A neuromagnetic study of the functional organization of the sensorimotor cortex

Simultaneous electroencephalography-functional magnetic resonance imaging for assessment of human brain function

Electroencephalography (EEG) and functional Magnetic Resonance Imaging (MRI) have long been used as tools to examine brain activity. Since both methods are very sensitive to changes of synaptic activity, simultaneous recording of EEG and fMRI can provide both high temporal and spatial resolution. Therefore, the two modalities are now integrated into a hybrid tool, EEG-fMRI, which encapsulates the useful properties of the two. Among other benefits, EEG-fMRI can contribute to a better understanding of brain connectivity and networks. This review lays its focus on the methodologies applied in performing EEG-fMRI studies, namely techniques used for the recording of EEG inside the scanner, artifact removal, and statistical analysis of the fMRI signal. We will investigate simultaneous resting-state and task-based EEG-fMRI studies and discuss their clinical and technological perspectives. Moreover, it is established that the brain regions affected by a task-based neural activity might not be limited to the regions in which they have been initiated. Advanced methods can help reveal the regions responsible for or affected by a developed neural network. Therefore, we have also looked into studies related to characterization of structure and dynamics of brain networks. The reviewed literature suggests that EEG-fMRI can provide valuable complementary information about brain neural networks and functions.

Functional connectivity of cerebellar dentate nucleus and cognitive impairments in patients with drug-naive and first-episode schizophrenia

Cognitive impairments are the hallmark of schizophrenia and prominent in the early episode stage. However, the underlying pathological mechanisms of cognitive impairments are not fully understood. This study aimed to investigate the abnormal functional connectivity (FC) of the cerebellar dentate nucleus (DN) and its correlation with cognitive impairments in patients with drug-naive and first-episode schizophrenia. Resting-state functional magnetic resonance imaging data were acquired in 47 patients and 43 healthy controls. Cognitive functions were assessed by number sequence span, verbal category fluency, digit-symbol coding tests. The results showed that the patients had deficits in all three cognitive tests compared to the controls. Furthermore, the increased FC of DN with the bilateral postcentral gyrus and decreased FC of DN with the right inferior temporal gyrus and regional cerebellum (e.g., Vermis 4-5 and Crus I) were observed in the patient group compared to the control group. Importantly, these abnormal DN FC significantly correlated with cognitive tests (e.g., number sequence span and digit-symbol coding) and clinical symptoms (e.g., negative symptom) in the patient group. The results suggested that abnormal FC of DN with cortical and subcortical regions was associated with cognitive impairments and symptom severity and might be an underlying neural mechanism in schizophrenia.

How meaning unfolds in neural time: Embodied reactivations can precede multimodal semantic effects during language processing

Research on how the brain construes meaning during language use has prompted two conflicting accounts. According to the 'grounded view', word understanding involves quick reactivations of sensorimotor (embodied) experiences evoked by the stimuli, with simultaneous or later engagement of multimodal (conceptual) systems integrating information from various sensory streams. Contrariwise, for the 'symbolic view', this capacity depends crucially on multimodal operations, with embodied systems playing epiphenomenal roles after comprehension. To test these contradictory hypotheses, the present magnetoencephalography study assessed implicit semantic access to grammatically constrained action and non-action verbs (n = 100 per category) while measuring spatiotemporally precise signals from the primary motor cortex (M1, a core region subserving bodily movements) and the anterior temporal lobe (ATL, a putative multimodal semantic hub). Convergent evidence from sensor- and source-level analyses revealed that increased modulations for action verbs occurred earlier in M1 (∼130-190 ms) than in specific ATL hubs (∼250-410 ms). Moreover, machine-learning decoding showed that trial-by-trial classification peaks emerged faster in M1 (∼100-175 ms) than in the ATL (∼345-500 ms), with over 71% accuracy in both cases. Considering their latencies, these results challenge the 'symbolic view' and its implication that sensorimotor mechanisms play only secondary roles in semantic processing. Instead, our findings support the 'grounded view', showing that early semantic effects are critically driven by embodied reactivations and that these cannot be reduced to post-comprehension epiphenomena, even when words are individually classified. Briefly, our study offers non-trivial insights to constrain fine-grained models of language and understand how meaning unfolds in neural time.

Whole-hand water flow stimulation increases motor cortical excitability: a study of transcranial magnetic stimulation and movement-related cortical potentials

Previous studies examining the influence of afferent stimulation on corticospinal excitability have demonstrated that the intensity of afferent stimulation and the nature of the afferents targeted (cutaneous/proprioceptive) determine the effects. In this study, we assessed the effects of whole-hand water immersion (WI) and water flow stimulation (WF) on corticospinal excitability and intracortical circuits by measuring motor evoked potential (MEP) recruitment curves and conditioned MEP amplitudes. We further investigated whether whole-hand WF modulated movement-related cortical activity. Ten healthy subjects participated in three experiments, comprising the immersion of participants' right hands with (whole-hand WF) or without (whole-hand WI) water flow, and no immersion (control). We evaluated MEP recruitment curves produced by a single transcranial magnetic stimulation (TMS) pulse at increasing stimulus intensities, short-interval intracortical inhibition (SICI), and intracortical facilitation (ICF) using the paired TMS technique before and after 15 min of intervention. Movement-related cortical potentials (MRCPs) were evaluated to examine primary motor cortex, supplementary motor area, and somatosensory cortex excitability upon movement before and after whole-hand WF. After whole-hand WF, the slope of the MEP recruitment curve significantly increased, whereas SICI decreased and ICF increased in the contralateral motor cortex. The amplitude of the Bereitschaftspotential, negative slope, and motor potential of MRCPs significantly increased after whole-hand WF. We demonstrated that whole-hand WF increased corticospinal excitability, decreased SICI, and increased ICF, although whole-hand WI did not change corticospinal excitability and intracortical circuits. Whole-hand WF modulated movement-related cortical activity, increasing motor cortex activation for the planning and execution of voluntary movements.

