Dipolar generators of the early scalp somatosensory evoked potentials to tibial nerve stimulation in human subjects

Opioid Specific Effects on Central Processing of Sensation and Pain: A Randomized, Cross-Over, Placebo-Controlled Study

Moderate to severe pain is often treated with opioids, but central mechanisms underlying opioid analgesia are poorly understood. Findings thus far have been contradictory and none could infer opioid specific effects. This placebo-controlled, randomized, 2-way cross-over, double-blinded study aimed to explore opioid specific effects on central processing of external stimuli. Twenty healthy male volunteers were included and 3 sets of assessments were done at each of the 2 visits: 1) baseline, 2) during continuous morphine or placebo intravenous infusion and 3) during simultaneous morphine + naloxone or placebo infusion. Opioid antagonist naloxone was introduced in order to investigate opioid specific effects by observing which morphine effects are reversed by this intervention. Quantitative sensory testing, spinal nociceptive withdrawal reflexes (NWR), spinal electroencephalography (EEG), cortical EEG responses to external stimuli and resting EEG were measured and analyzed. Longer lasting pain (cold-pressor test - hand in 2° water for 2 minutes, tetanic electrical), deeper structure pain (bone pressure) and strong nociceptive (NWR) stimulations were the most sensitive quantitative sensory testing measures of opioid analgesia. In line with this, the principal opioid specific central changes were seen in NWRs, EEG responses to NWRs and cold-pressor EEG. The magnitude of NWRs together with amplitudes and insular source strengths of the corresponding EEG responses were attenuated. The decreases in EEG activity were correlated to subjective unpleasantness scores. Brain activity underlying slow cold-pressor EEG (1-4Hz) was decreased, whereas the brain activity underlying faster EEG (8-12Hz) was increased. These changes were strongly correlated to subjective pain relief. This study points to evidence of opioid specific effects on perception of external stimuli and the underlying central responses. The analgesic response to opioids is likely a synergy of opioids acting at both spinal and supra-spinal levels of the central nervous system. Due to the strong correlations with pain relief, the changes in EEG signals during cold-pressor test have the potential to serve as biomarkers of opioid analgesia. PERSPECTIVE: This exploratory study presents evidence of opioid specific effects on the pain system at peripheral and central levels. The findings give insights into which measures are the most sensitive for assessing opioid-specific effects.

Manipulation of Dysfunctional Spinal Joints Affects Sensorimotor Integration in the Prefrontal Cortex: A Brain Source Localization Study

Objectives. Studies have shown decreases in N30 somatosensory evoked potential (SEP) peak amplitudes following spinal manipulation (SM) of dysfunctional segments in subclinical pain (SCP) populations. This study sought to verify these findings and to investigate underlying brain sources that may be responsible for such changes. Methods. Nineteen SCP volunteers attended two experimental sessions, SM and control in random order. SEPs from 62-channel EEG cap were recorded following median nerve stimulation (1000 stimuli at 2.3 Hz) before and after either intervention. Peak-to-peak amplitude and latency analysis was completed for different SEPs peak. Dipolar models of underlying brain sources were built by using the brain electrical source analysis. Two-way repeated measures ANOVA was used to assessed differences in N30 amplitudes, dipole locations, and dipole strengths. Results. SM decreased the N30 amplitude by 16.9 ± 31.3% (P = 0.02), while no differences were seen following the control intervention (P = 0.4). Brain source modeling revealed a 4-source model but only the prefrontal source showed reduced activity by 20.2 ± 12.2% (P = 0.03) following SM. Conclusion. A single session of spinal manipulation of dysfunctional segments in subclinical pain patients alters somatosensory processing at the cortical level, particularly within the prefrontal cortex.

Brain source connectivity reveals the visceral pain network

INTRODUCTION

Several brain structures have been consistently found to be involved in visceral pain processing. However, recent research questions the specificity of these regions and it has been suggested that it is not singular activations of brain areas, but their cross-communication that results in perception of pain. Moreover, frequency at which neurons are firing could be what separates pain from other sensory modalities which otherwise involve the same anatomical locations. In this test/retest study, we identified the network of sources and their frequencies following visceral pain.

METHODS

62-channel evoked potentials following electrical stimulation in oesophagus were recorded in twelve healthy volunteers on two separate days. Multichannel matching pursuit (MMP) and dipolar source localisation were used. Multiple sources responsible for one MMP component were considered to act synchronously as each MMP component is mono-frequency and has a single topography. We first identified components that were reproducible within subjects over recording sessions. These components were then analysed across subjects.

