Sensory and motor interfering influences on somatosensory evoked potentials

Microstructural alterations predict impaired bimanual control in Parkinson’s disease

Bimanual coordination is impaired in Parkinson’s disease affecting patients’ ability to perform activities of daily living and to maintain independence. Conveyance of information between cortical and subcortical areas is essential for bimanual coordination and relies on the integrity of cerebral microstructure. As pathological deposition of alpha-synuclein compromises microstructure in Parkinson’s disease, we investigated the relationship between microstructural integrity and bimanual coordination using diffusion-weighted MRI in 23 patients with Parkinson’s disease (mean age ± standard deviation: 56.0 ± 6.45 years; 8 female) and 26 older adults (mean age ± standard deviation: 58.5 ± 5.52 years). Whole-brain analysis revealed specific microstructural alterations between patients and healthy controls matched for age, sex, handedness, and cognitive status congruent with the literature and known Parkinson’s disease pathology. A general linear model revealed distinct microstructural alterations associated with poor bimanual coordination in Parkinson’s disease, corrected for multiple comparisons using a permutation-based approach. Integrating known functional topography, we conclude that distinct changes in microstructure cause an impediment of structures involved in attention, working memory, executive function, motor planning, motor control, and visual processing contributing to impaired bimanual coordination in Parkinson’s disease.

Evidence for prefrontal cortex hypofunctioning in schizophrenia through somatosensory evoked potentials

Patients with schizophrenia (SCZ) exhibit a variety of symptoms related to altered processing of somatosensory information. Little is known, however, about the neural substrates underlying somatosensory impairments in SCZ. This study endeavored to evaluate somatosensory processing in patients with SCZ compared to healthy individuals by generating somatosensory evoked potentials through stimulation of the right median nerve. The median nerve was stimulated by a peripheral nerve stimulator in 34 SCZ and 33 healthy control (HC) participants. The peripheral nerve stimulus (PNS) intensity was adjusted to 300 percent of sensory threshold and delivered at 0.1 Hz. The EEG data were acquired through 64-channels per 10-20 montage. We collected and averaged 100 trials and the recording electrodes of interest were the F3/F5 electrodes representing the dorsolateral prefrontal cortex (DLPFC) and C3/CP3 representing the somatosensory cortex (S1). In response to PNS, SCZ participants experienced over the DLPFC N30 amplitude that was significantly smaller than that of HC participants. By contrast, S1 N20 was of similar amplitude between the two groups. In addition, we found an association between N20 and N30 amplitudes in SCZ but not in HC participants. Our findings suggest that patients with SCZ demonstrate aberrant processing of somatosensory activation by the DLPFC locally and not due to a connectivity disruption between S1 and DLPFC. These results could help to develop a model through which to DLPFC hypofunctioning could be studied. Our findings may also help to identify a potential biological target to treat somatosensory information processing related deficits in SCZ.

Toward understanding the neurophysiological basis of peripersonal space: An EEG study on healthy individuals

The subcortical mechanisms subtending the sensorimotor processes related to the peripersonal space (PPS) have been well characterized, whereas less evidence is available concerning the cortical mechanisms. We investigated the theta, alpha and beta event-related spectral perturbations (ERSP) while holding the forearm in different positions into the PPS of the face. Fifty healthy individuals were subjected to EEG recording while being provided with median nerve electric stimulation at the wrist of the right hand held at different hand-to-face distances. Theta and beta rhythms were significantly perturbed depending on the hand-to-face distance, whereas alpha oscillations reflected a more general, non-specific oscillatory response to the motor task. The perturbation of theta and beta frequency bands may reflect the processes of top-down modulation overseeing the conscious spatiotemporal encoding of sensory-motor information within the PPS. In other words, such perturbation reflects the continuous update of the conscious internal representations of the PPS to build up a purposeful and reflexive motor response.

Neural Activations Associated With Friction Stimulation on Touch-Screen Devices

Tactile sensation largely influences human perception, for instance when using a mobile device or a touch screen. Active touch, which involves tactile and proprioceptive sensing under the control of movement, is the dominant tactile exploration mechanism compared to passive touch (being touched). This paper investigates the role of friction stimulation objectively and quantitatively in active touch tasks, in a real human-computer interaction on a touch-screen device. In this study, 24 participants completed an active touch task involved stroking the virtual strings of a guitar on a touch-screen device while recording the electroencephalography (EEG) signal. Statistically significant differences in beta and gamma oscillations in the middle frontal and parietal areas at the late period of the active touch task are found. Furthermore, stronger beta event-related desynchronization (ERD) and rebound in the presence of friction stimulation in the contralateral parietal area are observed. However, in the ipsilateral parietal area, there is a difference in beta oscillation only at the late period of the motor task. As for implicit emotion communication, a significant increase in emotional responses for valence, arousal, dominance, and satisfaction is observed when the friction stimulation is applied. It is argued that the friction stimulation felt by the participants' fingertip in a touch-screen device further induces cognitive processing compared to the case when no friction stimulation is applied. This study provides objective and quantitative evidence that friction stimulation is able to affect the bottom-up sensation and cognitive processing.

