Multichannel recording of cerebral potentials evoked by esophageal balloon distension in humans

Gut inference: A computational modelling approach

Neurocomputational theories have hypothesized that Bayesian inference underlies interoception, which has become a topic of recent experimental work in heartbeat perception. To extend this approach beyond cardiac interoception, we describe the application of a Bayesian computational model to a recently developed gastrointestinal interoception task completed by 40 healthy individuals undergoing simultaneous electroencephalogram (EEG) and peripheral physiological recording. We first present results that support the validity of this modelling approach. Second, we provide a test of, and confirmatory evidence supporting, the neural process theory associated with a particular Bayesian framework (active inference) that predicts specific relationships between computational parameters and event-related potentials in EEG. We also offer some exploratory evidence suggesting that computational parameters may influence the regulation of peripheral physiological states. We conclude that this computational approach offers promise as a tool for studying individual differences in gastrointestinal interoception.

Cortical changes to experimental sensitization of the human esophagus

Topographical organization in the neocortex shows experience-dependent plasticity. We hypothesized that experimental sensitization of the esophagus results in changes of the topographical distribution of the evoked potentials and the corresponding dipole source activities to painful stimulation. An endoscopic method was used to deliver 35 electrical stimuli at the pain threshold to a fixed area of the mucosa in 10 healthy volunteer men and women. The stimulations were repeated after 30 min (reproducibility experiment), and after 60 min following perfusion of 200 ml 0.1 N hydrochloric acid (sensitization experiment). During stimulation the electroencephalogram was recorded from 64 surface electrodes. The sensitization resulted in a decrease in the pain threshold (F=6.2; P=0.004). The topographic distribution of the evoked potentials showed reproducible negative (N1, N2) and positive (P1, P2) components. After acid perfusion a reduced latency and a change in localization was seen for the P1 subdivided into frontal and occipital components (F=29.5, P<0.001; F=53.7, P<0.001). Furthermore the sensitization resulted in a reduction of the latency for P2 (F=6.2, P=0.009). The source analysis showed consistent dipolar activity in the bilateral opercular-insular cortex before and after acid perfusion. For the anterior cingulate dipole there was a reduction in latency (P=0.03) and a posterior shift (P=0.0002) following acid perfusion. The findings indicate that short-term sensitization of the esophagus results in central neuroplastic changes involving the cingulate gyrus, which also showed pathological activation in functional diseases of the gut, thus reflecting the importance of this region in visceral pain and hyperalgesia.

Characterization of cortical potentials evoked by oesophageal balloon distention and acid perfusion in patients with functional heartburn

Oesophageal visceral hypersensitivity is thought to be important in generating symptoms in functional heartburn (FH). However, the neurophysiological mechanisms involved are poorly understood. The aim of this study was to compare the characteristics of oesophageal cortical evoked potentials (CEPs) induced by balloon distension and acid perfusion in FH and controls. We studied 21 FH patients and 12 healthy volunteers. Oesophageal mechanical stimulation was performed using the specially constructed mechanical pump. CEPs were recorded using the 10-20 international system of electroencephalogram recording. Oesophageal distention elicited recognizable, reproducible and muti-peak CEPs. CEP latencies for N1, P1 and N2 components were significantly shorter (P = 0.016, P = 0.003 and P = 0.031, respectively) in FH than in controls before perfusion. Acid perfusion significantly decreased the latencies of N1, P1 and N2 (P = 0.022, P = 0.007 and P = 0.041, respectively) and significantly increased the amplitude of P1-N2 components (P = 0.020) in FH patients, but not in controls. In conclusion, cortical evoked potential responses evoked by oesophageal distention and acid perfusion were greater in FH than in controls, suggesting that dysfunction of visceral neural pathways and/or alterations in cortical processing may produce and mediate oesophageal hypersensitivity in FH. These findings provide the evidence that central sensitization contributes to the development and maintenance of oesophageal hypersensitivity.