Neuromagnetic hand and foot motor sources recruited during action verb processing

The current study investigated sensorimotor involvement in the processing of verbs describing actions performed with the hands, feet, or no body part. Actual movements were used to identify neuromagnetic sources for hand and foot actions. These sources constrained the analysis of verb processing. While hand and foot sources picked up activation in all three verb conditions, peak amplitudes showed an interaction of source and verb condition at 200 ms after word onset, thereby reflecting effector-specificity. Specifically, hand verbs elicited significantly higher peak amplitudes than foot verbs in hand sources. Our results are in line with theories of embodied cognition that assume an involvement of sensorimotor areas in early stages of lexico-semantic processing, even for single words without a semantic or motor task.

Neuromagnetic activation following active and passive finger movements

The detailed time courses of cortical activities and source localizations following passive finger movement were studied using whole-head magnetoencephalography (MEG). We recorded motor-related cortical magnetic fields following voluntary movement and somatosensory-evoked magnetic fields following passive movement (PM) in 13 volunteers. The most prominent movement-evoked magnetic field (MEF1) following active movement was obtained approximately 35.3 ± 8.4 msec after movement onset, and the equivalent current dipole (ECD) was estimated to be in the primary motor cortex (Brodmann area 4). Two peaks of MEG response associated with PM were recorded from 30 to 100 msec after movement onset. The earliest component (PM1) peaked at 36.2 ± 8.2 msec, and the second component (PM2) peaked at 86.1 ± 12.1 msec after movement onset. The peak latency and ECD localization of PM1, estimated to be in area 4, were the same as those of the most prominent MEF following active movement. ECDs of PM2 were estimated to be not only in area 4 but also in the supplementary motor area (SMA) and the posterior parietal cortex (PPC) over the hemisphere contralateral to the movement, and in the secondary somatosensory cortex (S2) of both hemispheres. The peak latency of each source activity was obtained at 54-109 msec in SMA, 64-114 msec in PPC, and 84-184 msec in the S2. Our results suggest that the magnetic waveforms at middle latency (50-100 msec) after PM are different from those after active movement and that these waveforms are generated by the activities of several cortical areas, that is, area 4 and SMA, PPC, and S2. In this study, the time courses of the activities in SMA, PPC, and S2 accompanying PM in humans were successfully recorded using MEG with a multiple dipole analysis system.

Neural decoding of unilateral upper limb movements using single trial MEG signals

A brain machine interface (BMI) provides the possibility of controlling such external devices as prosthetic arms for patients with severe motor dysfunction using their own brain signals. However, there have been few studies investigating the decoding accuracy for multiclasses of useful unilateral upper limb movements using non-invasive measurements. We investigated the decoding accuracy for classifying three types of unilateral upper limb movements using single-trial magnetoencephalography (MEG) signals. Neuromagnetic activities were recorded in 9 healthy subjects performing 3 types of right upper limb movements: hand grasping, pinching, and elbow flexion. A support vector machine was used to classify the single-trial MEG signals. The movement types were predicted with an average accuracy of 66 ± 10% (chance level: 33.3%) using neuromagnetic activity during a 400-ms interval (-200 ms to 200 ms from movement onsets). To explore the time-dependency of the decoding accuracy, we also examined the time course of decoding accuracy in 50-ms sliding windows from -500 ms to 500 ms. Decoding accuracies significantly increased and peaked once before (50.1 ± 4.9%) and twice after (58.5 ± 7.5% and 64.4 ± 7.6%) movement onsets in all subjects. Significant variability in the decoding features in the first peak was evident in the channels over the parietal area and in the second and third peaks in the channels over the sensorimotor area. Our results indicate that the three types of unilateral upper limb movement can be inferred with high accuracy by detecting differences in movement-related brain activity in the parietal and sensorimotor areas.

Movement-related neuromagnetic fields and performances of single trial classifications

In order to clarify whether neurophysiological profiles affect the performance of brain machine interfaces (BMI), we examined the relationships between amplitudes of movement-related cortical fields (MRCFs) and decoding performances during movement. Neuromagnetic activities were recorded in nine healthy participants during three types of unilateral upper limb movements. The movement types were inferred by a support vector machine. The amplitude of MRCF components, motor field (MF), movement-evoked field I (MEFI), and movement-evoked field II (MEFII) were compared with the decoding accuracies in all participants. Decoding accuracies at the latencies of MF, MEFI, and MEFII surpassed the chance level in all participants. In particular, accuracies at MEFI and MEFII were significantly higher in comparison with that of MF. The amplitudes and decoding accuracies were strongly correlated (MF, r(s)=0.90; MEFI, r(s)=0.90; and MEFII, r(s)=0.87). Our results show that the variation of MRCF components among participants reflects decoding performance. Neurophysiological profiles may serve as a predictor of individual BMI performance and assist in the improvement of general BMI performance.

