Sources of cortical responses to painful CO(2) laser skin stimulation of the hand and foot in the human brain

Inhibition of cortical somatosensory processing during and after low frequency peripheral nerve stimulation in humans

OBJECTIVE

Transcutaneous low-frequency stimulation (LFS) elicits long-term depression-like effects on human pain perception. However, the neural mechanisms underlying LFS are poorly understood. We investigated cortical activation changes occurring during LFS and if changes were associated with reduced nociceptive processing and increased amplitude of spontaneous cortical oscillations post-treatment.

METHODS

LFS was applied to the radial nerve of 25 healthy volunteers over two sessions using active (1 Hz) or sham (0.02 Hz) frequencies. Changes in resting electroencephalography (EEG) and laser-evoked potentials (LEPs) were investigated before and after LFS. Somatosensory-evoked potentials were recorded during LFS and source analysis was carried out.

RESULTS

Ipsilateral midcingulate and operculo-insular cortex source activity declined linearly during LFS. Active LFS was associated with attenuated long-latency LEP amplitude in ipsilateral frontocentral electrodes and increased resting alpha (8-12 Hz) and beta (16-24 Hz) band power in electrodes overlying operculo-insular, sensorimotor and frontal cortical regions. Reduced ipsilateral operculo-insular cortex source activity during LFS correlated with a smaller post-treatment alpha-band power increase.

CONCLUSIONS

LFS attenuated somatosensory processing both during and after stimulation.

SIGNIFICANCE

Results further our understanding of the attenuation of somatosensory processing both during and after LFS.

Homotopic reduction in laser-evoked potential amplitude and laser-pain rating by abdominal acupuncture

BACKGROUND

The neural mechanism underlying the analgesic effect of acupuncture is largely unknown. We aimed at investigating the effect of abdominal acupuncture (AA) on the laser-evoked potential (LEP) amplitude and laser-pain rating to stimulation of body parts either homotopic or heterotopic to the treated acupoint.

METHODS

Laser-evoked potentials were recorded from 13 healthy subjects to stimulation of the right wrist (RW), left wrist (LW) and right foot (RF). LEPs were obtained before, during and after the AA stimulation of an abdominal area corresponding to the representation of the RW. Subjective laser-pain rating was collected after each LEP recording.

RESULTS

The amplitude of the N2/P2 LEP component was significantly reduced during AA and 15 min after needle removal to both RW (F = 4.14, p = .02) and LW (F = 5.48, p = .008) stimulation, while the N2/P2 amplitude to RF stimulation (F = 0.94, p = .4) remained unchanged. Laser-pain rating was reduced during AA and 15 min after needle removal only to RW stimulation (F = 5.67, p = .007).

CONCLUSION

Our findings showing an AA effect on LEP components to both the ipsilateral and contralateral region homotopic to the treated area, without any LEP change to stimulation of a heterotopic region, suggest that the AA analgesia is mediated by a segmental spinal mechanism.

SIGNIFICANCE

Although abdominal acupuncture has demonstrated to be effective in the reduction in laser-evoked potential (LEP) amplitude and laser-pain rating, the exact mechanism of this analgesic effect is not known. In the current study, we found that treatment of an area in the "turtle representation" of the body led to a topographical pattern of LEP amplitude inhibition that can be mediated by a segmental spinal mechanism.

Abnormal Circadian Modification of Aδ-Fiber Pathway Excitability in Idiopathic Restless Legs Syndrome

Restless legs syndrome (RLS) is characterized by unpleasant sensations generally localized to legs, associated with an urge to move. A likely pathogenetic mechanism is a central dopaminergic dysfunction. The exact role of pain system is unclear. The purpose of the study was to investigate the nociceptive pathways in idiopathic RLS patients. We enrolled 11 patients (mean age 53.2 ± 19.7 years; 7 men) suffering from severe, primary RLS. We recorded scalp laser-evoked potentials (LEPs) to stimulation of different sites (hands and feet) and during two different time conditions (daytime and nighttime). Finally, we compared the results with a matched control group of healthy subjects. The Aδ responses obtained from patients did not differ from those recorded from control subjects. However, the N1 and the N2-P2 amplitudes' night/day ratios after foot stimulation were increased in patients, as compared to controls (N1: patients: 133.91 ± 50.42%; controls: 83.74 ± 34.45%; p = 0.016; Aδ-N2-P2: patients: 119.15 ± 15.56%; controls: 88.42 ± 23.41%; p = 0.003). These results suggest that RLS patients present circadian modifications in the pain system, which are not present in healthy controls. Both sensory-discriminative and affective-emotional components of pain experience show parallel changes. This study confirms the structural integrity of Aδ nociceptive system in idiopathic RLS, but it also suggests that RLS patients present circadian modifications in the pain system. These findings could potentially help clinicians and contribute to identify new therapeutic approaches.

Anodal Transcutaneous Spinal Direct Current Stimulation (tsDCS) Selectively Inhibits the Synaptic Efficacy of Nociceptive Transmission at Spinal Cord Level

Recently studies have aimed at developing transcutaneous spinal direct current stimulation (tsDCS) as a non-invasive technique to modulate spinal function in humans. Independent studies evaluating its after-effects on nociceptive or non-nociceptive somatosensory responses have reported comparable effects suggesting that tsDCS impairs axonal conduction of both the spino-thalamic and the medial lemniscus tracts. The present study aimed to better understand how tsDCS affects, in humans, the spinal transmission of nociceptive and non-nociceptive somatosensory inputs. We compared the after-effects of anodal low-thoracic, anodal cervical and sham tsDCS on the perception and brain responses elicited by laser stimuli selectively activating Aδ-thermonociceptors of the spinothalamic system and vibrotactile stimuli selectively activating low-threshold Aβ-mechanoreceptors of the lemniscal system, delivered to the hands and feet. Low-thoracic tsDCS selectively and significantly affected the LEP-N2 wave elicited by nociceptive stimulation of the lower limbs, without affecting the LEP-N2 wave elicited by nociceptive stimulation of the upper limbs, and without affecting the SEP-N2 wave elicited by vibrotactile stimulation of either limb. This selective and segmental effect indicates that the neuromodulatory after-effects of tsDCS cannot be explained by anodal blockade of axonal conduction and, instead, are most probably due to a segmental effect on the synaptic efficacy of the local processing and/or transmission of nociceptive inputs in the dorsal horn.

