Altered reflex and perceptual responses within the brain-gut

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1 GASTROENTEROLOGY 2006;131: REVIEWS IN BASIC AND CLINICAL GASTROENTEROLOGY Neuroimaging of the Brain-Gut Axis: From Basic Understanding to Treatment of Functional GI Disorders EMERAN A. MAYER,*, BRUCE D. NALIBOFF,*,, and A. D. BUD CRAIG *Center for Neurovisceral Sciences & Women s Health; CURE: Digestive Disease Research Center; David Geffen School of Medicine at UCLA, Los Angeles, California; VA Greater Los Angeles Healthcare System, Los Angeles, California; and Barrow Neurological Institute, Phoenix, Arizona We are enthusiastic in the introduction of a new monthly review article series, entitled Reviews in Basic and Clinical Gastroenterology. Written by authorities in their respective fields, the objective of each review article is to provide an overview of a particular theme or topic for the broad scientific and clinical readership. Within a given topic, both basic and clinical aspects will be covered, accompanied by key figures and relevant references. The reader will also appreciate that topics will be interwoven from month to month as well. We hope you enjoy this section. David C. Metz, MD, Wafik El-Deiry, MD, PhD, and Anil K. Rustgi, MD Altered reflex and perceptual responses within the brain-gut axis have emerged as a generally accepted model to explain the cardinal symptoms of functional gastrointestinal (GI) syndromes. The ability to image the living human brain with various neuroimaging modalities has greatly enhanced our ability to study these brain gut interactions in health and disease. Reflex responses within the brain-gut axis, mediated by lamina I and vagal afferents, and efferents of the autonomic nervous system, play a crucial role in the maintenance of homeostasis during physiological perturbations caused by food intake, contractile activity, and metabolic products of the enteric flora. The insular cortex plays an important role in the conscious perception of all sensations arising from the body, while the dorsal anterior cingulate cortex (dacc), with its connection to effector systems, mediates the affective response and motivational drive. The magnitude and gain of these processes is highly influenced by central arousal systems and top-down corticolimbic modulation, mediating the effect of environmental context, emotions, cognitions, and memories on perception and gut function. The majority of neuroimaging studies of the brain gut axis in humans are consistent with the model of parallel processing of afferent information in insula and dacc. Newer hypothesis driven studies, studying the differential contributions of afferent input, central arousal systems, and cortico-limbic pontine interactions have greatly contributed to our understanding of brain gut interactions in health and disease. fmri mediated detection of changes in the activity and connectivity of brain regions involved in these different processes by pharmacological and behavioral therapies holds great promise for the development of novel approaches to functional GI disorders. Glossary Homeostatic Afferents The term refers to small-diameter sensory afferent fibers terminating in lamina I of the spinal cord that innervate all of the tissues and organs of the body, including the viscera, skin, muscle, joint, and teeth. All of these fibers signal changes in the physiological condition of the body and provide the essential sensory input that is crucial for the autonomic responses that maintain homeostasis. 1 Homeostatic Afferent Processing Network The term refers to a brain network that is consistently activated in response to homeostatic afferent fiber activation. This network includes the insular and dacc, thalamic nuclei (MDvc, VMb, VMpo), and the parabrachial nucleus (PBN). Since non-painful and painful visceral and somatic stimuli, as well as emotional stimuli, can activate this network, the term pain matrix commonly used to describe these regions in the literature no longer seems appropriate. Feelings Several specific meanings are used for this word in common usage. First, we experience different feelings from our bodies including satiety, abdominal pain, and discomfort which represent afferent sensory input from receptors and can be regarded as sensations (eg, visceral sensations). Second, we experience feelings associated with our ongoing emotional condition, otherwise known as mood or affective state, such as anxiety, contentment, or irascibility. We include also what we Abbreviations used in this paper: dacc, dorsal anterior cingulate cortex; DNIC, diffuse noxious inhibitory control; FD, functional dyspepsia; fmri, functional magnetic resonance imaging; IGLEs, intraganglionic laminar endings; LCC, locus coeruleus complex; MDvc, ventral caudal portion of the medial dorsal nucleus (of the thalamus); NCF, nucleus cuneiformis; NTS, nucleus tractus solitarius; pacc, perigenual cingulate cortex; PAG, periaqueductal gray; PBN, parabrachial nucleus; PET, positron emission tomography; PFC, prefrontal cortex; VMb, basal part of the ventral medial nucleus; VMpo, posterior part of the ventral medial nucleus (of the thalamus) by the AGA Institute /06/$32.00 doi: /j.gastro

2 1926 MAYER ET AL GASTROENTEROLOGY Vol. 131, No. 6 regard as homeostatic feelings, such as chilliness, achiness, or burning pain, that represent our physical condition. Third, we experience feelings associated with strong emotions elicited by social or environmental conditions, such as anger, sadness, happiness, and so on, and the evaluation of such conditions. These feelings in particular represent the awareness of our behavioral condition. The subjective experience of all of these types of feelings is completely dependent on self-awareness in humans. 2,3 Emotion The term is used to describe a neurobehavioral state adapted for the attainment of a particular goal or the resolution of particular conditions as described by Rolls. 4 It is characterized in humans as a motivation accompanied by a characteristic feeling and autonomic sequelae. Emotional behavior may occur without awareness or without a concomitant feeling, as during unconscious emotional actions, or as in animals that do not display self-awareness. Emotions are viewed as ongoing and continuously varying events. Homeostatic Emotions We define these as the motivations and feelings that are associated with changes in the body s physiological condition and with the autonomic responses and behaviors that occur in order to restore an optimal balance. 2 For example, if your body is hypoglycemic, you feel hunger and you are motivated to eat. Homeostatic emotions are the background emotions that affect our energy level, our mood, and our disposition. Spinal and vagal visceral afferent input to the central nervous system plays an important role in the generation of such emotions. I. Introduction Bidirectional brain-gut interactions play an important role in the regulation of many vital functions in health and disease. In health, brain-gut interactions play a crucial role in the regulation of digestive processes (including the regulation of food intake and bowel movements), in the modulation of the gut-associated immune system, and in the coordination of the overall physical and emotional state of the organism with digestive processes (reviewed in Mayer and Saper ). Altered brain-gut interactions are likely to underlie symptom generation in several functional GI disorders, including irritable bowel syndrome (IBS), functional dyspepsia (FD), non-cardiac chest pain, and cyclical vomiting syndrome. Although less well characterized, alterations in brain-gut interactions may also be involved in the modulation of disease activity in inflammatory bowel disease (reviewed in Mayer ) and in the pathophysiology of various eating disorders. 