, the first anticipation scan was followed by a delivered intense pressure stimulus

European Journal of Pain (2000), 4

160

(60 mmHg), followed by a second anticipation scan. The five scans producing images were ordered as follows: baseline, 45 mmHg, anticipation 1, 60 mmHg, anticipation 2.

Sample 2 A double balloon catheter was placed with one balloon in the rectum, per our standard procedure (Munakata et al., 1997) and the other placed in the sigmoid colon. Only the initial three scans (baseline, 45 mmHg, anticipation 1), which were identical to the first three scans of sample 1, were analyzed. In this protocol, the third scan was followed by inflation of the sigmoid balloon and data acquired after the initial three scans were therefore not analysed for the current report. There were two relevant differences between the groups. The low balloon inflation simulated scan after baseline was not conducted on the second sample, and the double balloon catheter (rectum and sigmoid colon) was placed in the second sample, as opposed to a single rectal balloon in the original sample. The sigmoid balloon was never inflated until after the data reported herein was collected.

Assessment of subjective stimulus ratings With the exception of the baseline scan, the degree of unpleasantness of visceral sensations were assessed at the conclusion of each scan by a previously validated written scale (Gracely et al., 1976) consisting of verbal descriptors arrayed along a 20 cm line in order of progressive intensity, such that the subject’s response could be readily transformed to numerical scores from 0 to 20 by direct measurement. While several other ratings were also collected at this point, only the ‘discomfort’ ratings will be reported herein. The ratings of one female IBS patient were lost due to computer error.

Assessment of rectal compliance Rectal compliance was calculated twice, using two different volume measurements. Target rectal compliance was determined at the first point of reaching steady state (target) pressure during European Journal of Pain (2000), 4

S. B ERMAN

ET AL .

the moderate stimulus. We calculated target rectal compliance using (Vtarget/P) where P = balloon pressure (45 or 60 mmHg) and Vtarget = balloon volume when target P first reached. Maximum rectal compliance was determined at the point of maximum rectal balloon volume during the 60 s stimulus. Similarly, we calculated maximum rectal compliance as (Vmax/P), where P = balloon pressure and Vmax = maximum balloon volume.

Receptive relaxation As previously described by our group (Lembo et al., 1994; Mertz et al., 1995), the receptive relaxation rate was estimated by measuring the change in rectal balloon volume observed during a phasic pressure stimulus and the corresponding change in time. Receptive relaxation rates were calculated during the 60 s stimulus using the equation RR = (Vend2Vtarget/Tend2Ttarget), where Vend = balloon volume at the end of stimulus, Vtarget = balloon volume when target P first reached, Tend = time of stimulus end (60 s), and Ttarget = time when target P first reached. PET image analysis PET data were subjected to statistical parametric mapping analysis by the method of Friston and colleagues (Friston et al., 1994; Friston et al., 1995a; Friston et al., 1995b) using the SPM96 software. In brief, images are spatially normalized into a standard space using linear and nonlinear transforms, and spatially smoothed to reduce noise variance. Parametric statistical models are assumed at each voxel, using multiple linear regression to describe the variability in the data in terms of experimental and confounding effects, and residual variance. Subject effects are defined as a confound and global activity is removed by proportional scaling. Hypotheses expressed in terms of the model parameters are assessed at each voxel with univariate statistics, thus producing a statistic image, or Statistical Parametric Map. Only voxels where modelling of the total experimental effects explains significant additional variance are tested for eigenimage analysis or individual contrasts. The multiple comparison problem of simultaneously assessing all voxels is addressed using the theory of

G ENDER

SPECIFIC BRAIN ACTIVATION IN

IBS

continuous random fields, assuming the statistic image to be a good lattice representation of an underlying continuous stationary random field. Results for the Euler characteristic allow computation of corrected p-values for each voxel and of corrected p-values for each cluster of suprathreshold contiguous voxels. Corrected cluster p-values jointly evaluate spatial extent (number of voxels, k) and peak activation (maximum Z-score).

Eigenimage analysis To characterize changes in functional connectivity during this task, principle components were derived by singular value decomposition of the N (voxels passing the model described in the next section thresholded at 0.05 uncorrected) by S (scans) array. In this technique, the first component accounts for the greatest amount of total variance–covariance. The second component accounts for the greatest portion of remaining variance–covariance, and so on. The N-dimensional vector of voxel loadings (spatial mode) for a component is the corresponding eigenvector of the N × N covariance matrix, taking scans as repeated measures for each voxel. Similarly, the S-dimensional vector of scan loadings (temporal mode) is the eigenvector of the S × S covariance matrix, taking voxels as repeated measures for each scan. In both spatial and temporal modes of a component, each eigenvector can be split into two arrays representing the positive and negative elements. For a particular mode, elements with the same sign are positively correlated; elements with opposite signs are negatively correlated. In spatial mode, images of the two arrays can be constructed. Voxels in the positive (negative) array increase (decrease) for scans with positive temporal loadings. Similarly, voxels in the negative (positive) array increase (decrease) for scans with negative temporal loadings.

