Brain activation responses to subliminal or supraliminal rectal stimuli and to auditory stimuli in irritable bowel syndrome

Authors

  • V. Andresen,

    1. Department of Medicine, Division of Hepatology, Gastroenterology, and Endocrinology, Charité– Universitätsmedizin Berlin, Berlin, Germany
    2. Department of Medicine, Division of Psychosomatic Medicine and Psychotherapy, Charité– Universitätsmedizin Berlin, Berlin, Germany
    Search for more papers by this author
  • D. R. Bach,

    1. Department of Medicine, Division of Hepatology, Gastroenterology, and Endocrinology, Charité– Universitätsmedizin Berlin, Berlin, Germany
    Search for more papers by this author
  • A. Poellinger,

    1. Department of Radiology, Charité– Universitätsmedizin Berlin, Berlin, Germany
    Search for more papers by this author
  • C. Tsrouya,

    1. Department of Medicine, Division of Hepatology, Gastroenterology, and Endocrinology, Charité– Universitätsmedizin Berlin, Berlin, Germany
    Search for more papers by this author
  • A. Stroh,

    1. Department of Radiology, Charité– Universitätsmedizin Berlin, Berlin, Germany
    Search for more papers by this author
  • A. Foerschler,

    1. Department of Neuroradiology, University of Leipzig, Leipzig, Germany
    Search for more papers by this author
  • P. Georgiewa,

    1. Department of Medicine, Division of Psychosomatic Medicine and Psychotherapy, Charité– Universitätsmedizin Berlin, Berlin, Germany
    Search for more papers by this author
  • C. Zimmer,

    1. Department of Neuroradiology, Klinikum rechts der Isar, Technische Universität München, Munich, Germany
    Search for more papers by this author
  • H. Mönnikes

    1. Department of Medicine, Division of Hepatology, Gastroenterology, and Endocrinology, Charité– Universitätsmedizin Berlin, Berlin, Germany
    2. Department of Medicine, Division of Psychosomatic Medicine and Psychotherapy, Charité– Universitätsmedizin Berlin, Berlin, Germany
    Search for more papers by this author

Hubert Mönnikes MD, PhD, Department of Medicine, Division of Hepatology, Gastroenterology, and Endocrinology, Augustenburger Platz 1, 13353 Berlin, Germany.
Tel: +49 30 450 653391; fax: +49 30 450 553991;
e-mail: hubert.moennikes@charite.de

Abstract

Abstract  Visceral hypersensitivity in irritable bowel syndrome (IBS) has been associated with altered cerebral activations in response to visceral stimuli. It is unclear whether these processing alterations are specific for visceral sensation. In this study we aimed to determine by functional magnetic resonance imaging (fMRI) whether cerebral processing of supraliminal and subliminal rectal stimuli and of auditory stimuli is altered in IBS. In eight IBS patients and eight healthy controls, fMRI activations were recorded during auditory and rectal stimulation. Intensities of rectal balloon distension were adapted to the individual threshold of first perception (IPT): subliminal (IPT −10 mmHg), liminal (IPT), or supraliminal (IPT +10 mmHg). IBS patients relative to controls responded with lower activations of the prefrontal cortex (PFC) and anterior cingulate cortex (ACC) to both subliminal and supraliminal stimulation and with higher activation of the hippocampus (HC) to supraliminal stimulation. In IBS patients, not in controls, ACC and HC were also activated by auditory stimulation. In IBS patients, decreased ACC and PFC activation with subliminal and supraliminal rectal stimuli and increased HC activation with supraliminal stimuli suggest disturbances of the associative and emotional processing of visceral sensation. Hyperreactivity to auditory stimuli suggests that altered sensory processing in IBS may not be restricted to visceral sensation.