RESULTS

MMP and source localisation showed three main brain networks; an early network at ~8.3 Hz and ~3.5 Hz involving brainstem, operculum, and pre-frontal cortex peaking at ~77 ms. This was followed by an operculum, amygdale, mid-cingulate, and anterior-cingulate network at ~4.5 Hz. Finally, there was an operculum and mid-cingulate network that persisted over the entire time interval, peaking at 245.5±51.4 ms at ~2.1 Hz.

CONCLUSION

This study gives evidence of operculum's central integrative role for perception of pain and shows that MMP is a reliable method to study upstream brain activity.

3D handheld laser scanner based approach for automatic identification and localization of EEG sensors

This paper describes and assesses for the first time the use of a handheld 3D laser scanner for scalp EEG sensor localization and co-registration with magnetic resonance images. Study on five subjects showed that the scanner had an equivalent accuracy, a better repeatability, and was faster than the reference electromagnetic digitizer. According to electrical source imaging, somatosensory evoked potentials experiments validated its ability to give precise sensor localization. With our automatic labeling method, the data provided by the scanner could be directly introduced in the source localization studies.

Topographic distribution of the tibial somatosensory evoked potential using coherence

The objective of the present study was to determine the adequate cortical regions based on the signal-to-noise ratio (SNR) for somatosensory evoked potential (SEP) recording. This investigation was carried out using magnitude-squared coherence (MSC), a frequency domain objective response detection technique. Electroencephalographic signals were collected (International 10-20 System) from 38 volunteers, without history of neurological pathology, during somatosensory stimulation. Stimuli were applied to the right posterior tibial nerve at the rate of 5 Hz and intensity slightly above the motor threshold. Response detection was based on rejecting the null hypothesis of response absence (significance level alpha= 0.05 and M = 500 epochs). The best detection rates (maximum percentage of volunteers for whom the response was detected for the frequencies between 4.8 and 72 Hz) were obtained for the parietal and central leads mid-sagittal and ipsilateral to the stimulated leg: C4 (87%), P4 (82%), Cz (89%), and Pz (89%). The P37-N45 time-components of the SEP can also be observed in these leads. The other leads, including the central and parietal contralateral and the frontal and fronto-polar leads, presented low detection capacity. If only contralateral leads were considered, the centro-parietal region (C3 and P3) was among the best regions for response detection, presenting a correspondent well-defined N37; however, this was not observed in some volunteers. The results of the present study showed that the central and parietal regions, especially sagittal and ipsilateral to the stimuli, presented the best SNR in the gamma range. Furthermore, these findings suggest that the MSC can be a useful tool for monitoring purposes.

Effects of spinal cord stimulation on the cortical somatosensory evoked potentials in failed back surgery syndrome patients

OBJECTIVE

To evaluate the functional activation of the somatosensory cortical regions in neuropathic pain patients during therapeutic spinal cord stimulation (SCS).

METHODS

In nine failed back surgery syndrome patients, the left tibial and the left sural nerves were stimulated in two sessions with intensities at motor and pain thresholds, respectively. The cortical somatosensory evoked potentials were analyzed using source dipole analysis based on 111 EEG signals.

RESULTS

The short-latency components of the source located in the right primary somatosensory cortex (SI: 43, 54 and 65ms) after tibial nerve stimulation, the mid-latency SI component (87ms) after sural nerve stimulation, and the mid-latency components in the right (approximately 161ms) and left (approximately 168ms) secondary somatosensory cortices (SII) were smaller in the presence of SCS than in absence of SCS. The long-latency source component arising from the mid-cingulate cortex (approximately 313ms) was smaller for tibial and larger for sural nerve stimuli during SCS periods compared to periods without SCS.

CONCLUSIONS

SCS attenuates the somatosensory processing in the SI and SII. In the mid-cingulate cortex, the effect of SCS depends on the type of stimulation and nerve fibers involved.

SIGNIFICANCE

Results suggest that the effects of SCS on cortical somatosensory processing may contribute to a reduction of allodynia during SCS.