Covariate analysis In the initial sample, we tested the hypothesis at each voxel that blood flow, expressed as normalized radioactive counts, covaried with the intensity of rectal pressure; 0 (baseline and

161

anticipation scans), 45, or 60 mmHg using a multiple subjects single covariate design. For the confirmation sample, we tested the simpler hypothesis that the moderate rectal pressure scan (45 mmHg) showed increased blood flow as compared to the scans without rectal pressure (0 mmHg). This is equivalent to the covariate analysis when only one non-zero value of the intensity covariate is used. By default, SPM maps all voxels registering differences that are significant at an alpha level of p < 0.01 (uncorrected). This initial strategy images on the order of 103 ‘activated’ voxels when there is no actual effect. Therefore, unless a specific local activation is hypothesized, we tabulate only sets of contiguous voxels generating cluster (extent/height) or voxel (height) significance of p < 0.05 after correction for multiple tests. We also applied other statistical strategies to inform interpretation of the standard SPM results. For example, p < 0.001 uncorrected was substituted for p < 0.01 as the initial cut-off to better localize the largest and strongest activations. The p < 0.01 uncorrected results were examined without further correction when a specific voxel was hypothesized, and also in order to more sensitively compare separate samples, thus yielding a more informative test of group differences. To visually assess recruitment of the insula, activated voxels were superimposed on representative magnetic resonance imaging (MR) coronal sections. Although all analyses quantified both activations and deactivations associated with rectal pressure, none of the deactivations attained corrected significance levels, and will not be discussed.

RESULTS Sample 1

Eigenimage analysis We first examined the hypothesis of functional gender differences through an eigenimage analysis (Friston et al., 1993). This recursive principal component analysis is useful for characterizing the principal sources of variance in a set of images. European Journal of Pain (2000), 4

162

S. B ERMAN

ET AL .

Eigenimage 1 (-ve) (a)

1.00

Component score

0.50

0.00

–0.50

–1.00 BL

45

A1

60

A2

Scan condition Eigenimage 1 (-ve) (b)

1.00

Component score

0.50

0.00

–0.50

–1.00 BL

45

A1

60

A2

Scan condition

FIG. 1. The principal eigenvector derived from the initial sample of male (A) and female (B) IBS patients. The five scans represent baseline (0 mm), 45 mmHg inflation, anticipation 1 (0 mm), 60 mm and anticipation 2 (0 mm). Rectal pressure scans are represented by green bars. Note that for both of these scans, activations are represented in the negative eigenvectors. The eigenimages associated with these vectors are presented above in three orthogonal maximum projection views from the right, top and back of the brain. The stereotactic space is defined by the atlas of Talairach and Tournoux (Talairach and Tournoux, 1988). The area of the insula is outlined in red. European Journal of Pain (2000), 4

G ENDER

SPECIFIC BRAIN ACTIVATION IN

TABLE 1.

IBS

Activations significant after correction for multiple tests.

p (corrected) Cluster-level (k, Z) A

0.003 (3785, 3.66) 0.127 (1015, 3.61)

B

0.018 (722, 439) 0.006 (1037, 4.12)

C

163

0.006 (3139, 3.93) 0.082 (1185, 3.62)

p (uncorrected)

Talairach anatomy

Voxel-level (Z)

Cluster

Voxel

x,y,z (mm)

0.559 (3.66) 0.561 (3.65) 0.690 (3.54) 0.613 (3.61) 0.885 (3.32) 0.948 (3.20) 0.040 (4.39) 0.742 (3.34) 0.772 (3.31) 0.107 (4.12) 0.282 (3.81) 0.402 (3.67) 0.137 (3.93) 0.195 (3.82) 0.357 (3.59) 0.328 (3.62) 0.866 (3.03) 0.999 (2.38)

0.000

0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.009

–18 20 14 –16 6 30 32 46 –2 42 2 4 54 6 –4 46 6 10 40 4 0 52 16 10 30 14 –12 –36 2 6 30 0 12 –22 –2 10 –28 –10 –6 –36 2 8 –14 22 14 40 –6 6 60 0 2 36 10 –6

0.004 0.005 0.001 0.001 0.133

BA 24 10 13 22 13 13 44 13 13? 13? 13 13 22 13

Structure Caudate Cingulate gyrus Middle frontal gyrus Insula Superior temporal gyrus Insula Insula Inferior frontal gyrus Inferior frontal gyrus Insula/claustrum/putamen Insula/claustrum/putamen Putamen Putamen Insula Caudate Insula Superior temporal gyrus Insula

A = SPM significant (threshold at 0.001 uncorrected, with a corrected p<0.05) activation by rectal pressure in six male IBS patients. When a voxel appears in the probability map of more than one structure, all are tabled. B = SPM results associated with the moderate rectal pressure stimulus in the confirmation sample of 11 male IBS patients. These two clusters contain the most voxels, the highest peaks, and the right and left insula (hypothesized activations). C = Activations in response to rectal pressure that were greater (threshold at 0.01 uncorrected, with corrected p<0.1) in 17 male IBS patients than in 13 female IBS patients.

There were six male and seven female IBS patients. The first component accounted for the large majority of variance–covariance in both males (76.9%) and females (78.1%). The corresponding temporal vectors for both genders are similar in that the primary features of the experimental manipulations are reflected in the scan loadings (Fig. 1). The scans that differ most are the baseline scan and the maximum (60 mmHg) inflation. The moderate (45 mmHg) inflation resembles the latter and the anticipation scans, where no pressure was actually applied, resemble the former. Rectal pressure scans in both male and female IBS patients load negatively on the first principal component. The spatial distribution of voxels that load negatively on this component, and thus are activated by rectal pressure, show a distinct gender difference (Fig. 1). Along with other structures, males show bilateral activation of the insula, outlined in red. In contrast, females show activation primarily in the left cerebellum.

