Available information about the cerebral cortical registration. Cerebral Cortical Registration of Subliminal Visceral Stimulation

Transcription

1 GASTROENTEROLOGY 2002;122: Cerebral Cortical Registration of Subliminal Visceral Stimulation MARK K. KERN and REZA SHAKER Division of Gastroenterology and Hepatology and the Digestive Disease Center, The Medical College of Wisconsin, Milwaukee, Wisconsin Background & Aims: Although brain registration of subliminal somatic stimulations such as masked visual stimuli and their influence on electrical and hemodynamic measures of cerebral activity have been reported previously, there have been no reports on cerebral cortical registration of subliminal visceral stimulation. Because studies evaluating the consequences of subliminal somatic stimulation have shown that subliminal stimulation can effect behavior, it is conceivable that such subliminal messages from the intestine could potentially influence intestinal sensory/motor function or effect the perception/interpretation of sensory signals originating from the gut. Methods: We studied the cerebral cortical functional magnetic resonance imaging (fmri) response to subliminal, liminal, and supraliminal rectal distention in healthy volunteers. Results: Study findings indicate that subliminal afferent signals originating from the gut are registered in the cerebral cortex without reaching the level of awareness. Locations of cortical activity caused by intestinal subliminal stimulation are similar to those of liminal and supraliminal stimulation but their intensity and volume are significantly lower (P < 0.05). Conclusions: Subliminal afferent signals originating from the gut are registered in the cerebral cortex and induce changes in measures of brain activity, such as hemodynamic changes detectable by fmri. Available information about the cerebral cortical registration of sensory signals originating from the gut in humans is incomplete and is generally derived from studies of perceived nonpainful and painful stimuli. 1 5 These studies of healthy subjects and patients with irritable bowel syndrome (IBD) 2,4 have shown activation of multiple cortical regions including the anterior cingulate gyrus, insular cortex, and the primary and secondary sensory motor cortices associated with perceived nonpainful and painful rectal distention. Under physiological conditions, stimulation of intestinal afferents do not normally reach the awareness level, but they may reach the brainstem and result in an unconscious triggering of gastrointestinal reflexes. However, it is not known whether these subliminal stimulations reach the cerebral cortex. Brain registration of subliminal somatic stimulations 6 9 such as masked visual stimuli and their effect on cortical function, as well as on electrical and hemodynamic measures of cortical activity 8 has been documented previously. Studies evaluating the consequences of subliminal somatic stimulations indicate that they are capable of affecting behavior. Subliminal visual presentation of abandonment stimuli, for example, results in a change in eating behavior in individuals with 14 and without 15 disordered eating habits. Because cerebral cortical registration of subliminal somatic stimuli induce changes in behavior, it is conceivable that such subliminal messages from the intestine potentially could result in consequences, such as affecting sensory/motor function of the intestine and possibly affect behavior and perception/interpretation of sensory signals originating from the gut. Two fundamental questions, therefore, need to be answered: (1) whether subliminal sensory signals from the intestine register in the cerebral cortex; and (2) does this cortical registration, if it occurs, result in any sensory/motor or behavioral consequences. The present study was undertaken to address the first of these questions. In this study, we set out to determine the cerebral cortical activity during subliminal rectal distention and compare the findings, if any, to the cortical activity induced by liminal and supraliminal stimulus intensities. Materials and Methods A total of 23 adult subjects (age, years; 8 men and 10 women) were studied. The study protocol was approved by the Human Research Review Committee of the Medical College of Wisconsin and subjects gave written informed consent before their studies. All subjects completed a detailed Abbreviations used in this paper: AFNI, Analysis of Functional Neuroimages; BA, Brodmann area; fmri, functional magnetic resource imaging; TR, repetition time by the American Gastroenterological Association /02/$35.00 doi: /gast