A dynamic network involving M1-S1, SII-insular, medial insular, and cingulate cortices controls muscular activity during an isometric contraction reaction time task

Magnetoencephalographic, electromyographic (EMG), work, and reaction time (RT) were recorded from nine subjects during visually triggered intermittent isometric contractions of the middle finger under two conditions: unloaded and loaded (30% of maximal voluntary contraction). The effect of muscle fatigue was studied over three consecutive periods under both conditions. In the loaded condition, the motor evoked field triggered by the EMG onset decreased with fatigue, whereas movement-evoked fields (MEFs) increased (P < 0.01). Fatigue was demonstrated in the loaded condition, since (i) RT increased due to an increase in the electromechanical delay (P < 0.002); (ii) work decreased from Periods 1 to 3 (P < 0.005), while (iii) the myoelectric RMS amplitude of both flexor digitorum superficialis and extensor muscles increased (P < 0.003) and (iv) during Period 3, the spectral deflection of the EMG median frequency of the FDS muscle decreased (P < 0.001). In the unloaded condition and at the beginning of the loaded condition, a parallel network including M1-S1, posterior SII-insular, and posterior cingulate cortices accounted for the MEF activities. However, under the effect of fatigue, medial insular and posterior cingulate cortices drove this network. Moreover, changes in the location of insular and M1-S1 activations were significantly correlated with muscle fatigue (increase of RMS-EMG; P < 0.03 and P < 0.01, respectively). These results demonstrate that a plastic network controls the strength of the motor command as fatigue occurs: sensory information, pain, and exhaustion act through activation of the medial insular and posterior cingulate cortices to decrease the motor command in order to preserve muscle efficiency and integrity.

Preoperative localization of hand motor cortex by adaptive spatial filtering of magnetoencephalography data

Object

The goal of this study was to examine the sensitivity and specificity in preoperative localization of hand motor cortex by imaging regional event-related desynchronization (ERD) of brainwaves in the β frequency band (15–25 Hz) involved in self-paced movement.

Methods

Using magnetoencephalography (MEG), the authors measured ERD that occurred before self-paced unilateral index finger flexion in 66 patients with brain tumors, epilepsy, and arteriovenous malformations.

Results

The authors applied an adaptive spatial filtering algorithm to MEG data and found that peaks of the tomographic distribution of β-band ERD sources reliably localized hand motor cortex compared with electrical cortical stimulation. They also observed high specificity in estimating contralateral hand motor cortical representations relative to somatosensory cortex. Neither presence nor location of tumor changed the qualitative or quantitative location of motor cortex relative to somatosensory cortex.

Conclusions

An imaging protocol using ERD obtained by adaptive spatial filtering of MEG data can be used for extremely reliable preoperative localization of hand motor cortex.

Cortical neuromagnetic activation accompanying two types of voluntary finger extension

We examined the amplitude and latency of movement-related cerebral field (MRCF) waveforms, the generator and afferent feedback of movement-evoked field 1 (MEF1), and the relationship between motor field neuromagnetic activity and electromyographic activity during performance of two types of voluntary index extension. Eight healthy, right-handed male volunteers participated in this study. Experiments for each subject consisted of recording of MRCFs following two types of finger movement. One (Task 1) involved voluntary extension of the right index finger to about 40 degrees . In the second (Task 2), an elastic band was placed on the right index fingertip, producing a resistance of about 1.5 times the electromyographic activity associated with the voluntary movement yielding extension to approximately 40 degrees . Peak amplitude and the ECD moment of the motor field differed significantly between the two tasks. In Task 2, the electromechanical delay from EMG onset to movement onset (77.8+/-16.2) was longer than in Task 1 (44.4+/-10.4). However, the latency from EMG onset to MEF1 peak was 87.6+/-8.5 ms in Task 2, and did not differ significantly from that in Task 1 (88.6+/-8.5). The ECDs of MEF1 were located significantly medial to N20 m and lateral and posterior to the motor field. These findings suggest that the ECD of MEF1 is located in area 3b, but is slightly different from N20 m, and that this MEF1 component activation is due not to the onset of joint movement but to that of muscular contraction.

Motor field sensitivity for preoperative localization of motor cortex

OBJECT

In this study the role of magnetic source imaging for preoperative motor mapping was evaluated by using a single-dipole localization method to analyze motor field data in 41 patients.

METHODS

Data from affected and unaffected hemispheres were collected in patients performing voluntary finger flexion movements. Somatosensory evoked field (SSEF) data were also obtained using tactile stimulation. Dipole localization using motor field (MF) data was successful in only 49% of patients, whereas localization with movement-evoked field (MEF) data was successful in 66% of patients. When the spatial distribution of MF and MEF dipoles in relation to SSEF dipoles was analyzed, the motor dipoles were not spatially distinct from somatosensory dipoles.

CONCLUSIONS

The findings in this study suggest that single-dipole localization for the analysis of motor data is not sufficiently sensitive and is nonspecific, and thus not clinically useful.

Is the movement-evoked potential mandatory for movement execution? A high-resolution EEG study in a deafferented patient

During simple self-paced index finger flexion with and without visual feedback of the finger, we compared the movement-evoked potentials of the completely deafferented patient GL with those of 7 age-matched healthy subjects. EEG was recorded from 58 scalp positions, together with the electromyogram (EMG) from the first dorsal interosseous muscle and the movement trace. We analyzed the movement parameters and the contralateral movement-evoked potential and its source. The patient performed the voluntary movements almost as well as the controls in spite of her lack of sensory information from the periphery. In contrast, the movement-evoked potential was observed only in the controls and not in the patient. These findings clearly demonstrate that the movement-evoked potential reflects cutaneous and proprioceptive feedback from the moving part of the body. They also indicate that in absence of sensory peripheral input the motor control switches from an internal "sensory feedback-driven" to a "feedforward" mode. The role of the sensory feedback in updating the internal models and of the movement-evoked potential as a possible cortical correlate of motor awareness is discussed.

