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

Connectivity of neural signals to the primary motor area during preparatory periods for movement following external and internal cues

PURPOSE

We investigated the connectivity of neural signals from movement-related cortical areas to the primary motor area (M1) in the hemisphere contralateral to the movement side during the period of movement-related magnetic fields before movement.

MATERIALS AND METHODS

Participants were 13 healthy adults, and nerual signals were recorded using magnetoencephalography. Spontaneous extension of the right wrist was performed at the participant's own pace and following a visual cue in internal (IC) and external (EC) cue tasks. The connectivity of neural signals to M1 from each movement-related motor area was assessed by Granger causality analysis (GCA). The GCA was performed on the neural activity elicited in a frequency band between 7.8 and 46.9 Hz during the pre-movement periods, which occurred durng the readiness field (RF) and the negative slope prime (NSp). F-values, as connectivity values obtained by GCA, were compared between the EC and IC cue tasks.

RESULTS

For NSp periods, the connectivity of neural signals from the left superior frontal area (SF-L) to M1 was dominant in the IC task, whereas that from the left superior parietal area (SP-L) to M1 was dominant in the EC task. The F value in the GCA from SP-L to M1 was greater in the EC task during RF than in the IC task during equivalent periods.

CONSLUSIONS

In the present study, there were differences in the connectivity of neural signals to M1 between IC and EC tasks. The present results suggested that the pattern of pre-movement neural activity that resulted in a movement was not uniform but differed between movement tasks just before the movement.

A review of EEG and MEG for brainnetome research

The majority of brain activities are performed by functionally integrating separate regions of the brain. Therefore, the synchronous operation of the brain's multiple regions or neuronal assemblies can be represented as a network with nodes that are interconnected by links. Because of the complexity of brain interactions and their varying effects at different levels of complexity, one of the corresponding authors of this paper recently proposed the brainnetome as a new -ome to explore and integrate the brain network at different scales. Because electroencephalography (EEG) and magnetoencephalography (MEG) are noninvasive and have outstanding temporal resolution and because they are the primary clinical techniques used to capture the dynamics of neuronal connections, they lend themselves to the analysis of the neural networks comprising the brainnetome. Because of EEG/MEG's applicability to brainnetome analyses, the aim of this review is to identify the procedures that can be used to form a network using EEG/MEG data in sensor or source space and to promote EEG/MEG network analysis for either neuroscience or clinical applications. To accomplish this aim, we show the relationship of the brainnetome to brain networks at the macroscale and provide a systematic review of network construction using EEG and MEG. Some potential applications of the EEG/MEG brainnetome are to use newly developed methods to associate the properties of a brainnetome with indices of cognition or disease conditions. Associations based on EEG/MEG brainnetome analysis may improve the comprehension of the functioning of the brain in neuroscience research or the recognition of abnormal patterns in neurological disease.

Reappraisal of field dynamics of motor cortex during self-paced finger movements

BACKGROUND

The exact origin of neuronal responses in the human sensorimotor cortex subserving the generation of voluntary movements remains unclear, despite the presence of characteristic but robust waveforms in the records of electroencephalography or magnetoencephalography (MEG).

AIMS

To clarify this fundamental and important problem, we analyzed MEG in more detail using a multidipole model during pulsatile extension of the index finger, and made some important new findings.

RESULTS

Movement-related cerebral fields (MRCFs) were confirmed over the sensorimotor region contralateral to the movement, consisting of a temporal succession of the first premovement component termed motor field, followed by two or three postmovement components termed movement evoked fields. A source analysis was applied to separately model each of these field components. Equivalent current diploes of all components of MRCFs were estimated to be located in the same precentral motor region, and did not differ with respect to their locations and orientations. The somatosensory evoked fields following median nerve stimulation were used to validate these findings through comparisons of the location and orientation of composite sources with those specified in MRCFs. The sources for the earliest components were evoked in Brodmann's area 3b located lateral to the sources of MRCFs, and those for subsequent components in area 5 and the secondary somatosensory area were located posterior to and inferior to the sources of MRCFs, respectively. Another component peaking at a comparable latency with the area 3b source was identified in the precentral motor region where all sources of MRCFs were located.

CONCLUSION

These results suggest that the MRCF waveform reflects a series of responses originating in the precentral motor area.

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.

Preparing to grasp emotionally laden stimuli

Background

Contemporary theories of motor control propose that motor planning involves the prediction of the consequences of actions. These predictions include the associated costs as well as the rewarding nature of movements’ outcomes. Within the estimation of these costs and rewards would lie the valence, that is, the pleasantness or unpleasantness of a given stimulus with which one is about to interact. The aim of this study was to test if motor preparation encompasses valence.