Event-related brain potentials elicited by high-speed cooling of the skin: A robust and non-painful method to assess the spinothalamic system in humans

OBJECTIVE

To investigate whether cool-evoked potentials (CEP) elicited by brisk innocuous cooling of the skin could serve as an alternative to laser-evoked potentials (LEP), currently considered as the best available neurophysiological tool to assess the spinothalamic tract and diagnose neuropathic pain.

METHODS

A novel device made of micro-Peltier elements and able to cool the skin at -300 °C/s was used to record CEPs elicited by stimulation of the hand dorsum in 40 healthy individuals, characterize the elicited responses, and assess their signal-to-noise ratio. Various stimulation surfaces (40 mm² and 120 mm²), cooling ramps (-200 °C/s and -133 °C/s) and temperature steps (20 °C, 15 °C, 10 °C, 5 °C) were tested to identify optimal stimulation conditions.

RESULTS

CEPs were observed in all conditions and subjects, characterized by a biphasic negative-positive complex maximal at the vertex (Cz), peaking 190-400 ms after stimulus onset, preceded by a negative wave over central-parietal areas contralateral to the stimulated hand. Their magnitude was modulated by stimulation surface, cooling ramp and temperature step.

CONCLUSION

Rapid innocuous skin cooling elicits robust CEPs at latencies compatible with the conduction velocity of Aδ-fibers.

SIGNIFICANCE

CEPs can be a complementary tool to the recording of LEPS for assessing the function of small-diameter Aδ-fibers and the spinothalamic tract.

Magnetoencephalographic study of event-related fields and cortical oscillatory changes during cutaneous warmth processing

Thermoreception is an important cutaneous sense, which plays a role in the maintenance of our body temperature and in the detection of potential noxious heat stimulation. In this study, we investigated event‐related fields (ERFs) and neural oscillatory activities, which were modulated by warmth stimulation. We developed a warmth stimulator that could elicit a warmth sensation, without pain or tactile sensation, by using a deep‐penetrating 980‐nm diode laser. The index finger of each participant (n = 24) was irradiated with the laser warmth stimulus, and the cortical responses were measured using magnetoencephalography (MEG). The ERFs and oscillatory responses had late latencies (∼1.3 s and 1.0–1.5 s for ERFs and oscillatory responses, respectively), which could be explained by a slow conduction velocity of warmth‐specific C‐fibers. Cortical sources of warmth‐related ERFs were seen in the bilateral primary and secondary somatosensory cortices (SI and SII), posterior part of the anterior cingulate cortex (pACC), ipsilateral primary motor, and premotor cortex. Thus, we suggested that SI, SII, and pACC play a role in processing the warmth sensation. Time–frequency analysis demonstrated the suppression of the alpha (8–13 Hz) and beta (18–23 Hz) band power in the bilateral sensorimotor cortex. We proposed that the suppressions in alpha and beta band power are involved in the automatic response to the input of warmth stimulation and sensorimotor interactions. The delta band power (1–4 Hz) increased in the frontal, temporal, and cingulate cortices. The power changes in delta band might be related with the attentional processes during the warmth stimulation.

Neurophysiological Comparison Among Tonic, High Frequency, and Burst Spinal Cord Stimulation: Novel Insights Into Spinal and Brain Mechanisms of Action

RATIONALE

Spinal cord stimulation (SCS) is an effective option for neuropathic pain treatment. New technological developments, as high-frequency (HF) and theta burst stimulation (TBS), have shown promising results, although putative mechanisms of action still remain debated.

METHODS

thirty patients with lower back pain were enrolled and underwent LF, HF, and TBS. Laser evoked potentials (LEPs) were recorded by using a Nd:YAG laser. Amplitudes and latencies of the main two components (N1, N2/P2) were compared among different experimental sessions. Changes in resting motor threshold (RMT), cortical silent period (cSP), short intracortical inhibition (SICI), and intracortical facilitation (ICF) were also evaluated.

RESULTS

TBS dampened LEP amplitudes compared with LF (N1: p = 0.032; N2/P2: p < 0.0001) and HF stimulation (N1: p = 0.029; N2/P2: p < 0.0001, Holm-Sidak post-hoc test). Concurrently, TBS increased N1 latency, when compared with baseline and LF stimulation (p = 0.009 and 0.0033). Whereas RMT and SICI did not change among experimental conditions, TBS significantly prolonged cSP duration compared with baseline (p = 0.002), LF (p = 0.048), and HF-SCS (p = 0.016); finally, both HF (p = 0.004) and TBS (p = 0.0039) increased ICF.

CONCLUSION

TBS modulates medial and lateral pain pathways through distinct mechanisms, possibly involving both GABA(a)ergic and Glutamatergic networks at an intracortical level. These results may have implications for therapy and for the choice of best stimulation protocol.

Evidence-based source modeling of nociceptive cortical responses: A direct comparison of scalp and intracranial activity in humans

BACKGROUND

Source modeling of EEG traditionally relies on interplay between physiological hypotheses and mathematical estimates. We propose to optimize the process by using evidence gathered from brain imaging and intracortical recordings.