7 Although the term brain-gut axis provides a general construct to explain how alterations in perception and in the autonomic regulation of the gut contribute to the cardinal symptoms of abdominal pain and discomfort reported by functional GI patients, the concept does not provide a comprehensive framework to understand other characteristic features of these disorders, such as the overlap between different functional GI syndromes, 8 and the overlap between functional GI syndromes with other visceral, somatic, and affective disorders (reviewed in Wesseley et al ). A broader framework is also needed to interpret the consistent widespread activations seen in brain imaging studies of visceral pain (in particular insula, dacc, and prefrontal cortex [PFC] regions), regardless of experimental paradigm, as well as in studies using somatic or emotional stimuli. Finally, there is a need for more hypothesis driven experimental designs to deconstruct brain circuits activated by visceral afferent stimuli into afferent processing networks, as well as overlapping networks activated by cognitive, emotional, and arousal components of the experimental setting. Such identification of specific brain circuits is the prerequisite for evaluation of pharmacologic and behavioral therapies using imaging technologies (for details, see part II). In the current review, we will discuss brain-gut interactions in health and disease based on the concept of homeostatic reflex regulation, the conscious perception of homeostatic responses, and the possible alteration of both processes in functional GI disorders. We will provide an overview of the functional neuroanatomical basis for this concept derived from preclinical research, and we will review published neuroimaging studies performed in humans which are consistent with many of the functional neuroanatomical concepts laid out in the first part of this review. Finally, we will discuss implications of this conceptualization for the development and evaluation of novel therapeutic approaches to functional GI disorders. The Concept of Homeostatic Emotions It has traditionally been taught that the gut and the skin differ starkly in their relationship with the brain. 10 This seems intuitively acceptable, because there are major differences: the gut and the skin certainly feel different; we can localize sensations on our skin, but we have difficulty pinpointing sensations from our viscera and generally refer them to somatic dermatomes; we have voluntary control over our limbs, but very little control over our gut. Nevertheless, recent neurobiological evidence suggests that there is a common pattern in the neural connections of all tissues of the body gut, skin, muscle, joint, teeth, and so on that is evolutionarily related to the primal need for the brain to maintain the integrity of the entire body. This process is called homeostasis. The pathways for homeostatic control of the body are evolutionarily ancient. Each of the various tissues of the body generates sensory inputs that signify changes in physiological conditions, and the brain integrates these signals to maintain a homeostatic balance of the entire body that optimizes the use of energy for survival. We humans uniquely experience feelings from each of the tissues of our bodies because evolution has produced brain mechanisms for the conscious perception of an image of the physiological condition of our bodies (ie, how you feel ). In the healthy person, and in the absence of arousal and attentional focus on a particular feeling (such focus does occur with painful stimulus intensities), these feelings and sensations contribute to background emotions 3 but do not distract from constant cognitive demands on the individual. In contrast, in the functional GI patients, there is an alteration in the attentional, perceptual, and affective response to such feelings from the digestive tract, typified by such expressions as I feel constipated, I feel bloated, I don t feel right. The classical term interoception, formerly used only to refer to visceral sensation, has recently been redefined, based on these new findings, to refer to the sense of the physiological condition of the body. The new neuroanatomical findings indicate that all of the feelings from our bodies reflect its physiological condition and

3 December 2006 THE BRAIN-GUT AXIS 1927 can be viewed as homeostatic emotions 2 or background emotions. 3 This includes such typical visceral sensations as hunger, thirst, vasomotor flush, satiation, fullness, and urgency, but also a range of other sensations such as temperature, itch, muscle ache, and pain. Like all emotions, these comprise both a feeling (sensation) and a motivation (affect). This is particularly relevant in the context of functional GI disorders, where symptoms are not limited to discomfort and pain attributed to the viscera, but frequently include a whole range of other feelings of physical and emotion discomfort. 9 The phylogenetically new pathways in human and primate brains generate both of these aspects directly, resulting in forebrain activation patterns that resemble the activation pattern common to all human emotions (that is, activation of the anterior insula and dacc). The redefined concept of interoception emerges directly from the functional anatomical data summarized below. This concept is strongly reminiscent of the natural philosophical concept of Gemeingefühl ( common sensation ) in the German literature of the 1800s, of the sense of the material me described by Sherrington in 1900, and of the idea that human emotions are based on feelings from the body, which is the essence of the James-Lange theory of emotion and its recent reformulation by Damasio. 3 In order to understand how these findings relate feelings from the gut to homeostasis and how homeostatic regulation controls feelings from the gut, the nature of homeostasis and homeostatic emotions must first be described. Homeostatic Emotions Drive Behavior The interplay between the feeling and the motivation of a homeostatic emotion has been studied in great detail with thermal stimuli, 1,2,11 14 but can easily be adapted to the stimuli relevant to the digestive system. For example, the affect (pleasantness, unpleasantness) experienced in connection with food stimuli is the perceptual correlate of obligatory motivations. In the prolonged absence of food and water needed to maintain metabolic balance, a person will experience increasing discomfort (even hunger pains). If kept in this situation long enough, the discomfort grows until it becomes an intractable motivation. When the person is finally able to eat and drink, it feels pleasant, and he/she is motivated to eat and drink until no longer hungry. Just as a cool stimulus feels wonderful if you are hot, but unpleasant if you are cold, so too, under opposing conditions of hunger or satiety, the affect generated by the availability of food and water strikingly inverts thus, a large intake of food feels wonderful if you are hungry, but the same meal can produce an unpleasant feeling of nausea if you are already full. In the same way, eating salt or sugar is pleasant (and thus motivated) if the body needs it, but unpleasant if it doesn t (so-called stimulus-specific satiety 4 ). In other words, the specific affective feeling that we perceive reflects