“Gating” effects of simultaneous peripheral electrical stimulations on human secondary somatosensory cortex: a whole-head MEG study

A thermal nociceptive patch in the S2 cortex of nonhuman primates: a combined functional magnetic resonance imaging and electrophysiology study

ABSTRACT

Human functional magnetic resonance imaging (fMRI) and behavioral studies have established the roles of cortical areas along the Sylvian fissure in sensing subjective pain. Yet, little is known about how sensory aspects of painful information are represented and processed by neurons in these regions and how their electrophysiological activities are related to fMRI signals. The current study aims to partially address this critical knowledge gap by performing fMRI-guided microelectrode mapping and recording studies in the homologous region of the parietal operculum in squirrel monkeys under light anesthesia. In each animal studied (n = 8), we detected mesoscale mini-networks for heat nociception in cortical regions around the lateral sulcus. Within the network, we discovered a ∼1.5 × 1.5-mm2-sized cortical patch that solely contained heat nociceptive neurons that aligned with the heat fMRI activation locus. These neurons responded slowly to thermal (heat and cold) nociceptive stimuli exclusively, continued firing for several seconds after the succession of stimulation, and exhibited multidigit receptive fields and high spontaneous firing rates. Similar to the fMRI responses, increasing temperatures in the nociceptive range led to a nonlinear increase in firing rates. The finding of a clustering of heat nociceptive neurons provides novel insights into the unique functional organization of thermal nociception in the S2 subregion of the primate brain. With fMRI, it supports the existence of a modality-preferred heat nociceptive patch that is spatially separated and intermingled with touch patches containing neurons with comparable receptive fields and the presence of functionally distinct mini-networks in primate opercular cortex.

Event-related potentials evoked by skin puncture reflect activation of Aβ fibers: comparison with intraepidermal and transcutaneous electrical stimulations

Background

Recently, event-related potentials (ERPs) evoked by skin puncture, commonly used for blood sampling, have received attention as a pain assessment tool in neonates. However, their latency appears to be far shorter than the latency of ERPs evoked by intraepidermal electrical stimulation (IES), which selectively activates nociceptive Aδ and C fibers. To clarify this important issue, we examined whether ERPs evoked by skin puncture appropriately reflect central nociceptive processing, as is the case with IES.

Methods

In Experiment 1, we recorded evoked potentials to the click sound produced by a lance device (click-only), lance stimulation with the click sound (click+lance), or lance stimulation with white noise (WN+lance) in eight healthy adults to investigate the effect of the click sound on the ERP evoked by skin puncture. In Experiment 2, we tested 18 heathy adults and recorded evoked potentials to shallow lance stimulation (SL) with a blade that did not reach the dermis (0.1 mm insertion depth); normal lance stimulation (CL) (1 mm depth); transcutaneous electrical stimulation (ES), which mainly activates Aβ fibers; and IES, which selectively activates Aδ fibers when low stimulation current intensities are applied. White noise was continuously presented during the experiments. The stimulations were applied to the hand dorsum. In the SL, the lance device did not touch the skin and the blade was inserted to a depth of 0.1 mm into the epidermis, where the free nerve endings of Aδ fibers are located, which minimized the tactile sensation caused by the device touching the skin and the activation of Aβ fibers by the blade reaching the dermis. In the CL, as in clinical use, the lance device touched the skin and the blade reached a depth of 1 mm from the skin surface, i.e., the depth of the dermis at which the Aβ fibers are located.

Results

The ERP N2 latencies for click-only (122 ± 2.9 ms) and click+lance (121 ± 6.5 ms) were significantly shorter than that for WN+lance (154 ± 7.1 ms). The ERP P2 latency for click-only (191 ± 11.3 ms) was significantly shorter than those for click+lance (249 ± 18.6 ms) and WN+lance (253 ± 11.2 ms). This suggests that the click sound shortens the N2 latency of the ERP evoked by skin puncture. The ERP N2 latencies for SL, CL, ES, and IES were 146 ± 8.3, 149 ± 9.9, 148 ± 13.1, and 197 ± 21.2 ms, respectively. The ERP P2 latencies were 250 ± 18.2, 251 ± 14.1, 237 ± 26.3, and 294 ± 30.0 ms, respectively. The ERP latency for SL was significantly shorter than that for IES and was similar to that for ES. This suggests that the penetration force generated by the blade of the lance device activates the Aβ fibers, consequently shortening the ERP latency.

Conclusions

Lance ERP may reflect the activation of Aβ fibers rather than Aδ fibers. A pain index that correctly and reliably reflects nociceptive processing must be developed to improve pain assessment and management in neonates.

Adhesive dentistry sensory stimulus technique as a neuromechanism for the treatment of orofacial pain associated to temporomandibular disorders: Case study

Purpose

The authors intend, by presenting a case study, emphasize the neuromodulation process of orofacial pain induced by the stimulation of the sensory and motor stimulation of the trigeminal nerve, which can play an important role on pain modulation.

