Somatic and limbic cortex activation in esophageal distention: A functional imaging study

Little is known about the cerebral representations of visceral sensations in humans. Using functional magnetic resonance imaging (fMRI), we mapped the cortical areas of the human brain that were activated by mechanical stimulation of the esophagus in 5 healthy volunteers. Stimulation probes were placed into the distal part of the esophagus and inflated to produce a local distention. The cerebral activation pattern was related to the strength and quality of the stimulus. The weakest stimulus accompanied by a well-localized albeit weak retrosternal sensation activated only the parietal opercular cortices, probably including the secondary somatosensory cortex (SII). Additional activation of the primary sensorimotor cortex (SI) at the level of the face and mouth representation as well as of the right premotor cortex was found during repetitive distention of the esophagus at 0.5 Hz. Repetitive stimulation at 1 Hz additionally activated the insulabilaterally. The strongest distention stimulus, which caused a painful retrosternal sensation, resulted in an activation of the anterior cingulate cortex. Our findings demonstrate that SII is the primary cortical target of visceral afferents originating in the esophagus. Limbic structures become engaged when the visceral senstion is unpleasant or painful.     

Different cortical organization of visceral and somatic sensation in humans

Sensory stimuli from the visceral domain exhibit perceptual characteristics different from stimuli applied to the body surface. Compared with somatosensation there is not much known about the cortical projection and functional organization of visceral sensation in humans. In this study, we determined the cortical areas activated by non-painful electrical stimulation of visceral afferents in the distal oesophagus, and somatosensory afferents in the median nerve and the lip in seven healthy volunteers using whole-head magnetoencephalography. Stimulation of somatosensory afferents elicited short-latency responses (≈ 20–60 ms) in the primary somatosensory cortex (SI) contralateral (median nerve) or bilateral (lip) to the stimulated side, and long-latency responses (≈ 60–160 ms) bilaterally in the second somatosensory cortex (SII). In contrast, stimulation of visceral oesophageal afferents did not evoke discernible responses in SI but well reproducible bilateral SII responses (≈ 70–190 ms) in close vicinity to long-latency SII responses following median nerve and lip stimuli. Psychophysically, temporal discrimination of successive stimuli became worse with increasing stimulus repetition rates (0.25 Hz, 0.5 Hz, 1 Hz, 2 Hz) only for visceral oesophageal, but not for somatosensory median nerve stimuli. Correspondingly, amplitudes of the first cortical response to oesophageal stimulation emerging in the SII cortex declined with increasing stimulus repetition rates whereas the earliest cortical response elicited by median nerve stimuli (20 ms SI response) remained unaffected by the stimulus frequency. Our results indicate that visceral afferents from the oesophagus primarily project to the SII cortex and, unlike somatosensory afferents, lack a significant SI representation. We propose that this cortical projection pattern forms the neurophysiological basis of the low temporal and spatial resolution of conscious visceral sensation.     

Primary and secondary somatosensory cortex responses to anticipation and pain: a magnetoencephalography study

Several brain regions, including the primary and secondary somatosensory cortices (SI and SII, respectively), are functionally active during the pain experience. Both of these regions are thought to be involved in the sensory-discriminative processing of pain and recent evidence suggests that SI in particular may also be involved in more affective processing. In this study we used MEG to investigate the hypothesis that frequency-specific oscillatory activity may be differentially associated with the sensory and affective components of pain. In eight healthy participants (four male), MEG was recorded during a visceral pain experiment comprising baseline, anticipation, pain and post-pain phases. Pain was delivered via intraluminal oesophageal balloon distension (four stimuli at 1 Hz). Significant bilateral but asymmetrical changes in neural activity occurred in the β-band within SI and SII. In SI, a continuous increase in neural activity occurred during the anticipation phase (20-30 Hz), which continued during the pain phase but at a lower frequency (10-15 Hz). In SII, oscillatory changes only occurred during the pain phase, predominantly in the 20-30 Hz β band, and were coincident with the stimulus. These data provide novel evidence of functional diversity within SI, indicating a role in attentional and sensory aspects of pain processing. In SII, oscillatory changes were predominantly stimulus-related, indicating a role in encoding the characteristics of the stimulus. We therefore provide objective evidence of functional heterogeneity within SI and functional segregation between SI and SII, and suggest that the temporal and frequency dynamics within cortical regions may offer valuable insights into pain processing.     

