Significance of the second somatosensory cortex in sensorimotor integration: enhancement of sensory responses during finger movements

Cortical maps of somatosensory perception in human

Tactile and movement-related somatosensory perceptions are crucial for our daily lives and survival. Although the primary somatosensory cortex is thought to be the key structure of somatosensory perception, various cortical downstream areas are also involved in somatosensory perceptual processing. However, little is known about whether cortical networks of these downstream areas can be dissociated depending on each perception, especially in human. We address this issue by combining data from direct cortical stimulation (DCS) for eliciting somatosensation and data from high-gamma band (HG) elicited during tactile stimulation and movement tasks. We found that artificial somatosensory perception is elicited not only from conventional somatosensory-related areas such as the primary and secondary somatosensory cortices but also from a widespread network including superior/inferior parietal lobules and premotor cortex. Interestingly, DCS on the dorsal part of the fronto-parietal area including superior parietal lobule and dorsal premotor cortex often induces movement-related somatosensations, whereas that on the ventral one including inferior parietal lobule and ventral premotor cortex generally elicits tactile sensations. Furthermore, the HG mapping results of the movement and passive tactile stimulation tasks revealed considerable similarity in the spatial distribution between the HG and DCS functional maps. Our findings showed that macroscopic neural processing for tactile and movement-related perceptions could be segregated.

Neural signatures of visuo-motor integration during human-robot interactions

Visuo-motor integration shapes our daily experience and underpins the sense of feeling in control over our actions. The last decade has seen a surge in robotically and virtually mediated interactions, whereby bodily actions ultimately result in an artificial movement. But despite the growing number of applications, the neurophysiological correlates of visuo-motor processing during human-machine interactions under dynamic conditions remain scarce. Here we address this issue by employing a bimanual robotic interface able to track voluntary hands movement, rendered in real-time into the motion of two virtual hands. We experimentally manipulated the visual feedback in the virtual reality with spatial and temporal conflicts and investigated their impact on (1) visuo-motor integration and (2) the subjective experience of being the author of one's action (i.e., sense of agency). Using somatosensory evoked responses measured with electroencephalography, we investigated neural differences occurring when the integration between motor commands and visual feedback is disrupted. Our results show that the right posterior parietal cortex encodes for differences between congruent and spatially-incongruent interactions. The experimental manipulations also induced a decrease in the sense of agency over the robotically-mediated actions. These findings offer solid neurophysiological grounds that can be used in the future to monitor integration mechanisms during movements and ultimately enhance subjective experience during human-machine interactions.

Engrafted glial progenitor cells yield long-term integration and sensory improvement in aged mice

Aging causes astrocyte morphological degeneration and functional deficiency, which impairs neuronal functions. Until now, whether age-induced neuronal deficiency could be alleviated by engraftment of glial progenitor cell (GPC) derived astrocytes remained unknown. In the current study, GPCs were generated from embryonic cortical neural stem cells in vitro and transplanted into the brains of aged mice. Their integration and intervention effects in the aged brain were examined 12 months after transplantation. Results indicated that these in-vitro-generated GPC-derived astrocytes possessed normal functional properties. After transplantation they could migrate, differentiate, achieve long-term integration, and maintain much younger morphology in the aged brain. Additionally, these GPC-derived astrocytes established endfeet expressing aquaporin-4 (AQP4) and ameliorate AQP4 polarization in the aged neocortex. More importantly, age-dependent sensory response degeneration was reversed by GPC transplantation. This work demonstrates that rejuvenation of the astrocyte niche is a promising treatment to prevent age-induced degradation of neuronal and behavioral functions.

Vibration Alert to the Brain: Evoked and Induced MEG Responses to High-Frequency Vibrotactile Stimuli on the Index Finger of Dominant and Non-dominant Hand

Background: In recent years, vibrotactile haptic feedback technology has been widely used for user interfaces in the mobile devices. Although functional neuroimaging studies have investigated human brain responses to different types of tactile inputs, the neural mechanisms underlying high-frequency vibrotactile perception are still relatively unknown. Our aim was to investigate neuromagnetic brain responses to high-frequency vibrotactile stimulation, using magnetoencephalography (MEG).

Methods: We measured 152-channel whole-head MEG in 30 healthy, right-handed volunteers (aged 20–28 years, 15 females). A total of 300 vibrotactile stimuli were presented at the tip of either the left index finger or the right index finger in two separate sessions. Sinusoidal vibrations at 150 Hz for 200 ms were generated with random inter-stimulus intervals between 1.6 and 2.4 s. Both time-locked analysis and time-frequency analysis were performed to identify peak responses and oscillatory modulations elicited by high-frequency vibrations. The significance of the evoked and induced responses for dominant and non-dominant hand stimulation conditions was statistically tested, respectively. The difference in responses between stimulation conditions was also statistically evaluated.

Results: Prominent peak responses were observed at 56 ms (M50) and at 100 ms (M100) for both stimulation conditions. The M50 response revealed clear dipolar field patterns in the contralateral side with significant cortical activations in the contralateral primary sensorimotor area, whereas the M100 response was not as prominent as the M50. Vibrotactile stimulation induced significant suppression of both alpha (8–12 Hz) and beta (20–30 Hz) band activity during the mid-latency period (0.2–0.4 s), primarily in sensorimotor areas contralateral to the stimulation side. In addition, a significant alpha enhancement effect in posterior regions was accompanied with alpha suppressions in sensorimotor regions. The alpha suppression was observed in a broader distribution of cortical areas for the non-dominant hand stimulation.

Conclusion: Our data demonstrate that high-frequency tactile vibrations, which is known to primarily activate Pacinian corpuscles, elicit somatosensory M50 and M100 responses in the evoked fields and induce modulations of alpha and beta band oscillations during mid-latency periods. Our study is also consistent with that the primary sensorimotor area is significantly involved in the processing of high-frequency vibrotactile information with contralateral dominance.