Covariate analysis Gender-specific covariate analyses quantified increases or decreases in rCBF associated with rectal pressure across the five scans (covariate = 0, 45, 0, 60, 0 mmHg). SPM results for males are shown in Figure 2. Activations that were significant after correction for multiple tests are shown in Table 1. Two clusters of voxels covaried with delivered rectal pressure in males. The largest cluster featured an independently significant peak voxel at 40, 4, 0 mm in the right frontotemporal cortex. The second cluster was in a homologous region of the left frontotemporal cortex. The peak voxel had the lowest rCBF in the baseline scan and the highest rCBF in the 60 mmHg inflation, an increase of 3.5%. For that voxel, the rCBF during both anticipation conditions was similar to the baseline scan, and rCBF in the 45 mmHg inflation was similar to that in the 60 mmHg inflation (Fig. 3). Both activated clusters were centred in the grey matter medial to the European Journal of Pain (2000), 4

164

S. B ERMAN

Sagittal

ET AL .

Coronal 72

R

0 0

–43

– 104

64

68

0

5 10 0 15 20 25 30

64

2 Transverse

4

0

Design matrix

FIG. 2. Voxels activated by rectal pressure in 6 male IBS patients thresholded at p<0.001 uncorrected. Coordinates roughly translate to millimetre distances from the anterior commissure with x representing mediallateral values (negative = left), y representing anterior-posterior values (negative = posterior) and z representing superior-inferior values (negative = inferior). In this ‘glass brain’ format, voxels passing the statistical threshold are black in coronal, sagittal and transverse views of the otherwise transparent template brain [see Fig. 1]. Activations in all figures can be roughly localized using the presented co-ordinates, approximately equivalent to millimetre translations from the origin at anterior commissure.

intersection of the Sylvian fissure with the circular sulcus of the insula (Fig. 4). In other words, they were in the central insula in Brodmann area (BA) 13. Although the sample of female IBS patients was 17% larger, no clusters or voxels reached a corrected probability value below 0.8. Even without correction for multiple tests, there were no European Journal of Pain (2000), 4

voxels in or near the insula that showed any correlation with rectal pressure. Sample 2

Eigenimage analysis Two principal components were generated to characterize the distributed brain systems that

G ENDER

SPECIFIC BRAIN ACTIVATION IN

IBS

165

Z = 2.98 p = 0.001

Z=4.39 p= 0.000 (0.04 corrected) 106

102 101

104 Adjusted response

100

Adjusted response

99 98 97 96

102

100

98

95 96

94 BL

45

A1

60

A2

Scan

FIG. 3. Peak response to rectal pressure in 6 male IBS patients (voxel 40, 4, 0).

covaried across the three scans; baseline 45 mmHg, and anticipation. In both genders, the first component accounted for about 55% of total variance and the second for the remainder. One component primarily discriminated the moderate rectal pressure stimulus from the baseline scan, while the other primarily discriminated it from the anticipation scan. Similar to the findings in sample 1, the insula and surrounding regions were prominent in both eigenimages associated with the rectal pressure stimulus for males, but not for females.

Covariate analyses The first sample suggested rectal pressure activates the insula in male IBS patients. This hypothesis was tested in the new sample at three levels of covariate analysis. First, we performed a direct replication testing the extent to which the 45 mmHg inflation produced more activation than the baseline and anticipation conditions at the peak voxel of the initial sample; 40, 4, 0. As shown in Figure 5, the 45 mmHg inflation increased rCBF more than 2% as compared to both other scans, confirming the expected activation at the p < 0.001 level. Second, we viewed the

BL

45

A1

Scan

FIG. 5. Test of the hypothesized activation of voxel 40, 4, 0 associated with the moderate rectal pressure stimulus in the second sample of 11 male IBS patients.

voxels activated at the 0.05 uncorrected alpha level in the Y = 4 mm coronal section. As can be seen in Figure 6, the bilateral activation was also replicated. However, the left-sided activation was somewhat lateral and superior to that seen in the initial sample, extending from insula into BA 43 in the precentral gyrus. (The medial–superior activation also seen in Figure 6 was not based on an a priori hypothesis and thus cannot be considered significant at this alpha level). Finally, we performed the standard wholebrain assessment, with correction for number of voxels tested. Only two clusters contained more than 305 voxels, and no individual voxel attained corrected significance. However, the largest cluster of suprathreshold voxels, which included parts of the left insula, was significant for spatial extent (3785 voxels, p = 0.003 corrected), indicating there is a real activation somewhere within the cluster. The second largest cluster was comprised of 1015 voxels with a peak at 42, 2, 4 (Z = 3.61, see Table 2). This location is almost identical to the right insula voxel activated maximally in the first sample of male IBS patients. In contrast to the male patients, the six female IBS patients produced no activation at voxel 40, European Journal of Pain (2000), 4

166

S. B ERMAN

Coronal

ET AL .

Coronal

4 3

3

Z-value

Z-value

2

2

1

1

0

0

FIG. 4. Activated voxels (p<0.001 uncorrected) associated with rectal pressure in six male IBS patients superimposed on a typical MRI coronal section 4 mm anterior to the anterior commissure. Vertical and horizontal lines cross at the voxel of peak activation. Note the bilateral ‘Ys’ formed by the intersection of the Sylvian fissure with the circular sulcus of the insula. –30, –12, –4

FIG. 6. Activated voxels (p<0.01 uncorrected) associated with the moderate rectal pressure stimulus in 11 male IBS patients superimposed on a typical MRI coronal section 4 mm anterior to the anterior commissure.

