The application of functional magnetic resonance imaging and positron-emission tomography has identified a network of brain areas that process visceral sensation from the esophagus (120), stomach (121), and the anorectum (122). Results of these studies suggest that visceral sensation is primarily represented in the secondary somatosensory cortex. Unlike somatic sensation, which has a strong homuncular representation in the primary somatosensory cortex, visceral representations in the primary somatosensory cortex are vague and diffuse (120). This might account for visceral sensation being poorly localized in comparison with somatic sensation. Nevertheless, visceral sensation is represented in paralimbic and limbic structures (e.g., anterior insular cortex, amygdala, and anterior and posterior cingulate cortex), and prefrontal and orbitofrontal cortices (123,124), areas that purportedly process the affective and cognitive components of visceral sensation. Neuroimaging data suggests that differences also exist in the cortical representation of various visceral organs and in the upper versus lower GI tract. For example, the primary sensory motor cortex shows more prominent upper gut representation and the prefrontal and orbitofrontal cortices show greater lower gut representation (125).
Differential cortical activation is also seen when comparing sensation from the visceral and somatic regions of the GI tract, for example, sensations from the esophagus versus the anterior chest wall (126) or the rectum versus the anal canal (122). Brain processing for eso-phageal and anterior chest wall sensations occurred in a common brain network consisting of secondary somatosensory and parietal cortices, thalamus, basal ganglia, and cerebellum (126). However, differential processing of sensory information from these two areas occurred within the insular, primary sensory, motor, and anterior cingulate and prefrontal cortices. This is consistent with knowledge that similarities exist for visceral and somatic pain experience and might also explain the individual's ability to distinguish between the two modalities and generate differential emotional, autonomic, and motor responses when each modality is individually stimulated.
Gender differences in cortical representation of visceral sensation also occurred among healthy volunteers. Activation in the sensory motor and parieto-occipital areas is common in both males and females following rectal distension; however, greater activation in the anterior cingulate/prefrontal cortices was found in women (127). These gender differences in the processing of sensory input substantiate reports that perceptual responses are exaggerated in female patients with chronic abdominal pain.
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