2 February 2002 SUBLIMINAL VISCERAL STIMULATION 291 health-related questionnaire before each study and did not have any present or previous history of gastrointestinal-related diseases. To study the brain response to rectal distention, cerebral cortical activity was monitored in all subjects using a blood oxygenation level dependent functional magnetic resource imaging (fmri) technique. Magnetic resonance imaging (MRI) echo planar and spoiled gradient recalled acquisition at steady state anatomical images were acquired in the sagittal plane for 13 contiguous slices, 10-mm in thickness, spanning the whole brain volume. MRI scanning was performed on a 1.5 Tesla General Electric Signa Scanner (GE Medical Systems, Milwaukee, WI). The scanner was equipped with a custom 3-axes head coil designed for rapid gradient field switching and a shielded, transmit/receive birdcage radio-frequency coil to acquire a time course of echo planar images across the entire brain volume with the desired slice specifications. Echo planar images resolved to pixels/slice at repetition time (TR) of 1 second and echo time of 40 milliseconds were obtained during 6 scanning sequences with a 1-minute interval between scans. Data Analysis A nonbiased method of detecting cerebral cortical regions of stimulus-related changes in oxy/deoxyhemoglobin concentration was used to correlate an idealized wave representative of the stimulus paradigm to the actual MRI-generated magnetic signals. Regions of cortical signal changes associated with rectal distention are shown graphically as color overlayed images stereotaxically mapped on the anatomical images in the Talairach-Tournoux coordinate system. 16 Correlation statistics, image registration, activity volume calculation, magnetic signal change graphics, and 3-dimensional display were facilitated by the Analysis of Functional Neuroimages (AFNI) software package written by Robert Cox of the Medical College of Wisconsin Biophysics Research Institute. 17 The AFNI software runs on a Pentium III based Linux workstation (Southwest Computers, Inc., Houston, TX). AFNI allows the user to display a 3-dimensional brick of MRI data transformed from images captured as a sequence of 2-dimensional images. In the correlation technique, a cross-correlation function is calculated between an idealized waveform that represents a theoretical cortical response to the stimulus paradigm and an experimental waveform that represents the actual, local cortical magnetic signal changes during an MRI scanning sequence. The result of this calculation is a measure of the similarity between the shape of the idealized waveform and the experimental waveform. The probability of inappropriately identifying an experimental waveform as being significantly correlated can also be calculated from the correlation analysis. The AFNI software was used to detect regions of cortical activity using the correlation technique described previously, as well as display and calculate cortical volumetric regions of correlated fmri signal change. Regions of activity were displayed as color maps overlaid on the 3-dimensional anatomical images. Our criteria for including a volume element, or voxel, as a region of cortical activity required the calculated correlation between the actual MRI time-series magnetic signal and the idealized response waveform to be greater than or equal to Furthermore, we applied the additional clustering requirement that a displayed region of correlated activity must be represented by a cluster of 3 or more contiguous correlated voxels. 18 In the present study, we used a pixel matrix for each sagittal image covering a mm field of view and a slice thickness of 10 mm to be able to include the whole cerebral cortex. Thus, one echo planar image voxel was mm 3. Using these criteria, an activated cluster must be greater than 423 mm 3 to be included in the analysis. This cluster criteria was applied to avoid including single voxel activities that may represent artifact. 19,20 To reduce magnetic signal artifact caused by subject motion, a gradient descent least squares method of image registration was used. 21 The data are expressed as mean standard error of the mean unless otherwise stated. The percent change in fmri signal intensity was calculated by determining the difference between the maximum and minimum magnetic field intensity in a correlated fmri time series and dividing this difference by the minimum intensity in that time series. The volume of cortical activity was calculated by counting the number of activated voxels and multiplying by the voxel volume. As stated, the volume of a single echo planar image voxel was 141 mm 3. The cortical mantle was divided into 4 regions designated by Brodmann area (BA) numbers