Two systems of resting state connectivity between the insula and cingulate cortex

The insula and cingulate cortices are implicated in emotional, homeostatic/allostatic, sensorimotor, and cognitive functions. Non‐human primates have specific anatomical connections between sub‐divisions of the insula and cingulate. Specifically, the anterior insula projects to the pregenual anterior cingulate cortex (pACC) and the anterior and posterior mid‐cingulate cortex (aMCC and pMCC); the mid‐posterior insula only projects to the posterior MCC (pMCC). In humans, functional neuroimaging studies implicate the anterior insula and pre/subgenual ACC in emotional processes, the mid‐posterior insula with awareness and interoception, and the MCC with environmental monitoring, response selection, and skeletomotor body orientation. Here, we tested the hypothesis that distinct resting state functional connectivity could be identified between (1) the anterior insula and pACC/aMCC; and (2) the entire insula (anterior, middle, and posterior insula) and the pMCC. Functional connectivity was assessed from resting state fMRI scans in 19 healthy volunteers using seed regions of interest in the anterior, middle, and posterior insula. Highly correlated, low‐frequency oscillations (< 0.05 Hz) were identified between specific insula and cingulate subdivisions. The anterior insula was shown to be functionally connected with the pACC/aMCC and the pMCC, while the mid/posterior insula was only connected with the pMCC. These data provide evidence for a resting state anterior insula–pACC/aMCC cingulate system that may integrate interoceptive information with emotional salience to form a subjective representation of the body; and another system that includes the entire insula and MCC, likely involved in environmental monitoring, response selection, and skeletomotor body orientation. Human Brain Mapp 2009. © 2008 Wiley‐Liss, Inc.     

Sensorimotor integration in human primary and secondary somatosensory cortices

We measured somatosensory evoked fields (SEFs) to electric median nerve stimuli from eight healthy subjects with a whole-scalp 122-channel neuromagnetometer in two different conditions: (i) ‘rest', with stimuli producing clear tactile sensation without any motor movement, and (ii) ‘contraction' with exactly the same stimuli as in ‘rest', but with the subjects maintaining sub-maximal isometric contraction in thenar muscles of the stimulated hand. The aim was to study the role of the primary (SI) and secondary somatosensory (SII) cortices in sensorimotor integration. The amplitude of the SI response N20m did not change with coincident isometric contraction, whereas P35m was significantly reduced. On the contrary, activation of contra- and ipsilateral SII cortices was significantly enhanced during the contraction. We suggest that isometric contraction facilitates activation of SII cortices to tactile stimuli, possibly by decreasing inhibition from the SI cortex. The enhanced SII activation may be related to tuning of SII neurons towards relevant tactile input arising from the region of the body where the muscle activation occurs.     

Attention modulates beta oscillations during prolonged tactile stimulation

The inter‐play between changes in beta‐band (14–30‐Hz) cortical rhythms and attention during somatosensation informs us about where and when relevant processes occur in the brain. As such, we investigated the effects of attention on somatosensory evoked and induced responses using vibrotactile stimulation and magnetoencephalographic recording. Subjects received trains of vibration at 23 Hz to the right index finger while watching a movie and ignoring the somatosensory stimuli or paying attention to the stimuli to detect a change in the duration of the stimulus. The amplitude of the evoked 23‐Hz steady‐state response in the contralateral primary somatosensory cortex (SI) was enhanced by attention and the underlying dipole source was located 2 mm more medially, indicating top‐down recruitment of additional neuronal populations for the functionally relevant stimulus. Attentional modulation of the somatosensory evoked response indicates facilitation of early processing of the tactile stimulus. Beta‐band activity increased after vibration offset in the contralateral primary motor cortex (MI) [event‐related synchronization (ERS)] and this increase was larger for attended than ignored stimuli. Beta‐band activity decreased in the ipsilateral SI prior to stimulus offset [event‐related desynchronization (ERD)] for attended stimuli only. Whereas attention modulation of the evoked response was confined to the contralateral SI, event‐related changes of beta‐band activity involved contralateral SI–MI and inter‐hemispheric SI–SI connections. Modulation of neural activity in such a large sensorimotor network indicates a role for beta activity in higher‐order processing.     

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.     