the metabolic (homeostatic) behavioral motivation that originates from the body s needs. Sensory inputs that relate the body s physiological condition, or homeostatic afferents, drive the homeostatic mechanisms that promote survival. The primordial means of regulating hunger, thirst, abdominal fullness, and the evacuation of waste products (stool and urine) in all vertebrates is motivated behavior, so the pathways that guide homeostatic sensory input to motivational processes must be ancient. Such behavioral motivation (and the accompanying autonomic changes) is generated in non-humanoid mammals by a signal to the forebrain (ie, the behavioral controller) from the main homeostatic afferent integration site in the brainstem, the parabrachial nucleus (PBN). In humans, evolution produced a new direct (spinothalamo-cortical) pathway to the forebrain that surmounts the primal pathway and which generates both an affective motivation and a sensation in the limbic motor and sensory cortices (Figure 1A). The basic homeostatic (interoceptive) feelings, or modalities, not only include hunger, thirst, stomach cramps, fullness, and rectal urgency, but all feelings from the body such as temperature, itch, and muscle ache. So, consistent with the view that an emotion in humans consists of both a sensation and a motivation with direct autonomic sequelae, 4,15,16 these feelings are the human percepts of distinct homeostatic emotions that directly relate the body s needs. As will be discussed below, the concept of homeostatic emotions, regardless of the valence of the emotion has direct relevance to the interpretation of findings in human neuroimaging studies of visceral stimuli. It is consistent with the reported activation of similar brain regions (insula, dacc) by a variety of both pleasant and unpleasant stimuli (reviewed in Vogt ). Pain and Other Feelings From the Body as Aspects of Homeostasis Pain, whether from the skin or from the gut, is often regarded as a distinct feeling, but from the perspective laid out above, pain can be viewed as another homeostatic feeling. It has characteristics exactly comparable to other feelings from the body. Pain normally originates with a change in the condition of tissues, a physiological imbalance that automatic (subconscious) homeostatic systems alone cannot rectify. It comprises both a sensation and an affective behavioral drive with accompanying autonomic adjustments. Depending upon conditions, pain can be unpleasant (as usual) or pleasant (such as when it relieves an intense itch). Pain also generates characteristic reflexive motor patterns, as do itch, hunger, and experimental gut stimuli. The behavioral motivation of pain is normally correlated with the intensity of the sensory input, but this can vary under different behavioral, autonomic, and emotional conditions, so that pain can become intolerable or it can disappear, similar to any other homeostatic emotion (eg, hunger). The modulation of the motivational aspect of a homeostatic emotion like pain can occur via inhibitory or excitatory prefrontal influences on brain circuits involving the dacc However, unremitting pain that outlasts its homeostatic role is pathological. 21 Viewing pain and other feelings from the body as homeostatic emotions provides a ready explanation of the interactions of these feelings from the body, with other homeostatic conditions (including the level of arousal, mood, and affect) since homeostasis is an integrated, dynamic process. This conceptual perspective also provides a firm basis for explaining the interactions of pain with emotional status or attention (ie, the psychological dimension of pain), and it unifies the different conditions that can cause different types of pain from different tissues under a common homeostatic function the maintenance of the integrity of the body. All animals respond with emotional behavior (including musculoskeletal and autonomic responses) to stimuli that in humans cause a feeling of pain. 16 An example relevant to preclinical studies in visceral pain mech-

4 1928 MAYER ET AL GASTROENTEROLOGY Vol. 131, No. 6 Figure 1. Ascending projections of homeostatic afferents. (A) Organization of interoceptive pathways. Small diameter afferents that travel with sympathetic and with parasympathetic efferents provide input to lamina I and NTS, respectively. In mammals, the activity of both types of afferents is integrated in the PBN, which projects to insular cortex. In non-human and human primates, a direct projection from lamina I and from the NTS exist to ventromedial thalamic nuclei (VMpo and VMb, respectively). Neurons in these nuclei project in a topographical fashion to the mid/posterior insula. In humans, this cortical image of the homeostatic state of the organism is re-represented in the anterior insula on the same side of the brain. These re-representations provide the substrate for a subjective evaluation of interoceptive state. PBN, parabrachial nucleus. Reprinted with permission from Craig (B) Spino-thalamo-cortical system. Summary diagram illustrating the projections in primates of homeostatic afferent pathways from lamina I (spinal) and NTS (vagal) to thalamic nuclei, and the 2 cortical regions involved in the sensory (insula) and motivational (ACC) dimensions of homeostatic emotions. NTS, nucleus tractus solitarius. ACC, anterior cingulate cortex. Modified from Craig anisms is the so-called visceromotor reflex seen in response to colorectal distention in rodents. 22 The new data described in the following text reveal that noxious stimuli, whether from the viscera or from the skin, are represented in an evolutionarily ancient neural pathway that has the primary purpose of driving homeostatic mechanisms at the enteric (for gut stimuli), spinal, and brainstem levels and generating integrated behavioral motivation at the forebrain level. In primates, novel thalamocortical projections have emerged from this basic homeostatic system that provides direct pathways to encephalized cortical mechanisms for highly resolved sensations and motivations. Notably, sub-primates have only the subcortical mechanisms that drive integrated homeostatic (emotional) behavior, that is, they do not have these direct telencephalic pathways that produce an interoceptive cortical representation of the precise physiological condition of the body in humans. In humans, these novel pathways are further elaborated, re-represented, and integrated with other forebrain emotional components in the anterior insula and orbitofrontal cortex (the fronto-insular region) so that a conscious perception of the entire emotional moment is formed that is based, evolutionarily and functionally, on the body s homeostatic condition, including the state of the gastrointestinal tract (Figure 1A B). As will be discussed in the clinical part of this review, this concept has considerable implications for the study of mechanisms underlying visceral pain and autonomic dysregulation in patients with functional GI disorders, using neuroimaging approaches. In the majority of these patients (as well as patients with a wide range of other functional disorders), the primary symptoms of abdominal pain and discomfort are directly re-