Alterations in knee sensorimotor brain functional connectivity contributes to ACL injury in male high-school football players: a prospective neuroimaging analysis

OBJECTIVE

This study's purpose was to utilize a prospective dataset to examine differences in functional brain connectivity in male high school athletes who suffered an anterior cruciate ligament (ACL) injury relative to their non-injured peers.

METHODS

Sixty-two male high school football players were evaluated using functional magnetic resonance imaging prior to their competitive season to evaluate resting-state functional brain connectivity. Three athletes later experienced an ACL injury and were matched to 12 teammates who did not go on to sustain an ACL injury (controls) based on school, age, height, weight, and year in school. Twenty-five knee-motor regions of interest (ROIs) were created to identify differences in connectivity between the two groups. Between-subject F and t tests were used to identify significant ROI differences using a false discovery rate correction for multiple comparisons.

RESULTS

There was significantly less connectivity between the left secondary somatosensory cortex and the left supplementary motor area (p = 0.025), right pre-motor cortex (p = 0.026), right supplementary motor area (p = 0.026), left primary somatosensory cortex (superior division; p = 0.026), left primary somatosensory cortex (inferior division; p = 0.026), and left primary motor cortex (p = 0.048) for the ACL-injured compared to the control subjects. No other ROI-to-ROI comparisons were significantly different between the groups (all p > 0.05).

CONCLUSION

Our preliminary data indicate a potential sensorimotor disruption for male football players who go on to experience an ACL injury. Future studies with larger sample sizes and complementary measures of neuromuscular control are needed to support these findings.

Functional and Structural Neuroplasticity Induced by Short-Term Tactile Training Based on Braille Reading

Neuroplastic changes induced by sensory learning have been recognized within the cortices of specific modalities as well as within higher ordered multimodal areas. The interplay between these areas is not fully understood, particularly in the case of somatosensory learning. Here we examined functional and structural changes induced by short-term tactile training based of Braille reading, a task that requires both significant tactile expertise and mapping of tactile input onto multimodal representations. Subjects with normal vision were trained for 3 weeks to read Braille exclusively by touch and scanned before and after training, while performing a same-different discrimination task on Braille characters and meaningless characters. Functional and diffusion-weighted magnetic resonance imaging sequences were used to assess resulting changes. The strongest training-induced effect was found in the primary somatosensory cortex (SI), where we observed bilateral augmentation in activity accompanied by an increase in fractional anisotropy (FA) within the contralateral SI. Increases of white matter fractional anisotropy were also observed in the secondary somatosensory area (SII) and the thalamus. Outside of somatosensory system, changes in both structure and function were found in i.e., the fusiform gyrus, the medial frontal gyri and the inferior parietal lobule. Our results provide evidence for functional remodeling of the somatosensory pathway and higher ordered multimodal brain areas occurring as a result of short-lasting tactile learning, and add to them a novel picture of extensive white matter plasticity.

Beyond the N1: A review of late somatosensory evoked responses in human infants

Somatosensory evoked potentials (SEPs) have been used for decades to study the development of somatosensory processing in human infants. Research on infant SEPs has focused on the initial cortical component (N1) and its clinical utility for predicting neurological outcome in at-risk infants. However, recent studies suggest that examining the later components in the infant somatosensory evoked response will greatly advance our understanding of somatosensory processing in infancy. The purpose of this review is to synthesize the existing electroencephalography (EEG) and magnetoencephalography (MEG) studies on late somatosensory evoked responses in infants. We describe the late responses that have been reported and discuss the utility of such responses for illuminating key aspects of somatosensory processing in typical and atypical development.

Time Course of Tactile Gating in a Reach-to-Grasp and Lift Task

Humans' sensory systems are bombarded by myriad events every moment of our lives. Thus, it is crucial for sensory systems to choose and process critical sensory events deemed important for a given task and, indeed, those that affect survival. Tactile gating is well known, and defined as a reduced ability to detect and discriminate tactile events before and during movement. Also, different locations of the effector exhibit different magnitudes of sensitivity changes. The authors examined that time course of tactile gating in a reaching and grasping movement to characterize its behavior. Tactile stimulators were attached to the right and left mid-forearms and the right index finger and fifth digit. When participants performed reach-to-grasp and lift targets, tactile acuity decreased at the right forearm before movement onset (F. L. Colino, G. Buckingham, D. T. Cheng, P. van Donkelaar, & G. Binsted, 2014 ). However, tactile sensitivity at the right index finger decreased by nearly 20% contrary to expectations. This result reflecting that there may be an additional source acting to reduce inhibition related to tactile gating. Additionally, sensitivity improved as movement end approached. Collectively, the present results indicate that predictive and postdictive mechanisms strongly influence tactile gating.

The role of tactile afference in shaping motor behaviour and implications for prosthetic innovation

The present review focusses on how tactile somatosensory afference is encoded and processed, and how this information is interpreted and acted upon in terms of motor control. We relate the fundamental workings of the sensorimotor system to the rehabilitation of amputees using modern prosthetic interventions. Our sense of touch is central to our everyday lives, from allowing us to manipulate objects accurately to giving us a sense of self-embodiment. There are a variety of specialised cutaneous mechanoreceptive afferents, which differ in terms of type and density according to the skin site. In humans, there is a dense innervation of our hands, which is reflected in their vast over-representation in somatosensory and motor cortical areas. We review the accumulated evidence from animal and human studies about the precise interplay between the somatosensory and motor systems, which is highly integrated in many brain areas and often not separable. The glabrous hand skin provides exquisite, discriminative detail about touch, which is useful for refining movements. When these signals are disrupted, such as through injury or amputation, the consequences are considerable. The development of sensory feedback in prosthetics offers a promising avenue for the full integration of a missing body part. Real-time touch feedback from motor intentions aids in grip control and the ability to distinguish different surfaces, even introducing the possibility of pleasure in artificial touch. Thus, our knowledge from fundamental research into sensorimotor interactions should be used to develop more realistic and integrative prostheses.