Materials and methods

A 25 year-old woman presenting orofacial pain was referred to the stomatology service at the Centro Hospitalar do Porto. After collecting the patient's anamnesis, the thermographic camera FLIR i7 was used to record the thermal status of the orofacial structures, before the adhesive dentistry sensory stimulus protocol, after 45 minutes, and after one week.

Results

This study suggests the relation of adhesive dentistry sensory stimulus technique in the neuromodulation of orofacial pain and its association with the temporomandibular disorders . As the tongue senses the stimulus of the resin composite placed on the palatal surface of the 1^st premolar, 2^nd premolar and 1^st molar of the maxilla, this can promote and induce an effect regarding a peripheral nerve neuromodulation resulting in a blockage of the nociceptive trigeminal pathway from temporomandibular disorders.

Conclusion

Orofacial pain is a common complaint among the patients that come to a dentistry appointment, which may have different diagnosis and treatments. A positive effect on the patient's symptomatology was confirmed clinically on subsequent dental appointments and monitored by infrared thermography.

Attention modulates the gating of primary somatosensory oscillations

Sensory gating (SG) is a well-studied phenomenon in which neural responses are reduced to identical stimuli presented in succession, and is thought to represent the functional inhibition of primary sensory information that is redundant in nature. SG is traditionally considered pre-attentive, but little is known about the effects of attentional state on this process. In this study, we investigate the impact of directed attention on somatosensory SG using magnetoencephalography. Healthy young adults (n ​= ​26) performed a novel somato-visual paired-pulse oddball paradigm, in which attention was directed towards or away from paired-pulse stimulation of the left median nerve. We observed a robust evoked (i.e., phase-locked) somatosensory response in the time domain, and three stereotyped oscillatory responses in the time-frequency domain including an early theta response (4-8 ​Hz), and later alpha (8-14 ​Hz) and beta (20-26 ​Hz) responses across attentional states. The amplitudes of the evoked response and the theta and beta oscillations were gated for the second stimulus, however, only the gating of the oscillatory responses was altered by attention. Specifically, directing attention to the somatosensory domain enhanced SG of the early theta response, while reducing SG of the later alpha and beta responses. Further, prefrontal alpha-band coherence with the primary somatosensory cortex was greater when attention was directed towards the somatosensory domain, supporting a frontal modulatory effect on the alpha response in primary somatosensory regions. These findings highlight the dynamic effects of attentional modulation on somatosensory processing, and the importance of considering attentional state in studies of SG.

Somatosensory gating is altered and associated with migraine chronification: A magnetoencephalographic study

Background

Brain excitability is changed in migraine but not fully characterized yet. This study explored if somatosensory gating is altered in migraine and linked to migraine chronification.

Methods

Paired electrical stimuli were delivered to the left index fingers of 21 patients with migraine without aura (MO), 22 patients with chronic migraine (CM), and 36 controls. The first and second responses to the paired stimuli were obtained from the contralateral primary (cSI), contralateral secondary (cSII) and ipsilateral secondary (iSII) somatosensory cortices to compute the gating ratios (second vs. first response strengths).

Results

The first and second cSI responses and gating ratios differed in all groups ( p < 0.05); the responses were typically smaller in the MO and CM groups. The cSI gating ratio increased as a continuum across controls (0.73 ± 0.04, p < 0.001), MO (0.83 ± 0.04) to CM (0.97 ± 0.06) and was higher in CM vs. controls ( p < 0.001). When MO and CM were combined, cSI gating ratio was associated with headache frequency (r = 0.418, p = 0.005). Paired responses and gating ratios of cSII and iSII did not differ among the groups.

Conclusions

Somatosensory gating is altered in migraine and associated with headache chronification. Further studies must clarify if this abnormal sensory modulation is a true gating deficit independent of low preexcitation level.

Oscillatory dynamics and functional connectivity during gating of primary somatosensory responses

KEY POINTS

Sensory gating is important for preventing excessive environmental stimulation from overloading neural resources. Gating in the human somatosensory cortices is a critically understudied topic, particularly in the lower extremities. We utilize the unique capabilities of magnetoencephalographic neuroimaging to quantify the normative neural population responses and dynamic functional connectivity of somatosensory gating in the lower extremities of healthy human participants. We show that somatosensory processing is subserved by a robust gating effect in the oscillatory domain, as well as a dynamic effect on interhemispheric functional connectivity between primary sensory cortices. These results provide novel insight into the dynamic neural mechanisms that underlie the processing of somatosensory information in the human brain, and will be vital in better understanding the neural responses that are aberrant in gait-related neurological disorders (e.g. cerebral palsy).