Methodology/Principal Findings

The readiness potential, an electrophysiological marker of motor preparation, was recorded before the grasping of pleasant, neutral and unpleasant stimuli. Items used were balanced in weight and placed inside transparent cylinders to prompt a similar grip among trials. Compared with neutral stimuli, the grasping of pleasant stimuli was preceded by a readiness potential of lower amplitude, whereas that of unpleasant stimuli was associated with a readiness potential of higher amplitude.

Conclusions/Significance

We show for the first time that the sensorimotor cortex activity preceding the grasping of a stimulus is affected by its valence. Smaller readiness potential amplitudes found for pleasant stimuli could imply in the recruitment of pre-set motor repertoires, whereas higher amplitudes found for unpleasant stimuli would emerge from a discrepancy between the required action and their aversiveness. Our results indicate that the prediction of action outcomes encompasses an estimate of the valence of a stimulus with which one is about to interact.

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.

Choosing the optimal trigger point for analysis of movements after stroke based on magnetoencephalographic recordings

The aim of this study was to select the optimal procedure for analysing motor fields (MF) and motor evoked fields (MEF) measured from brain injured patients. Behavioural pretests with patients have shown that most of them cannot stand measurements longer than 30 minutes and they also prefer to move the hand with rather short breaks between movements. Therefore, we were unable to measure the motor field (MF) optimally. Furthermore, we planned to use MEF to monitor cortical plasticity in a motor rehabilitation procedure. Classically, the MF analysis refers to rather long epochs around the movement onset (M-onset). We shortened the analysis epoch down to a range from 1000 milliseconds before until 500 milliseconds after M-onset to fulfil the needs of the patients. Additionally, we recorded the muscular activity (EMG) by surface electrodes on the extensor carpi ulnaris and flexor carpi ulnaris muscles. Magnetoencephalographic (MEG) data were recorded from 9 healthy subjects, who executed horizontally brisk extension and flexion in the right wrist. Significantly higher MF dipole strength was found in data based on EMG-onset than in M-onset based data. There was no difference in MEF I dipole strength between the two trigger latencies. In conclusion, we recommend averaging in respect to the EMG-onset for the analysis of both components MF as well as MEF.

Cannabis with high δ9-THC contents affects perception and visual selective attention acutely: an event-related potential study

OBJECTIVE

Cannabis intake has been reported to affect cognitive functions such as selective attention. This study addressed the effects of exposure to cannabis with up to 69.4mg Delta(9)-tetrahydrocannabinol (THC) on Event-Related Potentials (ERPs) recorded during a visual selective attention task.

METHODS

Twenty-four participants smoked cannabis cigarettes with four doses of THC on four test days in a randomized, double blind, placebo-controlled, crossover study. Two hours after THC exposure the participants performed a visual selective attention task and concomitant ERPs were recorded.

RESULTS

Accuracy decreased linearly and reaction times increased linearly with THC dose. However, performance measures and most of the ERP components related specifically to selective attention did not show significant dose effects. Only in relatively light cannabis users the Occipital Selection Negativity decreased linearly with dose. Furthermore, ERP components reflecting perceptual processing, as well as the P300 component, decreased in amplitude after THC exposure. Only the former effect showed a linear dose-response relation.

CONCLUSIONS

The decrements in performance and ERP amplitudes induced by exposure to cannabis with high THC content resulted from a non-selective decrease in attentional or processing resources.

SIGNIFICANCE

Performance requiring attentional resources, such as vehicle control, may be compromised several hours after smoking cannabis cigarettes containing high doses of THC, as presently available in Europe and Northern America.

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.

Symmetry of post-movement beta-ERS and motor recovery from stroke: a low-resolution EEG pilot study

The inter-hemispheric symmetry of electroencephalographic (EEG) post-movement beta-event-related synchronization (PMBS) after movements on a drawing board was studied in eight acute stroke subjects with mild hemiparesis and eight normal subjects. A follow-up testing was conducted 3 months after the initial recordings with a twofold purpose: (1) to validate the reproducibility of the experimental protocol in normal subjects; and (2) to study changes of inter-hemispheric PMBS-symmetry as a response to recovery of motor function. PMBS values were calculated and their topographic distributions illustrated at various time instances following movement offset. Significant PMBS patterns were present in all normal subjects, with only minor differences within consecutive recordings. The side of hemiparesis in acute stroke subjects could be distinguished (P = 0.04) on the basis of the signed symmetry index, a quantitative measure of lateralization. The follow-up testing on three recovered stroke subjects revealed a trend of changes in the lateralization towards the contralateral side of movement, an indication that the cortical organization of movement following recovery turned out as reported for normal subjects. Further clinical investigations need to be carried out to evaluate the relationship between recovery and PMBS symmetry on a large number of subjects, using the method presented here.