Use of magnitude-squared coherence to identify the maximum driving response band of the somatosensory evoked potential

The present study proposes to apply magnitude-squared coherence (MSC) to the somatosensory evoked potential for identifying the maximum driving response band. EEG signals, leads [Fpz'-Cz'] and [C3'-C4'], were collected from two groups of normal volunteers, stimulated at the rate of 4.91 (G1: 26 volunteers) and 5.13 Hz (G2: 18 volunteers). About 1400 stimuli were applied to the right tibial nerve at the motor threshold level. After applying the anti-aliasing filter, the signals were digitized and then further low-pass filtered (200 Hz, 6th order Butterworth and zero-phase). Based on the rejection of the null hypothesis of response absence (MSC(f) > 0.0060 with 500 epochs and the level of significance set at a = 0.05), the beta and gamma bands, 15-66 Hz, were identified as the maximum driving response band. Taking both leads together ("logical-OR detector", with a false-alarm rate of a = 0.05, and hence a = 0.0253 for each derivation), the detection exceeded 70% for all multiples of the stimulation frequency within this range. Similar performance was achieved for MSC of both leads but at 15, 25, 35, and 40 Hz. Moreover, the response was detected in [C3'-C4'] at 35.9 Hz and in [Fpz'-Cz'] at 46.2 Hz for all members of G2. Using the "logical-OR detector" procedure, the response was detected at the 7th multiple of the stimulation frequency for the series as a whole (considering both groups). Based on these findings, the MSC technique may be used for monitoring purposes.

Evidence for early activation of primary motor cortex and SMA after electrical lower limb stimulation using EEG source reconstruction

Compared to median nerve somatosensory evoked potentials (SEP), less is known about activity evoked by nerve stimulation of the lower limb. To understand the mechanisms and the physiology of sensor- and motor control it is useful to investigate the sensorimotor functions as revealed by a standardized functional status. Therefore, we investigated SEPs of the lower limb in 6 healthy male volunteers. For each side, tibial and peroneal nerves were stimulated transcutaneously at the fossa poplitea. The tibial nerves were also stimulated further distally at the ankle joint. Source localization was applied to 64-EEG-channel data of the SEPs. In contrast to somatosensory areas, which are activated after median nerve stimulation, we found dipoles adjacent to motor areas near Brodmann area 4 (BA 4) for SEP components P 32/40 and P 54/60 and near the supplementary motor area (SMA) for the N 75/83 component. These sources could reliably be distinguished for each individual subject as well as for the grand mean data set. Our data show that afferent projections from the lower limb mainly reach primary motor areas (BA 4) and only subsequently, with a delay of 40 ms, higher order motor areas such as SMA. We conclude that a focused view on SEP of the lower limb could be a useful tool to investigate pathological states in motor control or peripheral deafferentiation.

Attention induces reciprocal activity in the human somatosensory cortex enhancing relevant- and suppressing irrelevant inputs from fingers

OBJECTIVE

We studied whether attention regulates information processing in the human primary somatosensory cortex (SI) by selective enhancement of relevant- and suppression of irrelevant information.

METHODS

Under successive and simultaneous electric stimuli to both the right index and middle fingers, tactile stimuli were randomly (20%) presented on one of the two fingers in separate two runs exchanging the finger. Subjects were requested to discriminate the tactile stimuli in an attention task to induce attention to one finger and to ignore the stimuli in a control task to avoid such an attention focus. Somatosensory evoked magnetic fields were measured only for the two-finger electric stimulation and an early component (M50) was analyzed.

RESULTS

In spite of the two-finger simultaneous stimulation, attention to either the index or middle finger lowered or heightened the M50-sourse location, respectively. The attention task did not increase the M50 amplitude.

CONCLUSIONS

Attention to a finger enhanced selectively the representation of the finger in the SI cortex. However, this SI activity did not increase the M50 amplitude, suggesting that the attention suppressed another finger region receiving the unattended inputs.

SIGNIFICANCE

Attention regulates the SI activity by selectively enhancing the task-relevant information and by filtering out other noise inputs.

Plastic interactions between hand and face cortical representations in patients with trigeminal neuralgia: a somatosensory-evoked potentials study

Neurophysiological and neuroimaging studies suggest that pain may play a major role in determining cortical somatosensory rearrangements even in the adult brain. The re-organizational power of pain, however, has been tested in models in which massive deafferentation co-existed with pain (e.g. in phantom pain). Moreover, information on whether spinal and brainstem changes contribute to pain-related plasticity in humans is meagre. We used the non-invasive somatosensory evoked potentials technique in patients with right primary trigeminal neuralgia and no clinical signs of large-diameter fibers of trigeminal deafferentation to assess whether pain may induce plastic changes at multiple levels in the somatosensory system. Subcortical and cortical potentials evoked by stimulation of the right median and posterior tibial nerves ipsilateral to the facial pain were compared with those obtained following stimulation of the left median and tibial nerves and with those obtained in a control group tested in comparable conditions. Amplitudes of parietal N20 and P27 and frontal N30 potentials observed following stimulation of the right median nerve ipsilateral to the facial pain were greater than those of the left median nerve and showed a positive correlation with magnitude of pain. This right-left asymmetry was absent following stimulation of the patients' tibial nerves and in control subjects. No changes were found in spinal N13 and brainstem P14. That facial pain is associated with neuroplastic changes within the somatic cortical representation of the hand suggests a pain-related topographic cortical reorganisation.