5 December 2006 THE BRAIN-GUT AXIS 1929 Figure 2. Hierarchical organization of homeostatic reflex systems involving the sympathetic nervous system. (A) Homeostatic afferents that report the physiological condition of all tissues in the body, including the GI tract, terminate in lamina I of the dorsal horn. The ascending projections of these neurons provide the basis for reflex arcs at the spinal, medullary and mesencephalic levels. Limbic, paralimbic, and prefrontal centers provide modulatory influences on the gain of these reflexes. Reprinted with permission from Craig (B) Cortical modulation of homeostatic afferent input to the central nervous system. PFC regions (dorsolateral PFC [dlpfc], orbitofrontal cortex [orbfc]) modulate activity in limbic and paralimbic regions (amygdala [amy], ACC subregions, and hypothalamus [Hypoth]), which in turn regulate activity of descending inhibitory and facilitatory descending pathways through the PAG and pontomedullary nuclei. Activity in these corticolimbic pontine networks mediates the effect of cognitions and emotions on the perception of homeostatic feelings, including visceral pain and discomfort. lated to the altered sensory and affective perception of homeostatic feelings associated with the esophagus, stomach, or intestine (bloating, urgency, pain). In addition, altered homeostatic reflex responses may contribute to findings such alterations in regional gastrointestinal transit, and in reflex responses affecting gastrointestinal motility and secretion. Since rodents, the animals most commonly used in experiments to model these disorders, do not have the forebrain structures to generate the conscious emotional feelings of humans, findings obtained in such animals (using so-called pseudo-affective reflex responses) may reflect primarily the phylogenetically shared enteric, spinal and brainstem components of homeostatic pathways (eg, reflexes), but may provide less insight into the uniquely human experience of abdominal pain and discomfort. This limitation may be particularly important for disease relevant cortical modulation mediating cognitive and emotional influences on the experience of visceral sensations. It is conceivable that the relatively low yield of conventional drug discovery strategies that have heavily relied on testing in animal models to produce effective medications for functional GI disorders, may be related to this fundamental difference in the conscious experience of homeostatic feelings between humans and non-humanoid animals. II. Basic Aspects The Functional Neuroanatomy of Homeostatic Afferent Pathways Small-diameter primary afferent fibers that report the physiological status of all of tissues (including nociceptors, thermoreceptors, osmoreceptors, and metaboreceptors) terminate monosynaptically on projection neurons in lamina I of the spinal dorsal horn. Developmentally, these small-diameter afferents originate from a second wave of small dorsal root ganglion cells that emerge subsequent to the large cells that generate mechanoreceptors and proprioceptors, and their projection into the dorsal horn is temporally coordinated with the appearance of lamina I neurons. 23 The lamina I neurons originate from progenitors of interneurons in the lateral horn (the sympathetic cell column) and migrate to the top of the dorsal horn (aided by a ventromedial rotation of the entire dorsal horn) precisely at the right time to meet the incoming small

6 1930 MAYER ET AL GASTROENTEROLOGY Vol. 131, No. 6 diameter afferents. This coordinated development indicates that together the small-diameter afferents and the lamina I neurons constitute a cohesive system for homeostatic afferent activity that parallels the sympathetic nervous system. The small-diameter afferents in cranial parasympathetic nerves (eg, vagus, glossopharyngeal) terminate similarly in the medullary nucleus of the solitary tract (NTS). Notably, the small-diameter afferents report the physiological status of all tissues of the body, including viscera, muscle, as well as skin (the largest organ of the body), so that these are not simply spinal and vagal visceral afferents, 24 but rather can be called homeostatic afferents. The small-diameter afferent fibers that innervate the gut course peripherally with sympathetic and parasympathetic nerves (eg, splanchnic, pelvic, vagus), and with somatic nerves (eg, pudendal). Many innervate vessels in the mesentery and have a cardiovascular role, while others innervate the mesenteric ganglia and play a role in the communication between the central nervous system and the enteric nervous system, the virtually autonomous component of the autonomic nervous system that controls the gut. Many respond to chemical and mechanical stimuli over a broad range of thresholds and response slopes, so that they might be considered to represent a broad continuum of response properties. However, they can also be regarded as separable into different classes, including low-threshold, high-threshold (nociceptive), silent (unresponsive unless inflamed), and thermosensitive (primarily warm 25,26 ). These classes may correspond to different functional roles and different subjective feelings. Within the gut, there are also several distinct anatomical patterns of innervation, with some fibers ending within the longitudinal muscle sheets, others ending within the enteric ganglia (intraganglionic laminar endings, IGLEs), 27 and others in close proximity to enterochromaffin 28 or to mast cells. 29 Recent evidence from in vitro experiments suggests that these different terminal morphologies correspond to different patterns of physiological sensitivity. 30,31 The likelihood that distinctly different physiological classes of visceral afferents exist and activate different central pathways is supported by anatomical evidence showing that different neurons in the superficial dorsal horn are activated by renal artery occlusion and by renal vein occlusion, probably by responding selectively to renal osmoreceptors or mechanoreceptors, respectively. 32 The silent fiber type may be of particular interest with respect to clinical disorders, because such fibers from the skin (also called sleeping or mechanically insensitive ) have recently been identified as a distinct biological category that is directly responsible for the central sensitization caused by peripheral inflammation. 33 Notably, most of these findings stem from observations of afferent fibers that parallel the sympathetic system; vagal afferent fibers are less well studied, but include fibers innervating the gut that are metaboreceptive and chemoreceptive and are likely associated with the ingestive and nutritive functions of the gut. 27,34,35 Hierarchical Organization of Homeostatic Reflexes The lamina I neurons that receive the small-diameter fiber inputs have projections within the spinal cord and brainstem (Figure 2A) that clearly reveal their direct role in homeostasis. 1,2 In the spinal cord, their only major projection is to the sympathetic cell column of the thoracolumbar spinal cord, where autonomic pre-ganglionic output neurons are located. In the brainstem, they project exclusively to the recognized homeostatic integration sites (eg, caudal and rostral ventrolateral medulla, catecholamine cell groups A1-2 and A5-7, PBN, periaqueductal gray [PAG]), which also receive parasympathetic afferent activity by way of the NTS and which are heavily interconnected with the hypothalamus and amygdala. The NTS that receives small-diameter fibers from parasympathetic nerves similarly projects to all of these sites. These spinal and bulbar projections from lamina I and from the NTS provide the substrate for the hierarchical, modality-selective somato-autonomic reflexes activated by spinal small-diameter afferents that are crucial for homeostatic control of all tissues of the body. 36 The lamina I spino-bulbar projection includes all known lamina I cell types, reflecting input from all physiological classes of homeostatic afferent activity. Notably, these are not just emergency pathways, but are engaged in an ongoing basis; for example, respiration is linearly modulated by skin temperature, 37 and cardiorespiratory activity is directly affected by ongoing muscular activity. 