METHODS

We recorded laser-evoked potentials in 18 healthy participants, using high-density EEG. Brain sources were modeled during the first second poststimulus, constraining their initial position to regions where nociceptive-related activity has been ascertained by intracranial EEG. These comprised the two posterior operculo-insular regions, primary sensorimotor, posterior parietal, anterior cingulate/supplementary motor (ACC/SMA), bilateral frontal/anterior insular, and posterior cingulate (PCC) cortices.

RESULTS

The model yielded an average goodness of fit of 91% for individual and 95.8% for grand-average data. When compared with intracranial recordings from 27 human subjects, no significant difference in peak latencies was observed between modeled and intracranial data for 5 of the 6 assessable regions. Morphological match was excellent for operculo-insular, frontal, ACC/SMA and PCC regions (cross-correlation > 0.7) and fair for sensori-motor and posterior parietal cortex (c-c ∼ 0.5).

CONCLUSIONS

Multiple overlapping activities evoked by nociceptive input can be disentangled from high-density scalp EEG guided by intracranial data. Modeled sources accurately described the timing and morphology of most activities recorded with intracranial electrodes, including those coinciding with the emergence of stimulus awareness. Hum Brain Mapp 38:6083-6095, 2017. © 2017 Wiley Periodicals, Inc.

Human primary somatosensory cortex is differentially involved in vibrotaction and nociception

The role of the primary somatosensory cortex (S1) in vibrotaction is well established. In contrast, its involvement in nociception is still debated. Here we test whether S1 is similarly involved in the processing of nonnociceptive and nociceptive somatosensory input in humans by comparing the aftereffects of high-definition transcranial direct current stimulation (HD-tDCS) of S1 on the event-related potentials (ERPs) elicited by nonnociceptive and nociceptive somatosensory stimuli delivered to the ipsilateral and contralateral hands. Cathodal HD-tDCS significantly affected the responses to nonnociceptive somatosensory stimuli delivered to the contralateral hand: both early-latency ERPs from within S1 (N20 wave elicited by transcutaneous electrical stimulation of median nerve) and late-latency ERPs elicited outside S1 (N120 wave elicited by short-lasting mechanical vibrations delivered to index fingertip, thought to originate from bilateral operculo-insular and cingulate cortices). These results support the notion that S1 constitutes an obligatory relay for the cortical processing of nonnociceptive tactile input originating from the contralateral hemibody. Contrasting with this asymmetric effect of HD-tDCS on the responses to nonnociceptive somatosensory input, HD-tDCS over the sensorimotor cortex led to a bilateral and symmetric reduction of the magnitude of the N240 wave of nociceptive laser-evoked potentials elicited by stimulation of the hand dorsum. Taken together, our results demonstrate in humans a differential involvement of S1 in vibrotaction and nociception.NEW & NOTEWORTHY Whereas the role of the primary somatosensory cortex (S1) in vibrotaction is well established, its involvement in nociception remains strongly debated. By assessing, in healthy volunteers, the effect of high-definition transcranial direct current stimulation over S1, we demonstrate a differential involvement of S1 in vibrotaction and nociception.

Distinct Somatic Discrimination Reflected by Laser-Evoked Potentials Using Scalp EEG Leads

Discrimination is an important function in pain processing of the somatic cortex. The involvement of the somatic cortex has been studied using equivalent dipole analysis and neuroimaging, but the results are inconsistent. Scalp electroencephalography (EEG) can reflect functional changes of particular brain regions underneath a lead. However, the responses of EEG leads close to the somatic cortex in response to pain have not been systematically evaluated. The present study applied CO₂ laser stimulation to the dorsum of the left hand. Laser-evoked potentials (LEPs) of C4, T3, and T4 leads and pain ratings in response to four stimulus intensities were analyzed. LEPs started earlier at the C4 and T4 leads. The onset latency and peak latency of LEPs for C4 and T4 leads were the same. Only 10 of 22 subjects (45 %) presented equivalent current dipoles within the primary somatosensory or motor cortices. LEP amplitudes of these leads increased as stimulation intensity increased. The stimulus–response pattern of the C4 lead was highly correlated with pain rating. In contrast, an S-shaped stimulus–response curve was obtained for the T3 and T4 leads. The present study provides supporting evidence that particular scalp channels are able to reflect the functional characteristics of their underlying cortical areas. Our data strengthen the clinical application of somatic-cortex-related leads for pain discrimination.

Mapping multidimensional pain experience onto electrophysiological responses to noxious laser heat stimuli

The origin of the conscious experience of pain in the brain is a continuing enigma in neuroscience. To shed light on the brain representation of a multifaceted pain experience in humans, we combined multivariate analysis of subjective aspects of pain sensations with detailed, single-trial analysis of electrophysiological brain responses. Participants were asked to fully focus on any painful or non-painful sensations occurring in their left hand during an interval surrounding the onset of noxious laser heat stimuli, and to rate their sensations using a set of visual analogue scales. Statistical parametric mapping was used to compute a multivariate regression analysis of subjective responses and single-trial laser evoked potentials (LEPs) at subject and group levels. Standardized Low Resolution Electromagnetic Tomography method was used to reconstruct sources of LEPs. Factor analysis of subjective responses yielded five factors. Factor 1, representing pain, mapped firstly as a negative potential at the vertex and a positive potential at the fronto-temporal region during the 208-260ms interval, and secondly as a strong negative potential in the right lateral frontal and prefrontal scalp regions during the 1292-1340ms interval. Three other factors, labelled "anticipated pain", "stimulus onset time", and "body sensations", represented non-specific aspects of the pain experience, and explained portions of LEPs in the latency range from 200ms to 700ms. The subjective space of pain during noxious laser stimulation is represented by one large factor featuring pain intensity, and by other factors accounting for non-specific parts of the sensory experience. Pain is encoded in two separate latency components with different scalp and brain representations.