Neuromagnetic gamma-band activity in the primary and secondary somatosensory areas

To evaluate the gamma-band activity related to somatosensory processing, we recorded neuromagnetic signals from seven healthy subjects. The source power changes evoked by electrical stimulation of the median nerve were estimated with synthetic aperture magnetometry (SAM). Source power in the low gamma band (40 Hz) decreased in the contralateral primary somatosensory cortex (SI) for a few hundred milliseconds (i.e. middle and long latency) and then increased inversely. Source power in the high gamma band (70-90 Hz) increased simultaneously both in the contralateral SI and contra/ipsilateral secondary somatosensory cortex (SII) in 80-180 ms. These results suggest that low and high gamma oscillations work under independent mechanisms during somatosensory processing. In particular, high gamma oscillations may play an essential role in making a functional connection between SI and SII.     

Comparison between SI and SII responses as a function of stimulus intensity

In this MEG study we investigated the differences in responses to somatosensory electrical stimuli between primary (SI) and secondary (SII) sensory cortices using 10 different levels of stimulus intensity, starting from below the sensory threshold up to a weak painful level. SI dipole source linearly increased in amplitude as the stimulus intensity raised up to a strong motor level and then saturated at higher stimulation levels. The contralateral and ipsilateral SII dipole source strengths followed the stimulus intensity growing up to the motor threshold, but showed a decrease at the strong motor level, followed by an increase as the stimulus intensity raised towards the weak painful threshold. These results suggest different responses of SI and SII cortices as the intensity of stimulation rises from non-painful to painful values.     

Central representation of the RIII flexion reflex associated with overt motor reaction: an fMRI study.

Recent neuroimaging studies precised the functions of the brain regions included in the so-called "pain-matrix". They isolated brain structures mediating attentional, emotional, anticipatory, cognitive, and discriminative aspects of pain perception. Surprisingly, little attention was devoted to isolate the cerebral network associated with the motor response to pain. In this study, we used fMRI to measure BOLD signal changes in nine volunteers while they received low- (L-) and high- (H-) intensity painful electrical shocks on the (left) lower limb. High-intensity stimulation was associated with a significantly stronger pain sensation and with a pronounced motor (withdrawal) reflex. BOLD responses common to L- and H-stimulation intensities were found in the right prefrontal and right posterior parietal cortices. These did not correlate with subjective pain ratings and probably mediate attentional processes unrelated to pain intensity and withdrawal. In contrast, signal changes in insula, left SII cortices and right amygdala did correlate with pain ratings and are therefore likely to encode for pain intensity. High-intensity shocks selectively recruited a motor network, including vermis, MI, SI, and paracentral cortices bilaterally, right premotor, right SII and posterior cingulate cortices. These responses, assessed for the first time in a functional imaging study, emphazised on the presence of a motor component in what has been described as the pain-matrix. They should be considered as a motor component of pain-related processes activated in case of intense pain.     

Analysis of pain-related somatosensory evoked magnetic fields using the MUSIC (multiple signal classification) algorithm for magnetoencephalography.

We evaluated the effectiveness of the Multiple Signal Classification (MUSIC) algorithm by analysing pain-related somatosensory-evoked magnetic fields (SEFs) by 148-channel whole-head-type magnetoencephalography. MUSIC peaks of middle latency components were located around the primary somatosensory cortex (SI), contralateral to the stimulated finger. Long latency components were located around the bilateral secondary somatosensory cortices (SII) and cingulate gyri. Peaks at the SII and cingulate gyri were more prominent on very painful and moderately painful stimulation than on weak stimulation. The results were in very good agreement with results from single dipole estimation. These findings suggest that the MUSIC algorithm could be a useful tool for analysis of pain-related SEFs.     

tDCS modulates behavioral performance and the neural oscillatory dynamics serving visual selective attention

Transcranial direct‐current stimulation (tDCS) is a noninvasive method for modulating human brain activity. Although there are several hypotheses about the net effects of tDCS on brain function, the field's understanding remains incomplete and this is especially true for neural oscillatory activity during cognitive task performance. In this study, we examined whether different polarities of occipital tDCS differentially alter flanker task performance and the underlying neural dynamics. To this end, 48 healthy adults underwent 20 min of anodal, cathodal, or sham occipital tDCS, and then completed a visual flanker task during high‐density magnetoencephalography (MEG). The resulting oscillatory responses were imaged in the time‐frequency domain using beamforming, and the effects of tDCS on task‐related oscillations and spontaneous neural activity were assessed. The results indicated that anodal tDCS of the occipital cortices inhibited flanker task performance as measured by reaction time, elevated spontaneous activity in the theta (4–7 Hz) and alpha (9–14 Hz) bands in prefrontal and occipital cortices, respectively, and reduced task‐related theta oscillatory activity in prefrontal cortices during task performance. Cathodal tDCS of the occipital cortices did not significantly affect behavior or any of these neuronal parameters in any brain region. Lastly, the power of theta oscillations in the prefrontal cortices was inversely correlated with reaction time. In conclusion, anodal tDCS modulated task‐related oscillations and spontaneous activity across multiple cortical areas, both near the electrode and in distant sites that were putatively connected to the targeted regions.     