Neuromagnetic motor fields accompanying self-paced rhythmic finger movement at different rates

We have studied the effect of movement rate on MEG activity associated with self-paced finger movement in four subjects to determine whether the amplitude or latency of motor-evoked activity changes across a range of rates. Subjects performed a continuation paradigm at 21 distinct rates (range: 0.5-2.5 Hz) chosen because of their relevance for many types of sensorimotor coordination (e.g. musical performance). Results revealed a pair of field patterns whose topography and temporal dynamics were similar across all subjects. The strongest pattern was a movement-evoked field (MEF) that emerged during the response and exhibited one or two polarity reversals in time depending on the subject. The MEF complex was tightly coupled to the biphasic response profile but neither latency nor peak amplitude of each MEF component had significant dependence on the temporal duration between successive responses, i.e. movement rate. In contrast, the maximal amplitude of a second, weaker pattern decreased by over 50% when movement rates exceeded 1.1 Hz (inter-response interval <1 s). This pattern was characterized by a change in field line direction over the midline of the scalp and a gradual accumulation of amplitude prior to movement onset. Both characteristics are suggestive of a readiness field. The observed rate-dependent changes in this field may contribute to known transitions in sensorimotor coordination that emerge when the frequency of coordination is increased.

Preoperative magnetic source imaging for brain tumor surgery: a quantitative comparison with intraoperative sensory and motor mapping

Object

The aim of this study was to compare quantitatively the methods of preoperative magnetic source (MS) imaging and intraoperative electrophysiological cortical mapping (ECM) in the localization of sensorimotor cortex in patients with intraaxial brain tumors.

Methods

Preoperative magnetoencephalography (MEG) was performed while patients received painless tactile somatosensory stimulation of the lip, hand, and foot. The early somatosensory evoked field was modeled using a single equivalent current dipole approach to estimate the spatial source of the response. Three-dimensional magnetic resonance image volume data sets with fiducials were coregistered with the MEG recordings to form the MS image. These individualized functional brain maps were integrated into a neuronavigation system. Intraoperative mapping of somatosensory and/or motor cortex was performed and sites were compared.

In two subgroups of patients we compared intraoperative somatosensory and motor stimulation sites with MS imaging–based somatosensory localizations. Mediolateral projection of the MS imaging source localizations to the cortical surface reduced systematic intermodality discrepancies. The distance between two corresponding points determined using MS imaging and ECM was 12.5 ± 1.3 mm for somatosensory–somatosensory and 19 ± 1.3 mm for somatosensory–motor comparisons. The observed 6.5 mm increase in site separation was systematically demonstrated in the anteroposterior direction, as expected from actual anatomy. In fact, intraoperative sites at which stimulation evoked the same patient response exhibited a spatial variation of 10.7 ± 0.7 mm.

Conclusions

Preoperative MS imaging and intraoperative ECM show a favorable degree of quantitative correlation. Thus, MS imaging can be considered a valuable and accurate planning adjunct in the treatment of patients with intraaxial brain tumors.

Noninvasive identification of human central sulcus: a comparison of gyral morphology, functional MRI, dipole localization, and direct cortical mapping

The locations of the human primary hand cortical somatosensory and motor areas were estimated using structural and functional MRI, scalp-recorded somatosensory-evoked potential dipole localization, expert judgments based on cortical anatomy, and direct cortical stimulation and recording studies. The within-subject reliability of localization (across 3 separate days) was studied for eight normal subjects. Intraoperative validation was obtained from five neurosurgical patients. The mean discrepancy between the different noninvasive functional imaging methods ranged from 6 to 26 mm. Quantitative comparison of the noninvasive methods with direct intraoperative stimulation and recording studies did not reveal a significant mean difference in accuracy. However, the expert judgments of the location of the sensory hand areas were significantly more variable (maximum error, 39 mm) than the dipole or functional MRI techniques. It is concluded that because expert judgments are less reliable for identifying the cortical hand area, consideration of the findings of noninvasive functional MRI and dipole localization studies is desirable for preoperative surgical planning.

Preoperative magnetic source imaging for brain tumor surgery: a quantitative comparison with intraoperative sensory and motor mapping

OBJECT

The aim of this study was to compare quantitatively the methods of preoperative magnetic source (MS) imaging and intraoperative electrophysiological cortical mapping (ECM) in the localization of sensorimotor cortex in patients with intraaxial brain tumors.

METHODS

Preoperative magnetoencephalography (MEG) was performed while patients received painless tactile somatosensory stimulation of the lip, hand, and foot. The early somatosensory evoked field was modeled using a single equivalent current dipole approach to estimate the spatial source of the response. Three-dimensional magnetic resonance image volume data sets with fiducials were coregistered with the MEG recordings to form the MS image. These individualized functional brain maps were integrated into a neuronavigation system. Intraoperative mapping of somatosensory and/or motor cortex was performed and sites were compared. In two subgroups of patients we compared intraoperative somatosensory and motor stimulation sites with MS imaging-based somatosensory localizations. Mediolateral projection of the MS imaging source localizations to the cortical surface reduced systematic intermodality discrepancies. The distance between two corresponding points determined using MS imaging and ECM was 12.5 +/- 1.3 mm for somatosensory-somatosensory and 19 +/- 1.3 mm for somatosensory-motor comparisons. The observed 6.5 mm increase in site separation was systematically demonstrated in the anteroposterior direction, as expected from actual anatomy. In fact, intraoperative sites at which stimulation evoked the same patient response exhibited a spatial variation of 10.7 +/- 0.7 mm.

CONCLUSIONS

Preoperative MS imaging and intraoperative ECM show a favorable degree of quantitative correlation. Thus, MS imaging can be considered a valuable and accurate planning adjunct in the treatment of patients with intraaxial brain tumors.