Introduction

Visceral hypersensitivity is one of the underlying mechanisms in the pathophysiology of the irritable bowel syndrome (IBS),1,2 but it is uncertain whether peripheral components of the brain-gut axis, such as enteric receptors and afferent neurones, or cerebral networks, or both, play a role in this disturbance.3,4 Indeed, there is evidence that peripheral (possibly postinflammatory) sensitization mechanisms might be involved in the hypersensitivity of IBS.5,6 However, cerebral sensory processing of peripheral visceral stimuli has also been shown to be different between healthy subjects and IBS patients under different experimental conditions.7–12 However, the available data do not allow to delineate a uniform or unequivocal theory about the focus of abnormal sensory processing in IBS.13,14

Specific brain regions participate in the cerebral processing of visceral afferent signals. From the brainstem or thalamic nuclei, signals are projected to higher cortical processing regions15–17 of primary, discriminative perception (location, characteristic and intensity of the sensation), such as the primary (S1) and secondary (S2) somatosensory cortex and the insula.18,19 Signals are also projected directly or via the insular cortex20 to brain regions serving the secondary associative and emotional perception. Associative sensory processing (e.g. attention, orientation, sensory memory, and initiation of response) involves the prefrontal cortex (PFC),21 that may also participate in the control and modulation of pain.22 Emotional processing involves the limbic system, that is the anterior cingulate cortex (ACC), which conveys functions such as attention, fear, suffering, avoidance23,24 and also sensory inhibition via the endogenous opioid system,25 and subcortical structures such as the amygdala and the hippocampus.26,27

In functional brain imaging studies, altered cerebral processing of visceral stimuli has been detected in IBS patients,7–11 but as a rule, these studies have compared responses in patients and controls to the same, typically painful or noxious, stimulus intensities. However, any hypersensitivity induced by peripheral sensitization would also result in increased cerebral response patterns, when using fixed stimulus intensities.

To circumvent this problem, we evaluated the effects of rectal stimuli of different intensities in order to create the same perceived input from the gut by adapting the stimulation intensities relative to the individual's perception threshold (IPT). Thus, we separately analysed sensory processing of supraliminal and subliminal visceral stimuli in each individual.

To evaluate whether any detected differences in brain activation were specific for the processing of rectal stimuli, we additionally exposed subjects to non-visceral stimulation by auditory stimulation, which would be expected to directly incite cerebral processing networks through the eighth cranial nerve. In order to provide an adequate contrast to the potentially aversive rectal stimuli, the chosen stimuli were neutral spoken words.

Taken together, in the present study we tested two hypotheses: first, that cerebral response patterns to IPT-adapted supraliminal and subliminal stimulation are associated with differences in cerebral activations in IBS patients and controls; and second, that any observed differences in brain activation between IBS and controls would be specific for rectal stimuli.

Materials and methods

Subjects

Eight right-handed patients with IBS [five females, three males; mean age, 41.3 (27–64) years; diarrhoea-predominant, n = 5, alternating stools, n = 3] established by Rome II criteria28 were recruited at our institution's outpatient clinic for gastrointestinal functional disorders after a thorough diagnostic work-up including colonoscopy. None of the patients used centrally acting agents to treat bowel symptoms, and peripherally acting IBS treatments were stopped 7 days prior to the study.

As controls, eight right-handed healthy volunteers [five males, three females; mean age, 39.4 (24–54) years] were selected after exclusion of individuals with concomitant or previous gastrointestinal disorders by history and the IBS symptom score-questionnaire.29

In all subjects, written informed consent as approved by the institutional ethical committee was obtained, and concomitant psychiatric disorders were excluded by using the standardized German Diagnostic Interview for Psychiatric Disorders (DIPS),30 a structured clinical interview based on the Anxiety Disorders Interview Schedule.31

Personality Questionnaires

For the assessment of personality characteristics, all study participants were asked to fill in the following questionnaires: the German form of the State-Trait-Anxiety-Inventory (STAI)32 for the assessment of depression, neuroticism and complaints, the German form of the Beck's Depression Index (BDI)33 for the assessment of depression, and the German form of the NEO-Five-Factor-Inventory (NEO-FFI)34 for the assessment of neuroticism, openness to experience, extroversion, agreeableness, and conscientiousness.