and standard anatomical structures. The sensory/motor region was designated by BA 1, 2, 3, 4, 5, and 6; the anterior cingulate/prefrontal region by BA 8, 10, 24, and 32; the parietal/occipital region by BA 7, 18, 19, and 30. The insular cortex was designated as the portion of the cerebral cortex deep within the lateral fissure consisting of several long gyri paralleling the lateral fissure and 5 short gyri located more rostrally. Cortical activity volumes were calculated for each of these cortical regions and compared using analysis of variance with multiple t-testing by means of Tukey s correction. Studies were performed in 2 stages. Stage I: Evaluation of Cortical Activity During Subliminal and Supraliminal Rectal Distention Eighteen of 23 subjects participated in the following paradigm-driven rectal distention protocol, using a commercially available computer-controlled barostat (G and J Electronics, Inc., Willowdale, Ontario, Canada). A catheter-affixed polyethylene bag was positioned in the rectum before MRI scanning. The polyethylene bag was roughly cylindricalshaped with a length of 10 cm and a fully inflated diameter of 8 cm. Maximum bag volume was 500 ml and was infinitely compliant up to its distensible limit. The barostat device was kept outside of the scanner suite and was connected to the bag by a 30-foot polyethylene tube (3 mm outer diameter, 1.8 mm inner diameter). After inserting the catheter-affixed bag into the rectum, the perception threshold for each individual subject was determined. Air was incrementally pumped into the

3 292 KERN AND SHAKER GASTROENTEROLOGY Vol. 122, No. 2 rectal bag in 5 mm Hg steps and sustained for 10 seconds by means of the computer-controlled barostat. After each pressure step, the subject was asked if he or she felt anything. This step-wise procedure was continued until the subject reported feeling the inflated bag. The air in the bag was then evacuated and the step-wise perception threshold procedure was repeated 2 more times to ascertain the threshold. The barostat pressure recorded at this level was deemed to reflect the perception awareness threshold for rectal distention of that particular subject. All tested subjects reliably reproduced the same perception threshold pressure during the step-wise determination procedure. During the experimental scans, air was infused and evacuated from the rectal bag at the maximum possible flow rate of 60 ml/sec to maintain the desired constant distension pressure or nondistension (zero) pressure. Before every MRI scan, air was infused into the rectal bag until a minimal static pressure of 3 to 5 mm Hg was observed. A small quantity of air was then evacuated from the bag so that the pressure in the bag was zero relative to atmospheric pressure. Thus, the rectal bag before each scan was preloaded with air up to the volume where a nominal pressure was measured. An example of typically registered volumes and pressures follows: for a perception level pressure reading of 30 mm Hg, a bag air volume of 250 ml was registered. The subliminal distending pressure would then be 20 mm Hg, requiring a bag volume 220 ml of air, and the supraliminal pressure would be 40 mm Hg, which requires 275 ml of air. A zero pressure reading was established in the bag at a maximum volume of 175 ml of air. By starting each MRI scan with a preloaded volume of 175 ml of air in the rectal bag, the subliminal distending pressure could be achieved (or relieved) in less than 1 second, the perception level pressure in 1.3 seconds, and the supraliminal pressure in 1.7 seconds. Three barostat-controlled distention levels were tested: (1) 10 mm Hg below perception threshold (subliminal); (2) at the perception threshold (liminal); and (3) 10 mm Hg above the perception threshold (supraliminal). There was a 1-minute interval between each distention-scanning session. Subjects were asked whether or not they had any sensation after each distention scan. MRI data was acquired during six 120-second scan sessions of 20-second intervals of sustained distention alternated with 20 seconds of no distention. The order of distention level sessions was randomized in each subject. Two scans were performed at each distention level. Stage II: Validation of Cortical Activity During Subliminal Distention To test whether anticipation of a previously perceived rectal distention contributed to cortical activity during subliminal stimulation, we studied 5 of 23 subjects with the following paradigm after establishing their pressure threshold for perception awareness of balloon distention, as described previously. During a 120-second scanning session, alternating 20-second intervals of subliminal and no