Andrej Stancak

On the existence of different types of central beta rhythms below 30 Hz

EEGs were recorded from sensorimotor areas of 12 subjects performing unilateral self-paced brisk and slow finger movements. Two different beta components were found below 30 Hz: (i) One component, at about twice the frequency of the mu rhythm, showed desynchronization in parallel with the mu rhythm starting at about 2 s prior to movement. Measurements of bicoherence have shown that this beta component can be non-linearly related to the arch-shaped mu rhythm. (ii) Another beta component started to desynchronize about 1 s prior to movement, recovered very fast and displayed a pronounced post-movement beta synchronization (PMBS) within the first second after movement-offset. This PMBS was found in the frequency band between 12 and 26 Hz. The average percentage power increase at contralateral sensorimotor area was 232% (SD = 82%) for brisk and 265% (SD = 102%) for slow movements. The PMBS was of contralateral dominance and is interpreted as a correlate of active inhibition or idling of the primary motor area following movement execution.

Event-related desynchronisation of central beta-rhythms during brisk and slow self-paced finger movements of dominant and nondominant hand.

Changes in central beta-rhythms (14-29 Hz) during movement were investigated in 12 right-handed subjects by quantifying event-related desynchronisation (ERD). EEG was recorded from 24 closely spaced electrodes overlaying the left and right sensorimotor hand area. The subjects performed approximately 80 brisk (movement time < 0.21 s) and 80 slow (movement time 1.3-2.1 s) self-paced extensions of their left or right index finger. Beta-band power attenuation in the preparatory period (2.0-0.5 s before movement onset) was larger in the contralateral hemisphere in both types of movement and similar for both fingers. In the 0.4-s period before the onset of extensor muscle contraction, right-finger movements only showed a significant contralateral preponderance of beta-ERD. During movement an anterior ERD predominance in the right sensorimotor hand area and a widespread ERD in the left sensorimotor area was found for both fingers. The recovery and rebound of beta-rhythms showed contralateral preponderance which was expressed more in the right hemisphere, especially after left-finger movements. The results suggest that the dynamics of premovement desynchronisation and postmovement synchronisation of central beta-rhythms is related to hand dominance.

Desynchronization and recovery of β rhythms during brisk and slow self-paced finger movements in man

Event-related desynchronization (ERD) and recovery of EEG beta rhythms (15-26 Hz) were studied during slow and brisk self-paced index finger extension and flexion. beta rhythms started to recover earlier in brisk movements. Brisk movements showed no correlation between duration of EMG burst in the extensor muscle and the latency of recovery whereas slow movements did. In contrast to beta-ERD which was widespread, the recovery and rebound of beta rhythms occurred in a circumscribed focus close to the hand MI area.

Modulation of motor-cortex oscillatory activity by painful Aδ- and C-fiber stimuli

Spontaneous approximately 20-Hz oscillations, arising predominantly from the primary motor cortex (MI), are readily observed by magnetoencephalography (MEG). Prior studies have indicated that the level of the approximately 20-Hz rhythm reflects the functional state of the MI cortex: increased 20-Hz level is associated with increased inhibition and suppression of the rhythm with excitation of MI. Close interaction is suggested between pain and the motor system by the association of chronic pain with motor dysfunction and by the alleviation of pain by motor-cortex stimulation. We therefore explored the effect of noxious input on motor-cortex functions by recording MEG signals from nine healthy subjects during selective laser stimulation of Adelta- and C-fibers of the hand. The approximately 20-Hz level was suppressed in the contralateral MI cortex in all nine subjects after painful Adelta- and C-fiber stimuli (P < 0.001). The suppression started 180 +/- 10 ms (mean +/- SEM) after Adelta-fiber stimuli and 820 +/- 30 ms after C-fiber stimuli, and peaked 160-170 ms later. Similar, but about 50% weaker, suppression of the approximately 20-Hz oscillations occurred in seven out of nine subjects in the ipsilateral MI. These results suggest automatic, lateralized, excitation of the MI cortex by noxious Adelta- and C-fiber input.

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 effects of handedness and type of movement on the contralateral preponderance of μ-rhythm desynchronisation

Event-related desynchronisation (ERD) of mu-rhythm was studied in 12 right-handed and 11 left-handed subjects during brisk and slow self-paced index finger movements of dominant and nondominant hand. Electroencephalogram (EEG) was recorded from the sensorimotor hand area of both hemispheres. The contralateral preponderance of mu-rhythm ERD in the pre-movement period showed the following changes: (i) the contrasts between left- and right-finger movements were larger and earlier in the dominant than nondominant hemisphere in both handedness groups; (ii) right-handed subjects showed larger lateralisation of mu-rhythm ERD prior to right-finger as compared to left-finger movements, whereas about equal contralateral preponderance for both sides was found in the left-handed; (iii) the lateralisation of mu-rhythm ERD was lower prior to brisk as compared to slow movements, especially in the left-handed subjects. The results demonstrate that hand dominance, handedness and type of movement influence the proportion of pre-movement mu-rhythm desynchronisation in the left and right peri-rolandic area.