Integrated approach for studying adaptation mechanisms in the human somatosensory cortical network

Magnetoencephalography and independent component analysis (ICA) was utilized to study and characterize neural adaptation in the somatosensory cortical network. Repetitive punctate tactile stimuli were applied unilaterally to the dominant hand and face using a custom-built pneumatic stimulator called the TAC-Cell. ICA-based source estimation from the evoked neuromagnetic responses indicated cortical activity in the contralateral primary somatosensory cortex (SI) for face stimulation, while hand stimulation resulted in robust contralateral SI and posterior parietal cortex (PPC) activation. Activity was also observed in the secondary somatosensory cortical area (SII) with reduced amplitude and higher variability across subjects. There was a significant difference in adaptation rate between SI and higher-order somatosensory cortices for hand stimulation. Adaptation was significantly dependent on stimulus frequency and pulse index within the stimulus train for both hand and face stimulation. The peak latency of the activity was significantly dependent on stimulation site (hand vs. face) and cortical area (SI vs. PPC). The difference in the peak latency of activity in SI and PPC is presumed to reflect a hierarchical serial-processing mechanism in the somatosensory cortex.

Tactile gating in a reaching and grasping task

Abstract A multitude of events bombard our sensory systems at every moment of our lives. Thus, it is important for the sensory cortex to gate unimportant events. Tactile suppression is a well-known phenomenon defined as a reduced ability to detect tactile events on the skin before and during movement. Previous experiments found detection rates decrease just prior to and during finger abduction, and decrease according to the proximity of the moving effector. This study examined how tactile detection changes during a reach to grasp. Fourteen human participants used their right hand to reach and grasp a cylinder. Tactors were attached to the index finger, the fifth digit, and the forearm of both the right and left arm and vibrated at various epochs relative to a "go" tone. Results showed that detection rates at the forearm decreased before movement onset; whereas at the right index finger, right fifth digit and at the left index finger, left fifth digit, and forearm sites did not decrease like in the right forearm. These results indicate that the task affects gating dynamics in a temporally- and contextually dependent manner and implies that feed-forward motor planning processes can modify sensory signals.

Cortical somatosensory processing measured by magnetoencephalography predicts neurodevelopment in extremely low-gestational-age infants

BACKGROUND

Higher cortical function during sensory processing can be examined by recording specific somatosensory-evoked magnetic fields (SEFs) with magnetoencephalography (MEG). We evaluated whether, in extremely low-gestational-age (ELGA) infants, abnormalities in MEG-recorded SEFs at term age are associated with adverse neurodevelopment at 2 y of corrected age.

METHODS

SEFs to tactile stimulation of the index finger were recorded at term age in 30 ELGA infants (26.5 ± 1.2 wk, birth weight: 884 g ± 181 g). Neurodevelopment was evaluated at 2 y of corrected age. Controls were 11 healthy term infants.

RESULTS

In nine of the ELGA infants (30.0%), SEFs were categorized as abnormal on the basis of lack of response from secondary somatosensory cortex (SII). At 2 y, these infants had a significantly worse mean developmental quotient and locomotor subscale on the Griffiths Mental Development Scales than the ELGA infants with normal responses. Mild white matter abnormalities in magnetic resonance imaging at term age were detected in 21% of infants, but these abnormalities were not associated with adverse neurodevelopment.

CONCLUSION

Abnormal SII responses at term predict adverse neuromotor development at 2 y of corrected age. This adverse development may not be foreseen with conventional neuroimaging methods, suggesting a role for evaluating SII responses in the developmental risk assessment of ELGA infants.

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.

Reorganization of the human somatosensory cortex in hand dystonia

Background and Purpose:

Abnormalities of finger representations in the somatosensory cortex have been identified in patients with focal hand dystonia. Measuring blood flow with positron emission tomography (PET) can be use to demonstrate functional localization of receptive fields.

Methods:

A vibratory stimulus was applied to the right thumb and little finger of six healthy volunteers and six patients with focal hand dystonia to map their receptive fields using H₂¹⁵O PET.

Results:

The cortical finger representations in the primary somatosensory cortex were closer to each other in patients than in normal subjects. No abnormalities were found in secondary somatosensory cortex, but the somatotopy there is less well distinguished.

Conclusions:

These data confirm prior electrophysiological and functional neuroimaging observations showing abnormalities of finger representations in somatosensory cortex of patients with focal hand dystonia.

Intracortical modulation of somatosensory evoked fields during movement: evidence for selective suppression of postsynaptic inhibition

As accurate finger movements depend on guidance by afferent sensory feedback information, it is of interest to examine how the cortical processing of afferent signals is altered during movement states compared with rest. In the present study we evaluated afferent input to the primary somatosensory cortex (SI) in human subjects performing a finger opposition task. We recorded somatosensory evoked magnetic fields (SEFs) in 6 healthy subjects to stimulation of left and right median nerves in a resting condition and during active right-sided finger movements. At the left SI, the SEFs to right (moving hand) median nerve stimulation showed a selective and robust reduction of the P35m deflection during movement compared with rest, while there were only minor non-significant changes in the other SEF deflections, including N20m, which represents the 1st excitatory cortical event after stimulation. In contrast, at the right SI the SEFs to left (non-moving hand) median nerve stimulation were modified in the opposite direction: the P35m deflection was slightly enhanced during right-sided movement, there being no significant changes in the other deflections. The results thus show that the P35m SEF deflection can be selectively reduced during finger movements of the stimulated hand, and selectively enhanced if the movement is being performed with the fingers of the opposite hand. Because N20m was not changed, the modulation took place at the cortical level rather than in the afferent pathways. As the P35m SEF deflection likely represents postsynaptic IPSPs at SI, the results suggest that postsynaptic inhibition to somatosensory impulses from the moving part of the body is suppressed. Comparison of the present results with recent intracellular studies in behaving mice suggests that the P35m reduction specifically corresponds to a reduction in the activity of parvalbumin-containing fast-spiking inhibitory interneurons during movement. The results provide evidence that precision movements can be executed without this type of cortical postsynaptic inhibition.