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.

Low-Frequency MEG Activity and MRI Evaluation in Parkinson's Disease

BACKGROUND

Low - frequency activity has been reported in Parkinson disease (PD). We investigated the magnetoencephalographic (MEG) recordings from PD patients and healthy volunteers in the frequency domain.

METHODS

The MEG recordings were carried out in a magnetically shielded room with a whole-head 122-channel gradiometer. Nine patients suffering from PD and 9 age and gender matched healthy subjects were included in the study. MRI with T1-w and T2-w images, was available in patients' records.

RESULTS

Frequency analysis may be usefully combined in the analysis of data sets, which contain oscillatory components. Prominent low frequencies can be seen in the spectrum obtained from the PD patients whereas in the control group the frequency range was 6Hz in the majority of channels. MRI did not disclose specific findings in any case.

CONCLUSIONS

It is suggested that the MEG could be a complementary method in the diagnostic evaluation of PD using spatial distribution of the raw data in the frequency domain.

A comparative study of a theoretical neural net model with MEG data from epileptic patients and normal individuals

OBJECTIVE

The aim of this study was to compare a theoretical neural net model with MEG data from epileptic patients and normal individuals.

METHODS

Our experimental study population included 10 epilepsy sufferers and 10 healthy subjects. The recordings were obtained with a one-channel biomagnetometer SQUID in a magnetically shielded room.

RESULTS

Using the method of x2-fitting it was found that the MEG amplitudes in epileptic patients and normal subjects had Poisson and Gauss distributions respectively. The Poisson connectivity derived from the theoretical neural model represents the state of epilepsy, whereas the Gauss connectivity represents normal behavior. The MEG data obtained from epileptic areas had higher amplitudes than the MEG from normal regions and were comparable with the theoretical magnetic fields from Poisson and Gauss distributions. Furthermore, the magnetic field derived from the theoretical model had amplitudes in the same order as the recorded MEG from the 20 participants.

CONCLUSION

The approximation of the theoretical neural net model with real MEG data provides information about the structure of the brain function in epileptic and normal states encouraging further studies to be conducted.

Posterior parietal negativity preceding self-paced praxis movements

Studies of movement-related cortical potentials (MRCPs) for simple movements have shown a slowly rising negativity (Bereitschaftspotential, or BP) about 2 s prior to movement onset, centered in the bilateral sensorimotor area. However, complex movements may elicit a different temporal and spatial distribution of this pre-movement activity. In this study, 64-channel electroencephalography (EEG) was recorded while normal volunteers were asked to perform a simple thumb adduction once every 10--15 s for three 10--15 min blocks. Following this, they were asked to make tool-use movements (hammer, scissor, and screwdriver pantomime) in the same manner. Surface electromyography (EMG) was recorded on the thumb adductor and forearm flexor. MRCP was analyzed for the beginning part of the epoch (from 3.5 s to 1.5 s before EMG onset, with 0.5 s time bins) for differences in the amplitude and spatial distribution of the BP. Significant differences were seen from 3.0 s to 2.0 s before EMG onset, where the amplitude was greater for the more complex movements. On average, negativity began at 3.0 s before onset for praxis movements, and only 1.7 s before onset for thumb adduction. Additionally, the negativity seen for the complex movements had a distribution beginning over the left hemisphere posterior parietal area, whereas, thumb adduction movements had a more anterior distribution, over the bilateral sensorimotor area. The posterior parietal negativity (PPN) suggests that early parietal activity is essential for tool-use movements and is not a part of preparing simple movements.

Neuromagnetic analysis of the late phase of the readiness field for precise hand movements using magnetoencephalography