–32, –8, –4

–32, –4, –2

Coronal slices through the “Y” plane

–36, 0, 6

–36, 4, 8

–36, 8, –12

FIG. 7. Voxels more activated by rectal pressure in male than female IBS patients (p<0.001 uncorrected), superimposed on MRI coronal sections from 12 mm posterior to 8 mm anterior to the anterior commissure. Red crosshairs meet at the approximate center of the activated region, with the voxel coordinate given above each section. European Journal of Pain (2000), 4

G ENDER

SPECIFIC BRAIN ACTIVATION IN

IBS

4, 0 or any other voxel in right or left insula. The only significant result in the whole-brain analysis was a cluster consisting of 482 voxels (cluster p = 0.05 corrected) with a peak Z of 4.64 (voxel p = 0.018 corrected) at 64, –44, –2 in the right middle temporal gyrus (BA 21). In sum, the second sample replicated the insula activation sex difference seen in the first sample. Combined analysis of samples 1 and 2 Because the second sample of females was smaller than males, it is possible there was insufficient power to demonstrate insula activation. Therefore, we performed a combined analysis of the total 13 females and 17 males, with separate contrasts assessing rectal pressure activation in males, activation in females and their interaction. We reasoned that if there was insufficient power to demonstrate insula activation in 13 females, when it had been evident in separate samples of 7 and 11 males, that there was a real gender difference in insula activation. Moreover, the interaction allowed a direct test of this hypothesis. There were no differences in age between the gender groups (Males: range 27–61, mean age = 41.7, SD = 12; Females: range 26–65, mean age = 39.7, SD = 11).

Males Rectal pressure in the 17 males (thresholded at p < 0.01 uncorrected) was correlated with a large region (13 709 voxels, cluster p < 0.0005 corrected) with maxima in both right (40,6,4; Z = 5.48) and left insula (–38,2,8; Z = 5.46; both p < 0.0005 corrected). Surrounding areas were also activated as part of this large cluster, including lateral frontotemporal cortices and midline structures ranging from anterior cingulate to the posterior thalamus.

Females In the combined 13 females, after correction for multiple tests, there was no significant correlation of rectal pressure with any brain region. Moreover, no voxel anywhere near the right or left insula showed a significant correlation at p = 0.01 uncorrected.

167

Interaction analysis No voxels were activated more by rectal pressure in female IBS patients as opposed to males. In contrast, two regions located roughly at the right and left frontotemporal juncture were more activated in males than females (Table 3). The region on the left produced cluster-level significance after correction for multiple tests (p = 0.006), while its right hemisphere homologue was suggestive at the cluster level (p = 0.082 corrected). Serial coronal sections thresholded at p < 0.001 uncorrected locate both clusters principally in the insula (Fig. 7).

Subjective discomfort ratings The ratings of one female were lost and two males were excluded from these analyses due to ratings that were statistical outliers. Effects of gender (male vs female) and condition (45 mmHg inflation vs anticipation) were examined by ANOVA (SAS Inst. Inc., 1989) on the discomfort ratings of the remaining 15 males and 12 females. The 45 mmHg inflations were rated higher that the anticipation scans (F = 53.8, p < 0.0001). There was no significant effect of gender (p > 0.5), or interaction of gender with condition. Covariate analyses were conducted to identify regions where blood flow covaried with discomfort in males, females and their interaction. There was no significant covariation in the females. In males, discomfort ratings covaried with rCBF in two large clusters (p < 0.004) containing areas similar to those activated by pressure, but with fewer total voxels. One cluster was centred in right insula (cluster p = 0.012, peak 40,0,2 voxel p = 0.018). The other included left insula (cluster p = 0.001). In the interaction analysis, discomfort ratings were associated with significantly more activation in males as compared with females for 1732 voxels in the anterior cingulate (ACC; BA 32) and middle frontal gyrus (cluster p = 0.027). Subjective discomfort ratings were highly correlated with objective stimulus intensity. To attempt to differentiate brain regions related to discomfort from those related to pressure per se, additional European Journal of Pain (2000), 4

168

covariate analyses of the male IBS patients were conducted. The first analysis modelled visceral pressure as the covariate of interest, but specified discomfort ratings as a confounding covariate. The second modelled discomfort as the covariate of interest, with visceral pressure specified as a confounding covariate. After removal of variance associated with discomfort, pressure continued to activate one significant cluster (p = 0.034) which included large portions of the left insula, globus pallidus and middle frontal gyrus. In contrast, after removal of variance associated with pressure, discomfort activated one significant cluster (p = 0.028) which included an independently significant voxel (–18, 32, 26, p = 0.009) in the left ACC (BA 32). Although the cluster extended into middle and superior frontal gyri (BAs 8 and 9), the insula was not represented. These analyses were not conducted in females, since neither inflation nor discomfort produced significant activations. Potential confounding variables

Predominant bowel habit Bowel habit constitutes a potential confound in any report of IBS gender differences, since constipation–predominant bowel habit is more common in female patients (Thompson, 1997). There were no patients characterized by constipation in our confirmatory sample, but the original patients reported predominant constipation in only two of six males but four of seven females. To rule out the possibility that constipation symptoms are associated with attenuation or reversal of the reported insula activation, we reexamined the effect of the rectal pressure stimulus in females after excluding the four patients characterized by constipation. No cluster or voxel was significantly activated by the rectal pressure stimulus after correction for multiple tests. Furthermore, no voxel in or near the insula was activated at the more liberal criterion of p < 0.01 uncorrected.