Source analysis of beta-synchronisation and cortico-muscular coherence after movement termination based on high resolution electroencephalography

We hypothesized that post-movement beta synchronization (PMBS) and cortico-muscular coherence (CMC) during movement termination relate to each other and have similar role in sensorimotor integration. We calculated the parameters and estimated the sources of these phenomena.

We measured 64-channel EEG simultaneously with surface EMG of the right first dorsal interosseus muscle in 11 healthy volunteers. In Task1, subjects kept a medium-strength contraction continuously; in Task2, superimposed on this movement, they performed repetitive self-paced short contractions. In Task3 short contractions were executed alone. Time-frequency analysis of the EEG and CMC was performed with respect to the offset of brisk movements and averaged in each subject. Sources of PMBS and CMC were also calculated.

High beta power in Task1, PMBS in Task2-3, and CMC in Task1-2 could be observed in the same individual frequency bands. While beta synchronization in Task1 and PMBS in Task2-3 appeared bilateral with contralateral predominance, CMC in Task1-2 was strictly a unilateral phenomenon; their main sources did not differ contralateral to the movement in the primary sensorimotor cortex in 7 of 11 subjects in Task1, and in 6 of 9 subjects in Task2. In Task2, CMC and PMBS had the same latency but their amplitudes did not correlate with each other. In Task2, weaker PMBS source was found bilaterally within the secondary sensory cortex, while the second source of CMC was detected in the premotor cortex, contralateral to the movement. In Task3, weaker sources of PMBS could be estimated in bilateral supplementary motor cortex and in the thalamus.

PMBS and CMC appear simultaneously at the end of a phasic movement possibly suggesting similar antikinetic effects, but they may be separate processes with different active functions. Whereas PMBS seems to reset the supraspinal sensorimotor network, cortico-muscular coherence may represent the recalibration of cortico-motoneuronal and spinal systems.

Conflict caused by visual feedback modulates activation in somatosensory areas during movement execution

The role of sensory information in motor control has been studied, but the cortical processing underlying cross-modal relationship between visual and somatosensory information for movement execution remains a matter of debate. Visual estimates of limb positions are congruent with proprioceptive estimates under normal visual conditions, but a mismatch between the watched and felt movement of the hand disrupts motor execution. We investigated whether activation in somatosensory areas was affected by the discordance between the intended and an executed action. Subjects performed self-paced thumb movement of the left hand under normal visual and mirror conditions. The Mirror condition provided a non-veridical and unexpected visual feedback. The results showed activity in the primary somatosensory area to be inhibited and activity in the secondary somatosensory area (SII) to be enhanced with voluntary movement, and neural responses in the SII and parietal cortex were strongly affected by the unexpected visual feedback. These results provide evidence that the visual information plays a crucial role in activation in somatosensory areas during motor execution. A mechanism that monitors sensory inputs and motor outputs congruent with current intension is necessary to control voluntary movement.

Effects of motor intention on the perception of somatosensory events: A behavioural and functional magnetic resonance imaging study

The intention to execute a movement can modulate our perception of sensory events, and this modulation is observed ahead of both ocular and upper limb movements. However, theoretical accounts of these effects, and also the empirical data, are often contradictory. Accounts of “active touch”, and the premotor theory of attention, have emphasized how movement intention leads to enhanced perceptual processing at the target of a movement, or on the to-be-moved effector. By contrast, recent theories of motor control emphasize how internal “forward” model (FM) estimates may be used to cancel or attenuate sensory signals that arise as a result of self-generated movements. We used behavioural and functional brain imaging (functional magnetic resonance imaging, fMRI) to investigate how perception of a somatosensory stimulus differed according to whether it was delivered to a hand that was about to execute a reaching movement or the alternative, nonmoving, hand. The results of our study demonstrate that a somatosensory stimulus delivered to a hand that is being prepared for movement is perceived to have occurred later than when that same stimulus is delivered to a nonmoving hand. This result indicates that it takes longer for a tactile stimulus to be detected when it is delivered to a moving limb and may correspond to a change in perceptual threshold. Our behavioural results are paralleled by the results of our fMRI study that demonstrated that there were significantly reduced blood-oxygen-level-dependent (BOLD) responses within the parietal operculum and insula following somatosensory stimulation of the hand being prepared for movement, compared to when an identical stimulus was delivered to a nonmoving hand. These findings are consistent with the prediction of FM accounts of motor control that postulate that central sensory suppression of somatosensation accompanies self-generated limb movements, and with previous reports indicating that effects of sensory suppression are observed in higher order somatosensory regions.

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.

Passive tactile recognition of geometrical shape in humans: An fMRI study

Tactile shape discrimination involves frontal other than somatosensory cortex (Palva et al., 2005 [48]), but it is unclear if this frontal activity is related to exploratory concomitants. In this study, we investigated topographical details of prefrontal, premotor, and parietal areas during passive tactile recognition of 2D geometrical shapes in conditions avoiding exploratory movements. Functional magnetic resonance imaging (fMRI) was performed while the same wooden 2D geometrical shapes were blindly pressed on subjects' passive right palm in three conditions. In the RAW condition, shapes were pressed while subjects were asked to attend to the stimuli but were not trained to recognize them. After a brief training, in the SHAPE condition subjects were asked to covertly recognize shapes. In the RECOGNITION condition, they were asked to overtly recognize shapes, using response buttons with their opposite hand. Results showed that somatosensory cortex including contralateral SII, contralateral SI, and left insula was active in all conditions, confirming its importance in processing tactile shapes. In the RAW vs. SHAPE contrast, bilateral posterior parietal, insular, premotor, prefrontal, and (left) Broca's areas were more active in the latter. In the RECOGNITION, activation of (left) Broca's area correlated with correct responses. These results suggest that, even without exploratory movements, passive recognition of tactile geometrical shapes involves prefrontal and premotor as well as somatosensory regions. In this framework, Broca's area might be involved in a successful selection and/or execution of the correct responses.