Short latency cerebral response evoked by painful electrical stimulation applied to the human sigmoid colon and to the convergent referred somatic pain area

Background. The brain-gut interaction is important for the understanding of pain mechanisms related to gastroenterological diseases. Unfortunately little is known about the early cerebral events related to the processing of gut-evoked pain. The aims of this human study were (1) to investigate the early-evoked brain potentials (EPs) to painful sigmoid colon stimulation and (2) to evaluate the EPs evoked from the convergent referred skin pain area after this area was induced by the painful gut stimulation. The background for the second aim was to evaluate whether the convergent input between somatic and visceral structures could induce detectable short-term cortical reorganization. Methods. Eleven subjects (nine men) participated; the mean age was 39.5+/-11.9 years. The gut-evoked EPs (recorded from 31 scalp sites) were evoked by electrical stimulation 30 cm from the anal verge by a modified biopsy forceps, inserted through a sigmoidoscope. The painful gut stimulation elicited a characteristic pain pattern referred to the abdomen. The short latency somatosensory evoked potentials were evoked from the skin inside and outside the referred pain area elicited by gut stimulation. A total of 750 electrical stimuli were delivered to the gut at slight painful stimulus intensity and 500 stimuli were delivered to the skin. Results. Short-latency EPs to electrical gut stimulation with an onset of 50-60 ms could be recorded. The gut EP topography revealed three consecutive positive peaks (P63, P101, P145) towards the frontal area. Centroparietal negativities (N128 and N222) were found, which were followed by two central positivities (P269 and P352). The somatic and gut evoked EPs differed in morphology and topography, but the EPs to skin stimulation inside and outside the gut-evoked referred pain area did not differ significantly. Conclusion. Short latency (50-60 ms) EPs to painful electrical sigmoid colon stimulation were demonstrated, reflecting an early cortical processing of sensory input from the sigmoid colon. The early cortical processing of somatic input from experimentally evoked visceral referred pain areas did not cause any detectable short-term cortical reorganization.

Brain research in functional gastrointestinal disorders

The current understanding is that functional gastrointestinal disorders (FGIDs) result from dysregulation of the bidirectional communication between the gut and the brain (i.e., the brain-gut axis), modulated by various psychosocial and environmental factors (i.e., the biopsychosocial model). This concept has led to a growing interest in the research of brain function in relation to gut motor and sensory function. Brain research on the mechanisms that are involved in the generation of gastrointestinal symptoms includes studies of the gut response to brain stimulation with technique such as transcranial magnetic stimulation or studies of the brain response to gut stimulation by cortical evoked potentials, positron emission tomography, and functional magnetic resonance imaging. Studies using these techniques have shown that visceral/gut sensation involves activation of several brain regions that are associated with various brain functions, including sensation, cognition, and affect. The complexity of the brain response to visceral stimulation and the multidetermined nature of FGIDs make studies of brain function in FGID patients difficult and demands great caution in interpreting their results. Nevertheless, brain research in FGIDs is an emerging field and suggests that patients with irritable bowel syndrome differ from healthy subjects in the way that their brain response to visceral (e.g., rectal) distention. These studies emphasize the role of the central nervous system in conducting and processing visceral signals and suggest that alteration in brain processes involving perception and affective responses might be key factors in the pathogenesis of functional gastrointestinal symptoms.

Identification of the optimal parameters for recording cortical potentials evoked by mechanical stimulation of the human oesophagus

Cortical evoked potentials (CEP) have been recorded in response to both electrical stimulation (ES) and mechanical stimulation (MS) of the oesophagus. While the optimal parameters for recording reproducible oesophageal CEP to ES have recently been established, they have not yet been determined for MS, and reported CEP to MS show considerable variability. This study aimed to identify the optimal parameters required to record reproducible MS induced CEP. CEP were recorded from the vertex (Cz) in six subjects (one female; age range 23-47 years). MS was performed 5 cm above the lower oesophageal sphincter by rapidly inflating a 2-cm long silicone balloon at a frequency of 0.2 Hz. The rise time to maximum inflation was 165 ms. In order to determine the minimum number of stimuli required to produce optimal signal-to-noise quality, we acquired data in runs of 25, 50, 100 and 300 stimuli and to determine the stimulation intensity that produced the shortest latency and the largest amplitude CEP, we averaged four runs of 50 stimuli at five different intensities ranging from sensory threshold to pain. CEP reproducibility was then assessed in three subjects on three separate occasions using parameters determined from these measurements. We found that optimal signal-to-noise quality was achieved by averaging four runs of 50 stimuli; that P1 latency was shortest and P1-N1 amplitude largest at intensities of 75% and pain threshold and that highly reproducible CEP were obtained in all individuals. We conclude that it is possible to obtain highly reproducible oesophageal CEP to MS which can now be compared to those obtained by ES in order to identify which is most suitable for clinical studies.