Carpal tunnel syndrome modifies sensory hand cortical somatotopy: a MEG study

The adult somatosensory system has shown reorganizational abilities at cortical and subcortical levels after peripheral nerve lesions. In the present study the effects of carpal tunnel syndrome (CTS) are investigated as reflected on the somatotopy of the primary cortical hand representation. Position and intensity of cortical sources activated by the separate electrical stimulation of median nerve and Digits 1, 3, and 5 of both affected and non-affected hands are evaluated by magnetoencephalographic (MEG) technique. Correlation of MEG results with patient-, physician- and neurophysiological-oriented evaluations of CTS was carried out. Patients showed changes in cortical hand somatotopy in strict relationship to self-referred assessment of symptoms and hand disability in daily activities, including: 1) a more extended representation of the affected hand when paresthesias prevailed; and 2) a more restricted representation due to lateral shift of the little finger was observed when pain symptoms dominated the clinical picture. Contralateral to the side of CTS, the cortical sources activated by Digit 5-stimulation appeared significantly enhanced with respect to contralateral ones from non-affected hand. When comparing the amplitude of peripheral sensory nerve action potentials of median and ulnar nerves to that of cortical responses (i.e., ECD strengths of M20 and M30 components after stimulation of Digits 3 and 5), a significant selective amplification of M30 with respect to M20 and sensory nerve action potential (SNAP) appeared during Digit 3 stimulation compared to that observed for Digit 5. This has been interpreted as a central magnification mechanism in brain responsiveness, possibly revealing a safety factor enabling sensory perception despite the small peripheral signal due to nerve trunk dysfunction. Hum.

The movement-related potential in children with migraine and tension-type headache

Migraine is characterized by an elevated contingent negative variation (CNV) in adults and children. In the present study the movement-related potential preceding self-initiated movements, the Bereitschaftspotential, was investigated in 30 children (mean age 12 years) who were suffering from migraine and tension-type headache and in 16 healthy age-matched controls. Children pressed a button 80 times with the right index finger while movement-related potentials were recorded from frontal and central electrodes. Whereas healthy children evidenced positive movement-related potentials at left and midline positions, children with migraine and tension-type headache showed negative movement-related potentials at midline leads without lateralization. Negativity was even more pronounced in cases of migraine with than without aura symptoms.

Changes in electromyogram during upper limb muscle contraction induced by resistive loaded breathing in humans

Expiratory only resistive loaded breathing (RL) reduces high energy electromyogram (EMG) power (EH) during an isometric contraction of a leg extensor muscle, but not an arm flexor. An interaction between afferent activity during expiratory RL and inspiratory non-loaded phases of breathing, which the contraction spanned, could have accounted for the reduced EH in these long contractions. Therefore this study tested the hypothesis that brief arm extensor muscle contractions (70% of maximal force), performed during a single phase of expiratory RL, would also exhibit reduced EH. Surprisingly, EH in triceps, but not biceps brachii was reduced significantly when the contraction was performed during inspiratory RL rather than expiratory RL. The results suggest that either (a) short and prolonged contractions or (b) motor drive to arm and leg extensors are affected differently by RL.

Oscillatory cortical activity and movement-related potentials in proximal and distal movements

OBJECTIVES

Event-related desynchronization (ERD) of alpha- and beta-rhythms, the post-movement beta-synchronization and the cortical movement-related potentials were analyzed in distal (finger) and proximal (shoulder) movements.

METHODS

EEG was recorded in 7 healthy right-handed men using a 59-channel whole-head EEG system while subjects performed self-paced movements.

RESULTS

The amplitude of the Bereitschaftspotential (BP) was greater over the central midline area and smaller over the contralateral sensorimotor hand area in shoulder than in finger movements. The maximal alpha- and beta-ERD was localized at parietal electrodes in shoulder movements and over the left and right sensorimotor hand area in finger movements. The post-movement beta-ERS was greater in shoulder than in finger movements, especially at the electrode located 3.5 cm left of the central midline electrode. A significant correlation between the slope of the terminal portion of the BP (negative slope) and amplitude of the post-movement beta-synchronization was observed in shoulder but not in finger movements.

CONCLUSIONS

Enhancement of BP over the central midline electrode suggests increased activation of the supplementary motor area in proximal movements. The spatial distribution of the alpha- and beta-ERD and of the post-movement beta-ERS shows topographic differences which may refer to the somatotopic organization of the primary sensorimotor cortex with shoulder representation medial to hand and fingers. The correlation between the negative slope and the post-movement beta-ERS in proximal movements supports the view that the brief post-movement inhibition over the motor cortical area is related to the pre-movement activation of that area.

The role of higher-order motor areas in voluntary movement as revealed by high-resolution EEG and fMRI

In the human motor cortex structural and functional differences separate motor areas related to motor output from areas essentially involved in higher-order motor control. Little is known about the function of these higher-order motor areas during simple voluntary movement. We examined a simple finger flexion movement in six healthy subjects using a novel brain-imaging approach, integrating high-resolution EEG with the individual structural and functional MRI. Electrical source reconstruction was performed in respect to the individual brain morphology from MRI. Highly converging results from EEG and fMRI were obtained for both executive and higher-order motor areas. All subjects showed activation of the primary motor area (MI) and of the frontal medial wall motor areas. Two different types of medial wall activation were observed with both methods: Four of the subjects showed an anterior type of activation, and two of the subjects a posterior type of activation. In the former, activity started in the anterior cingulate motor area (CMA) and subsequently shifted its focus to the intermediate supplementary motor area (SMA). Approximately 120 ms before the movement started, the intermediate SMA showed a drop of source strength, and simultaneously MI showed an increase of source strength. In the posterior type, activation was restricted to the posterior SMA. Further, three of the subjects investigated showed activation in the inferior parietal lobe (IPL) starting during early movement preparation. In all subjects showing activation of higher-order motor areas (anterior CMA, intermediate SMA, IPL) these areas became active before the executive motor areas (MI and posterior SMA). We suggest that the early activation of the anterior CMA and the IPL may be related to attentional functions of these areas. Further, we argue that the intermediate part of the SMA triggers the actual motor act via the release of inhibition of the primary motor area. Our results demonstrate that a noninvasive, multimodal brain imaging technique can reveal individual cortical brain activity with high temporal and spatial resolution, independent of a priori physiological assumptions.