ABSTRACT

Sensory gating (SG) is a phenomenon in which neuronal responses to subsequent similar stimuli are weaker, and is considered to be an important mechanism for preventing excessive environmental stimulation from overloading shared neural resources. Although gating has been demonstrated in multiple sensory systems, the neural dynamics and developmental trajectory underlying SG remain poorly understood. In the present study, we adopt a data-driven approach to map the spectrotemporal amplitude and functional connectivity (FC) dynamics that support gating in the somatosensory system (somato-SG) in healthy children and adolescents using magnetoencephalography (MEG). These data underwent time-frequency decomposition and the significant signal changes were imaged using a beamformer. Voxel time series were then extracted from the peak voxels and these signals were examined in the time and time-frequency domains, and then subjected to dynamic FC analysis. The results obtained indicate a significant decrease in the amplitude of the neural response following the second stimulation relative to the first in the primary somatosensory cortex (SI). A significant decrease in response latency was also found between stimulations, and each stimulation induced a sharp decrease in FC between somatosensory cortical areas. Furthermore, there were no significant correlations between somato-SG metrics and age. We conclude that somato-SG can be observed in SI in both the time and oscillatory domains, with rich dynamics and alterations in inter-hemispheric FC, and that this phenomenon has already matured by early childhood. A better understanding of these dynamics may provide insight to the numerous psychiatric and neurologic conditions that have been associated with aberrant SG across multiple modalities.

Augmented Pain Processing in Primary and Secondary Somatosensory Cortex in Fibromyalgia: A Magnetoencephalography Study Using Intra-Epidermal Electrical Stimulation

The aim of this study was to investigate augmented pain processing in the cortical somatosensory system in patients with fibromyalgia (FM). Cortical evoked responses were recorded in FM (n = 19) and healthy subjects (n = 21) using magnetoencephalography after noxious intra-epidermal electrical stimulation (IES) of the hand dorsum (pain rating 6 on a numeric rating scale, perceptually-equivalent). In addition, healthy subjects were stimulated using the amplitude corresponding to the average stimulus intensity rated 6 in patients with FM (intensity-equivalent). Quantitative sensory testing was performed on the hand dorsum or thenar muscle (neutral site) and over the trapezius muscle (tender point), using IES (thresholds, ratings, temporal summation of pain, stimulus-response curve) and mechanical stimuli (threshold, ratings). Increased amplitude of cortical responses was found in patients with FM as compared to healthy subjects. These included the contralateral primary (S1) and bilateral secondary somatosensory cortices (S2) in response to intensity-equivalent stimuli and the contralateral S1 and S2 in response to perceptually-equivalent stimuli. The amplitude of the contralateral S2 response in patients with FM was positively correlated with average pain intensity over the last week. Quantitative sensory testing results showed that patients with FM were more sensitive to painful IES as well as to mechanical stimulation, regardless of whether the stimulation site was the hand or the trapezius muscle. Interestingly, the slope of the stimulus-response relationship as well as temporal summation of pain in response to IES was not different between groups. Together, these results suggest that the observed pain augmentation in response to IES in patients with FM could be due to sensitization or disinhibition of the cortical somatosensory system. Since the S2 has been shown to play a role in higher-order functions, further studies are needed to clarify the role of augmented S2 response in clinical characteristics of FM.

A spatiotemporal signature of cortical pain relief by tactile stimulation: An MEG study

Recently, the cortical mechanisms of tactile-induced analgesia have been investigated; however, spatiotemporal characteristics have not been fully elucidated. The insular-opercular region integrates multiple sensory inputs, and nociceptive modulation by other sensory inputs occurs in this area. In this study, we focused on the insular-opercular region to characterize the spatiotemporal signature of tactile-induced analgesia using magnetoencephalography in 11 healthy subjects. Aδ (intra-epidermal electrical stimulation) inputs were modified by Aβ (mechanical tactile stimulation) selective stimulation, either independently or concurrently, to the right forearm. The optimal inter-stimulus interval (ISI) for cortical level modulation was determined after comparing the 40-, 60-, and 80-ms ISI conditions, and the calculated cortical arrival time difference between Aδ and Aβ inputs. Subsequently, we adopted a 60-ms ISI for cortical modulation and a 0-ms ISI for spinal level modulation. Source localization using minimum norm estimates demonstrated that pain-related activity was located in the posterior insula, whereas tactile-related activity was estimated in the parietal operculum. We also found significant inhibition of pain-related activity in the posterior insula due to cortical modulation. In contrast, spinal modulation was observed both in the posterior insula and parietal operculum. Subjective pain, as evaluated by the visual analog scale, also showed significant reduction in both conditions. Therefore, our results demonstrated that the multisensory integration within the posterior insula plays a key role in tactile-induced analgesia.

Age-Related Reduced Somatosensory Gating Is Associated with Altered Alpha Frequency Desynchronization

Sensory gating (SG), referring to an attenuated neural response to the second identical stimulus, is considered as preattentive processing in the central nervous system to filter redundant sensory inputs. Insufficient somatosensory SG has been found in the aged adults, particularly in the secondary somatosensory cortex (SII). However, it remains unclear which variables leading to the age-related somatosensory SG decline. There has been evidence showing a relationship between brain oscillations and cortical evoked excitability. Thus, this study used whole-head magnetoencephalography to record responses to paired-pulse electrical stimulation to the left median nerve in healthy young and elderly participants to test whether insufficient stimulus 1- (S1-) induced event-related desynchronization (ERD) contributes to a less-suppressed stimulus 2- (S2-) evoked response. Our analysis revealed that the minimum norm estimates showed age-related reduction of SG in the bilateral SII regions. Spectral power analysis showed that the elderly demonstrated significantly reduced alpha ERD in the contralateral SII (SIIc). Moreover, it was striking to note that lower S1-induced alpha ERD was associated with higher S2-evoked amplitudes in the SIIc among the aged adults. Conclusively, our findings suggest that age-related decline of somatosensory SG is partially attributed to the altered S1-induced oscillatory activity.