Neuronal correlates of gastric pain induced by fundus distension: a 3T-fMRI study

Visceral hypersensitivity in gastric fundus is a possible pathogenesis for functional dyspepsia. The cortical representation of gastric fundus is still unclear. Growing evidence shows that the insula, but not the primary or secondary somatosensory region (SI or SII), may be the cortical target for visceral pain. Animal studies have also demonstrated that amygdala plays an important role in processing visceral pain. We used fMRI to study central projection of stomach pain from fundus balloon distension. We also tested the hypothesis that there will be neither S1 nor S2 activation, but amygdala activation with the fundus distension. A 3T-fMRI was performed on 10 healthy subjects during baseline, fullness (12.7 +/- 0.6 mmHg) and moderate gastric pain (17.0 +/- 0.8 mmHg). fMRI signal was modelled by convolving the predetermined psychophysical response. Statistical comparisons were performed between conditions on a group level. Gastric pain activated a wide range of cortical and subcortical structures, including thalamus and insula, anterior and posterior cingulate cortices, basal ganglia, caudate nuclei, amygdala, brain stem, cerebellum and prefrontal cortex (P < 0.001). A subset of these neuronal substrates was engaged in the central processing of fullness sensation. SI and SII were not activated during the fundus stimulation. In conclusion, the constellation of neuronal structures activated by fundus distension overlaps the pain matrices induced musculocutaneous pain, with the exception of the absence of SI or SII activation. This may account for the vague nature of visceral sensation/pain. Our data also confirms that the insula and amygdala may act as the central role in visceral sensation/pain, as well as in the proposed sensory-limbic model of learning and memory of pain.     

Characteristics of the human contra- versus ipsilateral SII cortex.

OBJECTIVES: In order to study the interaction between left- and right-sided stimuli on the activation of cortical somatosensory areas, we recorded somatosensory evoked magnetic fields (SEFs) from 8 healthy subjects with a 122 channel whole-scalp SQUID gradiometer. METHODS: Right and left median nerves were stimulated either alternately within the same run, with interstimulus intervals (ISIs) of 1.5 and 3 s, or separately in different runs with a 3 s ISI. In all conditions 4 cortical source areas were activated: the contralateral primary somatosensory cortex (SI), the contra- and ipsilateral secondary somatosensory cortices (SII) and the contralateral posterior parietal cortex (PPC). RESULTS: The earliest activity starting at 20 ms was generated solely in the SI cortex, whereas longer-latency activity was detected from all 4 source areas. The mean peak latencies for SII responses were 86-96 ms for contralateral and 94-97 ms for ipsilateral stimuli. However, the activation of right and left SII areas started at 61+/-3 and 62+/-3 ms to contralateral stimuli and at 66+/-2 and 63+/-2 ms to ipsilateral stimuli, suggesting a simultaneous commencing of activation of the SII areas. PPC sources were activated between 70 and 110 ms in different subjects. The 1.5 s ISI alternating stimuli elicited smaller SII responses than the 3 s ISI non-alternating stimuli, suggesting that a considerable part of the neural population in SII responds both to contra- and ipsilateral stimuli. The earliest SI responses did not differ between the two conditions. There were no significant differences in source locations of SII responses to ipsi- and contralateral stimuli in either hemisphere. Subaverages of the responses in sets of 30 responses revealed that amplitudes of the SII responses gradually attenuated during repetitive stimulation, whereas the amplitudes of the SI responses were not changed. CONCLUSIONS: The present results implicate that ipsi- and contralateral SII receive simultaneous input, and that a large part of SII neurons responds both to contra- and ipsilateral stimulation. The present data also highlight the different behavior of SI and SII cortices to repetitive stimuli.     