Functional plasticity in the human primary somatosensory cortex following acute lesion of the anterior lateral spinal cord: neurophysiological evidence of short-term cross-modal plasticity

The primary somatosensory cortex (S1) in adult animals and humans is capable of rapid modification after deafferentation. These plastic changes may account for a loss of tonic control by nociceptive inputs over inhibitory mechanisms within structures of the dorsal column-medial lemniscal system. Most studies, however, have been performed under conditions where deafferentation of C and A delta fibres coexists with large-diameter fibres deafferentation. In this study the effect of the acute lesion of one ascending anterior lateral column on neuronal activity within the dorsal column-medial lemniscal system was assessed by recording somatosensory evoked potentials (SEPs) in seven patients who underwent unilateral percutaneous cervical cordotomy (PCC) as treatment for drug-resistant malignant pain.Spinal, brainstem and cortical SEPs were recorded 2h before and 3h after PCC by stimulating the posterior tibial nerve at both ankles. Amplitudes of cortical potentials obtained by stimulation of the leg contralateral to PCC were significantly increased after PCC. No significant changes in spinal or brainstem potentials were observed. PCC did not affect SEP components obtained by stimulation of the leg ipsilateral to PCC. Our results suggest that nociceptive deafferentation may induce a rapid modulation of cortical neuronal activity along the lemniscal pathway, thus providing the first evidence in humans of short-term cortical plasticity across the spinothalamic and lemniscal systems.

Source generators of the early somatosensory evoked potentials to tibial nerve stimulation: an intracerebral and scalp recording study

OBJECTIVE

To investigate the location of the cerebral generators of the early scalp somatosensory evoked potentials (SEPs) after tibial nerve stimulation.

METHODS

Tibial nerve SEPs were recorded in 15 patients, suffering from Parkinson's disease, who underwent implantation of intracerebral (IC) electrodes in the subthalamic nucleus, in the globus pallidum or in the thalamic ventralis intermediate nucleus. SEPs were recorded both from the scalp surface and from the IC leads.

RESULTS

The lemniscal P30 response was recorded by all the electrodes. The IC waveforms included a negative N40IC response, followed by a positive (P50IC) and a negative (N60IC) potential. The N40IC, the P50IC and the N60IC potentials did not differ in latency from the P40, the N50 and the P60 responses recorded by the Cz electrode. In 6 patients, in which SEPs were recorded also during the voluntary movement of the stimulated foot (active gating), an amplitude reduction of the SEP components following the P30 potential was observed during movement at the vertex and in the IC traces. Instead, in the contralateral temporal traces the SEP components (N40temp and P50temp) were not modified by active gating, and in the ipsilateral parietal traces only the positive potentials at about 60ms of latency was decreased.

CONCLUSIONS

Two differently oriented generators are active in the contralateral hemisphere at both 40 and 50ms of latency after tibial nerve stimulation. One source is oriented perpendicularly to the mesial hemispheric surface and generates the potentials recorded by the contralateral temporal and the ipsilateral parietal leads; the other dipolar source is radial to the hemispheric convexity, and generates the potentials at the vertex and those recorded by the IC electrodes.

Characterizing somatosensory evoked potential sources with dipole models: Advantages and limitations

Several methods have been developed to investigate the cerebral generators of scalp somatosensory evoked potentials (SEPs), because simple visual inspection of the electroencephalographic signal does not allow for immediate identification of the active brain regions. When the neurons fired by the afferent inputs are closely grouped, as usually occurs in SEP generation, they can be represented as a dipole, that is, as a linear source with two opposite poles. Several techniques for dipolar source modeling, which use different algorithms, have been employed to build source models of early, middle-latency, and late cognitive SEPs. Modifications of SEP dipolar activities after experimental maneuvers or in pathological conditions have also been observed. Although the effectiveness of dipolar source analysis should not be overestimated due to the intrinsic limitations of the approach, dipole modeling provides a means to assess SEPs in terms of cerebral sources and voltage fields that they produce over the head.

Scalp distribution of the earliest cortical somatosensory evoked potential to tibial nerve stimulation: proposal of a new recording montage

OBJECTIVE

To investigate the most reliable method to record the earliest cortical somatosensory evoked potential (SEP) after tibial nerve stimulation. The 'gating' phenomenon was used to dissociate the overlapping cortical SEP components.