Corticofugal projections induce long-lasting effects on somatosensory responses in the trigeminal complex of the rat

The sensory information flow at subcortical relay stations is controlled by the action of topographic connections from the neocortex. To determinate the functional properties of the somatosensory corticofugal projections to the principal (Pr5) and caudal spinal (Sp5C) trigeminal nuclei, we performed unitary recordings in anesthetized rats. To examine the effect of these cortical projections we used tactile stimulation of the whisker and electrical stimulation of somatosensory cortices. Corticofugal anatomical projections to Pr5 and Sp5C nuclei were detected by using retrograde fluorescent tracers. Neurons projecting exclusively to Pr5 were located in the cingulate cortex while neurons projecting to both Sp5C and Pr5 nuclei were located in the somatosensory and insular cortices (>75% of neurons). Physiological results indicated that primary somatosensory cortex produced a short-lasting facilitating or inhibiting effects (<5 min) of tactile responses in Pr5 nucleus through activation of NMDA glutamatergic or GABA_A receptors since effects were blocked by iontophoretically application of APV and bicuculline, respectively. In contrast, stimulation of secondary somatosensory cortex did not affect most of the Pr5 neurons; however both cortices inhibited the nociceptive responses in the Sp5C nucleus through activation of glycinergic or GABA_A receptors because effects were blocked by iontophoretically application of strychnine and bicuculline, respectively. These and anatomical results demonstrated that the somatosensory cortices projects to Pr5 nucleus to modulate tactile responses by excitatory and inhibitory actions, while projections to the Sp5C nucleus control nociceptive sensory transmission by only inhibitory effects. Thus, somatosensory cortices may modulate innocuous and noxious inputs simultaneously, contributing to the perception of specifically tactile or painful sensations.

Aging-related decline in somatosensory inhibition of the human cerebral cortex

Primary somatosensory (SI) cortical inhibition to repetitive stimuli tends to decline with increasing age. However, aging effects on the inhibition mechanism of secondary somatosensory cortex (SII) remain elusive. We aimed to study the aging-related changes of cortical inhibition in the human somatosensory system. Neuromagnetic responses to paired-pulse electrical stimulation to the median nerve were recorded in 21 young and 20 elderly male adults. Paired-pulse suppression (PPS) of SI and SII activities was estimated by the ratio of the response to Stimulus 2 to the response to Stimulus 1. Based on equivalent current dipole modeling, PPS ratios of the contralateral (SIIc) and ipsilateral (SIIi) secondary somatosensory cortices were higher in elderly than in young subjects (p < 0.001 in SIIc and p = 0.034 in SIIi). At an individual basis, a higher PPS ratio in SIIc than in SI was found in 16 (80 %) out of the 20 elderly participants; in contrast, the PPS ratios of SIIc and SI cortices were similar in young participants (p = 0.031). In conclusion, a larger PPS ratio in elderly suggests an aging-related decline in somatosensory cortical inhibition. Furthermore, compared to SI, the electrophysiological responses of SII cortex are especially vulnerable to aging in terms of cortical inhibition to repetitive stimulation.

Neural correlates of somatosensory paired-pulse suppression: a MEG study using distributed source modeling and dynamic spectral power analysis

Paired-pulse stimulation has been used previously to evaluate cortical excitability and sensory gating. To help elucidate the neural network involved in paired-pulse suppression of somatosensory cortical processing, magnetoencephalographic (MEG) responses to paired-pulse electrical stimulation of the left median nerve of the wrists of 13 healthy males were recorded using an intra-pair interstimulus interval (ISI) of 500ms and an inter-pair ISI of 8s. Minimum norm estimates showed the presence of cortical activation in the bilateral primary somatosensory cortex, the post-central sulcus and the supplementary motor areas. Compared with the responses to the first stimulation, the responses to the second stimulation were attenuated in these areas with gating ratios (the amplitude ratios of the second response to the first response) of 0.54-0.69. By spectral power dynamic analysis, beta frequency oscillations were found to be associated with an early-latency (30-36ms) gating process in the contralateral primary somatosensory cortex and post-central sulcus, whereas theta and alpha oscillations were correlated with paired-pulse suppression of activations at 98-136ms in the ipsilateral primary somatosensory cortex, the bilateral post-central sulcus and the supplementary motor areas. In summary, it can be concluded that differential oscillatory activities are involved in the pair-pulse suppression in various somatosensory regions in response to repetitive external stimulations.