Double-Cone Coil TMS Stimulation of the Medial Cortex Inhibits Central Pain Habituation

Objective

The aim of this study was to investigate whether Transcranial Magnetic Stimulation (TMS) applied over the medial line of the scalp affects the subjective perception of continuous pain induced by means of electric stimulation. In addition, we wanted to identify the point of stimulation where this effect was maximum.

Methods

Superficial electrical stimulation was used to induce continuous pain on the dominant hand. At the beginning of the experiment we reached a pain rating of 5 on an 11-point numeric rating scale (NRS; 0 = no pain and 10 = maximum tolerable pain) for each subject by setting individually the current intensity. The TMS (five pulses at increasing intensities) was applied on 5 equidistant points (one per session) over the medial line of the scalp in 13 healthy volunteers using a double-cone coil to stimulate underlying parts of the brain cortex. In every experimental session the painful stimulation lasted 45 minutes, during which pain and distress intensities NRS were recorded continuously. We calculated the effect of adaptation and the immediate effect of the TMS stimulation for all locations. Additionally, an ALE (Activation Likelihood Estimation) meta-analysis was performed to compare our results with the neuroimaging literature on subjective pain rating.

Results

TMS stimulation temporarily decreased the pain ratings, and pain adaptation was suppressed when applying the TMS over the FCz site on the scalp. No effect was found for distress ratings.

Conclusions

The present data suggest that the medial cortex in proximity of the cingulated gyrus has a causal role in adaptation mechanisms and in processing ongoing pain and subjective sensation of pain intensity.

Pain modulation in waking and hypnosis in women: event-related potentials and sources of cortical activity

Using a strict subject selection procedure, we tested in High and Low Hypnotizable subjects (HHs and LHs) whether treatments of hypoalgesia and hyperalgesia, as compared to a relaxation-control, differentially affected subjective pain ratings and somatosensory event-related potentials (SERPs) during painful electric stimulation. Treatments were administered in waking and hypnosis conditions. LHs showed little differentiation in pain and distress ratings between hypoalgesia and hyperalgesia treatments, whereas HHs showed a greater spread in the instructed direction. HHs had larger prefrontal N140 and P200 waves of the SERPs during hypnotic hyperalgesia as compared to relaxation-control treatment. Importantly, HHs showed significant smaller frontocentral N140 and frontotemporal P200 waves during hypnotic hypoalgesia. LHs did not show significant differences for these SERP waves among treatments in both waking and hypnosis conditions. Source localization (sLORETA) method revealed significant activations of the bilateral primary somatosensory (BA3), middle frontal gyrus (BA6) and anterior cingulate cortices (BA24). Activity of these contralateral regions significantly correlated with subjective numerical pain scores for control treatment in waking condition. Moreover, multivariate regression analyses distinguished the contralateral BA3 as the only region reflecting a stable pattern of pain coding changes across all treatments in waking and hypnosis conditions. More direct testing showed that hypnosis reduced the strength of the association of pain modulation and brain activity changes at BA3. sLORETA in HHs revealed, for the N140 wave, that during hypnotic hyperalgesia, there was an increased activity within medial, supramarginal and superior frontal gyri, and cingulated gyrus (BA32), while for the P200 wave, activity was increased in the superior (BA22), middle (BA37), inferior temporal (BA19) gyri and superior parietal lobule (BA7). Hypnotic hypoalgesia in HHs, for N140 wave, showed reduced activity within medial and superior frontal gyri (BA9,8), paraippocampal gyrus (BA34), and postcentral gyrus (BA1), while for the P200, activity was reduced within middle and superior frontal gyri (BA9 and BA10), anterior cingulate (BA33), cuneus (BA19) and sub-lobar insula (BA13). These findings demonstrate that hypnotic suggestions can exert a top-down modulatory effect on attention/preconscious brain processes involved in pain perception.

Heightened eating drive and visual food stimuli attenuate central nociceptive processing

Hunger and pain are basic drives that compete for a behavioral response when experienced together. To investigate the cortical processes underlying hunger-pain interactions, we manipulated participants' hunger and presented photographs of appetizing food or inedible objects in combination with painful laser stimuli. Fourteen healthy participants completed two EEG sessions: one after an overnight fast, the other following a large breakfast. Spatio-temporal patterns of cortical activation underlying the hunger-pain competition were explored with 128-channel EEG recordings and source dipole analysis of laser-evoked potentials (LEPs). We found that initial pain ratings were temporarily reduced when participants were hungry compared with fed. Source activity in parahippocampal gyrus was weaker when participants were hungry, and activations of operculo-insular cortex, anterior cingulate cortex, parahippocampal gyrus, and cerebellum were smaller in the context of appetitive food photographs than in that of inedible object photographs. Cortical processing of noxious stimuli in pain-related brain structures is reduced and pain temporarily attenuated when people are hungry or passively viewing food photographs, suggesting a possible interaction between the opposing motivational forces of the eating drive and pain.