11,38 Spino-Thalamo-Cortical Pathway in Humans Ascending lamina I afferent activity is integrated mainly in several brainstem sites (A1, PBN, PAG) in sub-primates, which then provide an integrated signal to behavioral control regions in the forebrain. In primates, however, there is a novel, additional lamina I STT projection to a specific thalamo-cortical relay nucleus (VMpo) in posterolateral thalamus, which in turn projects to a discrete portion of dorsal posterior insular cortex (Figure 1). 2 This interoceptive cortex contains modalityselective representations of all afferent activity from lamina I (ie, sympathetic afferent input) and from the NTS (ie, parasympathetic input). In monkeys, this pathway is just visible, but in humans, it is greatly enlarged. Many functional imaging studies in humans confirm its role in pain, hunger, thirst, temperature, itch, muscle sensation, sensual touch, and cardiorespiratory activity, etc. (reviewed in Craig ), and it can be regarded as primary sensory cortex for the physiological condition of the body in primates. By contrast, in sub-primates the insular cortex appears to have a primordial role in modulating brainstem homeostatic integration (in PBN and other sites) and shows convergent, nonselective responses to homeostatic afferent inputs. The ascending afferent lamina I pathway in primates and humans also provides a direct thalamo-cortical pathway (by way of MDvc in medial thalamus) that activates the dacc (Figure 1). In sub-primates, the dacc receives only integrated homeostatic input from the brainstem. 2,39 On the basis of functional imaging and lesion studies in humans, the dacc can be directly associated with the affective aspect of visceral and somatic pain (unpleasantness as well as pleasantness), and with volition, behavioral motivation, and autonomic responses. 2,19,39 41 Its interconnections with PFC regions, insular, and ventral striatal regions, along with its strong descending projections to the brainstem, particularly the PAG, strongly support the idea that it can be regarded generally as the limbic behavioral motor cortex, just as the insula can be regarded as the limbic sensory cortex. 17,42

7 December 2006 THE BRAIN-GUT AXIS 1931 The VMpo also has a collateral projection to a portion of sensorimotor cortex (area 3a) that is intercalated between the primary somatosensory area and the primary motor area. Similarly, vagal afferent activation occurs in the lateral portion of area 3a of the primary sensorimotor region by way of the NTS and VMb. These projections can be associated with cortical control of the reflex actions of skeletal muscle in response to homeostatic afferent inputs, a role subsumed under the term viscero-somatic integration. The forebrain interoceptive representation of the body s condition in the dorsal posterior insula is successively rerepresented in the middle insula and then in the right (nondominant) anterior insula. 2 Functional imaging data show that the right anterior insula is associated with subjective awareness of homeostatic emotions (eg, visceral and somatic pain, temperature, sexual arousal, hunger, and thirst) as well as all emotions (eg, anger, fear, disgust, sadness, happiness, trust, love, empathy, social exclusion). This region is intimately interconnected with the dacc, which is co-active in such studies. Thus, the meta-representation of the state of the body in the right anterior insula can be regarded as an image of the material self as a feeling (sentient) entity that engenders emotional awareness, consistent with the James- Lange theory of emotion and with the somatic marker hypothesis of Damasio. 3 Thus, viewing feelings from the body as homeostatic emotions enables neuroanatomical explanations of interactions with other homeostatic functions and with emotions at the level of the forebrain. Finally, recent evidence showing asymmetry in the sympathetic and parasympathetic afferent re-representations and control mechanisms in the left and right insula and ACC seems to accord with the psychophysiological evidence for the forebrain asymmetry of positive and negative emotions, suggesting that natural homeostatic means for modulation of stressrelated exacerbation of painful syndromes, such as relaxed, slow breathing, may be based on the neuroanatomy of homeostatic emotions. 43 Figure 3. Schematic illustration of different, overlapping brain networks mediating the effects of cognitions and emotions on the perception of homeostatic feelings, including visceral pain and discomfort. Differential dysregulations of one or several of these networks could result in altered perception, even in the presence of normal visceral afferent input to the brain. Descending Modulation of Homeostatic Reflexes and Feelings Lamina I neurons not only receive input via somatic and visceral afferents, but also receive descending (facilitatory and inhibitory) modulation directly from brainstem pre-autonomic sources, including serotonergic nuclei within the rostral ventral medulla and the noradrenergic pontine locus coeruleus complex (LCC; Figure 2); indeed, lamina I and the spinal autonomic columns are the only regions in the spinal cord that receive descending modulation from the hypothalamus. Thus, the activity of lamina I neurons that ultimately produces the various feelings from the body is modulated by various tonic and phasically active, descending inhibitory, and facilitatory pathways whose primary purpose is control of homeostatic integration. Recent evidence (in rats) suggests that the gain of the spino-bulbo-spinal reflex loops originating from neurokinin 1-receptor-containing lamina I neurons is not constant but can be modulated by peripheral primary afferent inputs from inflamed tissue or damaged nerves, 47 adapting the homeostatic response to the overall state of the organism. Thus, since the descending forebrain control of these pre-autonomic brainstem regions originates in brain networks associated with attention, emotion generation (ie, the limbic system), and emotion regulation (PFC), these anatomic connections provide the basis for corticolimbic modulation of the afferent activity associated with feelings from the body, including pain, at the spinal level as well as at the brainstem and forebrain levels (Figure 2). III. Clinical Aspects Visceral pain and discomfort, including hunger, fullness, early satiety, nausea, bloating, or abdominal pain in humans is a subjective, conscious experience, that results from the modulation of homeostatic feelings by cognitive (attention, expectation), emotional (arousal, anxiety), and motivational factors, as well as memories of past experiences. Thus, the conscious experience is an image of the homeostatic state of the body represented in the insular cortex (Figure 1) and is further modified by these cortical and limbic inputs. In principal, altered perception of visceral stimuli could result from activity changes in visceral afferent signal processing areas alone (reflecting increased visceral afferent input to the brain from the gut), from alterations in distinct but overlapping pain modulation circuits ( central pain amplification ), or from variable combinations of these 2 overlapping circuitries (Figure 3). Activation of Regions Involved in Homeostatic Emotions in Studies of the Human Brain-Gut Axis Following the initial publication of a manuscript describing brain responses to expected and delivered rectal distention in humans in 1997, 51 there has been a series of studies describing brain regions activated during brief, experimental, visceral stimulation. The majority of these studies to date have used distention of the rectosigmoid colon, esophagus, and stomach. In addition, a few studies have been reported using