Pallidal deep brain stimulation and L-dopa effect on PET motor activation in advanced Parkinson's disease

BACKGROUND AND PURPOSE The antiakinetic effect of internal Globus pallidus deep brain stimulation (Gpi-DBS) in Parkinson's disease is not clear and not either how this effect is modulated by L-dopa. METHODS Left Gpi-DBS and/or L-dopa effect was studied with auditory paced right-handed sequential movements on (15)O-butanol positron emission tomography (PET) in five patients. Rest and for conditions during movements (DBS off/L-dopa off; DBS on/L-dopa off; DBS off/L-dopa on; DBS on/L-dopa on) were compared with statistical parametric mapping. RESULTS Gpi-DBS activated the right supplementary motor area/premotor (SMA/PMC), and right insular cortex (IC), and as L-dopa decreased the left sensorimotor cortex (M1/S1) activity. L-dopa increased the left ventrolateral thalamus (VLTH), and decreased the left superior parietal cortex (PC) activity. Gpi-DBS and L-dopa interaction showed right SMA/PMC, IC, and left PC activation, decrease of left VLTH, PMC, and dorsolateral prefrontal cortex (PFC) activity. CONCLUSIONS The improvement of bradykinesia with Gpi-DBS is secondary and contributed to the regress of M1/S1-related rigidity and compensatory SMA/PMC, and IC activation. L-dopa and Gpi-DBS alone each reduces M1/S1 overactivity. Interaction ignores this effect, moreover has akinetic effect in the left VLTH, PMC, and PFC. Motor improvement possibly related to left PC and compensatory right SMA/PMC, and IC activation.

Functional neuroimaging correlates of finger-tapping task variations: an ALE meta-analysis

Finger-tapping tasks are one of the most common paradigms used to study the human motor system in functional neuroimaging studies. These tasks can vary both in the presence or absence of a pacing stimulus as well as in the complexity of the tapping task. A voxel-wise, coordinate-based meta-analysis was performed on 685 sets of activation foci in Talairach space gathered from 38 published studies employing finger-tapping tasks. Clusters of concordance were identified within the primary sensorimotor cortices, supplementary motor area, premotor cortex, inferior parietal cortices, basal ganglia, and anterior cerebellum. Subsequent analyses performed on subsets of the primary set of foci demonstrated that the use of a pacing stimulus resulted in a larger, more diverse network of concordance clusters, in comparison to varying the complexity of the tapping task. The majority of the additional concordance clusters occurred in regions involved in the temporal aspects of the tapping task, rather than its execution. Tapping tasks employing a visual pacing stimulus recruited a set of nodes distinct from the results observed in those tasks employing either an auditory or no pacing stimulus, suggesting differing cognitive networks when integrating visual or auditory pacing stimuli into simple motor tasks. The relatively uniform network of concordance clusters observed across the more complex finger-tapping tasks suggests that further complexity, beyond the use of multi-finger sequences or bimanual tasks, may be required to fully reveal those brain regions necessary to execute truly complex movements.

Parallel input makes the brain run faster

In serial sensory processing, information flows from the thalamus via primary sensory cortices to higher-order association areas. However, association cortices also receive, albeit weak, direct thalamocortical sensory inputs of unknown function. For example, while information proceeds from primary (SI) to secondary (SII) somatosensory cortex in a serial fashion, both areas are known to receive direct thalamocortical sensory input. The present study examines the potential roles of such parallel input arrangements. The subjects were presented with median nerve somatosensory stimuli with the instruction to respond with the contralateral hand. The locations and time courses of the activated brain areas were first identified with magnetoencephalography (MEG). In a subsequent session, these brain areas were modulated with single-pulse transcranial magnetic stimulation (TMS) at 15-210 ms after the somatosensory stimulus while electroencephalography (EEG) was recorded. TMS pulses at 15-40 ms post-stimulus significantly speeded up reaction times and somatosensory-evoked responses, with largest facilitatory effects when the TMS pulse was given to contralateral SII at about 20 ms. To explain the results, we propose that the early somatosensory-evoked physiological SII activation exerts an SII-->SI influence that facilitates the reciprocal SI-->SII pathway - with TMS to SII we apparently amplified this mechanism. The results suggest that the human brain may utilize parallel inputs to facilitate long-distance cortico-cortical connections, resulting in accelerated processing and speeded reaction times. This arrangement could also allow very early top-down modulation of the bottom-up stream of sensory information.

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.

Somatosensory processes subserving perception and action

The functions of the somatosensory system are multiple. We use tactile input to localize and experience the various qualities of touch, and proprioceptive information to determine the position of different parts of the body with respect to each other, which provides fundamental information for action. Further, tactile exploration of the characteristics of external objects can result in conscious perceptual experience and stimulus or object recognition. Neuroanatomical studies suggest parallel processing as well as serial processing within the cerebral somatosensory system that reflect these separate functions, with one processing stream terminating in the posterior parietal cortex (PPC), and the other terminating in the insula. We suggest that, analogously to the organisation of the visual system, somatosensory processing for the guidance of action can be dissociated from the processing that leads to perception and memory. In addition, we find a second division between tactile information processing about external targets in service of object recognition and tactile information processing related to the body itself. We suggest the posterior parietal cortex subserves both perception and action, whereas the insula principally subserves perceptual recognition and learning.