Emotional modulation of experimental pain: a source imaging study of laser evoked potentials

Negative emotions have been shown to augment experimental pain. As induced emotions alter brain activity, it is not clear whether pain augmentation during noxious stimulation would be related to neural activation existing prior to onset of a noxious stimulus or alternatively, whether emotional stimuli would only alter neural activity during the period of nociceptive processing. We analyzed the spatio-temporal patterns of laser evoked potentials (LEPs) occurring prior to and during the period of cortical processing of noxious laser stimuli during passive viewing of negative, positive, or neutral emotional pictures. Independent component analysis (ICA) was applied to series of source activation volumes, reconstructed using local autoregressive average model (LAURA). Pain was the strongest when laser stimuli were associated with negative emotional pictures. Prior to laser stimulus and during the first 100 ms after onset of laser stimulus, activations were seen in the left and right medial temporal cortex, cerebellum, posterior cingulate, and rostral cingulate/prefrontal cortex. In all these regions, positive or neutral pictures showed stronger activations than negative pictures. During laser stimulation, activations in the right and left anterior insula, temporal cortex and right anterior and posterior parietal cortex were stronger during negative than neutral or positive emotional pictures. Results suggest that negative emotional stimuli increase activation in the left and right anterior insula and temporal cortex, and right posterior and anterior parietal cortex only during the period of nociceptive processing. The role of background brain activation in emotional modulation of pain appears to be only permissive, and consisting in attenuation of activation in structures maintaining the resting state of the brain.

Emotional conflict in a model modulates nociceptive processing in an onlooker: a laser-evoked potentials study

Observing models displaying facial expressions of pain elicits neural activity in onlookers' neural structures involved in first-hand experience of pain and in monitoring conflicting information. We investigated whether the purported conflict between the pain and its emotional expression in a model modulates cortical responses elicited by nociceptive laser stimuli in an onlooker. Seeing happy facial expressions, incongruent with the perceptual status attributed to the model, determined a significant reduction in the laser-evoked N2 potential. One of the main sources of this response is the anterior cingulate cortex, an area involved in pain perception, empathy for pain and conflict detection. A pre-activation of the anterior cingulate cortex due to the detection of the emotional conflict may, therefore, be responsible for the reduction of nociceptive-related response in the same brain area. Thus, top-down variables, like the appraisal of the others' emotional status, modulate onlookers' nociceptive-related neural activity.

Coronary microvascular function and cortical pain processing in patients with silent positive exercise testing and normal coronary arteries

ST-segment depression during exercise stress testing in asymptomatic subjects showing normal coronary arteries is considered a "false-positive" result. Coronary microvascular dysfunction, however, might be a possible cause of ST-segment depression in these cases. We assessed the coronary blood flow response to adenosine and to cold pressor test in the left anterior descending artery, using transthoracic Doppler echocardiography in 14 asymptomatic subjects with exercise-induced ST-segment depression and normal coronary arteries (group 1), 14 patients with microvascular angina (group 2), and 14 healthy subjects (group 3). Flow-mediated dilation was assessed in the brachial artery. Central pain processing was assessed using cortical laser evoked potentials during chest and right hand stimulation with 3 sequences of painful stimuli. The coronary blood flow response to adenosine was 1.8 ± 0.4, 1.9 ± 0.5, and 3.1 ± 0.9 in groups 1, 2, and 3, respectively (p <0.001). The corresponding coronary blood flow responses to the cold pressor test were 1.74 ± 0.4, 1.53 ± 0.3, and 2.3 ± 0.6 (p <0.001). The flow-mediated dilation was 5.5 ± 2.3%, 4.6 ± 2.4%, and 9.8 ± 1.2% in the 3 groups, respectively (p <0.001). The laser evoked potential N2/P2 wave amplitude decreased throughout the 3 sequences of stimulation in groups 1 and 3 but not in group 2 (chest, -19 ± 22%, +11 ± 42% and -36 ± 12%, p <0.001; right hand, -22 ± 25%, +12 ± 43% and -30 ± 20%, p = 0.009; in groups 1, 2, and 3). In conclusion, exercise stress test-induced ST-segment depression in asymptomatic subjects with normal coronary arteries cannot be considered as a simple false-positive result, because it can be related to coronary microvascular dysfunction. The different symptomatic state compared to patients with microvascular angina can, at least in part, be explained by differences in cortical processing of neural pain stimuli.

Habituation to pain in "medication overuse headache": a CO2 laser-evoked potential study

OBJECTIVE

Our aim was to investigate CO(2) laser-evoked potential (LEP) habituation to experimental pain in a group of patients affected by medication-overuse headache, with a history of episodic migraine becoming chronic, before and after treatment, consisting in acute medication withdrawal and a preventive treatment cycle.

BACKGROUND

One of the main features of LEPs in migraineurs is a lower habituation to repetitive noxious stimuli during the interictal phase.

METHODS

LEPs were recorded to stimulation of both the right hand and the right perioral region in 14 patients and in 14 healthy subjects. The habituation of both the N1 and the vertex N2/P2 components was assessed by measuring the LEP amplitude changes across 3 consecutive repetitions of 30 trials each.

RESULTS

In the 8 patients who had clinically improved after treatment, the N2/P2 amplitude habituation was significantly higher after treatment than before treatment following both hand (F = 43.2, P < .0001) and face stimulation (F = 6.9, P = .01). In these patients, the N2/P2 amplitude habituation after treatment was not different from that obtained in healthy controls (P = .18 and P = .73 for hand and face stimulation, respectively). On the contrary, in the patients who did not improve, the N2/P2 amplitude still showed reduced habituation after both hand (F = 3.1, P = .08) and face (F = 0.7, P = .4) stimulation.

CONCLUSION

The deficient habituation of the vertex N2/P2 complex was partly restored after successful treatment of medication-overuse headache, reflecting a modification in pain-processing pathways.

Cerebral excitability is abnormal in patients with painful chronic pancreatitis

BACKGROUND

We investigated whether patients with painful chronic pancreatitis (CP) present abnormalities in the cerebral response to experimental pain stimuli.

