**2.6 Feature extraction**

The filtered signals are used to calculate spectrum PSD, in detail, estimate the PSD in the band between 8 and 30 Hz, this is based on the fact that the beta rhythm has distinct topographies and responses to the limb movements, compared with the alpha rhythm, the oscillatory power of the mu rhythm in the sensorimotor cortex ipsilateral to the tasks increased, while that of the beta rhythm in the contralateral sensorimotor cortex decreased simultaneously. The Welch's averaged used for spectral estimation that is a modified periodogram method. With 1 s Hamming window and 50% overlap, Welch's method can reduce noise. Each trial is divided into five bands and then the power and variance of each band are calculated, and energy of the whole trial is calculated.