8 1932 MAYER ET AL GASTROENTEROLOGY Vol. 131, No. 6 chemical-induced discomfort in the esophagus. 52,53 Derbyshire, reviewing 15 relevant articles with 21 independent study samples 51,52,54 66 published a comprehensive review of these studies up to May of The studies included positron emission tomography (PET) and functional magnetic resonance imaging (fmri) assessments of an active stimulation condition compared with a non-distention or reduced stimulation baseline condition. Overall, the regions reported as being activated by the experimental visceral stimuli were comparable with those reported in studies of noxious somatic stimulation Consistent with the functional neuroanatomy of lamina I afferents outlined earlier, the review identified the insular cortex as the single most consistently activated brain region. In addition, a majority of studies also reported activation of cortical regions including the dacc, the primary sensory cortex, and PFC regions. Although activation of these areas were found for both upper and lower GI stimulation, lower GI stimulation was more consistently associated with activation of more rostral portions of the dacc that have direct connections with limbic and brainstem structures (including the PAG and amygdala) involved in autonomic regulation, while upper GI stimulation (primarily of the esophagus) was more consistently associated with activation of areas involved in sensory and motor processing including more posterior aspects of the dacc (eg, the midcingulate cortex). Two studies have looked at sex differences in response to rectal stimuli in healthy subjects. Kern et al studied brain responses in 28 healthy control subjects (15 women) to aversive rectal distention using fmri. 66 Subjects received individualized distention intensities that were either below (subliminal) or just above the perception threshold. In both sexes, increasing stimulus intensity was associated with increases in brain activation. Volume of cortical activity during distention was significantly greater in women at all distention levels. Men showed localized clusters of fmri activity primarily in the sensory motor cortex and parieto-occipital regions, whereas women also showed activity in the insular cortex, dacc, and PFC regions. Berman et al 71 studied brain responses in 13 healthy adults (6 women) during 15 s of cued rectal distention at 2 pressures: 25 mm Hg (uncomfortable), and 45 mm Hg (mild pain), as well as during an expectation condition (no distention). The 45-mm Hg pressure significantly activated the insula and dacc in both sexes. When the number of activated voxels, number of deactivated voxels, and ratio of deactivated voxels to total voxels affected were assessed via random effects mixed-model analyses combining subject data at the region level, greater insula activation in men compared with women was seen during the expectation condition and during the 25 mm Hg distention. In contrast, greater deactivations in women were seen in the amygdala (25 mm Hg distention) and midcingulate (45 mm Hg distention). Women had a significantly higher proportion of deactivated voxels than men in all 4 subcortical structures during the 25 mm Hg distention. In summary, studies published during the past 5 years using widely different experimental paradigms have confirmed the consistent activation of the central homeostatic afferent processing network consistent with the robust activation of visceral afferent pathways despite varying experimental paradigms and analysis techniques. 53,72 76 By contrast, the variable activations of PFC and limbic regions, and the seemingly contradictory results on sex differences in reported studies is reflective of the fact that experimental paradigms and analysis techniques varied, and that the majority of these studies did not take into account cognitive and emotional aspects of pain modulation. Similarities and Differences in Brain Responses to Visceral and Somatic Pain Stimuli Consistent with the concept of homeostatic emotions generated by visceral and somatic afferent stimulation, similar activation patterns of anterior insula and dacc in response to somatic (generally cutaneous heat or pressure) and visceral stimuli (balloon distention, acid perfusion) have been reported, even though some differences in the relative amounts of activation and location of activations within these regions was observed. 67 Several studies have directly compared the brain s response to visceral and somatic pain stimuli. For example, Bushnell s laboratory has examined the perceptual and central nervous system responses to visceral and cutaneous painful stimuli matched in terms of perceived intensity within the same chest dermatome The authors found that the visceral, mechanical stimulus was experienced as more unpleasant, diffuse, and variable than cutaneous thermal pain of similar intensity, independent of the duration of the stimulus. 77 The cutaneous heat pain evoked higher activations in the bilateral anterior insular cortex and ventrolateral PFC. However, pain in the same dermatome from distention of the esophagus was associated with activation of bilateral inferior primary somatosensory cortex, bilateral primary motor cortex and a more rostral region within the dacc. Evaluating differential brain responses to visceral and somatic stimuli of the lower body, Aziz s laboratory found similar brain activation to visceral (rectal) and somatic (anal) distention, although a greater activation of motor cortex by the somatic stimulus was observed. 61 Tracey s group used fmri scanning of the brain to compare the cortical processing of aversive visceral (rectal distention) and somatic heat stimuli in 10 healthy control subjects. 80 Similar to the findings in the chest, the relative unpleasantness of the subjective experience of the visceral mechanical stimuli was higher than that of the somatic thermal stimuli. Controlling for unpleasantness, similar activations were found for the 2 stimulation types including insula, dacc, and secondary motor area. Visceral stimuli were associated with deactivation of the perigenual cingulate cortex (pacc); a finding also reported in somatic pain studies, 81 with a relatively greater activation of the right anterior insula. Somatic (but not visceral) pain was associated with left dorsolateral PFC, a region concerned with cognitive processes. In a follow-up study, 82 the authors compared brainstem responses to intensity-matched rectal or cutaneous abdominal electrical stimuli. Similar significant activation associated with both stimuli was observed in several brainstem regions including the PAG, the PBN, the LCC, and the nucleus cuneiformis. However, 2 regions showed greater responses during the visceral pain condition: a significantly greater activation of a region that the authors identified as the nucleus cuneiformis (perhaps signifying activity in the PBN described above) and a significant correlation of the right PAG with anxiety ratings. The authors concluded from these findings that the observed differences might represent a greater nocifensive response and a greater emotive salience of visceral pain.