The functional architecture of S1 during touch observation described with 7 T fMRI

Recent studies indicate that the primary somatosensory cortex (S1) is active not only when touch is physically perceived but also when it is merely observed to be experienced by another person. This social responsivity of S1 has important implications for our understanding of S1 functioning. However, S1 activity during touch observation has not been characterized in great detail to date. We focused on two features of the S1 functional architecture during touch observation, namely the topographical arrangement of index and middle finger receptive fields (RFs), and their dynamic shrinkage during concurrent activation. Both features have important implications for human behavior. We conducted two fMRI studies at 7 T, one where touch was physically perceived, and one where touch was observed. In the two experiments, participants either had their index finger and/or middle finger stimulated using paintbrushes, or just observed similar touch events on video. Our data show that observing and physically experiencing touch elicits overlapping activity changes in S1. In addition, observing touch to the index finger or the middle finger alone evoked topographically arranged activation foci in S1. Importantly, when co-activated, the index and middle finger RFs not only shrank during physical touch perception, but also during touch observation. Our data, therefore, indicate a similarity between the functional architecture of S1 during touch observation and physical touch perception with respect to single-digit topography and RF shrinkage. These results may allow the tentative conclusion that even primary somatosensory experiences, such as physical touch perception, can be shared amongst individuals.

Sadness enhances the experience of pain and affects pain-evoked cortical activities: an MEG study

Pain is a multidimensional phenomenon. Previous psychological studies have shown that a person's subjective pain threshold can change when certain emotions are recognized. We examined this association with magnetoencephalography. Magnetic field strength was recorded with a 306-channel neuromagnetometer while 19 healthy subjects (7 female, 12 male; age range = 20-30 years) experienced pain stimuli in different emotional contexts induced by the presentation of sad, happy, or neutral facial stimuli. Subjects also rated their subjective pain intensity. We hypothesized that pain stimuli were affected by sadness induced by facial recognition. We found: 1) the intensity of subjective pain ratings increased in the sad emotional context compared to the happy and the neutral contexts, and 2) event-related desynchronization of lower beta bands in the right hemisphere after pain stimuli was larger in the sad emotional condition than in the happy emotional condition. Previous studies have shown that event-related desynchronization in these bands could be consistently observed over the primary somatosensory cortex. These findings suggest that sadness can modulate neural responses to pain stimuli, and that brain processing of pain stimuli had already been affected, at the level of the primary somatosensory cortex, which is critical for sensory processing of pain.

PERSPECTIVE

We found that subjective pain ratings and cortical beta rhythms after pain stimuli are influenced by the sad emotional context. These results may contribute to understanding the broader relationship between pain and negative emotion.

Facial macrosomatognosia and pain in a case of Wallenberg's syndrome: selective effects of vestibular and transcutaneous stimulations

Macro- and micro-somatognosia refer to rare disorders of the cerebral representation of the body whereby patients perceive body parts as disproportionately large or small. Here we report the experimental study of a patient who, following a left lateral medullary stroke (Wallenberg's syndrome, including vestibular deficits) complained of a persistent somatosensory illusory sensation of swelling, confined to the left side of his face (i.e., left macrosomatognosia). This hemifacial somatosensory distortion was associated with a left facial anesthesia, and a neuropathic pain affecting the three branches of the left trigeminal nerve. In this study, we first document quantitatively the patient's somatosensory illusion by using a somatosensory-to-visual matching task in which the patient modified the picture of his own face to fit his left-sided somatosensory misperception. The patient's performance revealed that macrosomatognosia was confined to the second branch of the left trigeminal nerve. Perception of the size of visual objects was comparatively preserved. Second, we investigated the effects of two peripheral stimulations, which may affect the spatial component of somatosensory deficits (caloric vestibular stimulation, CVS; transcutaneous electrical nervous stimulation, TENS) and pain (TENS). Left CVS abolished the facial somatosensory illusion, for about 30min, but had no effect on the left facial pain. Conversely, left TENS substantially reduced the neuropathic pain during stimulation, but had no effect on macrosomatognosia, indicating a double dissociation between the two disorders. These results reveal that facial macrosomatognosia may be regarded as a high-order deficit of somatosensory perception of the shape and volume of the face, which fits the definition of 'hyperschematia' (i.e., when the body takes up too much room) originally proposed by Bonnier (1905). Our data also indicate that CVS may favor the restoration of the conscious representation of the shape and size of the face. Overall, these findings lend support to the view that afferent inputs from the vestibular system can affect in a specific fashion the activity of cerebral structures involved in the building up and updating of the topological description of body parts.

Somatosensory processing of the tongue in humans

We review research on somatosensory (tactile) processing of the tongue based on data obtained using non-invasive neurophysiological and neuroimaging methods. Technical difficulties in stimulating the tongue, due to the noise elicited by the stimulator, the fixation of the stimulator, and the vomiting reflex, have necessitated the development of specialized devices. In this article, we show the brain activity relating to somatosensory processing of the tongue evoked by such devices. More recently, the postero-lateral part of the tongue has been stimulated, and the brain response compared with that on stimulation of the antero-lateral part of the tongue. It is likely that a difference existed in somatosensory processing of the tongue, particularly around primary somatosensory cortex, Brodmann area 40, and the anterior cingulate cortex.