Characteristics of sensori-motor interaction in the primary and secondary somatosensory cortices in humans: a magnetoencephalography study

We studied sensori-motor interaction in the primary (SI) and secondary somatosensory cortex (SII) using magnetoencephalography. Since SII in both hemispheres was activated following unilateral stimulation, we analyzed SIIc (contralateral to stimulation) as well as SIIi (ipsilateral to stimulation). Four tasks were performed in human subjects in which a voluntary thumb movement of the left or right hand was combined with electrical stimulation applied to the index finger of the left or right hand: L(M)-L(S) (movement of the left thumb triggered stimulation to the left finger), L(M)-R(S) (movement of the left thumb triggered electrical stimulation to the right finger), R(M)-R(S) (movement of the right thumb triggered electrical stimulation to the right finger), and R(M)-L(S) (movement of the right thumb triggered electrical stimulation to the left finger). Stimulation to the index finger only (S condition) was also recorded. In SI, the amplitude of N20m and P35m was significantly attenuated in the R(M)-R(S) and L(M)-L(S) tasks compared with the S condition, but that for other tasks showed no change, corresponding to a conventional gating phenomenon. In SII, the R(M)-L(S) task significantly enhanced the amplitude of SIIc but reduced that of SIIi compared with the S condition. The L(M)-L(S) and R(M)-R(S) tasks caused a significant enhancement only in SIIi. The L(M)-R(S) task enhanced the amplitude only in SIIc. The laterality index showed that SII modulation with voluntary movement was more dominant in the hemisphere ipsilateral to movement but was not affected by the side of stimulation. These results provided the characteristics of activities in somatosensory cortices, a simple inhibition in SI but complicated changes in SII depending on the side of movement and stimulation, which may indicate the higher cognitive processing in SII.

Haptic selective attention by foot and by hand

Nonvisual perceptions of a wielded object's spatial properties are based on the quantities expressing the object's mass distribution, quantities that are invariant during the wielding. The mechanoreceptors underlying the kind of haptic perception involved in wielding - referred to as effortful, kinesthetic, or dynamic touch - are those embedded in the muscles, tendons, and ligaments. The present experiment's focus was the selectivity of this muscle-based form of haptic perception. For an occluded rod grasped by the hand at some intermediate position along its length, participants can attend to and report selectively the rod's full length, its partial lengths (fore or aft of the hand), and the position of the grip. The present experiment evaluated whether participants could similarly attend selectively when wielding by foot. For a given rod attached to and wielded by foot or attached to (i.e. grasped) and wielded by hand, participants reported (by magnitude production) the rod's whole length or fractional length leftward of the point of attachment. On measures of mean perceived length, accuracy, and reliability, the degree of differentiation of partial from full extent achieved by means of the foot matched that achieved by means of the hand. Despite their neural, anatomical, and experiential differences, the lower and upper limbs seem to abide by the same principles of selective muscle-based perception and seem to express this perceptual function with equal facility.

Sensorimotor integration in S2, PV, and parietal rostroventral areas of the human sylvian fissure

We explored cortical fields on the upper bank of the Sylvian fissure using functional magnetic resonance imaging (fMRI) and magnetoencephalography (MEG) to measure responses to two stimulus conditions: a tactile stimulus applied to the right hand and a tactile stimulus with an additional movement component. fMRI data revealed bilateral activation in S2/PV in response to tactile stimulation alone and source localization of MEG data identified a peak latency of 122 ms in a similar location. During the tactile and movement condition, fMRI revealed bilateral activation of S2/PV and an anterior field, while MEG data contained one source at a location identical to the tactile-only condition with a latency of 96 ms and a second rostral source with a longer latency (136 ms). Furthermore, Region-of-interest analysis of fMRI data identified increased bilateral activation in S2/PV and the rostral area in the tactile and movement condition compared with the tactile only condition. An area of cortex immediately rostral to S2/PV in monkeys has been called the parietal rostroventral area (PR). Based on location, latency, and conditions under which this field was active, we have termed the rostral area of human cortex PR as well. These findings indicate that humans, like non-human primates, have a cortical field rostral to PV that processes proprioceptive inputs, both S2/PV and PR play a role in somatomotor integration necessary for manual exploration and object discrimination, and there is a temporal hierarchy of processing with S2/PV active prior to PR.

Pain processing is faster than tactile processing in the human brain

Pain signals threat and drives the individual into a behavioral response that significantly depends on a short stimulus-response latency. Paradoxically, the peripheral and spinal conduction velocities of pain are much slower than of tactile information. However, cerebral processing times and reaction times of touch and pain have not yet been fully assessed. Here we show that reaction times to selective nociceptive cutaneous laser stimuli are substantially faster than expected from the peripheral conduction velocities. Furthermore, by using magnetoencephalography, we found that latencies between earliest stimulus-evoked cortical responses and reaction times are approximately 60 ms shorter for nociceptive than for tactile stimuli. These findings reveal that cerebral processing of pain is substantially faster than processing of tactile information and relatively compensates for the slow peripheral and spinal conduction velocities of pain. Our observation shows how the cerebral organization of pain processing enhances motor responses to potentially harmful stimuli and thereby subserves the particular behavioral demands of pain.

Imaging motor recovery after stroke

Most patients show improvement in the weeks or months after a stroke. Recovery is incomplete, however, leaving most with significant impairment and disability. Because the brain does not grow back to an appreciable extent, this recovery occurs on the basis of change in function of surviving tissues. Brain mapping studies have characterized a number of processes and principles relevant to recovery from stroke in humans. The findings have potential application to improving therapeutics that aim to restore function after stroke.