9 December 2006 THE BRAIN-GUT AXIS 1933 Two studies have looked at differences in central processing of somatic and visceral experimental stimuli in IBS patients. These studies follow from a series of psychophysiological studies showing increased perception of visceral stimulation in IBS, 83,84 but less consistent findings regarding IBS sensitivity to noxious somatic stimuli. However, depending on the somatic pain stimulus used, different investigators have reported normal, reduced, 89 and enhanced 90 perceptual responses to somatic pain stimuli. One of the two imaging studies done to date comparing visceral and somatic stimuli in IBS used thermal pain 90 and the other cutaneous pressure. 89 Verne et al studied brain responses with fmri to phasic aversive rectal distention and to tonic cutaneous heat applied to the foot in 9 IBS patients and in a group of healthy age- and sex-matched controls. 90 Both types of nociceptive stimuli evoked greater neural activity in brain regions of patients compared with controls in dacc and insula, as well as in thalamus and somatosensory regions and in the PFC. Enhanced brain responses to both types of stimuli were observed within the same brain structures. The authors interpreted these findings as supporting their hypothesis that visceral and cutaneous hypersensitivity in IBS patients is related to increased afferent processing in ascending pathways, rather than to altered cognitive and/or emotional modulation at higher brain levels. A somewhat different conclusion was reached from a study in female patients with IBS with (n 10) and without (n 10) a comorbid diagnosis of fibromyalgia. 91 Brain responses to somatic pressure and to aversive rectal distention were evaluated with H 2 15 O-PET. The somatic stimulus was perceived as less aversive than the visceral stimulus by the IBS patients, while IBS fibromyalgia patients rated both stimuli as equally aversive. Group differences in regional brain activation were only observed within the dacc (not in the insula), where IBS patients showed a greater response to visceral stimuli and IBS fibromyalgia patients showed a greater response to somatic stimuli. The authors concluded from their findings that chronic stimulus-specific enhancement of dacc responses to sensory stimuli in both syndromes may be associated with cognitive enhancement of either visceral (IBS) or somatic (IBS fibromyalgia) sensory input. The fact that no group differences were observed in primary sensory areas (thalamus, somatosensory cortex, insula) is consistent with the concept that afferent input that reaches the brain is not different between the 2 patient populations, while arousal and attentional mechanisms may differ. In summary, a growing number of brain imaging studies have addressed the question of how brain responses to somatic and visceral pain stimuli may differ, both in healthy control subjects, as well as in patients with IBS. Consistently, greater subjective affective rating of visceral pain stimuli (in terms of unpleasantness), and activation of the homeostatic afferent network by both types of stimuli has been observed in most studies. In contrast, a consistent difference in brain processing of visceral and somatic stimuli has not emerged. For example, evidence for the expected greater activation of limbic and paralimbic brain regions, in particular the dacc for visceral stimuli, or differences in arousal and antinociceptive mechanisms between visceral and somatic stimuli have not been reported. This lack of consistency may be due in large part to differences in study design (imaging modality, study paradigms, nature of stimuli used, previous exposure of subjects to similar stimuli, sex of participants, etc.) and in the relatively small number of studies that have directly compared the 2 modalities. Based on the concept of homeostatic emotions presented earlier, one would expect that visceral stimuli of matched intensity may be associated with greater unpleasantness and motivational drive, a greater activation of dacc, and greater autonomic responses, compared with somatic stimuli. Modulation of Homeostatic Afferent Processing Network by Cognitive and Emotional Stimuli The brain has multiple ways to modulate the perception of afferent information and this modulation is influenced by the environmental context, the emotional state of the individual (eg, fear, anxiety, or anger), cognitive factors (eg, expectation, attention), or memories of previous sensory events (conditioned responses) (Figure 2B). As outlined in the basic section of this review, these top-down influences can modulate homeostatic afferent input to the central nervous system at multiple levels (Figure 2A). Considerable progress has been made both on a preclinical and, more recently, on a clinical level to identify brain regions, circuits, and mechanisms that play a role in the facilitation and inhibition of the subjective pain experience. 46,92 Both clinical and preclinical studies support a role for right orbitofrontal and right ventrolateral PFC in mediating inhibition of emotional and pain responses (reviewed in Lieberman et al ) and for dorsolateral PFC in pain modulation. 18,93 Emotional Modulation Phillips et al 94 studied the effects of emotional context on responses to nonpainful esophageal distention in 8 healthy subjects (7 males) on brain activation using fmri. Activation within the right anterior insular cortex and bilateral dacc by the visceral stimulus was significantly greater while simultaneously viewing fearful faces compared with neutral faces. In a second paradigm, subjective discomfort and brain responses in another 8 healthy male subjects during the same esophageal stimulus were studied while viewing faces with low, moderate, or high intensity of fear expression. During the high intensity fearful visual stimulus, significantly greater discomfort, anxiety, and greater brain activation in the left dacc and bilateral anterior insular cortex was seen. These findings clearly demonstrated the powerful effect of emotional context on the perceptual, emotional, and brain response to an innocuous visceral stimulus. Specifically, they demonstrated clearly the modulatory role of emotion regulation circuits on the homeostatic afferent processing network. Cognitive Modulation Attention. Aziz s group 95 examined the modulatory role of attention on the brain responses to nonpainful visceral (esophageal) distention in 7 healthy volunteers (6 males). Brain responses to phasic visual and esophageal stimuli were presented simultaneously while subjects were asked to focus their attention on either the esophageal or the visual stimulus (selective attention) or both (divided attention). Selective attention on the esophageal stimulus was associated with activation of sensory (somatosensory cortex) and cognitive (dacc) networks, while selective attention on the visual stimulus activated the visual cortex. During the divided attention task, more brain regions in the sensory and cognitive domains were activated to

10 1934 MAYER ET AL GASTROENTEROLOGY Vol. 131, No. 6 Figure 4. Central arousal networks and visceral hypersensitivity. Normalization of discomfort visual analogue scale ratings (left upper graph), and discomfort threshold (right upper graph) in 24 IBS patients undergoing experimental rectal balloon distention every 3 months over a 12-month period. Brain images obtained in a subset of 12 individuals during the initial and the fourth experimental session showed decreases in central arousal circuits involving the mid-cingulate cortex (MCC), amygdala (not shown), and dorsal pons (including LCC region). Similar decreases were observed during an expectation condition and during the actual distention. Reprinted with permission from Naliboff et al. 101 process esophageal stimuli, in comparison to those processing visual stimuli. These findings emphasize the importance of attentional processes in the modulation of sensory information from the body and the relative biological importance placed on visceral sensation, compared with other sensory modalities. Expectation. A variety of studies in the somatic pain literature have evaluated brain responses to the expectation of an aversive stimulus. 59,75,81,96,97 These studies suggest that the brain can either up-regulate or down-regulate sensory and limbic brain regions based on previous experience, familiarity with the stimulus and expected intensity. 81 Several early studies have shown some evidence of activation of the homeostatic afferent network during conditioned anticipation of a visceral stimulus. 59,75 In a preliminary report, Berman et al studied the brain fmri BOLD response to anticipated (cue condition) and delivered mild and moderate rectal distention in 12 healthy women and 14 female IBS patients. 