Steady-state activation in somatosensory cortex after changes in stimulus rate during median nerve stimulation

Passive electrical stimulation activates various human somatosensory cortical systems including the contralateral primary somatosensory area (SI), bilateral secondary somatosensory area (SII) and bilateral insula. The effect of stimulation frequency on blood oxygenation level-dependent (BOLD) activity remains unclear. We acquired 3-T functional magnetic resonance imaging (fMRI) in eight healthy volunteers during electrical median nerve stimulation at frequencies of 1, 3 and 10 Hz. During stimulation BOLD signal changes showed activation in the contralateral SI, bilateral SII and bilateral insula. Results of fMRI analysis showed that these areas were progressively active with the increase of rate of stimulation. As a major finding, the contralateral SI showed an increase of peak of BOLD activation from 1 to 3 Hz but reached a plateau during 10-Hz stimulation. Our finding is of interest for basic research and for clinical applications in subjects unable to perform cognitive tasks in the fMRI scanner.

Cortical brain responses during passive nonpainful median nerve stimulation at low frequencies (0.5-4 Hz): an fMRI study

Previous findings have shown that the human somatosensory cortical systems that are activated by passive nonpainful electrical stimulation include the contralateral primary somatosensory area (SI), bilateral secondary somatosensory area (SII), and bilateral insula. The present study tested the hypothesis that these areas have different sensitivities to stimulation frequency in the condition of passive stimulation. Functional MRI (fMRI) was recorded in 24 normal volunteers during nonpainful electrical median nerve stimulations at 0.5, 1, 2, and 4 Hz repetition rates in separate recording blocks in pseudorandom order. Results of the blood oxygen level-dependent (BOLD) effect showed that the contralateral SI, the bilateral SII, and the bilateral insula were active during these stimulations. As a major finding, only the contralateral SI increased its activation with the increase of the stimulus frequency at the mentioned range. The fact that nonpainful median-nerve electrical stimuli at 4 Hz induces a larger BOLD response is of interest both for basic research and clinical applications in subjects unable to perform cognitive tasks in the fMRI scanner.

Conditioning transcutaneous electrical nerve stimulation induces delayed gating effects on cortical response: A magnetoencephalographic study

The present study was undertaken to investigate after-effects of 7 Hz non-painful prolonged stimulation of the median nerve on somatosensory-evoked fields (SEFs). The working hypothesis that conditioning peripheral stimulations might produce delayed interfering ("gating") effects on the response of somatosensory cortex to test stimuli was evaluated. In the control condition, electrical thumb stimulation induced SEFs in ten subjects. In the experimental protocol, a conditioning median nerve stimulation at wrist preceded 6 electrical thumb stimulations. Equivalent current dipoles fitting SEFs modeled responses of contralateral primary area (SI) and bilateral secondary somatosensory areas (SII) following control and experimental conditions. Compared to the control condition, conditioning stimulation induced no amplitude modulation of SI response at the initial stimulus-related peak (20 ms). In contrast, later response from SI (35 ms) and response from SII were significantly weakened in amplitude. Gradual but fast recovery towards control amplitude levels was observed for the response from SI-P35, while a slightly slower cycle was featured from SII. These findings point to a delayed "gating" effect on the synchronization of somatosensory cortex after peripheral conditioning stimulations. This effect was found to be more lasting in SII area, as a possible reflection of its integrative role in sensory processing.

Anticipatory electroencephalography alpha rhythm predicts subjective perception of pain intensity

This high-resolution electroencephalography (EEG) study tested the hypothesis that the suppression of rolandic alpha power before predictable painful stimulation affects the subject's subsequent evaluation of pain intensity, as a reflection of the influence of expectancy processes on painful stimulus processing. High-resolution EEG data were recorded (126 channels) from 10 healthy adult volunteers during the expectancy of a painful CO(2)-laser stimulation at the right wrist. Surface laplacian estimation enhanced the EEG spatial information content over 6 scalp regions of interest (left frontal, right frontal, left central, right central, left parietal, and right parietal areas). Spectral power was computed for 3 alpha sub-bands with reference to the individual alpha frequency peak (about 5-7 Hz for alpha 1, 7-9 Hz for alpha 2, and 9-11 Hz for alpha 3). The suppression of the alpha power before the painful stimulation [as reflected by the event-related desynchronization (ERD)] indexed the anticipatory cortical processes. Results showed maximum (negative) correlations between the alpha 2 and alpha 3 ERD amplitude at the left central area and the subjective evaluation of pain intensity (P < .001). The stronger the anticipatory alpha 2 and alpha 3 ERD, the higher the subjective evaluation of pain intensity. For alpha 3, that correlation was confirmed even when the effect of habituation across the recording session was taken into account. These results suggest that the anticipatory suppression of the alpha rhythms over the contralateral primary sensorimotor cortex predicts subsequent subjects' evaluation of pain intensity, in line with its crucial role for the discrimination of that intensity.