Cortical processing of irrelevant somatosensory information from the leg is altered by attention during early movement preparation

The ability to actively suppress, or gate, irrelevant sensory information is needed for safe and efficient walking in sensory-rich environments. Both attention and the late phase of motor preparation alter somatosensory evoked potentials (SEPs) in healthy adults. The aim of this study was to examine the effect of attention on the processing of irrelevant somatosensory information during the early phase of preparation of plantarflexion movements. Young healthy individuals received tibial nerve stimulation while electroencephalography (EEG) recorded SEPs over the Cz electrode. Three conditions were tested in both legs: 1) Rest, 2) Attend To the stimulated limb, and 3) Attend Away from the stimulated limb. In conditions 2 and 3, vibration (80 Hz) was applied over the medial soleus muscle to cue voluntary plantarflexion movements of the stimulated (Attend To) or non-stimulated leg (Attend Away). Only SEPs delivered during early preparation were averaged for statistical analysis. Results demonstrated a main effect of condition for the N40 and N70 indicating that SEP amplitudes in the Attend To condition were smaller than rest (p ≤ 0.02). For the P50, no interaction effects or main effects were found (p ≥ 0.08). There was no main effect of leg for any component measured. The results indicate that gating of irrelevant sensory information during early preparation occurs in the leg when attention is directed within the same limb. If attention alters the somatosensory stimuli from a leg movement, then directing attention may affect safe community walking.

Repetitive Passive Finger Movement Modulates Primary Somatosensory Cortex Excitability

Somatosensory inputs induced by repetitive passive movement (RPM) modulate primary motor cortex (M1) excitability; however, it is unclear whether RPM affects primary somatosensory cortex (S1) excitability. In this study, we investigated whether RPM affects somatosensory evoked potentials (SEPs) and resting state brain oscillation, including alpha and beta bands, depend on RPM frequency. Nineteen healthy subjects participated in this study, and SEPs elicited by peripheral nerve electrical stimulation were recorded from the C3’ area in order to assess S1 excitability (Exp. 1: n = 15). We focused on prominent SEP components such as N20, P25 and P45-reflecting S1 activities. In addition, resting electroencephalograms (EEGs) were recorded from C3’ area to assess the internal state of the brain network at rest (Exp. 2: n = 15). Passive abduction/adduction of the right index finger was applied for 10 min at frequencies of 0.5, 1.0, 3.0, and 5.0 Hz in Exp. 1, and 1.0, 3.0, and 5.0 Hz in Exp. 2. No changes in N20 or P25 components were observed following RPM. The 3.0 Hz-RPM decreased the P45 component for 20 min (p < 0.05), but otherwise did not affect the P45 component. There was no difference in the alpha and beta bands before and after any RPM; however, a negative correlation was observed between the rate of change of beta power and P45 component at 3.0 Hz-RPM. Our findings indicated that the P45 component changes depending on the RPM frequency, suggesting that somatosensory inputs induced by RPM influences S1 excitability. Additionally, beta power enhancement appears to contribute to the P45 component depression in 3.0 Hz-RPM.

N30 Somatosensory Evoked Potential Is Negatively Correlated with Motor Function in Parkinson's Disease

Objective

The aim of this study was to investigate frontal N30 status in Parkinson’s disease (PD) and to examine the correlation between the amplitude of frontal N30 and the severity of motor deficits.

Methods

The frontal N30 was compared between 17 PD patients and 18 healthy volunteers. Correlations between the amplitude of frontal N30 and the Unified Parkinson’s Disease Rating Scale (UPDRS) motor score of the more severely affected side was examined.

Results

The mean latency of the N30 was not significantly different between patients and healthy volunteers (p = 0.981), but the mean amplitude was lower in PD patients (p < 0.025). There was a significant negative correlation between the amplitude of N30 and the UPDRS motor score (r = -0.715, p = 0.013).

Conclusions

The frontal N30 status indicates the motor severity of PD. It can be a useful biomarker reflecting dopaminergic deficits and an objective measurement for monitoring the clinical severity of PD.