Individual perception threshold

After a rectal enema, fasting subjects were rectally intubated with a deflated soft polyethylene bag (maximal volume, 600 mL; MAK-LA, Los Angeles, CA, USA) attached to a thin plastic tube (length, 5 m; diameter of inflation and measurement channels, 4 mm each) that was fixed with tape to the buttock. Subjects were tested in supine position corresponding to the position on the MRI-scanner table. After 15 min of rest, the rectal balloon distension protocol was started using a computer-driven barostat device (Dual Distender II; G&J, Willowdale, Canada). To avoid anticipation effects, subjects were instructed that a random sequence of distensions would be applied anywhere between no sensation at all and possibly painful sensations.

The IPT was established by using a distension protocol starting with the ascending method of limits followed by a tracking method35 as soon as the stimulus was clearly perceived for the first time, and by calculating the mean value of six threshold distension pressures. Each distension lasted for 20 s followed by a rest of 10 s. After 15 s of each distension period, subjects were asked to rate their perception on a scale from 1 (no perception) to 6 (pain).

Stimulation protocol for the functional MRI scanning

As repetitive rectal stimulation may induce sensitization mechanisms,36 IPT assessments and functional magnetic resonance imaging (fMRI) stimulation experiments were performed on different days. Rectal balloon stimulation during fMRI was performed with the same barostat device used for IPT assessment. The stimulation protocol consisted of a block design of subliminal, liminal, and supraliminal stimulus intensities separated by resting phases with a balloon pressure of 1 mmHg (Fig. 1) to ensure short inflation times. Each distension period lasted 1 min, each resting phase 24 s. For each subject, pressure intensities of balloon distensions were adapted to the individual IPT. Thus, subliminal distensions were performed at a pressure of IPT −10 mmHg, liminal distensions at the IPT, and the supraliminal at IPT +10 mmHg (Fig. 1). These gradients were chosen to unequivocally separate the stimulation states without repeating the subjective stimulus intensity assessment during fMRI experiments (in order to minimize stimulation of attention processes). If IPT values were below 15 mmHg, distension pressures were defined to be 5, 15 and 25 mmHg to make sure the lowest value was different from resting phases. Following the rectal stimulation, we added a sequence of auditory stimuli with spoken words (applied through headphones) in two blocks of 48 s each, separated by resting phases. The words have been chosen from a list of German words that have been evaluated as neutral by a group of 50 German-speaking healthy volunteers, e.g. the German words (each with two syllables) for armchair, garden, car, plate, glasses, etc.

Figure 1.

Stimulation protocol. The rectal stimulation during the fMRI acquisition was performed in a block design consisting of six balloon distensions of 60 s each separated by resting phases (R) of 24 s each. Stimulation intensities were adapted to the individual perception threshold (IPT): subliminal stimulation (sub) = 10 mmHg below the IPT, liminal stimulation (lim) = IPT pressure, supraliminal stimulation (supra) = 10 mmHg above the IPT. During resting phases balloon pressure returned to baseline (B) = 1 mmHg.

MRI acquisition

Magnetic resonance images were collected on a 1.5 T whole body scanner (Siemens Magnetom Vision, Erlangen, Germany) with a standard head coil. A vacuum pad was used to minimize head movements. First, a T1-weighted localizer scan was recorded. Next, T2-weighted oblique scans were obtained (TR/TE 4500/128 ms, field of view 230 mm), primarily to aid Talairach transformation for data analysis. For the functional scans, an echo-planar sequence (TR/TE 4000/66 ms; flip angle 90°; field of view 230 mm; matrix 128 × 128; slice thickness 6 mm, interslice gap 0.6 mm; in-plane resolution 1.8 × 1.8 mm) was used. A total of 120 images per slice were acquired. Sixteen slices adjusted at a transverse-to-coronal angle of approximately 20° covering the whole brain with the exception of the most superior frontal and superior parietal lobe, inferior temporal pole, and cerebellum (most superior z about 60 and most inferior z about −25 according to Talairach and Tournoux37) were obtained for all studies. Structural 3D data sets were acquired using a T1-weighted sagittal sequence with isotropic voxels (TR/TE 11.4/4.4 ms; flip angle 15°; number of slices 160, matrix 256 × 256, field of view 256 mm, voxel size 1 mm3).