distentions were performed. This was followed by a scan of 120 seconds of no distention. Two scans were performed for each paradigm. Subsequent to this, a 240-second scan was obtained using perception awareness threshold (liminal) and subperception threshold (subliminal) distention pressures with varying intervals of rest between distentions. Results Cortical activity was detected for all perceived and unperceived intensities in all subjects. The distention pressure of the barostat bag at the subperception level averaged mm Hg. The distention pressure of the barostat bag at and above perception levels averaged , and mm Hg, respectively. Neither of 2 perceived distentions was reported to be associated with discomfort or pain. Stage I: Evaluation of Cortical Activity During Subliminal and Supraliminal Rectal Distention Cerebral cortical fmri activity during subliminal rectal distention. In all studied volunteers, subliminal rectal distention was associated with a detectable increase in cerebral cortical fmri activity of 3.8% 0.15% over the baseline. As seen in Table 1, cortical activity was generally bilateral and included the sensory/motor, parieto-occipital and anterior cingulate/prefrontal regions as well as the insular cortex. Comparison of cerebral cortical fmri activity during subliminal, liminal, and supraliminal rectal distention. Rectal distention to the liminal (Table 2) and supraliminal levels (Table 3) induced cerebral cortical activity in regions similar to those of subliminal stimulus (Figure 1). Inspection of the BA data in Tables 1, 2, and 3 shows the overall similarity in regions of activity caused by subliminal, liminal, and supraliminal rectal distention. However, increases in stimulus intensity resulted in increases in volume of cortical activity. In 12 of 18 subjects, these increases in volume of cortical activity resulted in representation of additional BAs in some subjects, although these areas, as seen in Tables 1, 2, and 3, might have been activated previously in other subjects during subliminal stimulus. In the remaining 6 subjects, the increase in cortical activity remained confined within the BAs observed during subliminal stimulations. To further compare the distribution of BA representation for the 3 levels of distension, a 2 contingency analysis was performed. The results of this analysis showed no significant differences in the distribution of BA representation for the 3 levels of rectal distension ( , P 1.00). Comparison of the total volume of bilateral cerebral cortical activity associated with various levels of rectal distention showed a direct relationship between

4 February 2002 SUBLIMINAL VISCERAL STIMULATION 293 Table 1. Rectal Distention to Subliminal Levels Subjects Brodmann areas I 1 B B B B B 2 B B B B B 3 B B B B B B B 4 B B B B 5 B B B B B B 6 B B B B B B B B 7 B B B B B 8 B B B B B B B 9 B B B B B B 10 B B B B B B B 11 B B B B B B B B 12 B B B B 13 B B B B B B 14 B B B B B R 15 B B B B B B 16 B B B B B L 17 B B B B B B B 18 B B L, left hemispheric activity; R, right hemispheric activity; B, bilateral activity; I, insular cortex; Numbers indicate Brodmann Areas (BA). Sensory/motor, BA 1,2,3,4,5,6; anterior cingulate/prefrontal, BA 8,10,24,32; parietal/occipital, BA 7,18,19,30. the stimulus strength and cortical activation volume (Figure 2). Analysis of variance demonstrated significant differences between the volume of activated voxels in response to the 3 levels of distention (P 0.05). Multiple pair-wise comparisons with Tukey correction demonstrated that higher levels of distention resulted in larger volumes of cortical activity (P 0.05). Moreover, the average fmri signal change also showed distention-level dependence (Figure 3). Analysis of variance showed significant differences among the average maximum percent fmri signal increase in response to subliminal, liminal, and supraliminal rectal distention (Figure 4). Multiple pair-wise comparison with Tukey correction showed that the higher the stimulus intensity the higher the percent maximum fmri signal change (P 0.05). Table 2. Rectal Distention to Liminal Levels Subjects Brodmann areas I 1 B B B B B B 2 B B B B B B B 3 B B B B B B B 4 B B B B B B B 5 B B B B B B B 6 B B B B B B B B 7 B B B B B 8 B B B B B B B 9 B B B B B B B 10 B B B B B B B B 11 B B B B B B B B 12 B B B B B 13 B B B B B B 14 B B B B B B 15 B B B B B B B 16 B B B B B B B L 17 B B B B B B B B B B B B 18 B B B B B B B L, left hemispheric activity; R, right hemispheric activity; B, bilateral activity; I, insular cortex; Numbers indicate Brodmann Areas (BA). Sensory/Motor, BA 1,2,3,4,5,6; Anterior Cingulate/Prefrontal, BA 8,10,24,32; Parietal/Occipital, BA 7,18,19,30.