Reduced topological efficiency in cortical-basal Ganglia motor network of Parkinson's disease: a resting state fMRI study

Parkinson's disease (PD) is mainly characterized by dopamine depletion of the cortico-basal ganglia (CBG) motor circuit. Given that dopamine dysfunction could affect functional brain network efficiency, the present study utilized resting-state fMRI (rs-fMRI) and graph theoretical approach to investigate the topological efficiency changes of the CBG motor network in patients with PD during a relatively hypodopaminergic state (12 hours after a last dose of dopamimetic treatment). We found that PD compared with controls had remarkable decreased efficiency in the CBG motor network, with the most pronounced changes observed in rostral supplementary motor area (pre-SMA), caudal SMA (SMA-proper), primary motor cortex (M1), primary somatosensory cortex (S1), thalamus (THA), globus pallidus (GP), and putamen (PUT). Furthermore, reduced efficiency in pre-SMA, M1, THA and GP was significantly correlated with Unified Parkinson's Disease Rating Scale (UPDRS) motor scores in PD patients. Together, our results demonstrate that individuals with PD appear to be less effective at information transfer within the CBG motor pathway, which provides a novel perspective on neurobiological explanation for the motor symptoms in patients. These findings are in line with the pathophysiology of PD, suggesting that network efficiency metrics may be used to identify and track the pathology of PD.

Modulation of the N30 generators of the somatosensory evoked potentials by the mirror neuron system

The N30 component of the somatosensory evoked potential is known to be modulated by sensory interference, motor action, movement ideation and observation. We introduce a new paradigm in which the observation task of another person's hand movement triggers the somatosensory stimulus, inducing the N30 response in participants. In order to identify the possible contribution of the mirror neuron network (MNN) to this early sensorimotor processing, we analyzed the N30 topography, the event-related spectral perturbation and the inter-trial coherence on single electroencephalogram (EEG) trials, and we applied swLORETA to localize the N30 sources implicated in the time-frequency domain at rest and during observation, as well as the generators differentiating these two contextual brain states. We found that N30 amplitude increase correlated with increased contralateral precentral alpha, frontal beta, and contralateral frontal gamma power spectrum, and with central and precentral alpha and parietal beta phase-locking of ongoing EEG signals. We demonstrate specific activation of the contralateral post-central and parietal cortex where the angular gyrus (BA39), an important MNN node, is implicated in this enhancement during observation. We conclude that this part of the MNN, involved in proprioceptive processing and more complex body-action representations, is already active prior to somatosensory input and may enhance N30.

Knowing each random error of our ways, but hardly correcting for it: an instance of optimal performance

Random errors are omnipresent in sensorimotor tasks due to perceptual and motor noise. The question is, are humans aware of their random errors on an instance-by-instance basis? The appealing answer would be ‘no’ because it seems intuitive that humans would otherwise immediately correct for the errors online, thereby increasing sensorimotor precision. However, here we show the opposite. Participants pointed to visual targets with varying degree of feedback. After movement completion participants indicated whether they believed they landed left or right of target. Surprisingly, participants' left/right-discriminability was well above chance, even without visual feedback. Only when forced to correct for the error after movement completion did participants loose knowledge about the remaining error, indicating that random errors can only be accessed offline. When correcting, participants applied the optimal correction gain, a weighting factor between perceptual and motor noise, minimizing end-point variance. Together these results show that humans optimally combine direct information about sensorimotor noise in the system (the current random error), with indirect knowledge about the variance of the perceptual and motor noise distributions. Yet, they only appear to do so offline after movement completion, not while the movement is still in progress, suggesting that during movement proprioceptive information is less precise.

Regional alterations of brain microstructure in Parkinson's disease using diffusion tensor imaging

This study tested the hypothesis that diffusion tensor imaging can detect alteration in microscopic integrity of white matter and basal ganglia regions known to be involved in Parkinson's disease (PD) pathology. It was also hypothesized that there is an association between diffusion abnormality and PD severity and subtype. Diffusion tensor imaging at 4 Tesla was obtained in 12 PD and 20 control subjects, and measures of fractional anisotropy and mean diffusivity were evaluated using both region-of-interest and voxel-based methods. Movement deficits and subtypes in PD subjects were assessed using the Motor Subscale (Part III) of the Unified Parkinson's Disease Rating Scale. Reduced fractional anisotropy (P < .05, corrected) was found in PD subjects in regions related to the precentral gyrus, substantia nigra, putamen, posterior striatum, frontal lobe, and the supplementary motor areas. Reduced fractional anisotropy in the substantia nigra correlated (P < .05, corrected) with the increased rating scale motor scores. Significant spatial correlations between fractional anisotropy alterations in the putamen and other PD-affected regions were also found in the context of PD subtypes index analysis. Our data suggest that microstructural alterations detected with diffusion tensor might serve as a potential biomarker for PD.