Taste in the monkey cortex

The sense of taste in humans differs substantially from that of rodents, from which a preponderance of gustatory electrophysiology derives. To establish a more appropriate neural model for human gustation, we recorded the activity of single neurons in the primary taste cortex in 11 alert cynomolgus macaques. Taste cells composed 6% of all neurons encountered. Another 24% responded during mouth and jaw movements, and 4% were sensitive to tactile stimulation of the mouth. Smaller numbers responded during olfactory or visual stimulation, or when the monkey extended his tongue. Taste cells could be divided into four statistically independent groups, corresponding to those most responsive to glucose (38%), NaCl (34%), quinine (22%), or HCI (5%). The location of a taste cell did not predict its response profile, i.e., there was no clear topographic organization of taste sensitivity. We established neural thresholds and intensity-response functions to the basic stimuli and determined that-with the exception of HCl, to which the macaque is relatively insensitive-they were similar to those reported by human subjects. We then turned to the coding of taste quality, as inferred in macaques from the patterns of neural activity elicited by each of greater than 100 stimuli. The results proved generally faithful to human reports of the perceived qualities of these same tastants. Finally, an investigation of taste mixtures revealed a degree of mixture suppression and interaction among basic qualities similar to those reported by humans. We conclude that the alert macaque offers a reliable neural model for human gustation.

The electrical and magnetical cerebral responses evoked by electrical stimulation of the esophagus and the location of their cerebral sources

OBJECTIVES

After electrical stimulation of the esophagus cerebral responses are recordable, their cortical source is under discussion. Brain mapping using electroencephalography recordings demonstrated partially controversial results. Sources of evoked responses can be localized more easily using magnetoencephalography than electroencephalography.

METHODS

We examined 22 volunteers by recording electrical somatosensory potentials after electrical stimulation of the esophagus. In 9 of these 22 subjects additional recording of magnetic fields was performed and the sources of the evoked magnetic fields were computed.

RESULTS

The evoked potentials after electrical stimulation of the esophagus had a similar latency as the previously published data. The source localization done by magnetoencephalography suggest that first a region of the postcentral gyrus is activated which is temporo-lateral to the primary somatosensory cortex of the pharynx. This region is suggested to be the primary somatosensory region of the esophagus. This source was followed by a source in the parietal operculum thought being part of the secondary somatosensory cortex. Simultaneously the insular cortex was activated pointing to a parallel neuronal pathway to the central autonomic nervous system.

CONCLUSION

After electrical stimulation of the esophagus somatosensory cortical areas of the temporal postcentral gyrus and the operculum are activated. In parallel activation of the insular cortex as part of the central autonomic network was found.

The effect of hyperglycaemia on cerebral potentials evoked by rapid rectal distension in healthy humans

BACKGROUND

Acute hyperglycaemia affects the perception of sensations arising from the gastrointestinal tract. The mechanisms responsible for this effect are unknown. Recordings of cerebral evoked potentials (EPs) can be used to assess the integrity of visceral afferent pathways. Our aim was to determine whether hyperglycaemia affects EPs elicited by rectal distension in healthy humans.

MATERIALS AND METHODS

Twelve healthy men, aged 19-31 years, were studied. A manometric catheter, incorporating a rectal balloon, was positioned 7-10 cm from the anal verge. Balloon distensions at both 'low' ( approximately 20 mL) and 'high' ( approximately 28 mL) volumes were performed, in a single-blind, randomized order, during both euglycaemia (4 mmol L-1) and hyperglycaemia (12 mmol L-1). EPs were recorded from a midline scalp electrode (Cz, International 10-20 system) and averaged for each series of 50 distensions. EP latencies and interpeak amplitudes were calculated.

