Brain activity associated with skilled finger movements: multichannel magnetic recordings

The psychophysiology of "covert" goal-directed behavior

Covert behavior is defined as behavior that is not directly visible and is thus comparable to a type of behavioral silence that requires modern psychophysiological techniques to reveal. Goal-directed behavior is teleologically purposive. Fundamentally, there are two approaches to accounting for purposeful behavior. One is the cybernetic approach, which views behavior as homeostatic and largely reflexive. The other one views behavior as a cognitive process that involves an interaction between neural events representing the previous experience, the present state of the individual, and the occurrence of particular features in the environment. This review, based on published data, presents a non-invasive psychophysiological method for investigating the electrical brain activity associated with those "silent" behaviors such as intention, evaluation of results, and memorization. Movement-related potentials (MRPs) are ideal for studying these processes. The MRPs are recorded during the execution of the skilled performance task (SPT). This task requires the execution of fast ballistic movements with the thumbs of both hands, learning a precise and short time interval between the two thumb presses, and scoring the highest number of target performances. The subject receives real-time feedback about the results of his performance. The MRPs associated with this task and present during covert behavior are the Bereitschaftspotential (BP) present before the onset of movement and the Skilled Performance Positivity (SPP) after movement, which coincides with the subject's awareness of the success or failure of his performance. These potentials show a maturational trend, reaching the adult form around the age of 10 when formal and abstract thinking progress. SPT and MRPs are particularly suitable to study neurodevelopmental disorders. Children with developmental dyslexia show abnormal MRPs, both in latency and amplitude, in different brain areas.

Load effect on background rhythms during motor execution: A magnetoencephalographic study

We investigated the effect of load against self-paced movement on cortical involvement for motor execution. Ten right-handed healthy volunteers were requested to perform brisk extension of the right index finger at self-paced intervals exceeding 10s for three load conditions: 0g, 50g and 100g. Movement-related magnetic fields were recorded using an MEG system. The signals were band-pass-filtered through 18-23Hz and rectified before averaging with respect to EMG onset. We analyzed the time course and %change of peak amplitude with reference to the baseline amplitude in event-related desynchronization (ERD) or synchronization (ERS) in each hemisphere. Maximum response was observed around the left somatomotor area for all conditions. ERD did not show any significant difference before the movement onset among the three load conditions. For %change, ERS in the post-movement period was significantly larger for the 100g load condition than for the 0g load condition, and that was significantly greater over the left than over the right hemisphere. These findings indicate that the load has little effect on pre-movement desynchronization, whereas it affects the post-movement synchronization on background rhythms.

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.

Cortical neuromagnetic fields evoked by voluntary and passive hand movements in healthy adults

Neuromagnetic fields were recorded from the left cerebral hemisphere of six healthy right-handed subjects under three different conditions: (1) externally triggered rapid voluntary extension and flexion of the right hand, (2) passive extension and flexion of the right hand, and (3) stimulation of the skin of the right index finger by means of air pressure. Location analysis using the current density analysis did not reveal any differences between motor evoked field I (MEF I) in active and passive movements, and met the maximum of cerebral activation in the contralateral precentral region. In contrast, the sensory evoked field was located clearly in the contralateral postcentral region. Additionally, a significantly shorter latency of MEF I (with respect to movement onset) was observed in flexion compared with extension in both passive and active movements. These results support the assumption that MEF I is generated by cortical activation resulting from proprioceptive, probably muscle spindle, input. The current density analysis has proved to be an appropriate method for investigating movement-related fields. Furthermore, the described method seems to be appropriate for evaluating the processes of cortical reorganization and the influence of neurorehabilitation within longitudinal studies in patients with lesions in motor centers of the brain.

Movement-related slow cortical magnetic fields and changes of spontaneous MEG- and EEG-brain rhythms

Cortical activity was recorded from 5 healthy adults with a 122-channel whole-head magnetometer while the subjects performed during unilateral finger movements at self-paced intervals exceeding 6 s. The readiness field (RF) started over the contralateral somatomotor area 0.3-1 s prior to the movement onset in subjects (Ss) 1, 2, and 4, and culminated in the motor field (MF) 30 ms after it (Ss 1-4). These signals were followed by movement evoked fields MEFI (Ss 1-5) and MEFII (Ss 1-4) at 100-150 ms and 200-250 ms after the movement onset, respectively. One subject showed clear RF over the ipsilateral hemisphere as well. The contralateral dominance of the RF contrasted the more symmetric distribution of the simultaneously recorded electric Bereitschaftspotential (BP). The RF onset never preceded the BP onset. We suggest that BP receives contribution from the early bilateral activation of the crown of the precentral gyrus, whereas RF reflects later activity of the fissural motor cortex. Spontaneous oscillations in the background activity (spontaneous activity) of approximately 10 Hz started to dampen 2-3 s prior to the movement onset in the somatomotor areas of both hemispheres with contralateral predominance (S1 and S3), and returned to a steady level 0.8-2 s after the movement onset in all subjects. Higher frequency bands in the same area displayed a prominent rebound about 1 s after the movement onset in 4 subjects. Execution of self-paced movements is evidently expressed differently in the slow movement-related fields and in the cortical spontaneous activity.

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.

Bilateral activation of the human somatomotor cortex by distal hand movements

We recorded cortical magnetic signals, simultaneously over the whole scalp, from 6 healthy subjects during 3 motor tasks to track the varying proportion of contra- vs. ipsilateral activation. The subjects performed self-paced index finger flexions, simultaneous flexion of 4 fingers, and a sequence of rapid digit movements in different sessions. Index finger and 4-finger movements were associated with phasic bilateral dampening of spontaneous 10 and 20 Hz rhythms along the central sulcus, starting approximately 1 sec before the movement in the contralateral hemisphere. A rebound occurred within 1 sec after the index finger and 4-finger flexions; the rapid finger movements resulted in a persistent blocking of the rhythms. Averaging with respect to movement onset showed a slow bilateral frontal readiness field starting about 0.5 sec prior to motion onset. It was followed, within 200 msec after movement onset, by phasic movement-evoked fields (MEFs) which were bilateral during the tasks involving several fingers. The contra- vs. ipsilateral MEF amplitude ratio C/I decreased from 4.0 during index finger movements to 0.6 during rapid finger flexions, reflecting the enhanced activation of the ipsilateral primary somatomotor cortex with increasing complexity of movement.

Cortical magnetic and electric fields associated with voluntary finger movements

Multichannel recordings of both movement-related magnetic fields (MRMFs) and movement-related cortical potentials (MRCPs) were simultaneously recorded in association with voluntary unilateral self-paced index finger abduction movement in two normal volunteers. 1) Slow magnetic field (readiness field; RF) can be detected several hundred msec before the movement onset, and its field distribution indicates the existence of the largest generator source over the contralateral primary motor area. Taken together with the vertex-maximal Bereitschaftspotential which corresponds to the earlier part of the RF, the complexity of this magnetic field suggested by relatively low correlation value in single dipole model indicates the co-activation of other underlying generators besides this largest dipole. 2) The utilization of MRMF with MRCP facilitates the separation of two distinct electrophysiological events in proximity to the movement onset, which are difficult to be determined by the technique of MRCP only. Those are the motor field (MF) and the movement evoked field I (MEFI) in MRMF, and the parietal peak motor potential (ppMP) and the frontal peak motor potential (fpMP) in MRCP, which occur approximately 20 and 100 msec after EMG onset, respectively. These two subcomponents may imply the culmination of motor cortex and sensory feedback activation, respectively. Combined study of MRMF and MRCP will provide better definition of cortical events related to voluntary movement than the study of either modality alone.