Interaction of Tactile Input in the Human Primary and Secondary Somatosensory Cortex—A Magnetoencephalographic Study

Interaction of simultaneous tactile input at two finger sites in primary (SI) and secondary somatosensory cortex (SII) was studied by whole-head magnetoencephalography. Short pressure pulses were delivered to fingers of the right and left hand at an interstimulus interval of 1.6 s. The first phalanx of the left digit 1 and four other sites were stimulated either separately or simultaneously. We compared four sites with increasing distance: the second phalanx of left digit 1, left digit 5, and digits 1 and 5 of the right hand. The temporal evolution of source activity in the contralateral SI and bilateral SII was calculated using spatiotemporal source analysis. Interaction was assessed by comparing the source activity during simultaneous stimulation with the sum of the source activities elicited by separate stimulation. Significant suppressive interaction was observed in contralateral SI only for stimuli at the same hand, decreasing with distance. In SII, all digits of the same and the opposite hand interacted significantly with left digit 1. When stimulating bilaterally, SII source waveforms closely resembled the time course of the response to separate stimulation of the opposite hand. Thus, in bilateral simultaneous stimulation, the contralateral input arriving first in SII appeared to inhibit the later ipsilateral input. Similarly, the separate response to input at two unilateral finger sites which arrived slightly earlier in SII dominated the simultaneous response. Our results confirm previous findings of considerable overlap in the cortical hand representation in SII and illustrate hemispheric specialization to contralateral input when simultaneous stimuli occur bilaterally.

Mu-rhythm changes in brisk and slow self-paced finger movements

WE analysed whether type of movement (brisk vs slow) and active muscle force are encoded in the time course of mu-rhythm desynchronization during self-paced finger movements. Ten subjects performed 100 brisk and slow extensions of the right index finger. The time course of mu-rhythm desynchronization in the contralateral sensorimotor area before movement was identical for both types of movements. Brisk movements accompanied by a stronger extensor muscle contraction were preceded by larger desynchronization. The onset of mu-rhythm recovery was related to the duration of the extensor EMG burst in both types of movement. The results suggest that both amplitude and duration of the extensor muscle contraction are encoded in the time course of mu-rhythm desynchronization.

The effects of external load on movement-related changes of the sensorimotor EEG rhythms

The effects of external load opposing brisk voluntary extension of the right index finger on the EEG rhythms in the left and right sensorimotor hand area were studied in 13 right-handed subjects. Four levels of external loads corresponding to the weights of 0 g (no load), 30 g, 80 g and 130 g were used. The effects of external load on EEG rhythms were the following: (i) prior to movement, the desynchronisation of beta-rhythms (18-25 Hz) over the contralateral sensorimotor area was greater under the two largest loads as compared to the 0 g load. However, beta-desynchronisation at ipsilateral electrodes was larger under the 80 g load than under the 130 g load, presumably due to a transcallosally mediated inhibition exerted by the highly excited contralateral motor area; (ii) the mu-rhythm desynchronisation continued over both hemispheres for about 0.3-0.4 s after movement and the largest load was accompanied by the longest mu-rhythm desynchronisation; (iii) the post-movement beta-synchronisation was also longer under the heaviest load (130 g) as compared to the no-load condition (0 g), especially in subjects who prolonged their total movement time under the heaviest load. The results show that (i) the movement-related desynchronisation and synchronisation of sensorimotor EEG rhythms is influenced by external load opposing finger movement, and (ii) the effects of external load differ for the mu- and beta-rhythms.

Desynchronization of cortical rhythms following cutaneous stimulation: effects of stimulus repetition and intensity, and of the size of corpus callosum.

OBJECTIVE Event-related desynchronization (ERD) and synchronization (ERS) of the Rolandic electroencephalographic (EEG) rhythms following brief, innocuous electrocutaneous stimulation were studied with respect to stimulus intensity and repetition and the size of corpus callosum (CC). METHODS EEG was recorded using 82 closely spaced electrodes in 13 right-handed subjects. The subjects received 650 brief electrical stimuli to the right index finger at irregular intervals (6-12 s) in 5 blocks. The intensities of the stimuli varied randomly at 20, 30, 50, 65 and 80% of pain threshold. RESULTS Mu- and beta-ERD of 0.3-0.6 s latency over the contra- and ipsilateral S1/M1 area was observed in all subjects. Post-stimulus beta-ERS over the contra- and ipsilateral frontal cortices with a peak latency of 0.6-0.8 s was found in 9 subjects. Stimuli presented in the second half of the experiment were followed by a smaller ipsilateral mu-ERD and smaller contra- and ipsilateral beta-ERD than stimuli applied in the first block. Mu- and beta-ERD and beta-ERS distinguished weak (20%) from intermediate and strong stimuli (>35%) but not the intermediate from strong stimuli. The amplitude of ipsilateral beta-ERS correlated positively with the size of intermediate truncus of CC (r(9)=0.71, P<0.05). In contrast, ipsilateral ERD showed no significant correlations with the size of CC. CONCLUSIONS Habituation of ipsilateral mu-ERD and bilateral beta-ERD and beta-ERS suggests that these cortical responses are parts of the orienting response, and fail to disentangle fine intensity gradations. Ipsilateral beta-ERS appears to be mediated by the transcallosal fiber system.