RESULTS

Polyphasic EPs were recorded in all but one subject. Although the blood glucose concentration had no significant effect on the latencies of the EP peaks elicited by either 'low'- or 'high'-volume balloon distension, the interpeak amplitude (P1-N1) was greater during hyperglycaemia than during euglycaemia at the 'low' balloon volume (6.3 +/- 1.2 microV vs. 4.8 +/- 1.0 microV, P < 0.05). The blood glucose concentration had no significant effect on the perception of rectal balloon distension.

CONCLUSIONS

We conclude that in normal subjects acute hyperglycaemia increases the amplitude of the cerebral EP elicited by rectal balloon distension at low balloon volumes, suggesting that the effects of hyperglycaemia on gastrointestinal sensation may be mediated by central mechanisms.

Cortical activation during oesophageal stimulation: a neuromagnetic study

We investigated the neuromagnetic responses to mechanical stimulation of the oesophagus. In six healthy right-handed volunteers (mean age 31.6 years) the proximal and distal oesophagus were stimulated by electronically controlled pump-inflation of a silicone balloon once every 4.5-5.5 sec (dwell time 145 msec). The balloon volume was adjusted to induce different sensation levels (i) just above threshold of perception, (ii) strong sensation and (iii) painful sensation. Evoked magnetic brain responses were recorded time-locked to stimulus onset with a Neuromag-122TM whole-head neuromagnetometer and modelled as equivalent current diploe (ECD) sources. ECDs were superimposed on individual magnetic resonance imaging (MRI) scans. Magnetic brain responses following distal oesophageal stimulation were adequately explained by a time-varying 2-4 dipole model with unilateral or bilateral sources in second somatosensory cortex and later sources in the frontal cortex. With increasing stimulus intensities, latencies of the sources decreased and amplitudes increased. Proximal oesophageal stimulation led to activation of source areas spatially similar to those of distal oesophageal stimulation but with shorter response latencies. Both painful and nonpainful mechanical stimulation of the oesophagus activate the second somatosensory cortex (SII). Evidence for topographic organization of oesophageal afferents in SII is poor.

Cognitive evoked potentials to anticipated oesophageal stimulus in humans: quantitative assessment of the cognitive aspects of visceral perception

Evoked potential studies provide an objective measure of the neural pathways involved with perception. The effects of cognitive factors, such as anticipation or awareness, on evoked potentials are not known. The aim was to compare the evoked potential response to oesophageal stimulation with the cortical activity associated with anticipation of the same stimulus. In 12 healthy men (23.5 +/- 4 years), oesophageal electrical stimulation (15 mA, 0.2 Hz, 0.2 msec) was applied, and the evoked potentials recorded using scalp electrodes. A computerized model of randomly skipped stimuli (4:1 ratio) was used to separately record the evoked potentials associated with stimulation and those associated with an anticipated stimulus. The electrical stimulus represented the nontarget stimulus and the skipped impulse the target (anticipatory) stimulus. This anticipatory evoked potential was also compared to auditory P300 evoked potentials. Reproducible evoked potentials and auditory P300 responses were elicited in all subjects. Anticipatory evoked potentials (peak latency 282.1 +/- 7.9 msec, amplitude 8.2 +/- 0.7 microV, P < 0.05 vs auditory P300 evoked potential) were obtained with the skipped stimulus. This anticipatory evoked potential was located frontocentrally, while the auditory P300 potential was located in the centro-parietal cortex. The anticipatory evoked potential associated with expectation of an oesophageal stimulus, although of similar latency to that of the auditory P300 evoked response, originates from a different cortical location. The recording of cognitive evoked potentials to an expected oesophageal stimulus depends on attention to, and awareness of, the actual stimulus. Anticipatory evoked potentials to GI stimuli may provide an objective electrophysiological tool for the assessment of the cognitive factors associated with visceral perception.