Centrifugal regulation of human cortical responses to a task-relevant somatosensory signal triggering voluntary movement

Many studies have reported a movement-related modulation of response in the primary and secondary somatosensory cortices (SI and SII) to a task-irrelevant stimulation in primates. In the present study, magnetoencephalography (MEG) was used to examine the top-down centrifugal regulation of neural responses in the human SI and SII to a task-relevant somatosensory signal triggering a voluntary movement. Nine healthy adults participated in the study. A visual warning signal was followed 2 s later by a somatosensory imperative signal delivered to the right median nerve at the wrist. Three kinds of warning signal informed the participants of the reaction which should be executed on presentation of the imperative signal (rest or extension of the right index finger, extension of the left index finger). The somatosensory stimulation was used to both generate neural responses and trigger voluntary movement and therefore was regarded as a task-relevant signal. The responses were recorded using a whole-head MEG system. The P35m response around the SI was reduced in magnitude without alteration of the primary SI response, N20m, when the signal triggered a voluntary movement compared to the control condition, whereas bilateral SII responses peaking at 70-100 ms were enhanced and the peak latency was shortened. The peak latency of the responses in the SI and SII preceded the onset of the earliest voluntary muscle activation in each subject. Later bilateral perisylvian responses were also enhanced with movement. In conclusion, neural activities in the SI and SII evoked by task-relevant somatosensory signals are regulated differently by motor-related neural activities before the afferent inputs. The present findings indicate a difference in function between the SI and SII in somatosensory-motor regulation.

Neuromagnetic SII responses do not fully reflect pain scale

To elucidate the role of somatosensory cortices in coding pain magnitude, we recorded the neuromagnetic responses of ten subjects to mild, moderate, and severe pain stimulation by delivering thulium-laser pulses on the dorsum of the left hand. The stimulus intensities for producing different pain levels were determined individually, and the mean values across subjects were 255, 365, and 490 mJ for mild, moderate, and severe pain, respectively. We obtained 40 responses for each intensity condition, and analyzed the averaged cortical signals by multi-dipole modeling. All subjects showed consistent activation over the bilateral secondary somatosensory (SII) cortices for each intensity level, peaking around 150-230 ms, with 15-ms earlier on the contralateral hemisphere. The SII dipole strength was significantly larger for the moderate than for the mild pain stimulation, but lacked further increase as the pain magnitude elevated to the severe level. In contrast, the primary somatosensory cortical response was detected in only half of our subjects, and thus it seemed difficult to evaluate its role in pain intensity coding. Our results suggest that activation strength in human SII cortices reflects the magnitude of peripheral noxious inputs only up to the moderate level, and some other cerebral correlates may get involved in sensing a further increment of pain magnitude.

Centrifugal regulation of a task-relevant somatosensory signal triggering voluntary movement without a preceding warning signal

A warning signal followed by an imperative signal generates anticipatory and preparatory activities, which regulate sensory evoked neuronal activities through a top-down centrifugal mechanism. The present study investigated the centrifugal regulation of neuronal responses evoked by a task-relevant somatosensory signal, which triggers a voluntary movement without a warning signal. Eleven healthy adults participated in this study. Electrical stimulation was delivered to the right median nerve at a random interstimulus interval (1.75-2.25 s). The participants were instructed to extend the second digit of the right hand as fast as possible when the electrical stimulus was presented (ipsilateral reaction condition), or extend that of the left hand (contralateral reaction condition). They also executed repetitively extension of the right second digit at a rate of about 0.5 Hz, irrespective of electrical stimulation (movement condition), to count silently the number of stimuli (counting condition). In the control condition, they had no task to perform. The amplitude of short-latency somatosensory evoked potentials, the central P25, frontal N30, and parietal P30, was significantly reduced in both movement and ipsilateral reaction conditions compared to the control condition. The amplitude of long-latency P80 was significantly enhanced only in the ipsilateral reaction condition compared to the control, movement, contralateral reaction, and counting conditions. The long-latency N140 was significantly enhanced in both movement and ipsilateral reaction conditions compared to the control condition. In conclusion, short- and long-latency neuronal activities evoked by task-relevant somatosensory signals were regulated differently through a centrifugal mechanism even when the signal triggered a voluntary movement without a warning signal. The facilitation of activities at a latency of around 80 ms is associated with gain enhancement of the task-relevant signals from the body part involved in the action, whereas that at a latency of around 140 ms is associated with unspecific gain regulation generally induced by voluntary movement. These may be dissociated from the simple effect of directing attention to the stimulation.

Stroke recovery and its imaging

Functional imaging of stroke recovery is a unique source of information that might be useful in the development of restorative treatments. Several features of brain function change spontaneously after stroke. Current studies define many of the most common events. Key challenges for the future are to develop standardized approaches to help address certain questions, determine the psychometric qualities of these measures, and define the clinical usefulness of these methods.

Neural basis of aging: the penetration of cognition into action control

Although functional imaging studies have frequently examined age-related changes in neural recruitment during cognitive tasks, much less is known about such changes during motor performance. In the present study, we used functional magnetic resonance imaging to investigate age-related changes in cyclical hand and/or foot movements across different degrees of complexity. Right-handed volunteers (11 young, 10 old) were scanned while performing isolated flexion-extension movements of the right wrist and foot as well as their coordination, according to the "easy" isodirectional and "difficult" nonisodirectional mode. Findings revealed activation of a typical motor network in both age groups, but several additional brain areas were involved in the elderly. Regardless of the performed motor task, the elderly exhibited additional activation in areas involved in sensory processing and integration, such as contralateral anterior insula, frontal operculum, superior temporal gyrus, supramarginal gyrus, secondary somatosensory area, and ipsilateral precuneus. Age-related activation differences during coordination of both segments were additionally observed in areas reflecting increased cognitive monitoring of motor performance, such as the pre-supplementary motor area, pre-dorsal premotor area, rostral cingulate, and prefrontal cortex. In the most complex coordination task, the elderly exhibited additional activation in anterior rostral cingulate and dorsolateral prefrontal cortex, known to be involved in suppression of prepotent response tendencies and inhibitory cognitive control. Overall, these findings are indicative of an age-related shift along the continuum from automatic to more controlled processing of movement. This increased cognitive monitoring of movement refers to enhanced attentional deployment, more pronounced processing of sensory information, and intersensory integration.

