EEG beta suppression and low gamma modulation are different elements of human upright walking

Cortical involvement during upright walking is not well-studied in humans. We analyzed non-invasive electroencephalographic (EEG) recordings from able- bodied volunteers who participated in a robot-assisted gait-training experiment. To enable functional neuroimaging during walking, we applied source modeling to high-density (120 channels) EEG recordings using individual anatomy reconstructed from structural magnetic resonance imaging scans. First, we analyzed amplitude differences between the conditions, walking and upright standing. Second, we investigated amplitude modulations related to the gait phase. During active walking upper mu (10-12 Hz) and beta (18-30 Hz) oscillations were suppressed [event-related desynchronization (ERD)] compared to upright standing. Significant beta ERD activity was located focally in central sensorimotor areas for 9/10subjects. Additionally, we found that low gamma (24-40 Hz) amplitudes were modulated related to the gait phase. Because there is a certain frequency band overlap between sustained beta ERD and gait phase related modulations in the low gamma range, these two phenomena are superimposed. Thus, we observe gait phase related amplitude modulations at a certain ERD level. We conclude that sustained mu and beta ERD reflect a movement related state change of cortical excitability while gait phase related modulations in the low gamma represent the motion sequence timing during gait. Interestingly, the center frequencies of sustained beta ERD and gait phase modulated amplitudes were identified to be different. They may therefore be caused by different neuronal rhythms, which should be taken under consideration in future studies.