Effects of octreotide on esophageal visceral perception and cerebral evoked potentials induced by balloon distension

OBJECTIVE

Octreotide, a somatostatin analog, is antinociceptive and increases perception threshold in the rectum. The aim of this study was to determine whether octreotide alters esophageal sensory thresholds and cortical evoked potentials (CEPs) resulting from intraesophageal balloon distension.

METHODS

Twelve healthy volunteers (six men and six women, median age 25 yr, range 21-60 yr) underwent a randomized, double-blind, placebo-controlled trial of octreotide 100 microg s.c. versus saline. A 30-mm balloon was inserted 5 cm above the lower esophageal sphincter without topical anesthesia. The balloon was inflated at a rate of 170 cc/s to a maximum of 30 cc in 2 cc steps. Both pressure and volume were recorded. Patients reported first sensation (S1) and maximally tolerated pain (S2). Two cycles were performed both preinjection and 40 min postinjection. Evoked potentials were recorded from Cz to linked ears over 50 balloon inflation cycles (volume = S2).

RESULTS

Threshold volume to first sensation (S1) was significantly increased after octreotide injection [median (interquartile range): 24 (14-26) cc vs 13 (9-21) cc, p < 0.02]. No significant alteration in volume causing pain (S2) was noted after octreotide injection [29 (25-30+) cc vs 22 (19-29) cc]. Neither were volumes causing either first sensation [18 (11-24) cc vs 13 (9-18) cc] or pain [27 (23-30) cc vs 23 (21-25) cc] significantly altered by placebo injection. Neither amplitude nor latency of any of the three peaks of the evoked potential recordings differed significantly between postplacebo and postoctreotide recordings.

CONCLUSION

Octreotide significantly increased esophageal perception thresholds to balloon distension. It did not alter pain thresholds, nor were cortical evoked potentials to painful stimulation altered in normal subjects.

Identification of the optimal parameters for recording cortical evoked potentials to human oesophageal electrical stimulation

Cortical evoked potentials in response to stimulation of the oesophagus may prove to be a powerful technique for assessing the oesophageal afferent pathway in health and disease. However, in order to maximize the potential of this technique it is essential that the optimal parameters for recording oesophageal CEP are established. The aim was to determine the optimal parameters required to record reproducible CEP. CEP were recorded from the vertex in eight subjects (age range 23-44 years). Electrical stimulation was performed 5 cm above the lower oesophageal sphincter using a bipolar ring electrode at 0.2 Hz. Protocol 1: to determine the stimulation intensity which generates the largest amplitude and shortest latency, two runs of 50 stimuli were applied at increasing intensities. Protocol 2: to determine the number of stimuli for optimal signal to noise ratio, 10 runs of 50 stimuli were recorded. Individual runs were averaged. Protocol 3: to determine the optimal inter-run interval, CEP evoked by 200 stimuli were averaged using randomly chosen inter-run intervals. Protocol 4: CEP reproducibility using parameters determined from Protocols 1-3 was assessed in three subjects on three separate occasions. The results were as follows: Protocol 1; P1 latency was shortest and P1-N1 amplitude largest at an intensity of 75% above threshold. Protocol 2; optimal signal-to-noise was achieved by averaging four runs of 50 stimuli. Protocol 3; the optimal interstudy interval was 10 min. Protocol 4; highly reproducible CEP were obtained in all individuals. Using these optimal parameters, it is possible to obtain highly reproducible oesophageal CEP to ES which can now be used for clinical study.

Effects of two anticholinergic drugs, trospium chloride and biperiden, on motility and evoked potentials of the oesophagus

BACKGROUND

Anticholinergic drugs are known to impair the motor function of the oesophagus but their effects on the oesophageal afferent pathways are unknown.

AIM

To determine the effects of a peripherally-acting (trospium chloride) and a centrally-acting (biperiden) anticholinergic drug on the motility and the evoked potentials of the oesophagus.

METHODS

Nine healthy volunteers were randomized to receive 1.2 mg trospium chloride (TC), 5 mg biperiden (BIP) or saline i.v. Primary peristalsis was elicited by swallowing a 5 mL water bolus and secondary peristalsis by insufflation of 20 mL air, 10 times each. Oesophageal potentials were evoked by electrical stimulation in the distal and proximal oesophagus (30 stimulations at 0.4 Hz, two runs).