PERSPECTIVE

This electroencephalographic study showed that anticipatory activation/deactivation of sensorimotor cortex roughly predicts subjective evaluation of pain. This motivates further investigation on possible implications for the understanding of central chronic pain. Chronic pain patients might exaggerate the anticipatory activation of sensorimotor cortex to negligible pain stimuli.

Abnormal reactivity of the primary somatosensory cortex during the experience of pain in complex regional pain syndrome: a magnetoencephalograhic case study

A 49-year-old male worker developed persistent pain in his left wrist after work strain injuries. Clinical symptoms met with criteria for Complex Regional Pain Syndrome (CRPS) type I. In the present study, the effect of the experience of pain on the somatotopy of the primary cortical hand representation was investigated. Somatosensory evoked magnetic fields (SEF) elicited by non-painful tactile stimulation at the index finger of the affected and the unaffected hand were recorded when experiencing pain elicited by a moderate physical load condition (holding a 1.6 kg object in the hand). It was shown that MEG and subjective responses to innocuous tactile stimuli were reduced when simultaneous nociceptive stimulation was applied. These findings suggest a gating effect in the central nervous system elicited by concurrent simultaneous information from two different somatosensory modalities (pain and tactile). The results revealed the existence of nociceptive-induced plastic changes in the central nervous system associated with CRPS type I.

Modulation of somatosensory event-related potential components in a tactile-visual cross-modal task

Human tactile discrimination studies have shown that visual stimuli enhance tactile performance. Other studies on event-related potentials showed that somatosensory N140 was enhanced when attention of human subjects was directed to tactile stimuli. Therefore, we hypothesized that N140 would be modulated when human subjects performed tactile cross-modal delay tasks. Scalp-event-related potentials were recorded from normal subjects performing either a tactile-tactile unimodal, or a tactile-visual cross-modal delayed matching-to-sample task. Identical tactile stimuli were used in both tasks. N140 component evoked by the tactile stimuli was enhanced in the cross-modal task. Enhancement of this component was also observed in control cross-modal tasks. The results suggest that tactile-visual cross-modal association affects tactile sensory-perceptual processes in humans.

Differential modulation in human primary and secondary somatosensory cortices during the preparatory period of self-initiated finger movement

To elucidate the mechanisms underlying sensorimotor integration, we investigated modulation in the primary (SI) and secondary (SII) somatosensory cortices during the preparatory period of a self-initiated finger extension. Electrical stimulation of the right median nerve was applied continuously, while the subjects performed a self-initiated finger extension and were instructed not to pay attention to the stimulation. The preparatory period was divided into five sub-periods from the onset of the electromyogram to 3000 ms before movement and the magnetoencephalogram signals following stimulation in each sub-period were averaged. Multiple source analysis indicated that the equivalent current dipoles (ECDs) were located in SI and bilateral SII. Although the ECD moment for N 20 m (the upward deflection peaking at around 20 ms) was not significantly changed, that for P 30 m (the downward deflection peaking at around 30 m) was significantly smaller in the 0- to -500-ms sub-period than the -2000- to -3000-ms sub-period. As for SII, the ECD moment for the SII ipsilateral to movement showed no significant change, while that for the contralateral SII was significantly larger in the 0- to -500-ms sub-period than the -1500- to -2000-ms or -2000- to -3000-ms sub-period. The opposite effects of movement on SI and SII cortices indicated that these cortical areas play a different role in the function of the sensorimotor integration and are affected differently by the centrifugal process.

Nociceptive and non-nociceptive sub-regions in the human secondary somatosensory cortex: An MEG study using fMRI constraints

Previous evidence from functional magnetic resonance imaging (fMRI) has shown that a painful galvanic stimulation mainly activates a posterior sub-region in the secondary somatosensory cortex (SII), whereas a non-painful sensory stimulation mainly activates an anterior sub-region of SII [Ferretti, A., Babiloni, C., Del Gratta, C., Caulo, M., Tartaro, A., Bonomo, L., Rossini, P.M., Romani, G.L., 2003. Functional topography of the secondary somatosensory cortex for non-painful and painful stimuli: an fMRI study. Neuroimage 20 (3), 1625-1638.]. The present study, combining fMRI with magnetoencephalographic (MEG) findings, assessed the working hypothesis that the activity of such a posterior SII sub-region is characterized by an amplitude and temporal evolution in line with the bilateral functional organization of nociceptive systems. Somatosensory evoked magnetic fields (SEFs) recordings after alvanic median nerve stimulation were obtained from the same sample of subjects previously examined with fMRI [Ferretti, A., Babiloni, C., Del Gratta, C., Caulo, M., Tartaro, A., Bonomo, L., Rossini, P.M., Romani, G.L., 2003. Functional topography of the secondary somatosensory cortex for non-painful and painful stimuli: an fMRI study. Neuroimage 20 (3), 1625-1638.]. Constraints for dipole source localizations obtained from MEG recordings were applied according to fMRI activations, namely, at the posterior and the anterior SII sub-regions. It was shown that, after painful stimulation, the two posterior SII sub-regions of the contralateral and ipsilateral hemispheres were characterized by dipole sources with similar amplitudes and latencies. In contrast, the activity of anterior SII sub-regions showed statistically significant differences in amplitude and latency during both non-painful and painful stimulation conditions. In the contralateral hemisphere, the source activity was greater in amplitude and shorter in latency with respect to the ipsilateral. Finally, painful stimuli evoked a response from the posterior sub-regions peaking significantly earlier than from the anterior sub-regions. These results suggested that both ipsi and contra posterior SII sub-regions process painful stimuli in parallel, while the anterior SII sub-regions might play an integrative role in the processing of somatosensory stimuli.