5 294 KERN AND SHAKER GASTROENTEROLOGY Vol. 122, No. 2 Table 3. Rectal Distention to the Supraliminal Levels Subjects Brodmann areas I 1 B B B B B B B 2 B B B B B B B 3 B B B B B B B 4 B B B B B B B 5 B B B B B B B 6 B B B B B B B B 7 B B B B B 8 B B B B B B B B 9 B B B B B B B 10 B B B B B B B B 11 B B B B B B B B 12 B B B B B B 13 B B B B B B 14 B B B B B B 15 B B B B B B 16 B B B B B B B L 17 B B B B B B B B B B B B 18 B B B B B B B L, left hemispheric activity; R, right hemispheric activity; B, bilateral activity; I, insular cortex. Numbers indicate Brodmann Areas (BA). Sensory/motor: BA 1,2,3,4,5,6; Anterior cingulate/prefrontal: BA 8,10,24,32; Parietal/occipital: BA 7,18,19,30. Stage II: Validation of Cortical Activity During Subliminal Distention In all 5 subjects, distention of the rectal wall to 10 mm Hg below the perception threshold without a prior experience of perceived distention invariably resulted in cerebral cortical fmri activity in regions similar to those observed in stage I (Figure 5). The increase in fmri signal intensity among these 5 volunteers averaged 3.9% 0.28% over the baseline. There were no correlated signal changes during the nondistention scan periods. Subliminal distentions interspersed with supraliminal distentions with varying interdistention time intervals also induced increases in cerebral cortical activity independent of the rest intervals (Figure 6). Discussion In this study, we determined the cerebral cortical fmri response to subliminal rectal distention in healthy volunteers. Study findings indicate that afferent signals originating from the rectum, but not strong enough to reach the perception level, are registered in the cerebral cortex and can be detected by fmri. However, the volume and maximum fmri signal intensity change of Figure 2. Comparison of total cortical activity volume in response to subliminal, liminal, and supraliminal rectal distention. The volume of cortical activity was directly related to the intensity of stimulation. Figure 4. Comparison of maximum percent fmri signal increase in response to subliminal, liminal, and supraliminal rectal distentions. There was a direct relationship between the intensity of the stimulus and magnitude of fmri signal change (*P 0.05).

6 February 2002 SUBLIMINAL VISCERAL STIMULATION 295 activated cortical areas caused by subliminal afferent signals are significantly smaller compared with those that reach the perception level. The notion of cortical registration of stimulation without awareness has long been the subject of considerable debate and dates back to nearly 300 years ago. 22 Earlier studies of the phenomenon called masked priming have reported that masked visual words that are presented so briefly that they cannot be seen, may facilitate the subsequent processing of related words. 23,24 Recent studies 8 using modern techniques for evaluation of brain activity have shown that masked visual stimuli induce a measurable change on the electrical and hemodynamic measures of the brain activity without being consciously perceived. These studies have also shown that the unconscious activity induced by masked visual stimuli not only include areas involved in sensory processing, but also areas of motor programming. 8 Cerebral cortical registration without conscious perception of the visceral stimulation has not been systematically evaluated. Previous studies have shown that esophageal acid perfusion, without inducing heartburn, induced cortical activity, whereas cortical activity was not observed during saline infusion, supporting the notion of cortical registration of subliminal esophageal chemical stimulation. 19 The findings of the present study corroborate this notion and confirm the existence of cortical perception without consciousness of subliminal afferent signals induced by distention of the intestine. These findings have physiologic ramifications and indicate a more expanded involvement of the cerebral cortex in gastrointestinal sensory function than generally recognized and raise the possibility of the influences of these subliminal visceral stimulations on gastrointestinal-related central nervous system sensory and motor functions. From a clinical perspective, the cortical registration of subliminal lower gut stimulation provides a unique opportunity for studying