METHODS

Contact heat-evoked potentials (CHEPS) were recorded in 15 patients with CP and in 15 healthy volunteers during repetitive stimulation of the upper abdominal region (pancreatic 'viscerotome') and the right forearm (heterologous area). Three sequences of painful stimuli were applied at each site. Subjective pain scores were assessed by a visual analogue scale. Habituation was calculated as the relative change in CHEPS amplitudes between the first and the third stimulation sequence.

RESULTS

As expected pain scores decreased in healthy volunteers during successive stimulations at both sites (i.e., habituation), while in the CP group, they remained unchanged. The cerebral response consisted of an early-latency, low-amplitude response (N1, contralateral temporal region) followed by a late, high-amplitude, negative-positive complex (N2/P2, vertex). During successive stimulation of the pancreatic area, N2/P2 amplitude increased 25% in CP patients, while they decreased 20% in healthy volunteers (p = 0.006). After stimulation of the forearm, N2/P2 amplitudes increased 3% in CP patients compared to a decrease of 20% in healthy volunteers (p = 0.06).

CONCLUSIONS

Taken together, CP patients had an abnormal cerebral response to repetitive thermal stimuli. This was most prominent after stimulation of the upper abdominal area. As this area share spinal innervation with the pancreatic gland, these findings likely mirror distinctive abnormalities in cerebral pain processing.

The influence of transient spatial attention on the processing of intracutaneous electrical stimuli examined with ERPs

OBJECTIVE

Determine the influence of transient spatial attention on the processing of intracutaneous electrical stimuli.

METHODS

Electrical stimuli, a single pulse or five pulses, were presented at the index fingers of the left or right hand. The to-be-attended hand and stimulated finger varied randomly from trial to trial. Participants had to press a foot pedal only when the relevant stimulus, varied between participants, occurred at the attended hand. EEG was measured to extract relevant ERP components.

RESULTS

The N100 and N150 were enhanced for attended as compared to unattended stimuli. The N100, N150, P260, and the P500 were enlarged for five pulse as compared to single pulse stimuli. The P260, which is thought to reflect a call for attention, was enhanced for unattended as compared to attended stimuli. Source analyses indicate that attentional effects on the N100, N150, and P260 may be related to changes in activity in secondary somatosensory areas and the anterior cingulate cortex.

CONCLUSIONS

A transient manipulation of spatial attention increases cortical activity induced by attended relative to unattended intracutaneous electrical stimuli, but initially unattended stimuli appear to induce an enhanced orienting effect.

SIGNIFICANCE

Initially unattended intracutaneous electrical stimuli seem to induce a call for attention.

The primary somatosensory cortex largely contributes to the early part of the cortical response elicited by nociceptive stimuli

Research on the cortical sources of nociceptive laser-evoked brain potentials (LEPs) began almost two decades ago (Tarkka and Treede, 1993). Whereas there is a large consensus on the sources of the late part of the LEP waveform (N2 and P2 waves), the relative contribution of the primary somatosensory cortex (S1) to the early part of the LEP waveform (N1 wave) is still debated. To address this issue we recorded LEPs elicited by the stimulation of four limbs in a large population (n=35). Early LEP generators were estimated both at single-subject and group level, using three different approaches: distributed source analysis, dipolar source modeling, and probabilistic independent component analysis (ICA). We show that the scalp distribution of the earliest LEP response to hand stimulation was maximal over the central-parietal electrodes contralateral to the stimulated side, while that of the earliest LEP response to foot stimulation was maximal over the central-parietal midline electrodes. Crucially, all three approaches indicated hand and foot S1 areas as generators of the earliest LEP response. Altogether, these findings indicate that the earliest part of the scalp response elicited by a selective nociceptive stimulus is largely explained by activity in the contralateral S1, with negligible contribution from the secondary somatosensory cortex (S2).

Cortical activation changes during repeated laser stimulation: a magnetoencephalographic study

Repeated warm laser stimuli produce a progressive increase of the sensation of warmth and heat and eventually that of a burning pain. The pain resulting from repetitive warm stimuli is mediated by summated C fibre responses. To shed more light on the cortical changes associated with pain during repeated subnoxious warm stimution, we analysed magnetoencephalographic (MEG) evoked fields in eleven subjects during application of repetitive warm laser stimuli to the dorsum of the right hand. One set of stimuli encompassed 10 laser pulses occurring at 2.5 s intervals. Parameters of laser stimulation were optimised to elicit a pleasant warm sensation upon a single stimulus with a rise of skin temperature after repeated stimulation not exceeding the threshold of C mechano-heat fibres. Subjects reported a progressive increase of the intensity of heat and burning pain during repeated laser stimulation in spite of only mild (4.8°C) increase of skin temperature from the first stimulus to the tenth stimulus. The mean reaction time, evaluated in six subjects, was 1.33 s, confirming involvement of C fibres. The neuromagnetic fields were modelled by five equivalent source dipoles located in the occipital cortex, cerebellum, posterior cingulate cortex, and left and right operculo-insular cortex. The only component showing statistically significant changes during repetitive laser stimulation was the late component of the contralateral operculo-insular source peaking at 1.05 s after stimulus onset. The amplitude increases of the late component of the contralateral operculo-insular source dipole correlated with the subjects' numerical ratings of warmth and pain. Results point to a pivotal role of the contralateral operculo-insular region in processing of C-fibre mediated pain during repeated subnoxious laser stimulation.

Features of somatosensory manifestations induced by intracranial electrical stimulations of the human insula

OBJECTIVE

To study the clinical manifestations induced by intracranial electrical stimulation of the insular cortex in epileptic patients submitted to invasive stereo-electroencephalography (SEEG) recordings.