Modulation of the response to a somatosensory stimulation of the hand during the observation of manual actions

Observation of hand movements has been repeatedly demonstrated to increase the excitability of the motor cortical representation of the hand. Little attention, however, has been devoted to its effect on somatosensory processing. Movement execution is well known to decrease somatosensory cortical excitability, a phenomenon termed 'gating'. As executed and observed actions share common cortical representations, we hypothesized that action observation (hand movements) should also modulate the cortical response to sensory stimulation of the hand. Seventeen healthy subjects participated in these experiments in which electroencephalographic (EEG) recordings of the somatosensory steady-state response (SSSR) were obtained. The SSSR provides a continuous measure of somatosensory processing. Recordings were made during a baseline condition and five observation conditions in which videos showed either a: (1) hand action; (2) passive stimulation of a hand; (3) static hand; (4) foot action; or (5) static object. The method employed consisted of applying a continuous 25 Hz vibratory stimulation to the index finger during the six conditions and measuring potential gating effects in the SSSR within the 25 Hz band (corresponding to the stimulation frequency). A significant effect of condition was found over the contralateral parietal cortex. Observation of hand actions resulted in a significant gating effect when compared to baseline (average gating of 22%). Observation of passive touch of the hand also gated the response (17% decrease). In conclusion, the results show that viewing a hand performing an action or being touched interferes with the processing of somatosensory information arising from the hand.

Frontal phasic and oscillatory generators of the N30 somatosensory evoked potential

The N30 component of somatosensory evoked potentials has been recognized as a crucial index of brain sensorimotor processing and has been increasingly used clinically. Previously, we have shown that the N30 is accompanied by both an increase of the power spectrum of the ongoing beta-gamma EEG (event related synchronization, ERS) and by a reorganization (phase-locking) of the spontaneous phase of this rhythm (inter-trials coherency, ITC). In order to localize its sources taking into account both the phasic and oscillatory aspects of the phenomenon, we here apply swLORETA methods on averaged signals of the event-related potential (ERP) from a 128 scalp-electrodes array in time domain and also on raw EEG signals in frequency domain at the N30 peak latency. We demonstrate that the two different mechanisms that generate the N30 component power increase (ERS) and phase locking (ITC) across EEG trials are spatially localized in overlapping areas in the precentral cortex, namely the motor cortex (BA4) and the premotor cortex (BA6). From this common region, the generator of the N30 event-related potential expands toward the posterior part of BA4, the anterior part of BA6 and the prefrontal cortex (BA9). These latter areas also present significant ITC sources in the beta-gamma frequency range, but without significant power increase of this rhythm. This demonstrates that N30 results from network activity that depends on distinct oscillating and phasic generators localized in the frontal cortex.

Non-dominant hand movement facilitates the frontal N30 somatosensory evoked potential

Background

Previous literature has shown that the frontal N30 is increased during movement of the hand contralateral to median nerve stimulation. This finding was a result of non-dominant left hand movement in right-handed participants. It is unclear however if the effect depends upon non-dominant hand movement or if this is a generalized phenomenon across the upper-limbs. This study tests the effect of dominant and non-dominant hand movement upon contralateral frontal and parietal somatosensory evoked potentials (SEPs) and further tests if this relationship persists in left hand dominant participants. Median nerve SEPs were elicited from the wrist contralateral to movement in both right hand and left hand dominant participants alternating the movement hand in separate blocks. Participants were required to volitionally squeeze (~ 20% of a maximal voluntary contraction) a pressure-sensitive bulb every ~3 seconds with the hand contralateral to median nerve stimulation. SEPs were continuously collected during the task and individual traces were grouped into time bins relative to movement according to the timing of components of the Bereitschaftspotential. SEPs were then averaged and quantified from both FCZ and CP3/4 scalp electrode sites during both the squeeze task and at rest.

Results

The N30 is facilitated during non-dominant hand movement in both right and left hand dominant individuals. There was no effect for dominant hand movement in either group.

Conclusions

N30 amplitude increase may be a result of altered sensory gating from motor areas known to be specifically active during non-dominant hand movement.