Functional MRI activation of somatosensory and motor cortices in a hand-grafted patient with early clinical sensorimotor recovery

The aim of this study was to investigate somatosensory and motor cortical activity with functional MRI (fMRI) in a hand-grafted patient with early clinical recovery. The patient had motor fMRI examinations before transplantation, and motor and passive tactile stimulations after surgery. His normal hand and a normal group were studied for comparison. A patient with complete brachial plexus palsy was studied to assess the lack of a fMRI signal in somatosensory areas in the case of total axonal disconnection. Stimulating the grafted hand revealed significant activation in the contralateral somatosensory cortical areas in all fMRI examinations. The activation was seen as early as 10 days after surgery; this effect cannot be explained by the known physiological mechanisms of nerve regeneration. Although an imagination effect cannot be excluded, the objective clinical recovery of sensory function led us to formulate the hypothesis that a connection to the somatosensory cortex was rapidly established. Additional cases and fundamental studies are needed to assess this hypothesis, but several observations were compatible with this explanation. Before surgery, imaginary motion of the amputated hand produced less intense responses than executed movements of the intact hand, whereas the normal activation pattern for right-handed subjects was found after surgery, in agreement with the good clinical motor recovery.

Hand posture modulates cortical finger representation in SII

Somatosensory magnetic fields evoked by electrical stimuli of the thumb or the index finger were recorded using a whole head magnetoencephalography (MEG) system in 10 subjects performing different finger postures, open hand posture and close hand posture for picking up a small object. The mean Euclidean distances between the ECD (equivalent current dipole) locations for the thumb and index finger in the secondary somatosensory cortex (SII) across the subjects were 8.5 +/- 2.1 mm in the close hand posture and 11.2 +/- 2.6 mm in the open hand posture. The distance was significantly shorter in the close hand posture (paired t test, P = 0.002, n = 8). However, the distances of the P38m and P60m components in the primary somatosensory cortex (SI) were not significantly different between the two hand postures (P38m: 13.4 +/- 5.6 mm in the open and 13.5 +/- 3.9 mm in the close; P60m: 12.4 +/- 2.6 mm in the open and 16.2 +/- 5.3 mm in the close). This shortening of the spatial distance between the cortical finger representations suggests a similarity in humans of the rapid changes in the dynamics of cortical circuits reported in animal studies. In addition, the overlap of the cortical finger representations, which might be suggested by the shortening of the distance between the ECDs in SII, is likely to play a role in information integration between sensory inputs from the thumb and index finger.

Functional topography of the secondary somatosensory cortex for nonpainful and painful stimulation of median and tibial nerve: an fMRI study

Functional magnetic resonance imaging (fMRI) was used to study the cortical activity of the bilateral secondary somatosensory cortex (SII) during nonpainful (motor threshold) and painful electrical stimulation of median and tibial nerves. fMRI recordings were performed in eight normal young adults. The aim was at evaluating the working hypothesis of a spatial segregation of nonpainful and painful populations not only in the "hand" representation of SII [Ferretti, A., Babiloni, C., Del Gratta, C., Caulo, M., Tartaro, A., Bonomo, L., Rossini, P.M., Romani, G.L., 2003. Functional topography of the secondary somatosensory cortex for nonpainful and painful stimuli: an fMRI study. NeuroImage 20, 1625-1638.] but also in its "foot" representation. Results showed that, in both "hand" and "foot" representations of bilateral SII, the activity elicited by the painful stimulation was localized more posteriorly with respect to that elicited by the nonpainful stimulation. A fine spatial analysis of the SII responses revealed a clear somatotopic organization in the bilateral SII subregion especially reactive to the nonpainful stimuli (i.e., segregation of the hand and foot representations). In contrast, it was not possible to disentangle the "hand" and "foot" representations of SII for painful stimuli. These results extended to the SII "foot" representation previous evidence of a spatial segregation in the SII "hand" representation of subregions for the painful and nonpainful stimuli. Furthermore, they suggest that noxious information is not somatotopically represented in human bilateral SII, at least as inferred from fMRI data at 1.5 T.