De Bentley

Caudal cingulate cortex involvement in pain processing: an inter-individual laser evoked potential source localisation study using realistic head models

&NA; Electrophysiological studies have revealed a source of laser pain evoked potentials (LEPs) in cingulate cortex. However, few studies have used realistically shaped head models in the source analysis, which account for individual differences in anatomy and allow detailed anatomical localisation of sources. The aim of the current study was to accurately localise the cingulate source of LEPs in a group of healthy volunteers, using realistic head models, and to assess the inter‐individual variability in anatomical location. LEPs, elicited by painful CO2 laser stimulation of the right forearm, were recorded from 62 electrodes in five healthy subjects. Dipole source localisation (CURRY 4.0) was performed on the most prominent (P2) peak of each LEP data set, using head models derived from each subjects structural magnetic resonance image (MRI). For all subjects, the P2 LEP peak was best explained by a dipole whose origin was in cingulate cortex (mean residual variance was 3.9±2.4 %). For four out of five subjects, it was located at the border of the caudal division of left anterior cingulate cortex (area 24/32′) with left posterior cingulate cortex (area 23/31). For the fifth subject the dipole was centred in right posterior cingulate cortex (area 31). This study demonstrates that the location of the cingulate source of LEPs is highly consistent across subjects, when analysed in this way, and supports the involvement of caudal cingulate regions in pain processing.

Differential effects on the laser evoked potential of selectively attending to pain localisation versus pain unpleasantness.

OBJECTIVE To determine the effects on the laser evoked potential (LEP) of selectively attending to affective (unpleasantness) versus sensory-discriminative (localisation) components of pain. METHODS LEPs, elicited by painful CO2 laser stimulation of two areas of the right forearm, were recorded from 62 electrodes in 21 healthy volunteers, during three tasks that were matched for generalised attention: Localisation (report stimulus location), Unpleasantness (report stimulus unpleasantness), Control (report pain detection). LEP components are named by polarity, latency, and electrode. RESULTS N300-T7 peak amplitude was significantly greater during Localisation than Unpleasantness. The difference in N300-T7 amplitude between Localisation and Control approached significance, suggesting an increased amplitude in Localisation compared with Control, rather than a reduced amplitude in Unpleasantness. Peak amplitude, latency, and topography of N300-FCz, P450, P600-800 (early P3) and P800-1000 (late P3) did not differ significantly between tasks. CONCLUSIONS These results suggest that the N300-T7 LEP peak reflects the activity of cerebral generators involved in the localisation of pain. The topography of N300-T7 is consistent with a source in contralateral secondary somatosensory cortex/insula and maybe primary somatosensory cortex. SIGNIFICANCE This study confirms a role of the lateral pain system in the localisation of pain, and distinguishes it from stimulus novelty or attention.

Selective attention to pain: a psychophysical investigation.

Laboratory research suggests that the processing of painful stimuli can be modulated by selective attention to a particular sensory modality. However, alternative accounts for previous findings remain possible in terms of task-switching and spatial attention effects. In the present study, we examined whether attention can be selectively directed to the pain modality in order to facilitate the processing of the sensory-discriminative aspects of painful laser heat stimuli when these alternatives were ruled out. Participants made speeded spatial discrimination responses to an unpredictable sequence of painful laser heat and visual stimuli presented on the left forearm. On each trial, a symbolic cue predicted the likely modality for the upcoming target on the majority of trials. Participants responded more rapidly when the target was presented in the expected as opposed to the unexpected modality, demonstrating that selective attention can modulate the processing of painful stimuli. These findings are discussed in relation to contemporary theories of crossmodal attention and multisensory information-processing.