Rectal compliance There was no significant difference between male and female IBS patients in mean target European Journal of Pain (2000), 4

S. B ERMAN

ET AL .

compliance (male: 1.87 ± 0.26 ml/mmHg; female: 2.20 ± 0.30 ml/mmHg; p = 0.41), or mean maximum compliance (male: 3.26 ± 0.43 ml/mmHg; female: 4.39 ± 0.58 ml/mmHg; p = 0.12). There were also no significant differences in receptive relaxation rate (male: 1.26 ± 0.25 ml/min; female: 1.89 ± 0.38 ml/mmHg; p = 0.16).

DISCUSSION In the current study we provide evidence suggesting that moderate tonic distension of the rectum of patients suffering from irritable bowel syndrome produces gender-specific differences in brain activation patterns, despite similar ratings of stimulus intensities and resulting discomfort. In the population studied, male but not female IBS patients show bilateral activation of the insula, a brain region receiving extensive visceral (noxious and non-noxious, spinal and vagal) information, which also plays an important role in autonomic responses (Cechetto & Saper, 1990; Yasui et al., 1991) and emotional processing (Casey et al., 1994; Augustine, 1996).

Sources of information and limitations The information was obtained through controlled application of a graded rectal pressure stimulus and collection of patient ratings and images of resulting changes in cerebral blood flow, conceptualized as a measure of regional neural activation. The use of objective and subjective covariates of the rectal pressure manipulation provides important clues to the physiology of responsive brain systems. However, the methodology has limited spatial and temporal resolution. If brain responses are too topographically diffuse, too transient or too heterogeneous, they will not be well characterized. Additionally, the possibility of sample-specific activations are a potential problem in all brain activation studies that seek to generalize to larger populations. In this study, confirmation in a second sample of a regional hypothesis derived from an initial group of subjects mitigates against this possibility.

G ENDER

SPECIFIC BRAIN ACTIVATION IN

IBS

One additional limitation comes from the fact that the baseline scan was the first scan for all subjects. First scan effects (novelty, anxiety, etc.) are a large source of variance in dynamic PET, and many results obtained by comparing a single initial baseline scan with subsequent scans are likely to be dependent on scan order. To mitigate these confounds, we did not rely on paired comparisons of the initial baseline scan with all subsequent conditions. Zero pressure scans were recorded after (anticipation) as well as before (baseline) each inflation scan. Therefore, the pressure covariate design we utilized minimizes effects of scan order.

Comparison to the PET pain literature Imaging studies of somatic pain (Derbyshire et al., 1997) suggest fairly consistent activation of a network of brain regions including the anterior cingulate, insula and thalamus, and sometimes activation of prefrontal cortex, lenticular nucleus, and cerebellum, even though considerable differences exist between reported studies. Additionally, parietal activation in both primary somatosensory cortex and inferior parietal lobe have been linked to non-nociceptive sensory–discriminatory and attentional aspects of painful somatic stimuli (Derbyshire et al., 1997). The current study differs significantly from previous published studies in several important features: (1) The experimental treatment was a tonic pressure stimulus which for most patients mimicked the physiological sensation of rectal fullness; (2) In most cases, the stimulus was not rated as painful; (3) The stimulus was mechanically applied to the viscera; (4) The stimulus was applied to the body region (distal colon) associated with chronic pain and discomfort in these patients, and which has seldom been studied with PET. Despite these salient differences in experimental conditions, a combined analyses of all 30 IBS patients revealed that a very similar network of brain areas was activated by rectal pressure as is activated by somatic pain. That is, the thalamus, insula, anterior cingulate (ACC) and prefrontal cortex, cerebellum, and inferior parietal lobe were all represented.

169

Role of the insula in viscerosomatic sensory processing As far back as the pioneering studies of Wilder Penfield, stimulation of the insular cortex in humans has been reported to elicit experience of visceral sensations (Penfield & Boldrey, 1937; Penfield & Faulk, 1955). Based on extensive tracer studies and functional studies in animals, including non-human primates, a role of the insula as visceral sensory and visceral motor cortex has been established (Cechetto & Saper, 1990; Yasui et al., 1991). In addition, several studies suggest a role for the anterior insula in processing of emotionally relevant context, such as disgust (Phillips et al., 1997), and the recollection of affect-laden autobiographical information (Fink et al., 1996). In a recent review of brain areas activated by painful stimuli, Derbyshire and associates (1997) found the insula to be reported second only to the anterior cingulate cortex. Activation of the insula has been reported in 10 out of 17 brain imaging studies involving noxious stimuli including: heat stimuli applied to the hand (Derbyshire et al., 1997), arm (Coghill et al., 1994; Svensson et al., 1997; Paulson et al., 1998), or foot (Xu et al., 1997), shock to the fingers (Howland et al., 1995), oesophageal balloon inflation (Aziz et al., 1997), capsaicin injection to the forehead (May et al., 1998), formalin injections to the face in the cat (Kuroda et al., 1995), vagus nerve stimulation in the rat (Ito, 1998), and ongoing neuropathic pain (Hsieh et al., 1995). Taken together, the extant literature suggests an important role of the insula in brain networks dealing with perceptual/homeostatic responses to pain, but also with affective/cognitive responses to pain, and with the memory of such responses. Cognitive activations could be associated with an increased concern, focus and emotional response to currently experienced or remembered pain episodes. The wide range of studies eliciting insula activation with painful and with other affectively negative stimuli may result from the insula being involved in multiple, physiologically diverse brain networks, or by a more general system such as production or discrimination of pain or negative affect. The fact that many of the relevant indices, such as tissue damage and European Journal of Pain (2000), 4