98 Distention increased activity in the homeostatic brain regions and decreased activity in the infragenual cingulate. As in previous studies comparing IBS patients and control subjects, the increases were more extensive in the IBS patients, with significant differences in midcingulate and dorsal brainstem. During cued anticipation of distention, activity decreased in the insula, dacc, amygdala, and dorsal brainstem in healthy women, but not in IBS patients, consistent with a top-down modulation of homeostatic afferent networks by cortical regions. Three self-rated measures of negative affect during scanning were higher in IBS patients than healthy women (P.001), and the anticipatory BOLD decreases in bilateral dorsal brainstem, centered in the pontine LCC, were inversely correlated with all 3 measures. The amplitude of anticipatory decrease in the LCC was associated with greater activation by subsequent distention in right orbitofrontal cortex and bilateral supragenual ACC regions previously implicated in cognitive coping. These findings suggest that in healthy women, the brain decreases activity within the homeostatic brain matrix in expectation of a certain, inescapable pelvic pain stimulus. A failure to generate this down-regulation in IBS patients may be related to differences in cortically mediated coping styles, emotional factors, and linked arousal systems. Yaguez et al 99 studied brain responses during different phases of visceral aversive conditioning in 8 healthy volunteers (5 males) using fmri. The authors used a classical conditioning paradigm in which different colored circles were used as conditioned stimuli and were paired with painful esophageal distention (learning phase), airpuff to the wrist, or nothing. Brain responses during the learning phase (delivery of aversive esophageal distention) were seen in the homeostatic brain matrix and in somatosensory cortex. During the anticipation and extinction phase of the paradigm, brain activity resembled that seen during actual esophageal distention, including activation in insula and dacc. These findings emphasize the importance of cognitive influences, such as expectation and memory recall in top-down modulation of brain regions involved in the processing of homeostatic information from the body. Hypervigilance. Several lines of evidence indicate that IBS patients and other functional disorders have hypervigilance for symptom-relevant sensations. 100 Repeated exposure to experimental visceral stimuli can lead to decreased hypervigilance and, therefore, discomfort. In a longitudinal study of IBS patients exposed to 6 sessions of rectal inflations over a 1-year period, we examined regional cerebral blood flow to the inflations and anticipation of inflations using H 15 2 O-PET at the first and last session. 101 As shown in Figure 4, subjective ratings of the rectal inflations normalized over the 12 months

11 December 2006 THE BRAIN-GUT AXIS 1935 of the study, while IBS symptom severity did not, indicating decreased vigilance independent of changes in perceived disease activity. In response to rectal distention, stable activation of regions of the homeostatic afferent network (including thalamus and anterior insula) was observed over the 12-month period, while activity in limbic, paralimbic, and pontine regions decreased. During the anticipation condition, there were significant decreases in dacc, amygdala, and dorsal brainstem (perhaps involving the LCC) activation at 12 months. One way to interpret these findings is that brain regions processing the feeling and the motivational dimension of the homeostatic emotion were affected differentially by the habituation process: while insula activation remained constant, dacc activation progressively decreased. An analysis examining the covariation of these brain regions, as well as preliminary results from an effective connectivity modeling approach to the data, 102 supported the hypothesis of changes in an arousal network including limbic, pontine, and cortical areas underlying the decreased perception seen over the multiple stimulation studies. In summary, brain imaging studies of cognitive and affective modulation of perceptual and brain responses to visceral stimuli strongly support the concept of a homeostatic afferent network in the brain. The fact that components of this network can be modulated differentially by top-down corticolimbic influences has important implications for a better understanding of symptom generation/modulation in functional GI disorders. Figure 5. Role of corticolimbic inhibitory network in pain modulation. Medial frontal cortical (FC) mediation of the impact of right lateral FC on the PAG. The dotted line indicates that the relationship between right lateral PFC [RLPF] and the PAG is indirect. Unstandardized coefficients with associated standard errors in parentheses are reported. The coefficients in brackets indicate the direct effect of RLFC on PAG prior to the inclusion of the mediating medial FC. ** indicates P.01. FC, frontal cortex. PAG, periaqueductal gray. RLFC, right lateral frontal cortex. Reprinted with permission from Mayer et al. 109 Activation of Brain Networks Associated With Descending Pain Modulation Since the beginning of the 20 th century it has been known that the brain can tonically inhibit spinal cord excitability, thereby regulating the amount of peripheral sensory information reaching the central nervous system. More recent evidence has demonstrated the activity of both pain inhibitory and facilitatory mechanisms that can tonically and phasically regulate spinal cord excitability While top-down tonic pain inhibitory modulation appears to predominate in healthy individuals during basal conditions, an up-regulation of descending pain facilitatory systems has been demonstrated in the maintenance of hyperalgesia in animal models of peripheral nerve injury. 47 An alteration in the balance between inhibitory and facilitatory pain modulatory systems has been proposed as a possible mechanism underlying chronic pain syndromes such as fibromyalgia 103 and IBS. 104,105 Zambreanu et al were the first to demonstrate the activation of brainstem regions in the context of central sensitization in healthy human volunteers. 106 Using 3T fmri, they compared whole brain responses, including the brainstem, to punctuate mechanical stimulation in an area of secondary hyperalgesia (induced by heat/capsaicin sensitization model) or in a control area. They found greater activation during stimulation of the hyperalgesic region in several cortical regions, including posterior insula, ACC, and posterior cingulate cortex, as well as thalamus and pons. The brainstem activation was localized to the NCF (possibly involving PBN) and the PAG, brain regions that receive input from corticolimbic networks (including the rostral ACC), send projections to the rostroventral medulla and are part of a cortico-limbic-pontine pain modulation circuit 107,108 (see also Figures 2 and 3). There is preliminary evidence to suggest that patients with IBS may also show abnormal activation of brain circuits involved in pain modulation. 74,109,110 Two studies were performed by Wilder-Smith and coworkers 74,110 to study the central correlates of heterotopic pain inhibition. In the first study, they performed an fmri study in 10 female patients with IBS (5 constipated-, 5 diarrheapredominant bowel habit) and 10 female healthy control subjects to test the hypothesis that IBS patients show abnormal activation of diffuse noxious inhibitory control (DNICs) systems in response to a noxious stimulus. 74 DNIC activation can be quantified by the perceptual modulation of a painful stimulus (in this case noxious rectal balloon distention) by a secondary heterotypically applied nociceptive stimulus (in this case ice water immersion of the foot). They found that subjective pain ratings of rectal volume distention by the heterotypic cold pain stimulus was reduced in healthy controls but not in the IBS patient group, suggesting an inadequate activation of DNICs in the IBS patients. Following the heterotypic cold stimulus, a complex set of differences in response to rectal pain was found among the controls and the 2 IBS bowel habit based sub-groups. These included decreased activation in insula, thalamus, and PAG in the control group (perhaps reflecting the DNIC process) that was absent in the IBS patients. In the second study, similar perceptual results were found and differences were also found between IBS and controls during expectation of rectal pain, without actual distention. 110 More recent brain imaging studies in patient populations provide more direct support for alterations in cortico-limbicpontine pain modulation networks in IBS patients leading to visceral hypersensitivity. Mayer et al 109 examined 3 groups of male subjects, ulcerative colitis patients with quiescent disease (n 9), patients with IBS (n 9), and healthy male controls (n 9), during actual and expected but undelivered rectal distentions using H 2 15 O-PET. This study found similar responses in all 3 groups in the homeostatic afferent network (anterior insula and dacc). However, IBS patients compared with both the ulcerative colitis and control groups showed consistently greater activation of limbic/paralimbic brain regions (amygdala, hypothalamus, ventral/rostral ACC, dorsomedial PFC) sugges-