The aim of this study was to elucidate the cortical regulation of precise finger movements by using magnetoencephalography, with particular emphasis on the late phase of the readiness field. Magnetic brain signals were recorded non-invasively by 306 channel magnetoencephalography during the following two tasks. The first task, a simple task, was to bend the right thumb once as quickly as possible. The second task, a precise one, was to alternately oppose the thumb with the index finger and the middle finger of right hand. In this study, we confirmed that the differences between the two tasks were observed in the late phase of the readiness field, especially in the magnetic field 600 ms before the onset of movement. The activity of the magnetic field of the precise movement task was higher than the activity of the simple movement task. There were obvious differences in the spatial and temporal aspects of the left hemisphere. In the simple movement, the premotor area or motor area was activated in the late phase of the time window. The average latency from the EMG onset was -98.6+/-34.0 ms (n = 5). In the precise movement, the prefrontal area and the SMA were activated in the early and/or middle phases of the time window. The average latency from the EMG onset was -292.0+/-14.9 ms (n = 3) for the prefrontal cortex and -167.8+/-38.3 ms (n = 4) for the SMA. The premotor area or motor area was activated in the late stage of the RF. The average latency from the EMG onset was -111.6+/-61.4 ms (n = 5). Many studies have been performed on the movement-related readiness field. However, the activity of the prefrontal area and the SMA had not previously been studied in the late phase of the readiness field. Our study indicated that the prefrontal area and the SMA played important roles immediately before the onset of precise finger movement. The integration of the prefrontal area, the SMA, and the premotor area is important for the onset of precise finger movement.

Cortical activation in area 3b related to finger movement: an MEG study

To evaluate cortical activation reflecting sensory feedback after finger movement, we recorded movement-related cerebral fields (MRCFs) following voluntary finger movement and somatosensory evoked fields for mixed (median) and pure cutaneous (radial) nerve stimulations (mSEFs and rSEFs) in six normal subjects. Equivalent current dipoles for movement-evoked field 1 (MEF1) in MRCFs and the component (70m) obtained in mSEFs, not clearly in rSEFs, were similarly distributed in each subject. They were located in area 3b, but both mean locations were significantly (p < 0.01) medial to N20m in mSEFs. MEF1 and 70m reflect similar cortical activities related to finger movement and have the same neuronal generator in area 3b, which is different from that of N20m.

Magnetoencephalographic representation of the sensorimotor hand area in cases of intracerebral tumour

OBJECTIVE

To assess the clinical value of magnetoencephalography (MEG) in localising the primary hand motor area and evaluating cortical distortion of the sensorimotor cortices in patients with intracerebral tumour.

METHODS

10 normal volunteers (controls) and 14 patients with an intracerebral tumour located around the central region were studied. Somatosensory evoked magnetic fields (SEFs) following median nerve stimulation, and movement related cerebral magnetic fields (MRCFs) following index finger extension, were measured in all subjects and analysed by the equivalent current dipole (ECD) method to ascertain the neuronal sources of the primary sensory and motor components (N20m and MF, respectively). These ECD locations were defined as the primary hand sensory and motor areas and the positional relations between these two functional areas in controls and patients were investigated.

RESULTS

The standard range of ECD locations of MF to N20m was determined in controls. In 11 of the 14 patients, MRCFs could identify the primary motor hand area. ECD locations of MF were significantly closer to the N20m in the medial-lateral direction in patients than in controls. In patients with a tumour located below the sensorimotor hand area, relative ECD locations of MF to N20m moved anteriorly over the standard range determined in the control subjects. These MEG findings correlated well with radiological tumour locations. The mean estimated ECD strength of MF was significantly lower in patients than in controls.

CONCLUSIONS

MRCF was useful in localising the primary motor hand area in patients with intracerebral tumour. The relative ECD locations of MF to N20m describe the anatomical distortion of the sensorimotor cortex.

MEG alpha activity decrease reflects destabilization of multistable percepts

Multistable stimuli offer the possibility to investigate visual awareness, since they evoke spontaneous alternations between different perceptual interpretations of the same stimulus and, therefore, allow to dissociate perceptual from stimulus-driven mechanisms. In the present study, we used an ambiguous motion paradigm and compared endogenous reversals of perceived motion direction which occur spontaneously during constant ambiguous stimulation with exogenous reversals that were induced externally by changes of stimulation. Contrasting the two conditions allowed to investigate processes that trigger endogenous reversals, since the related activity should be absent in the exogenous reversal condition. We employed ambiguous dot patterns which can easily be transformed to present two stable motion directions in order to induce exogenous pattern reversals. Whole-head MEG was recorded from 10 subjects. We analyzed event-related fields (ERFs) and oscillatory activity in the alpha and gamma ranges. The results showed P300-like slow waves in response to both endogenous and exogenous reversals reflecting the conscious recognition of pattern reversals. Analyses in the gamma-band did not reveal any significant modulations. The alpha activity showed different time courses for endogenous and exogenous reversals. While the exogenous alpha activity decreased in temporal relation to the pattern reversal, the endogenous alpha activity displayed a continuous decrease starting in the time interval preceding the reversal. This time course of the endogenous alpha activity is consistent with a bottom-up approach to figure reversals, since it reflects a process of destabilization of the actual percept until a switch of visual awareness occurs.