170

discomfort, are highly correlated under normal circumstances makes it difficult to specify the limits of insular functions. In the current study, a significant activation of the insula by visceral pressure was observed after removal of the effects of highly correlated ratings of visceral discomfort. In contrast, after removal of effects of actual pressure, discomfort was not associated with insula activation. ACC activation was significantly correlated with unpleasantness ratings after removal of effects of actual pressure, but not with actual pressure after removal of effects of discomfort. These findings are in agreement with those reported by Rainville and associates (1997), which showed a correlation of the same anterior cingulate region with unpleasantness ratings of stimuli eliciting somatic pain. Gender differences in insula activation The use of imaging techniques to study gender differences in pain is a recent development that has not yet generated consistent results. In particular, insula activation by a painful somatic stimulus has been very recently reported to show no gender difference (Derbyshire et al., 1998) to be greater in women (Paulson et al., 1998), and to be greater in men (Becerra et al., 1998). However, the current stimulus was applied to the pelvic region, where gender differences in pain perception have been most consistent (Unruh, 1996; Berkley, 1997a&b). While Paulson and associates (1998) found that phasic heat pain generated greater insula activation in healthy females, as compared to males, females also rated the stimuli as more intense than males did. Our data confirmed that the insula is part of a network demanding increased blood flow with increased intensity and discomfort. This suggests the sex difference in insula activation reported by Paulson and associates may have been a function of perceived intensity. In contrast, the current study used a mostly non-noxious stimulus in a visceral region where these patients report chronic pain and discomfort, and males and females rated the stimuli as equally uncomfortable, and equally intense. Given the salient role of the insula across species in processing, integration and autonomic European Journal of Pain (2000), 4

S. B ERMAN

ET AL .

responses to aversive somatic and visceral afferent signals, it is unlikely that the current findings signal a lack of involvement of this brain region in the processing of visceral information in female IBS patients. However, several possible explanations for the observed differences in insula activation between male and female IBS patients can be suggested and some of these hypotheses may be tested in future studies. If insula activation by a visceral stimulus is more transient in female IBS patients, the signal integrated over 90 s could show a smaller or even absent rCBF increase. A different time course of visceral afferent fiber activation in the rectum could be related to differences in compliance or receptive relaxation rates during isobaric distension. However, we did not find significant differences between male and female patients in such mechanoelastic properties of the rectum. Insula activation by visceral stimulation may be more variable in females, resulting in an apparent less significant mean activation. Such variability may be related to hormonal factors. Becerra and colleagues (1998) recently demonstrated increased activation of the insula in response to thermal pain in male, as compared to female normal volunteers. However, the gender difference was found only when women were tested in the menstrual stage where oestrogen and progesterone peaked. It is therefore conceivable that the lack of significant insula activation in the current study is related to greater variability of insula activation in females compared to males. This greater variability could be related to variation in female sex hormones due to age, menstrual status, hormone replacement or intake of oral contraceptives. Finally, the observed gender differences may be related to differences in the processing of the emotional content of the stimulus (Phillips et al. 1997, Fink et al. 1996; Lane et al. 1997). Even though not specifically tested in the current study, it is conceivable that the female patients attributed less negative emotion to the test stimulus compared to the male patients. In summary, we have demonstrated greater insula activation by a tonic moderately aversive visceral stimulus in male as compared to female IBS patients. Future investigations of this effect

G ENDER

SPECIFIC BRAIN ACTIVATION IN

IBS

should benefit from consideration of hormonal influences and from study designs addressing possible differences in the time course of regional brain activation.

ACKNOWLEDGEMENT Supported by National Institutes of Health Grants DK48351 (EAM) and NR04881 (BDN).

REFERENCES Aggarwal A, Cutts TF, Abell TL, Cardoso S, Familoni B, Bremer J, Karas J. Predominant symptoms in irritable bowel syndrome correlate with specific autonomic nervous system abnormalities. Gastroenterology 1994; 106: 945–950. Andersson HI, Ejlertsson G, Leden I, Rosenberg C. Chronic pain in a geographically defined general population: Studies of differences in age, gender, social class, and pain localization. Clin J Pain 1993; 9: 174–182. Augustine JR. Circuitry and functional aspects of the insular lobe in primates including humans. Brain Res Rev 1996; 22: 229–244. Aziz Q, Andersson JL, Valind S, Sundin A, Hamdy S, Jones AK, Foster ER, Langstrom B, Thompson DG. Identification of human brain loci processing esophageal sensation using positron emission tomography. Gastroenterology 1997; 113: 50–59. Bassotti G, Bucaneve G, Betti C, Morelli A. Sudden awakening from sleep: effects on proximal and distal colonic contractile activity in man. Eur J Gastroenterol Hepatol 1990; 2: 475–478. Bassotti G, Gaburri M. Manometric investigation of highamplitude propagated contractile activity of the human colon. Am J Physiol 1988; 255: G660–G664. Becerra L, Comite A, Breiter H, Gonzalez RG, Borsook D. Differential CNS activation following a noxious thermal stimulus in men and women: An fMRI study. Soc Neurosci Abstr 1998; 24: 1136. (Abstract) Berkley KJ. Sex differences in pain. Behav Brain Sci 1997a; 20: 371–380. Berkley KJ. Female vulnerability to pain and the strength to deal with it. Behav Brain Sci 1997b; 20: 473–479. Casey KL, Minoshima S, Berger KL, Koeppe RA, Morrow TJ, Frey KA. Positron emission tomographic analysis of cerebral structures activated specifically by repetitive noxious heat stimuli. J Neurophysiol 1994; 71: 802–807. Cechetto DF, Saper CB. Role of the cerebral cortex in autonomic function. Central regulation of autonomic function. New York: Oxford University Press, 1990: 208–223. Coghill RC, Talbot JD, Evans AC, Meyer E, Gjedde A, Bushnell MC, Duncan GH. Distributed processing of pain and vibration by the human brain. J Neurosci 1994; 14: 4095–4108. Derbyshire SWG, Jones AKP, Gyulai F, Clark S, Townsend D, Firestone LL. Pain processing during three levels of noxious stimulation produces differential patterns of central activity. Pain 1997; 73: 431–445.