Cortical activation during fast repetitive finger movements in humans: steady-state movement-related magnetic fields and their cortical generators

OBJECTIVE

To study the cortical physiology of fast repetitive finger movements.

METHODS

We recorded steady-state movement-related magnetic fields (ssMRMFs) associated with self-paced, repetitive, 2-Hz finger movements in a 122-channel whole-head magnetometer. The ssMRMF generators were determined by equivalent current dipole (ECD) modeling and co-registered with anatomical magnetic resonance images (MRIs).

RESULTS

Two major ssMRMF components occurred in proximity to EMG onset: a motor field (MF) peaking at 37+/-11 ms after EMG onset, and a postmovement field (post-MF), with inverse polarity, peaking at 102+/-13 ms after EMG onset. The ECD for the MF was located in the primary motor cortex (M1), and the ECD for the post-MF in the primary somatosensory cortex (S1). The MF was probably closely related to the generation of corticospinal volleys, whereas the post-MF most likely represented reafferent feedback processing.

CONCLUSIONS

The present data offer further evidence that the main phasic changes of cortical activity occur in direct proximity to repetitive EMG bursts in the contralateral M1 and S1. They complement previous electroencephalography (EEG) findings on steady-state movement-related cortical potentials (ssMRCPs) by providing more precise anatomical information, and thereby enhance the potential value of ssMRCPs and ssMRMFs for studying human sensorimotor cortex activation non-invasively and with high temporal resolution.

A neuromagnetic normative data set for hemispheric sensory hand cortical representations and their interhemispheric differences

Somatotopy of human hand primary sensory cortex has been studied neuromagnetically [C. Baumgartner, A. Doppelbauer, L. Deecke, D.S. Barth, J. Zeitlhofer, G. Lindinger, W.W. Sutherling, Neuromagnetic investigation of somatotopy of human hand somatosensory cortex, Exp. Brain Res. 87 (1991) 641-648.] [1]. Investigation of sensory cortex devoted to the hand will be of major importance in relation to clinical recovery after sensorimotor deficits as well as an index of plasticity phenomena following alterations of peripheral nerves inputs. Here a normative data set has been constructed, on the basis of the neuromagnetic investigation of the primary sensory hand cortical representation in the two hemispheres of 20 healthy volunteers. This can be used to evaluate interhemispheric differences of the 'sensory' hand areas during experimental paradigms in the healthy as well as following patients with monohemispheric lesions. The localizations in each hemisphere of the cortical Equivalent Current Dipoles (ECDs) activated with the shortest latencies (N20m and P30m components) by separate stimulation of left and right median nerve, thumb and little finger were analysed. By considering the ECDs to thumb and little finger stimulation the boundaries of the hand cortical representation in primary sensory cortex, the 'hand extension' was measured as the distance between the two. For all the considered parameters (related to N20m and P30m ECDs: latency, strength, spatial position in the individual head, 'hand extension', interhemispheric differences) the appropriate variable distribution was considered and by including the 98% of the healthy population normative limits were calculated.

Anatomical congruence of metabolic and electromagnetic activation signals during a self-paced motor task: a combined PET-MEG study

We have investigated the degree of spatial correlation between the cerebral blood flow variations measured by positron emission tomography (PET) and the electromagnetic sources as measured by magnetoencephalography (MEG) in five subjects while performing a self-paced right index finger tapping task. Data were processed independently for each technique using both single-case and intersubject analysis. PET and MEG were coregistered with anatomical magnetic resonance images for each subject. Both extension and flexion motor-related fields were extracted from the MEG signal. Using the single dipole model we identified the motor evoked field 1 (MEF1) in all subjects and the motor field (MF) in three subjects. Individual and intersubject averaged PET data showed consistent contralateral primary sensorimotor (PSM) hand area and bilateral supplementary motor area activation. MEG individual and intersubject averaged results demonstrated that both MEF1 and MF dipoles were localized within the PSM PET activated area. Individual PSM mass center to dipole distance was 12 and 15.3 mm on average for the MEF1 and the MF component, respectively. For the same components, the intersubject averaged analysis shows distances between the PET Z-score maximum and the dipole locations of 6.3 and 15.0 mm, respectively. These results show that PET and MEG MEF1 activation signals spatially coincide within instrumental, registration, and modeling errors.

A comparison between electric source localisation and fMRI during somatosensory stimulation

The present study investigates the electric source localisation of somatosensory evoked potentials (SEPs) using high-resolution EEG (61 scalp electrodes) considering the individual brain morphology as obtained from magnetic resonance images (MRI). A comparison with the activation maps in fMRI under the same somatosensory stimulation paradigm was done. The somatosensory evoked potentials (SEPs) to electrical stimulation of the right median nerve were collected from the scalp of 8 healthy right-handed subjects. The source reconstruction for the 20 ms SEP component was performed by using a single moving dipole model as a source model and a spherical three-shell model as a head model. In 6 of the subjects fMRI was performed using the same electric stimulation of the right median nerve. The source location of the 20 ms SEP component was found to be within the postcentral gyrus. The fMRI activation maps were also located in the postcentral gyrus when using the same somatosensory stimulation paradigm. The appropriateness of using high-resolution EEG and fMRI in the functional localisation of the primary somatosensory cortex is discussed.