Data analysis

For data analysis, we used the Brainvoyager software.38 The 2D functional data were reconstructed within the 3D structural data set. The 3D data were then transformed into the standardized Talairach spaced brain.37 Finally, the reconstructed data set underwent subsequent procedures of motion correction, intensity scaling and detrending.

Statistical maps were constructed using the general linear model module of the Brainvoyager software. The stimulation conditions were used as predictors, and the contrasts of each stimulation condition vs rest were analysed. For the between-group comparison, predictors were defined for each stimulus type as the interaction of the group with this stimulus type. Therefore, each predictor represented a larger signal increase of one group at the referred stimulus type.

Activated clusters were only accepted if they showed highly significant (P < 0.001) activation increase (within-group) or activation difference (between-group). In order to correct for multiple comparisons, a minimal cluster size of 6 voxels was defined.

Definition of regions of interest

As regions of interest (RoI) for the data analysis (Tables 2–4), we selected brain areas known to be involved in primary visceral sensory processing and brain regions known to be involved in secondary associative and emotional processing. All regions were neuroanatomically predefined by an expert neuroradiologist according to the coordinates of Talairach and Tournoux,37 and according to neuroanatomical visualization.

Table 2.  Significant activations (P < 0.001) in the sensory perception areas in healthy controls (A) and IBS patients (B)
RoIConditionSideNo. of clustersNo. of voxels*x*y*z*% Signal change*
  1. sub, Subliminal vs rest; liminal, liminal vs rest; supra, supraliminal vs rest; RoI, regions of interest; R, right; L, left; B, bilateral; S1, primary somatosensory cortex; S2, secondary somatosensory cortex; insula ant./post., insula anterior/posterior; x + y + z, coordinates of the Talairach space.

  2. *Data presented for the largest cluster.

(A) Controls
  ThalamusSupraB32907−14−2270.28
  S1SupraB6616−40−18510.33
  S2SupraB21611−53−13120.23
  Insula ant.SupraB3904−351350.25
  Insula post.SupraB22125−38−1050.21
(B) IBS patients
  ThalamusSupraR113711−1400.11
  S1LiminalR15442−14490.11
  S2SupraB380−59−18160.14
  Insula ant.SupraB21467361060.18
  Insula post.SupraB3118938−400.15
Table 3.  Significant activations (P < 0.001) in higher sensory processing areas in healthy controls (A) and IBS patients (B)
RoIConditionBASideNo. of clustersNo. of voxels*x*y*z*% Signal change*
  1. sub, Subliminal vs rest; liminal, liminal vs rest; supra, supraliminal vs rest; RoI, region of interest; BA, Brodman area; R, right; L, left; B, bilateral; antCC, perigenual anterior cingulate cortex (posterior border: coronary slice through the genu corporis callosi); amCC, anteromedial cingulate cortex (posterior border: coronary slice through the anterior commission); PFC, prefrontal cortex; Amy, amygdala; HC, hippocampus; x + y + z, coordinates of the Talairach space.

  2. *Data presented for the largest cluster.