171

Derbyshire SWG, Jones AKP, Townsend D, Gyulai F, Smith GS, Firestone L. Gender differences in the central response to pain controlled for stimulus intensity. Soc Neurosci Abstr 1998; 24: 528. (Abstract) Derogatis LR, Lazarus L. SCL-90-R, brief symptom inventory, and matching clinical rating scales. Lawrence Erlbaum Associates, Inc. 1994; 217–218. Drossman DA, Li Z, Andruzzi E, Temple R, Talley NJ, Thompson WG, Whitehead WE, Janssens J, Funch-Jensen P, Carazziari E, Richter JE, Koch GG. U.S. householder survey of functional GI disorders: Prevalence, sociodemography and health impact. Dig Dis Sci 1993; 38: 1569–1580. Eggen AE. The Tromso Study: Frequency and predicting factory of analgesic drug use in a free-living population (12–56 years). J Clin Epidemiol 1993; 46: 1297–1304. Ektor-Anderson J, Janzon L, Sjolund B. Chronic pain and the sociodemographic environment: Results from the Pain Clinic at Malmo General Hospital in Sweden. Clin J Pain 1993; 9: 183–188. Fink GR, Markowitsch HJ, Reinkemeier M, Bruckbauer T, Kessler J, Heiss WD. Cerebral representation of one’s own past: Neural networks involved in autobiographical memory. J Neurosci 1996; 16: 4275–4282. Friston KJ, Ashburner J, Frith CD, Poline J-B, Heather JD, Frackowiak RSJ. Spatial registration and normalisation of images. Hum Brain Mapping 1995a; 2: 165–189. Friston KJ, Frith CD, Liddle PF, Frackowiak RS. Functional connectivity: The principal-component analysis of large (PET) data sets. J Cereb Blood Flow Metab 1993; 13: 5–14. Friston KJ, Holmes AP, Worsley KJ, Poline JP, Frith CD, Frackowiak RSJ. Statistical parametric maps in functional imaging: a general linear approach. Hum Brain Mapping 1995b; 2: 189–210. Friston KJ, Worsley KJ, Frackowiak RS, Mazziotta JC, Evans AC. Assessing the significance of focal activations using their spatial extent. Hum Brain Mapping 1994; 1: 210–220. Furukawa Y, Cook IJ, Panagopoulos V, McEvoy RD, Sharp DJ, Simula M. Relationship between sleep patterns and human colonic motor patterns. Gastroenterology 1994; 107: 1372–1381. Giamberardino MA, Berkley KJ, Iezzi S, deBigontina P, Vecchiet L. Pain threshold variations in somatic wall tissues as a function of menstrual cycle, segmental site and tissue depth in non-dysmenorrheic women, dysmenorrheic women and men. Pain 1997; 71: 187–197. Gracely RH, McGrath P, Dubner R. Validity and sensitivity of ratio scales of sensory and affective verbal pain descriptors: Manipulation of affect by diazepam. Pain 1976; 2: 19–29. Howland EW, Wakai RT, Mjaanes BA, Balog JP, Cleeland CS. Whole head mapping of magnetic fields following painful electric finger shock. Brain Res Cog Brain Res 1995; 2: 165–172. Hsieh J-C, Belfrage M, Stone-Elander S, Hansson P, Ingvar M. Central representation of chronic ongoing neuropathic pain studied by positron emission tomography. Pain 1995; 63: 225–236. Ito SI. Possible representation of somatic pain in the rat insular visceral sensory cortex: A field potential study. Neurosci Lett 1998; 241: 171–174. Jackson NA, Houghton LA, Whorwell PJ. The menstrual cycle affects rectal visceral sensitivity in patients with irritable bowel syndrome (IBS) but not healthy volunteers. Gastroenterology 1997; 112: A1132. European Journal of Pain (2000), 4