Reproducibility and validity of electric source localisation with high-resolution electroencephalography

The present study investigates the reproducibility and validity of the EEG source localisation of somatosensory evoked potentials (SEPs) using high-resolution EEG (61 scalp electrodes) and a source reconstruction on the basis of the individual brain morphology as obtained from magnetic resonance images (MRIs). The somatosensory evoked potentials (SEPs) to electrical stimulation of the right median nerve were repeatedly collected from the scalp of one healthy subject in 9 replications run on 9 different days. The source reconstruction for the 19 ms SEP component was performed by using a single moving dipole model as a source model. Two different head models were used: a spherical 3 shell model and a more realistically shaped 3 compartment model computed using the boundary element method (BEM). The source locations of the 19 ms SEP component were found to be highly reproducible using both head models: the mean standard deviation of the dipole locations was found to be 2.6 mm for the 3 shell model and 4 mm for the more realistically shaped head model. By projection into the individual MRI, the dipoles resulting from either head models were found to be located within the postcentral gyrus. The electric source locations were consistent with the maximum of the task-specific changes seen in a functional magnetic resonance imaging (fMRI) experiment when using the same somatosensory stimulation protocol.

Sensory feedback contributes to early movement-evoked fields during voluntary finger movements in humans

Neuromagnetic field changes accompanying voluntary movement in humans ('movement-evoked fields' or MEFs) were recorded over the scalp using a whole-head MEG system during the performance of self-paced finger movements in order to determine the contribution of sensory feedback to the generation of these brain responses. It was found that cooling the subject's arm resulted in delays of 8 ms or more in the latency of the early movement-evoked field component (MEFI). These delays were attributed to increases in conduction times in the afferent pathways as confirmed by electrically evoked somatosensory responses and suggest a peripheral origin of the MEFI. In a second experiment, we demonstrated the effects of sensory input to the contralateral hand during a simple button pressing task in 4 subjects. The results indicated that responses over the hemisphere ipsilateral to the side of movement which resembled previously reported ipsilateral MEFs can be elicited by the spread of mechanical stimulation to opposite side of the body when a mechanical trigger is used. These experiments provide further evidence that early movement-evoked fields produced by unilateral finger movements are observed primarily over the contralateral somatosensory cortex and represent sensory feedback to the somatosensory cortex from the periphery.

Reliability of dipole localization for the movement-evoked field component MEF I

The movement-evoked field I (MEF I) component is the largest and most stable neuromagnetic component accompanying self-paced movements. In order to use MEG for studying dynamic changes in the cortical organization of movements, data about the reliability and variability of these neuromagnetic components for individual subjects must be established during different sessions. For this aim, three male subjects were requested to perform self-paced flexions of their index finger and thumb in repeated sessions while the MEG was recorded by a 31 channel system. The MEF I was identified for each session and a single equivalent dipole was calculated for this component. The dipole localizations of the various sessions were compared. The standard deviation of the localization for all persons and all values amounts to 4.0-5.2 mm for the three spatial dimensions. Our data suggest that the spatial distance between two single focal sources fitted to the MEF I must be greater than 14 mm to be interpreted as distinct. However, the neuromagnetic field structure and the resulting dipole localization of the MEF I component are quite stable and could be used for the evaluation of cortical plasticity.

Somatosensory evoked magnetic fields and potentials following passive toe movement in humans

The somatosensory evoked magnetic fields (SEFs) and evoked potentials (SEPs) following passive toe movement were studied in 10 normal subjects. Five main components were identified in SEFs recorded around the vertex around the foot area of the primary sensory cortex (SI). The first and second components, 1M and 2M, were identified at approximately 35 and 46 ms. Equivalent current dipoles (ECDs) of both 1M and 2M were estimated around SI in the hemisphere contralateral to the movement toe, and were probably generated in area 3a or area 2, which mainly receive inputs ascending through muscle and joint afferents. The large inter-individual difference of 1M and 2M in terms of ECD orientation was probably due to a large anatomical variance of the foot area of SI. The third and fourth components, 3M and 4M, were identified at approximately 62 ms and 87 ms, respectively. They appeared to be a single large long-duration component with two peaks. Since the 3M and 4M components were significantly larger than the 1M and 2M components in amplitude and their ECD location was significantly superior to that of 1M and 2M, we suspected that they were generated in different sites from those of 1M and 2M, probably area 3b or area 4. Four components, 1E, 2E, 3E and 4E, were identified in SEPs, which appeared to correspond to 1M, 2M, 3M and 4M, respectively. The variation observed in the scalp distribution of the primary component, 1E, could be accounted for by the variation of the orientation of ECD of the 1M component. There was a large difference in the waveform of the long-latency component (longer than 100 ms) between SEFs and SEPs. The 5E of SEPs was a large amplitude component, but the 5M of SEFs was small or absent. We speculate that this long-latency component was generated by multiple generators.

Spatial properties and interhemispheric differences of the sensory hand cortical representation: a neuromagnetic study

We performed a neuromagnetic investigation of the sensory hand cortical representation in the two hemispheres of 20 healthy volunteers. The localizations within the brain hemispheres of the cortical Equivalent Current Dipoles (ECDs) activated with the shortest latencies (N20 m and P30 m components) by separate stimulation of contralateral median nerve, thumb and little finger were analysed. The ECD spatial coordinates were in agreement with the known somatotopy of the sensory homunculus: little finger more medial and posterior, thumb more lateral and anterior, median nerve in-between. By considering the ECDs to thumb and little finger stimulation the boundaries of the hand cortical representation in primary sensory cortex, the 'hand extension' was evaluated as the distance between the two. This parameter was similar on the two hemispheres, the 'hand extension' being 17 mm and 12 mm for N20 m and P30 m components, respectively, with a standard deviation of 5 mm. We provide for the first time the ECDs localization of left and right median nerve, thumb and little finger, as well as the 'hand extension' values, and their interhemispheric differences as a normative data set describing the organization of primary sensory cortical areas reserved to the hand in the healthy population. This approach permits objective measurements of absolute values, as well as of interhemispheric differences, of the sensory hand area following a monohemispheric lesion as well as to non-invasively follow-up its reorganization during clinical recovery.