(A) Controls
    antCCSubBA24B2437−33220.37
BA32B337633500.4
SupraBA24B276533260.26
    amCCSupraBA24B21864414320.25
BA32B21303341230.3
    PFCsubBA10B3346340−60.43
BA11B229446−60.32
LiminalBA10R1285043−11.07
BA46R145504320.9
SupraBA10B72011555110.52
BA46L23663−3738200.34
(B) IBS patients
  amCCSupraBA24B212722390.17
BA32R110337480.15
  PFCLiminalBA10L2231−85330.32
SupraBA10B722583855130.25
BA11B253−1923−150.31
BA46B332964437170.19
  Amy/HCSupraR120522−11−130.13
Table 4.  Between-group differences: significantly higher (P < 0.001) activation in controls when compared with IBS patients (A) and in IBS patients when compared with controls (B)
ConditionRoISideNo. of clustersNo. of voxels*x*y*z*
  1. sub, Subliminal vs rest; liminal, liminal vs rest; supra, supraliminal vs rest; RoI, region of interest; BA, Brodman area; R, right; L, left; B, bilateral; S1, primary somatosensory cortex; S2, secondary somatosensory cortex; ACC, anterior cingulate cortex; PFC, prefrontal cortex; HC, hippocampus; x + y + z, coordinates of the Talairach space.

  2. *Data presented for the largest cluster.

(A) Controls > IBS patients
  SubACC (BA24 + 32)R227535−4
PFC (BA10)B486549−3
  LiminalPFC (BA10,11 + 46)R3472322−12
  SupraS1B282−24−2356
S2L1125−64−1616
ACC (BA24 + 32)B3747−64710
PFC (BA10)B37848529
(B) IBS patients > controls
  LiminalACC (BA32)B21789425
PFC (BA10)B37368492
  SupraS1R12423−2957
PFC (BA10)R2166372725
HCR123322−1413

Results

Individual perception threshold of first sensation

The mean IPT of first sensation of the rectal barostat distension was 16.4 mmHg (SD 6.2) in IBS and 19.6 mmHg (SD 4.7) in healthy controls (P = 0.29).

Personality characteristics

The personality traits were not significantly different between IBS patients and healthy controls. Results are summarized in Table 1. Descriptively, IBS patients had higher values for anxiety (P = 0.17), depression (P = 0.19) and neuroticism (P = 0.11) relative to healthy controls (Table 1).

Table 1.  Personality characteristics: results are presented as mean ± SD of each test for the healthy control group, the IBS group, and the reference population (as listed in the corresponding references for each test: Refs (32–34)
 ControlsIBS patientsIBS patients vs controls (P=)Reference population
  1. STAI, State-Trait-Anxiety-Inventory; with X2, Trait-Anxiety Subtest32; BDI, Beck's Depression Index33; NEO, NEO-Five-Factor-Inventory.34

STAI X234.8 ± 6.141.3 ± 11.00.1735.7 ± 9.4
BDI5.4 ± 5.38.7 ± 4.40.196.5 ± 5.2
NEO: Neuroticism1.7 ± 0.52.2 ± 0.60.111.8 ± 0.7
NEO: Extroversion2.8 ± 0.42.6 ± 0.30.242.4 ± 0.6
NEO: Openness to experience2.5 ± 0.52.4 ± 0.30.732.7 ± 0.5
NEO: Agreeableness2.4 ± 0.62.4 ± 0.30.812.4 ± 0.5
NEO: Conscientiousness3.0 ± 0.63.2 ± 0.50.462.6 ± 0.6

Within-group analysis

Response to subliminal stimulation  In controls, subliminal rectal distension induced significant activations in the perigenual part of the ACC (antCC), and the PFC (P < 0.001, Tables 2 and 3, Fig. 2). By contrast, in IBS patients no cerebral activation at subliminal distension was observed (Tables 2 and 3).

Figure 2.

Subliminal rectal stimulation in healthy controls induced significant activation of the perigenual anterior cingulate cortex (ACC) [=anterior cingulate cortex (antCC)] and the prefrontal cortex (PFC) (P < 0.001, sagittal slice at x = −1).

Response to liminal stimulation  In controls, liminal rectal distension induced significant, but small activations in the PFC (P < 0.001, Table 3), while IBS patients responded to liminal stimulation with larger activations in the PFC and S1 (P < 0.001, Tables 2 and 3).