172

Jackson NA, Houghton LA, Whorwell PJ, Currer B. Does the menstrual cycle affect anorectal physiology? Dig Dis Sci 1994; 39: 2607–2611. James FR, Large RG, Bushnell JA, Wells JE. Epidemiology of pain in New Zealand. Pain 1991; 44: 279–283. Klonoff EA, Landrine H, Brown M. Appraisal and response to pain may be a function of its bodily location. J Psychosom Res 1993; 37: 661–670. Kuroda R, Yorimae A, Yamada Y, Nakatani J, Takatsuji K. c-fos expression after formalin injection into the face in the cat. Stereotactic Functional Neurosurg 1995; 65: 152–156. Lane RD, Reiman EM, Ahern GL, Schwartz GE, Davidson RJ. Neuroanatomical correlates of happiness, sadness, and disgust. Am J Psychiatry 1997; 154: 926–933. Lee OY, Schmulson M, Mayer EA, Chang L, Naliboff BD. Gender related differences in irritable bowel syndrome symptoms. Gastroenterology 1999b; 116: A1026. (Abstract) Lembo T, Munakata J, Mertz H, Niazi N, Kodner A, Nikas V, Mayer E. Evidence for the hypersensitivity of lumbar splanchnic afferents in irritable bowel syndrome. Gastroenterology 1994; 107: 1686–1696. Lembo T, Naliboff B, Munakata J, Fullerton S, Saba L, Tung S, Schmulson M, Mayer EA. Symptoms and visceral perception in patients with pain-predominant irritable bowel syndrome. Am J Gastroenterol 1999 (In Press). Malagelada JR. Altered visceral sensation in functional dyspepsia and related syndromes. In: Mayer EA, Raybould HE, editors. Basic and Clinical Aspects of Chronic Abdominal Pain. Amsterdam: Elsevier, 1993: 55–59. Mangel AW, Northcutt AR. The safety and efficacy of Alosetron, a 5HT3 receptor antagonist, in female irritable bowel syndrome patients. Alim Pharmacol Therapeutics 1999; 13: 77–82. May A, Kaube H, Buchel C, Eichten C, Rijntjes M, Juptner M, Weiller C, Diener HC. Experimental cranial pain elicited by capsaicin: A PET study. Pain 1998; 74: 61–66. Mayer EA, Gebhart GF. Basic and clinical aspects of visceral hyperalgesia. Gastroenterology 1994; 107: 271–293. Mayer EA, Naliboff B, Munakata J, Silverman DH. Braingut mechanisms of visceral sensitivity. In: Corazziari E, editor. NeuroGastroenterology. New York: de Gruyter, 1996: 17–31. Mertz H, Naliboff B, Munakata J, Niazi N, Mayer EA. Altered rectal perception is a biological marker of patients with irritable bowel syndrome. Gastroenterology 1995; 109: 40–52. Munakata J, Naliboff B, Harraf F, Kodner A, Lembo T, Chang L, Silverman DH, Mayer EA. Repetitive sigmoid stimulation induces rectal hyperalgesia in patients with irritable bowel syndrome. Gastroenterology 1997; 112: 55–63. Naliboff BD, Munakata J, Chang L, Mayer EA. Toward a biobehavioral model of visceral hypersensitivity in irritable bowel syndrome. J Psychosomatic Res 1998; 45: 485–492. Naliboff BD, Munakata J, Fullerton S, Gracely RH, Kodner A, Harraf F, Mayer EA. Evidence for two

European Journal of Pain (2000), 4

S. B ERMAN

ET AL .

distinct perceptual alterations in irritable bowel syndrome. Gut 1997; 41: 505–512. Paulson PE, Minoshima S, Morrow TJ, Casey KL. Gender differences in pain perception and patterns of cerebral activation during noxious heat stimulation in humans. Pain 1998; 76: 223–229. Penfield W, Boldrey E. Somatic motor and sensory representation in the cerebral cortex of man as studied by electrical stimulation. Brain 1937; 60: 389–443. Penfield W, Faulk ME. The insula. Further observations on its function. Brain 1955; 78: 445–470. Phillips ML, Young AW, Senior C, Brammer M, Andrew C, Calder AJ, Bullmore ET, Perrett DI, Rowland D, Williams SC, Gray JA, David AS. A specific neural substrate for perceiving facial expressions of disgust. Nature 1997; 389: 495–498. Rainville P, Duncan GH, Price DD, Carrier B, Bushnell MC. Pain affect encoded in human anterior cingulate but not somatosenory cortex. Science 1997; 277: 968–971. Richter JE, Bradley LA. The irritable esophagus. In: Mayer EA, Raybould HE, editors. Basic and Clinical Aspects of Chronic Abdominal Pain. Amsterdam: Elsevier, 1993: SAS Inst. Inc. SAS/STAT Users Guide, version 6.07. Cary, N.C.: SAS Institute, Inc., 1989: Silverman DH, Munakata JA, Ennes H, Mandelkern MA, Hoh CK, Mayer EA. Regional cerebral activity in normal and pathological perception of visceral pain. Gastroenterology 1997; 112: 64–72. Svensson P, Minoshima S, Beydoun A, Morrow TJ, Casey KL. Cerebral processing of acute skin and muscle pain in humans. J Neurophysiol 1997; 78: 450–460. Talairach J, Tournoux P. Co-planar stereotaxic atlas of the human brain: 3 dimensional porportional system: an approach to cerebral imaging. New York: Thieme Medical Publishers, 1988. Thompson WG. Gender difference in irritable bowel syndromes. Eur J Gastroenterol Hepatol 1997; 9: 299–302. Thompson WG, Dotevall G, Drossman DA. Irritable bowel syndrome: guidelines for the diagnosis. Roma 88 Working Team report presented at the International Congress on Gastroenterology. Rome, 1988. Unruh AM. Gender variations in clinical pain experience. Pain 1996; 65: 123–167. Von Korff M, Dworkin SF, Le Resche L, Kruger A. An epidemiologic comparison of pain complaints. Pain 1988; 32: 173–183. Von Korff M, Dworkin SF, Le Resche L. Graded chronic pain status: An epidemiologic evaluation. Pain 1990; 40: 279–291. Xu X, Fukuyama H, Yazawa S, Mima T, Hanakawa T, Magata Y, Kanda M, Fujiwara N, Shindo K, Nagamine T, Shibasaki H. Functional localization of pain perception in the human brain studied by PET. NeuroReport 1997; 8: 555–559. Yasui Y, Saper CB, Cechetto DF. Autonomic responses and efferent pathways from the insular cortex in the rat. J Comp Neurology 1991; 303: 355–374.