a variety of physiologic and pathologic conditions devoid of ancillary activation of cortical regions related to the awareness of stimuli, such as those induced by pain, fear, anxiety, and memory recall. Similar to liminal and supraliminal stimulations, subliminal signals arising from the rectum were registered in 4 regions: sensory motor, parieto-occiptal, anterior cingulated, and the insular cortex. Although the brain imaging techniques used in the present study give no direct evidence of the neural pathway that mediated the observed cortical activation during subliminal rectal distention, one can speculate that activation of the insular, anterior cingulate/prefrontal, and parieto/occipital regions may have been mediated by afferent projections from spinal and/or vagal pathways. However, because primary and secondary sensory/motor cortical regions are supplied by spinal afferents alone, one could surmise the spinal afferents to be the pathway for cortical activation of the sensory/motor region during subliminal rectal distention. However, such activity could have been alternatively induced via intracortical relay from insular cortical activity mediated by vagal afferents. Whether the cortical registration of subliminal intestinal stimulation is mediated through those that mediate the liminal and supraliminal stimuli remains to be ascertained. The magnitude of signal intensity changes during this study ranged from an average of 3.7% to 15% over the baseline, depending on stimulus intensity. The relatively short TR and thick slices are possible contributing factors to the reported percent signal change values. Our choice of slice thickness and number were made to optimally scan the whole brain using sampling frequencies conducive to monitoring fmri signal changes of initially unknown frequency characteristics. Percent changes in fmri signal intensity in the 1% 2% range at 1.5T are typical of event-related paradigm studies. The stimulation pattern used in our study was essentially a block trial paradigm with extended intervals of stimulation compared with the relatively brief application of stimuli associated with event-related techniques. A wide range of magnetic signal intensity increases has been reported for block trial fmri studies. For perceived and/or painful rectal distention, values have been reported of 6% (1) at 1.5 T, TR 64 ms, 7-mm slice thickness. For painful thermal stimulation, 3% increases have been reported at 1 T, TR 91 ms, 1-cm slice thickness. 25 For painful cold stimuli, 5% increases have been reported at 1.5 T, TR 320 ms, 4-mm slice thickness. 26 The effect of MR slice order and thickness has been recently studied 27 and showed the multislice echo planar imaging technique to have no sensitivity to inflow. This study of the visual cortex also reported percent signal changes as large as 12% for 1.8-mm slices for TRs of 1.5, 3, and 6 seconds using a 2T scanner. Because in the present study the experimental condition, slice thickness, and recording parameters were kept constant for all types of stimuli, the potential overestimation of signal intensity change would apply similarly to subliminal, as well as liminal and supraliminal registration. Anticipation or anxiety caused by the experimental circumstances could be argued to have played a role in inducing cortical activity during subliminal distentions.

7 296 KERN AND SHAKER GASTROENTEROLOGY Vol. 122, No. 2 Figure 1. Š Figure 3. Time course of the average percent of fmri signal change in all activated regions and for all subjects during the 3 intensities of rectal distention. Gray, subliminal; black, liminal; red, supraliminal; gray rectangles signify the intervals of rectal distention. Figure 5. The colored areas on the anatomical and echo planar MRI images are regions of magnetic signal change during subliminal rectal distention from a representative male subject. The average magnetic signal time course for the 10 red voxels in the green box shown on the echo planar image is shown in the upper graph at right (mean SEM) where the gray regions on the graph represent intervals of sustained rectal distention. For the same 10 voxels, the average time course was calculated for the scanning sequence immediately following the subliminal distention scan wherein no rectal distention was performed.