Anatomical localization and intra-subject reproducibility of laser evoked potential source in cingulate cortex, using a realistic head model

OBJECTIVES To (i) accurately localize the cingulate source of late laser evoked potentials (LEPs) using a realistic head model incorporating the individuals anatomy and (ii) assess the within-subject reproducibility of this source. METHODS Late LEPs, elicited by painful CO2 laser stimulation of the right forearm, were recorded from 62 electrodes in one healthy subject. This was repeated 9 times, over 3 different days. Dipole source localization (CURRY 4.0) was performed on the most prominent (P2) peak of each LEP data set, using a head model derived from the subjects structural magnetic resonance image. RESULTS In all cases the P2 LEP peak was best explained by a dipole located close to the border of the caudal division of left anterior cingulate cortex with left posterior cingulate cortex (mean residual variance was 1.7+/-0.4%). The maximum standard deviation from the mean dipole location was 3.2 mm. CONCLUSIONS This study demonstrates that the location of the cingulate source of late LEPs is highly reproducible within this subject, when analyzed in this way, and suggests involvement of caudal cingulate regions in pain processing.

Categories of placebo response in the absence of site-specific expectation of analgesia

&NA; Experimental placebo analgesia is induced by building an expectation of reduced pain in a specific body part, usually using an inert cream in the guise of a local anaesthetic in conjunction with conditioning. We investigated non‐site‐specific placebo analgesia by conditioning subjects to expect the anaesthetic cream on one arm, without specifying if they will definitely receive the cream, or to which arm it might be applied. Painful heat pulses (150 ms) from a CO2 laser were delivered randomly to both arms. A treatment group (n = 24) underwent three experimental blocks (pre‐cream, conditioning after cream, and post‐conditioning). During the conditioning block, the intensity of the stimulus was reduced on one arm only. In the post‐conditioning block it was returned to the painful level. We evaluated the change of intensity rating post‐conditioning compared to the pre‐cream block. In contrast to a control group (n = 16), the treatment group reported a significant reduction in intensity ratings (F1,38 = 12.1; p = 0.001). In the treatment group, we observed a range of placebo responses: unilateral responders (33.3%), subjects with a placebo response in the conditioned arm only; bilateral responders (33.3%), subjects reporting reduction in the intensity ratings in both arms, and non‐responders, whose intensity ratings were not influenced by conditioning. We discuss these responses in terms of different levels of expected analgesia, facilitated by the absence of a site‐specific focus for the treatment. We suggest this allowed the individuals suggestibility to influence their assessment of the pain experience by combining different levels of expectation with the information from the actual pain stimulus.

Source localisation of 62-electrode human laser pain evoked potential data using a realistic head model

Laser evoked potentials (LEPs), elicited by painful laser stimulation of the right forearm, were recorded from 62 electrodes in a single healthy subject. The positions of the electrodes on the scalp were co-registered with the subjects structural magnetic resonance image (MRI) of the brain. Spatio-temporal dipole modelling, using a head model derived from the MRI, estimated sources in left posterior cingulate, posterior parietal and anterior insular cortices. The parietal source peaked in activity at 260 ms, which explained the N1/N2 peaks of the LEPs. The cingulate source was the most strongly activated, at 400 ms, and accounted for the P2 LEP component. The insular source showed late, prolonged activation, peaking in magnitude at 850 ms. This is the first study to report scalp-recorded LEP generators in posterior parietal and insular cortices. Although these sources require replication, they are consistent with other functional imaging studies.

Selective modulation of nociceptive processing due to noise distraction

&NA; This study investigates the effects of noise distraction on the different components and sources of laser‐evoked potentials (LEPs) whilst attending to either the spatial component (localisation performance task) or the affective component (unpleasantness rating task) of pain. LEPs elicited by CO2 laser stimulation of the right forearm were recorded from 64 electrodes in 18 consenting healthy volunteers. Subjects reported either pain location or unpleasantness, in the presence and absence of distraction by continuous 85 dBa white noise. Distributed sources of the LEP peaks were identified using Low Resolution Electromagnetic Tomography (LORETA). Pain unpleasantness ratings and P2 (430 ms) peak amplitude were significantly reduced by distraction during the unpleasantness task, whereas the localisation ability and the corresponding N1/N2 (310 ms) peak amplitude remained unchanged. Noise distraction (at 310 ms) reduced activation in the anterior cingulate cortex (ACC) and precuneus during attention to localisation and unpleasantness, respectively. This suggests a complimentary role for these two areas in the control of attention to pain. In contrast, activation of the occipital pole and SII were enhanced by noise during the localisation and unpleasantness task, respectively, suggesting that the presence of noise was associated with increased spatial attentional load. This study has shown selective modulation of affective pain processing by noise distraction, indicated by a reduction in the unpleasantness ratings and P2 peak amplitude and associated activity within the medial pain system. These results show that processing of the affective component of pain can be differentially modulated by top‐down processes, providing a potential mechanism for therapeutic intervention.