METHODS

We retrospectively studied the clinical manifestations induced by intracerebral electrical stimulations of the insular cortex in 96 patients. In order to precisely localize the position of the electrodes a postoperative 3D CT scan or a 3D MRI was obtained and then the images were merged with the preoperative MRI in the same stereotactic referenced system.

RESULTS

A total of 341 electrical bipolar stimulations were performed. The most frequently induced symptom was a somatosensory manifestation (70%), mainly tingling and electric sensation involving the contralateral face and arm. Motor responses represented the 8% of the total amount, as well as auditory phenomena. Language dysfunction accounted for 2% of responses. Autonomic and gustatory phenomena represented respectively 1% of responses.

CONCLUSIONS

We found a great prevalence of somatosensory manifestations whereas other types of clinical modifications were extremely infrequent.

SIGNIFICANCE

Our data support a prominent somatosensory role of the human insular cortex and provide a precise characterization of the different types of sensory manifestations induced by intracranial electrical stimulation of the human insula.

Functional neuroanatomy of the insular lobe

The insula is the fifth lobe of the brain and it is the least known. Hidden under the temporal, frontal and parietal opercula, as well as under dense arterial and venous vessels, its accessibility is particularly restricted. Functional data on this region in humans, therefore, are scarce and the existing evidence makes conclusions on its functional and somatotopic organization difficult. 5 patients with intractable epilepsy underwent an invasive presurgical evaluation with implantation of diagnostic invasive-depth electrodes, including insular electrodes that were inserted using a mesiocaudodorsal to laterorostroventral approach. Altogether 113 contacts were found to be in the insula and were stimulated with alternating currents during preoperative monitoring. Different viscerosensitive and somatosensory phenomena were elicited by stimulation of these electrodes. A relatively high density of electrode contacts enabled us to delineate several functionally distinct areas within the insula. We found somatosensory symptoms to be restricted to the posterior insula and a subgroup of warmth or painful sensations in the dorsal posterior insula. Viscerosensory symptoms were elicited by more anterior electrode contacts with a subgroup of gustatory symptoms occurring after stimulation of electrode contacts in the central part of the insula. The anterior insula did not show reproducible responses to stimulation. In line with previous studies, we found evidence for somato- and viscerosensory cortex in the insula. In addition, our results suggest that there is a predominantly posterior and central distribution of these functions in the insular lobe.

Baroreceptor activation attenuates attentional effects on pain-evoked potentials

Focused attention typically enhances neural nociceptive responses, reflected electroencephalographically as increased amplitude of pain-evoked event-related potentials (ERPs). Additionally, pain-evoked ERPs are attenuated by hypertension and baroreceptor activity, through as yet unclear mechanisms. There is indirect evidence that these two effects may interact, suggesting that baroreceptor-related modulation of nociception is more than a low-level gating phenomenon. To address this hypothesis, we explored in a group of healthy participants the combined effects of cue-induced expectancy and baroreceptor activity on the amplitude of pain-evoked ERPs. Brief nociceptive skin stimuli were delivered during a simple visual task; half were preceded by a visual forewarning cue, and half were unpredictable. Nociceptive stimuli were timed to coincide either with systole (maximum activation of cardiac baroreceptors) or with diastole (minimum baroreceptor activation). We observed a strong interaction between expectancy and cardiac timing for the amplitude of the P2 ERP component; no effects were observed for the N2 component. Cued stimuli were associated with larger P2 amplitude, but this effect was abolished for stimuli presented during baroreceptor activation. No cardiac timing effect was observed for un-cued stimuli. Taken together, these findings suggest a close integration of cognitive-affective aspects of expectancy and baroreceptor influences on pain, and as such may cast further light on mechanisms underlying mental and physiological contributions to clinical pain.

Somatotopic organization of pain responses to direct electrical stimulation of the human insular cortex

The question whether pain encoding in the human insula shows some somatotopic organization is still pending. We studied 142 patients undergoing depth stereotactic EEG (SEEG) exploration of the insular cortex for pre-surgical evaluation of epilepsy. 472 insular electrical stimulations were delivered, of which only 49 (10.5%) elicited a painful sensation in 38 patients (27%). Most sites where low intensity electric stimulation produced pain, without after-discharge or concomitant visually detectable change in EEG activity outside the insula, were located in the posterior two thirds of the insula. Pain was located in a body area restricted to face, upper limb or lower limb for 27 stimulations (55%) and affected more than one of these regions for all others. The insular cortex being oriented parallel to the medial sagittal plane we found no significant difference between body segment representations in the medio-lateral axis. Conversely a somatotopic organization of sites where stimulation produced pain was observed along the rostro-caudal and vertical axis of the insula, showing a face representation rostral to those of upper and lower limbs, with an upper limb representation located above that of the lower limb. These data suggest that, in spite of large and often bilateral receptive fields, pain representation shows some degree of somatotopic organization in the human insula.

Oscillatory characteristics of nociceptive responses in the SII cortex

OBJECTIVE

This study is aimed to explore the frequency characteristics of pain-evoked neuromagnetic responses in the secondary somatosensory (SII) cortices.

METHODS

Thulium-laser nociceptive stimuli to the left hand dorsum of 10 right-handed healthy adults. The pain stimuli were rated as mild, moderate, and severe levels according to subjects' reports on a 10-point visual analog scale. We analyzed their cortical responses with wavelet-based frequency analyses and equivalent current dipole (ECD) modeling.

RESULTS

For each pain level, we found an increase of theta (4-8 Hz) and alpha (8-13 Hz) power in bilateral SII areas at 180-210 ms after stimulus onset. The power was larger for the moderate than for the mild pain level (p < 0.05), but there was no statistical power difference of these oscillations between moderate and severe pain stimulus conditions (p = 0.7). Within the SII area, we did not observe particular difference in theta and alpha ECD locations between varying pain level conditions.