Response to supraliminal stimulation  In controls, supraliminal rectal distension induced significant activations in all of the primary discriminative sensory perception areas: thalamus, S1, S2, anterior and posterior insula (P < 0.001, Table 2, Fig. 3). In addition, significant activations were also observed in regions of secondary associative and emotional sensory processing, specifically in a small cluster in the perigenual ACC (antCC) and in two large clusters in the caudal ACC (amCC), and in the PFC (P < 0.001, Table 3, Fig. 3). There was no significant activation of the amygdala and hippocampus in controls.

Figure 3.

Supraliminal rectal stimulation induced significant activation in specific sites of the caudal anterior cingulate cortex (ACC) [=anteromedial cingulate cortex (amCC)] and of the prefrontal cortex (PFC) in healthy controls (panel A) but not in patients with IBS (panel B). In contrast, both groups responded with significant activations in the secondary somatosensory cortex (S2) and in the insula (panels A + B) (P < 0.001, sagittal slice at x = −1, coronary slice at y = 0).

IBS patients responded to supraliminal stimulation with significant activations in the same primary sensory perception areas: thalamus, S2 (but not S1), and anterior and posterior insula (P < 0.001, Table 2, Fig. 3) and in secondary associative and emotional sensory processing areas: PFC and caudal ACC (amCC). In addition, IBS patients responded to supraliminal stimulation with significant activation of the amygdala and hippocampus (P < 0.001, Table 3).

Comparison between IBS and health

For all stimulus intensities, group differences were mainly observed in regions of higher emotional and associative sensory processing.

At subliminal rectal stimulation, there were significantly higher activations in healthy controls in regions of secondary sensory processing, i.e. ACC and PFC (P < 0.001, Table 4), compared with IBS patients.

At liminal rectal stimulation, main differences between IBS patients and healthy controls were significantly higher activations in IBS patients in regions of secondary sensory processing, i.e. ACC and PFC (P < 0.001, Table 4).

At supraliminal stimulation, the main differences between IBS patients and healthy controls were significantly higher activations of the ACC and the PFC in healthy controls, and a significantly higher activation of the hippocampus in IBS patients (P < 0.001, Table 4, Fig. 4).

Figure 4.

Group difference at supraliminal rectal stimulation: significantly higher activation of the hippocampus in IBS patients (P < 0.001, coronary slice at y = −14).

Response to auditory stimulation with neutral spoken words  In response to neutral spoken words, healthy controls responded with a significant, but very small activation in the PFC (P < 0.001).

However, only IBS patients responded with significant activations (P < 0.001) in the ACC, hippocampus, insula and a large significant activation in the PFC (Table 5).

Table 5.  Significant activations (P < 0.001) in higher sensory processing areas in healthy controls (A) and IBS patients (B) in response to auditory stimulation with neutral words
RoISideNo. of clustersNo. of voxels*x*y*z*% Signal change*
  1. RoI, region of interest; R, right; L, left; B, bilateral; ACC, anterior cingulate cortex; PFC, prefrontal cortex; HC, hippocampus; x + y + z, coordinates of the Talairach space.

  2. *Data presented for the largest cluster.

(A) Controls
  PFCR145265560.40
(B) IBS patients
  ACCR222316460.19
  PFCB81408046−110.18
  HCB265227−27190.14
  InsulaR184734950.20

Discussion

In this study, we used an individually adapted stimulation design with subjectively comparable rectal distension intensities, to avoid differences in peripheral sensory reception but rather reflect alterations in cerebral sensory processing networks. Our main observations were:

  • 1At subliminal rectal stimulation, only controls revealed significant activation of the ACC and PFC, suggesting decreased inhibitory sensory processing functions in IBS.
  • 2In response to liminal rectal stimulation, IBS patients responded with higher activations in secondary associative sensory processing regions, suggesting a hyperreactivity to first rectal sensation in IBS patients in this experimental setting.
  • 3Supraliminal rectal stimulation induced significantly higher activation of ACC and PFC in healthy controls suggesting differences in associative and emotional processing of visceral stimuli. Only IBS patients had a significant activation of the hippocampus supporting the concept that in IBS visceral sensation may have a higher emotional impact.
  • 4Auditory stimulation with neutral words, as neutral non-visceral control stimulation, induced an increased activation of ACC, PFC and hippocampus in IBS patients suggesting a generalized hyperreactivity in emotional processing brain regions that is not specific for visceral sensation. This also suggests that the reduced activations during subliminal and supraliminal rectal stimulation were not the result of a generalized hyporeactivity of these brain areas in the IBS group.

Processing of subliminal rectal stimulation

Earlier studies detected cortical activations in response to subliminal rectal stimuli in primary and secondary sensory processing regions in IBS patients as well as in healthy controls.39,40 In contrast, we observed significant activations of the perigenual ACC (antCC) and the PFC only in healthy controls. Differences in the analysis (group analysis vs single subject analysis) and lower stimulation intensities in our study could account for these discrepancies between the current study and prior literature.

As both the ACC and PFC are likely involved in modulation and control of sensory input,22,25 activations of these two regions at subliminal levels of stimulation may represent inhibitory sensory modulation functions that contribute to keep weaker signals below the level of conscious perception. Hence, the lack of ACC and PFC activation in IBS patients could indicate a disturbed sensory modulation. Alternatively, as all subjects anticipated the delivery of rectal stimulation during the experiments, activations during subliminal stimulation could reflect anticipation processes. This is consistent with the role of the perigenual ACC in coding for the onset uncertainty of an impending stimulus.39 This concept would also be supported by the lack of activation of any primary discriminative sensory regions. In this respect, our observations are in accordance with earlier data showing that anticipation of a painful sensation during a sham rectal stimulation induced ACC activation in controls but not in IBS patients.7 It has been proposed that the lack of ‘preparation’ to expected stimuli could reflect a disturbance in sensory inhibition.7 Taken together, our findings indicate that IBS patients may have an impaired sensory processing of subliminal visceral afferent signals. As the majority of physiological visceral stimuli remain subliminal, these findings may point to a fundamental visceral sensory disturbance in IBS, which should be further evaluated in future studies.

Processing of liminal rectal stimulation

Liminal rectal stimulation induced larger activations in secondary sensory processing regions in IBS when compared with healthy controls. This is in contrast to the observations at both subliminal and supraliminal stimulation and may appear to be a surprising finding. However, in an experimental setting where subjects are expecting or anticipating rectal stimulation of unknown degree of severity or unpleasantness, these findings could be attributed to a hyperreactivity of IBS patients to the first sensation of rectal stimuli. This is consistent with the concept of an increased attention of IBS patients towards gut sensation.40,41 Corresponding observations of increased emotional processing activity in IBS patients have been reported in response to painful rectal stimuli.8,9 It is conceivable that activations in emotional sensory processing regions in IBS patients may partly reflect an increased emotional reactivity to a stimulus independent of the actual stimulus intensity. In fact, the antidepressant drug, amitriptyline, reduces the rectal pain-induced activations in these regions only during stress, suggesting that the drug targets emotional processing rather than sensory processing.42

Processing of supraliminal rectal stimulation

In response to supraliminal rectal stimulation, activations at higher intensities in controls relative to IBS patients, particularly in the ACC and PFC, partly confirm,7,10,11,43,44 or contradict8,9,42 earlier findings. Different activation-results between various brain imaging studies might reflect differences in the nature and degree of sensory processing alterations in IBS.13,14 The use of different stimulation protocols and imaging techniques complicate interpretation of the prior literature. Moreover, various functions are co-located within some of these brain regions, and different activations may reflect responses of functionally different subunits within a region.8,9,22,25,43

While there are many, in part conflicting data on activation differences of the ACC and PFC, activation differences of the hippocampus between IBS and health are scarcely reported, and only in the context of rectal pain.44 This is the first report of higher activation of the hippocampus in IBS patients relative to healthy controls in response to non-painful supraliminal rectal stimulation. With regard to the otherwise lower activation responses in the IBS patients in our study, this appears to be particularly striking and needs confirmation in future studies. However, as the hippocampus is involved in the regulation of emotional distress,45 its increased activation in IBS patients could reflect the clinical impression that, at least to a subgroup of IBS patients, visceral stimuli may have an increased stressful emotional impact.