Movement gating of beta/gamma oscillations involved in the N30 somatosensory evoked potential

Evoked potential modulation allows the study of dynamic brain processing. The mechanism of movement gating of the frontal N30 component of somatosensory evoked potentials (SEP) produced by the stimulation of the median nerve at wrist remains to be elucidated. At rest, a power enhancement and a significant phase-locking of the electroencephalographic (EEG) oscillation in the beta/gamma range (25-35 Hz) are related to the emergence of the N30. The latter was also perfectly identified in presence of pure phase-locking situation. Here, we investigated the contribution of these rhythmic activities to the specific gating of the N30 component during movement. We demonstrated that concomitant execution of finger movement of the stimulated hand impinges such temporal concentration of the ongoing beta/gamma EEG oscillations and abolishes the N30 component throughout their large topographical extent on the scalp. This also proves that the phase-locking phenomenon is one of the main actors for the N30 generation. These findings could be explained by the involvement of neuronal populations of the sensorimotor cortex and other related areas, which are unable to respond to the phasic sensory activation and to phase-lock their firing discharges to the external sensory input during the movement. This new insight into the contribution of phase-locked oscillation in the emergence of the N30 and in its gating behavior calls for a reappraisal of fundamental and clinical interpretation of the frontal N30 component.

Evidence-based medicine evaluation of electrophysiological studies of the anxiety disorders

We provide a systematic, evidence-based medicine (EBM) review of the field of electrophysiology in the anxiety disorders. Presently, electrophysiological studies of anxiety focus primarily on etiological aspects of brain dysfunction. The review highlights many functional similarities across studies, but also identifies patterns that clearly differentiate disorder classifications. Such measures offer clinical utility as reliable and objective indicators of brain dysfunction in individuals and indicate potential as biomarkers for the improvement of diagnostic specificity and for informing treatment decisions and prognostic assessments. Common to most of the anxiety disorders is basal instability in cortical arousal, as reflected in measures of quantitative electroencephalography (qEEG). Resting electroencephalographic (EEG) measures tend to correlate with symptom sub-patterns and be exacerbated by condition-specific stimulation. Also common to most of the anxiety disorders are condition-specific difficulties with sensory gating and the allocation and deployment of attention. These are clearly evident from evoked potential (EP) and event-related potential (ERP) electrical measures of information processing in obsessive compulsive disorder (OCD), post-traumatic stress disorder (PTSD), panic disorder (PD), generalized anxiety disorder (GAD) and the phobias. Other'ERP measures clearly differentiate the disorders. However, there is considerable variation across studies, with inclusion and exclusion criteria, medication status and control group selection not standardized within condition or across studies. Study numbers generally preclude analysis for confound removal or for the derivation of diagnostic biomarker patterns at this time. The current trend towards development of databases of brain and cognitive function is likely to obviate these difficulties. In particular, electrophysiological measures of function are likely to play a significant role in the development and subsequent adaptations of DSM-V and assist critically in securing improvements in nosological and treatment specificity.

Direct evidence for cortical suppression of somatosensory afferents during visuomotor adaptation

Upon exposure to novel visuomotor relationships, the information carried by visual and proprioceptive signals becomes discrepant, often disrupting motor execution. It has been shown that degradation of the proprioceptive sense (arising either from disease or experimental manipulation) enhances performance when drawing with mirror-reversed vision. Given that the central nervous system can exert a dynamic control over the transmission of afferent signals, reducing proprioceptive inflow to cortical areas could be part of the normal adaptive mechanisms deployed in healthy humans upon exposure to novel visuomotor environments. Here we address this issue by probing the transmission of somatosensory afferents throughout the course of adaptation to a visuomotor conflict, by recording median nerve somatosensory evoked potentials. We show that early exposure to tracing with mirror-reversed vision is accompanied by substantial proprioceptive suppression occurring in the primary somatosensory cortex (S1). This proprioceptive gating is gradually alleviated as performance increases with adaptation, returning to baseline levels. Peripheral and spinal evoked potentials were not modulated throughout, suggesting that the gating acted to reduce cortico-cortico excitability directly within S1. These modulations provide neurophysiological evidence for flexibility in sensory integration during visuomotor adaptation, which may functionally serve to reduce the sensory conflict until the visuo-proprioceptive mapping is updated.

Adaptive changes of rhythmic EEG oscillations in space implications for brain-machine interface applications

The dramatic development of brain machine interfaces has enhanced the use of human brain signals conveying mental action for controlling external actuators. This chapter will outline current evidences that the rhythmic electroencephalographic activity of the brain is sensitive to microgravity environment. Experiments performed in the International Space Station have shown significant changes in the power of the astronauts' alpha and mu oscillations in resting condition, and other adaptive modifications in the beta and gamma frequency range during the immersion in virtual navigation. In this context, the dynamic aspects of the resting or default condition of the awaken brain, the influence of the "top-down" dynamics, and the possibility to use a more constrained configuration by a new somatosensory-evoked potential (gating approach) are discussed in the sense of future uses of brain computing interface in space mission. Although, the state of the art of the noninvasive BCI approach clearly demonstrates their ability and the great expectance in the field of rehabilitation for the restoration of defective communication between the brain and external world, their future application in space mission urgently needs a better understanding of brain neurophysiology, in particular in aspects related to neural network rhythmicity in microgravity.

