Modulation of event-related desynchronization during kinematic and kinetic hand movements

Abstract

Background: Event-related desynchronization/synchronization (ERD/ERS) is a relative power decrease/increase of electroencephalogram (EEG) in a specific frequency band during physical motor execution and mental motor imagery, thus it is widely used for the brain-computer interface (BCI) purpose. However what the ERD really reflects and its frequency band specific role have not been agreed and are under investigation. Understanding the underlying mechanism which causes a significant ERD would be crucial to improve the reliability of the ERD-based BCI. We systematically investigated the relationship between conditions of actual repetitive hand movements and resulting ERD.

Methods: Eleven healthy young participants were asked to close/open their right hand repetitively at three different speeds (Hold, 1/3 Hz, and 1 Hz) and four distinct motor loads (0, 2, 10, and 15 kgf). In each condition, participants repeated 20 experimental trials, each of which consisted of rest (8-10 s), preparation (1 s) and task (6 s) periods. Under the Hold condition, participants were instructed to keep clenching their hand (i.e., isometric contraction) during the task period. Throughout the experiment, EEG signals were recorded from left and right motor areas for offline data analysis. We obtained time courses of EEG power spectrum to discuss the modulation of mu and beta-ERD/ERS due to the task conditions.

Results: We confirmed salient mu-ERD (8-13 Hz) and slightly weak beta-ERD (14-30 Hz) on both hemispheres during repetitive hand grasping movements. According to a 3 × 4 ANOVA (speed × motor load), both mu and beta-ERD during the task period were significantly weakened under the Hold condition, whereas no significant difference in the kinetics levels and interaction effect was observed.

Conclusions: This study investigates the effect of changes in kinematics and kinetics on resulting ERD during repetitive hand grasping movements. The experimental results suggest that the strength of ERD may reflect the time differentiation of hand postures in motor planning process or the variation of proprioception resulting from hand movements, rather than the motor command generated in the down stream, which recruits a group of motor neurons.

Figure 1

Kinematics pattern of the instructed hand grasping movement. Participants were instructed to relax during the rest and preparation periods, whereas they were asked to grasp their right hand at three distinct speeds (i.e., Hold, Slow, and Fast) and four different grasping loads (0, 2, 10, and 15 kgf) during the task period.

Figure 2

Layout of EEG electrodes. Eight active dry EEG electrodes were placed C3/C4 and the surrounding area based on the international 10–20 system. In offline analysis EEG data were re-treated by bipolar spatial derivation between C3/C4 and the neighbor electrodes.

Figure 3

Time course of ERD/ERS. Each figure shows a time course of the relative power decrease (ERD) and increase (ERS) on C3 under each speed and motor load condition. This is a typical result of a participant (Subject G). The horizontal axis indicates the time aligned at the onset of the task period (0 s), and the vertical axis indicates the frequency. The colorbar indicates the percentage of ERD/ERS. The mu-ERD can be observed during the task period. In the Hold condition, the mu-ERD disappears and mu-ERS are alternatively confirmed in the middle of the task period.

Figure 4

Effects of kinematics and kinetics on the resulting mu and beta-ERD over the contralateral (C3) and ipsilateral (C4) motor areas. Each figure demonstrates the statistical comparisons of the relative power decrease (ERD) during the task period under the different speeds and loads conditions averaged across subjects (n=11). Statistically significant difference was confirmed between the Hold and other speed conditions. On the other hand, there were no significant difference among kinetics conditions. ***(p < 0.001), **(p < 0.01), *(p < 0.05), and n.s. (p > 0.1).