The bereitschaftspotential paradigm in investigating voluntary movement organization in humans using magnetoencephalography (MEG)

In 1965, Kornhuber and Deecke first described the bereitschaftspotential (BP), a paradigm for investigating the organization of voluntary movement in humans, using electroencephalography (EEG). This paradigm has since been used in many studies for investigating motor control in healthy humans and patients. Over the last years, the advantages of magnetoencephalography (MEG) have been applied to the BP paradigm by a number of researchers. The main advantage of magnetoencephalography over electroencephalography is that MEG has a higher localization accuracy. This is due to the fact that the different structures of the head (brain, liquor cerebrospinalis, skull and scalp) influence the magnetic fields less than the volume current flow that causes the EEG. Additionally, the MEG is reference free, so that the localization of sources with a given precision is easier for MEG than it is for EEG. The present protocol shows in detail how the bereitschaftspotential paradigm can be applied using MEG. Some additional paradigms for investigating motor plasticity, somatosensory gating, Parkinson disease, and the efference copy theory are suggested as well.

The impact of a decision process upon scalp recorded premovement potential

Twelve normal subjects were asked to select one finger after a warning signal and to move it. Using different modes of averaging the EEG responses, it is shown that: (i) the selection of a finger is accompanied by a distinct negative slow potential consisting of a rapid set-up and a following plateau which is the most prominent under the Fz electrode, and (ii) the decision process, when dispersed within a large interval in repeated trials, gives rise to a slow progressive increase of negativity resembling the early component of premovement slow cortical potential from the classical self-paced paradigm.

Post-movement synchronization of beta rhythms in the EEG over the cortical foot area in man

Post-movement synchronization of the electroencephalogram (EEG) was studied in nine right-handed subjects who performed voluntary self-paced dorsal flexions with the right and left foot. The findings revealed that foot movement results in enhanced beta oscillations after movement. These beta bursts showed subject-specific resonance frequencies in the range between 12 and 32 Hz and were localized to electrode Cz and to one electrode 2.5 cm more anterior. Comparison of left and right foot movement revealed a larger post-movement beta synchronization (PMBS) with left foot movement, but no differences concerning the topographical distribution of the PMBS.

Internal consistency of dipole localizations for the human movement-evoked magnetic field component 1 (MEF 1)

The present magnetoencephalographic study was conducted in order to assess the accuracy of dipole localizations for the movement-evoked field component 1 (MEF 1). Three male subjects were requested to perform self-paced flexions of their index finger and thumb in repeated sessions of 60 trials while the neuromagnetic field was recorded by a 31 channel system. Single moving dipole localizations were performed for the MEF 1. The error within single sessions was calculated by split-half reliability and window-homogeneity in a total of 61 sessions. The mean spatial deviation between both halves amounted to 3.8 mm. The window-homogeneity was found to be 2 mm deviation/10 ms.

Changes in movement-related brain activity during transient deafferentation: a neuromagnetic study

Neuromagnetic fields from the left cerebral hemisphere of three healthy, right-handed subjects were investigated preceding and during voluntary index finger movements performed every 8-15 s under two different experimental conditions: before (stage A) and during (stage B) anesthetic block of median and radial nerves at the wrist. The anesthesia caused blocking of cutaneous receptors and some of the proprioreceptors from a wide hand area, including the entire index finger. However, the index finger movements were not impaired because the muscles participating in the task were not anesthetized. The magnetic signals of the brain sources corresponding to the main components of the movement-related neuromagnetic fields (motor field, MF and movement-evoked field I, MEFI) were mapped and localized using a moving dipole model. In the three investigated subjects the MF and MEFI dipole sources were stronger (30% on average) during stage B than during stage A. No significant changes in spatial coordinates of the estimated dipole locations between stages A and B were observed. This was true for both MF and MEFI. The results show that the MEFI reflects not only proprioceptive input from the periphery but cutaneous inputs as well. In this way the results support the view that cutaneous inputs play a specific role in the cortical control of movement.

Neuromagnetic fields of the brain evoked by voluntary movement and electrical stimulation of the index finger

Neuromagnetic fields from the left cerebral hemisphere of five healthy, right-handed subjects were investigated under two different experimental conditions: (1) electrical stimulation of the right index finger (task somatosensory evoked fields, task SEF's), and (2) voluntary movement of the same finger referred to as movement-related fields, (MRFs). The two conditions were, performed in random order every 5-8 s. In addition, the task SEF's were compared to control SEF's recorded at the beginning of the experiment in order to find the optimal dewar position for localizing the central sulcus. The magnetic signals of the sources corresponding to the main components of the somatosensory evoked fields (early ones at 24 ms and at 34 ms, and late ones after 50 ms) and movement-related fields (motor field, MF and movement-evoked field I-MEF I) were mapped and localized by means of a moving dipole model. In four out of five subjects the MEF I dipoles were found to be located deeper than the early task SEF dipoles. In addition, all of the task SEF's components were found to exhibit larger amplitudes than the control SEF's components. The results are discussed in respect to the ability to selectively analyze contributions of mainly proprioceptive (area 3a) and cutaneous (area 3b) areas in the primary somatosensory cortex using magnetoencephalography. An additional finding of the study was that all of the task SEF's components were found to exhibit larger amplitudes than the control SEF's components.

Comparing localization of conventional functional magnetic resonance imaging and magnetoencephalography

The technique of functional magnetic resonance imaging (FMRI) allows the measurement of functional cerebral blood flow changes occurring with specific tasks. However, the spatial relationship between neuronal activity and functional cerebral blood flow changes is not known yet. This study compares the centre of neuronal activation (measured by magnetoencephalography) with that of the blood flow response (measured by FMRI) to unilateral motor stimulation in eight subjects. The results show a mean localization difference of 1.6 cm and demand application of methodological improvements as recently suggested.