Adaptive changes in the neuromagnetic response of the primary and association somatosensory areas following repetitive tactile hand stimulation in humans

Cortical adaptation in the primary somatosensory cortex (SI) has been probed using different stimulation modalities and recording techniques, in both human and animal studies. In contrast, considerably less knowledge has been gained about the adaptation profiles in other areas of the cortical somatosensory network. Using magnetoencephalography (MEG), we examined the patterns of short‐term adaptation for evoked responses in SI and somatosensory association areas during tactile stimulation applied to the glabrous skin of the hand. Cutaneous stimuli were delivered as trains of serial pulses with a constant frequency of 2 Hz and 4 Hz in separate runs, and a constant inter‐train interval of 5 s. The unilateral stimuli elicited transient responses to the serial pulses in the train, with several response components that were separated by independent component analysis. Subsequent source reconstruction techniques identified regional generators in the contralateral SI and somatosensory association areas in the posterior parietal cortex (PPC). Activity in the bilateral secondary somatosensory cortex (i.e., SII/PV) was also identified, although less consistently across subjects. The dynamics of the evoked activity in each area and the frequency‐dependent adaptation effects were assessed from the changes in the relative amplitude of serial responses in each train. We show that the adaptation profiles in SI and PPC areas can be quantitatively characterized from neuromagnetic recordings using tactile stimulation, with the sensitivity to repetitive stimulation increasing from SI to PPC. A similar approach for SII/PV has proven less straightforward, potentially due to the tendency of these areas to respond selectively to certain stimuli. Hum Brain Mapp, 2013. © 2012 Wiley Perodicals, Inc.     

Interactions between visual and motor areas during the recognition of plausible actions as revealed by magnetoencephalography

Several studies have shown activation of the mirror neuron system (MNS), comprising the temporal, posterior parietal, and sensorimotor areas when observing plausible actions, but far less is known on how these cortical areas interact during the recognition of a plausible action. Here, we recorded neural activity with magnetoencephalography while subjects viewed point‐light displays of biologically plausible and scrambled versions of actions. We were interested in modulations of oscillatory activity and, specifically, in coupling of oscillatory activity between visual and motor areas. Both plausible and scrambled actions elicited modulations of θ (5–7 Hz), α (7–13 Hz), β (13–35 Hz), and γ (55–100 Hz) power within visual and motor areas. When comparing between the two actions, we observed sequential and spatially distinct increases of γ (～65 Hz), β (～25 Hz), and α (～11 Hz) power between 0.5 and 1.3 s in parieto‐occipital, sensorimotor, and left temporal areas. In addition, significant clusters of γ (～65 Hz) and α/β (～15 Hz) power decrease were observed in right temporal and parieto‐occipital areas between 1.3 and 2.0 s. We found β‐power in sensorimotor areas to be positively correlated on a trial‐by‐trial basis with parieto‐occipital γ and left temporal α‐power for the plausible but not for the scrambled condition. These results provide new insights in the neuronal oscillatory activity of the areas involved in the recognition of plausible action movements and their interaction. The power correlations between specific areas underscore the importance of interactions between visual and motor areas of the MNS during the recognition of a plausible action. Hum Brain Mapp 35:581–592, 2014. © 2012 Wiley‐Periodicals, Inc.     

The role of somatosensory cortical regions in the processing of painful gastric fundic distension: an update of brain imaging findings

Abstract  Painful gastric distension is processed in a network consisting of brainstem, thalamus, insula, anterior cingulate cortex, (lateral) orbitofrontal and prefrontal cortex, superior temporal cortex and cerebellum. However, the role of primary and secondary somatosensory cortical regions (SI/SII) in the processing of visceral sensation or pain in general and gastric sensation in particular remains unclear. The aim of this study was to localize activations in the SI/SII area from our previously published functional brain imaging studies on gastric distension more precisely, using newly available cytoarchitectonic probability maps of SI/SII, implemented in the SPM Anatomy toolbox. In healthy volunteers, we found two clusters to be overlapping with SII (mainly the OP4 subregion) and, to a lesser extent, SI, although this overlap was small in size. In functional dyspepsia patients, we found two clusters to be overlapping with SII (mainly OP4), of which the cluster in the right hemisphere also overlapped with SI. These findings were confirmed in a conjunction analysis of both groups. Activation in right SI/SII was significantly higher in healthy volunteers when formally compared to patients. These results provide more detailed information on the brain processing of gastric sensation, supporting the hypothesis that SI/SII are involved. This is in line with some previously published studies on visceral sensation, but at variance with some other studies. Methodological differences between the brain imaging studies on gastric distension may account for these somewhat discrepant findings.     