Pure phase-locking of beta/gamma oscillation contributes to the N30 frontal component of somatosensory evoked potentials

Background

Evoked potentials have been proposed to result from phase-locking of electroencephalographic (EEG) activities within specific frequency bands. However, the respective contribution of phasic activity and phase resetting of ongoing EEG oscillation remains largely debated. We here applied the EEGlab procedure in order to quantify the contribution of electroencephalographic oscillation in the generation of the frontal N30 component of the somatosensory evoked potentials (SEP) triggered by median nerve electrical stimulation at the wrist. Power spectrum and intertrial coherence analysis were performed on EEG recordings in relation to median nerve stimulation.

Results

The frontal N30 component was accompanied by a significant phase-locking of beta/gamma oscillation (25–35 Hz) and to a lesser extent of 80 Hz oscillation.

After the selection in each subject of the trials for which the power spectrum amplitude remained unchanged, we found pure phase-locking of beta/gamma oscillation (25–35 Hz) peaking about 30 ms after the stimulation. Transition across trials from uniform to normal phase distribution revealed temporal phase reorganization of ongoing 30 Hz EEG oscillations in relation to stimulation. In a proportion of trials, this phase-locking was accompanied by a spectral power increase peaking in the 30 Hz frequency band. This corresponds to the complex situation of 'phase-locking with enhancement' in which the distinction between the contribution of phasic neural event versus EEG phase resetting is hazardous.

Conclusion

The identification of a pure phase-locking in a large proportion of the SEP trials reinforces the contribution of the oscillatory model for the physiological correlates of the frontal N30. This may imply that ongoing EEG rhythms, such as beta/gamma oscillation, are involved in somatosensory information processing.

Hypofunctioning of sensory gating mechanisms in patients with obsessive-compulsive disorder

BACKGROUND

In obsessive-compulsive disorder (OCD) patients, functional abnormalities in basal ganglia/precentral circuitries cause cortical hyperexcitability and lack of inhibitory control. These loops can be partly explored by median-nerve somatosensory evoked potentials (SEPs), which functionally reflect the brain responsiveness to somatosensory stimuli. In healthy humans, SEPs' amplitude during voluntary finger movements is lower than during muscular relaxation (i.e., sensory gating). Cortical hyperexcitability in OCD could be eventually responsible for a reduction of sensory gating. This might have pathophysiologic implications for motor compulsions.

METHODS

Median-nerve SEPs were recorded in 11 OCD patients and 9 healthy volunteers during muscle relaxation ("Relax") or finger movements of the stimulated hand ("Move"). Latencies and amplitudes of pre- and postcentral SEP components were compared between groups during "Relax" and "Move" conditions.

RESULTS

In OCD patients, the responsiveness to sensory stimuli was enhanced for precentral SEPs. Sensory gating ("Relax" vs. "Move") in control subjects involved both pre- and postcentral SEPs, the former being reduced in amplitude by approximately 60%. In OCD patients, sensory gating was spatially restricted to precentral SEP components and was significantly reduced compared with control subjects (approximately 30%).

CONCLUSIONS

Enhanced precentral SEPs and hypofunctioning of centrifugal sensory gating in OCD might reflect the inability to modulate sensory information due to a "tonic" high level of cortical excitability of motor and related areas, likely resulting from basal ganglia dysfunction. This might offer new insights into the pathophysiology of OCD.

Differential modulation of the short- and long-latency somatosensory evoked potentials in a forewarned reaction time task

OBJECTIVE

We investigated modulation of the short- and long-latency somatosensory evoked potentials (SEPs) in a forewarned reaction time task.

METHODS

A pair of warning (auditory) and imperative stimuli (somatosensory) was presented with a 2 s interstimulus interval. In movement condition, subjects responded by grip movement with the ipsilateral hand to the somatosensory stimulation when the imperative stimulus was presented. In counting condition, they silently counted the number of imperative stimuli. The SEPs in response to the imperative stimuli were recorded.

RESULTS

Frontal N30 and central N60 amplitudes were significantly smaller in the movement than in the counting or rest conditions. None of the short-latency components differed between the counting and rest conditions. In contrast to the short-latency components, P80 was significantly larger in the counting than in the rest condition, and showed a further increase from the counting to the movement condition. The N140 amplitude was significantly larger in the movement than the rest condition, but was not changed between the counting and the rest conditions.