METHODS

In 11 subjects we recorded the scalp SEPs at rest, during the voluntary (active gating) and passive (passive gating) foot movement and during the isometric calf muscle contraction (isometric gating).

RESULTS

At the vertex the P40 amplitude was reduced in all the gating conditions. Instead, both the P40 response recorded in the parietal region ipsilateral to the stimulation (indicated as P40par) and the fronto-temporal N37 potential were reduced in amplitude only during the passive foot movement.

CONCLUSIONS

The same behaviour of the N37 and P40par potentials suggests that they can represent the opposite counterparts of the same dipolar generator. Instead, the real P40 amplitude, which is affected in all the gating conditions, is recorded at the vertex and might be generated by a different source. We conclude that the montage obtained by referring a temporal electrode contralateral to the stimulation to an ipsilateral parietal lead can reliably record the earliest cortical component (N37/P40par) after tibial nerve stimulation.

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

OBJECTIVES

To investigate whether the same dipolar model could explain the scalp CO(2) laser evoked potential (LEP) distribution after either hand or foot skin stimulation.

METHODS

LEPs were recorded in 14 healthy subjects after hand and foot skin stimulation and brain electrical source analysis of responses obtained in each individual was performed.

RESULTS

A 5 dipolar sources model explained the scalp LEP topography after both hand and foot stimulation. In particular, we showed that the co-ordinates of the two earliest activated dipoles were compatible with source locations in the upper bank of the Sylvian fissure on both sides. These sources did not change their location when the stimulation site was moved from the upper to the lower limb. The other 3 dipoles of our model were activated in the late LEP latency range with a biphasic profile and a location compatible with activation of the cingulate gyrus and deep temporo-insular structures.

CONCLUSIONS

The dipolar model previously proposed for the hand stimulation LEPs can also satisfactorily explain the LEP distribution obtained after foot stimulation. The earliest activated Sylvian dipolar sources did not change their location when the upper or lower limb was stimulated, as expected from the close projections of hand and foot in the second somatosensory area. No source in the primary somatosensory area was necessary to model the scalp topography of LEPs to hand and foot stimulation.

Effect of movement on dipolar source activities of somatosensory evoked potentials

The early scalp somatosensory evoked potentials (SEPs) to median and tibial nerve stimulation were recorded at rest and during voluntary movement of the stimulated hand and foot, respectively. Both tibial and median nerve SEP distributions at rest could be explained by four-dipole models, in which one dipole was activated at the same latency as the subcortical far field and the three remaining dipolar sources were located in the perirolandic region contralateral to the stimulated side. Voluntary movement reduced all cortical dipoles in strength, while the subcortical one remained unchanged, suggesting that the effect of movement occurs above the cervicomedullary junction. In animals, cutaneous inputs are suppressed during movement and we therefore interpreted the depression of activity in the primary somatosensory cortex induced by movement as due to selective "gating" of cutaneous afferents. Because the reduction in strength of the cortical dipoles was generally lower during passive than active movement, both centrifugal and centripetal mechanisms probably contribute to the phenomenon of "gating."

Brain electrical source analysis of primary cortical components of the tibial nerve somatosensory evoked potential using regional sources

Tibial nerve somatosensory evoked potentials (SEPs) show higher amplitudes ipsilateral to the side of stimulation, whereas subdural recordings revealed a source in the foot area of the contralateral hemisphere. We now investigated this paradoxical lateralization by performing a brain electrical source analysis in the P40 time window (34-46 ms). The tibial nerve was stimulated behind the ankle (8 subjects). On each side, 2048 stimuli were applied twice. SEPs were recorded using 32 magnetic resonance imaging (MRI)-verified electrode positions (bandpass 0.5-500 Hz). In each case, the P40 amplitude was higher ipsilaterally (0.45 +/- 0.14 microV) than contralaterally (-0.49 +/- 0.16 microV). The best fitting regional source, however, was always located in the contralateral hemisphere with a mean distance of 8.2 +/- 4.3 mm from the midline. The positivity pointed ipsilaterally shifting from a frontal orientation (P37) to a parietal direction (P40). The P40 dipole moment was 2.5 times stronger than the dipole moment of P37, which makes P40 most prominent in EEG recordings. However, with its oblique dipole orientation compared to the tangential P37 dipole, it is systematically underestimated in MEG. Dipole orientations explained interindividual variability of scalp potential distribution. SEP amplitudes were smaller when generated in the dominant (left) hemisphere. This is explained by deeper located sources (5.4 +/- 1.6 mm) with a more tangential orientation (delta theta = 17.5 +/- 2.3 degrees) in the left hemisphere.