RESULTS

Both anticholinergic drugs reduced by a similiar amount the contraction amplitudes (TC 17 mmHg, BIP 25 mmHg, saline 67 mmHg; P < 0.01) and the rate of secondary contractions (TC 60%, BIP 70%, saline 95%; P < 0.01). In contrast, only biperiden prolonged the latencies of the evoked potentials (N1 peak, distal oesophagus: BIP 191 ms, TC 102 ms, saline 101 ms; P < 0.01; P1 peak: BIP 322 ms, TC 161 ms, saline 144 ms; P < 0.01).

CONCLUSIONS

Both anticholinergic drugs depress oesophageal motility, but only the centrally-acting anticholinergic drug biperiden modifies the oesophageal evoked potentials, suggesting a central cholinergic transmission of the oesophageal afferent pathways.

Cortical localisation of magnetic fields evoked by oesophageal distension

Magnetoencephalographic source localisation techniques were used to measure oesophageal evoked magnetic fields from the cerebral cortex in 3 subjects. By using rapid balloon distension as a stimulus, a comparison of proximal and distal oesophageal cortical representation was made. The distal oesophagus was represented bilaterally in the insular cortex and SII as well as the inferior aspect of SI. The proximal oesophagus was represented unilaterally in superior and inferior SI, insular cortex and SII. Significantly, the superior portion of SI was consistently activated in subjects following stimulation of the proximal oesophagus, but similar activation was not found in response to distal stimulation. This may reflect the contribution from somatic afferent fibres in the striate muscle of the proximal segment. In conclusion, vagal afferents appear to contribute more to cortical activation following stimulation of the distal rather than the proximal oesophagus, while spinal afferents appear to be activated by both proximal and distal oesophageal stimulation.

Estimation of habituation and signal-to-noise ratio of cortical evoked potentials to oesophageal electrical and mechanical stimulation

Electrical and mechanical stimulation of the oesophagus has been recently proposed to examine the physiological effects of autonomic stimulation in humans. Cortical evoked potentials (EPs) to oesophageal stimulation provide an assessment of afferent fibres and central processing. However, habituation takes place during averaging of cortical EPs and reduces the signal-to-noise ratio (SNR) as the number of stimuli increases. The SNR of cortical EPs to oesophageal stimulation is computed for 15 normal subjects. Habituation is characterised by the Euclidean distance between the EEG response to single stimuli and the averaged EP, to serve as an objective measure of similarity between the averaged EP and the single-stimulus EEG. With electrical stimulation, the SNR is highest (0.41 +/- 0.21) for 1-12 stimuli and then significantly decreases to 0.2 +/- 0.08 for 13-24 stimuli (p < 0.001). With balloon distension (BD), the SNR is highest (0.22 +/- 0.16) for 1-12 stimuli and lowest (0.12 +/- 0.14) for 13-24 stimuli, but these SNRs are not significantly different from each other. Both electrical and mechanical stimulation of the oesophagus produce rapidly adapting EPs. The SNR of the EPs is higher with electrical stimulation than with BD. The EPs response to BD has a higher variability and is more noisy. Consequently, these results suggest that the overall cortical EP response to electrical stimulation of the oesophagus is more reproducible than that due to balloon distension.

Source modeling of esophageal evoked potentials

Evoked potentials can be recorded from the scalp after stimulation of the esophagus by balloon distension. The purpose of this study was to estimate the number and localization of sources contributing to the esophageal evoked potential (EEP). The EEP was recorded from 32 scalp electrodes in 5 healthy subjects. Spatio-temporal dipole modeling was performed in the time interval from 185 msec to 525 msec after stimulation (mean values). The EEP was best explained by the combined activity of 1 dipole located relatively high in the midline and 2 lateral dipoles. Given the anatomical projection of esophageal sensory fibers and the location of these dipoles the sources were probably located in the cingulate gyri and insular cortex. There was no evidence that sources in the lower brain-stem contributed to the scalp recorded EEP.