CONCLUSIONS

The 4-13 Hz activities, peaking from 180 to 210 ms, are oscillatory correlates of SII activation in response to nociceptive stimulation, but their power may code the magnitude of pain stimuli only up to moderate level, as rated subjectively. This measure could be potentially used to evaluate SII activation in further pain studies.

Laser evoked potential recording from intracerebral deep electrodes

OBJECTIVE

To investigate whether recording from deep intracerebral (IC) electrodes can disclose laser evoked potential (LEP) components generated under the cerebral cortex.

METHODS

LEPs were recorded to hand and/or perioral region stimulation from 7 patients suffering from Parkinson's disease, who underwent implant of IC electrodes in the globus pallidum pars interna (GPi), in the subthalamic nucleus (STN) and in the pedunculopontine nucleus (PPN). LEPs were obtained from the IC electrode contacts and from the Cz vertex, referred to the nose.

RESULTS

The scalp traces showed a triphasic response (P1-N2-P2). The IC electrodes recorded two main components (ICP2 and ICN2), showing the same latencies as the scalp N2 and P2 potentials, respectively. The ICP2-ICN2 complex was sometimes preceded by a ICP1 wave at the same latency of the scalp P1 response.

CONCLUSIONS

The LEP components recorded from the IC electrodes mirrored the ones picked up from the Cz lead, thus suggesting that they are probably generated by the opposite pole of the same cortical sources producing the scalp responses.

SIGNIFICANCE

In the IC traces, there was no evidence of earlier potentials possibly generated within the thalamus or of subcortical far-field responses. This means that the nociceptive signal amplification occurring within the cerebral cortex is necessary to produce identifiable LEP components.

Aesthetic value of paintings affects pain thresholds

Pain is modulated by cognitive factors, including attention and emotions. In this study we evaluated the distractive effect of aesthetic appreciation on subjectively rated pain (visual analogue scale;VAS) and multi-channel evoked potentials induced by CO(2) laser stimulation of the left hand in twelve healthy volunteers. Subjects were stimulated by laser in the absence of other external stimulation (baseline condition) and while looking at different paintings they had previously rated as beautiful, neutral or ugly. The view of paintings previously appreciated as beautiful produced lower pain scores and a clear inhibition of the P2 wave amplitude, localized in the anterior cingulate cortex; the inhibition of P2 wave amplitude was lesser or not significant during the presentation of the ugly or neutral paintings, respectively. Dipole source localization analysis of the LEP peaks showed significant changes during different conditions, with a shift from the posterior to the anterior right cingulated cortex while looking at paintings previously rated as beautiful. Our results provide evidence that pain may be modulated at cortical level by the aesthetic content of the distracting stimuli.

Learning potentiates neurophysiological and behavioral placebo analgesic responses

Expectation and conditioning are supposed to be the two main psychological mechanisms for inducing a placebo response. Here, we further investigate the effects of both expectation, which was induced by verbal suggestion alone, and conditioning at the level of N1 and N2-P2 components of CO2 laser-evoked potentials (LEPs) and subjective pain reports. Forty-four healthy volunteers were pseudorandomly assigned to one of three experimental groups: Group 1 was tested with verbal suggestion alone, Group 2 was tested with a conditioning procedure, whereby the intensity of painful stimulation was reduced surreptitiously, so as to make the volunteers believe that the treatment was effective, Group 3 was a control group that allowed us to rule out phenomena of sensitization and/or habituation. Pain perception was assessed according to a Numerical Rating Scale (NRS) ranging from 0=no pain sensation to 10=maximum imaginable pain. Both verbal suggestions (Group 1) and conditioning (Group 2) modified the N2-P2 complex, but not the N1 component of LEPs. However, the suggestion-induced LEP changes occurred without subjective perception of pain decrease. Conversely, the N2-P2 amplitude changes that were induced by the conditioning procedure were associated with the subjective perception of pain reduction. Compared to natural history, conditioning produced more robust reductions of LEP amplitudes than verbal suggestions alone. Overall, these findings indicate that prior positive experience plays a key role in maximizing both behavioral and neurophysiological placebo responses, emphasizing that the placebo effect is a learning phenomenon which affects the early central nociceptive processing.

How Do Brain Areas Communicate During the Processing of Noxious Stimuli? An Analysis of Laser-Evoked Event-Related Potentials Using the Granger Causality Index

Several imaging techniques have identified different brain areas involved in the processing of noxious stimulation and thus in the constitution of pain. However, only little is known how these brain areas communicate with one another after activation by stimulus processing and which areas directionally affect or modulate the activity of succeeding areas. One measure for the analysis of such interactions is represented by the Granger Causality Index (GCI). In applying time-varying bivariate and partial variants of this concept (tvGCI), the aim of the present study was to investigate the interaction of neural activities between a set of scalp electrodes that best represent the brain electrical neural activity of major cortical areas involved in the processing of noxious laser-heat stimuli and their variation in time. Bivariate and partial tvGCIs were calculated within four different intervals of laser-evoked event-related potentials (LEPs) including a baseline period prior to stimulus application and three intervals immediately following stimulus application, i.e., between 170 and 200 ms (at the N2 component), between 260 and 320 ms (P2 component), and between 320 and 400 ms (P3 component of LEPs). Results show some similarities, but also some striking differences between bivariate and partial tvGCIs. These differences might be explained by the nature of bivariate and partial tvGCIs. However, both tvGCI approaches revealed a directed interaction between medial and lateral electrodes of the centroparietal region. This result was interpreted as a directed interaction between the anterior cingulate cortex and the secondary somatosensory cortex and the insula, structures that are significantly involved in the constitution of pain.