Douglas Cheyne

Dynamic cortical activity in the human brain reveals motor equivalence

That animals and humans can accomplish the same goal using different effectors and different goals using the same effectors attests to the remarkable flexibility of the central nervous system. This phenomenon has been termed ‘motor equivalence’,, an example being the writing of a name with a pencil held between the toes or teeth. The idea of motor equivalence has reappeared because single-cell studies in monkeys have shown that parameters of voluntary movement (such as direction) may be specified in the brain, relegating muscle activation to spinal interneuronal systems,. Using a novel experimental paradigm and a full-head SQUID (for superconducting quantum interference device) array to record magnetic fields corresponding to ongoing brain activity, we demonstrate: (1), a robust relationship between time-dependent activity in sensorimotor cortex and movement velocity, independent of explicit task requirements; and (2) neural activations that are specific to task demands alone. It appears, therefore, that signatures of motor equivalence in humans may be found in dynamic patterns of cortical activity.

A MEG analysis of the P300 in visual discrimination tasks

Based on recent research that indicated that P300 scalp topography varies as a function of task and/or information to be processed, this study examined scalp-recorded magnetic fields correlated with the P300 by means of whole-head magnetoencephalography. Subjects performed two discrimination tasks, in which targets, defined on either object or spatial characteristics of the same visual stimuli, had to be discriminated. Based on the across-subject root mean square (RMS) functions a sequence of 4 components could be identified in both tasks, N1m, P3m, and two later components, which, based on their estimated neuronal sources, were classified as representing motor processes during and following the manual responses to target stimuli. Reliable between-task differences in source localization were obtained for the P3m component, but not for the other components. Inferior-medial sources were found for the P3m evoked by both spatial and object targets, with these sources being located about 3.5 cm more anterior for object targets. These results suggest that different neuronal sources, possibly located in subcortical regions in the vicinity of the thalamus, contribute to the P3m evoked by target stimuli defined by either object or spatial stimulus characteristics.

Frequency-dependent spatial distribution of human somatosensory evoked neuromagnetic fields

Using synthetic aperture magnetometry (SAM), we examined the spatial distribution of frequency changes in magnetoencephalography signal rhythms on individual magnetic resonance images following somatosensory stimulation. SAM is a novel statistical spatial filtering method that uses an adaptive beamformer. Electrical stimulation of the right median nerve demonstrated high-frequency event-related synchronization (ERS) in the 50-200-Hz range, consistently localized in the contralateral primary sensorimotor area in all subjects (n=7). Event-related desynchronization (ERD) was demonstrated in the 8-13, 13-25 and 25-50-Hz ranges bilaterally in the area surrounding the central sulcus. The differences in the spatial distribution as well as the frequency bands between ERS and ERD suggest that ERS and ERD reflect the responses of different cell assemblies rather than a frequency shift of the same cell assembly.

Event-related changes in neuromagnetic activity associated with syncopation and synchronization timing tasks.

For low rhythmic rates (1.0 to ∼2.0 Hz), subjects are able to successfully coordinate finger flexion with an external metronome in either a syncopated (between the beats) or synchronized (on each beat) fashion. Beyond this rate, however, syncopation becomes unstable and subjects spontaneously switch to synchronization to maintain a 1:1 stimulus/response relationship. We used a whole‐head magnetometer to investigate the spatiotemporal dynamics of neuromagnetic activity (MEG) associated with both coordinative patterns at eight different rates spanning the range 1.0–2.75 Hz. Timing changes in the event‐related fields accompanied transitions from syncopation to synchronization and followed the placement of the motor response within each stimulus/response cycle. Decomposition of event‐related fields into component auditory and motor brain responses revealed that the amplitude of the former decreased with increasing coordination rate whereas the motor contribution remained approximately constant across all rates. Such an interaction may contribute to changes in auditory‐motor integration that cause syncopation to become unstable. Examination of event‐related changes in high frequency bands revealed that MEG signal power in the beta band (15–30 Hz) was significantly lower during syncopated coordination in sensors covering the contralateral sensorimotor area suggesting a dependence of beta rhythm amplitude on task difficulty. Suppression of beta rhythms was also stronger during synchronization preceded by syncopation, e.g., after subjects had switched, when compared with a control condition in which subjects synchronized throughout the entire range of rates. Hum. Brain Mapping 14:65–80, 2001.

Spatiotemporal analysis of neuromagnetic events underlying the emergence of coordinative instabilities.