8 February 2002 SUBLIMINAL VISCERAL STIMULATION 297 Š Figure 1. Composite regions of cortical activity shown superimposed in color on a stereotaxic brain volume rendered in 3 dimensions. The right upper quadrant of the brain volume has been removed just above the midaxial plain (Talairach-Tournoux plane at z 17mm) and to the right of the midsagittal plane. The activity shown represents the composite activity of all stage I volunteers in these planes in response to (A) subliminal, (B) liminal, and (C) supraliminal distentions. Also shown are the gross locations of the cortical regions defined by BA. The sensory/motor region (SM) was designated by BA 1, 2, 3, 4, 5, and 6; the anterior cingulate/prefrontal region (ACPF) by BA 8, 10, 24, and 32; the parietal/occipital region (PO) by BA 7, 18, 19, and 30. The insular cortex (I) was designated as the portion of the cerebral cortex deep within the lateral fissure consisting of several long gyri paralleling the lateral fissure and 5 short gyri located more rostrally. In regard to anxiety, considering the fact that the fmri technique measures magnetic signal changes and anxiety would most likely be present during the entire sequence of scanning including periods of subliminal distention and no distention, association of the observed cortical fmri activity during subliminal distentions with anxiety would be unlikely. In addition, no changes in signal intensity were observed during the nondistention period. In regard to anticipation, stage II of the experiments was designed to address these concerns. In these experiments, we varied the sequence of distentions, as well as the interval between distentions with respect to subliminal and liminal distentions. Cortical activity was seen during subliminal distentions that were not preceded by a prior sequence during which volunteers had experienced awareness of distention. In addition, cortical activity associated with subliminal rectal distention was significantly correlated with the onset of subliminal distention even when the intervals between various liminal and supraliminal stimulations were varied. Therefore, it does not seem that anxiety or anticipation were significant contributing factors in the detected cortical activity during subliminal rectal distention. Figure 6. Average waveform for all correlated voxels in a representative subject. Intervals of perception-level rectal distention (tall gray regions on the graph) and subliminal distention (shorter gray regions) are separated by variable time intervals. Still, subliminal distentions at unexpected time intervals induce cortical activity. The distending pressure that was associated with awareness in the current study is higher than some of the previously reported studies. 4,5,28,29 This discrepancy is possibly caused by differences in the length of the connecting tube (in our case approximately 30 feet) that was necessitated by the need to keep the barostat device outside of the scanner suite. Therefore, the pressure registered at the perception level reported in the current study may not accurately reflect the actual distention pressure for perception, but provided a quantifiable measure for inducing uniform subliminal and supraliminal distentions in all study subjects. Because of these technical limitations, determination of the range for subliminal pressures that cause cortical activity could not be undertaken and awaits improvements in barostatic recording systems and catheter assemblies. In summary, subliminal afferent signals originating from the gut are registered in the cerebral cortex and induce changes in measurements of brain activity such as blood oxygenation level changes detectable by fmri. Locations of cortical activity caused by subliminal gut stimulation are similar to those of liminal and supraliminal stimulation but their intensity and volume are significantly lower. The possible influence of cortical registration of subliminal intestinal visceral stimulation on gut sensory/motor function as well as perception/interpretation of sensory signals originating from the intestine, merits further consideration. References 1. Baciu MV, Bonaz BL, Papillon E, Bost RA, Le Bas J-F, Fournet J, Segabarth CM. Central processing of rectal pain: a functional MR imaging study. Am J Neuroradiol 1999;20: Berman S, Munakata J, Naliboff BD, Chang L, Mandelkern M, Chang L, Mayer EA. Gender differences in regional brain response to visceral pressure in IBS patients. Eur J Pain 2000;4: Bouras EP, O Brien TJ, Camilleri M, O Connor MK, Mullan BP. Cerebral topography of rectal stimulation using single photon emission computed tomography. Am J Physiol Gastrointest Liver Physiol 1999;277:G687 G Mertz H, Morgan V, Tanner G, Pickens D, Price R, Shyr Y, Kessler R. Regional cerebral activation in irritable bowel syndrome and control subjects with painful and nonpainful rectal distention. Gastroenterology 2000;118: Silverman DHS, Munakata JA, Ennes H, Mandelkern MA, Hoh CK, Mayer EA. Regional cerebral activity in normal and pathological perception of visceral pain. Gastroenterology 1997;112:64 72.