CONCLUSIONS

The attenuation of the frontal N30 and central N60, and the enhancement of the P80 and possibly the N140 resulted from the centrifugal mechanism. The present findings may show the different effects of voluntary movement on the early and subsequent cortical processing of the relevant somatosensory information requiring a behavioral response.

SIGNIFICANCE

The present study demonstrated the differential modulation of short- and long-latency components of SEPs in a forewarned reaction time task.

Effect of tactile interference stimulation of the ear in human primary somatosensory cortex: a magnetoencephalographic study

OBJECTIVE

To confirm the somatotopic representation of the ear in the primary somatosensory cortex (SI), we studied the tactile interference effects on somatosensory evoked magnetic fields (SEFs) following stimulation of the ear (Helix, Lobulus and Tragus).

METHODS

We applied tactile interference stimulation to the neck or face area continuously and concurrently while a time-locked electrical stimulation was applied to the ear. If the amplitude would be reduced by the interference, this would indicate that the cortical representation for both the time-locked electrical stimulation and the continuous interference stimulation overlapped. A two or 3-source model, Source 1 in the neck area of SI, Source 2 in the face area of SI, and Source 3 in the secondary somatosensory cortex (SII), was found to be the most appropriate by brain electric source analysis (BESA).

RESULTS

Amplitudes of Sources 1 and 2 in most interference conditions were decreased. Source 1 following stimulation of all 3 sites was significantly reduced when the interference was applied to the neck area. Source 2 following stimulation of all 3 sites was significantly reduced when the interference was applied to the face area.

CONCLUSIONS

These findings showed that the interference effect was found in both the neck and face areas of SI following the ear stimulation.

SIGNIFICANCE

The representation of the ear in SI might be located in both the neck and face areas.

The selective gating of the N30 cortical component of the somatosensory evoked potentials of median nerve is different in the mesial and dorsolateral frontal cortex: evidence from intracerebral recordings

OBJECTIVE

The somatosensory evoked potentials of the median nerve (SEP) were registered intracerebrally in 12 subjects to elucidate the origin of N30 component and its behavior in the motor 'gating' tasks.

METHODS

The recordings were done from the electrodes which were inserted within the cortex of frontal lobe in the pre-surgical phase of epilepsy surgery. The registrations focused on the precentral N30 SEP component and its behaviour under the 'gating' paradigms. Two different 'gating' paradigms, motor and mental, were used and the SEP then were recorded in 3 conditions: (1) normal (N) paradigm, during which the subjects were instructed not to perform any movement by the stimulated hand, or to mentally simulate the movement; (2) active movement (AM) paradigm, during which the subjects were instructed to perform the active movement as the internal motor sequence test by the fingers of the hand of the stimulated limb; (3) mental movement simulation (MMS), during which the subjects were instructed to only mentally simulate the movements performed in the previous paradigm, and this 'virtual' movement also involved the hand of the stimulated limb. The recordings were done at least twice in each paradigm and averaged runs of 2000 artefact-free sweeps were used for the analysis.

RESULTS

The results demonstrated that the precentral N30 component of SEP is generated only in the pre-motor area, either dorsolaterally or mesially, which consists of Brodmann's areas 6 and 8, and their borders. Only the N30 potentials recorded there in 7 subjects had a shape and character of 'near-field' potential. The behaviour of the N30 component when recorded in the AM and MMS paradigms was different depending on the fact of whether they were recorded dorsolaterally or mesially. When there was a clear 'near-field' N30 potential recorded mesially, there was a certain gating present during the AM paradigm, i.e. during the performance of movement. However, the gating caused by the mental movement simulation in the MMS paradigm was substantially more expressed, and the N30 wave practically disappeared in some cases. On the contrary, the gating of the N30 wave, recorded in the frontal dorsolateral premotor cortex (DLPC), was almost complete when the AM (active movement) paradigm was employed, and it was only partial when the MMS paradigm (mental movement simulation) was employed.

CONCLUSIONS

The results of N30 registrations in our group of patients strongly support the theory of separate generator (or generators) of the N30 wave within the premotor cortex. They also brought forward evidence that the dorsolateral premotor cortex (Brodmann's areas 6 and 8) serves as the substrate of the 'motor execution' process, and the mesial frontal cortex (Brodmann's area 6) serves as the substrate of the 'motor planning' process. Further research should focus on the mutual registration of neurophysiological phenomena and imaging phenomena to obtain new data, which will be able to more precisely elucidate the workings of the premotor cortex during the whole process of motor performance.

Abnormal gating of somatosensory inputs in essential tremor

OBJECTIVE

To study whether sensorimotor cortical areas are involved in Essential Tremor (ET) generation.

BACKGROUND

It has been suggested that sensorimotor cortical areas can play a role in ET generation. Therefore, we studied median nerve somatosensory evoked potentials (SEPs) in 10 patients with definite ET.