A full-head 143-channel superconducting quantum interference device was used to study changes occurring in the magnetic activity of the human brain during performance of an auditory-motor coordination task in which the rate of coordination was systematically increased. Previous research using the same task paradigm demonstrated that spontaneous switches in timing behavior that arise with higher coordination rates are accompanied by qualitative changes in spatiotemporal brain activity measured by electro- and magnetoencephalography. Here we show how these patterns can be decomposed into basic physiological events, i.e., evoked brain responses to acoustic tones and self-initiated finger movements. The frequency dependence of the amplitudes of these component responses may shed new light onto why spontaneous timing transitions occur in the first place.

Motor cortex activity and predicting side of movement: neural network and dipole analysis of pre-movement magnetic fields

Neuromagnetic fields were recorded from human subjects during the performance of left and right voluntary finger movements. Modeling of current dipole sources indicated symmetric activation of both motor cortices beginning 600 ms prior to movement onset. This activity became lateralized to the contralateral hemisphere 200-300 ms prior to movement onset, the period during which an artificial neural network showed increased ability to predict side of movement within single trials. The results describe the mechanism of lateralization of cortical brain activity preceding voluntary movement and provide further evidence of the involvement of ipsilateral motor cortex in unilateral movements.

Neuromagnetic fields preceding unilateral movements in dextrals and sinistrals.

MOVEMENT -related magnetic fields were recorded with a whole-head magnetoencephalographic system in three dextrals and three sinistrals during right or left index finger extension. The motor field (MF) demonstrated an asymmetrical isofield map pattern with larger field reversal over the contralateral hemisphere for dominant hand movement and an almost symmetrical pattern for non-dominant hand movement in each subject. The equivalent current dipole moment of the MF for the contralateral hemisphere was significantly larger than the ipsilateral hemisphere for dominant hand movement, and almost equal for both hemispheres for non-dominant hand movement. These results were congruent for both dextrals and sinistrals, suggesting a more important role of the hemisphere contralateral to the dominant hand in unilateral voluntary movement, regardless of handedness.

Modulation of somatosensory evoked magnetic fields by sensory and motor interferences.

Modulatory influences of skilled exploratory finger movements on somatosensory evoked magnetic fields evoked by median nerve stimulation were investigated in six healthy subjects using a whole head magnetometer (MEG) system. The exploratory finger movements caused major changes in the somatosensory evoked fields. The most prominent effect was a reversal of the dipolar magnetic field around 30ms after median nerve stimulation. Similar but less pronounced effects were exerted by repetitive finger movements and tactile stimulation of the hand. A dipole analysis and super-imposition of resulting sources on individual MRI scans showed that all somatosensory evoked fields up to 60ms after stimulation, and the modulation of these responses were located in the primary somatosensory cortex (SI).

Cerebral motor control in patients with gliomas around the central sulcus studied with spatially filtered magnetoencephalography

Objective: Application of spatially filtered magnetoencephalography (MEG) to investigate changes in the mechanism of cerebral motor control in patients with tumours around the central sulcus. Methods: MEG records were made during a repetitive hand grasping task in six patients with gliomas around the central sulcus and in four control subjects. Power decreases in the α (8–13 Hz), β (13–30 Hz), and low γ bands (30–50 Hz) during the motor tasks (event related desynchronisation, ERD) were analysed statistically with synthetic aperture magnetometry. The tomography of ERD was superimposed on the individual’s magnetic resonance image. Results: β ERD was consistently localised to the contralateral primary sensorimotor cortex (MI/SI) in control subjects, whereas the α and low γ ERD showed considerable intersubject variability. β ERD in patients during non-affected side hand movement was also localised to the contralateral MI/SI, but exclusively to the ipsilateral hemisphere during affected side hand movement. Conclusions: The altered pattern of ERD in the patient group during affected side hand movement suggests recruitment of diverse motor areas, especially the ipsilateral MI/SI, which may be required for the effective movement of the affected hand.

Measurements of extremely low frequency brain magnetic fields associated with four-tone memory processes

Recently magnetic-field measurements of the brain have become a useful tool to study higher brain function. The purpose of this study is to propose source models of both the memorizing process and the recognition process. The components of magnetic fields perpendicular to the surface of the head were measured by using a whole cortex type of DC SQUID with a third-derivative gradiometer. The authors observed the extremely slow components of MEG activities during both the memorizing process and the recognition process. They estimated the sources using two current dipoles, and compared the two cases.