9 298 KERN AND SHAKER GASTROENTEROLOGY Vol. 122, No Bar M, Biederman I. Localizing the cortical region mediating visual awareness of object identity. Proc Natl Acad Sci U S A 1999;96: Brazdil M, Rektor I, Dufek M, Jurak P, Daniel P. Effect of subthreshold target stimuli on event-related potentials. Electroencephalogr Clin Neurophysiol 1998;107: Dehaene S, Naccache L, Le Clec H G, Koechlin GE, Mueller M, Dehaene-Lambertz G, van de Moortele PF, Bahan D. Imaging unconscious semantic priming. Nature 1998;395: Whalen PJ, Rauch SL, Etcoff NL, McInerney SC, Lee MB, Jenike MA. Masked presentations of emotional facial expressions modulate amygdala activity without explicit knowledge. J Neurosci 1998;18: Bunce SC, Bernat E, Wong PS, Shevrin H. Further evidence for unconscious learning: preliminary support for the conditioning of facial EMG to subliminal stimuli. J Psychiatr Res 1999;33: de Houwer J, Hendricks H, Baeyens F. Evaluative learning with subliminally presented stimuli. Consciousness Cognition 1997;6: Draine SC, Greenwald AG. Replicable unconscious semantic priming. J Exp Psychol 1998;127: Elliott R, Dolan RJ. Neural response during preference and memory judgments for subliminally presented stimuli: a functional neuroimaging study. J Neurosci 1998;18: Patton CJ. Fear of abandonment and binge eating: a subliminal psychodynamic activation investigation. J Nerv Ment Dis 1992; 180: Waller G, Mijatovich S. Preconscious processing of threat cues: impact on eating among women with unhealthy eating attitudes. Int J Eat Disord 1998;24: Talairach J, Tournoux P. Co-planer sterotaxic atlas of the human brain. New York, Thieme Medical, Cox RW, Hyde JS. Software tools for analysis and visualization of fmri data. NMR Biomed 1997;10: Binkofski F, Schnitzler A, Enck P, Frieling T, Posse S, Seitz RJ, Freud HJ. Somatic and limbic cortex activation in esophageal distention: a functional magnetic resonance imaging study. Ann Neurol 1998;44: Kern M, Birn R, Jaradeh S, Jesmanowicz A, Cox R, Hyde JS, Shaker R. Identification and characterization of cerebral cortical response to esophageal mucosal acid exposure and distention. Gastroenterology 1998;115: Kern MK, Jaradeh S, Arndorfer RC, Shaker R. Cerebral cortical representation of reflexive and volitional swallowing in humans. Am J Physiol Gastrointest Liver Physiol 2001;280:G354 G Cox RW, Jesmanowicz A. Real-time 3-D image registration for functional MRI. Magn Reson Med 1999;42: Merikle PM. Perception without awareness: critical issues. Am Psychol 1992; 47: Forster KI, Davis C. Repetition priming and frequency attenuation in lexical access. J Exp Psychol Learn Memory Cogn 1984;10: Marcel AJ. Conscious and unconscious perception: experiments on visual masking and word recognition. Cogn Psychol 1983;15: Jones AP, Hughes DG, Brettle DS, Robinson L, Sykes JR, Aziz Q, Hamdy S, Thompson DG, Derbyshire SW, Chen AC, Jones AK. Experiences with functional magnetic resonance imaging at 1 tesla. Br J Radiol 1988;71: Kwan CL, Crawley AP, Mikulis DJ, Davis KD. An fmri study of the anterior cingulate cortex and surrounding medial wall activations evoked by noxious cutaneous heat and cold stimuli. Pain 2000; 85: Howseman AM, Grootoonk S, Porter DA, Ramdeen J, Holmes AP, Turner R. The effect of slice order and thickness on fmri activation data using multislice echo-planar imaging. NeuroImage 1999;9: Lagier E, Delvaux M, Vellas B, Fioramonti J, Bueno L, Alberede JL, Flexinos J. Influence of age on rectal tone and sensitivity to distension in healthy subjects. Neurogastroenterol Motil 1999; 11: Sloots CE, Felt-Bersma RJ, Cuesta MA, Meuwissen SG. Rectal visceral sensitivity in healthy volunteers: influences of gender, age and methods. Neurogastroenterol Motil 2000;12: Received March 26, Accepted October 18, Address requests for reprints to: Reza Shaker, M.D., Division of Gastroenterology and Hepatology, Froedtert Memorial Lutheran Hospital, 9200 W. Wisconsin Avenue, Milwaukee, Wisconsin rshaker@mcw.edu; fax: (414) Supported in part by NIH grant R01 DK25731.