Acoustic noise in functional magnetic resonance imaging reduces pain unpleasantness ratings

Functional magnetic resonance imaging (fMRI) is increasingly used in cognitive studies. Unfortunately, the scanner produces acoustic noise during the image acquisition process. Interference from acoustic noise is known to affect auditory, visual and motor processing, raising the possibility that acoustic interference may also modulate processing of other sensory modalities such as pain. With the increasing use of fMRI in the investigation of the mechanisms of pain perception, particularly in relation to attention, this issue has become highly relevant. Pain is a complex experience, composed of sensory-discriminative, affective-motivational and cognitive-evaluative components. The aim of this experiment was to assess the effect of MRI scanner noise, compared to white noise, on the affective (unpleasantness) and the sensory-discriminative (localisation) components of pain. Painful radiant heat from a CO(2) laser was delivered to the skin of the right forearm in 24 healthy volunteers. The volunteers attended to either pain location or pain unpleasantness during three conditions: i) no noise, ii) exposure to MRI scanner noise (85 dB) or iii) exposure to white noise (85 dB). Both MRI scanner noise and white noise significantly reduced unpleasantness ratings (from 5.1 +/- 1.6 in the control condition to 4.7 +/- 1.5 (P = 0.002) and 4.6 +/- 1.6 (P < 0.001) with scanner and white noise respectively), whereas the ability to localise pain was not significantly affected (from 85.4 +/- 9.2% correct in the control condition to 83.1 +/- 10.3% (P = 0.06) and 83.9 +/- 9.5% (P = 0.27) with MRI scanner and white noise respectively). This phenomenon should be taken into account in the design of fMRI studies into human pain perception.

Dipole source localisation using independent component analysis: single trial localisation of laser evoked pain

The accuracy of the inverse solution that finds the spatial location of the generating sources from averaged scalp-recorded event related potentials (ERPs) relies on assumptions about the ERP signals and the sources. We provide evidence that using independent component analysis (ICA) as a signal decomposition filter prior to applying the inverse solution reveals sources that cannot be detected by conventional source localisation methods. Five clusters of sources emerged: a single source cluster in caudal cingulate and bilateral sources in secondary somatosensory cortex (SII), inferior parietal cortex, premotor cortex and insular cortex. The locations of the source dipoles were consistent with findings using fMRI and PET but have not all been previously detected in a single electrophysiological study. In addition, the time-course of the activation of these dipoles was estimated. The results suggest that using ICA to localise single trial data is a powerful tool for exploring the spatiotemporal dynamics of rapid and complex brain processes.

285 SENSORY MODULATION OF LASER EVOKED POTENTIALS BY TRYPTOPHAN DEPLETION

Introduction: Pain associated with menstruation. A most frequent gynecological disorder, it is classified into primary and secondary. Its pathogenesis is not always understood. Prostaglandins seem to be intimately involved in primary dysmenorrhoea. Medication is usually required for all cases of moderate to severe painful menstruation. Apart from pharmacological agents, several techniques have been used including relaxation therapy, hypnosis, manipulation, psychotherapy, acupuncture, biofeedback techniques, surgery. Material and methods: It was a descriptive study. The study included 215 virgin students of reproductive age (19–22 yrs). The study subjects completed a questionnaire. Results: The median age of menarche was 12 years and the mean was11.92. Frequency of primary dysmenorrhea was 62 %(mild24.5%, moderate 22.5%, severe 15%) but the daily activity was hampered only in 15%. They had poor knowledge about dysmenorea and menstruation abnormality. 35.5% treated themselves with iboprophen, codeine vitb6, mefnamic acid and . . . and some of them used from herbal drugs and some of them only had rest and eat some sweet. Conclusion: It seems the rate of dysmenorea in our study was like other studies, and our girls needed education about the abnormalities of menstruation.