Specificity of brain activation responses in IBS

This fMRI study includes a neutral non-visceral, i.e. auditory, control stimulus in addition to the rectal stimulation experiment in order to evaluate the specificity of cerebral response patterns to visceral stimuli. We observed that IBS patients have significantly increased activations of associative and emotional processing brain regions in response to hearing neutral words. This observation has important implications. First, the reduced activations during subliminal and supraliminal rectal stimulation were not the result of a general cerebral hyporeactivity of the IBS group. Our results also confirm earlier observations of Blomhoff et al.46 and Berman et al.47 who demonstrated a cerebral hyperreactivity to emotional and neutral auditory stimuli in IBS patients. A similar auditory hyperreactivity is also found in patients with migraine48 and post-traumatic stress disorders,49 disorders often found to co-exist with IBS.50 Interestingly, the cerebral response patterns to neutral words correspond to the response to painful rectal stimulation in other studies,8,9 and to the response to first sensation of rectal stimuli in our study. These findings suggest that an increased reactivity of the emotional sensory processing regions in IBS patients may not be specific for visceral sensation and may even be independent of the emotional impact of the stimulus itself. Rather it is conceivable that differences in the emotional state might be a contributing factor. This in accordance with the findings by Blomhoff et al., who detected a greater increase of central reactivity to auditory stimuli in IBS patients with concomitant phobic anxiety disorders.51 Moreover, previous rectal stimulation studies have demonstrated that both external and internal psychological factors may influence cerebral activation responses in IBS patients.12,42,52 Although the IBS patients in our study had higher levels of anxiety, depression and neuroticism, we failed to show statistically significant differences between the groups. However, we cannot exclude a type II error due to the small sample sizes.

Overall, the current observations indicate that differences between IBS patients and healthy controls in brain areas involved in emotional stimulus-processing may not be specific for visceral sensation, but might reflect a generalized increase in emotional sensitivity and affective reactivity. This is consistent with the frequent association of IBS with psychological or extra-intestinal functional disorders.50

We acknowledge that such brain imaging studies have some pitfalls, as recently identified by Drossman53: the sample size of n = 8 for each group is rather small, and the bowel dysfunctions are heterogeneous in the IBS group. There is some evidence that diarrhoea- vs constipation-predominant IBS patients have different cerebral response patterns to rectal stimulation.44 Moreover, the stimulus application and recording within a block design has some intrinsic disadvantage, because the cortical activity is averaged over the whole stimulation period. Thereby, any specific activation only occurring for a short time fraction (e.g. at the beginning of the stimulation) might be missed. However, analogous methods were used in previous studies assessing cerebral processing of rectal stimuli. Therefore, our results are comparable with prior literature. Future studies should include more patients, and allow for different subgroup analyses and a thorough evaluation of psychometric variables to further elucidate the nature of altered visceral and non-visceral sensory processing in IBS.

In summary, our results indicate that the associative and emotional cerebral processing of visceral sensory stimuli is altered in IBS even at the level of subliminal stimuli. Hyperreactivity of emotional processing brain regions to auditory stimuli suggests that altered sensory processing in IBS may not be specific for or restricted to visceral sensation.

Acknowledgments

The authors would like to thank Norbert Brombacher and Mathias Moosman, Department of Neurology, Charité– Universitätsmedizin Berlin, for their advice and support.

Ancillary