GABA-ergic modulation of prefrontal spatio-temporal activation pattern during emotional processing: a combined fMRI/MEG study with placebo and lorazepam

Various prefrontal cortical regions have been shown to be activated during emotional stimulation, whereas neurochemical mechanisms underlying emotional processing in the prefrontal cortex remain unclear. We therefore investigated the influence of the GABA-A potentiator lorazepam on prefrontal cortical emotional-motor spatio-temporal activation pattern in a combined functional magnetic resonance imaging/magnetoencephalography study. Lorazepam led to the reversal in orbito-frontal activation pattern, a shift of the early magnetic field dipole from the orbito-frontal to medial prefrontal cortex, and alterations in premotor/motor cortical function during negative and positive emotional stimulation. It is concluded that negative emotional processing in the orbito-frontal cortex may be modulated either directly or indirectly by GABA-A receptors. Such a modulation of orbito-frontal cortical emotional function by lorazepam has to be distinguished from its effects on cortical motor function as being independent from the kind of processing either emotional or nonemotional.

Studying semantics in the brain: the rapid stream stimulation paradigm

Event-related potentials (ERPs) provide information about the temporal course of cognitive processes in the brain. They have proved to be a valuable tool in order to explore semantic aspects of word processing. However, to date, research in this field has been mostly concerned with the study of post-lexical features by means of the N400-paradigm. We introduce here the rapid stream stimulation paradigm, in which stimuli reflecting different levels of linguistic information are presented to subjects at a high rate of stimulation. The present protocol shows in detail how this paradigm can be applied. The application of the rapid stream stimulation paradigm evokes the recognition potential (RP), an ERP component that peaks at around 260 ms after stimuli onset and seems to be reflecting lexical selection processes. Results of studies that revealed the sensibility of the RP to visual-semantic aspects and the location of its neural generators within basal extrastriate areas are reported. Although some research has been conducted with the rapid stream stimulation paradigm much remains still to be done. Some of the possibilities that this paradigm offers are further discussed.

Button-pressing affects P300 amplitude and scalp topography

BACKGROUND

Scant and equivocal research exists examining the effects of button-pressing on P300. Button-pressing may decrease P300 latency and amplitude. The melding of motor potentials and P300 may also confound studies of P300 topography, such as studies of temporal scalp-area asymmetries in schizophrenia.

METHOD

P300 was measured on button-press and silent-count tasks in control subjects. An estimate of motor activity was constructed from a simple reaction time task, with reaction times matched to the button-press task. The motor estimate was subtracted from the button-press P300 to assess Kok's (1988) additive model. Lastly, lateral P300 from schizophrenia patients was compared with each condition's P300.

RESULTS

P300 was smaller and its topography different in the button-pressing task relative to silent-counting. The motor-correction procedure generated a P300 with normal topography. Comparison of the button-press P300 in controls to the silent-count P300 in schizophrenia patients reduced a significant lateral asymmetry to trend level. This asymmetry was significant after the correction procedure.

CONCLUSIONS

Button-pressing generates smaller P300 than silent-counting. Also, P300 topography in button-pressing tasks is confounded by motor potentials. The distortion can be corrected with a motor potential estimate. Motor potentials can occlude differences in P300 topography between groups.

Regulating action: alternating activation of midline frontal and motor cortical networks

OBJECTIVES

Focal electrical fields recorded over the midline prefrontal cortex have been found to index rapid evaluative decisions, including the recognition of having made an error in a speeded response task. The nature of these electrical fields and how they are related to cortical areas involved in response execution remains to be clarified.

METHODS

As subjects performed a speeded response task the EEG was recorded with a 128-channel sensor array. By filtering out the large slow waves of the event-related potential, we found that the error-related negativity (Ne/ERN) arises from a midline frontal oscillation that alternates with oscillations over lateral sensorimotor cortex. Electrical source analyses were used to determine the brain sources involved in the generation of these oscillations.

RESULTS

The results show that the midline and lateral oscillations have a period of about 200 ms (theta), and they are present for both correct and error responses. When an error is made, the midline error oscillation is recruited strongly, and it becomes correlated with the motor oscillation. Source analyses localized the midline error oscillation to centromedial frontal cortex and the lateral oscillation to sensorimotor cortices.

CONCLUSIONS

Because of the similarity between the midline oscillation observed in the present study and frontal midline theta, the nature of the Ne/ERN may be clarified by the frontal midline theta literature. The correlation between the midline and sensorimotor oscillations suggests a possible mechanism for how midline frontal evaluative and monitoring networks contribute to action regulation.

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.