METHODS

To distinguish SEP changes due to hand movements from those specifically related to central mechanisms of tremor, SEPs were recorded at rest, during postural tremor and during active and passive movement of the hand. Moreover, we recorded SEPs from 5 volunteers who mimicked hand tremor. The traces were further submitted to dipolar source analysis.

RESULTS

Mimicked tremor in controls as well as active and passive hand movements in ET patients caused a marked attenuation of all scalp SEP components. These SEP changes can be explained by the interference between movement and somatosensory input ('gating' phenomenon). By contrast, SEPs during postural tremor in ET patients showed a reduction of N20, P22, N24 and P24 cortical SEP components, whereas the fronto-central N30 wave remained unaffected.

CONCLUSIONS

Our findings suggest that in ET patients the physiological interference between movement and somatosensory input to the cortex is not effective on the N30 response. This finding thus indicates that a dysfunction of the cortical generator of the N30 response may play a role in the pathogenesis of ET.

Finger movement is associated with attenuated cutaneous reflexes recorded from human first dorsal interosseous muscle

Cutaneomuscular reflexes (CMR) have been recorded from the first dorsal interosseous muscle (1DI) of the preferred hand, somatosensory evoked potentials (SEP) were recorded from the contralateral sensory cortex and the sensory nerve action potential (SNAP) was recorded from the median nerve of 15 adult subjects whilst electrically stimulating the digital nerves of the index finger. Subjects performed the following tasks (a) a sustained abduction of the index finger against resistance at 10-20 % maximum voluntary contraction (MVC), and (b) abduction of the index finger as in (a) whilst performing self paced low amplitude tapping of the (i) index finger, (ii) thumb, (iii) middle finger, (iv) little finger and (v) ipsilateral foot. The E2 component of the CMR and the N20/P25 components of the SEP were significantly reduced during finger tapping (P < 0.05). This reduction was independent of which finger was tapping (P > 0.05). There was a significant (qualitative) relationship between the decrease in the size of the E2 component of the CMR and the N20/P25 components of the SEP ((2) test; P < 0.05). There were no significant changes in E1 and I1 (P > 0.05). The size of the SNAP was independent of task (P > 0.05). The size of the E1, I1, E2 components of the CMR, and the N20/P25 components of the SEP were unaltered during foot tapping (P > 0.05, n = 4). We conclude that the decrease in size of the E2 component associated with finger tapping results from gating of the digital nerve input.

Modulation of human corticomotor excitability by somatosensory input

In humans, somatosensory stimulation results in increased corticomotoneuronal excitability to the stimulated body parts. The purpose of this study was to investigate the underlying mechanisms. We recorded motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) from abductor pollicis brevis (APB), first dorsal interosseous (FDI), and abductor digiti minimi (ADM) muscles. MEP amplitudes, recruitment curves (RC), intracortical inhibition (ICI), intracortical facilitation (ICF), resting (rMT) and active motor thresholds (aMT) were recorded before and after a 2-h period of ulnar nerve electrical stimulation at the wrist. Somatosensory input was monitored by recording somatosensory evoked potentials. To differentiate excitability changes at cortical vs. subcortical sites, we recorded supramaximal peripheral M-responses and MEPs to brainstem electrical stimulation (BES). In order to investigate the involvement of GABAergic mechanisms, we studied the influence of lorazepam (LZ) (a GABA(A) receptor agonist) relative to that of dextromethorphan (DM) (an NMDA receptor antagonist) and placebo in a double-blind design. We found that somatosensory stimulation increased MEP amplitudes to TMS only in the ADM, confirming a previous report. This effect was blocked by LZ but not by either DM or placebo and lasted between 8 and 20 min in the absence of (i) changes in MEPs elicited by BES, (ii) amplitudes of early somatosensory-evoked potentials or (iii) M-responses. We conclude that somatosensory stimulation elicited a focal increase in corticomotoneuronal excitability that outlasts the stimulation period and probably occurs at cortical sites. The antagonistic effect of LZ supports the hypothesis of GABAergic involvement as an operating mechanism.

Pediatric phantom vision (Charles Bonnet) syndrome

Visual symptomatology in childhood often presents diagnostic difficulties. Recurrent paroxysmal visual complaints, although typically associated with migraine, may also signal other disorders. We describe a 9-year-old partially sighted male with paroxysmal zoopsias resulting from Charles Bonnet syndrome. This condition is characterized by paroxysmal visual hallucinations occurring in patients with chronic visual impairment, akin to the phantom-limb phenomenon. This pediatric case is the fourth report of this condition. We have reviewed the other cases.