Short communication: mapping of somatosensory cortices with functional magnetic resonance imaging in anaesthetized macaque monkeys

Functional magnetic resonance imaging (fMRI) in macaque monkeys is emerging as a potent candidate to bridge the gap between data from human fMRI studies and data from anatomy, electrophysiology and lesion studies in monkeys. The primary (SI) and secondary (SII) somatosensory cortices are the principal regions for somatosensory information processing and contain systematic representations of the body surface map (somatotopy). To examine the functional organization of the somatosensory cortices in anaesthetized macaque monkeys with fMRI, we asked whether focal and differential activation could be observed in SI and SII in response to tactile stimulation with two parameters: body sides (right and left) and body regions (hand and face). We found that changes in stimulus parameters elicited differential focal activation in both SI and SII in two ways. First, the hand and face stimulation activated SI and SII in the contralateral, but not in the ipsilateral, hemisphere. Second, the hand and face stimulation differentially activated two adjacent regions in both SI and SII. These fMRI results appear to correlate with previous mapping studies by other methods in the macaque somatosensory cortices. This study shows the feasibility of fMRI studies in mapping multiple sensory areas in monkeys by which we can distinguish between adjacent functionally distinct regions.     

Mechanical vibratory stimulation of feline forepaw skin induces long-lasting potentiation in the secondary somatosensory cortex

We investigated the long-lasting effects of mechanical vibratory stimulation of the skin on the excitability of feline cortical neurons in the forelimb areas of the primary (SI) and secondary (SII) somatosensory cortices. Conditioning mechanical stimuli were 300 bursts of 10 pulses at 200 Hz delivered with a 10-s interburst interval from a mechanical stimulator. Test field potentials and unit discharges were evoked by electrical stimulation to the ventral posterolateral thalamic nucleus (VPL) or by single mechanical stimuli applied to the skin. In SII, the mechanical burst stimulation to the skin increased the amplitudes of field potentials and the frequency of unit discharges elicited by single mechanical stimuli applied to the skin. The vibratory conditioning stimulus also produced a similar potentiation of the VPL-evoked field potentials (126-139% increase in amplitude, P < 0.05) with an associated increase in firing rates of extracellularly recorded neuronal activity (117%, P < 0.001). These potentiations persisted through the entire experimental period of 120 min. The translaminar current source density analysis calculated from the VPL-evoked field potentials increased to 127% of the control value (P < 0.01). In contrast, in SI we observed no significant changes in the field potential amplitudes or in the currents generated in superficial layers (91-117%). Taken together with the previous finding that tetanic electrical stimulation of VPL induces long-lasting potentiation of the VPL-evoked cortical responses in SII but not of those in SI, the present results suggest that SII has a large capacity for the rapid functional plasticity involved in the learning that occurs during repeated tactile experiences.     

Effects of muscle contraction on somatosensory event-related EEG power and coherence changes.

Effects of isometric muscle contraction on amplitude and coherence changes of EEG rhythms during repetitive cutaneous electrical stimulation were analyzed in 10 right-handed subjects. Subjects received electrical stimuli at intensity above pain threshold to their right middle finger while either squeezing a rubber tube with the right index finger and thumb, or keeping their ipsilateral hand muscles relaxed. EEG was recorded using 111 closely spaced electrodes. Somatosensory stimuli were followed by reduction of the relative 8-12 and 16-24 Hz band power (at 0.2-0.4 s) in bilateral primary sensorimotor cortices (S1/M1) and medial frontal cortex, and by a subsequent increase in 16-24 Hz band power (at 0.9 s). Isometric muscle contraction strongly suppressed these band power changes. The 8-12 and 16-24 Hz mean coherence in a wide region surrounding the contralateral S1/M1 and in the medial frontal cortex showed an initial decrease, partially paralleling band power changes, and later an increase. Ipsilateral S1/M1 showed a decrease in 8-12 Hz coupling only with the central and frontal electrodes of the same hemisphere. Muscle contraction reduced all coherence changes, but enhanced the 8-12 Hz coherence between ipsilateral S1/M1 and posterior parietal cortex. Early post-stimulus decrease of oscillatory coupling between S1/M1 and premotor cortex and between S1/M1 and medial frontal cortex suggests that these cortical regions act rather independently during processing of somatosensory information, and synchronize only later when the band power in contralateral S1/M1 increases. Motor cortex activation associated with ipsilateral hand muscle contraction interferes with cortical processing of somatosensory stimuli in S1/M1 cortices.