2.4.3. ERS/ERD analysis

To measure mean event-related changes in the power spectrum, event-related spectral perturbation (ERSP) analysis was conducted by time–frequency decomposition (TFD) using the short-time Fourier transform (STFT) with a fixed Hanning window of 200 ms. For each trial, the time–frequency estimate at each point of the time plane was computed from −200 to 1,000 ms in the time domain and from 0.1 to 30 Hz in the frequency domain. The resulting spectrogram represented signal power as a joint function of time and frequency at each time–frequency point; this can be defined as:

For each trial, the baseline interval was set from −200 to 0 ms. Baseline correction was achieved by the subtraction method in the time–frequency domain:

where R¯(f) is the average power spectral density of the baseline interval at each frequency. A value of P_bl(t, f) <0 was considered to represent ERD and a value of P_bl(t, f) >0 was considered to represent ERS. ERD/ERS values for the theta (4–7 Hz), alpha (8–13 Hz), and beta (14–30 Hz) frequency bands at Fpz, FZ, CZ, and PZ were selected for analysis.