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.

Differential generators for N20m and P35m responses to median nerve stimulation

To study the spatial and behavioral dynamics of cortical sources for N20m and P35m at varying stimulus intensities, we measured neuromagnetic cortical responses to left electric median nerve stimulation at the wrist in 17 male healthy adults. The stimulus intensity levels were individually determined according to sensory threshold (ST) for perceiving electric pulses. Using equivalent current dipole (ECD) modeling, we analyzed the peak latencies, amplitudes, and locations of ECDs from 14 subjects for N20m and P35m elicited at 2 ST, 3 ST, and 4 ST. Compared with N20m, P35m was localized 3.3 +/- 0.6 mm more superiorly at 2-4 ST, and 2.9 +/- 1.2 mm more medially at 3-4 ST. Superimposed over subjects' own MR images, N20m ECDs were localized in the area of 3b contralateral to stimulus side in all 17 subjects at 3 ST, whereas P35m ECDs were localized either in the postcentral (in 14 subjects) or in the precentral areas (in 3 subjects). We found no clear correlation between N20m and P35m in terms of peak latencies as well as the corresponding growth of activation strengths along with stepwise increase in stimulus intensity. Our results imply that the two early SEF components, N20m and P35m, have differential cortical generators, with distinctive neurophysiological behaviors in response to varying stimulus intensity levels.

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.

Cortical representation of dermatomes: MEG-derived maps after tactile stimulation

Mechanical stimulation of skin receptors is known to evoke cortical responses arising from the somatosensory cortex. Here we present a magnetoencephalographic (MEG) study where dermatomal somatosensory-evoked fields (DSSEFs) were recorded after mechanical stimulation of sacral (S1), lumbar (L3), thoracic (Th7), and cervical (C4) dermatomes in three healthy volunteers. All MEG measurements were repeated in order to test the replicability of the results. DSSEFs were successfully measured and modeled in all three participants. The topography and temporal dynamics of cortical responses derived after stimulation of each dermatome are described. We found that cortical-evoked responses can be reliably recorded using MEG after mechanical stimulation of dermatomes when a sufficiently large skin region within the dermatome is stimulated. Primary sensory cortex response (SI) to each of the four dermatomes was replicable and showed stability over time. The MEG-derived individual maps of activation confirm the somatotopic representation of dermatomes in primary sensory cortex and the utility of MEG recordings in disentangling the interactions between primary and secondary sensory cortex during somatic perception.

EEG source analysis and fMRI reveal two electrical sources in the fronto-parietal operculum during subepidermal finger stimulation

Using functional magnetic resonance imaging (fMRI) and electroencephalographic (EEG) source dipole analysis in 10 normal subjects, two electrical source dipoles in the contralateral fronto-parietal operculum were identified during repetitive painful subepidermal stimulation of the right index finger. The anterior source dipole peaking at 79 +/- 8 ms (mean +/- SD) was located in the frontal operculum, and oriented tangentially toward the cortical surface. The posterior source dipole peaking at 118 +/- 12 ms was located in the upper bank of the Sylvian fissure corresponding to the second somatosensory cortex (S2). The orientations of the posterior source dipoles displayed large variability, but differed significantly (P < 0.05) from the orientations of the anterior source dipoles. Electrical sources and fMRI clusters were also observed in ipsilateral fronto-parietal operculum. However, due to low signal-to-noise ratio of ipsilateral EEG sources in individual recordings, separation of sources into anterior and posterior clusters was not performed. Combined fMRI and source dipole EEG analysis of individual data suggests the presence of two distinct electrical sources in the fronto-parietal operculum participating in processing of somatosensory stimuli. The anterior region of the fronto-parietal operculum shows earlier peak activation than the posterior region.

Activation of human SII cortex during exploratory finger movement and hand clenching tasks

We used electric median nerve stimuli to elucidate the functional properties of neurons in the human secondary somatosensory cortex during exploration of small objects and muscle contraction. Somatosensory evoked fields were recorded from nine healthy subjects with a 204-channel neuromagnetometer. Electrical stimuli were applied once every 3 s to the left median nerve at the wrist. The conditions during the stimulation were rest (control session), exploration of small objects (exploration session) and clenching the hand while the wrist was being electrically stimulated (clench session). The strengths of equivalent current dipoles of evoked fields from the secondary somatosensory cortex were increased during the exploration session, but those of evoked fields were decreased by the clench session.

Temporal dynamics of alpha and beta rhythms in human SI and SII after galvanic median nerve stimulation. A MEG study

In this MEG study, we investigated cortical alpha/sigma and beta ERD/ERS induced by median nerve stimulation to extend previous evidence on different resonant and oscillatory behavior of SI and SII (NeuroImage 13 [2001] 662). Here, we tested whether simple somatosensory stimulation could induce a distinctive sequence of alpha/sigma and beta ERD/ERS over SII compared to SI. We found that for both alpha/sigma (around 10 Hz) and beta (around 20 Hz) rhythms, the latencies of ERD and ERS were larger in bilateral SII than in contralateral SI. In addition, the peak amplitude of alpha/sigma and beta ERS was smaller in bilateral SII than in contralateral SI. These results indicate a delayed and prolonged activation of SII responses, reflecting a protracted information elaboration possibly related to SII higher order role in the processing of somatosensory information. This temporal dynamics of alpha/sigma and beta rhythms may be related to a sequential activation scheme of SI and SII during the somatosensory information processes. Future studies should evaluate in SII the possible different functional significance of alpha/sigma with respect to beta rhythms during somatosensory processing.